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NATIONAL TRANSMISSION &
DESPATCH COMPANY LTD.
GSO TRAINING CENTRE TARBELA
PROTECTION & INSTRUMENTATION
1st Semester
By:
Muhammad Mustafa
Additional Manager
Protection & Instrumentation
March, 2013
In the name of Almighty Allah,
The most merciful and the most beneficial
Dedicated To:
All those who helped me
in preparation of this
book
TABLE OF CONTENTS
1. General
1
1-1
Safety Principles
1-2
Electric Shock Hazards
1-3
1.2.1
Shock Current Equation
1.2.2
Effects of Electrical Shock
1.2.3
Path of Shock Current
FIRST AID
1.3.1 AIMS OF FIRST AID
1.3.2 First Aid for Electric Shock
1.3.3 Artificial respiration
1.3.4 Cardiopulmonary Resuscitation (CPR)
1-4
Low voltage hazards
1-5
Touch and Step Potential
1.6
Grounding & Bonding
1-7
Absolute limits of approach
1-8
1.7.1
Position of the Workman
1.7.2
Platforms and Aerial Devices
Clothing and Use of Personal Protective Equipment (PPE)
1-9
Fire Safety
1.9.1 Fire Extinguishers
1.9.2 In-Plant Training
1.9.3 Extinguisher Maintenance Tips
1.9.4 Hazardous Locations
2. Fundamentals of Electricity
2-1
AC and DC Supply
2-2
Types of Load
2-3
Ohm’s Law
2-4
Series and Parallel Combination of Resistors
2.4.1 Resistivity and its Dependence upon Temperature
2-5
2.4.2
Color Code for Carbon Resistances
2.4.3
Rheostat
2.4.4
Thermistors
2.4.5
Varastors
2.4.6
Electrical Power and Power Dissipation in Resistors
Single Phase and Three Phase Supply
20
2-6
Star and Delta Connections
2-7
Electrostatic and Electromagnetic Induction
3. Basic Concept of an Electrical Power System and Basic Components of Power
System
3-1
Power System Concept
3-2
Components of Power System
4. Electrical Measuring Instruments
4-1
Galvanometer
4-2
Voltmeter
4-3
Ammeter
4-4
Ohmmeter
4-5
Multi-meters
4-6
Clip-on ammeters
4-7
Cathode Ray Oscilloscopes
4-8
Safety Precautions
4-9
Important Points when Using Insulation Resistance Tester
4.10
Temperature Correction of Ri Readings
35
38
5. Basic Requirements Control Circuits
5.1
Auxiliary Switches
5.2
Device Function Numbers
5.3
Basic Requirements Control Circuits
5.4
5.3.1 Control the Closing
5.3.2 Control the Tripping
5.3.3 Trip Free Feature
5.3.4 Anti-Pumping Feature
5.3.5 Anti-Slam Feature
5.3.6 Reliability
5.3.7 General Maintenance of Breaker Control Relays
5.3.8 ASA Definitions
Overload Protection
5.5
5.6
5.7
Over Current or Short Circuit Protection
Contactor
Maintenance of Contactor
6. P&I Tools & Plant (T&P)
6.1
53
P&I Tools
6.1.1
P&I Personnel Tools
6.1.2 P&I Test Equipments
6.2
Shop Rules
6.3
Care and Up Keep of Tools
60
6.4
Safety Precautions when Applying Voltage from Test Equipment Sources
6.4.1
6.4.2
6.4.3
6.4.4
Testing Apparatus in its In-Service Location
Testing in a Location other in the In-Service Position
Signs and Guards Required during Voltage Testing of Apparatus
Special Precautions
7. Introduction of Grid Station Main and Auxiliary Equipment
7-1
Transformers
7-1-1 Use of Power Transformer
7-1-2 Types of Transformer
7-2
Circuit Breakers
7.2.1 Working Principle of Circuit Breaker
7.2.2 Types of Circuit Breaker
7-3
Disconnect Switches/Isolators
7-4
Lightning Arrester
7-5
Batteries and Battery Chargers
7-6
Station Grounding System
7-7
AC&DC Supply System
7-8
Power and Control Cables
7.8.1
7-9
Purposes of Shielding / Shield Grounding
Bus Bars
67
7.9.1 Bus Bar Schemes
8. Transformers
8.1
79
Fundamental Theory
8.1.2 Main Constructional Parts of Transformer
8.1.3 Ideal Transformer
8.1.4 Ideal Transformer Model
8.1.5 Theory of Transformer
8.1.6 Equivalent Circuit of Transformer
8.1.7 Losses in Transformer
8.2
Auto Transformer
8.3
Tertiary Winding of Transformer
8.4
Transformer Connections/Transformer Bank Connections, and Winding
Connections/Vector Groups
8.5
Phase sequence
8.6
Parallel operation of transformers
8.7
Transformer Tests
8.7.1
Polarity Test
8.7.2
Insulation Resistance Test
8.7.3 Transformer Turn Ratio Test
8.7.4 Open Circuit Test
8.7.5 Short Circuit Test
8.7.6 Verification of Vector Group
8.8
Cooling Systems
9. Current Transformers
125
9-1
Fundamental Theory
9-2
Types of Current Transformers
9-3
Errors in Current Transformers
9-4
Current Transformers Connections
9-5
Current Transformer Parameters
9-6
Accuracy Limit Factor and Instrument Security Factor of Current Transformer
9-7
Current Transformer Tests
9.6.1
Continuity Test
9.6.2 Insulation Resistance Test
9.6.3 Current Ratio Test
9.6.4 Polarity Test
9.6.5 Saturation Test
9.6.6 Circuit Verification Test
10. Potential transformers (PT) and Capacitor voltage transformers (CVT) 141
10.1
Fundamental Theory
10.2
Types of Potential Transformers
10.3
Errors in Potential Transformers
10.4
Potential Transformer Parameters
10.5
Potential Transformers Connections
10.6
Potential Transformer Tests
10.6.1 Continuity Test
10.7
10.6.2
Polarity test
10.6.3
Insulation resistance test
10.6.4
Voltage ratio test
10.6.5
Circuit verification
Potential Transformer Supply Supervision
11. Introduction to Protection
11-1
Introduction
11-2
11-3
Purpose of Protection Relaying
Principles of Protection Relaying
11-4
Functions of Protective Relaying
11-5
Protection Equipment
149
11-6
11.7
The Functional Requirements of the Relay
11.6.1
Reliability
11.6.2
Selectivity
11.6.3
Stability
11.6.4
Speed
11.6.6
Sensitivity
Relaying Terminology
11.7.1
Relaying Operation
11.7.2
Relay Resetting
11.7.3
Relay Pickup, Relay Dropout
11.7.4
Normally Open, Normally Closed Contacts
11.7.5
Pallet Switches
11.7.6
Relay Seal-In
11.7.7
Inverse Time and Definite Time Relays
11.7.8
Relay Target
11.7.9
Reach
11.7.10 Direct Under Reach
11.7.11 Permissive Overreach
11.7.12 Echo
11.7.13 Automatic Reclosing
11-8
Device Numbers and their Universal Nomenclature
11-9
Relay Protective Schemes
12. Over-Current Protection
12-1
Over Current Relays
12-2
Types of Over Current Relays
12-3
Operating Principals of Over Current Relays
12-4
Setting Calculations
12-5
Over Current Relay Testing
12-5-1
Pick- up/Drop off
12-5-2
Operating Time
13. Differential Protection
13-1
Faults in Power Transformer
13-2
Differential Protection
13-3
Principle of Differential Protection
13-4
Types of differential relays
13-5
Magnetizing Inrush Current
13-6
Balance of Differential Relay for Various Vector Groups
162
177
13-7
13-8
Differential relay testing
13-7-1
Pickup/ Drop off
13-7-2
Operating time
13-7-3
Percent slope
13-7-4
Percent Second Harmonics
Practical Connections of Differential on a Power Transformer
14. Under-Frequency relay
14-1
Under-Frequency Protection
14-2
Operating Principles
14-4
Under Frequency Relay Testing
14-4-1
Pickup/Drop off
14-4-2
Operating Time
185
15. Over-fluxing Relay
15-1
Causes of Over-fluxing in Transformer
15.2
Effect of Over Fluxing in Transformers
15-3
Operating Principles
16. Trip Circuit Supervision Relay
16-1
Trip Circuit Supervision Protection
188
193
16-2
Operating Principles
17. Restricted Earth Fault Relay
17-1
Restricted Earth fault Protection
17-2
Operating Principles
18. Breaker failure protection
18-1
Breaker Fail Protection
18-2
Operating Principles
19. Bus Bar Protection
19-1
Introduction
19-1
Bus Bar Faults
19-3
Bus Bar Protection Requirements
19-4
Types of Bus Bar Protection Systems
195
198
203
1.
General
1
1.1
THE SAFETY PRINCIPLES
Consider a work place. If, these places look familiar, you are right. However, we are not
interested in places, but the people who work in them. People like you and me. This program
then, is about safety and how to make it a part of everything you do.
A good place to begin our program is with the Safety Backs or as we will refer to them from now
on the “Safety Principles”.
There are five Safety Principles and it is expected at the end of this session that you will be able
to:
 State all five principles.
 Explain how the principles are applied in the work place.
 Demonstrate their application in a work assignment selected by your supervisor.
Now, what are the five Safety Principles? Well, here they are:
Safety Principles:
1. Know and identify the hazards.
2. Eliminate the hazards wherever practical.
3. Control the hazards when they cannot be eliminated.
4. Prevent or minimize the injuries when controlling the hazards.
5. Minimize the severity of injury if an injury has occurred.
The Safety Principles are logical and straightforward and it is very, very important that they are
applied in sequence to every job. The details of each principle are as follows:
Safety Principle - 1: Know and identify the hazard.
What do we mean? Well the requirement here is to ensure that you are knowledgeable of the
various hazards in the work place. The physical work environment is designed with the intent of
ensuring that unnecessary hazards are not present. However, sometimes it’s just not possible to
ensure that any given local work environment is hazard free. It’s of the utmost importance that
you be fully aware of the hazards associated with any work you are assigned.
Being knowledgeable of the hazard is not enough you must also be able to identify the hazards.
In other words, “knowing the hazard does not necessarily mean that you can identify them”.
Here is an example. You may know that asbestos is a potential hazard, but can you identify
asbestos? Probably not!
Although you know that asbestos is a hazard, you may have to get the help of someone qualified
to identify it.
No doubt you can think of other hazardous materials which might be difficult to identify. So,
identifying the hazard may mean having the hazard identified if you can’t do it yourself.
Safety Principle - 2: Eliminate the hazards wherever practical.
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In many cases, it will be practical to eliminate or remove the hazards that have been identified.
Let’s suppose that you are a mechanical maintainer and you are given a job to replace some
tubes in the boiler. The boiler will be shut down, and the tubes allowed cooling and draining.
Thus we have in effect eliminated two hazards. They are:
 The heat, and
 The pressurized water in the tubes
Now other hazards may still remain but we eliminated those that were practical to eliminate.
Ok, let’s try it from a different point of view. This time an E&I Maintainer is given a job to
change a wall receptacle in the business office. One of the steps the Maintainer will take is to
isolate and de-energize the receptacle circuit from the power source. By doing so, he has
eliminated the electrical hazard.
Again, other hazards may remain in his immediate work environment but he has eliminated a
hazard that was practical to eliminate.
Safety Principle - 3: Control the hazards when they cannot be eliminated.
Control the hazards when they cannot be eliminated. Sometimes it’s not practical to eliminate or
remove all the hazards associated with a particular job. When that is the case, you must take the
steps to control those hazards that remain.
For example, you are to perform a routine job in an area of the plant where the noise is
considered to be excessive.
 You have identified the hazard to be noise.
 In this case elimination was investigated and found to be impractical.
 So, now you must control the hazard. Period
But how do you control noise? Well control of the hazard can be achieved by following a work
procedure that will limit your exposure to the noise to an acceptably safe period of time.
Again take the example of drilling into a piece of steel. Certainly you can’t eliminate the
potential hazards due to the mechanical motion of the drill bit or the flying metal particles.
However, you can control the hazards by using a proper technical work procedure.
Safety Principle - 4: Prevent or minimize injuries when controlling the hazards.
This principle recognizes the need for a back-up control, measure to support Safety Principle#3.
Which is? That’s right, controlling the hazards that cannot be eliminated. What do we mean by
back-up control measure?
Well, we can control a noise hazard by ensuring that the time spent in an excessively noisy
environment is limited to a safe value, which in essence, is Safety Principle - 3. We can now
apply Safety Principle - 4 by making sure that the worker wears adequate hearing protection
during his entire stay in the noisy environment. That’s the back-up control measure.
Safety Principle - 5: Minimize the severity of the injury if an injury has occurred.
The principle dictates that you must know what to do to minimize the injury to yourself or a
fellow worker after an injury has occurred.
This may require:
3



Applying first aid to the injured person if you are qualified.
Getting someone who is qualified to perform first aid to the injured.
Providing emergency rescue assistance, if necessary.
Precisely what needs to be done depends a great deal on the extent of the injury and the
consequences of not taking immediate action.
And there you have it! Really what could be more logical and straight forward?
Yes, the five Safety Principles.
1. Know and identify the hazards.
2. Eliminate the hazards wherever practical.
3. Control the hazards when they cannot be eliminated.
4. Prevent or minimize the injuries when controlling the hazards.
5. Minimize the severity of injury if an injury has occurred.
We expect you to know and apply them on every job. As a matter of fact, you would be wise to
apply them off the job as well.
Looking at the safety principles in sequence, which, by the way, is how they must always be
applied, you will observe that principles - 1, 2, and 3 concentrate on the identification,
elimination and/or control of hazards.
Principle - 4 and 5 are directed toward the minimization of injuries.
You must certainly be aware that your primary safety objective is to prevent human injury. This
can be done through hazard identification, elimination and/or control. However, if human
injuries do occur, you must be able to minimize those injuries.
The five safety principles are designed to create and maintain a safer working environment for
all of us. Learn and apply the “Safety Principles” in everything you do.
1.2
ELECTRICAL SHOCK HAZARD
The basic shock hazards presented by electricity to human being are:
1. Physical movement caused by involuntary muscular reactions stimulated by the passage
of current through the body, or reactions caused by the sensation of the passage of
current. At high current values a person might be “thrown” from the circuit, at medium
currents he may not be able to let go, and at low “perceptible” currents he might pull back
or “jump”. Spark discharges, although not dangerous in themselves, can cause
involuntary body movement. Unplanned physical movement can cause falls, slips and
others injuries involving body mechanics.
2. Actual physical damage to the body caused by the passage of an electrical current. Tissue
distraction occurs due to the heat produced by the current flowing through the body
resistance.
4
3. Cassation of the proper functioning of vital organs due to the passage of current through
the body.
All involve the passage of an electric current through the body. Let us first examine,
then, how this is likely to happen.
1.2.1 SHOCK CURRENT EQUATION
When a voltage is impressed across any two points of the body, a resultant current will flow
between these points. This condition is setup by bodily proximity or contact with an electric
circuit or component that is “alive”. A circuit or component that is alive is one that has an
electric potential impressed upon it at some level above that of ground potential. The most usual
type of contact is between the circuit and ground, i.e. the hand touching a live circuit while
standing on the ground. This situation is shown in diagrammatically in Fig (1). The circuit
formed is a relatively simple one to which Ohm’s Law can readily be applied. The resultant
current that will pass through the body is of vital importance in determining the severity of the
damage to the body; therefore, the circuit should be examined with the object of determining this
current:
Where, V = Voltage to ground of source
RL = Resistance of circuit
RC = Contact resistance
RB = Resistance of body
RN = Resistance of ground
The source voltage V to ground is readily determined and can be considered to be constant. RL,
the resistance of the circuit is constant and can be determined with some difficulty. RN, the
resistance of the ground return path is constant and can be determined accurately only with great
difficulty. The contact resistance RL is a variable depending upon the area of the contact, the
resistance of gloves and soles of shoes, the condition of the skin, i.e. moist or dry. RB, the body
resistance varies considerably with the individual, with his condition at the instant of contact,
with the magnitude and frequency of the voltage that is applied and with the duration of contact.
The body resistance is the greatest unknown in the equation.
It is very difficult to determine the current that will pass through the body for any particular
situation; yet, it is the current that determines the severity of the damage of the individual and
whether he will live or die.
Unfortunately, the best known factor in our life-or-death equation is the source voltage and,
therefore, we commonly talk in terms of voltage and tend to relate it to a measurement of hazard,
i.e. “220 000 V circuits are more dangerous than 220 V circuits”. This is because live HV circuits
are generally inaccessible and are conspicuously dangerous, i.e. intense electrostatic field warns
these approaching, whereas, LV circuits give no warning prior to contact and are generally
5
easily accessible. THIS CAN LEAD ONE TO BE CARELESS AROUND LOWER
VOLTAGES. Individuals have been killed by contact with ordinary house circuits of 110 V AC
and by electrical apparatus in industry using as 42 V DC.
6
Fig (1)
7
1.2.2 EFFECTS OF ELECTRICAL SHOCK
The chart in Fig (2) outlines the probable effects of various magnitudes of shock current. Any
amount of current over 10 mA (0.01 A) is capable of producing a painful to severe shock. As the
current is increased the shock becomes increasingly severe. When a current flows through the
body, the muscles in its path tend to contract. This contraction may be so severe that the victim
cannot release his grasp on the live circuit.
The fact that the victim cannot let go is very important because in a few seconds blisters will
form on the skin at the contact points and the skin resistance reduces substantially thus allowing
the current through the body to increase.
At values as low as 20 mA (0.020 A) breathing become labored, finally ceasing completely at
values even below 75 mA (0.75 A).
At values approaching 100 mA (0.10 A) ventricular fibrillation of the heart occurs. This is an
uncoordinated twitching of the walls of the heart’s ventricles; in this condition the heart is not
pumping and death will most certainly occur unless the victim receives specialized medical
treatment which is not readily available.
Strangely enough, 200 mA (0.2 A) the muscular contractions are so severe that the heart is
forcibly clamped during the period that the current is flowing. This clamping action prevents the
heart from going into ventricular fibrillation and the victim has a chance for a survival.
8
Fig (2)
9
1.2.3 PATH OF SHOCK CURRENT
The path of the current through the body as well as the magnitude of the current is important
factors in shock damage. For instance, if the current does not pass through any vital organs, the
victim will no doubt be somewhat better off. The current limiting effect of the outer skin can be
considerably greater on a calloused palm than a sensitive area on another part of the body. The
outer layer of the skin provides most of the body’s current limiting resistance. Between the ears,
for example, the internal resistance (less the skin resistance) is only about 100 ohms, while from
hand to foot it is about 500 ohms. The outer skin resistance may vary from 1000 ohms for wet
skin to over 1000, 000 ohms for dry skin.
1.3
FIRST AID
First aid is the provision of initial care for an illness or injury. It is usually performed by nonexpert, but trained personnel to a sick or injured person until definitive medical treatment can be
accessed. Certain self-limiting illnesses or minor injuries may not require further medical care
past the first aid intervention. It generally consists of a series of simple and in some cases,
potentially life-saving techniques that an individual can be trained to perform with minimal
equipment.
While first aid can also be performed on all animals, the term generally refers to care of human
patients.
1.3.1 AIMS OF FIRST AID
The key aims of first aid can be summarized in three key points:



Preserve life: The overriding aim of all medical care, including first aid, is to save lives
Prevent further harm: Also sometimes called prevent the condition from worsening, or
danger of further injury, this covers both external factors, such as moving a patient away
from any cause of harm, and applying first aid techniques to prevent worsening of the
condition, such as applying pressure to stop a bleed becoming dangerous.
Promote recovery: First aid also involves trying to start the recovery process from the
illness or injury, and in some cases might involve completing a treatment, such as in the
case of applying a plaster to a small wound
First aid training also involves the prevention of initial injury and responder safety, and the
treatment phases.
1.3.2 FIRST AID FOR ELECTRIC SHOCK
Shock is a common occupational hazard associated with working with electricity. A person who
has stopped breathing is not necessarily dead but is in immediate danger. Life is dependent on
oxygen, which is breathed into the lungs and then carried by the blood to each and every body
cell. Since body cells cannot store oxygen and since the blood can hold only a limited amount
(and only for a short time), death will surely result from continued lack of breathing.
10
However, the heart may continue to beat for some time after breathing has stopped, and the
blood may still be circulated to the body cells. Since the blood will, for a short time, contain a
small supply of oxygen, the body cells will not die immediately. For a very few minutes, there is
some chance that the person's life may be saved.
The only logical, permissible delay is that required to free the victim from contact with the
electricity in the quickest, safest way. This step, while it must be taken quickly, must be done
with great care; otherwise, there may be two victims instead of one.
In the case of portable electric tools, lights, appliances, equipment, or portable outlet extensions,
the victim should be freed from contact with the electricity by turning off the supply switch or by
removing the plug from its receptacle. If the switch or receptacle cannot be quickly located, the
suspected electrical device may be pulled free of the victim. Other persons arriving on the scene
must be clearly warned not to touch the suspected equipment until it is reenergized.
The injured person should be pulled free of contact with stationary equipment (such as a bus bar)
if the equipment cannot be quickly reenergized or if the survival of others relies on the electricity
and prevents immediate shutdown of the circuits. This can be done quickly and easily by
carefully applying the following procedures:
1. Protect yourself with dry insulating material.
2. Use a dry board, belt, clothing, or other available nonconductive material to free the
victim from electrical contact. DO NOT touch the victim until the source of electricity
has been removed.
Once the victim has been removed from the electrical source, it should be determined whether
the person is breathing. If the person is not breathing, a method of artificial respiration is used.
1.3.3 ARTIFICIAL RESPIRATION
The process by which a person who has stopped breathing can be saved is called artificial
respiration (ventilation). The purpose of artificial respiration is to force air out of the lungs and
into the lungs, in rhythmic alternation, until natural breathing is reestablished. Records show that
seven out of ten victims of electric shock were revived when artificial respiration was started in
less than three minutes. After three minutes, the chances of revival decrease rapidly.
Artificial respiration should be given only when the breathing has stopped. Do not give artificial
respiration to any person who is breathing naturally. You should not assume that an individual
who is unconscious due to electrical shock has stopped breathing. To tell if someone suffering
from an electrical shock is breathing, place your hands on the person's sides at the level of the
lowest ribs. If the victim is breathing, you will usually be able to feel movement.
Once it has been determined that breathing has stopped, the person nearest the victim should start
the artificial respiration without delay and send others for assistance and medical aid.
11
Practical demonstration will be made by using SCHAFER method of artificial respiration
practice received by an electrical short victim.
1.3.4 CARDIOPULMONARY RESUSCITATION (CPR):
Sometimes victims of electrical shock suffer cardiac arrest or heart stoppage as well as loss of
breathing. Artificial respiration alone is not enough in cases where the heart has stopped. A
technique known as CPR has been developed to provide aid to a person who has stopped
breathing and suffered a cardiac arrest. Because you are working with electricity, the risk of
electrical shock is higher than in other occupations. You should, at the earliest opportunity, take
a course to learn the latest techniques used in CPR. The techniques are relatively easy to learn
and are taught in courses available through the Civil Defense Organization.
A heart that is in fibrillation cannot be restricted by closed chest cardiac massage. A special
device called a defibrillator is available in some medical facilities and ambulance services.
Muscular contractions are so severe with 200 mA and over that the heart is forcibly clamped
during the shock. This clamping prevents the heart from going into ventricular fibrillation,
making the victim's chances for survival better.
1.4
LOW VOLTGE HAZARD
Signs are often placed on electrical equipment displaying the words “Danger – High – Voltage”.
In our minds, we would do well to register another sign “Danger – Low – Voltage!” From
experience, the victims of high voltage shock usually respond to artificial respiration more
readily than to the victims of low voltage shock. The only conclusion to be drawn here is that
220 V can be just as lethal as 11,000 V.
Why Low Voltage is considered more dangerous than H.V? Generally electric hazard occurs due
to low voltage, rather than high voltage because high voltages are inaccessible as they give
warning to approaching persons prior to contact due to their electrostatic fields. But low voltages
give no such warning and are felt only when they are approached or touched. Therefore, low
voltages are considered more dangerous than high voltages.
1.5
TOUCH AND STEP POTENCIALS
Fig (3A) and (3B) illustrate the potential gradient that can exist along the earth during heavy
fault currents into a ground rod (electrode).
The heavy faults currents flowing down through the portable-temporary grounds cause the tower,
the ground rod and the earth in its immediate vicinity to rise well above ground potential. Since
the earth some distance away remains at normal ground potential a voltage gradient exists across
the surface of the earth in the immediate area.
In Fig (3A) touch potential hazard would be experienced by man ‘A’. It is the voltage
experienced by a person standing on earth and touching the structure, while a ground fault is
occurring.
12
The man ‘B’ in this figure would experience step potential hazard. The situation is much the
same as that described for touch potential, except in this case the person experiences the
potentials from foot to foot when he straddles between the ground grid and the earth.
If man ‘A’ was standing on a ground gradient mat fastened to the ground rod or tower, no
potential difference would exist between his hands and feet and he would be protected during the
fault condition.
Furthermore, if a ground grid system with a perimeter extending beyond the work area, such as
used in station ground networks, replaced the single ground rod, both man ‘A’ and ‘B’ would be
protected.
Station designers consider “touch and step potential” hazards in station ground network design
and incorporate gradient control methods into individual station network design.
A well designed station ground network provides low overall impedance to ground, a current
carrying capacity consistent with fault currents obtainable and a uniform or near uniform
potential of all earth surfaces within its perimeter during heavy fault current conditions.
Fig (3)
1.6
GROUNDING AND BONDING
The distinction between “grounding,” to clear the fault, and “bonding”, to save the man, can be
illustrated by referring to Fig (4). For the purpose of illustration only, one phase on a three-phase
circuit is shown; the other two phases would be similarly grounded. The work includes checking
the HV bushing connection, the LV bushing connection, the disconnect switch, and the bus
connector which will not be opened.
The portable-temporary ground on the HV side will provide a high capacity circuit to station
ground, and as well bond the workman because the transformer case he is standing on is also
solidly fastened to station ground, The same applies to the LV bushing connection, to the bus and
the blade side of the disconnect switch. The third ground on the jaw, side of the switch provides
both grounding and bonding for the man on the switch structure and bus are grounded to station
ground.
13
If the circuit becomes accidentally energized, the portion of the fault current which returns to its
source through the ground system and the earth would cause a potential difference between the
station ground networks including everything connected to it and the surrounding earth. This
potential difference could achieve several thousand volts for the duration of the fault.
However, since the station ground network is designed so there will be no appreciable difference
of potential in its various parts during the fault conditions, the greatest voltage gradient will exist
in the immediate vicinity of the buried conductors around the perimeter of the ground network
where the concentration of current is highest.
This gradient will produce a potential difference between the scaffold, touching the earth, and the
grounded bus above it. For this reason, the man on the scaffold must be bonded by connecting
the scaffold to either the bus or station ground by means of a portable-temporary ground. The
safety of this man is ensured by bonding the scaffold he is standing on to the bus he is touching.
The man is “outside” the grounds, but he is safely bonded and the circuit is bonded.
1.7
Fig (4)
ABSOLUTE LIMITS OF APPROACH
Of prime importance is the maintenance of the absolute limits of approach in using live line
tools. This is defined here as the length of clean dry epoxiglas insulation (or polypropylene rope
on the mannen stick) between the live component of the tool attached to the conductor and the
closest part of the person using the tool. The tools selected must be at least long enough to ensure
that these clearances are maintained. The table of absolute limits is shown hereunder as in Table
“A”.
Table A
Nominal Voltage Range
Absolute Limit of Approach
14
750
up to 15,000
Over 15,000 up to 50,000
Over 50,000 up to 150,000
Over 150,000 up to 250,000
Over 250,000 up to 550,000
0.31 m (1foot)
0.46 m (1.5 feet)
0.92 m (3 feet)
1.22 m (4 feet)
2.75 m (9 feet)
Caution: It is absolutely necessary to consider other live equipment or circuits that may be in
the work area when selecting live line tools and planning the work procedures. Additional
clearance should always be provided to allow for unplanned or inadvertent movement.
1.7.1 POSITION OF THE WORKMAN
Usually the workman will be standing on a grounded surface and be at ground potential when
performing live line operations. He should position himself in a convenient location where he
has good footing, easy access to the work and from where he can maintain, at all times, the
absolute limits of approach to all live parts in the work area.
In many station situations; accesses to equipment is difficult or awkward; work clearances are
limited and the fault current capability is very high. Live line operations in these circumstances
must be strictly limited to those which are absolutely essential and every precaution must be
taken not only for electric shock hazard protection but also to avoid an arc from forming, which
could have disastrous results.
1.7.2 PLATFORMS AND AERIAL DEVICES
When working from a non-insulated work platform including a ladder truck, the same general
principles will apply in the use of live line tools. Additional hazards exist, however, in these
circumstances, particularly in high voltage switchyards.
The work platform may be less stable and provide poorer footing than when standing on the
ground or belted onto a structure. Additional care must be taken to achieve firm balance and
control of the live line tools by the correct positioning of the workman with respect to the work
platform and the conductor being worked on.
In addition, the workmen will likely be within the influence of the HV electrostatic field of the
circuit being worked on of or a neighboring circuit. The capacitive coupling effect will tend to
cause his body to become charged at a voltage somewhat above ground potential. The resulting
shocks that he feels when his body comes in contact with ground potential, although not
dangerous in themselves, could be seriously distracting or even cause him to lose his balance.
To avoid this hazard, the workman should wear approved conductive boots and ensure that the
15
metal surface that he is standing on is grounded, thus continuously draining the charge that
tends to build up in his body.
When the absolute limit of approach to high voltage conductors is maintained, the electrostatic
field is not strong enough to cause a continuous current flow through his body to ground, to
exceed the “perception threshold” value (the minimum continuity with a feeling) provided that
the worker maintains continuity with a grounded surface.
There are two very important factors to consider when positioning the Ariel device:
It must be positioned so that those working from it will maintain the absolute limits of approach
as described above and, in addition, it is also necessary to maintain more stringent clearances
between the aerial devices itself and all live circuits. These further restrictions are outlined in
Table B.
Table-B
Nominal Voltage Range
750 up to 15,000
Over 15,000 up to 50,000
Over 50,000 up to 150,000
Over 150,000 up to 250,000
Over 250,000 up to 550,000
Limit of Approach
0.92 m (3 feet)
1.22 m (4 feet)
2.44 m (8 feet)
3.05 m (10 feet)
4.58 m (15 feet)
Station crews also use insulated aerial devices or bucket trucks. They are used primarily as work
platforms to gain to access to station structures and equipment. Table B does not apply to the use
of insulated bucket trucks. The general rule for work in stations is THE ABSOLUTE LIMITS
OF APPROACH (see table A) FROM THE LIVE APARATUS TO THE MAN IN THE
BUCKET SHALL BE MAINTAINED AT ALL TIMES.
Exception might be made from time to time in specific instances for men who receive
specialized training and become qualified to work within these distances, e.g. bare hand work.
The continuous current flow through the body due to electrostatic induction increases as one
comes closer to an energized high voltage conductor. Special techniques must sometimes be
employed to continuously bypass this current around the worker or shield him from the
electrostatic field. This effect becomes much stronger as the voltage of the circuit increases.
16
1.8
CLOTHING AND USE OF PERSONAL PROTECTIVE EQUIPMENT (PPE)
Clothing should fit snugly to avoid danger of becoming entangled in moving machinery or
creating a tripping or stumbling hazard. See Fig (5). Clothing should fit snugly to avoid danger
of becoming entangled in moving machinery or creating a tripping or stumbling hazard.
Fig (5)
Recommended safe work clothes include:
1. Thick-soled work shoes for protection against sharp objects such as nails. Wear work
with safety toes if the job required. Make sure the soles are oil resistant if the shoes are
subject to oil and grease.
2. Rubber boots for damp locations
3. A hat or cap. Wear an approved safety helmet (hard hat) if the job requires
Do not wear Tie, confine long hair or keep hair trimmed and avoid placing the head in close
proximity to rotating machinery. Do not wear jewelry. Gold and silver are excellent conductors
of electricity.
1.9
FIRE SAFETY
The chance of fire is greatly decreased by good housekeeping. Keep rags containing oil,
gasoline, alcohol, shellac, paint, and varnish in a covered metal container. Keep debris in a
designated area away from the building. Sound an alarm if a fire occurs. Alert all workers on the
job and then call the fire department. After calling the fire department, make a reasonable effort
to contain the fire.
17
1.9.1 FIRE EXTINGUISHERS
Always read instructions before using a fire extinguisher.
Always use the correct fire extinguisher for the class of fire.
See Fig (6). Fire extinguishers are normally red. Fire
extinguishers may be located on a red background so they
can be easily located. Always use the correct fire
extinguisher for the class of fire.
Be ready to direct firefighters to the fire. Inform them of any
special problems or conditions that exist, such as downed
electrical wires or leaks in gas lines. Report any
accumulations of rubbish or unsafe conditions that could be
fire hazards. Also, if a portable tool bin is used on the job, a
good practice is to store a CO2 extinguisher in it.
1.9.2 IN-PLANT TRAINING
Fig (6)
A selected group of personnel (if not all personnel) should be acquainted with all extinguisher
types and sizes available in a plant or work area. Training should include a tour of the facility
indicating special fire hazard operations.
In addition, it is helpful to periodically practice a dry run, discharging each type of extinguisher.
Such practice is essential in learning how to activate each type, knowing the discharge ranges,
realizing which types are affected by winds and drafts, familiarizing oneself with discharge
duration, and learning of any precautions to take as noted on the nameplate.
1.9.3 EXTINGUISHER MAINTENANCE TIPS
18
Inspect extinguishers at least once a month. It is common to find units that are missing, damaged,
or used. Consider contracting for such a service. Make contract for annual maintenance with a
qualified service agency. Never attempt to make repairs to extinguishers. This is the chief cause
of dangerous shell ruptures.
1.9.4 HAZARDOUS LOCATIONS
The use of electrical equipment in areas where explosion hazards are present can lead to an
explosion and fire. This danger exists in the form of escaped flammable gases such as naphtha,
benzene, propane, and others. Coal, grain, and other dust suspended in air can also cause an
explosion. Any hazardous location requires the maximum in safety and adherence to local,
provincial, and federal guidelines and laws, as well as in-plant safety rules.
To sum it all up:
“Working with electricity can be dangerous. However,
electricity can be safe if properly respected”
So Be Careful Out There!
19
2.
20
Fundamental of
Electricity
2.1
DC AND AC SUPPLY
Electricity is a form of energy called Electrical Energy. There are two types of electricity, Static
and Dynamic. Dynamic Electricity can be either Direct Current (DC) or Alternating Current
(AC).
STATIC ELECTRICITY
When two non-conductors such as silk cloth and glass rod are rubbed together, some electrons
are freed. Both materials become electrically charged. One is lacking electron and is positively
charged. The other has extra electrons and is negatively charged. These charges remain on the
surface of the material and do not move unless the two materials touch or are connected by a
conductor. Since there is no electricity flow, this is called Static Electricity.
DYNAMIC ELECTRICITY
21
When electrons are freed from their atoms and flow in a material, this is called dynamic
electricity.
DC SUPPLY
If the free electrons flow in one direction, the electricity is called Direct Current (DC). This is
the type of current produced by the vehicle’s battery. So an electrical supply whose amplitude
remains constant with respect to time is called Direct Current supply. Voltages and currents
remain constant over time (subject to ripple & transients).
AC SUPPLY
If the free electrons change direction from one positive to negative and back repeatedly with
time, the electricity is called alternating Current (AC). This is the type of current produced by
the vehicle’s alternator. So an electrical supply whose amplitude varies with respect to time and
repeats its shape is called alternating current supply. Voltages and currents are sinusoidal
(subject to higher frequency harmonics and transients). In Pakistan, all AC has a fixed frequency
of 50 Hz.
2.2
TYPES OF LOAD
Loads are classified as follows:
TYPES BY NATURE



Resistive
Inductive
Capacitive
TYPES BY CONNECTIONS
 Single Phase
 Poly Phase
2.3
OHM’S LAW
When a battery is connected across a conductor, an electric current begins to flow through it.
How much current flows through a conductor when a certain potential difference is set up across
its ends?
22
The answer to this question was given by German Physicist George Simon Ohm. He showed by
elaborate experiments that the current through a metallic conductor is directly proportional to the
potential difference across its ends. This fact is known as Ohm’s law which states that
“The current flowing through a conductor is directly proportional to the potential difference
applied across its ends provided the physical state such as temperature etc. is kept constant”.
Symbolically Ohm’s law can be written as
IαV
It implies that
V=IR
(1)
Where R is the constant of proportionality, is called the resistance of the conductor. The value of
the resistance depends upon the nature of the conductor, dimensions and the physical state of the
conductor. In fact the resistance is the measure of the opposition to the motion of electrons due to
their continuous bumping with lattice atoms. The unit of resistance is ohm. A conductor has a
resistance of one ohm if a current of one ampere flow through it when a potential difference of
one volt is applied across its ends. The symbol of ohm is Ω. If I is measured in ampere, V in
volts, then R is measured in ohms i.e.
R (ohms) = V (volts) / I (amperes)
A sample of conductor is used to obey Ohm’s law if its resistance R remains constant that is,
graph of its V verses I is exactly a straight line Fig (1). A conductor which strictly obeys Ohm’s
law is called Ohmic Conductor. However, there are certain devices, which do not obey Ohms
law i.e., they are Non-Ohmic Conductors. The examples of such devices are filament of a bulb
and semiconductor diodes.
23
Let us apply a certain potential difference across the terminals of a filament lamp and measure
the resulting current passing through it. If we repeat the measurement for the different values of
potential difference and draw a graph of voltage V verses current I, it will be seen that the graph
is not a straight line Fig (2). It means that a filament is a non- Ohmic device. This deviation of
I-V graph from a straight line is due to increase in resistance of the filament with temperature-a
topic which will be discussed in next section.
As the current passing through the filament is increased from zero, the graph is a straight line in
the initial stage because the change in the resistance of the filament with temperature due to
small current is not appreciable. As the current is further increased, the resistance of the filament
continues to increase due to rise in its temperature.
Fig (2)
24
Another example of non-Ohmic device is a semiconductor diode. The current-voltage plot of
such a diode is shown in Fig (3). The graph is not a straight line so semiconductor is also nonOhmic device.
2.4
SERIES AND PARALLEL COMBINATION OF RESISTORS
In an electric circuit, usually, a number of resistors are connected together. There are two
arrangements in which resistors can be connected with each other; one is known as series
arrangement and other one as parallel arrangement.
If the resistors are connected end to end such that the same current flow through all of them, they
are said to be connected in series as shown in Fig (4).
There equivalent resistance Re is given by
Re=R1 + R2 + R3 + ------
(2)
In parallel arrangement a number of resistors are
connected side by side with their ends joined together at two common points as shown in Fig (5).
Fig (4)
The equivalent resistance Re of the arrangement is given by
1/Re= 1/R1+1/R2+1/R3+ ------
(3)
25
Fig (5)
2.4.1 RESISTIVITY AND ITS DEPENDENCE UPON TEMPERATURE
It has been experimentally seen that the resistance R of a wire is directly proportional to its
length L and inversely proportional to its cross sectional area A. Expressing mathematically we
have
R α L/A
or
R=ρL/A
(4)
Where ρ is the constant of proportionality, known as resistivity or specific resistance of the
material of the wire. It may be noted that resistance is the
characteristic of a particular wire whereas the resistivity is the
property of the material of which the wire is made. From Equation
(4), we have
ρ = RA/L
(5)
The above equation gives the definition of resistivity as the
resistance of a meter cube of a material. The SI unit of resistivity is
ohm-meter (Ω-m)
Conductance is another quantity used to describe the electrical
properties of materials. In fact conductance is the reciprocal of
resistance i.e.
Conductance = 1/Resistance
26
The SI unit of conductance is mho or siemens.
Likewise conductivity, σ is the reciprocal of resistivity i.e.
σ= 1/ρ
Table (1)
The SI unit of conductivity is ohm_1 m_1 or mho m_1. Resistivity of various materials is given in
Table (1).
It may be noted from Table (1) that silver and copper are two best conductors. That is the reason
that most electric wires are made of copper.
The resistivity of a substance depends upon the temperature also. It can be explained by recalling
that the resistance offered by a conductor to the flow of electric current is due to collisions,
which the free electrons encounter with atoms of lattice. As the temperature of the conductor
rises, the amplitude of vibrations of the atoms in the lattice increases and hence, the probability
of their collisions with free electrons also increases. One may say that the atoms then offer a
bigger target, i.e., the collision cross-section of the atoms increases with temperature. This makes
the collisions between the free electrons and the atoms of the lattice more frequent and hence, the
resistance of the conductor increases. Experimentally the change in resistance of a metallic
conductor with temperature is found to be nearly linear over a considerable range of temperature.
Above and below 0oC as shown in Fig (6).
27
Over such a range the fractional change in resistance per Kelvin is known as the temperature
coefficient of resistance i.e.
α = (Rt – Ro)/ Rot
(6)
Where Ro and Rt are resistances at temperature 0oC and toC. As resistivity ρ depends on the
temperature, Eq. (6) gives
Rt=ρL/A
and Ro=ρL/A
Substituting the values of Rt and Ro in Eq. (6) we get
As α = ρt – ρo/ ρot
(7)
Where ρo is the resistivity of the conductor at 0oC and ρt is the resistivity at toC.
There are some substances like silicon, germanium etc., whose resistance decreases with increase
in temperature i.e., these substances have negative temperature coefficients.
2.4.2 COLOR CODE FOR CARBON RESISTANCES
Carbon resistors are more common in electronic equipments. These consists of high grade
ceramic rod or cone (called the substrate) on which is deposited resistive thin film of carbon. The
numerical values of their resistances are indicated by a color code which consists of bands of
different colors printed on the body of the resistor. The color used in this code and the digit
represented by them are given in Table (2).
Usually the code consists of four bands, as shown in Fig (7). Starting from left to right, the color
bands are interpreted as follows:
1. The first band indicates the first digit in the numerical value of the resistance.
2. The second band gives the second digit.
3. The third band is decimal multiplier i.e., it gives the number of zeroes after two digits.
4. The fourth band gives resistance tolerance. Its color is either gold or silver.
Silver band indicates tolerance of 10%, a gold shows a tolerance of 5%. If there is no
fourth band, tolerance is understood to be 20%. By tolerance, we mean the possible
variation from the marked value. For example, a 1000 resistor with a tolerance of 10%
will have an actual resistance anywhere between 900Ω and 1100Ω.
28
Fig (7)
Color
Value
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Gray
8
White
9
Ta
ble (2)
2.4.3 RHEOSTAT
It is a wire wound variable resistance. It consists of a bare Manganine wire wound over an
insulating cylinder. The ends of the wire are connected to two fixed terminals A and B as in Fig
(8). A third terminal is connected to a sliding contact C which can also be moved over the wire.
A rheostat can be used as a variable resistor as well as potential divider. To use it as a variable
resistor one of the fixed terminals such as A and the sliding terminal C is used. If the sliding
29
contact is shifted away from the terminal A, the length and hence the resistance included in the
circuit is increased and vice versa.
Fig (8)
A rheostat is also used as potential divider. This is illustrated in Fig (9). A potential difference
V is applied across the ends A and B of the rheostat with the help of a battery. If R is the
resistance of the wire AB, the current I passing through it is given by I=V/R.
Fig (9)
The potential difference between the portions BC of the wire AB is given by
VBC = current X resistance
= (V/R) r = (r/R) V
(8)
Where r, is the resistance of the portion BC of the wire. The circuit shown in the Fig (8) is
known as potential divider. Eq. (8) shows that this circuit can provide at its output terminals a
potential difference varying from zero to the full potential difference of the battery depending on
the position of the sliding contact. As the sliding contact C is moved towards end B, the length
30
and hence the resistance r of the portion of the wire decreases which according to Eq. (8),
decreases VBC. On the other hand if the sliding contact C is moved towards the end A, the output
voltage VBC increases.
2.4.4 THERMISTORS
A thermistor is a heat sensitive resistor. Most thermistors have negative temperature of resistance
i.e. the resistance of such thermistors decreases with increase in temperature. Thermistors with
positive temperature of coefficient are also available.
Thermistors are made by heating under high pressure semiconductor ceramic made from
mixtures of metallic oxides of manganese, nickel, copper, cobalt, iron etc. these are pressed into
desired shapes and then baked at high temperatures. Different types of thermistors are shown in
Fig (10) they may be in the form of beads, rods or washers.
Fig (10)
Thermistors with high negative temperature of resistance are very accurate for measuring low
temperatures especially near 10K. The higher resistance at low temperature enables more
accurate measurement possible.
Thermistors have wide applications as temperature sensors i.e., they convert changes of
temperature into electrical voltages which are duly processed.
2.4.5 VARISTORS
A varistor is an electronic component with diode-like non-linear voltage-current characteristics.
The name is a portmanteau of variable resistor. Varistors are often used to protect circuits against
excessive transient over-voltages by incorporating them into the circuit in such a way that, when
triggered, they will shunt the current created by the high voltage away from sensitive
components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s
function is to conduct significantly increased current when voltage is excessive.
31
Only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable
resistor include the potentiometer and the rheostat.
The most common type of varistor is the metal-oxide varistor (MOV). This contains a ceramic
mass of Zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth,
cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary
between each grain and its neighbor forms a diode junction, which allows current to flow in only
one direction. The mass of randomly oriented grains is electrically equivalent to a network of
back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate
voltage is applied across the electrodes, only a minute current flows, caused by reverse leakage
through the diode junctions. When a large voltage is applied, the diode junction breaks down due
to a combination of thermionic emission and Electron Tunneling, and a large current flow. The
result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has
a high resistance at low voltages and a low resistance at high voltages.
A varistor remains non-conductive as a shunt-mode device during normal operation when the
voltage across it remains well below its "clamping voltage", so varistors are typically used to
suppress line voltage surges. However, a varistor may not be able to successfully limit a very
large surge from an event such as a lightning strike where the energy involved is many orders of
magnitude greater than it can handle. Follow-through current as a result of a strike may generate
excessive current that completely destroys the varistor. Lesser surges still degrade it, however.
Degradation is defined by manufacturer's life-expectancy charts that relate current, time and
number of transient pulses. The main parameter affecting varistor life expectancy is its energy
(Joule) rating. As the energy rating increases, its life expectancy typically increases
exponentially, the number of transient pulses that it can accommodate increases and the
"clamping voltage" it provides during each transient decreases.
The probability of catastrophic failure can be reduced by increasing the rating, either by using a
single varistor of higher rating or by connecting more devices in parallel. A varistor is typically
deemed to be fully degraded when its "clamping voltage" has changed by 10%. In this condition
it is not visibly damaged and it remains functional (no catastrophic failure).
In general, the primary case of varistor breakdown is localized heating caused as an effect of
thermal runaway. This is due to a lack of conformity in individual grain-boundary junctions,
which leads to the failure of dominant current paths under thermal stress. If the energy in a
transient pulse (normally measured in joules) is too high, the device may melt, burn, vaporize, or
otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum
Ratings" in manufacturer's data-sheet are significantly exceeded.
HIGH VOLTAGE VARISTOR
32
Important parameters are the varistor's energy rating in joules, operating voltage, response time,
maximum current, and breakdown (clamping) voltage. Energy rating is often defined using
standardized transients such as 8/20 microseconds or 10/1000 microseconds, where 8
microseconds is the transient's front time and 20 microseconds is the time to half value. To
protect communication lines (such as telephone lines) transient suppression devices such as 3 mil
carbon blocks (IEEE C62.32), ultra-low capacitance varistors or avalanche diodes are used. For
higher frequencies such as radio communication equipment, a gas discharge tube (GDT) may be
utilized.
A typical surge protector power strip is built using MOVs. The cheapest kind may use just one
varistor, from hot (live, active) to neutral. A better protector would contain at least three
varistors; one across each of the three pairs of conductors (hot-neutral, hot-ground, neutralground). A power strip protector in the United States should have a UL1449 3rd edition approval
so that catastrophic MOV failure would not create a fire hazard.
2.4.6 ELECTRICAL POWER AND POWER DISSIPATION IN RESISTORS
Consider a circuit consisting of a battery E connected in series with a resistance R Fig (11).
Fig (11)
A steady current flow through the circuit and a steady potential difference V exists between the
terminals A and B of the resistor R. terminal A, connected to the positive pole of the battery, and
is at higher potential than the terminal B. In this circuit the battery is continuously lifting charge
uphill through the potential difference V. using the meaning of potential difference, the work
done in moving a charge ΔQ up through the potential difference V is given by
Work done =ΔW= V X ΔQ
(9)
33
This is the energy supplied by the battery. The rate at which the battery is supplying energy is the
power output or electrical power of the battery. Using the definition of power, we have
Electrical Power = Energy supplied/Time taken = V (ΔQ/ΔT)
Since I= ΔQ/ΔT
(11)
Electrical power = V x I
(12)
(10)
Eq. (12) is a general equation relation for power delivered from a source of current I operating on
a voltage V. in the circuit shown in Fig (11) the power supplied by the battery is expended or
dissipated in the resistor R. the principle of the conservation of electrical energy tells us that the
power dissipated in the resistor is also given by Eq. (12)
Power dissipated (P) = V x I
(13)
Alternative equation for calculating power can be found by substituting V=IR, I=V/R in turn in
Eq. (13)
P=V x I = IR x I = I2R
P = V x I = V x (V/R) = V2/R
Thus we have three equations for calculating the power dissipated in resistor.
P= V x I = I2R = V2/R
(14)
If V is expressed in volts and I in amperes, the power is expressed in Watts.
2-5
SINGLE PHASE AND THREE PHASE SUPPLY
SINGLE PHASE SUPPLY
It is that system in which a single phase and two wire system is used. The first wire is used for
phase and the second wire is used for the completion of current path.
THREE PHASE SUPPLY
34
It is that system in which three phases are used instead of a single phase system. Such systems
are used where bulk load is available and more rotational torque is needed.
2-6
STAR AND DELTA CONNECTIONS
Three single phases are connected together in such a manner that either we get a Delta (Δ) or
Star (Y) connection.
DELTA (Δ) CONNECTION
In a three phase system, three single phases are connected in such a manner that the head of one
phase is joined with the tail of the other and so on such that all the three phases are connected
together and finally we get a closed loop like a delta. Supply is taken from the three corners of
the delta. The only constraint with such a system is that it cannot be use for single phase loads.
In a Delta Supply System:
VLL = VPh
IL = √3 IPh
STAR (Y) CONNECTION
In a three phase system, three single phases are connected in such a manner that the heads or tails
of all the single phases are connected together and finally we get a common point called Neutral
or Star point and three branches. Three phase load is connected between the three branches and
single phase load is connected between any phase and the Neutral point.
In a Star Supply System:
35
VLL =√3 VPh
IL = IPh
2-7
Electrostatic and Electromagnetic Induction
As soon as Oersted discovered that electric currents produce magnetic fields, many scientists
began to look for the reverse effect, that is, to cause an electric current by means of a magnetic
field. In 1831, Michel Faraday in England and at the same time Joseph Henry in USA observed
that an EMF is set up in a conductor when it moves across a magnetic field. If the moving
conductor was connected to a sensitive galvanometer, it would show an electric current flowing
through the circuit as long as the conductor is kept moving in the magnetic field. The EMF
produced in the conductor is induced EMF, and the current generated is called the induced
current. This phenomenon is known as electromagnetic induction.
36
3.
Basic Concept of an
Electrical Power System
37
and Basic Components
of Power System
3.1
POWER SYSTEM CONCEPT
In a simplest power system, a generator and load is required. The need of transferring power
from generating station to load centre requires an additional transmission system of suitable
voltage level.
Interaction of these power components including generation, transmission and distribution give
rise to a power system. This may be a three phase balanced system energized at different voltage
levels for a convenience of maximum availability, minimum losses and safety of the system. In
an AC power system active power always flow from a leading power angle towards a lagging
power angle.
Generation power is transmitted to the load obeying power equation.
P
Where, V1 =
V2 =
= (V1 V2 Sin θ) /X1
sending end voltage
Receiving and voltage
38
X1 =
Reactance of the line
θ
Power angle, Phase difference between V1 and V2
=
For an efficient transmission, sending end voltage must lead the receiving end voltage or there
must be a potential difference. Operating a power system includes a large number of control
functions.
1. Control of frequency and thus control of active power.
2. Control of voltage and thus control of reactive power.
3. Control of network switching under normal operating conditions.
In a three phase AC system, generator produces three balanced voltages having 120o phase
difference, called positive sequence system. In a normal system three phase currents and voltages
have equal magnitudes and same phase angle between them.
Generator produces this set of voltages system, sometime called a positive sequence system. In
power system generator can produce both active and reactive power, it can also absorb reactive
power. Other sources of reactive power may be static capacitors and synchronous condensers.
The angle between the voltage and current is referred as phase angle between similar voltages
due to impedances of the power system is termed as power angle or load angle. Each type of
power may have different direction of flow on the same transmission line.
A normal system is always treated as a balanced three phase network with given characteristics:
1.
2.
3.
4.
Same phase angle between phases
Same nominal voltages
Same normal rated current
Frequency within limit
During an abnormal condition the balance of system may disturb, resulting in unbalanced
currents and voltages appearing in the system, due to any of the following causes
1.
2.
3.
4.
5.
6.
Symmetrical faults
Un-symmetrical faults
Switching over voltages
Ferranti effects on long lines
Lightning over voltages
Shifting of system neutral
39
3.2
COMPONENTS OF POWER SYSTEM
GENERATION
TRANSMISSION
LOAD
Hydel
Extra High Voltage
Industrial
Thermal
High Voltage
Commercial
Nuclear
Medium Voltage
Agricultural
Solar
Low Voltage
Domestic
Wind
Tidal
40
4.
Electrical Measuring
Instruments
41
4-1
GALVANOMETER
A galvanometer is an electrical instrument use to detect the passage of a current. Its working
depends upon the fact that when a conductor is placed in an electric field, it experiences a force
as soon as the current passes through it. Due to this force, a torque τ acts upon the conductor if it
is in the form of a coil or loop.
τ = NIBA cosα
(1)
Where N is the number of turns in the coil, A is its area, I is the current passing through it, B is
the magnetic field in which the coil is placed such that its plane makes an angle α with the
direction of B. due to action of the torque, the coil rotates and thus it detects the current. The
construction of a moving coil galvanometer is shown in Fig (1).
42
Fig (1)
A rectangular coil C is suspended between the concave shaped poles N and P of a U-shaped
magnet with the help of a fine metallic suspension wire. The rectangular coil is made of
enameled copper wire. It is wounded on a frame of non-magnetic material. The suspension wire
F is also used as one current lead of the coil. The other terminal of the coil is connected to a
loosely wound spiral E which serves as the second current lead. A soft iron cylinder D is place
inside the coil to make the field radial and stronger near the coil as shown in Fig (1).
When a current is passed through the coil, it is acted upon by a couple which tends to rotate the
coil. This couple is known as deflecting couple and is given by NIBA cosα. As the coil is placed
in a radial magnetic field in which the plane of the coil is always parallel to the field Fig (1), so α
is always zero. This makes cosα =1 and thus,
Deflecting couple = NIBA
As the coil turns under the action of deflecting couple, the suspension wire Fig (1) is twisted
which gives rise to a torsional couple. It tends to untwist the suspension and restore the coil to its
original positions this couple is known as restoring couple. The restoring couple of the
suspension wire is proportional to the angle of deflection θ as long as the suspension wire obeys
Hooke’s law. Thus
Restoring torque = cθ
(1)
Whereas, the constant c of the suspension wire is known as torsional couple and is defined as
couple for unit twist.
Under the effect of these two couples, coil comes to rest when
Deflecting torque = Restoring torque
NIBA = cθ
Or, I = (cθ)/BAN
Thus
(2)
I α θ, since c/BAN = Constant
Thus the current passing through the coil is directly proportional to the angle of deflection.
43
There are two methods commonly used for observing the angle of deflection of the coil. In
sensitive galvanometers the angle of deflection is observed by means of small mirror attached to
the coil along with lamp and scale arrangement Fig (2). A beam of light from the lamp is
directed towards the mirror of the galvanometer. After reflection from the mirror it produces a
spot on a translucent scale placed at a distance of one meter from the galvanometer. When the
coil rotates, the mirror attached to coil also rotates and spot of light moves on the scale. The
displacement of the spot of light on the scale is proportional to the angle of deflection (provided
the angle of deflection is small).
Fig (2)
The galvanometer used in school and college laboratories is a pivoted type galvanometer. In this
type of galvanometer, the coil is pivoted between two jeweled bearings. The restoring torque is
provided by two hair springs which also serve as current leads. A light Aluminum pointer is
attached to the coil which moves over a scale as shown in Fig (3). It gives angle of the deflection
of the coil.
44
Fig (3)
It is obvious from Eq.(2) that a galvanometer can be made more sensitive (to give large
deflection for a given current) if c/BAN is made small. Thus, to increase sensitivity of a
galvanometer, c can be increased or B, A and N may be increased. The couple c for unit twist of
the suspension wire can be decreased by increasing length and by decreasing its diameter. This
process, however, cannot be taken too far, as the suspension must be strong enough to support
the coil.
Another method to increase the sensitivity of galvanometer is to increase N, the number of turns
of the coil. In case of suspended coil type galvanometer, the number of turns cannot be increased
beyond a limit because it will make the coil heavy. To compensate for the loss of sensitivity, in
case fewer turns are used in the coil, we increase the value of the magnetic field employed. We
define current sensitivity of a galvanometer as the current, in microamperes, required to produce
one millimeter deflection on a scale placed one meter away from the mirror of the galvanometer.
When the current passing through the galvanometer is discontinued, the coil will not come to rest
as soon as the current flowing through the coil is stopped. It keeps on oscillating about its mean
position before coming to rest. In the same way if the current is established suddenly in a
galvanometer, the coil will shoot beyond its final equilibrium position and will oscillate several
45
times before coming to rest at its equilibrium position. As it is annoying and time consuming to
wait for the coil to come to rest, artificial ways are employed to make the coil come to rest
quickly after the current passed through it or the current is stopped from flowing through it, is
called stable or a dead beat galvanometer.
4-2
VOLTMETER
A voltmeter is an electrical device which measures the potential difference in volts between two
points. This, too, is made by modifying galvanometer. Since a voltmeter is always connected in
parallel, it must have a very high resistance so that it will not short the circuit across which the
voltage is to be measured. This is achieved by connecting a very high resistance R h placed in
series with the meter-movement Fig (4). Suppose we have a meter-movement whose resistance is
Rg and which deflects full scale with a current Ig. In order to make a voltmeter from it which has
a range of V volts, the value of the high resistance Rh should be such thus full scale deflection
will be obtained when it is connected across V volts. Under this condition the current through the
meter-movement is Ig. Applying Ohms law Fig (4) we have
V = Ig(Rg+Rh)
Rh = (V/Ig) - Rg
(3)
Fig (4)
If the scale of the galvanometer is calibrated from 0 to V volts, the combination of galvanometer
and the series resistor as a voltmeter with range 0-V volts by properly arranging the resistance Rh
any voltage can be measured. Thus, we see that a voltmeter possesses high resistance.
It may be noted that a voltmeter is always connected across the two points between which the
potential difference is to be measured. Before connecting a voltmeter, it should be assured that
46
its resistance is very high in comparison with the resistance of the circuit across which it is
connected otherwise it will load a circuit and will alter the potential difference which is required
to be measured.
4-3
AMMETER
An ammeter is an electrical instrument which is used to measure current in amperes. This is
basically a galvanometer. The portion of the galvanometer whose motion causes the needle of
the device to move across a scale is usually known as meter-movement. Most (meter movements
are very sensitive and full scale deflection is obtained with a current of few milli amperes only.
So an ordinary galvanometer cannot be used for measuring large currents without proper
modification.
Suppose we have a galvanometer whose meter-movement (coil) has a resistant Rg and which
gives full scale deflection when current Ig is passed through it. From Ohm’s law, we know that
the potential difference Vg which causes a current Ig to pass through the galvanometer is given by
Vg=IgRg
If we want to convert this galvanometer into an ammeter which can measure a maximum current
I, it is necessary to connect a low value bypass resistor called shunt. The shunt resistance is of
such a value so that the current Ig for full scale deflection of the galvanometer passes through
galvanometer and the remaining current (I – Ig) passes through the shunt in this situation as
shown in Fig (5)
The shunt resistance Rs can be calculated from the fact that as the meter-movement and the shunt
are connected in parallel with each other, the potential difference across the meter-movement is
equal to the potential difference across the shunt. Therefore,
IgRg = (I – Ig)Rs
Or
Rs = (Ig Rg)/ (I – Ig)
(4)
47
Fig (5)
The resistance of the shunt is usually so small that a piece of copper wire serves the purpose. The
resistance of the ammeter is the combined resistance of the galvanometer meter-movement and
the shunt. Usually it is very small. An ammeter must have a very low resistance so that it does
not disturb the circuit in which it is connected in series in order to measure the current.
4-4
OHMMETER
It is a useful device for rapid measurement of resistance consists of a galvanometer, and
adjustable resistance rs and a cell connected in series Fig (6). The series resistance rs is so
adjusted that when terminals c and d are short circuited, i.e., when R is equal to 0, the
galvanometer gives full scale deflection. So the extreme graduation of the usual scale of the
galvanometer is marked 0 for resistance measurement. When terminals c and d are not joined, no
current passes through the galvanometer and its deflection is zero. Thus zero of the scale is
marked as infinity as shown in Fig (6).
Now a known resistance R is connected across the terminals c and d. the galvanometer deflects
to some intermediate point. This point is calibrated as R. in this way the whole scale is calibrated
into resistance. The resistance to be measured is connected across the terminals c and d. the
deflection on the calibrated scale reads the value of the resistance directly.
48
Fig (6)
4-5
MULTIMETER-AVO METER It is an instrument which can measure current in
amperes, potential difference in volts and resistance in ohms. It basically consist of a sensitive
moving coil galvanometer which is converted into a multi-range ammeter, voltmeter or
ohmmeter accordingly as measuring circuit or a voltage measuring circuit or a resistance
measuring circuit is connected with galvanometer with the help of a switch known as function
switch as shown in Fig (7). Here X and Y are the main terminals of the AVO meter which are
connected with the circuit in which measurement is required. FS is the function selector switch
which connects the galvanometer with relevant measuring circuit.
Fig (7)
49
VOLTAGE MEASURING PART OF AVO METER
The voltage measuring part of the AVO meter is actually multi-range voltmeter. It consist of a
number of resistance of which can be connected in series with the moving coil galvanometer as
shown in Fig (8). The value of each resistance depends upon the range of the voltmeter which it
controls
Alternating voltages are also measured by AVO meter. AC voltage is first converted into DC
voltage by using diode as rectifier and then measured as usual.
Fig (8)
CURRENT MEASURING PART OF AVO METER
The current measuring part of the AVO meter is actually a multi-range ammeter. It consists of a
number of low resistances connected in parallel with the galvanometer. The values of these
resistances depend upon the range of the ammeter as shown in Fig (9).
The circuit also has a range selection switch RS which is used to select particular range of the
current.
50
Fig (9)
RESISTANCE MEASURING PART OF AVO METER
The resistance measuring part of AVO meter is, in fact, a multi-range ohm meter. Circuit for
each range of this meter consists of a battery of emf Vo and a variable resistance rs connected in
series with galvanometer of resistance Rg. when the function switch is switched to position X3,
this circuit is connected with the terminals X, Y of the AVO meter as shown in Fig (10).
Before measuring an unknown resistance by an ohmmeter it is first zeroed which means that we
short circuit the terminals X and Y and adjust rs, to produce full scale deflection.
Fig (10)
DIGITAL MULTIMETER (DMM)
Another useful device to measure resistance, current and voltage is an
electronic instrument called digital multi-meter. It is a digital version of an
AVO meter. It has become a very popular testing device because the digital
51
values are displayed automatically with decimal point, polarity and the unit for V, A or Ω. These
meters are generally easier to use because they eliminate the human error that often occurs in
reading the dial of an ordinary AVO meter. A portable DMM is shown in Fig (11).
Fig (11)
4-6
CLIP-ON AMMETERS
Clip-on Ammeters work on the principal that an Ammeter is connected to the circuit through a
Current Transformer, thus avoiding the breakage of the circuit needed for current measurement.
The Clip-on Ammeter may be of Analogue or Digital type.
4-7
CATHODE RAY OSCILLOSCOPES
Cathode Ray Oscilloscopes (CRO) is a very versatile electronic instrument which is, in fact, a
high speed graph plotting device. It works by the deflected beam of electrons as they pass
through a uniform electric field between the two sets of parallel plates as shown in the Fig (12).
The deflected beam then falls on a fluorescent screen where it makes a visible spot
Fig (12)
It can display graphs of functions which rapidly vary with time. It is called cathode ray
oscilloscope because it traces the desired waveform with a beam of electrons which are also
called cathode rays.
The beam of the electrons is provided by an electron gun which consists of an indirectly heated
cathode, a grid and three anodes. The filament F heats the cathode C which emits electrons. The
52
anodes A1, A2, A3 with high positive potential with respect to cathode, accelerate as well as focus
the electron beam to fix spot on the screen S. the grid G is at negative potential with respect to
cathode. It controls the number of electrons which are accelerated by anodes, and thus it controls
the brightness of the spot formed on the screen.
Now we would explain how the waveform of various voltages is formed in CRO.
The two sets of deflecting plates, shown in Fig (12) are usually referred as x and y deflection
plates because a voltage between the x plates deflects the beam horizontally on the screen i.e.,
parallel to x-axis. A voltage applied across the y plates deflects the beam vertically on the screen
i.e., along the y-axis. The voltage that is applied across the x-plates is usually provided by a
circuit that is built in the CRO. It is known as sweep or time base generator. Its output waveform
is a saw tooth voltage of period T Fig (13).
The voltage increases linearly with time for period T and then drops to zero. As this voltage is
impressed across the x-plates, the spot is directed linearly, with time along with x-axis for a time
T. then the spot returns to its starting point on the screen very quickly because a saw tooth
voltage rapidly falls to its initial value at the end of each period. We can actually see the spot
moving on the x-axis. If the time period T is very short, we just see a bright line on the screen.
If a sinusoidal voltage is applied across the y plates when, simultaneously, the time base voltage
is impressed across the x plates, the sinusoidal voltage, which itself gives rise to a vertical line,
will now spread out and will now appear as a sinusoidal trace on the screen. The pattern will
appear stationary only if the time T is equal to or some multiple of the time of one cycle of the
voltage on y plates. It is thus necessary to synchronize the frequency of the time base generator
53
with the frequency of the voltage at y plates. This is possible by adjusting the synchronization
controls provided on the front panel of the CRO.
USES OF CRO
The CRO is used for displaying the waveform of a given voltage. Once the waveform is
displayed, we can measure the voltage, its frequency and phase. For example, Fig (14) shows the
waveform of an alternating voltage. As the y-axis is calibrated in volts and the x-axis in time, we
can easily find the instantaneous value and the peak value of the voltage. The time period can
also be determined by using the time calibration of x-axis. Information about the phase
difference between two voltages can be obtained by simultaneously displaying their waveforms.
For example, the waveforms of two voltages are shown in Fig (15). These waveforms show that
when the voltage of I is increasing, that of II is decreasing and vice versa. Thus the phase
difference between these voltages is 180o.
Fig (14)
4-8
Fig (15)
SAFETY PRECAUTIONS
Proper Use of an Ammeter
1. Avoid careless handling
2. Always connect ammeter in series in the circuit whose current is to be measured.
3. Always first set the range selector switch at the highest range and then stepwise decrease the
range until suitable scale reading is obtained.
54
Proper Use of a VOLTMeter
1. Avoid careless handling
2. Always connect voltmeter in parallel to the circuit, whose voltage is to be measured.
3. Always first set the range selector switch at the highest range and then stepwise decrease the
range until suitable scale reading is obtained.
4. Leads of voltmeter must be free of defects.
Proper Use of an ohmMeter
1. Never use ohmmeter on energized circuit
2. Zero adjustment must be made for each range. For this, short its leads together and then bring
pointer to zero by variable resistor by using screw driver (or by turning the knob provided).
3. Avoid careless handling
4. For checking continuity of wires etc. it is better to use smallest range of ohmmeter.
PROPER USE OF A MULTIMETER OR (AVO METER)
1. Adjust the selector switch to the quantity which is to be measured (i.e. current, voltage or
resistance and A.C voltage or DC voltage etc.).
2. When multi-meter is not in use, put its selector switch to off position because of following
reasons.
a. Meter damage is avoided if it is connected to high voltages by someone.
b. Needle movement is damped and hence needle is protected from damage during
transportation etc.
Note: If off position is not provided, then return the selector switch to the highest voltage
range.
3. Avoid careless handling
4. When measuring V and I, always set range selection switch at the highest range and then
switch successively to lower ranges until a suitable range is obtained.
5. When measuring resistance (or continuity) of a circuit with multi-meter, the circuit must be
de-energized before inserting the meter leads in it.
6. Leads of multi-meter must be clean, dry and free of defects.
4.9
IMPORTANT POINTS WHEN USING INSULATION RESISTANCE TESTER
1. Insulation Resistance Tester must never be used in energized circuit.
2. Accuracy of Insulation Resistance Tester must be checked before using it. Accuracy of
Insulation Resistance Tester is checked by performing following two tests:
55
3.
4.
5.
6.
a. Zero Check: For this short Insulation Resistance Tester leads together and crank it,
the pointer of it must deflect towards zero position.
b. Infinity Check: For this leave Insulation Resistance Tester leads open circuited and
crank it, its needle must deflects towards infinity position.
The equipment under test must be discharged and grounded after test to discharge the
capacitor (insulation) and to release the energy which it may have absorbed due to the
dielectric absorption phenomenon.
Insulation Resistance Tester leads must be clean, dry and free of defects.
Guard terminal of Insulation Resistance Tester must never be touched during test.
The rating of Insulation Resistance Tester should not be more than the rating of equipment
under test.
PROCEDURE:
The H (or line or +) terminal of Insulation Resistance Tester is connected to the equipment
whose insulation resistance is to be measured; the E (Earth) terminal of it is connected to the
terminal of equipment which is grounded. Then it is cranked until its needle is stabilized and
reading is taken / recorded.
Insulation Resistance Tester readings are always corrected to 200C standard temperature because
Ri varies inversely with temperature. To convert Ri to 200C following rule is adopted:
a. For every 100C rise of temperature from 200C, Ri becomes half of its value.
b. For every 100C fall of temperature from 200C, Ri becomes double of its value.
One Mega-ohm reading for 1kv rated equipment at 20 C0 is considered satisfactory.
4.10
TEMPERATURE CORRECTION OF Ri READINGS
Example
Ri reading of 66KV equipment at 400C was measured to be 20 Mega-ohms. Can this equipment
be energized, explain.
Solution
Ri at 400C = 20 Mega-ohms
Ri at 300C = 40 Mega-ohms
Ri at 200C = 80 Mega-ohms
The equipment can be energized, since for 66KV equipment Ri reading of 66 Mega-ohms is
satisfactory. This reading is 80 Mega-ohms.
56
Example
Ri reading of 220KV insulation measured at –50C was 1000 Mega-ohms. Is this good reading?
Explain.
Solution
Ri at –50C = 1000 Mega-ohms
Ri at 50C = 500 Mega-ohms
Ri at 150C = 250 Mega-ohms
Ri at 250C = 125 Mega-ohms
Ri at 200C = (250 + 125)/2 = 375/2 = 187.5 Mega-ohms
This is not a good reading as the satisfactory reading for 220kV equipment is 220 Mega-ohms at
200C.
57
5.
Basic Control Circuits
58
5.1
AUXILIARY SWITCHES
The names of various automatic switches are:
1.
2.
3.
4.
5.
6.
Level switch
Flow switch
Position or limit switch
Pressure switch
Temperature switch
Speed switch (centrifugal switch)
IMPORTANT LETTERS
Letters ‘a’, ‘b’, ‘aa’ and ‘bb’ are used in diagrams to represent switches or auxiliary switches.
aIt is closed when main device (Circuit Breaker, Isolator or Contactor/Relay) is closed
/energized and it is opened when main device is open or de-energized. Sometimes, ‘a’ is
also called normally open contact of a device.
bIt is closed when main device is opened and vice versa. ‘b’ is also called normally closed
contact of a device.
aa-
It is always open. It only closes for a very short time when the driving force; (air
pressure, hydraulic pressure or spring) operating the main device is in action. It returns to
its original position when driving force ceases.
bb-
It is opposite to ‘aa’ i.e it is always closed. It opens for a very short time when driving
force operating the main device is in action. It re-closes or re-sets when driving force is
ceased
59
5.2
DEVICE FUNCTION NUMBERS
Function
No.
Type of Relay
2
Time delay relay
3
Interlocking relay
21
Distance relay
25
Check synchronizing relay
27
Under voltage relay
30
Enunciator relay
32
Directional power (Reverse power) relay
37
Low forward power relay
40
Field failure (loss of excitation) relay
46
Negative phase sequence relay
49
Machine or Transformer Thermal relay
50
Instantaneous Over current relay
51
A.C IDMT over current relay
52
Circuit breaker
52a
Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52b
Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55
Power Factor relay
56
Field Application relay
59
Overvoltage relay
60
Voltage or current balance relay
64
Earth fault relay
67
Directional relay
68
Locking relay
74
Alarm relay
76
D.C Over current relay
78
Phase angle measuring or out of step relay
79
AC Auto reclose relay
81
Frequency relay
81U
under frequency relay
60
81O
5.3
over frequency relay
83
Automatic selective control or transfer relay
85
Carrier or pilot wire receive relay
86
Tripping Relay
87
Differential relay
87G
Generator differential relay
87GT
overall differential relay
87U
UAT differential relay
87NT
Restricted earth fault relay (provided on HV side of Generator transformer)
95
Trip circuit supervision relay
99
Over flux relay
186A
Auto reclose lockout relay
186B
Auto recluse lockout relay
BASIC REQUIRMENTS OF CONTROL CIRCUITS
For circuit breakers there are a number of basic requirements which are desirable in the control
circuit. These features can be found in the motor, solenoid, spring (stored energy) and
pneumatically-operated Circuit Breakers.
A good understanding in these control circuit features will allow an intelligent approach to
trouble-shooting. Examination of the circuit diagram of the modern breakers will relatively
complicated network of switches, contactors and coils, the correct functioning of which is
essential. Each individual component of a circuit has a definite function to perform, thus
removing any one element will cause some type of faulty operation.
When the maintenance electrician knows the function of each component, he also knows what
type of faulty operation to expect when that component is inoperative. Conversely when maloperation of a breaker is found, the likely component or components at fault will be known.
5.3.1 CONTROL THE CLOSING
The closing circuit must do more than merely close the breaker, it must control this closing. To
do this, the following features are necessary.
61
Initiate The Closing Stroke: Means must be provided in the circuit to energize the closing device,
for example, the solenoid of the solenoid-operated breaker or the motor of a motor-operated
breaker.
Cut-Off: The closing power must be cut-off or disconnected automatically at the end of the
closing stroke. This is necessary to prevent overheating of the closing device. Solenoid coils used
on circuit breakers have only a short time rating, thus if a closing coil is left energized for more
than 15 seconds. It will overheat and suffer damage to the insulation. For this reason it cannot be
left to the operator to decide when to cut off the closing power since it left on too long, damage
will result. The alternative where the closing power could be left on for too short a time is
covered in Seal-In.
Seal-In: It is desirable to have the control circuit ensure that the breaker will fully close each
time that closing operation is initiated, if the breaker is closed by a simple switch. Simply
speaking; it completes the operation automatically started by us manually so as not to hold the
push button all the times.
5.3.2 CONTROL THE TRIPPING
Initiate Trip Stroke: Means must be provided to trip the breaker. This may involve energizing a
solenoid coil to trip a latch or in case of air blast breakers, to admit air to the blast valves and
contacts.
Cut-Off: For the reasons noted in above, means must be provided to automatically disconnect the
trip coil.
5.3.3 TRIP FREE FEATURE
When closing a breaker, the closing device (for example, the solenoid in a solenoid-operated
breaker) is energized and the plunger operates through the linkage to close the breaker contacts.
At the end of the closing stroke, appreciable time is required to de-energize the solenoid coil. In
the event that the breaker has been close on a faulted circuit, it must be reopened as quickly as
possible. If a breaker can trip automatically upon receiving a trip signal before closing operation
is complete, it is said to be “trip free”.
Various arrangements are provided to obtained trip-free action. Solenoid-operated breakers are
sometimes provided with action. Solenoid-operated breakers are sometimes provided with a
collapsible linkage. Pneumatically-operated breakers may be equipped with two latches, one of
which is unlatched during a normal trip operation and the other is only unlatched for a trip signal
62
while the breaker is closing. Other breakers use a large dump valve to quickly exhaust the air
under the closing piston. Many motor-operated breakers obtain a fast trip-free action by use of a
relay energized from the trip circuit to open the closing circuit. These methods would be known
as mechanically trip free, pneumatically trip free and electrically trip free respectively.
5.3.4 ANTI-PUMPING FEATURE
When a breaker is closed and a trip-free operation results, the close and trip stroke will be
completed in a very short time for a modern pneumatically-operated high voltage breaker, the
complete operation will take less than one-half of one second, thus it is quite likely that the
operator will still have the control switch in the closed position. Means must therefore be
provided to prevent the breaker from closing a second time, even though the operator is holding
the control switch in the closed position. This is usually accomplished by the use of a sealed in
relay which can only be released which in the closed position. This is usually accomplished by
the use of a sealed in relay which can only be released by opening the closing control switch.
When this feature is incorporated in the control circuit, the breaker is said to be “pump free” or
“anti-pumping”. Following a trip-free operation of the breaker, the operator must release the
control switch before a second attempt to close the breaker can be made.
5.3.5 ANTI-SLAM FEATURE
This feature prevents the energization of closing coil or tripping coil of an already closed breaker
or tripped breaker respectively.
1. In closing circuit this feature is mostly achieved through auxiliary switch b.
2. In opening circuit, this feature is mostly achieved through auxiliary switch a.
5.3.6 RELIABILITY
A circuit breaker is a protective device. It will be called upon to open faulty circuits infrequently,
however while it may stand inoperative for long periods; it must be relied upon to operate
correctly in time of trouble. Reliability for such a protective device is essential. For this reason a
battery supply is always used to provide the tripping power and in most cases for closing.
The control circuits usually have a separate trip and close bus. This is to give extra reliability to
the trip circuit. On 115 kV and above circuit breakers there are dual trip buses thus, if a closing
control circuit fuse fails during a closing stroke, the trip circuit or circuits are not affected.
The above requirement of extra reliability during tripping is also seen in the size of fuses used in
the trip and closed circuits, for example, on a solenoid-operated breaker the fuses in the closing
circuit will be rated at slightly less than value of current obtained by dividing the voltage by
63
resistance. The fuses must be so rated to provide a measure of protection for the short time rated
closing coil. During a normal closing operation, the closing coil will be energized for less than
one second. To have the closing fuses blow in approximately six seconds, it is necessary to use a
size which is actually less than the maximum current that the closing coil will normally draw.
Conversely, in the trip circuit the fuses will be rated at several times the current obtained by
voltage to resistance ratio. If the trip coil is not automatically disconnected at the end of the trip
stroke, the trip coil may carry current for a long period and being are short time rated coil, it will
be damaged. The fuses in this circuit will open only due to some fault condition. In order to gain
more reliable tripping, we do not protect the short time rated trip coil. The trip fuses are not put
in the breaker but are located in the control building.
5.3.7 GENERAL MAINTENANCE OF BREAKER CONTROL RELAYS
Frequent reference is made in this reference material to relays. These are control relays located in
the operating mechanism housing. They are concerned entirely with the sequence of the
mechanism of the breaker. The control circuit relays are all located on the breaker side of the
four-pole isolating switch. Such relays are a part of the breaker, being required for the breaker’s
correct functioning as much as possible, for example, a trip coil or interrupter and as such are the
responsibility of the maintenance electrician.
Other relays remote from the breaker determine under what system conditions the breaker will be
tripped. These protective relays, together with the interposing relays where such are used, form
the protective network and are the responsibility of the Meter and Relay Department. The
dividing line between the breaker control circuit relays and protective relays is well defined and
there should be not confusion in this regard.
5.3.8 ASA DEFINATIONS
RELAY: A relay is a device that is operative by a variation in the conditions of one electric
circuit to effect the operations of other devices in the same or another electric circuit.
CONTROL RELAY: A control relay is a relay which functions to initiate or to permit the next
desired operation in a control circuit or scheme.
PROTECTIVE RELAY: A protective relay is a relay, the principal function of which is to
protect service from interruption by removing defective components or to prevent or limit
damage to apparatus.
64
5.4 OVERLOAD PROTECTION
In order to avoid damage to motors etc. due to temperature rise because of overloading and
defective bearings etc., overload protection features are incorporated in motor control circuits. It
should be noted that over load relay or element is always incorporated in the power circuit but its
contact is installed in the control circuit. Due to this, the life of contact increases as it breaks
small current because in control circuit current is small. Over load relays are mostly operated
thermally and may be of bimetallic strip type or solder pot type.
5.5
OVER CURRENT OR SHORT CIRCUIT PROTECTION
The function of over current protective devices is to protect motors and its circuit elements etc.
from damage in case of phase-phase short circuits or phase-ground faults etc. The over-current
device must be capable of carrying the starting current of motors. Mostly fuses and magnetic
devices are used as over current protective devices. Rating of fuse should not exceed 300% of
full load current.
5.6
CONTACTOR
Contractor is a device which is operated electrically and controlling the operation of other
circuits magnetically. Contactor may also be called as an ON-OFF Switch. Contactor has two
types of contacts:
1. Main Contacts: These are used in power circuits and hence must be strong to carry the full
load current of motor continuously without undue heating.
2. Auxiliary Contacts: These are small and used in control circuits only. These may be NO or
NC and are used as seal in contact (NO), interlocking contact (NC) and for indications etc.
5.7
MAINTENANCE OF CONTACTOR
Contactor maintenance mainly includes
1. Removal of rust or deposits etc. from contacts with emery paper and dry cloth. Never file the
contacts as it will remove the elkonite from the contacts.
2. Checking of contacts alignment
3. Free movement of moving contacts assembly with binding etc.
4. Checking of connections at terminal points.
65
6.
66
P&I Tools & Plant
(T&P)
6.1
P&I TOOLS
The following is a list of tools that will be required by a P & I man when maintaining the
equipment within his area of responsibility. It is broken into two areas.
1
Personnel Tools: These are the tools each man should have at his end only on his
disposal. They should be his responsibility alone.
2
Common
Test
Equipments:
These
equipments
are
purchased
by
WAPDA/NTDC/GENCOs/DISCOs and kept in divisional stores for use by individuals when
required. The responsibility for this equipment is that of the XEN in-charge and a definite
method of checking the equipment’s in/out should be established along with a system of periodic
inventory check.
3
It is vital to maintenances program that these test equipments must be kept in a perfect
condition and any broken or worn parts be replaced or repaired as soon as possible.
4
All test equipments should be kept in an enclosed cabinet free from dust, moisture and
excessive vibration.
6.1.1 P&I PERSONNEL TOOLS
67
1. Electronic Digital Meter
2. Leather Tool Carrying Case, 20” x 10” x 12” high.
3. Soldering Iron-220 volt 40-50 Watt:
4. 1 roll 60/40 Flux Core Solders.
5. Insulating Tap (Black).
6. Continuity Tester (Buzzer).
7. Jeweler Screwdrivers Set (7 pieces / set).
8. Wire Strippers And Crimpers.
9. Nut Drivers 5 / 16”, ¼”, 11/32”, 3/16”, ½” and 7/16.
10. L-Type, Hex Head Set-9 Pieces.
11. Set of Screw Drivers Slot Head, Phillips.
12. Pliers, Side Cutting, Long Nose.
13. Potential Test Indicators (750 volts AC / DC) Neon Bulb Type.
14. Various Assortments of Length of Clip Cords (RED + BLACK) (6” To 6’) with Alligator
Clips and Insulated Rubbers Covers Wire should be No: 18 AWG.
15. Multiplex Scrappers 7½” length – 1 “wide.
16. Hacksaw, 10” length and Spare Blades.
17. Center Punch.
18. Slip Joint Pliers, 5” Length.
19. Curved Nose Pliers – 6” Length.
20. Adjustable Wrenches 100 mm, 200 mm.
21. Heavy Duty Screw Driver Set (Slot, Phillips).
22. Vise Grip Pliers 7”.
23. Utility Knife.
24. Screw Holding Screw Driver 4”, 6”
P&I TEST EQUIPMENTS
1. CT Analyzer
2. Universal Relay Test Set
3. Secondary Injection Relay Test Set
4. Primary Current Injection Test Set
5. Digital Clamp Meter
6. AC/DC Clamp on Meter
7. Digital Multi-Meter
8. Phase Angle Meter
9. Oscilloscope Digital, Dual Channel
10. Variac, Single Phase
11. Variac, Three Phase
12. Analog Multi-Meter
6.2
SHOP RULES
1. Do not use shop machinery until you are trained.
68
2.
3.
4.
5.
6.
Grease / oil on floor must be removed immediately to avoid slipping / falling.
Keep all tools in their proper place.
Wear eye protection when necessary
All the tools must be kept clean.
After the completion of job with tools, dry and clean them and return them to their proper
location.
7. Do not use damaged tools.
8. Long hairs / loose clothing should be confined when working around rotating machines.
6.2
CARE AND UP KEEP OF TOOLS
1. All the tools should be stored in a proper toolbox and each should be placed in their own
compartment.
2. Tools should not be thrown in toolbox in order to avoid damage to itself and to other
tools.
3. Tools must be kept away from heat because heat reduces hardness of tools.
4. All the tools should be kept clean. Dust or rust must never be allowed to accumulate on
tools.
5. After completion of work the tools should be wiped with a clean cloth moistened with
machine oil and then each tool should be stored in its proper place.
6. Each tool must be used only for that job for which it is made.
7. Ordinary plastic insulated tools must never be relied upon for electrical insulation.
8. Always purchase best quality tools regardless of their cost because good quality tools last
for long time and give good continuous service.
6.3
SAFETY PRECAUTIONS
EQUIPMENT SOURCES
WHEN
APPLYING
VOLTAGE
FROM
TEST
Employees, particularly maintenance staff, quite often have occasion to perform tests on isolated
apparatus by application of an external voltage source over which such maintenance staff have
exclusive control.
This information outlines the general and special precautions required during testing of apparatus
which involves application of a potential from a voltage source external to the system, i.e., from
a test equipment source. Written procedures based on these principles should be developed for
all repetitive tests performed by maintenance personnel, and other special or non-repetitive tests
where this is practical.
69
The external voltage source may consist of a high DC voltage test set, AC high voltage set,
“megger” type insulation resistance testers up to 10 kV range, etc. Tests involving a low AC
voltage source may also produce high voltage by inductive action.
6.4.1 TESTING APPARATUS IN ITS IN-SERVICE LOCATION
Observe the following precautions:
1. Where work safety depends on the isolation of apparatus, the apparatus to be tested shall
be de-energized prior to the connection of the test equipment.
2. This condition should be guaranteed by the appropriate form of work protection (where
such is required by the Work Protection Code) to ensure safe working conditions for
those operating the test equipment and for those working outside the immediate test area.
3. The equipment shall remain in the de-energized condition throughout the work period
accept as may be required to allow authorized testing.
4. In the case where isolation of apparatus is necessary to carry out a test, but de-energizing
is not (e.g., testing of instrument transformers from the secondary windings) other
precautions, as detailed in specific work practices and procedures, must be followed to
ensure safe working conditions.
6.4.2 TESTING IN A LOCATION OTHER IN THE IN-SERVICE POSITION
When electrical apparatus has been moved from its normal position to a shop or other area,
safety precautions appropriate to the new location must be taken before such equipment is
energized in any way for testing. The person in charge of any voltage testing shall be responsible
for establishing proper safe working conditions of the test or testing.
6.4.2.1 SIGNS AND GUARDS REQUIRED DURING VOLTAGE TESTING OF
APPARATUS RESPONSIBILITY OF PERSON IN CHARGE
The person in charge of any voltage testing is responsible for assuring that unauthorized persons
are barred from the test area while testing is in progress.
TYPES OF BARRIERS
Apparatus under voltage testing must be barricaded by suitable barriers displaying approved
warning signs, or alternatively approved written procedures must be followed for specific
repetitive tests.
70
Rigid-type barriers or rope mesh barriers may be employed. This type of safety barrier is
available from Central Stores. Cotton tape, when supplemented by warning signs and the posting
of observers, may be used as an alternative to such barriers.
Wherever, a complete barricade is not practical, as for example, when testing a high voltage
cable from one junction to another, then adequate alternative must be devised. One alternative
for this case would be to have each end monitored and observed by test personnel who are in
radio contact.
It is deemed not a requirement to barricade the equipment for Insulation Resistance tests at 2500
V or less.
WARNING SIGNS
An approved sign, “Caution HV Testing Keep Away”, is available from Stores, for use where,
warning signs are required.
WHEN BARRIERS AND SIGNS ARE REQUIRED
Barriers and signs should only be in place while actual testing is in progress and should be
removed promptly upon completion of the test. Adherence to this practice promotes respect for
such warnings.
TEST OPERATOR
Test shall be undertaken only by persons qualified by experience or training to do this type of
work.
Where possible, the controls for the test equipment should be located outside the main barrier so
that the person conducting the testing will be clear of the test area while the test is in progress.
6.4.3 SPECIAL PRECAUTIONS
TEST EQUIPMET LEADS
When special leads are not provided with the test equipment, a limp bare braided copper
conductor shall be used for high voltage connections. An approved bare conductor is available
from Central Stores. Solid drawn wire of the so-called “binding wire” type shall not be used for
71
high voltage test leads. This type of wire is not considered safe because of its tendency to kink
and break, thereby permitting a loose end to spring back through the air while still at high
potential.
Bare test leads are preferable to insulated leads in that the bare wire encourages more respect.
When insulated leads are necessary to be allowed the test to be performed, the insulation should
be capable of withstanding the maximum test voltage.
GROUNDING
Particular attention should be paid to how the test set and apparatus are connected to ground. The
frame of the apparatus must be connected to the ground of the test equipment, and both must be
attached to a station ground. Alternative procedures to ensure adequate safety shall be instituted
where such ground connections would make performance of the test impractical. Where a station
ground is not available, a temporary driven ground shall be used.
An approved grounding device shall be used to ground the accessible high voltage test supply
when the actual testing is not being performed.
TERMINATION POINTS
Under no circumstances shall an external supply be applied to a circuit without all termination
points of the circuit being known and isolated or protected.
RESIDUAL CHARGE FOLLOWING HIGH DC VOLTAGE TEST
The characteristics of large generator or motor windings and power cables are such that they may
retain dangerous charges for long charges for long times of periods of following application of a
high DC voltage. Equipment which has been tested with high DC voltage shall be grounded and
left grounded for a significant period of time or until by test it has been proven safe, since
otherwise there may be a voltage buildup over a period of time. A significant period of time is
defined as no less than four hours.
To prevent possible damage due to high peak discharge currents, the equipment which has been
tested should be grounded initially through an approved grounding device incorporating a
resistance between its connection to the equipment and the ground, by an approved procedure for
the specific application. The resistance grounding device should remain in service for not less
than 15 minutes, or alternatively, until the voltage has been reduced to a level of ten percent or
less of the test potential prior to making a solid connection to ground. After the solid ground
connection has been connected, the temporary working ground shall be reattached.
72
When it is necessary to reassemble the permanent connections to the equipment before
expiration of the recommended discharge time, work may be carried out on these connections
provided they are kept grouped solidly during the course of the work by means of approved
grounding devices. When it is impractical to make the permanent connection while the
grounding device is in place, an approved procedure shall be devised which may include special
tests and/or experience for that specific application.
TRANSFORMER WINDINGS
To carry out voltage tests on major equipment, all voltage transformer windings shall be
disconnected from the apparatus and grounded prior to test. All current transformer secondary
windings are to be short circuited and grounded prior to test.
It should be noted that application of even a low voltage AC supply to the low voltage terminals
of voltage transformation apparatus, such as voltage transformers and power transformers, could
result in the induction of a voltage of sufficient magnitude to cause an unsafe condition.
Similarly, the application of either a DC or AC voltage to the primary terminals of a current
transformer can cause high voltage at the terminals if these are left open circuited.
NOTE: DC voltages are not recommended for use on transformers, where DC
currents pass through a winding. This could cause a high voltage hazard.
Where DC currents must be passed through a winding, the opposite winding on the same core
leg (phase) must be short circuited.
GROUNDING OF COMPONENTS NOT UNDER TEST
Care shall be taken to ensure that all un-energized metallic parts in the vicinity, such as
conductors or shields not subjected to the test voltage, are connected to the ground. This must be
emphasized since charges can build up on ungrounded metallic equipment
73
7.
74
Introduction of Grid
Station Main and
Auxiliary Equipment
7-1
TRANSFORMERS
Electrical transformer is a static device which transforms electrical energy from one circuit to
another without any direct electrical connection and with the help of mutual induction between to
windings. It transforms power from one circuit to another without changing its frequency but
may be in different voltage level.
7-1-1 USE OF POWER TRANSFORMER
Generation of Electrical Power in low voltage level is very much cost effective. Hence Electrical
Power is generated in low voltage level. Theoretically, this low voltage leveled power can be
transmitted to the receiving end. But if the voltage level of a power is increased, the current of
the power is reduced which causes reduction in ohmic or I2R losses in the system, reduction in
cross sectional area of the conductor i.e. reduction in capital cost of the system and it also
75
improves the voltage regulation of the system. Because of these, low leveled power must be
stepped up for efficient. This is done by step up transformer at the sending side of the power
system network. As this high voltage power may not be distributed to the consumers directly,
this must be stepped down to the desired level at the receiving end with help of step down
transformer. These are the use of electrical power transformer in the electrical power system.
7-1-2 TYPES OF TRANSFORMER
Transformers can be categorized in different ways, depending upon their purpose, use,
construction etc. The types of transformer are as follows:
Step Up Transformer & Step Down Transformer - Generally used for stepping up and down
the voltage level of power in transmission and distribution power network.
Three phase transformer & Single Phase Transformer - Former is generally used in three
phase power system as it is cost effective than later but when size matters it is preferable to use
three phase transformer as it is easier to transport three single phase unit separately than one
single three phase unit.
Electrical Power Transformer, Distribution Transformer & Instrument Transformer Transformer generally used in transmission network is normally known as Power Transformer,
Distribution Transformer is used in distribution network and this is lower rating transformer and
Current Transformer & Potential Transformer, we use for relay and protection purpose in
electrical power system and in different instruments in industries are called instrument
transformer.
Two Winding Transformer & Auto-Transformer - Former is generally used where ratio
between High Voltage and Low Voltage is greater than 2. It is cost effective to use later where
the ratio between High Voltage and Low Voltage is less than 2.
Outdoor Transformer & Indoor Transformer - Transformers designed for installing at
outdoor is Outdoor Transformer and Transformers designed for installing at indoor is Indoor
Transformer.
7-2
CIRCUIT BREAKERS
Circuit Breaker is a switching device which can be operated manually as well as automatically
for controlling and protection of electrical power system respectively. As the modern power
system deals with huge currents, special attention should be given during designing of circuit
breaker to safe interruption of arc produced during the operation of circuit breaker
The modern power system deals with huge power network and huge numbers of associated
electrical equipment. During short circuit fault or any other types of electrical fault these
equipment as well as the power network suffer a high stress of fault current in them which may
76
damage the equipment and networks permanently. For saving these equipments and the power
networks the fault current should be cleared from the system as quickly as possible. Again after
the fault is cleared, the system must come to its normal working condition as soon as possible for
supplying reliable quality power to the receiving ends. In addition to that for proper controlling
of power system, different switching operations are required to be performed. So for timely
disconnecting and reconnecting different parts of power system network for protection and
control, there must be some special type of switching devices which can be operated safely under
huge current carrying condition. During interruption of huge current, there would be large arcing
in between switching contacts, so care should be taken to quench these arcs in safe manner.
Circuit breaker is the special device which does all the required switching operations during
current carrying condition.
7.2.1 WORKING PRINCIPLE OF CIRCUIT BREAKER
Circuit breaker mainly consists of fixed contacts and moving contacts. In normal "ON" condition
of circuit breaker, these two contacts are physically connected to each other due to applied
mechanical pressure on the moving contacts. There is an arrangement stored potential energy in
the operating mechanism of circuit breaker which is realized if switching signal given to the
breaker. The potential energy can be stored in the circuit breaker by different ways like by
deforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of
potential energy, it must be released during operation. Release of potential energy makes sliding
of the moving contact at extremely fast manner.
All circuit breaker have operating coils (tripping coils and close coil), whenever these coils are
energized by switching pulse, and the plunger inside them displaced. This operating coil plunger
is typically attached to the operating mechanism of circuit breaker, as a result the mechanically
stored potential energy in the breaker mechanism is released in forms of kinetic energy, which
makes the moving contact to move as these moving contacts mechanically attached through a
gear lever arrangement with the operating mechanism. After a cycle of operation of circuit
breaker the total stored energy is released and hence the potential energy again stored in the
operating mechanism of circuit breaker by means of spring charging motor or air compressor or
by any other means.
Till now we have discussed about mechanical working principle of circuit breaker. But there are
electrical characteristics of a circuit breaker which also should be considered in this discussion of
operation of circuit breaker.
The circuit breaker has to carry large rated or fault power. Due to this large power there is
always dangerously high arcing between moving contacts and fixed contact during operation of
circuit breaker.
Again as we discussed earlier the arc in circuit breaker can be quenched safely if the dielectric
strength between the current carrying contacts of circuit breaker increases rapidly during every
current zero crossing of the alternating current. The dielectric strength of the media in between
contacts can be increased in numbers of ways, like by compressing the ionized arcing media
77
since compressing accelerates the deionization process of the media, by cooling the arcing media
since cooling increase the resistance of arcing path or by replacing the ionized arcing media by
fresh gasses. Hence a numbers of arc quenching processes should be involved in operation of
circuit breaker.
7.2.2 TYPES OF CIRCUIT BREAKER
According to different criteria there are different types of circuit breaker
Classification Based on Arc Quenching Media:
1.
2.
3.
4.
Oil Circuit Breaker
Air Circuit Breaker
SF6 Circuit Breaker
Vacuum Circuit Breaker
Classification Based on Service:
1.
2.
Outdoor Circuit Breaker
Indoor Circuit Breaker
Classification Based on Operating Mechanism of circuit breaker:
1.
2.
3.
Spring Operated Circuit Breaker
Pneumatic Circuit Breaker
Hydraulic Circuit Breaker
Classification Based on Voltage level of installation:
1.
2.
3.
High Voltage Circuit Breaker
Medium Voltage Circuit Breaker
Low Voltage Circuit Breaker
7.3
DISCONNECT SWITCHES/ISOLATORS
In electrical engineering, a disconnector or isolator switch or disconnect switch is used to make
sure that an electrical circuit can be completely de-energized for service or maintenance. Such
switches are often found in electrical distribution and industrial applications where machinery
must have its source of driving power removed for adjustment or repair. High-voltage isolation
switches are used in electrical substations to allow isolation of apparatus such as circuit breakers
and transformers, and transmission lines, for maintenance. Often the isolation switch is not
intended for normal control of the circuit and is used only for isolation.
Isolator switches have provisions for a Padlock so that inadvertent operation is not possible. In
high voltage or complex systems, these padlocks may be part of a trapped-key interlocked to
78
ensure proper sequence of operation. In some designs the isolator switch has the additional
ability to earth the isolated circuit thereby providing additional safety. Such an arrangement
would apply to circuits which inter-connect power systems where both end of the circuit need to
be isolated.
The major difference between an isolator and a circuit breaker is that an isolator is an off-load
device intended to be opened only after current has been interrupted by some other control
device. Safety regulations of the utility must prevent any attempt to open the disconnector while
it supplies a circuit.
7-4
LIGHTNING ARRESTER
A lightning arrester (in Europe: surge arrester) is a device used on electrical power system and
communications systems to protect the insulation and conductors of the system from the
damaging effects lightning. The typical lightning arrester has a high voltage terminal and a
ground terminal. When a lightning surge (or switching surge, which is very similar) travels along
the power line to the arrester, the current from the surge is diverted through the arrestor, in most
cases to earth.
If protection fails or is absent, lightning that strikes the electrical system introduces thousands of
kilovolts that may damage the transmission lines, and can also cause severe damage to
transformers and other electrical or electronic devices. Lightning-produced extreme voltage
spikes in incoming power lines can damage electrical home appliances.
A lightning arrester may be a spark gap or may have a block of a semiconducting material such
as Silicon Carbide or Zinc Oxide. Some spark gaps are open to the air, but most modern varieties
are filled with a precision gas mixture, and have a small amount of radioactive material to
encourage the gas to ionize when the voltage across the gap reaches a specified level. Other
designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp)
connected between the protected conductor and ground, or voltage-activated solid-state switches
called varistors or MOVs.
Lightning arresters built for power system consist of a porcelain tube several feet long and
several inches in diameter, typically filled with disks of zinc oxide. A safety port on the side of
the device vents the occasional internal explosion without shattering the porcelain cylinder.
Lightning arresters are rated by the peak current they can withstand the amount of energy they
can absorb, and the break over voltage that they require to begin conduction. They are applied as
part of a lightning protection system, in combination with air terminals and bonding.
7-5
BATTERIES AND BATTERY CHARGERS
79
Supply of power to protection and control circuits is provided from storage batteries due to
reliability point of view.
The simplest operating unit to produce emf chemically is called a cell, whereas several cells
constitute a battery. Electrochemical devices consist of two dissimilar electrodes immersed in a
conducting solution, normally known as electrolyte that is capable of storing electrical energy.
The voltage of the cell depends upon the material of electrolyte, while the current and power
capacity of a cell depends upon the plate area and weight of active material in the electrodes.
Main types of storage batteries are:
1. Lead Acid Batteries
2. Alkaline Batteries
Active Parts of Lead Acid Battery:
1. Grid (Lead Antimony)
2. Positive Plates (Lead Per Oxide- PbO2)
3. Negative Plates (Lead- Pb)
4. Electrolyte (Sulphuric Acid-H2SO4)
Chemical Reactions
At Anode:
PbO2 + H2SO4↔ PbSO4 + H2O + ½O2
At Cathode:
Pb + H2SO4↔ PbSO4 + H2O
7-6
STATION GROUNDING SYSTEM
Earthing or grounding is the term used for electrical connection to general mass of earth in such
a manner as to ensure, at all times, an immediate discharge of energy without danger. A
grounding system to be totally effective must satisfy the following conditions:
A. Provide a low impedance path to ground for personnel and equipment protection and
effective circuit relaying.
B. Withstand and dissipate repeated fault and surge currents.
C. Provide corrosion allowance or corrosion resistance to various soil chemicals to insure
continuous performance during the life of the equipment being protected.
Types of Earthing:
A. Solid or Effective Earthing
B. Resistance Earthing/Reactance Earthing
80
Classification of Earthing
A. System or Neutral Earthing: The neutral point of generator, transformer, transmission and
distribution system or circuit, rotating machines etc. is connected to earth either directly
or through a resistance, or a reactance.
B. Equipment Earthing: Equipment Earthing means connecting the non current carrying
metallic parts in the neighborhood of electrical circuits to earth.
Resistance to current through an earth electrode system has the following three components:
A. Resistance of the ground rod itself and connections to it.
B. Contact resistance between the ground rod and earth adjacent to it.
C. Resistance of the surrounding earth.
7-7
AC & DC Supply System
In any substation AC and DC supply system plays a very important role for protection, control
and for all auxiliary services.
AC Supply System
For AC supply, normally a dedicated panel is specified in a substation which is only for the
substation and no external load is connected to it in order to avoid interruptions on it. On the LT
side two transformers are provided exclusively for the substation auxiliary services. For
reliability purposes, load is fed from one transformer; however in case load can be shifted to the
other transformer either from HT or LT side. Then we have distribution panels, from where load
is distributed throughout the substation through appropriate Circuit Breakers/ Miniature Circuit
Breakers.
DC Supply System
81
For DC supply system, Rectifiers, Batteries and Distribution Panels are provided in the
substation. In important substations, normally Two sets of Batteries along with Three Rectifiers
(One as standby) are provided for reliability purposes.
110 Volts Batteries
Two Sets
110 Volts Rectifiers
Three Sets
220 Volts Batteries
Two Sets
220 Volts Rectifiers
Three Sets
In 500 kV substations, Four sets of Batteries and Six Rectifiers (One as standby for Two banks).
Even, in case of emergency, loads of the same rating can be coupled with one Rectifier/Battery.
7.8
POWER CABLES
There are four main parts of cable:1.
2.
3.
4.
Conductor
Insulation
Shield or Sheath
Protective Covering
7.8.1 PURPOSES OF SHIELDING / SHIELD GROUNDING
The application of conducting (copper etc) and semiconducting (metabolized paper tap or
containing carbon or silicon etc) materials over the conductor insulation is called shielding. The
main purpose of shield is to keep even voltage gradient across the insulation in order to avoid
damage to insulation by corona or ionization.
Now shield may have induced voltages in it, so shield must be grounded in order to discharge
these induced voltages. When shield is grounded, it provides some more advantages as well,
which are:
1.
2.
3.
Provides earth return path in case of phase to ground fault
Human safety
Protects the cable from external high voltages, produced by lightening etc
Shield must be grounded at one place only (especially in single phase cable) in order to avoid
flow of current in shield and hence damage to it due to overheating. Shielding idea was given by
Martin Hochsadter in 1915. He gave the idea that put shield around the conductor of each phase
82
and then ground all shields. Such cables are called H-cables. Such cable fails phase to ground. In
these cables it is very rare that cable may fail phase to phase.
In a very long cable, sectionalized are used. In sectionalized shield each section is insulated from
each other and then each section is grounded at one place only.
7.9
BUS-BARS
There are two types of bus bars used in grid station, which are:
1.
The Flexile or Stranded Bus Bar
2.
The Rigid Bus Bar (may be tubular or solid)
1.
Flexible or Stranded Bus Bar: It is used where:
A. Longer spans are involved.
B. Where sufficient clearances are needed to allow for conductor sways and.
C. It is used as a long drop from horizontal bus to equipment bushing.
In the flexible bus bar sag must be enough to account for temperature variations without
affecting the clearances between phases and phases to ground.
2.
Rigid Bus Bar: It is used where:
A. Heavy currents are involved
B. Short or less Spacing is available
To account for thermal expansion /contraction of rigid bus provision must be made by means of
expansion joints and clamps to permit bus to slide both ways in order to avoid damage to
equipment bushing and isolators etc.
7.9.1 BUS BAR SCHEMES
SINGLE BUS SYSTEM
Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer
bay are connected to only one single bus as shown.
83
Fig (1)
84
SINGLE BUS SYSTEM WITH BUS SECTIONALIZER
FIG) (2
DOUBLE BUS SYSTEM
FIG (3)
DOUBLE BREAKER BUS SYSTEM
85
Fig (4)
ONE AND A HALF BREAKER BUS SYSTEM
FIG (5)
MAIN AND TRANSFER BUS SYSTEM
86
FIG (6)
DOUBLE BUS SYSTEM WITH BYPASS ISOLATORS
87
FIG (7)
RING BUS SYSTEM
Fig (8)
88
8.
Transformers
8.1
BASIC THEORY OF TRANSFORMER
The working principle of transformer is very simple. It depends upon Faraday's laws of
Electromagnetic Induction. Actually mutual induction between two or more winding is
responsible for transformation action in an electrical transformer. According to Faraday's laws,
"Rate of change of flux linkage with respect to time is directly proportional to the induced EMF
in a conductor or coil".
89
Say you have one winding which is supplied by an alternating electrical source. The alternating
current through the winding produces a continually changing flux or alternating flux surrounds
the winding. If any other winding is brought nearer to the previous one, obviously some portion
of this flux will link with the second. As this flux is continually changing in its amplitude and
direction, there must be a change in flux linkage in the second winding or coil. According to
Faraday's laws of Electromagnetic Induction, there must be an EMF induced in the second. If the
circuit of the latter winding is closed, there must be a current flow through it. This is the simplest
form of electrical transformer and this is most basic of working principle of transformer.
For better understanding we are trying to repeat the above explanation in more brief here.
Whenever we apply alternating current to an electric coil, there will be an alternating flux
surrounding that coil. Now if we bring another coil nearby this first one, there will be an
alternating flux linkage with that second coil. As the flux is alternating, there will be obviously a
rate of change of flux linkage with respect to time in the second coil. Naturally EMF will be
induced in it as per Faraday's laws of electromagnetic induction. This is the most basic concept
of working principle of transformer.
The winding which takes electrical power from the source, is generally known as Primary
Winding of transformer. Here in our above example, it is first winding.
Fig (1)
The winding which gives the desired output voltage due to mutual induction in the transformer,
is commonly known as Secondary Winding of Transformer. Here in our example it is second
winding
90
Fig (2)
The above mentioned form of transformer is theoretically possible but not practically, because in
open air very tiny portion of the flux of the first winding will link with second winding so the
current flows through the closed circuit of latter, will be so small that it may be difficult to
measure.
The rate of change of flux linkage depends upon the amount of linked flux, with the second
winding. So it is desired to link almost all flux of primary winding, to the secondary winding.
This is effectively and efficiently done by placing one low reluctance path common to both the
winding. This low reluctance path is transformer core, through which maximum number of flux
produced by the primary is passed through and linked with the secondary winding. This is most
basic electrical transformer.
8.2
MAIN CONSTRUCTIONAL PARTS OF TRANSFORMER
Three main parts of a transformer are,
1. Primary Winding of transformer - which produces magnetic flux when it is connected to
electrical source.
2. Magnetic transformer core - the magnetic flux produced by the primary winding, will pass
through this low reluctance path linked with secondary winding and creates a closed magnetic
circuit.
3. Secondary Winding of transformer - the flux, produced by primary winding, passes through
the core, will link with the secondary winding. This winding is also wound on the same core and
gives the desired output of the transformer.
8.1.3 IDEAL TRANSFORMER
An Ideal Transformer is an imaginary transformer which does not have any loss in it, means no
core losses, copper losses and any other losses in transformer. Efficiency of this transformer is
considered as 100%.
8.1.4 IDEAL TRANSFORMER MODEL
Ideal Transformer Model is developed by considering a transformer which does not have any
loss. That means the windings of the transformer are purely inductive and core of transformer is
loss free. There is zero leakage reactance of transformer. As we said, whenever we place a low
reluctance core inside the windings, maximum amount of flux passes through this core; but still
there is some flux which does not pass through the core but passes through the insulation used in
91
the transformer. This flux does not take part in the transformation action of the transformer. This
flux is called leakage flux of transformer.
In an Ideal Transformer, this leakage flux is considered as nil. That means 100% flux passes
through the core and linked with both primary and secondary windings of transformer. Although
every winding is supposed to be purely inductive, but it has some resistance which causes
voltage drop and I2R loss in it. In such ideal transformer model, the winding are also considered,
ideal that means resistance of the winding is zero.
Fig (3)
Now if an alternating source voltage V1 is applied in the primary winding of that Ideal
Transformer, there will be a counter self EMF, E1 induced in the primary winding which is
purely 180o in phase opposition with supply voltage V1.
Fig (4)
For developing counter EMF, E1 across the primary winding it draws current from the source to
produces required magnetizing flux. As the primary winding is purely inductive, that current is in
90o lags from the supply voltage V1. This current is called magnetizing current of transformer Iμ
92
Fig (5)
This alternating current, Iμ produces an alternating magnetizing flux Φ which is proportional to
that current and hence in phase with it. As this flux is also linked with secondary winding
through the transformer core, there will be another EMF, E2 induced in the secondary winding,
this is mutually induced EMF. As the secondary is placed on the same core where the primary
winding is placed, the EMF induced in the secondary winding of transformer, E2 is in the phase
with primary EMF, E1 and in phase opposition with source voltage, V1.
8.1.5 THEORY OF TRANSFORMER
We have discussed about theory of ideal transformer for better understanding of actual
elementary theory of transformer. Now we will go through one by one practical aspects of an
electrical transformer and try to draw vector diagram of transformer in every step. As we said
that in ideal transformer, there are no core losses in transformer. i.e. loss free transformer core.
But in practical transformer there are hysteresis and eddy current losses in transformer core.
THEORY OF TRANSFORMER ON NO-LOAD, AND HAVING NO WINDING
RESISTANCE AND NO LEAKAGE REACTANCE OF TRANSFORMER
Let us consider one electrical transformer with only core losses. That means that it has only core
losses but no copper lose and no leakage reactance of transformer. When an alternating source is
applied in the primary, the source will supply the current for magnetizing the transformer core.
But this current is not the actual magnetizing current, little bit greater than actual magnetizing
current. Actually total current supplied from the source has two components one is magnetizing
current which is merely utilized for magnetizing the core and other component of the source
current, is consumed for compensating the core losses in transformer. Because of this core loss
component, the source current in transformer on no-load condition, supplied from the source as
source current is not exactly at 90o lags of supply voltage but it lags behind an angle θ, which is
less than 90o.
If total current supplied from source is Io, it will have one component in phase with supply
voltage V1 and this component of the current Iw is core loss component. This component is taken
in phase with source voltage, because it is associated with active or working losses in
transformer. Other component of the source current is denoted as Iμ. This component produces
the alternating magnetic flux in the core, so it is watt-less means it is reactive part of the
93
transformer source current. Hence Iμ will be in quadrature with V1 and in phase with alternating
flux Φ.
Hence, total primary current in transformer on no-load condition can be represented as
Io = Iμ + Iw and,
|Iμ| = |Io|cosθ
|Iw| = |Io|sinθ
|Io| = ( |Iμ|2 + |Iw|2 )½
Now you have seen how simple to explain the theory of transformer on no-load.
Fig (6)
THEORY OF TRANSFORMER ON LOAD BUT HAVING NO WINDING RESISTANCE
AND LEAKAGE REACTANCE
Now we will examine the behavior of above said transformer on load that means load is
connected to the secondary terminals. Consider, transformer having core loss but no copper loss
and leakage reactance. Whenever load is connected to the secondary winding, load current will
start to flow through the load as well as secondary winding. This load current solely depends
upon the characteristics of the load and also upon secondary voltage of the transformer. This
current is called secondary current or load current; here it is denoted as I2. As I2 is flowing
through the secondary, a self MMF in secondary winding will be produced. Here it is N2I2,
where, N2 is the number of turns of the secondary winding of transformer.
94
Fig (7)
This MMF or magneto motive force in the secondary winding produces flux φ2. This φ2 will
oppose the main magnetizing flux and momentarily weakens the main flux and tries to reduce
primary self induced EMF, E1. If E1 falls down below the primary source voltage V1, there will
be extra current flow from source to primary winding. This extra primary current I2′ produces
extra flux φ′ in the core which will neutralize the secondary counter flux φ2. Hence the main
magnetizing flux of core, Φ remains unchanged irrespective of load.
So the total current drawn by this transformer from source can be divided into two components,
first one is utilized for magnetizing the core and compensate the core loss i.e. Io. It is no load
component of the primary current. Second one is utilized for compensating the counter flux of
the secondary winding. It is known as load component of the primary current. Hence total no
load primary current I1 of an electrical transformer having no winding resistance and leakage
reactance can be represented as follows
I1 = Io + I2′
Where θ2, is the angle between Secondary Voltage and Secondary Current of a transformer. Now
we will precede one further step towards more practical aspects of a transformer.
THEORY OF TRANSFORMER ON LOAD, WITH RESISTIVE WINDING, BUT NO
LEAKAGE REACTANCE
Now, consider the winding resistance of transformer but no leakage reactance. So far we have
discussed about the transformer which has ideal windings means winding with no resistance and
leakage reactance, but now we will consider one transformer which has internal resistance in the
winding but no leakage reactance. As the windings are resistive, there would be a voltage drop in
the windings.
95
Fig (8)
We have proved earlier that total primary current from the source on load is I1. The voltage drop
in the primary winding with resistance, R1 is R1I1. Obviously induced EMF across primary
winding E1, is not exactly equal to source voltage V1. E1 is less than V1 by voltage drop I1R1.
V1 = E1 + I1R1
Again in the case of secondary, the voltage induced across the secondary winding, E 2 does not
totally appear across the load since it also drops by an amount I2R2, where R2 is the secondary
winding resistance and I2 is secondary current or load current.
Similarly voltage equation of the secondary side of the transformer will be
V2 = E2 − I2R2
THEORY OF TRANSFORMER ON LOAD, WITH RESISTANCE AS WELL AS LEAKAGE
REACTANCE IN TRANSFORMER WINDINGS
Now we will consider the condition, when there is leakage reactance of transformer as well as
winding resistance.
96
Fig (9)
Let leakage reactance of primary and secondary windings of the transformer is X1 and X2
respectively.
Hence total impedance of primary and secondary winding with resistance R1 and R2 respectively,
can be represented as,
Z1 = R1 + jX1 (impedance of primary winding)
Z2 = R2 + jX2 (impedance of secondary winding)
We have already established the voltage equation of a transformer on load, with only resistances
in the windings; where voltage drops in the windings occur only due to resistive voltage drop.
But when we consider leakage reactance of transformer windings, voltage drop occurs in the
winding not only because of resistance, it is because of transformer windings. Hence, actual
voltage equation of a transformer can easily be determined by just replacing resistances R1 & R2
in the previously established voltage equations by Z1 and Z2.
Therefore, the voltage equations are,
V1 = E1 + I1Z1 & V2 = E2 − I2Z2
V1 = E1 + I1(R1 + jX1) ⇒ V1 = E1 + I1R1 + jI1X1
V2 = E2 - I2(R2 + jX2) ⇒ V2 = E2 - I2R2 − jI2X2
Resistance drops are in the direction of current vector but reactive drop will be in perpendicular
to the current vector as shown in the above vector diagram of transformer.
97
8.1.6 EQUIVALENT CIRCUIT OF TRANSFORMER
Equivalent impedance of Transformer is essential to be calculated as because the electrical
transformer is an electrical power system equipment so for estimating different parameters of
electrical power system, it may be required to calculate total internal impedance of an electrical
transformer viewing from primary side or secondary side as per requirement. This calculation
requires equivalent circuit of transformer referred to primary or equivalent circuit of transformer
referred to secondary sides respectively. Percentage impedance is also very essential parameter
of transformer. Special attention is to be given to this parameter during installing a transformer in
an existing electrical power system. Percentage impedance of different power transformers
should be properly matched during parallel operation of these transformers.
The percentage impedance can be derived from equivalent impedance of transformer so it can be
said that equivalent circuit of transformer is also required during calculation of % impedance.
EQUIVALENT CIRCUIT OF TRANSFORMER REFERRED TO PRIMARY
For drawing equivalent circuit of transformer referred to primary, first we have to establish
general equivalent circuit of transformer then we will modify it for referring from primary side.
For doing this we first recall the complete vector diagram of a transformer which is shown in the
figure below.
Fig.(10)
Let us consider the transformation ratio be
K = N1/N2 = E1/E2
98
The applied voltage to the primary is V1 and voltage across the primary winding is E1. Total
current supplied to primary is I1. So the voltage V1 applied to the primary is partly dropped by
I1Z1 or I1R1 + j.I1X1 before it appears across primary winding. The voltage appeared across
winding is countered by primary induced EMF E1.
So voltage equation of this portion of transformer can be written as
V1 - (I1R1 + j.I1X1) = E1
The equivalent circuit for that equation can be drawn as below,
Fig (11)
From the vector diagram above it is found that total primary current I1 has two components one
is no - load component Io and other is load component I2′. As this primary current have two
components or branches so there must be a parallel path with primary winding of transformer.
This parallel path of current is known as excitation branch of equivalent circuit of transformer.
The resistive and reactive branches of the excitation circuit can be represented as
Ro = E1 / Iw and Xo = E1 / Iμ.
Fig (12)
The load component I2′ flows through the primary winding of transformer and induced voltage
across the winding is E1 as shown in the figure. This induced voltage E1 transforms to secondary
and it is E2 and load component of primary current I2′ is transformed to secondary as secondary
99
current I2. Current of secondary is I2. So the voltage E2 across secondary winding is partly
dropped by I2Z2 or I2R2 + j.I2X2 before it appears across load. The load voltage is V2.
The complete equivalent circuit of transformer is shown below.
Fig (13)2
Now if we see the voltage drop in secondary from primary side then it would be K-times greater
and would be written as K.Z2.I2.
Again I2′.N1 = I2.N2
⇒ I2 = I2′.N1 / N2
⇒ I2 = K.I2′
Therefore,
K.Z2.I2 = K.Z2.K.I2′
= K2.Z2.I2′
From above equation, Secondary impedance of transformer referred to primary is,
Hence,
and
Z2′ = K2.Z2
R2′ = K2.R2
X2′ = K2.X2
So the complete equivalent circuit of transformer referred to primary is shown in the figure
below,
100
Fig (14)
APPROXIMATE EQUIVALENT CIRCUIT OF TRANSFORMER
Since Io is very small compared to I1, it is less than 5% of full load primary current, Io changes
the voltage drop insignificantly. Hence, it is good approximation to ignore the excitation circuit
in approximate equivalent circuit of transformer. The winding resistance and reactance being in
series can now be combined into equivalent resistance and reactance of transformer referred to
any particular side. In this case it is side 1 or primary side. Here V2′ = K.V2
Fig (15)
EQUIVALENT CIRCUIT OF TRANSFORMER REFERRED TO SECONDARY
101
In similar way approximate equivalent circuit of transformer referred to secondary can be drawn.
Where, equivalent impedance of transformer referred to secondary, can be derived as
Z1′ = Z1 / K2
Therefore,
R1′ = R1 / K2 and
X1′ = X1 / K2
Here, V1′ = V1 / K
Fig(16)
8.1.7 LOSSES IN TRANSFORMER
As the electrical transformer is a static device, mechanical loss in transformer normally does not
come into picture. We generally consider only electrical losses in transformer. Loss in any
machine is broadly defined as difference between input power and output power.
When input power is supplied to the primary of transformer, some portion of that power is used
to compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy Current
loss in transformer core and some portion of the input power is lost as I2R loss and dissipated as
heat in the primary and secondary winding, as because these windings have some internal
resistance in them. The first one is called core loss or iron loss in transformer and later is known
as ohmic loss or copper loss in transformer. Another loss occurs in transformer, known as Stray
Loss, due to Stray fluxes link with the mechanical structure and winding conductors.
COPPER LOSS IN TRANSFORMER
Copper loss is I2R loss, in primary side it is I12R1 and in secondary side it is I22R2 loss, where I1
& I2 are primary & secondary current of transformer and R1 & R2 are resistances of primary &
secondary winding. As the both primary & secondary currents depend upon load of transformer,
so copper loss in transformer vary with load.
CORE LOSSES IN TRANSFORMER
Hysteresis loss and eddy current loss both depend upon magnetic properties of the materials used
to construct the transformer core and its design. So these losses in transformer are fixed and do
not depend upon the load current. So core losses in transformer which is alternatively known as
iron loss in transformer and can be considered as constant for all range of load.
Hysteresis loss in transformer is denoted as,
Wh = KhfBm1.6
watts
102
Eddy Current loss in transformer is denoted as,
Where,
Kh = Hysteresis Constant.
Ke = Eddy Current Constant.
Kf = form Constant.
We = Kef2Kf2Bm2
watts
Copper loss can simply be denoted as,
IL2R2′ + Stray loss
Where,
IL = I2 = load of transformer, and
R2′ is the resistance of transformer referred to secondary.
Now we will discuss Hysteresis loss and Eddy Current loss in little bit more details for better
understanding the topic of losses in transformer
HYSTERESIS LOSS IN TRANSFORMER
Hysteresis loss in transformer can be explained in different ways. We will discuss two of them,
one is physical explanation other is mathematical explanation.
PHYSICAL EXPLANATION OF HYSTERESIS LOSS
The magnetic transformer is made of ′Cold Rolled Grain Oriented Silicon Steel′. Steel is very
good ferromagnetic material. Such kinds of materials are very sensitive to be magnetized. That
means whenever magnetic flux passes through, it will behave like magnet. Ferromagnetic
substances have numbers of domains in their structure. Domains are very small region in the
material structure, where all the dipoles are paralleled to same direction. In other words, the
domains are like small permanent magnet situated randomly in the structure of substance. These
domains are arranged inside the material structure in such a random manner, that net resultant
magnetic field of the said material is zero. Whenever external magnetic field or MMF is applied
to that substance, these randomly directed domains are arranged themselves in parallel to the axis
of applied MMF. After removing this external MMF, maximum numbers of domains again come
to random positions, but few of them still remain in their changed position. Because of these
unchanged domains the substance becomes slightly magnetized permanently. This magnetism is
called "Spontaneous Magnetism".
To neutralize this magnetism some opposite MMF is required to be applied. The magneto motive
force or MMF applied in the transformer core is alternating. For every cycle, due to this domain
reversal there will be extra work done. For this reason, there will be a consumption of electrical
energy which is known as Hysteresis loss of transformer.
MATHEMATICAL EXPLANATION OF HYSTERESIS LOSS IN TRANSFORMER
DETERMINATION OF HYSTERESIS LOSS
Consider a ring of ferromagnetic specimen of circumference L meter, cross - sectional area a m2
and N turns of insulated wire as shown in the picture beside,
103
Fig (17)
Let us consider, the current flowing through the coil is I A,
Magnetizing force,
H = NI/L or I = HL/N
Let, the flux density at this instant is B,
Therefore, total flux through the ring,
Φ = BXa Wb
As the current flowing through the solenoid is alternating, the flux produced in the iron ring is
also alternating in nature, so the EMF (e′) induced will be expressed as,
Fig (18)
e = - N (dφ/dt)
104
e = -N (dBa/dt)
e = -Na (dB/dt)
According to Lenz’s law this induced EMF will oppose the flow of current, therefore, in order to
maintain the current I in the coil, the source must supply an equal and opposite EMF. Hence
applied EMF,
e = e’ = Na (dB/dt)
Energy consumed in short time dt, during which the flux density has changed,
= e.I.dt
= Na (dB/dt) x I x dt
= Na (dB/dt) x (HL/N) x dt
= aLH.dB Joules
Thus, total work done or energy consumed during one complete cycle of magnetism,
W = aL ʃ H.dB, where limits of integration are from zero to Bmax
Now aL is the volume of the ring and H.dB is the area of elementary strip of B - H curve shown
in the figure above,
ʃ H.dB = total area enclosed by Hysteresis Loop.
Therefore, Energy consumed per cycle = volume of the ring X area of hysteresis loop.
In the case of transformer, this ring can be considered as magnetic core of transformer. Hence
this work done is nothing but electrical energy loss in transformer core and this is known as
hysteresis loss in transformer.
EDDY CURRENT LOSS
In transformer we supply alternating current in the primary, this alternating current produces
alternating magnetizing flux in the core and as this flux links with secondary winding there will
be induced voltage in secondary, resulting current to flow through the load connected with it.
Some of the alternating fluxes of transformer may also link with other conducting parts like steel
core or iron body of transformer etc. As alternating flux links with these parts of transformer,
105
locally induced EMF will result. Due to these EMFs there will be currents which will circulate
locally in those parts of the transformer. These circulating current will not contribute in output of
the transformer and dissipated as heat. This type of energy loss is called eddy current loss of
transformer.
8.2
AUTO TRANSFORMER
THEORY OF AUTO TRANSFORMER
Auto transformer is kind of electrical transformer where primary and secondary shares same
common single winding. In Auto Transformer, one single winding is used as primary winding as
well as secondary winding. But in two windings transformer two different windings are used for
primary and secondary purpose. A diagram of auto transformer is shown below, winding AB of
total turns N1 is considered as primary winding. This winding is tapped from point C and the
portion BC is considered as secondary. Let's assume the number of turns in between point B and
C is N2.
If V1 voltage is applied across the winding i.e. in between A and C
V1/N1
So voltage per turn in this winding is
Hence, the voltage across the portion BC of the winding, will be
(V1/N1) x N2 = V2
V2/V1 =N2/N1 = Constant = k
As BC portion of the winding is considered as secondary, it can easily be understood that value
of constant k-is nothing but turns ratio or voltage ratio of that Auto Transformer.
When load is connected between secondary terminals i.e. between B and C, load current I2 starts
to flow. The current in the secondary winding or common winding is the difference of I2 & I1.
106
Fig (19)
COPPER SAVINGS IN AUTO TRANSFORMER
Now we will discuss savings of copper in an auto transformer compared to conventional two
windings electrical transformer. We know that weight of copper of any winding depends upon its
length and cross-sectional area. Again length of conductor in winding is proportional to its
number of turns and cross-sectional area varies with rated current.
So weight of copper in winding is directly proportional to product of number of turns and rated
current of the winding.
Therefore, weight of copper in the section AC proportional to
(N1 − N2)I1
and similarly, weight of copper in the section BC proportional to
N2( I2 − I1)
Hence, total weight of copper in the winding of Auto Transformer proportional to
(N1 − N2)I1 + N2( I2 − I1)
= N1I1 − N2I1 + N2I2 − N2I1
= N1I1 + N2I2 − 2N2I1
= 2N1I1 − 2N2I1 (Since, N1I1 = N2I2)
= 2(N1I1 − N2I1)
In similar way it can be proved, the weight of copper in two winding transformer is proportional
to,
N1I1 + N2I2 = 2N1I1
(Since, in a transformer N1I1 = N2I2)
107
Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer
respectively,
Hence, Wa /Wtw = 2(N1I1 − N2I1)/2(N1I1)
= N1I1 − N2I1/N1I1
= 1 – N2I1/N1I1
= 1 – N2/N1
=1–k
∴ Wa = Wtw(1 − k)
⇒Wa = Wtw − kWtw
∴ Saving of copper in auto transformer compared to two winding transformer,
⇒Wtw − Wa = kWtw
108
Fig (20)
Auto transformer employs only single winding per phase as against two distinctly separate
windings in a conventional power transformer.
ADVANTAGES OF USING AUTO TRANSFORMER
For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the
corresponding size of two winding transformer. For transformation ratio say 20 however the size
would be 95%. The saving in cost is of course not in the same proportion. The saving of cost is
appreciable when the ratio of transformer is low, that is lower than 2.
DISADVANTAGES OF USING AUTO TRANSFORMER
But auto transformer has the following disadvantages:
1. Because of electrical conductivity of the primary and secondary windings the lower voltage
circuit is liable to be impressed upon by higher voltage. To avoid breakdown in the lower voltage
circuit, it becomes necessary to design the low voltage circuit to withstand higher voltage.
2. The leakage flux between the primary and secondary windings is small and hence the
impedance is low. This results into severer short circuit currents under fault conditions.
3. The connections on primary and secondary sides have necessarily to be same, except when
using interconnected starring connections. This introduces complications due to changing
primary and secondary phase angle particularly in the case-by-case of delta / delta connection.
109
4. Because of common neutral in a star / star connected auto transformer it is not possible to
earth neutral of one side only. Both their sides have to have their neutrality either earth or
isolated.
5. It is more difficult to preserve the electromagnetic balance of the winding when voltage
adjustment tapping are provided. It should be known that the provision of adjusting tapping on
an auto transformer increases considerably the frame size of the transformer. If the range of
tapping is very large, the advantages gained in initial cost is lost to a great event
8.3
TERTIARY WINDING OF TRANSFORMER
In some high rating transformer, one winding, in addition to its primary and secondary winding,
is used. This additional winding, apart from primary and secondary windings, is known as
Tertiary Winding of Transformer. Because of this third winding, the transformer is called Three
Winding Transformer or 3 Winding Transformer.
ADVANTAGES OF USING TERTIARY WINDING IN TRANSFORMER
Tertiary Winding is provided in electrical transformer to meet one or more of the following
requirements
1. It reduces the unbalancing in the primary due to unbalancing in three phase load
2.
It
redistributed
the
flow
of
fault
current
3. Sometime it is required to supply an auxiliary load in different voltage level in addition to its
main secondary load. This secondary load can be taken from tertiary winding of three winding
transformer.
4. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in
limitation of fault current in the event of a short circuit from line to neutral.
STABILIZATION BY TERTIARY WINDING OF TRANSFORMER
In star - star transformer comprising three single units or a single unit with 5 limb core offers
high impedance to the flow of unbalanced load between the line and neutral. This is because, in
both of these transformers, there is very low reluctance return path of unbalanced flux.
If any transformer has N - turns in winding and reluctance of the magnetic path is RL, then,
MMF = N.I = ΦRL ....... (1)
Where
Again,
I
and
Φ
are
current
and
flux
induced voltage V = 4.44ΦfN
in
the
transformer.
⇒V∝Φ
⇒ Φ = K.V (Where K is constant)....... (2)
110
Now, from equation (1) & (2), it can be rewritten as,
N.I = K.V.RL
⇒ V/I = N/(K.RL)
⇒ Z = N/(K.RL)
⇒ Z ∝ 1/RL
From, this above mathematical expression it is found that impedance is inversely proportional to
reluctance. The impedance offered by the return path of unbalanced load current, is very high
where very low reluctance return path is provided for unbalanced flux.
Fig (21)
In other words, very high impedance to the flow of unbalanced current in 3 phases system
between line and neutral. Any unbalanced current in three phase system can be divided in to
three sets of components likewise positive sequence, negative sequence and zero sequence
components. The zero sequence current actually co-phasial current in three lines. If value of cophasial current in each line is Io, then total current flows through the neutral of secondary side of
transformer is In = 3.Io. This current cannot be balanced by primary current as the zero sequence
current cannot flow through the isolated neutral star connected primary. Hence the said current in
the secondary side set up a magnetic flux in the core.
As we discussed earlier in this chapter low reluctance path is available for the zero sequence flux
in a bank of single phase units and in the 5 limb core consequently the impedance offered to the
zero sequence current is very high. The delta connected tertiary winding of transformer permits
the circulation of zero sequence current in it. This circulating current in this delta winding
balance the zero sequence component of unbalance load, hence prevent unnecessary
development of unbalance zero sequence flux in the transformer core. In few words it can be said
111
that, placement of tertiary winding in star - star - neutral transformer considerably reduces the
zero sequence impedance of transformer.
RATING OF TERTIARY WINDING OF TRANSFORMER
Rating of tertiary winding of transformer depends upon its use. If it has to supply additional load,
its winding cross-section and design philosophy is decided as per load and three phase dead short
circuit on its terminal with power flow from both sides of HV & MV.
In case it is to be provided for stabilizing purpose only, its cross - section and design has to be
decided from thermal and mechanical consideration for the short duration fault currents during
various fault conditions single line-to-ground fault being the most onerous.
8.4
TRANSFORMER CONNECTIONS/TRANSFORMER BANK CONNECTIONS, AND
WINDING CONNECTIONS/VECTOR GROUPS
It is found that generation, transmission and distribution of electrical power are more economical
in three phase system than single phase system. For three phase system three single phase
transformers are required. Three phase transformation can be done in two ways, by using single
three phase transformer or by using a bank of three single phase transformers. Both are having
some advantages over other. Single three phase transformer costs around 15% less than bank of
three single phase transformers. Again former occupies less space than later. For very big
transformer, it is impossible to transport large three phase transformer to the site and it is easier
to transport three single phase transformers which is erected separately to form a three phase
unit. Another advantage of using bank of three single phase transformers is that, if one unit of the
bank becomes out of order, then the bank can be run as open delta.
A Varity of connection of three phase transformer are possible on each side of both a single three
phase transformer or a bank of three single phase transformers.
MARKING OR LABELING THE DIFFERENT TERMINALS OF TRANSFORMER
Terminals of each phase of HV side should be labeled as capital letters, A, B, C, and those of LV
side should be labeled as small letters, a, b, c. Terminal polarities are indicated by suffixes 1 & 2.
Suffix 1’s indicates similar polarity ends and so do 2’s.
STAR-STAR TRANSFORMER
Star-Star Transformer is formed in a three phase transformer by connecting one terminal of each
phase of individual side, together. The common terminal is indicated by suffix 1 in the figure
below. If terminal with suffix 1 in both primary and secondary are used as common terminal,
voltages of primary and secondary are in same phase. That is why this connection is called zero
degree connection or 0o - connection.
If the terminals with suffix 1 are connected together in HV side as common point and the
terminals with suffix 2 in LV side are connected together as common point, the voltages in
112
primary and secondary will be in opposite phase. Hence, Star-Star Transformer connection is
called 180o - Connection, of three phase transformer.
Fig (22)
DELTA-DETLA TRANSFORMER
In delta-delta transformer, 1 suffixed terminals of each phase primary winding will be connected
with 2 suffixed terminal of next phase primary winding.
113
Fig (23)
If primary is HV side, then A1 will be connected to B2, B1 will be connected to C2 and C1 will be
connected to A2. Similarly in LV side 1 suffixed terminals of each phase winding will be
connected with 2 suffixed terminals of next phase winding. That means, a1 will be connected to
b2, b1 will be connected to c2 and c1 will be connected to a2. If transformer leads are taken out
from primary and secondary 2 suffixed terminals of the winding, then there will be no phase
difference between similar line voltages in primary and secondary. This delta-delta transformer
connection is zero degree connection or 0o - Connection.
But in LV side of transformer, if, a2 is connected to b1, b2 is connected to c1 and c2 is connected
to a1. The secondary leads of transformer are taken out from 2 suffixed terminals of LV
windings, and then similar line voltages in primary and secondary will be in phase opposition.
This connection is called 180o connection, of three phase transformer.
STAR-DELTA TRANSFORMER
Here in star delta transformer, star connection in HV side is formed by connecting all the 1
suffixed terminals together as common point and transformer primary leads are taken out from 2
suffixed terminals of primary windings.
The delta connection in LV side is formed by connecting 1 suffixed terminals of each phase LV
winding with 2 suffixed terminal of next phase LV winding. More clearly, a1 is connected to b2,
b1 is connected to c2 and c1 is connected to a2. The secondary (here it considered as LV) leads are
taken out from
114
Fig (24)
2 suffixed ends of the secondary windings of transformer. The transformer connection diagram is
shown in the figure beside.
It is seen from the figure that the sum of the voltages in delta side is zero. This is a must as
otherwise closed delta would mean a short circuit. It is also observed from the phasor diagram
that, phase to neutral voltage (equivalent star basis) on the delta side lags by −30 o to the phase to
neutral voltage on the star side; this is also the phase relationship between the respective line to
line voltages. This star delta transformer connection is therefore known as −30o connection.
STAR–DELTA
+30O CONNECTION IS ALSO
POSSIBLE BY CONNECTING
SECONDARY TERMINALS IN FOLLOWING SEQUENCE. A2 IS CONNECTED TO B1,
B2 IS CONNECTED TO C1 AND C2 IS CONNECTED TO A1. THE SECONDARY
LEADS OF TRANSFORMER ARE TAKEN OUT FROM 2 SUFFIXED TERMINALS OF
LV WINDINGS,
DELTA-STAR TRANSFORMER
Delta-Star transformer connection of three phase transformer is similar to star–delta connection.
If anyone interchanges HV side and LV side of star–delta transformer in diagram, it simply
becomes delta–star connected three phase transformer. That means all small letters of star delta
connection should be replaced by capital letters and all small letters by capital in delta star
transformer connection.
115
Fig (25)
DELTA-ZIGZAG TRANSFORMER
The winding of each phase on the star connected side is divided into two equal halves in delta
zig-zag transformer. Each leg of the core of transformer is wound by half winding from two
different secondary phases in addition to its primary winding.
STAR-ZIGZAG TRANSFORMER
The winding of each phase on the secondary star in a star zigzag transformer is divided into two
equal halves. Each leg of the core of transformer is wound by half winding from two different
secondary phases in addition to its primary winding.
CHOICE BETWEEN STAR CONNECTION AND DELTA CONNECTION OF THREE
PHASE TRANSFORMER
In star connection with earthed neutral, phase voltage i.e. phase to neutral voltage, is 1/√3 times
of line voltage i.e. line to line voltage. But in the case of delta connection phase voltage is equal
to line voltage. Star connected high voltage side electrical transformer is about 10% cheaper than
that of delta connected high voltage side transformer.
Let’s explain, let, the voltage ratio of transformer between HV & LV is K, voltage across HV
winding is VH and voltage across LV winding is VL and voltage across transformer leads in HV
side say Vp and in LV say Vs.
116
IN STAR-STAR TRANSFORMER
VH = Vp/√3 and VL = Vs/√3
⇒ Vp / Vs = VH / VL = K
⇒ VH = K. VL
Voltage difference between HV & LV winding,
VH − VL = Vp − Vs = (K − 1).Vs
IN STAR-DELTA TRANSFORMER
VH = Vp/√3 and VL = Vs
Voltage difference between HV & LV winding,
VH − VL = Vp/√3 − Vs = (K/√3 − 1).Vs
IN DELTA-STAR TRANSFORMER
VH = Vp and VL = Vs/√3
Voltage difference between HV & LV winding,
VH − VL = Vp − Vs/√3 = (K − 1/√3) .Vs
For 132/33KV transformer K = 4, Therefore,
Case – 1:
Voltage difference between HV & LV winding,
(4 − 1) Vs = 3. Vs
Case – 2:
Voltage difference between HV & LV winding,
(4/√3 − 1) Vs = 1.3 Vs
Case – 3:
Voltage difference between HV & LV winding,
(4 − 1/√3) Vs = 3.42 Vs
In case 2 voltage differences between HV and LV winding is minimum therefore potential
stresses between them is minimum, implies insulation cost in between these windings is also
less. Hence for step down purpose Star–Delta transformer connection is most economical, design
for transformer. Similarly it can be proved that for Step up purpose Delta - Star three phase
transformer connection is most economical.
8.5
PHASE SEQUENCE
117
In three phase system, we have all the three phasors, 120o apart. There are three types of phase
sequences, which are
1. Positive Sequence
2. Negative Sequence
3. Zero Sequence
Fig (26)
8.6
PARALLEL OPERATION OF TRANSFORMERS
It is economical to install a numbers of smaller rated transformers in parallel than installing a
bigger rated electrical transformer.
Fig (27)
118
This has mainly the following advantages,
1. To maximize electrical power system efficiency: Generally electrical transformer gives
the maximum efficiency at full load. If we run numbers of transformers in parallel, we
can switch on only those transformers which will give the total demand by running nearer
to its full load rating for that time. When load increases we can switch on, one by one
other transformers connected in parallel to fulfill the total demand. In this way we can run
the system with maximum efficiency.
2. To maximize electrical power system availability: If numbers of transformers run in
parallel we can take shutdown on any one of them for maintenance purpose. Other
parallel transformers in system will serve the load without total interruption of power.
3. To maximize power system reliability: If any one of the transformers run in parallel, is
tripped due to fault, other parallel transformers is the system will share the load hence
power supply may not be interrupted if the shared loads do not make other transformers
to over load.
4. To maximize electrical power system flexibility: Always there is a chance of increasing
or decreasing future demand of power system. If it is predicted that power demand will
be increased in future, there must be a provision of connecting transformers in system in
parallel to fulfill the extra demand because it is not economical from business point of
view to install a bigger rated single transformer by forecasting the increased future
demand as it is unnecessary investment of money. Again if future demand is decreased,
transformers running in parallel can be removed from system to balance the capital
investment and its return.
CONDITIONS FOR PARALLEL OPERATION OF TRANSFORMERS
When two or more transformers are run in parallel they must satisfy the following conditions for
satisfactory performance. These are the conditions for parallel operation of transformers.
1.
2.
3.
4.
Same voltage ratio of transformer
Same percentage impedance
Same polarity
Same phase sequence
1.
Same Voltage Ratio
If two transformers of different voltage ratios are connected in parallel with same primary supply
voltage, there will be a difference in secondary voltages. Now say the secondary of these
transformers are connected to same bus, there will be a circulating current between secondary’s
and therefore between primary’s also. As the internal impedance of transformer is small, a small
voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R
loss.
119
2.
Same Percentage Impedance
The current shared by two transformers running in parallel should be proportional to their MVA
ratings. Again, electric current carried by these transformers are inversely proportional to their
internal impedance. From these two statements it can be said that impedance of transformers
running in parallel are inversely proportional to their MVA ratings. In other words percentage
impedance or per unit values of impedance should be identical for all the transformers running in
parallel.
3.
Same Polarity
Polarity of all transformers running in parallel should be same otherwise huge circulating current
flows in the transformer but no load will be fed from these transformers. Polarity of transformer
means the instantaneous direction of induced EMF in secondary. If the instantaneous directions
of induced secondary EMF in two transformers are opposite to each other when same input
power is fed to the both of the transformers, the transformers are said to be in opposite polarity.
If the instantaneous directions of induced secondary EMF in two transformers are same when
same input power is fed to both the transformers, the transformers are said to be in same polarity.
4.
Same Phase Sequence
The phase sequence or the order in which the phases reach their maximum positive voltage must
be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will
be short circuited.
The above said conditions must be strictly followed for parallel operation of transformers but
totally identical percentage impedance of two different transformers is difficult to achieve
practically that is why the transforms run in parallel may not have exactly same percentage
impedance but the values would be as nearer as possible.
8.5
TRANSFORMER TESTS
TYPES OF TRANSFORMER TESTING
Tests
Type
Routine
Special Tests
Done
At
Factory
Tests
Tests
Type Test Of Transformer: To prove that the transformer meets customer’s specifications and
design expectations, the transformer has to go through different testing procedures in
manufacturer premises. Some transformer tests are carried out for confirming the basic design
expectation of that transformer. These tests are done mainly on a randomly selected unit units in
a lot. Type test of transformer confirms main and basic design criteria of a production lot.
Routine Tests of Transformer: Routine tests of transformer are mainly for confirming
operational performance of individual unit in a production lot. Routine tests are carried out on
every unit manufactured.
120
Special Tests Of Transformer: Special tests of transformer are done as per customer
requirement to obtain information useful to the user during operation or maintenance of the
transformer.
TESTS DONE AT SITE
1.
2.
3.
1.
Pre
Periodic/
Emergency Tests
Commissioning
Condition
Monitoring
Tests
Tests
PRE-COMMISSIONING TEST OF TRANSFORMER
In addition to these, the transformer also goes through some other tests, performed on it, before
actual commissioning of the transformer at site. These tests are done to assess the condition of
transformer after installation and compare the test results of all the low voltage tests with the
factory test reports
8.7.1 POLARITY TEST
Polarity of a transformer is defined as the relative instantaneous direction of current in its
terminals. Terminals have same polarity if current entering in the primary terminal and that
leaving from the secondary terminal is such that they form a continuous circuit. Terminals
having same polarity are marked with same number i.e. 1 or 2 e.g. H1, X1 and H2, X2 or with
same color.
Polarity of Transformer may be changed by interchanging the connection of either HV or LV
side. Polarity of Transformer is needed when:
1. Transformers are connected in parallel for load sharing
2. When single phase transformers are connection in Delta-Star, Star-Star etc.
3. When Transformer is under ratio test.
Polarity of 3-phase Transformer is indicated by phase marking or vector diagram or vector
groups.
POLARITY MEASUREMENT OF SINGLE PHASE TRANSFORMERS
It is carried out to know about the physical location of H1, H2, X1 and X2 Terminals etc.
Polarity of Single Phase Transformers can be measured by one of the following three methods:
1. D.C Method
2. A.C Method
3. Ratio Meter Method
121
PROCEDUTRE FOR POLARITY CHECKING OF SINGLE PHASE TRANSFORMER BY
D.C. METHOD
On HV side of transformer, select H1 and H2 terminals according to your own choice and
connect them with 1.5 volts Dry Cell through a Switch. Then connect DC voltmeter to LV side
in such a way that its positive terminal is connected to that terminal which is adjacent opposite to
that HV Terminal to which positive of Cell is connected. After making connections as directed
above, close switch “S”. If Voltmeter needle deflects towards clockwise direction (or upscale or
right side) then polarity of transformer is “subtractive” (i.e. H1 and X1 terminals of transformer
are adjacent opposite to each other).If voltmeter needle deflects towards anticlockwise direction
(or down scale or left side), then polarity of transformer is “additive” (i.e. H1 and X1 terminals of
transformer are diagonally opposite to each other).
1. Always energize HV side of transformer
2. This method is limited to low capacity transformer only because in high capacity
transformers danger of induction kick is present due to collapsing of flux when switch is
put off.
3. For reducing the danger of inductive kick
4. Use low capacity and low voltage battery i.e. always use 1.5v dry cell. Never use station/
truck battery.
5. Don’t touch the leads during testing. After completion of test immediately disconnect
battery from the circuit
PROCEDURE FOR POLARITY CHECKING OF SINGLE PHASE TRANSFORMERS BY
A.C. METHOD
On HV side of transformer, select H1 and H2 terminals according to your own choice and
connect them with a 220 volts AC supply through a Switch. Jumper any two adjacent HV and
LV terminals, while connect the remaining HV and LV terminal through an AC voltmeter-V1.
Also connect another AC voltmeter-V2 across the H1 and H2. After making connections, close
switch “S”. If voltmeter-V1 reading is less than voltmeter-V2 reading, then polarity of
transformer is “subtractive” (i.e. H1 and X1 terminals of transformer are adjacent opposite to each
other). If voltmeter-V1 reading is more than voltmeter-V2 reading, then polarity of transformer is
“additive” (i.e. H1 and X1 terminals of transformer are diagonally opposite to each other).
1. Always energize HV side of transformer
2. Jumper is put to those HV and LV terminals which are adjacent opposite to each other
3. Voltmeters V1 and V2 must be calibrated before the start of test
PROCEDURE FOR POLARITY TEST OF SINGLE PHASE TRANSFORMER BY
RATIOMETER
122
Select H1 and H2 terminals of transformer on HV side according to your own choice. Connect H1
and H2 terminals of Ratio-meter to H1 and H2 terminals of transformer respectively. Then
connect X1 terminal of Ratio meter to that LV terminal of transformer which is adjacent opposite
to its H1 terminal and connect X2 terminal of Ratio-meter to the other LV terminal of
transformer. Give supply to Ratio meter and follow the Ratio-meter manual for polarity
checking, and mark the polarity accordingly.
8.7.2 INSULATION RESISTANCE TEST
The main purpose of this test is to detect major faults of major insulation. To assess that
transformer can be energized or not i.e. to know about the condition of transformer major
insulations. For this Ri reading of 1 Mega-ohm/1kV at 20oC oil temperature is considered
satisfactory. Insulation Resistance Test is performed on transformer when
1. Newly installed
2. Under fault
Insulation resistance test of transformer is essential type test. This test is carried out to ensure the
healthiness of overall insulation system of an electrical power transformer.
PROCEDURE OF INSULATION RESISTANCE TEST OF TRANSFORMER
1. First disconnect all the line and neutral terminals of the transformer.
2. Test Set leads to be connected to LV and HV bushing studs to measure Insulation
Resistance, Ri value in between the LV and HV windings.
3. Test Set leads to be connected to HV bushing studs and transformer tank earth point to
measure Insulation Resistance, Ri value in between the HV windings and earth.
4. Test Set leads to be connected to LV bushing studs and transformer tank earth point to
measure Insulation Resistance, Ri value in between the LV windings and earth.
It is unnecessary to perform insulation resistance test of transformer per phase wise in three
phase transformer. Ri values are taken between the windings collectively as because all the
windings on HV side are internally connected together to form either star or delta and also all the
windings on LV side are internally connected together to form either star or delta.
MEASUREMENTS ARE TO BE TAKEN AS FOLLOWS
For Auto Transformer:
HV-IV to LV,
HV-IV to E,
LV to E
For Two Winding Transformer: HV to LV,
HV to E,
LV to E
For Three Winding Transformer: HV to IV,
HV to E,
HV to LV,
IV to E,
IV to LV,
LV to E
123
Oil temperature should be noted at the time of insulation resistance test of transformer. Since the
Ri value of transformer insulating oil may vary with temperature. Ri values to be recorded at
intervals of 15 seconds, 1 minute and 10 minutes.
With the duration of application of voltage, Ri value increases. The increase in Ri is an indication
of dryness of insulation.
Absorption Coefficient
Polarization Index
=
=
1
minute
value/
15
10 minutes value / 1 minute value
sec
value
8.7.3 TRANSFORMER TURN RATIO TEST
The performance of a transformer largely depends upon perfection of specific turns or voltage
ratio of transformer. So Transformer Ratio Test is an essential type test of transformer. This test
also performed as routine test of transformer. So for ensuring proper performance of electrical
power transformer, voltage and turn ratio test of transformer are one of the vital tests. Actually
the no load voltage ratio of transformer is equal to the turn ratio. So ratio test of transformer is
same as no load voltage ratio.
PROCEDURE OF TRANSFORMER RATIO TEST
The procedure of Transformer Ratio Test is simple. We just apply three phase 415 V supply to
HV winding, with keeping LV winding open. Then we measure the induced voltages at HV and
LV terminals of transformer to find out actual voltage ratio of transformer.
We repeat the test for all tap position separately.
1. First, the tap changer of transformer is kept in the lowest position and LV terminals are
kept open.
2. Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each
phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously.
3. After measuring the voltages at HV and LV terminals, the tap changer of transformer
should be raised by one position and repeat test.
4. Repeat the same for each of the tap position separately.
The above transformer ratio test can also be performed by portable Transformer Turns Ratio
(TTR) Meter. They have an in built power supply, with the voltages commonly used being very
low, such as 8-10 V and 50 Hz. The HV and LV windings of one phase of a transformer are
connected to the instrument, and the internal bridge elements are varied to produce a null
indication on the detector.
124
Let's have a discussion on Transformer Turns Ratio (TTR) Meter method of turn ratio test of
transformer.
A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of
induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %
Fig (28)
Theoretical Turn Ratio = HV Winding Voltage/ LV Winding Voltage
This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the
adjustable transformer as shown in the figure above and it should be changed until a balance
occurs in the percentage error indicator. The reading on this indicator implies the deviation of
measured turn ratio from expected turn ratio in percentage.
Deviation in % = (Measured Turn Ratio - Expected Turn Ratio)/ Expected Turn Ratio x 100
Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an
associated high excitation current.
Open turns in HV winding will indicate very low exciting current and no output voltage since
open turns in HV winding causes no excitation current in the winding means no flux hence no
induced voltage. But open turn in LV winding causes, low fluctuating LV voltage but normal
excitation current in HV winding. Hence open turns in LV winding will be indicated by normal
levels of exciting current, but very low levels of unstable output voltage.
The turn ratio test of transformer also detects high resistance connections in the lead circuitry or
high contact resistance in tap changers by higher excitation current and a difficulty in balancing
the bridge.
125
8.7.4 OPEN CIRCUIT TEST
The connection diagram for open circuit test on transformer is shown in the figure. A voltmeter,
wattmeter, and an ammeter are connected in LV side of the transformer as shown.
Fig (29)
The voltage at rated frequency is applied to that LV side with the help of a variac of variable
ratio auto transformer. The HV side of the transformer is kept open. Now with help of variac
applied voltage is slowly increase until the voltmeter gives reading equal to the rated voltage of
the LV side. After reaching at rated LV side voltage, all three instruments reading (Voltmeter,
Ammeter and Wattmeter) are recorded. The ammeter reading gives the no load current Ie. As no
load current Ie is quite small compared to rated current of the transformer, the voltage drops due
to this current then can be taken as negligible. Since, voltmeter reading V1 can be considered
equal to secondary induced voltage of the transformer. The input power during test is indicated
by wattmeter reading.
As the transformer is open circuited, there is no output hence the input power here consists of
core losses in transformer and copper loss in transformer during no load condition. But as said
earlier, the no load current in the transformer is quite small compared to full load current so
copper loss due to the small no load current can be neglected. Hence the wattmeter reading can
be taken as equal to core losses in transformer. Let us consider wattmeter reading is Po.
Po = V1 2/Rm, Where Rm is shunt branch resistance of transformer
If, Zm is shunt branch impedance of transformer, then,
Zm = V1/ Ie,
therefore, if shunt branch reactance of transformer is Xm, then,
126
(1/ Xm)2 = (1/ Zm)2 - (1/ Rm)2
These values are referred to the LV side of transformer as because the test is conducted on LV
side of transformer. These values could easily be referred to HV side by multiplying these values
with square of transformation ratio. Therefore it is seen that the open circuit test on transformer
is used to determine core losses in transformer and parameters of shunt branch of the equivalent
circuit of transformer.
8.7.5 SHORT CIRCUIT TEST
The connection diagram for short circuit test on transformer is shown in the figure. A voltmeter,
wattmeter, and an ammeter are connected in HV side of the transformer as shown. The voltage at
rated frequency is applied to that HV side with the help of a variac of variable ratio auto
transformer.
Fig (30)
The LV side of the transformer is short circuited. Now with help of variac applied voltage is
slowly increase until the ammeter gives reading equal to the rated current of the HV side. After
reaching at rated current of HV side, all three instruments reading (Voltmeter, Ammeter and
Wattmeter) are recorded. The ammeter reading gives the primary equivalent of full load current
IL. As the voltage, applied for full load current in short circuit test on transformer, is quite small
compared to rated primary voltage of the transformer, the core loss in transformer can be taken
as negligible here. Let’s, voltmeter reading is Vsc. The input power during test is indicated by
wattmeter reading. As the transformer is short circuited, there is no output hence the input power
here consists of copper losses in transformer. Since, the applied voltage Vsc is short circuit
voltage in the transformer and hence it is quite small compared to rated voltage so core loss due
to the small applied voltage can be neglected. Hence the wattmeter reading can be taken as equal
to copper losses in transformer. Let us consider wattmeter reading is Psc.
127
Psc = Re.IL2, Where Re is equivalent resistance of transformer
If, Ze is equivalent impedance of transformer, then
Ze = Vsc/ IL, therefore, if equivalent reactance of transformer is Xe, then
Xe2 = Ze2 - Re2
These values are referred to the HV side of transformer as because the test is conducted on HV
side of transformer. These values could easily be referred to LV side by dividing these values
with square of transformation ratio.
Therefore it is seen that the Short Circuit test on transformer is used to determine losses in
transformer at full load and parameters of approximate equivalent circuit of transformer.
8.7.6 VERIFICATION OF VECTOR GROUP
In three phase transformers, it is essential to carry out a Vector Group Test of transformer.
Proper vector grouping in a transformer is an essential criterion for parallel operation of
transformers.
There are several internal connections of transformers available in market. These connections
give various magnitudes and phase of the secondary voltage; the magnitude can be adjusted for
parallel operation by suitable choice of turn ratio, but the phase divergence cannot be
compensated. So we have to choose those transformers for parallel operation whose phase
sequence and phase divergence are same. All the transformers with same vector group have same
phase sequence and phase divergence between primary and secondary. So before procuring one
electrical power transformer, one should ensure the vector group of the transformer, whether it
will be matched with the existing system or not. The Vector Group Test of transformer confirms
the requirements.
The vector group of transformer is an essential property for successful parallel operation of
transformer. Hence every transformer must undergo Vector Group Test at factory site for
ensuring the customer specified vector group of transformer.
The phase sequence or the order, in which the phases reach their maximum positive voltages,
must be identical for two paralleled transformers. Otherwise, during the cycle, each pair of
phases will be short circuited.
128
Several secondary connections are available in respect of various primary three phase
connections in a three phase transformer. So for same primary applied three phase voltage there
may be different three phase secondary voltages with various magnitudes and phases for
different internal connection of the transformer. Let's have a discussion in detail by example for
better understanding.
We know that, the primary and secondary coils on any one limb have induced EMFs that are in
time-phase. Let's consider two transformers of same number primary turns and the primary
windings are connected in star. The secondary numbers of turns per phase in both transformers
are also same. But the first transformer has star connected secondary and other transformer has
delta connected secondary. If same voltages are applied in primary of both transformers, the
secondary induced EMF in each phase will be in same time-phase with that of respective primary
phase, as because the primary and secondary coils of same phase are wound on the same limb in
the transformer core. In first transformer, as the secondary is star connected, the secondary line
voltage is √3 times of induced voltage per secondary phase coil.
But in case of second transformer, where secondary is delta connected, the line voltage is equal
to induced voltage per secondary phase coil. If we go through the vector diagram of secondary
line voltages of both transformers, we will easily find that there will be a clear 30 o angular
difference between the line voltages of these transformers. Now, if we try to run these
transformers in parallel then there will be circulating current flows between the transformers as
because there is a phase angle difference between their secondary line voltages. This phase
difference cannot be compensated. Thus two sets of connections giving secondary voltages with
a phase displacement cannot be intended for parallel operation of transformer.
The following table gives the connections for which from the view point of phase sequence and
angular divergences, transformer can be operated parallel. According to their vector relation, all
three phase transformers are divided into different vector group of transformer. All transformers
of a particular vector group can easily be operated in parallel if they fulfill other condition for
parallel operations of transformers.
GROUP
Connection
Yy0
Connection
Dd0
0
(0o)
129
Yd1
Dy1
1
( 30o)
GROUP
Connection
11
( - 30o)
Yd11
Connection
Dy11
130
Fig (31)
PROCEDURE OF VECTOR GROUP TEST OF TRANSFORMER
Let’s have a YNd11 transformer.
1. Connect neutral point of star connected winding
with
earth.
2. Join 1U of HV and 2W of LV together.
3. Apply 415V, three phase supply to HV terminals.
4. Measure voltages between terminals
2U–1N, 2V–1N, 2W–1N, which means
voltages
between each LV terminal and HV neutral
5. Also measure voltages between terminals 2V–1V,
2W–1W and 2V–1W.
Fig (32)
For YNd11 transformer, we will find,
2U–1N
>
2V–1W > 2V–1V or 2W–1W
2V–1N
>
2W–1N
The Vector Group Test of transformer for other group can also be done in similar way.
131
8.8
TRANSFORMER COOLING SYSTEM
The main source of heat generation in transformer is its copper loss or I2R loss. Although there
are other factors contributing heat in transformer such as hysteresis & eddy current losses but
contribution of I2R loss dominates them. If this heat is not dissipated properly, the temperature of
the transformer will rise continually which may cause damages to paper insulation and liquid
insulation of transformer. So it is essential to control the temperature within permissible limit to
ensure the long life of transformer by reducing thermal degradation of its insulation system. In
electrical transformer we use external transformer cooling system to accelerate the dissipation
rate of heat of transformer.
DIFFERENT TRANSFORMER COOLING METHODS
For accelerating cooling different transformer cooling methods are used depending upon their
size and ratings. We will discuss these one by one below,
ONAN COOLING OF TRANSFORMER
This is the simplest transformer cooling system. The full form of ONAN is "Oil Natural Air
Natural". Here natural convectional flow of hot oil is utilized for cooling. In convectional
circulation of oil, the hot oil flows to the upper portion of the transformer tank and the vacant
place is occupied by cold oil. This hot oil which comes to upper side will dissipate heat in the
atmosphere by natural conduction, convection & radiation in air and will become cold. In this
way the oil in the transformer tank continually circulate when the transformer is loaded.
Fig (33)
132
As the rate of dissipation of heat in air depends upon dissipating surface of the oil tank, it is
essential to increase the effective surface area of the tank, so additional dissipating surface in the
form of tubes or radiators are connected to the transformer tank. This is known as radiator of
transformer or radiator bank of transformer. We have shown below a simplest form of Natural
Cooling or ONAN Cooling arrangement of an earthing transformer.
Fig (34)
ONAF COOLING OF TRANSFORMER
Heat dissipation can obviously be increased, if dissipating surface is increased but it can be made
further faster by applying forced air flow on that dissipating surface. Fans blowing air on cooling
surface is employed. Forced air takes away the heat from the surface of radiators and provides
better cooling than natural air. The full form of ONAF is "Oil Natural Air Forced". As the heat
dissipation rate is faster and more in ONAF transformer cooling method than ONAN cooling
system, electrical transformer can be loaded more without crossing the permissible temperature
limits.
133
Fig (35)
OFAF COOLING OF TRANSFORMER
In Oil Forced Air Natural cooling system of transformer, the heat dissipation is accelerated by
using forced air on the dissipating surface but circulation of the hot oil in transformer tank is
natural convectional flow.
Fig (36)
The heat dissipation rate can be still increased further if this oil circulation is accelerated by
applying some force. In OFAF cooling system the oil is forced to circulate within the closed loop
of transformer tank by means of oil pumps. OFAF means "Oil Forced Air Forced" cooling
methods of transformer.
The main advantage of this system is that it is compact system and for same cooling capacity
OFAF occupies much less space than farmer two systems of transformer cooling. Actually in Oil
Natural cooling system, the heat comes out from conducting part of the transformer is displaced
from its position, in slower rate due to convectional flow of oil but in forced oil cooling system
the heat is displaced from its origin as soon as it comes out in the oil, hence rate of cooling
becomes faster.
134
OFWF COOLING OF TRANSFORMER
We know that ambient temperature of water is much less than the atmospheric air in same
weather condition. So water may be used as better heat exchanger media than air. In OFWF
cooling system of transformer, the hot oil is sent to oil to water heat exchanger by means of oil
pump and there the oil is cooled by applying showers of cold water on the heat exchanger's oil
pipes. OFWF means "Oil Forced Water Forced" cooling in transformer.
ODAF COOLING OF TRANSFORMER
ODAF or Oil Directed Air Forced Cooling of Transformer can be considered as the improved
version of OFAF. Here forced circulation of oil directed to flow through predetermined paths in
transformer winding. The cool oil entering the transformer tank from cooler or radiator is passed
through the winding where gaps for oil flow or pre-decided oil flowing paths between insulated
conductors are provided for ensuring faster rate of heat transfer. ODAF or Oil Directed Air
Forced Cooling of Transformer is generally used in very high rating transformer.
ODWF COOLING OF TRANSFORMER
ODAF or Oil Directed Water Forced Cooling of Transformer is just like ODAF only difference
is that here the hot oil is cooled in cooler by means of forced water instead of air. Both of these
transformer cooling methods are called Forced Directed Oil Cooling of transformer
135
9.
Current Transformers
136
9-1
FUNDAMENTAL THEORY
Relay and meters cannot be connected directly to the power system (i.e. Primary of the Power
System, the value of voltage may be 132 KV, 220 KV and 500 KV on the primary side while
current may be also in hundreds of value. To prepare such relays having high values of V or I is
not possible. Therefore such type of devices have been invented which can give a replica of
actual primary side voltage or current on secondary side within safe Limits Such devices are
known as Instrument Transformers, because those work in conjunction with meters and relays.
Fig (1)
Instrument transformers means current transformers & Voltage transformers are used in
electrical power system for stepping down currents and voltages of the system for metering and
protection purpose. Actually relays and meters used for protection and metering, are not
designed for high currents and voltages.
137
High currents or voltages of the system cannot be directly fed to relays and meters. Current
Transformer steps down rated system current to 1 A or 5 A; similarly Voltage Transformers
steps down system voltages to 110V. The relays and meters are generally designed for 1 A, 5 A
and 110V.
A current transformer (CT) is an instrument transformer in which the secondary current is
substantially proportional to primary current and differs in phase from it by ideally zero degree.
A current transformer functions with the same basic working principle of electric power
transformer as we discussed earlier, but here is some difference. If an electrical power
transformer or other general purpose transformer, primary current varies with load or secondary
current. In case of current transformer, primary current is the system current and this primary
current or system current transforms to the CT secondary, hence secondary current or burden
current depends upon primary current of the current transformer.
In a power transformer, if load is disconnected, there will be only magnetizing current flows in
the primary. The primary of the power transformer takes current from the source proportional to
the load connected with secondary. But in case of Current transformer, the primary is connected
in series with power line. So current through its primary is nothing but the current flows through
that power line. The primary current of the CT, hence does not depend upon whether the load or
burden is connected to the secondary or not or what is the impedance value of burden. Generally
current transformer has very few turns in primary where as secondary turns are large in number.
Say Np is number of turns in CT primary and Ip is the current through primary. Hence the
primary AT is equal to NpIp AT.
If number of turns in secondary and secondary current in that CT are Ns and Is respectively then
Secondary AT is equal to NsIs AT.
In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT.
So from the above statement it is clear that if a CT has one turn in primary and 400 turns in
secondary winding, if it has 400 A current in primary then it will have 1A in secondary burden.
Thus the turn ratio of the CT is 400/1A.
There are different cores in a CT e.g. if 132 KV CT has three core
1st full core is 1S1…………………... 1S3 (15 VA Metering Core)
2nd full core is 2S1………………….. 2S3 (30 VA for-51)
3rd full core is 3S1 ………………….. 3S3 (60 VA for-87)
1st core is used for metering (for energy meters) and has 15 AV burden.
138
2nd core has 30 VA burden and is used to give CT to over-current relay.
3rd core has more VA equal to 60 VA and used for differential protection and impedance relay.
The characteristics of protection core and metering core are different; have different designs and
cannot be interchanged with each other when more fault current occurs then mechanical system
of meters pivot, jewel etc. damages and hence metering core is made smaller and should saturate
earlier therefore. But protection core should give exact replica of system current at fault
conditions protection core would saturate later.
9.2
TYPES OF CURRENT TRANSFORMERS
BAR PRIMARY TYPE
Its primary is in the form of solid straight rod, and its core on which winding is done is of
rectangular shape. Such type of CTs are also called window type CTs, it has only one winding in
its primary.
Fig (2)
RING TYPE
It is similar to window type CTs with the difference that its core is round shaped. Its primary is
in the form of a rod like bar primary type. Bushing CT of a power transformer is ring type.
Bushing CTs in a bush of a power transformer are also called free standing CTs.
139
Fig (3)
WOUND TYPE
Fig (4)
It is that CT that has both primary and secondary windings. The primary winding should have at
least one or more turns but the secondary winding of CT has many turns.
MULTI RATIO OR MULTI TAP
Such CTs having different CT ratios
140
Fig (5)
MULTI WOUND OR MULTICORE
CTs having different windings in, secondary are called multi core CTs. Some winding are used
for metering and some for protection.
Fig (6)
In this figure a CT having 3 cores in shown. This CT’S has one winding for each core. Hence in
this case No of winding = No for core. This CT is multi wound and multi core.
A CT which is multi ratio as well as multi core can have its CT ratio as e.g. 100-200-400/5/5/5
TO CONVERT SIMPLE CT INTO MULTI RATIO CT
From Secondary Winding
Tap off the secondary winding.
141
Fig (7)
If secondary windings are two in number then if secondary winding is tapped off from center
then 50/5A CT ratio is obtained.
Fig (8)
142
TO FORM MULTI RATIO CT
From Primary Side
In some high voltage CTs ratio can be changed from primary also. CT in this figure has two
conductors on primary side. If P1 connected with C1 and P2 with C2 i.e. parallel connection then
CT ratio becomes 600/5. When parallel connection is done then N1 = 1.
Fig (9)
For series connection, connect C1 C2, so that two turns are available on primary sides and ratio
becomes 300/5A.
According to the relation
N2/N1 = I1/ I2, N1 and I1 are inversely proportional, hence if N1 increases, I1 decreases and vice
versa.
9.3
ERROR IN CURRENT TRANSFORMERS
In an actual current transformer, errors with which we are connected can best be considered
through a study of phasor diagram for a CT,
Fig (10)
143
Is
Secondary
Current
Es
Secondary
induced
EMF
Ip
primary
Current
Ep
primary
induced
EMF
KT - turns ratio = numbers of secondary turns/number of primary turns
Io
Excitation
Current
Im
magnetizing
component
of
Io
Iw
core
loss
component
of
Io
Φm - main flux.
Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of the
phasors Es and Ep are proportional to secondary and primary turns. The excitation current Io
which is made up of two components Im and Iw. The secondary current Io lags behind the
secondary induced EMF Es by an angle Φ s. The secondary current is now transferred to the
primary side by reversing Is and multiplied by the turns ratio KT. The total current flows through
the primary Ip is then vector sum of KT Is and Io.
RATIO ERROR IN CURRENT TRANSFORMER
From the above phasor diagram it is clear that primary current Ip is not exactly equal to the
secondary current multiplied by turn ratio, i.e. KTIs. This difference is due to the primary current
is contributed by the core excitation current. The error in current transformer introduced due to
this difference is called Current Error of CT or sometimes Ratio Error in Current Transformer.
Hence, the Percentage Current Error = (Ip – KT. Is)/ IP
PHASE ANGLE ERROR IN CURRENT TRANSFORMER
For an ideal current transformer the angle between the primary and reversed secondary current
vector is zero. But for an actual current transformer there is always a difference in phase between
two due to the fact that primary current has to supply the component of the exiting current. The
angle between the above two phases in termed as Phase Angle Error in CT. Here in the pharos
diagram it is β, the phase angle error is usually expressed in minutes.
CAUSE OF ERROR IN CURRENT TRANSFORMER
The total primary current is not actually transformed in CT. One part of the primary current is
consumed for core excitation and remaining is actually transformed with turn ratio of CT so
there is error in current transformer means there are both Ratio Error in Current Transformer as
well as a Phase Angle Error in Current Transformer.
144
HOW TO REDUCE ERROR IN CURRENT TRANSFORMER
It is desirable to reduce these errors, for better performance. For achieving minimum error in
current transformer, one can adopt the following,
1. Using a core of high permeability and low hysteresis loss magnetic materials
2. Keeping the rated burden to the nearest value of the actual burden
3. Ensuring minimum length of flux path and increasing cross–sectional area of the core,
minimizing joint of the core
4. Lowering the secondary internal impedance
9.4
CURRENT TRANSFORMERS CONNECTIONS
Current Transformers are either Delta Δ connection or Star or Y connection
DELTA CONNECTION
Fig (11)
If compared 1 and 2 current become 180o out of phase
STAR CONNECTION
145
Fig (12)
If transformation is ∆ to Y (DY1 or DY11) or transformation is Y to ∆ (YD1 or YD11), then 30o
phase shift takes place.
146
9.5
CURRENT TRANSFORMER PARAMETERS
RATED PRIMARY CURRENT OF CURRENT TRANSFORMER
This is the value of rated primary current of Current Transformer on which it is designed to
perform best. Hence rated primary current of Current Transformer is an optimum value of
primary current at which, error of the Current Transformer is minimum and losses are also less
that means in few words, performance of the Current Transformer is best; with optimum heating
of the transformer
RATED SECONDARY CURRENT OF CURRENT TRANSFORMER
Like rated primary current, this is the value of secondary current due to which errors in the
Current Transformer is minimum. In other words, Rated Secondary Current of Current
Transformer is the value of secondary current on which the best performance of the Current
Transformer is based
RATED BURDEN OF CURRENT TRANSFORMER
It is the impedance connected to a Current Transformer. It is the total VA values connected to a
Current Transformer. This includes VA of Current Transformer’s winding (internal VA), VA of
the cable from Current Transformer to relays, meters, plus VA of the instruments connected to
Current Transformer.
Whatever is connected externally with the secondary of a Current Transformer is called burden
of Current Transformer. Rated burden of Current Transformer is the value of the burden to be
connected with the secondary of Current Transformer including connecting load resistance
expressed in VA or ohms on which accuracy requirement is based.
Fig (13)
This total VA value should not increase than the capacity of CT.
Usually capacity of metering core
= 15 VA
147
Capacity of core for O/C relay
= 30 VA
Capacity of core for different relay
= 60 VA
RATED FREQUENCY OF CURRENT TRANSFORMER
The value of the system frequency on which the Current Transformer operates
RATED SHORT CIRCUIT CURRENT OF A CURRENT TRANSFORMER
In some abnormal condition like huge short circuit fault, the Current Transformer faces a huge
current, flow through the Current Transformer primary. Although this fault current will not
continue for long time as because every fault in the system is cleared by electrical protection
system within very short time. So Current Transformer should be designed in such a way that it
can withstand this huge fault current at least for certain amount of time. It is unnecessary to
design any equipment for withstanding short circuit current for long period of time since the
short circuit fault is cleared by protection switch gear within fraction of second.
For Current Transformer Rated Short Circuit Current is defined as the RMS value of primary
current which the Current Transformer will withstand for a rated time with its secondary winding
short circuited without suffering harmful effects.
RATED VOLTAGE FOR CURRENT TRANSFORMER
The RMS value of the voltage used to designate the Current Transformer for a particular highest
system voltage is Rated Voltage for Current Transformer. The voltage of electrical power system
is increased if load of the system is reduced. As per standard, the system voltage can be raised up
to 10% above the normal voltage during no load condition. So every electrical equipment is such
designed so that it can withstand this highest voltage. As Current Transformer is an electrical
equipment, it should also be designed to withstand highest system voltage.
INSTRUMENT SECURITY FACTOR
ISF or Instrument Security Factor is the ratio of Instrument Limit Primary Current to the rated
Primary Current. Instrument Limit Current of a metering Current Transformer is the maximum
value of primary current beyond which Current Transformer core becomes saturated. Instrument
Security Factor of Current Transformer is the significant factor for choosing the metering
Instruments which to be connected to the secondary of the Current Transformer. Security or
Safety of the measuring unit is better, if ISF is low. If we go through the example below it would
be clear to us.
Suppose one Current Transformer has rating 100/1A and ISF is 1.5 and another Current
Transformer has same rating with ISF 2. That means, in first Current Transformer, the metering
core would be saturated at 1.5x100 or 150 A, whereas is second Current Transformer, core will
be saturated at 2x100 or 200A. That means whatever may be the primary current of both Current
Transformers; secondary current will not increase further after 150 & 200A of primary current of
148
the Current Transformers respectively. Hence maximum secondary current of the Current
Transformers would be 1.5 & 2.0 A.
As the maximum current can flow through the instrument connected to the first Current
Transformers is 1.5A which is less than the maximum value of current can flow through the
instrument connected to the second Current Transformers i.e. 2A. Hence security or safety of the
instruments of first Current Transformers is better than later.
Another significance of ISF is during huge electrical fault, the short circuit current, flows
through primary of the Current Transformers does not affect destructively, the measuring
instrument attached to it as because, the secondary current of the Current Transformers will not
rise above the value of rated secondary current multiplied by ISF
ACCURACY LIMIT FACTOR
For protection core of a Current Transformer, the ratio of Accuracy Limit Primary Current to the
Rated Primary Current is called Accuracy Limit Factor of Current Transformer. Broadly,
Accuracy Limit Primary Current is the maximum value of primary current, beyond which core of
the Protection Core of Current Transformer starts saturating. The value of Rated Accuracy Limit
Primary Current is always many times more than the value of Limit Primary Current. Actually
Current Transformer transforms the fault current of the electrical power system for operation of
the protection relays connected to the secondary of that Current Transformer.
If the core of the Current Transformer becomes saturated at lower value of primary current, as in
the case of Metering core of Current Transformer, the system fault will not reflect properly to the
secondary, which may cause, the relays remain inoperative even the fault level of the system is
large enough. That is why the core of the protection Current Transformer is made in such a way
that saturation level of that core must be high enough. But still there is a limit, because it is
impossible to make one magnetic core with infinitely high saturation level and secondly most
important reason is that although the protection core should have high saturation level but that
must be limited up to certain level otherwise total transformation of primary current during huge
fault may badly damage the protective relays. So it is clear from above explanation, Rated
Accuracy Limit Primary Current, should not be so less, that it will not at all help the relays to be
operated on the other hand this value must not be so high that it can damage the relays.
So, Accuracy Limit Factor or ALF should not have the value nearer to unit and at the same time
it should not be as high as 100. The standard values of ALF as per IS-2705 are 5, 10, 15, 20 &
30.
KNEE POINT VOLTAGE OF CURRENT TRANSFORMER
Knee Point Voltage of Current Transformer is significance of saturation level of a Current
Transformer core mainly used for protection purposes. The sinusoidal voltage of rated frequency
is applied to the secondary terminals of Current Transformer, with other winding being open
149
circuited which when increased by10%, cause the exiting current to increase by 50%. The CT
core is made of CRGO steel. It has its own saturation level.
The EMF induced in the CT secondary windings is
E2 = 4.44φfT2
Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T 2 is the number of
turns of the secondary winding. The flux in the core, is produced by excitation current Ie. We
have a non-liner relationship between excitation current and magnetizing flux. After certain
value of excitation current, flux will not further increase so rapidly with increase in excitation
current. This non-liner relation curve is also called B-H curve. Again from the equation above, it
is found that, secondary voltage of a Current Transformer is directly proportional to flux φ.
Hence one typical curve can be drawn from this relation between secondary voltage and
excitation current as shown below,
It is clear from the curve that, linear relation between V & Ie is maintained from point A & K.
The point A is known as Ankle Point and point K is known as Knee Point.
Fig (14)
In Differential and Restricted Earth Fault (REF) protection scheme, accuracy class and ALF of
the Current Transformer may not ensure the reliability of the operation. It is desired that,
Differential and REF relays should not be operated when fault occurs outside the protected
transformer. When any fault occurs outside the Differential protection zone, the faulty current
flows through Current Transformers of both sides of Electrical Power Transformer. Both LV &
HV Current Transformers have magnetizing characteristics. Beyond the Knee Point, for slight
increase in secondary EMF a large increase in excitation current is required. So after this knee
point excitation current of both Current Transformers will be extremely high, this may cause
mismatch between secondary current of LV & HV Current Transformers. This phenomenon may
cause unexpected tripping of Power Transformer.
So the magnetizing characteristics of both LV & HV sides Current Transformers, should be same
that means they have same knee point voltage Vk as well as same excitation current Ie at Vk/2. It
can be again said that, if both knee point voltage of current transformer and magnetizing
characteristic of Current Transformers of both sides of Power Transformer differ, there must be a
mismatch in high excitation currents of the Current Transformers during fault which ultimately
150
causes the unbalancing between secondary current of both groups of Current Transformers and
transformer trips.
So for choosing Current Transformers for Differential Protection of Transformer, one should
consider Current Transformer PS Class rather its convectional protection class. PS stands for
Protection Special which is defined by Knee Point voltage of current transformer Vk and
excitation current Ie at Vk/2.
WHY CT SECONDARY SHOULD NOT BE KEPT OPEN
The electrical power system load current always flows through current transformer primary;
irrespective of whether the Current Transformer is open circuited or connected to burden at its
secondary.
Fig (15)
If Current Transformer secondary is open circuited, all the primary current will behave as
excitation current, which ultimately produce huge voltage. Every Current Transformer has its
own Non-Linear magnetizing curve, because of which secondary open circuit voltage should be
limited by saturation of the core. If the RMS voltage across the secondary terminals is measured,
the value may not appear to be dangerous. As the Current Transformer primary current is
sinusoidal in nature, it zero 100 times per second, as frequency of the current is 50Hz. The rate
of change of flux at every current zero is not limited by saturation and is high indeed. This
develops extremely high peaks or pulses of voltage. These high peaks of voltage may not be
measured by conventional voltmeter. But these high peaks of induced voltage may breakdown
the Current Transformer insulation, and may case accident to personnel. The actual open circuit
voltage peak is difficult to measure accurately because of its very short peaks. That is why
Current Transformer secondary should not be kept open.
9.6
CURRENT TRANSFORMER TESTS
151
9.6.1 CONTINUITY TEST
Check the continuity of the Secondary side of the CT with an ohmmeter.
9.6.2 INSULATION RESISTANCE TEST
The main purpose of this test is to detect major faults of major insulation. To assess that Current
Transformer can be energized or not i.e. to know about the condition of Current Transformer
major insulations. For this Ri reading of 1 Mega-ohm/1kV at 20oC oil temperature is considered
satisfactory. Insulation Resistance Test is performed on transformer when,
1. Newly installed
2. Under fault
Check the insulation of Current Transformer with an appropriate voltage of the Insulation
Resistance Tester: Measurements are to be taken as follows:
1. Primary to Earth
2. Secondary to Earth
3. Primary to Secondary
9.6.3 RATIO TEST
It is the ratio between primary windings turns and secondary winding turns. In other words it is
the ratio between primary winding current and secondary winding current. As current ratio and
turns ratio are inversely proportional. So write it as
I1/I2 = N2/ N1
For example, a Current Transformer of ratio 100/5 means that
I2 = 100A;
I2 = 5A;
and
N1 = 1 turn
N2 = 20 turns
Check the ratio of the Current Transformer with the help of a Current Injection Test Set.
9.6.4 POLARITY OF CURRENT TRANSFORMERS
152
Polarity of a transformer is defined as the relative instantaneous direction of current in its
terminals. Terminals have same polarity if current entering in the primary terminal and that
leaving from the secondary terminal is such that they form a continuous circuit. Terminals
having same polarity are marked with same number i.e. 1 or 2 e.g. P1, S1 and P2, S2
Fig (16)
Assume the polarity marks on the Current Transformer are correct. Primary as P1-P2 and
Secondary as S1-S2. Connect a DC Voltmeter to the Secondary S1-S2, such that Positive and
Negative terminals of the Voltmeter are towards S1 and S2 of the Current Transformer
respectively. Connect negative of the 1.5 Volts DC cell to the P2 and touch P1 terminal of the
Current Transformer with the Positive terminal of the cell. The DC Voltmeter will show one of
the following deflections:
Forward or upscale, if the polarity is correct
Reverse or downscale, if the polarity is reversed.
The correct polarity is opposite of the marked polarity.
9.6.5 CURRENT TRANSFORMER SATURATION
Current Transformer saturation is a point where the excitation impedance collapses and whole of
the primary current is utilized in exciting the core of Current Transformer i.e. IP becomes, the Iexe
and secondary output of the Current Transformer ceases (reduces/finishes). The cause of Current
Transformer saturation is fault current which flows on fault or if Current Transformer is opened
accidentally at secondary side. DC transients are present in the fault current which superimpose
on AC quantity having less time but high magnitude. These transients saturate Current
Transformer core.
ANKLE POINT
Ankle point is a point where VE and IE are linear, i.e. VE and IE are direct proportional. Operating
point of CT should be in linear portion.
KNEE POINT
It is a point where increase of 10% in Vexc causes an increase of 50% in Iexc and this is the point
of the saturation of CT. Measuring CT operates between zero and ankle point and saturation
153
level is low. Protection CT operates satisfactory up to knee point and saturation level is high.
Protection CT cannot be used instead of measuring CT and vice versa.
Fig (17)
9.6.6 CIRCUIT VERIFICATION TEST
To be done in Laboratory.
10.
154
Potential
Transformers
10.1
FUNDAMENTAL THEORY
Potential Transformers or Voltage Transformers are used in electrical power system for stepping
down the system voltage to a safe value which can be fed to low ratings meters and relays.
Commercially available relays and meters used for protection and metering, are designed for low
voltages.
155
Fig (1)
A Voltage Transformer theory or Potential Transformer theory is just like theory of general
purpose step down transformer. Primary of this transformer is connected across the phases or and
ground depending upon the requirement. Just like the transformer, used for stepping down
purpose, Potential Transformer has fewer turns winding at its secondary. The system voltage is
applied across the terminals of primary winding of that transformer, and then proportionate
secondary voltage appears across the secondary terminals of the Potential Transformer. The
secondary voltage of the Potential Transformer is generally 110V. In an ideal Potential
Transformer or Voltage Transformer when rated burden is connected across the secondary the
ratio of primary and secondary voltages of transformer is equal to the turn ratio and furthermore
the two terminal voltages are in precise phase opposite to each other. But in actual transformer
there must be an error in the voltage ratio as well as in the phase angle between primary and
secondary voltages.
The errors in Potential Transformer or Voltage Transformer can best be explained by phasor
diagram.
10.2
TYPES OF POTENTIAL TRANSFORMERS
There are two types of Potential Transformer:
1. Conventional Voltage Transformer
2. Coupling Voltage Transformer
10.3
ERROR IN POTENTIAL TRANSFORMER
156
Fig(2)
Is
Secondary
Current
Es
Secondary
induced
emf
Vs
Secondary
terminal
voltage
Rs
Secondary
winding
resistance
Xs
Secondary
winding
reactance
Ip
Primary
current
Ep
primary
induced
emf
Vp
Primary
terminal
voltage
Rp
Primary
winding
resistance
Xp
Primary
winding
reactance
KT - turns ratio = numbers of primary turns/number of secondary turns
Io
Excitation
Current
Im
magnetizing
component
of
Io
Iw
core
loss
component
of
Io
Φm
main
flux
β - phase angle error
As in the case of Current Transformer and other purpose electrical power transformer, total
primary current Ip is the vector sum of excitation current and the current equal to reversal of
secondary current multiplied by the ratio 1/KT
Hence,
Ip = Io + Is/KT
If Vp is the system voltage applied to the primary of the PT then voltage drops due to resistance
and reactance of primary winding due to primary current Ip will comes into picture. After
157
subtracting this voltage drop from Vp, Ep will appear across the primary terminals. This Ep is
equal to primary induced emf. This primary emf will transform to the secondary winding by
mutual induction and transformed emf is Es. Again this Es will be dropped by secondary winding
resistance and reactance, and resultant will actually appear across the burden terminals and it is
denoted as Vs
So if system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in reality
actual secondary voltage of PT is Vs.
Ratio Error In Potential Transformer
The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio
error in a potential transformer, it can be expressed as ,
(Vp − KT.Vs
% voltage error =
) /Vp X 100 %
Phase Angle Error In Potential Transformer
The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors
KT.Vs is the phase error
CAUSE OF ERROR IN POTENTIAL TRANSFORMER
Fig (3)
The voltage applied to the primary of the potential transformer first drops due to internal
impedance of primary. Then it appears across the primary winding and then transformed
158
proportionally to its turns ratio, to secondary winding. This transformed voltage across secondary
winding will again drops due to internal impedance of secondary, before appearing across burden
terminals. This is the reason of errors in potential transformer.
10.4
POTENTIAL TRANSFORMER PARAMETERS
Rated Voltage For Potential Transformer
The RMS value of the voltage used to designate the Potential Transformer for a particular
highest system voltage is Rated Voltage for Potential Transformer. The voltage of electrical
power system is increased if load of the system is reduced. As per standard, the system voltage
can be raised up to 10% above the normal voltage during no load condition. So every electrical
equipment; is such designed so that it can withstand this highest voltage. As Potential
Transformer is electrical equipment, it should also be designed to withstand highest system
voltage.
Rated Burden Of Potential Transformer
Rated burden of Voltage Transformer is the value of the burden to be connected with the
secondary of Voltage Transformer including connecting load resistance expressed in VA or
ohms on which accuracy requirement is based.
Rated Frequency Of Potential Transformer
The value of the system frequency on which the Voltage Transformer operates.
10.5
POTENTIAL TRANSFORMERS CONNECTIONS
Open Delta Connection
In case of balanced toads on RYB phases the voltage between open delta i.e. pts A and B is equal
to zero volts.
Voltage across AB pt is called corner voltage which is usually 4 or 5 volts.
In balanced condition,
VRL0o + VYL120o + VBL240o = 0
If unbalanced condition, VRL0o + VYL120o + VBL240o = 0
159
Fig (4)
Broken Delta Connection
The above arrangement is called broken delta arrangement. We can get 3-phase supply from
Two Potential Transformers (from Two-phases). It means two single phase Potential
Transformers are used while one Potential Transformers is saved, so that much saving of cost.
Fig (5)
This situation is used in 11KV feeder system only for metering because metering is in normal
healthy condition. This phenomenon is not used for protection purposes, means that this type of
Potential Transformers is not used for protective relays.
The Potential Transformers used for protective relay must have three windings. But when ground
fault occurs then zero sequence voltages cannot be generated if used for protection purposes
having two windings. This type of connection can be used for low loads only.
10.6
POTENTIAL TRANSFORMER TESTS
10.6.1 CONTINUITY TEST
160
Check the continuity of the Primary and Secondary side of the Potential Transformer with an
ohmmeter.
10.6.2 POLARITY TEST
It is carried out to know about the physical location of H1, H2, X1 and X2 Terminals etc. On HV
side of Potential Transformer, select H1 and H2 terminals according to your own choice and
connect them with 1.5 volts Dry Cell through a Switch. Then connect DC voltmeter to LV side
in such a way that its positive terminal is connected to that terminal which is adjacent opposite to
that HV Terminal to which positive of Cell is connected. After making connections as directed
above, close switch “S”.


If Voltmeter needle deflects towards clockwise direction (or upscale or right side) then
polarity of transformer is “subtractive” (i.e. H1 and X1 terminals of transformer are
adjacent opposite to each other).
If voltmeter needle deflects towards anticlockwise direction (or down scale or left side),
then polarity of transformer is “additive” (i.e. H1 and X1 terminals of transformer are
diagonally opposite to each other).
10.6.3 INSULATION RESISTANCE TEST
Check the insulation of Potential Transformer with an appropriate voltage of the Insulation
Resistance Tester:
(i)
Primary to earth
(ii)
Secondary to earth
(iii)
Primary to Secondary
10.6.4 VOLTAGE RATIO TEST
Check the ratio of the Potential Transformer with one of the following methods
1. Back To Back Testing Method
2. Single Phase Testing Method
1.
Back To Back Testing Method
In this method there are to Potential Transformers, one with known ratio and the other one with
unknown ratio.
161
Connect Primary sides of both the Potential Transformers through a jumper wire. Now voltage is
applied through a variac to the secondary of that Potential Transformer whose ratio is known.
Then voltage at the secondary of unknown Potential Transformer is measured. Voltage Ratio of
the unknown Potential can now easily be calculated.
2.
Single Phase Testing Method
If a Potential Transformer of known Ratio is not available, then single phase to ground supply is
applied to the Potential Transformer. Then voltage at the secondary of under test Potential
Transformer is measured. Measured and Calculated Voltage Ratio can then be compared.
10.6.5 CIRCUIT VERIFICATION
To be done in Laboratory
10.7
POTENTIAL TRANSFORMER SUPPLY SUPERVISION
The Potential Transformer supply supervision is carried out with one of the following methods:
1. Balanced Beam Method
2. Logic Gates
Both the methods will be discussed during studies of the Distance Relays
10.8
CAPACITOR VOLTAGE TRANSFORMERS
The basic electrical diagram for a typical CCVT is shown in Fig. 1. The primary side consists of
two capacitive elements C1and C2connected in series. The Potential Transformer provides a
secondary voltage vo for protective relays and measuring instruments. The inductance Lc is
chosen to avoid phase shifts between viand vo at power frequency. However, small errors may
occur due to the exciting current and the CCVT burden (Zb)
162
Fig (6)
163
11.
Introduction to
Protection
11.1
INTRODUCTION
The purpose of an electrical power system is to generate and supply electrical energy to
consumers. The system should be designed and managed to deliver this energy to the utilization
points with both reliability and economy. Severe disruption to the normal routine of modern
164
society is likely if power outages are frequent or prolonged, placing an increasing emphasis on
reliability and security of supply. As the requirements of reliability and economy are largely
opposed, power system design is inevitably a compromise. A power system comprises many
diverse items of equipment. Fig (1) illustrates the diversity of equipment that is found.
Fig (1)
Many items of equipment are very expensive, and so the complete power system represents a
very large capital investment. To maximize the return on this outlay, the system must be utilized
as much as possible within the applicable constraints of security and reliability of supply. More
fundamental, however, is that the power system should operate in a safe manner at all times. No
matter how well designed, faults will always occur on a power system, and these faults may
represent a risk to life and/or property. Fig (2) shows the onset of a fault on an overhead line.
The destructive power of a fault arc carrying a high current is very great; it can burn through
copper conductors or weld together core laminations in a transformer or machine in a very short
time, some tens or hundreds of milliseconds. Even away from the fault arc itself, heavy fault
currents can cause damage to plant if they continue for more than a few seconds. The provision
of adequate protection to detect and disconnect elements of the power system in the event of
fault is therefore an integral part of power system design. Only by so doing can the objectives of
the power system be met and the investment protected. Fig (3) provides an illustration of the
consequences of failure to provide appropriate protection. This is the measure of the importance
of protection systems as applied in power system practice and of the responsibility vested in the
Protection Engineer.
165
Fig (2)
Fig (3)
11.2 PURPOSE OF PROTECTION RELAYING
The purpose of protection system is like a brain in human bodies. As brain is informed through
five senses and then it decides the action, in a similar manner, relay senses through Current
Transformer and/or Potential Transformer and then decides what to do.
11.3
PRINCIPLES OF PROTECTION SYSTEM
An electrical relay is a device which operates when the electrical quantity to which it responds
changes in a prescribed manner. If such a relay is used in protection of electrical equipment or
components of power system, it is called a protective relay.
Relays are used in three purposes
1. Protection
2. Control
166
3. Regulation
11.4 FUNCTIONS OF PROTECTIVE RELAYING
1.
2.
3.
4.
11.5
To detect the presence of a fault
To identify the faulted components
To initiate the appropriate circuit breaker
To remove the defective components from service
PROTECTION EQUIPMENT
The definitions that follow are generally used in relation to power system protection:
1. Protection System: a complete arrangement of protection equipment and other devices
required to achieve a specified function based on a protection principal (IEC 60255-20)
2. Protection Equipment: a collection of protection devices (relays, fuses, etc.). Excluded
are devices such as Current Transformers, Circuit Breakers, Contactors, etc.
3. Protection Scheme: a collection of protection equipment providing a defined function
and including all equipment required to make the scheme work (i.e. relays, Current
Transformers, Circuit Breakers, Batteries, etc.)
In order to fulfill the requirements of protection with the optimum speed for the many different
configurations, operating conditions and construction features of power systems, it has been
necessary to develop many types of relay that respond to various functions of the power system
quantities. For example, observation simply of the magnitude of the fault current suffices in
some cases but measurement of power or impedance may be necessary in others. Relays
frequently measure complex functions of the system quantities, which are only readily
expressible by mathematical or graphical means. Relays may be classified according to the
technology used:
1.
2.
3.
4.
Electromechanical
Static
Digital
Numerical
The different types have somewhat different capabilities, due to the limitations of the technology
used.
In many cases, it is not feasible to protect against all hazards with a relay that responds to a
single power system quantity. An arrangement using several quantities may be required. In this
167
case, either several relays, each responding to a single quantity, or, more commonly, a single
relay containing several elements, each responding independently to a different quantity may be
used.
11.6
THE FUNCTIONAL REQUIREMENTS OF THE RELAY
11.6.1 RELIABILITY
The most important requisite of protective relay is reliability since they supervise the circuit for a
long time before a fault occurs; if a fault then occurs, the relays must respond instantly and
correctly. Incorrect operation can be attributed to one of the following classifications:
1. Incorrect design/settings
2. Incorrect installation/testing
3. Deterioration in service
Design
The design of a protection scheme is of paramount importance. This is to ensure that the system
will operate under all required conditions, and (equally important) refrain from operating when
so required (including, where appropriate, being restrained from operating for faults external to
the zone being protected). Due consideration must be given to the nature, frequency and duration
of faults likely to be experienced, all relevant parameters of the power system (including the
characteristics of the supply source, and methods of operation) and the type of protection
equipment used. Of course, no amount of effort at this stage can make up for the use of
protection equipment that has not itself been subject to proper design.
Settings
It is essential to ensure that settings are chosen for protection relays and systems which take into
account the parameters of the primary system, including fault and load levels, and dynamic
performance requirements etc. The characteristics of power systems change with time, due to
changes in loads, location, type and amount of generation, etc. Therefore, setting values of relays
may need to be checked at suitable intervals to ensure that they are still appropriate. Otherwise,
unwanted operation or failure to operate when required may occur.
Installation
The need for correct installation of protection systems is obvious, but the complexity of the
interconnections of many systems and their relationship to the remainder of the installation may
168
make checking difficult. Site testing is therefore necessary; since it will be difficult to reproduce
all fault conditions correctly, these tests must be directed to proving the installation. The tests
should be limited to such simple and direct tests as will prove the correctness of the connections,
relay settings, and freedom from damage of the equipment. No attempt should be made to 'type
test' the equipment or to establish complex aspects of its technical performance
Testing
Comprehensive testing is just as important, and this testing should cover all aspects of the
protection scheme, as well as reproducing operational and environmental conditions as closely as
possible. Type testing of protection equipment to recognized standards fulfils many of these
requirements, but it may still be necessary to test the complete protection scheme (relays, Current
Transformers, Potential Transformer and other ancillary items) and the tests must simulate fault
conditions realistically.
Deterioration In Service
Subsequent to installation in perfect condition, deterioration of equipment will take place and
may eventually interfere with correct functioning. For example, contacts may become rough or
burnt owing to frequent operation, or tarnished owing to atmospheric contamination; coils and
other circuits may become open-circuited, electronic components and auxiliary devices may fail,
and mechanical parts may seize up. The time between operations of protection relays may be
years rather than days. During this period defects may have developed unnoticed until revealed
by the failure of the protection to respond to a power system fault. For this reason, relays should
be regularly tested in order to check for correct functioning.
Testing should preferably be carried out without disturbing permanent connections. This can be
achieved by the provision of test blocks or switches. The quality of testing personnel is an
essential feature when assessing reliability and considering means for improvement. Staff must
be technically competent and adequately trained, as well as self-disciplined to proceed in a
systematic manner to achieve final acceptance. Important circuits that are especially vulnerable
can be provided with continuous electrical supervision; such arrangements are commonly
applied to circuit breaker trip circuits and to pilot circuits.
Modern digital and numerical relays usually incorporate self testing/ diagnostic facilities to assist
in the detection of failures. With these types of relay, it may be possible to arrange for such
failures to be automatically reported by communications link to a remote operations centre, so
that appropriate action may be taken to ensure continued safe operation of that part of the power
system and arrangements put in hand for investigation and correction of the fault.
Protection Performance
Protection system performance is frequently assessed statistically. For this purpose each system
fault is classed as an incident and only those that are cleared by the tripping of the correct circuit
breakers are classed as 'correct'. The percentage of correct clearances can then be determined.
169
This principle of assessment gives an accurate evaluation of the protection of the system as a
whole, but it is severe in its judgment of relay performance. Many relays are called into
operation for each system fault, and all must behave correctly for a correct clearance to be
recorded. Complete reliability is unlikely ever to be achieved by further improvements in
construction. If the level of reliability achieved by a single device is not acceptable, improvement
can be achieved through redundancy, e.g. duplication of equipment. Two complete, independent,
main protection systems are provided, and arranged so that either by itself can carry out the
required function. If the probability of each equipment failing is x/unit, the resultant probability
of both equipments failing simultaneously, allowing for redundancy, is x2. Where x is small the
resultant risk (x2) may be negligible. Where multiple protection systems are used, the tripping
signal can be provided in a number of different ways. The two most common methods are:
All protection systems must operate for a tripping operation to occur (e.g. ‘two-out-of-two’
arrangement)
Only one protection system need operate to cause a trip (e.g. ‘one-out-of two’ arrangement)
The former method guards against palpation while the latter guards against failure to operate due
to an unrevealed fault in a protection system. Rarely, three main protection systems are provided,
configured in a ‘two-out-of three’ tripping arrangement, to provide both reliability of tripping,
and security against unwanted tripping. It has long been the practice to apply duplicate protection
systems to bus bars, both being required to operate to complete a tripping operation. Loss of a
busbar may cause widespread loss of supply, which is clearly undesirable. In other cases,
important circuits are provided with duplicate main protection systems, either being able to trip
independently.
On critical circuits, use may also be made of a digital fault simulator to model the relevant
section of the power system and check the performance of the relays used.
11.6.2 SELECTIVITY
The relay must be able to discriminate (select) between those conditions for which prompt
operation is required and those for which no operation, or time delayed operation is required.
When a fault occurs, the protection scheme is required to trip only those circuit breakers whose
operation is required to isolate the fault. This property of selective tripping is also called
'discrimination' and is achieved by two general methods.
Time Grading
170
Protection systems in successive zones are arranged to operate in times that are graded through
the sequence of equipments so that upon the occurrence of a fault, although a number of
protection equipments respond, only those relevant to the faulty zone complete the tripping
function. The others make incomplete operations and then reset. The speed of response will often
depend on the severity of the fault, and will generally be slower than for a unit system.
Unit Systems
It is possible to design protection systems that respond only to fault conditions occurring within a
clearly defined zone. This type of protection system is known as 'unit protection'. Certain types
of unit protection are known by specific names, e.g. restricted earth fault and differential
protection. Unit protection can be applied throughout a power system and, since it does not
involve time grading, is relatively fast in operation. The speed of response is substantially
independent of fault severity. Unit protection usually involves comparison of quantities at the
boundaries of the protected zone as defined by the locations of the current transformers. This
comparison may be achieved by direct hard-wired connections or may be achieved via a
communications link.
However certain protection systems derive their 'restricted' property from the configuration of
the power system and may be classed as unit protection, e.g. earth fault protection applied to the
high voltage delta winding of a power transformer. Whichever method is used, it must be kept in
mind that selectivity is not merely a matter of relay design. It also depends on the correct
coordination of Current Transformers and relays with a suitable choice of relay settings, taking
into account the possible range of such variables as fault currents, maximum load current, system
impedances and other related factors, where appropriate.
11.6.3 STABILITY
The term ‘stability’ is usually associated with unit protection schemes and refers to the ability of
the protection system to remain unaffected by conditions external to the protected zone, for
example through load current and external fault conditions.
11.6.4 SPEED
The relay must operate at the required speed. It should neither be too slow which may not result
in damage to the equipment nor should it be too fast which may result in undesired operation.
The function of protection systems is to isolate faults on the power system as rapidly as possible.
The main objective is to safeguard continuity of supply by removing each disturbance before it
leads to widespread loss of synchronism and consequent collapse of the power system.
As the loading on a power system increases, the phase shift between voltages at different bus
bars on the system also increases, and therefore so does the probability that synchronism will be
171
lost when the system is disturbed by a fault. The shorter the time a fault is allowed to remain in
the system, the greater can be the loading of the system. It worth to note that phase faults have a
more marked effect on the stability of the system than a simple earth fault and therefore require
faster clearance.
System stability is not, however, the only consideration. Rapid operation of protection ensures
that fault damage is minimized, as energy liberated during a fault is proportional to the square of
the fault current times the duration of the fault. Protection must thus operate as quickly as
possible but speed of operation must be weighed against economy. Distribution circuits, which
do not normally require a fast fault clearance, are usually protected by time-graded systems.
Generating plant and EHV systems require protection gear of the highest attainable speed; the
only limiting factor will be the necessity for correct operation, and therefore unit systems are
normal practice.
11.6.6 SENSITIVITY
The relaying equipment must be sufficiently sensitive so that it operates reliably when required
under the actual conditions that produces least operating tendency. Sensitivity is a term
frequently used when referring to the minimum operating level (current, voltage, power etc.) of
relays or complete protection schemes. The relay or scheme is said to be sensitive if the primary
operating parameter(s) is low. With older electromechanical relays, sensitivity was considered in
terms of the sensitivity of the measuring movement and was measured in terms of its voltampere consumption to cause operation. With modern digital and numerical relays the
achievable sensitivity is seldom limited by the device design but by its application and Current
Transformer/Potential Transformer parameters.
11.7
RELAYING TERMINOLOGY
11.7.1 RELAYING OPERATION
An electromechanical relay is said to have operated when sufficient current has passed through
the operating coil to cause movement of the mechanical components and move the contacts to
open or close, depending on the design and purpose of the relay. For solid sated relays, the relay
is said to have operated when the quantity to which it responds has reached the value where the
172
logic circuit initiates action to cause a set of contacts to open or close, depending on the purpose
of the relay.
11.7.2 RELAY RESETTING
Most electromechanical relays operate against a restraint spring or gravity, with the result that
when the actuating quantity disappears, or is reduced below preset pickup value, the relays will
reset. Theses relays are called “self-resetting”. However, some relays once they have operated
will not reset themselves. These are known as “manually-resetting” or “lockout relays”. Solid
state relays are similar, in that once the actuating quantity disappears or drops below the pickup
value, the logic circuit allow resetting of the contacts.
11.7.3 RELAY PICKUP, RELAY DROPOUT
If the actuating quantity applied to a relay is gradually increased, a point will be reached at which
the relay will operate. This minimum operating value is called the “relay pickup” value. If the
actuating quantity is then gradually decreased, a point will be reached where the relay contacts
reopen. This value is called the “relay dropout” value.
11.7.4 NORMALLY OPEN, NORMALLY CLOSED CONTACTS
A contact which is open if the relay has not operated is called a “normally open” contact; if it is
closed when the relay has not operated; it is called a “normally closed” contact. On electrical
drawings, all contacts are shown “open” or “closed” as they are when the relay is not operated,
even though in normal operation it may be that the relay is picked up.
11.7.5 PALLET SWITCHES
Pallet switches are auxiliary switches provided in circuit breakers and in certain disconnect
switches and linked to the operating mechanisms in such a way that they are opened or closed by
the operation of the main device. Those switches which open when the device opens are called
“a” pallet switch; those which open when the device closes are called “b” pallet switch.
11.7.6 RELAY SEAL-IN
173
Under certain circumstances, it may be desirable to insure that once a relay as operated, it
remains in the operated position (or picked up) for a definite period of time or until certain other
events have occurred. In such cases, a relay “seal-in” is provided.
11.7.7 INVERSE TIME AND DEFINITE TIME RELAYS
A definite time relay is one in which the time delay introduced remains constant from one
operation to the next regardless of the severity of fault conditions.
An inverse time relay is one in which the rate of travel of the moving contact assembly increases
with an increase in magnitude of the actuating quantity, i.e. the time required to close the
contacts decreases as the fault current increases.
11.7.8 RELAY TARGET
To analyze relay scheme performance during fault condition, it is necessary to know which
relays operated. This information is obtained from relay targets. These targets are operation
indicators which are “flagged” either mechanically by the movement of the moving contact
assembly, or electrically by the flow current through coil in series with the main contact. The
targets are generally brightly colored devices, mostly fluorescent red or orange, but some older
installations may have white targets.
11.7.9 REACH
This is a term applied to transmission lines and implies an impedance setting which is equivalent
to a certain line distance (impedance per line kilometer is a calculable quantity). The relay is set
according to the formula:
Z = E/I
11.7.10 DIRECT UNDER REACH
In this scheme, the fault detector is set at both ends of the line to cover only 85% of the line
distance. When the relays operate at either end, they initiate a transfer trip signal to the remote
end.
11.7.11 PERMISSIVE OVERREACH
This is a term applied to a relay scheme where the fault detectors at either end of a transmission
line are set to see more than 100% of the line and are, therefore, subject to operation for external
174
faults. To prevent operation on external faults, a permissive signal must be received from the
terminal before tripping can take place.
11.7.12 ECHO
This is a term used with a permissive overreach scheme when a line is open at one terminal and
the fault detectors at that terminal do not detect (or see) a fault and therefore, no permission to
trip is sent. To insure fast tripping, the permissive signal is echoed back to the detecting terminal
from the open terminal.
11.7.13 AUTOMATIC RECLOSING
Most faults on a power system are transient, lightning account for most of these. In order to limit
customer outages and to maintain a stable system, automatic reclosure schemes are employed to
get lines back in service after a transient fault. If a fault is permanent, the line will trip and stay
out after the reclose operation. Automatic reclosing may be short or long time. Reclosure may be
selected in the following modes using the newer type of reclosure schemes:
1. No reclosure
2. Under voltage – reclosure is permitted after the circuit breaker is tripped if no voltage is
present on the element tripped.
3. Synchrocheck – relosure is permitted if the voltages across the breaker to be reclosed are
in synchronism.
4. Voltage presence – reclosure is permitted after the breaker is tripped if voltage is present
on the element previously isolated.
11.8
DEVICE NUMBERS AND THEIR UNIVERSAL NOMENCLATURE
2
Time delay relay
3
Interlocking relay
21
Distance relay
25
Check synchronizing relay
27
Under voltage relay
30
Enunciator relay
32
Directional power (Reverse power) relay
37
Low forward power relay
175
40
Field failure (loss of excitation) relay
46
Negative phase sequence relay
49
Machine or Transformer Thermal relay
50
Instantaneous Over current relay
51
A.C IDMT over current relay
52
Circuit breaker
52a
Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52b
Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55
Power Factor relay
56
Field Application relay
59
Overvoltage relay
60
Voltage or current balance relay
64
Earth fault relay
67
Directional relay
68
Locking relay
74
Alarm relay
76
D.C Over current relay
78
Phase angle measuring or out of step relay
79
AC Auto reclose relay
81
Frequency relay
81U
under frequency relay
81O
over frequency relay
83
Automatic selective control or transfer relay
85
Carrier or pilot wire receive relay
86
Tripping Relay
87
Differential relay
87G
Generator differential relay
87GT overall differential relay
87U
UAT differential relay
87NT Restricted earth fault relay (provided on HV side of Generator transformer)
95
Trip circuit supervision relay
99
Over flux relay
186A Auto reclose lockout relay
176
186B Auto recluse lockout relay
11.9
RELAY PROTECTIVE SCHEMES
Primary Protection
Transformers, lines, reactors, buses and generators are protested by at least one sensitive relay
package which will trip quickly (about 20 ms) when a fault occurs. These relays are first line of
defance against damage to the system.
Backup Protection
All power circuits are protected by a second or a backup relay package which is more or less
independent of the other set (primary protection). The backup operates with an intentional time
delay.
Duplicate Protection
Now-a-days relay scheme have back up relaying as such. The new standard to protect a power
system consists of two independent relay schemes where neither of them has intentional time
delay.
Fig (4)
In addition Breaker Failure Protection is provided on all high voltage and some low voltage
breakers.
177
Fig (5)
Zone Protection
Power system is divided into zones which can be protected by a specialized group of relays and
which can also be separated at the rest of the system. If a zone is protected by two zone relays, it
will be called on Overlay Zone.
DETERMINING THE TYPE OF THE FAULT
Protective relay must be able to distinguish between and abnormal fault values. Disturbances to
the normal operating conditions, give rise to following changes in system parameters’ (some or
all).
1.
2.
3.
4.
5.
6.
Change in current
Change in voltage
Change in impedance (fall)
Voltage vector displacement
Temperature rise
Frequency deviation
FAULT DETECTING RELAYING
Purpose of these relays is to detect the presence of faults and then initiating the tripping scheme.
These relays may be of solid state or electromechanical type
178
13.
Over-Current
Protection
12.1
OVER CURRENT RELAYS
During short circuits on the power system, fault current is many fold of normal current and if not
removed from the system, it will cause damage or total collapse of the power system. Protection
against an abnormal current is the earliest protective system, developed and used till now all over
the world.
Over current relay basically sense the increase in flow of power system current, compare it with
a preset value decided during the fault calculation and relay setting process, then generate a trip
command for the related circuit breaker to isolate the faulty portion of the power system. Usually
the magnitude of current increases during fault, hence such relays are generally as over current
179
instead of a current relay. Under IEC (International Electro-technical Commission) standards
these types of relays are allocated with a designation number “51”, in all protection references
which include system drawings and manuals. Over current relays are connected to the power
system with help of current transformers.
12.2
GENERATIONS AND TYPES OF OVER CURRENT RELAYS
Since their development, four generation of these relays are in simultaneous use around the
world including power utilities in our country without any generation gap.
1.
2.
3.
4.
Electromechanical, the first generation relays.
Electronic with analogue design, the second generation.
Digital with micro-processor design, the third generation.
Numerical with software based design.
Depending on the time characteristics, these relays are further divided into four major types,
which include:
1. Instantaneous over current protection without any time delay, only include relay reaction
time, usually 10ms.
2. Definite over current relay with real time setting.
3. Inverse definite minimum time, IDMT, operating time varies inversely with magnitude of
fault current, hence called inverse over current relays.
4. Directional over current relays.
Over current relays have wide range of applications in big motors, generators, transformers, and
feeder/ transmission lines protection.
12.3
OPERATING PRINCIPALS OF OVER CURRENT RELAYS
ELECTROMACHENICAL - THE FIRST GENERATION
These relays remained popular in earlier power system as well as today. A large number of
relays and designs having robust characteristics and tropical weather condition resistant qualities,
make these are ready choice of protection engineers. These includes CDG (GEC), S type (BBC)
etc.
These electromechanical designs are popular in all time characteristics categories including:
1. Inverse time or IDMT.
2. Instantaneous
3. Definite time
OPERATING PRINCIPLES
ELECTROMECHANICAL CURRRENT DETECTING RELAYS
180
There may be a great variation in the physical appearance of the electromechanical relays. Their
operation is based upon one of the two fundamental principles.
1. Electromagnetic attraction
2. Electromagnetic induction
1.
ELECTROMAGNETIC ATTRACTION
A.
PLUNGER TYPE
Fig (1)
When a current is passed through an operating coil a magnetic field produced which, if the
current is sufficient, will draw the iron plunger in the solenoid. The moving contact moves
upward and completes the trip circuit. In the relay, operating current can be varied by adjusting
screw. This screw changes the magnetic field strength by moving soft iron sleeve inward or out
ward. These relay has a fix time.
B.
HINGED ARMATURE OR ATTRACTED ARMATURE TYPE
Figure below illustrates the essential components of hinged armature relay. Principle is similar to
plunger type.
Fig (2)
181
C.
INDUCTION DISC TYPE
This relay is basically a single phase induction motor which develops enough torque to turn its
disc when the current is of the sufficient magnitude. These relays are of two basic types:
i.
WOUND TYPE
This relay consists of two windings like a single phase capacitor motor. In this principle current
is split into two components by adding a capacitor in a starting winding.
ii.
SHADED POLE TYPE
In this method, instead of splitting the current, magnetic field is split in to two components to get
a couple for rotary motion the important components are shown in the figure. When current is
more than pickup value, disc start rotating and closes normally open contacts. Operating time
depends upon the speed of the disc rotating. There is inverse relation in current and time. Large
current means more torque and thus fast speed. Thus moving contact takes less time to close the
contact because of high speed. Distance, between moving and fix contact subdivided into ten
equal parts and is called calibrated on a dial, called a Time Dial. The settings are called time
dial settings. Therefore, such relays are called inverse time relays or IDMT (Inverse Definite
Minimum Time) relays.
Fig (3)
If we want to operate that the different current values, the operating coil is tapped off in such a
manner that the Ampere Turns remain constant at all such values. Hence the characteristics
remain same throughout the current range. Induction disc relays are therefore both time varying
and current varying relays.
182
Fig (4)
Winding taps are connected to Current Transformer circuit via a PLUG. Therefore these currents
settings are called PLUG SETTING. Its’ current time characteristics are shown in the relay
literature for different time dial settings (TDS). To avoid number of graphs for different plug
settings; time is plotted against plug setting multiples.
ELECTRONIC WITH ANALOGUE DESIGN - THE SECOND GENERATION
Early solid state design permits all the processing in analog form, for decision making level
detector and comparator circuitry is used. In analog electronic and digital relays, input
transformers are used in three phase gating circuits mainly in two configurations.
1. Output dependent on highest instantaneous voltage.
2. Output dependent on highest instantaneous current.
DIGITAL WITH MICRO-PROCESSOR DESIGN - THE THIRD GENERATION
These relays use the numerical techniques to derive the protection and control functions. With
the flexibility of the microprocessor, many characteristics of the inverse type relays including
Very Long Inverse, Normal Inverse, Very Inverse, Extremely Inverse, Definite Time and
Instantaneous elements are designed with in the product using independent algorithms.
In Digital relays four analog derived inputs, inclusive of neutral are multiplexed, then sampled
eight or less times per frequency cycle. The Fourier derived power frequency component returns
the RMS value of the measured quantity for further use.
These values are processed in analog to digital (A/D) convertor which is synchronized to the
power frequency measurement. In addition current is measured once per power frequency cycle
183
and Fourier is used to extract the fundamental component. The logic inputs are then filtered to
ensure that induced AC current in the external wiring to these inputs does not cause an incorrect
response. Sometimes opto-isolation is used in input and output circuits.
NUMERICAL WITH SOFTWARE BASED DESIGN
The numerical over-current time protection is usually equipped with a 16-bit microprocessor.
This provides fully digital processing of all functions for data acquisition of measured values.
The transducers of the measured value input section transforms the current from the current
transformers of the switchgear being protected. This unit then matches them to the internal
processing level of the relay. Apart from the galvanic and low capacitive isolation provided by
the input transformers, filters are provided for the suppression of interference. Filters are
designed according to bandwidth and processing speed to suit the measured value processing.
The matched analog values are then passed to the analog input section. This section contains
input amplifiers for each input, analog to digital convertors, and memory circuit for the data
transfer to the microprocessor. Apart from control and supervision of measured values, the
microprocessor processes the actual protective functions.
These include:
1.
2.
3.
4.
5.
6.
Scanning limit values and time sequences
Filtering and formation of the measured quantities
Calculation of negative and positive sequence currents for unbalanced load detection
Calculation of RMS values for overload detection
Decision regarding trip and re-close commands
Storage of measured quantities during a fault for analysis
Binary inputs and outputs to and from the processor are channeled via the input/output elements.
From these the processor receives information from the switchgear or from other equipments
about remote resetting, blocking signals and membrane key pad.
Output includes trip and re-close commands to the circuit breakers and visual indications i.e.
LEDs and alphanumerical display on the front. Usually integrated membrane key board in
connection with a built in alphanumerical LCD display enables communication with the
processor. All setting values and plant data are entered into the protection from this panel. The
dialog with the relay can also be carried out via the serial interface by means of personal
computer using RS232 Serial Bus or a Universal Serial Bus in modern relays.
In Numerical relays configuration is in software, some relays are provided with fixed
configuration. Some advance designs have more flexibility where user can make changes to the
184
internal logic of the relay by setting “software” links provided in software and called “Flexible
logic”, where you can generate your own inverse curves according to a specific requirement by
changing the variables of a particular algorithm. This process is sometimes called “Marshalling
of Numerical Relay”
12-4
SETTING CALCULATIONS
A-II
AA-1
B-II
BB-1C-II C
600-1200-2400/1A 600-1200-2400/1A
C-1
D
300-600-1200/5A
400-800-1600/1A
D-
1
ωω
~~ωω
ωωω
400-800-1600/5A
150A
400-800-1600/1A
250A
300-600-1200/5A
200A
150A
Fig (5)
D-1:Load=150A
10% of Over Load=1.1 x 150 =165A
CT Ratio= 300/5A
Plug Setting= 165 x 5/300 = 2.75A
ATIf = 4000A
Ifsec = 4000x5/300 = 66.67A
M.O.P.S. =Ifsec/P.S = 66.67/2.75 =24.24
LetToper. =0.1 Sec
Toper. =0.14 x T.D.S/( MOPS)0.02 - 1
T.D.S = 0.1[ (24.24)0.02 - 1/ 0.14 ]
= 0.1 [ 1.066 – 1/0.14 ]
= 0.1 [ 0.066/0.14 ] = 0.1 x 0.47
T.D.S = 0.047 Sec
ATIf = 6000A
185
Ifsec =6000x5/300 =100A
MOPS = Ifsec/P.S
MOPS = 100/2.75 = 36.36
Toper.= 0.14 x TDS/ (MOPS)0.02 - 1
Toper.= 0.14 x 0.047 /(36.36)0.02 - 1 = 0.00658 / 0.0745 = 0.09 Sec
AT If = 1000A
Ifsec = 1000 x 5 /300 =16.67A
MOPS = 16.67/2.75 =6.06
Toper.=0.14 x TDS/(MOPS)0.02 - 1 = 0.14 x 0.047 /(6.06)0.02 – 1
Toper. = 0.00658 / 1.0367 – 1 = 0.00658 /0.0367 =0.18 Sec
C-1:Load = 150A
10 % overload = 1.1 x 150 = 165A
C.T Ratio = 400/1A
Plug Setting = 165 x 1/400 = 0.41A
ATIf = 4000A
Ifsec =4000 x 1/400 =10A
MOPS = Ifsec/P.S = 10/0.41 =24.39
Let
Toper.= Toper. D-1 + D-1 C.B Mechanism operating time + D-1 C.B Arc Quenching Media + Safety
Margin
Toper.C-1 = 0.1 + 0.05 + 0.02 + 0.05 =0.22Sec
Toper.=0.14 x TDS/(MOPS)0.02 - 1
T.D.S = 0.22 [(24.39)0.02 - 1/0.14 ]
= 0.22[1.0659 – 1/0.14 ] = 0.22 x 0.471
186
T.D.S = 0.104 Sec
ATIf = 6000A
Ifsec = 6000 x 1/400 = 15A
MOPS= 15/0.41 = 36.59
Toper.= 0.14 xTDS/(MOPS)0.02 - 1
Toper.= 0.14 x 0.104/(36.59)0.02 -1 = 0.0145 / 0.0746 = 0.19Sec
ATIf =1000A
Ifsec= 1000 x 1/400 =2.5A
MOPS = 2.5/0.41 = 6.097
Toper.= 0.14 x 0.104/(6.097)0.02 - 1
Toper.=0.0146/0.0368 = 0.397Sec
C-II:Load = 350A
10% overload =110% x 350 = 385A
C.T. Ratio = 400/1A
Plug Setting = 385 x 1/400 =0.96A
ATIf = 4000A
Ifsec = 4000 x 1/400 = 10A
MOPS = 10/0.96 = 10.42
Toper = Toper
Margin
C-1
+ C.B Mechanism operating time + C-1 C.B Arc Quenching Media + Safety
Toper = 0.22 + 0.05 + 0.02 + 0.05 = 0.34 Sec
Toper = 0.14 x TDS/(MOPS)0.02 - 1
T.D.S = 0.34 [(10.42)0.02 - 1/0.14 = 0.34[0.0479/0.14] = 0.34 x 0.342 = 0.116 Sec
ATIf= 6000A
187
Ifsec= 6000 x 1/400 =15A
MOPS = 15/0.96 =15.63
Toper = 0.14 x TDS/(MOPS)0.02- 1= 0.14 x 0.116/(15.63)0.02- 1
Toper = 0.0162 / 0.0565 = 0.287 Sec
ATIf = 1000A
Ifsec = 1000 x 1/400 = 2.5A
MOPS = 2.5/0.96 = 2.6
Toper = 0.14 x 0.116/(2.6)0.02- 1
Toper = 0.0162/0.0193 = 0.839 Sec
B-1:Load =350A
10% overload =110% x 350 = 385A
C.T Ratio = 600/5A
Plug Setting = 385 x 5/600 = 3.21A
ATIf = 4000A
Ifsec = 4000 x 5/600 = 33.33A
MOPS =33.33/3.21 = 10.38
Toper B-1= Toper C-II+ C-II C.B Mechanism Opening time +C-II C.B Arc Quenching Media + Safety
Margin
Toper B-1 =0.34 + 0.05 + 0.02 + 0.05 =0.46 Sec
AT If=6000A
Ifsec =6000 x 5/600 =50A
MOPS = 50/3.21 =15.58
Toper =0.14 x 0.16/(15.58)0.02- 1
Toper =0.0224/0.0564 = 0.397 Sec
188
ATIf =1000A
Ifsec= 1000 x 5/600 = 8.33A
MOPS = 8.33/3.21 = 2.6
Toper = 0.14 x 0.16/(2.6)0.02- 1 = 0.0224/1.0193
Toper = 0.0224/0.0193 = 1.16 Sec
B-II:Load =600A
10% overload =110% x 600 =660A
C.T Ratio = 800/5A
Plug Setting =660 x 5/800 =4.125A
ATIf = 4000A
Ifsec= 4000 x 5/800 =25A
MOPS =25/4.125 = 6.06
Toper B-II = Toper B-1 + B-1 C.B Mechanism opening time + B-1 Arc Quenching Media + Safety
Margin
Toper B-II =0.46 + 0.05 + 0.02 + 0.05 = 0.58 Sec
Toper = 0.14 x TDS /(MOPS)0.02- 1
T.D.S = 0.58[(6.06)0.02- 1/0.14] = 0.58[0.0367/0.14]
T.D.S = 0.58 x 0.262 = 0.152 Sec
ATIf=6000A
Ifsec= 6000 x 5/800 = 37.5A
MOPS = 37.5/4.125 =9.09
Toper = 0.14 x 0.152/(9.09)0.02 - 1 = 0.02128/0.045 = 0.47 Sec
ATIf= 1000A
Ifsec = 1000 x 5/800 = 6.25A
189
MOPS = 6.25/4.125 = 1.52
Toper = 0.14 x 0.152/(1.52)0.02 -1 = 0.02128/1.0084 – 1
Toper = 0.02128/0.0084 = 2.53 Sec
A-1:Load = 600A
10% overload = 110% x 600 = 660A
C.T. Ratio = 1200/1A
Plug Setting = 660 x 1/1200 = 0.55A
AT If = 4000A
Ifsec = 4000 x 1/1200 = 3.33A
MOPS = 3.33/0.55 = 6.06
Toper A-1 = Toper B-II + B-II C.B Mechanism operating time + B-II C.B Arc Quenching Media +
Safety Margin
Toper A-1 = 0.58 + 0.05 +0.02 + 0.05 = 0.7 Sec
Toper = 0.14 x T.D.S/(MOPS)0.02 – 1
T.D.S = 0.7[(6.06)0.02 – 1/0.14] = 0.7[0.03669/0.14]
T.D.S = 0.7 x 0.262 = 0.18 Sec
AT If = 6000A
Ifsec = 6000 x 1/1200 = 5A
MOPS = 5/0.55 = 9.09
Toper = 0.14 0.18/(9.09)0.02 – 1 = 0.0252/0.0451= 0.56 Sec
AT If = 1000A
Ifsec = 1000 x 1/1200 = 0.83A
MOPS = 0.83/0.55 = 1.51
Toper =0.14 x 0.18/(1.51)0.02 – 1 = 0.0252/0.0083 = 3.04 Sec
190
A-II:Load = 750A
10%overload = 110% x 750 = 825A
C.T ratio = 1200/1A
Plug Setting = 825 x 1/1200 = 0.687A
AT If = 4000A
Ifsec = 4000 x 1/1200 = 3.33A
MOPS = 3.33/0.687 = 4.85
Toper A-II = Toper A-1 + A-1 C.B Mechanism operating time + A-1 C.B Arc Quenching Media +
Safety Margin
Toper A-II = 0.7 + 0.05 +0.02 + 0.05 = 0.82 Sec
Toper = 0.14 x T.D.S/(MOPS)0.02 – 1
T.D.S = 0.82[(4.85)0.02 – 1/0.14]= 0.82[0.0321/0.14]
T.D.S = 0.82 x 0.229 = 0.188 Sec
AT If = 6000A
Ifsec = 6000 x 1/1200 = 5A
MOPS = 5/0.687 = 7.28
Toper = 0.14 x 0.188/(7.28)0.02 – 1 = 0.0263/0.0405 = 0.65 Sec
AT If =1000A
Ifsec= 1000 x 1/1200 = 0.83A
MOPS = 0.83/0.687 = 1.21
Toper = 0.14 x 0.188/(1.21)0.02 – 1 = 0.0263/0.0038 = 6.92 Sec
12-5
OVER CURRENT RELAY TESTING
The following Tests are to be carried out on various Over Current Relays in Laboratory
12-5-1 Pick- Up/Drop Off
12-5-2 Operating Time
191
13.
Differential Protection
Relay
192
13.1
FAULTS IN POWER TRANSFORMERS
Transformer should be protected against.
1. Internal faults
2. Through faults
Internal fault occurs as a result of failure of the insulation, providing a short circuit path
between phases and often to the ground icon core. The heavy fault current can cause damage to
winding can even burn the core. Differential protection provides the best against internal faults.
Through faults are cleared by protective devices downstream. However, failure of downstream
relays could place a serve over load on the transformer. These currents due to through faults can
cause mechanical and thermal damage. All the protections for through faults must operate before
the transformer reaches at thermal limit according to transformer damage curve. Normally
differential Protection is applied to all transformers of 10MVA and above and smaller
transformers of importance.
13.2
DIFFERENTIAL PROTECTION
The Differential Protection of Transformer has many advantages over other schemes of
protection.
The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay.
But if any fault occurs in the transformer but not in oil then it cannot be detected by Buchholz
relay. Any flash over at the bushings are not adequately covered by Buchholz relay. Differential
relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for
detecting any internal fault in the transformer but Differential Protection scheme detects the
same in faster way.
The differential relays normally response to those faults which occur inside the differential
protection zone of transformer.
13.3
PRINCIPLE OF DIFFERENTIAL PROTECTION
Principle of Differential Protection scheme is one simple conceptual technique. The differential
relay actually compares between primary current and secondary current of power transformer, if
any unbalance found in between primary and secondary currents the relay will actuate and inter
trip both the primary and secondary circuit breaker of the transformer.
193
Suppose you have one transformer which has primary rated current Ip and secondary current Is. If
you install Current Transformer of ratio Ip/1A at primary side and similarly, Current
Transformers of ratio Is/1A at secondary side of the transformer. The secondary of these both
Current Transformers are connected together in such a manner that secondary currents of both
Current Transformers will oppose each other. In other words, the secondary of both Current
Transformers should be connected to same current coil of differential relay in such a opposite
manner that there will be no resultant current in that coil in normal working condition of the
transformer. But if any major fault occurs inside the transformer due to which the normal ratio of
the transformer is disturbed then the secondary current of both transformer will not remain the
same and one resultant current will flow through the current coil of the differential relay, which
will actuate the relay and inter trip both the primary and secondary circuit breakers.
To correct phase shift of current because of star-delta connection of transformer winding in case
of three phase transformer, the Current Transformer secondary should be connected in delta and
star respectively. If the equipment within the “Protection Zone” is functioning correctly, then the
sum of currents entering the Zone must equal the sum of currents leaving i.e. their difference
must be zero, and the relay will be inoperative.
13.4
TYPES OF DIFFERENTIAL RELAYS
There are two types of differential relays:
1. Plain Differential Relay
2. Percentage Differential relay
These relays can be best understood by an example. Consider a 132/11kV; YY connected Power
Transformer having no phase shift. Full load current ratings are 100 A and 1200 A respectively
Current Transformer ratio on HV side is taken as 100/5 and on LV side 1200/5. If polarities of
transformer are taken as subtractive, the single phase schematic circuit will be as under.
Fig (1)
The current in primary side Current Transformer loop is 5A with the direction according to
convention that “if primary current is entering ‘P1’ then secondary current leaves ‘S1’.
194
It is clear that 5A primary Current Transformer current balanced 5A secondary side Current
Transformer current. Current flowing through differential relay is
I operate = I1 – I2
I operate = 10 – 10
I operate = 0
Hence Differential scheme is perfectly balanced. However, if current direction in any loop is
reversed, the
I operate = I1 – (–I2)
I operate = I1 + I2
I operate = 5+5
I operate = 10 A, and the scheme will trip.
This shows the importance of current direction in differential schemes. Now the scheme is put
under first test i.e.
THROUGH FAULT TEST: Consider an out of Zone Fault of 2400A,
Fig (2)
I operate = I1- I2
I operate = 10-10
I operate = 0, and the schemes remains still balanced.
INTERNAL FAULT TEST (WITH SINGLE FEED): Consider an Internal Fault of 2400A,
195
Fig (3)
I operate = I1- I2
I operate = 10-0
I operate = 10
The difference of current flows through relay because I2 = 0, as no current flows in the secondary
of LV Current Transformers and the scheme will trip.
INTERNAL FAULT TEST (WITH DOUBLE FEED): Consider an Internal Fault, assume that
impedance on both side of fault is same and both sources share equal current.
Fig (4)
I operate = I1– 12
I operate = 10– (–10)
I operate =10 + 10
I operate = 20A, hence the scheme will trip too.
196
Consider such a relay having different character Current Transformers with two ampere
mismatch current. In case of plain differential scheme relay should operate.
There are many factors still left that can affect the balance of scheme.
1.
2.
3.
4.
5.
6.
7.
If CTs are not of identical design. Difference can flow to unbalance the scheme.
Saturation of one of the two CT’S.
The magnetizing in rush currents in T/FS.
Phase shifting in star delta connected transformers:
Connections of CT’S, phase.
Selection of phase if HV and LV side CT currents are not equal.
Effect of transformer taps.
The relay discussed is called a plain differential relay. To avoid problems of Current
Transformer saturation or varying character Current Transformers, solution is to employ two
restrain coils and one operating coil in each current loop. The restraining coils produce Restrain
Torque or a Negative Torque while the operating coil produces an Operating Torque or Positive
Torque. Restrain Torque strength can be changed by using a tapped winding.
Fig (5)
Consider such a relay having different character Current Transformers with 2 A mismatch
current.
197
Fig (6)
In case of plain differential scheme, relay should operate, but in modified scheme a restraining
current of
Ires = (I1 + I2)/2
Ires = (13+15)/2
Ires = 14A, flows in restraining coil and
I operate = I1 – I2
I operate = 15-13
I operate = 2 A, flows in operating coil
The stronger restraining field keeps the relay un-operative. Similarly if any CT saturates at such
high fault currents relay will behave similarly.
%age = Iop /I rest × 100
%age = 2/14 × 100
%age = 14.3%
Relay will restrain if set more than 14.3% slope. Normal relays are provided with 15, 30 and 45
percent slope. Higher the slope lesser the sensitivity of relay at 10 times fault current.
I res = (130 + 150)/2
I res = 140A
I operate = 150 – 130
I operate = 20
% age = (20/140) × 100
% age = 14.3%
198
Such relays are, therefore, termed as Percentage Differential relays because operating current is a
fix percentage of restrain current for same slope.
Fig (7)
From the figure it is clear that for a fixed restraining current of 14 A operating coil pick up
currents are different. At 15 % slope, relay is more sensitive than at 45% slope.
13.5
MAGNETIZING INRUSH CURRENT
When a power transformer is energized, transient magnetizing current flows for few cycles
having instantaneous peaks of 8 to 20 times those of full load current. Duration of which depends
upon
1.
2.
3.
4.
5.
Size of transformer
Size of power system
Source resistance
Residual flux level
Core material
On analysis it revealed it Second Harmonic components are dominant.
Component
2nd Harmonic
3rd Harmonic
4th Harmonic
%age
63%
26%
5.1%
Since inrush current flows with only one side of transformer energized. The effect is similar to
the fault on the system. Therefore, Harmonic restraint circuitry is essential to avoid maloperation of differential relays. This can be achieved by two means:
1. Time delay: One method for preventing tripping due to inrush the operating coil of the
differential relay is de-sensitized for a few milliseconds when the primary breaker is
closed.
199
2. Second Harmonic Restraint Circuit: Second harmonic in the inrush current are filtered
out using band pass filters then applied in restraining circuits to restrain the relay further,
when inrush current is flowing.
Fig (8)
But still exits a problem, whether these harmonics are generated by a power transformer or a
saturated Current Transformer to avoid malfunction in such conditions, relay should differentiate
between these conditions by Zero Detection Method. The wave shape of second harmonic
currents of a Current Transformer compared with a fault current wave from suggest that
magnetizing in rush wave normally stays at zero crossing for quite a sometime. This zero time is
different for a Current Transformer wave form and a Power Transformer wave form.
Hence relay is made to restrain if zero is detected in a cycle for more than a certain period
(typically for 1/4th of a cycle).
Fig (9)
13-6
BALANCE OF DIFFERENTIAL RELAY FOR VARIOUS VECTOR GROUPS
For correct application of differential protection requires Current Transformer ratio and winding
connections of Current Transformer secondary (Phasing) to match those of transformer. For this
purpose polarity convention used is, if current is entering P1 (dot) then it should leave from S1
(dot) i.e. Subtractive Polarity. The Hard Rule is that Current Transformer Secondary circuit
should be a Replica of Primary System, to consider
1. Minimizing of Difference in Current Magnitudes
200
2. Phase Shift Compensation
3. Zero Sequence Currents Trapping
All these requirements can only be achieved using matching or interposing Current
Transformers. It is a small transformer usually with four independent primary and secondary
windings.
Fig (10)
Using required turn on a matching Current Transformer, loop currents can be increased or
decreased depending on the difference of current at the relay. Phase shifting connections are also
be carried out of matching Current Transformers instead of main Current Transformers, to lower
the Current Transformer secondary burden, because matching Current Transformers are located
near the relay. By doing so I2R losses in a delta connection are avoided.
Delta-Star Power Transformer: To compensate for Delta–Star transformer phase shift of 30o and
Zero Sequence Current trap, the rule is to connect:
Delta side Current Transformers of Power Transformer in Star and
Star side Current Transformers of Power Transformer in Delta
Fig(11)
Delta-Delta Power Transformer: Star connected Current Transformers are used on both sides to
reduce I2R losses and hence burden on Current Transformers. As
Total Current Transformer burden =
Relay Resistance + Lead Resistance + Current Transformers Secondary Resistance
201
Star-Star Power Transformer: Current Transformers are now connected in delta on both sides to
provide Zero Sequence Current trap on both side to reduce Current Transformers burden.
Matching Current Transformers are used near differential relay in 1:1 ratio (if current change is
not required), and connect their relay side winding in delta.
13-7
DIFFERENTIAL RELAY TESTING
The following Tests are to be carried out on various Differential Relays in Laboratory
13-7-1 Pickup/ Drop off
13-7-2 Operating time
13-7-3 Percent slope
13-7-4 Percent Second Harmonics
13-8
PRACTICAL CONNECTIONS OF DIFFERENTIAL ON A POWER TRANSFORMER
Practical demonstration will be carried out with the 66 kV/ 11 kV, 1MVA Power Transformer
14.
202
Under Frequency
Relay
14.1
UNDER FREQUENCY PROTECTION
System stability is an important objective, whenever there is an overloading on the power
system; load on the system is needed to be reduced.
As, Ns = (120*f)/P
Or f = (Ns*P)/120
Due to increase in load, the speed of the generator goes down, and so does the frequency. In
order to improve the frequency, the speed of the generator needs to be increased. This can be
achieved through different means depending upon the type of generation. However, this process
203
has certain limits. Similarly, sometimes, a situation will be encountered when due to sudden
rejection of load, the frequency of the system increase. In order to monitor the variation of
frequency from a predetermined range, Frequency Protection is used.
Normally, in our system, we face under frequency problem or overloading. In order to control
the load, load shedding is carried out. Load shedding can be carried out either manually or
automatic.
In manual load shedding, Power Demand and Power Generated are compared. If Power Demand
is more than Power Generation, then load shedding of the deficit amount is carried out by
dividing the power to be shed among different areas according to priority.
Sometimes due to system constraints or climatic conditions, predictions become difficult, and
forced load shedding is carried out. However there are situations, when there is no time for
manual load shedding and the frequency is supposed to cross a critical stage, where total system
might collapse. In such situations, automatic load shedding is carried out.
An under frequency load shedding protection system is incorporated for stably managing a
power system by recovering a power system frequency to within a predetermined range when the
power system frequency drops. An under frequency level detection unit judges an under
frequency level of the power system frequency when the power system frequency drops resulting
from power generation shortage in the power system and a load shedding unit sequentially sheds
loads determined in advance based on a staying time when the power system frequency stays at
any one of the under frequency levels judged by the under frequency level detection unit and
sheds on this occasion more loads quickly when the under frequency level at which the power
system frequency stays is large.
14.2
OPERATING PRINCIPLES
Potential Transformer supply is applied to the relay. A rate of change of frequency detection unit
is provided, which judges, when the power system frequency deviates resulting from load
rejection, loading or a fault on the power system. When the power system frequency varies, the
frequency relay detects this change it acts accordingly. The relay might be over frequency or
204
under frequency or featuring both. Similarly the relay be single stage or multistage, depending
upon the requirements. Frequency relays are generally applied:
1. To monitor continuously the frequency of an electrical power system
2. Used in graded load shedding system
14-3
UNDER FREQUENCY RELAY TESTING
The following Tests are to be carried out on various Under Frequency Relays in Laboratory
14-3-1 PICKUP/DROP OFF
14-3-2 OPERATING TIME
205
15.
Over Fluxing Relay
206
15.1
CAUSES OF OVER FLUXING IN TRANSFORMER
As per present day transformer design practice, the peak rated value of the flux density is kept
about 1.7 to 1.8 Tesla, while the saturation flux density of CRGD steel sheet of core of electrical
power transformer is of the order of 1.9 to 2 Tesla which corresponds to about 1.1 times the rated
value. If during operation, an electrical power transformer is subjected to carry rather swallow
more than above mentioned flux density as per its design limitations, the transformer is said to
have faced over fluxing problem and consequent bad effects towards its operation and life.
Depending upon the design and saturation flux densities and the thermal time constants of the
heated component parts, a transformer has some over excitation capacity. I.S. specification for an
electrical power transformer does not stipulate the short time permissible over excitation, though
in a roundabout way it does indicate that the maximum over fluxing in transformer shall not
exceed 110%.
The flux density in a transformer can be expressed by
B
=
where,
C
V
=
f = Frequency.
C
A
=
Induced
V/f,
constant,
voltage,
The magnetic flux density is, therefore, proportional to the quotient of voltage and frequency
(V/f). Over fluxing can, therefore, occur either due to increase in voltage or decrease infrequency of both.
The probability of over fluxing is relatively high in step up transformers in Power stations
compared to step down transformers in Sub-Stations, where voltage and frequency usually
remain constant. However, under very abnormal system condition, over-fluxing trouble can arise
in step-down Sub-Station transformers as well.
15.2
EFFECT OF OVER FLUXING IN TRANSFORMERS
The flux in a transformer, under normal conditions is confined to the core of transformer because
of its high permeability compared to the surrounding volume. When the flux density in the core
of transformer increases beyond saturation point, a substantial amount of flux is diverted to steel
structural parts and into the air. At saturation flux density the core steel will over heat.
207
Structural steel parts which are un-laminated and are not designed to carry magnetic flux will
heat rapidly. Flux flowing in unplanned air paths may link conducing loops in the windings,
loads, tank base at the bottom of the core and structural parts and the resulting circulating
currents in these loops can cause dangerous temperature increase. Under conditions of excessive
over fluxing the heating of the inner portion of the windings may be sufficiently extreme as the
exciting current is rich in harmonies. It is obvious that the levels of loss which occur in the
winding at high excitation cannot be tolerated for long if the damage is to be avoided.
Physical evidences of damage due to over fluxing will vary with the degree of over excitation,
the time applied and the particular design of transformer. The Table given below summarizes
such physical damage and probable consequences.
Sr.
No.
1
2
3
4
Component
involved
Physical evidences
Consequences
Discoloration or metallic
parts
and
adjacent
Contamination of oil and surfaces of
Metallic support and insulation.
insulation. Mechanical weakening of
surfaces structure for
insulation. Loosening of Mechanical
core and coils
Possible carbonized material
structure
in
oil.
Evolution
of
combustible gas.
Windings
Discoloration
winding Electrical
and
mechanical.
insulation. Evolution of gas. Weakening of winding insulation
Lead conductors.
Discoloration of conductor
Electrical and mechanical weakening
insulation or support.
of insulation. Mechanical Weakening
of support.
Evolution of gas.
Core laminations.
Discoloration of insulating
material in contact with
Electrical weakening of major
core.
insulation (winding to core).
Increased inter-laminar eddy loss.
Discoloration
and
carbonization
of
organic/lamination
208
insulation.
Evaluation of gas.
5
Tank
Contamination of oil if paint inside
tank is blistered.
Blistering of paints
It may be seen that metallic support structures for core & coil, windings, lead conductors, core
laminations, tank etc. may attain sufficient temperature with the evolution of combustible gas in
each case due to over-fluxing of transformer and the same gas may be collected in Buchholz
Relay with consequent Alarm/Trip depending upon the quantity of gas collected which again
depends upon the duration, the transformer is subjected to over fluxing.
Due to over fluxing in transformer its core becomes saturated as such induced voltage in the
primary circuit becomes more or less constant. If the supply voltage to the primary is increased
to abnormal high value, there must be high magnetizing current in the primary circuit. Under
such magnetic state of condition of transformer core linear relations between primary and
secondary quantities (viz. for voltage and currents) are lost. So there may not be sufficient and
appropriate reflection of this high primary magnetizing current to secondary circuit as such
mismatching of primary currents and secondary currents is likely to occur, causing differential
relay to operate as we do not have over fluxing protection for sub-Station transformers.
STIPULATED WITHSTAND-DURATION OF OVER FLUXING IN TRANSFORMERS
Over fluxing in transformer has sufficient harmful effect towards its life which has been
explained. As over fluxing protection is not generally provided in step-down transformers of
Sub-Station, there must be a stipulated time which can be allowed matching with the transformer
design to withstand such over fluxing without causing appreciable damage to the transformer and
other protections shall be sensitive enough to trip the transformer well within such stipulated
time, if cause of over fluxing is not removed by this time.
It is already mentioned that the flux density 'B' in transformer core is proportional to V/f ratio.
Power transformers are designed to withstand (Vn/fn x 1.1) continuously, where Vn is the normal
highest RMS voltage and fn is the standard frequency. Core design is such that higher V/f causes
higher core loss and core heating. The capability of a transformer to withstand higher V/f values
i.e. over-fluxing effect, is limited to a few minutes as furnished below in the Table
F = (V/f)/(Vn/fn)
1.1
1.2
1.25
1.3
1.4
209
Duration of with stand limit
(minutes)
continuous
2
1
0.5
0
From the table above it may be seen that when over fluxing due to system hazards reaches such
that the factor F attains a values 1.4, the transformer shall be tripped out of service
instantaneously otherwise there may be a permanent damage.
15.3
OPERATING PRINCIPLES
The condition arising out of over fluxing does not call for high speed tripping. Instantaneous
operation is undesirable as this would cause tripping on momentary system disturbances which
can be borne safely but the normal condition must be restored or the transformer must be isolated
within one or two minutes at the most.
Flux density is proportional to V/f and it is necessary to detect a ratio of V/f exceeding unity, V
and f being expressed in per unit value of rated quantities. In a typical scheme designed for over
fluxing protection, the system voltage as measured by the Voltages Transformer is applied to a
resistance to produce a proportionate current; this current on being passed through a capacitor,
produces a voltage drop which is proportional to the functioning in question i.e. V/f and hence to
the flux in the power transformer. This is accompanied with a fixed reference D.C. voltage
obtained across a Zener diode. When the peak A.C. signal exceeds the D.C. reference it triggers
a transistor circuit which operates two electromechanical auxiliary elements. One is initiated
after a fixed time delay, the other after an additional time delay which is adjustable. The over
fluxing protection operates when the ratio of the terminal voltage to frequency exceeds a
predetermined setting and resets when the ratio falls below 95 to 98% of the operating ratio. By
adjustment of a potentiometer, the setting is calibrated from 1 to 1.25 times the ratio of rated
volts to rated frequency.
The output from the first auxiliary element, which operates after fixed time delay available
between 20 to 120 seconds second output relay operates and performs the tripping function.
It is already pointed out that high V/f occurs in Generator Transformers and Unit-Auxiliary
Transformers if full excitation is applied to generator before full synchronous speed is reached.
V/f relay is provided in the automatic voltage regulator of generator. This relay blocks and
prevents increasing excitation current before full frequency is reached.
When applying V/f relay to step down transformer it is preferable to connect it to the secondary
(L.V. side of the transformer so that change in tap position on the H.V. is automatically taken
care of. Further the relay should initiate an Alarm and the corrective operation is done / got done
210
by the operator. On extreme eventuality the transformer controlling breaker may be allowed to
trip.
16.
211
Trip Circuit
Supervision Relay
16.1
TRIP CIRCUIT SUPERVISION PROTECTION
In a protection system the trip circuit of the circuit breaker is crucial. If an interruption occurs in
the trip circuit a possible network fault will not be disconnected and would have to be cleared by
another protection upstream in the power system. The supervision function is particularly
important when there is only one tripping coil and circuit breaker tripping is vital. For instance,
for generator’s circuit breakers or any other important circuit breaker in distribution networks.
The supervision relay type Trip Circuit Supervision is intended for a continuous supervision of
circuit breaker trip circuit and to give an alarm for loss of auxiliary supply, faults on the trip-coil
or its wires independent of the breaker position, faults on the breaker auxiliary contacts and
faults in the supervision relay itself.
212
This guide describes methods to protect a power system from faults that are not cleared because
of failure of a power circuit breaker to operate or interrupt when called upon. The discussion is
limited to those instances where the breaker does not clear the fault after a protective relay has
issued a command to open or trip the circuit.
16-2
OPERATING PRINCIPLES
Trip Circuit Supervision Relay monitors the healthiness of the trip circuit of breaker. Basically
the circuit drives a small current through the breaker trip coil and monitors it continuously. If
there is any failure in the trip circuit or trip coil circuit, the relay will sense and operate its output
contacts. Certain time delay is introduced to prevent false indication during breaker
operation.
213
17.
Restricted
Earth Fault Relay
214
17.1
RESTRICTED EARTH FAULT PROTECTION OF TRANSFORMER
An external fault in the star side will result in current flowing in the line Current Transformer
of the affected phase and at the same time balancing current flows in the neutral Current
Transformer; hence the resultant electric current in the relay is therefore zero. So this REF relay
will not be actuated for external earth fault. But during internal fault the neutral Current
Transformer only carries the unbalance fault current and operation of Restricted Earth Fault
Relay takes place. This scheme of Restricted Earth Fault Protection is very sensitive for internal
earth fault of electrical power transformer. The protection scheme is comparatively cheaper than
differential protection scheme
17.2
OPERATING PRINCIPLES
Restricted earth fault protection is provided in electrical power transformer for sensing internal
earth fault of the transformer. In this scheme the Current Transformer secondary of each phase of
electrical power transformer are connected together as shown in the figure. Then common
terminals are connected to the secondary of a Neutral Current Transformer or NCT. The Current
Transformer connected to the neutral of power transformer is called Neutral Current
Transformer or simply NCT. Whenever there is an unbalancing in between three phases of the
power transformer, a resultant unbalance current flow through the close path connected to the
common terminals of the Current Transformer secondary. An unbalance current will also flow
through the neutral of power transformer and hence there will be a secondary current in NCT
because of this unbalance neutral current. In Restricted Earth Fault scheme the common
terminals of phase Current Transformers are connected to the secondary of NCT in such a
manner that secondary unbalance current of phase Current Transformers and the secondary
current of NCT will oppose each other. If these both currents are equal in amplitude there will
not be any resultant current circulate through the said close path. The Restricted Earth Fault
Relay is connected in this close path. Hence the relay will not respond even there is an
unbalancing in phase current of the power transformer.
215
Fig (1)
Fig (2)
216
Fig (3)
18.
217
Breaker Failure
Protection
18-1
BREAKER FAIL PROTECTION
A failure occurs when the circuit-breaker fails to correctly open and clear the fault after single or
three-pole trip commands have been issued by the protection unit. It is then necessary to trip the
relevant bus bar zone (section) to ensure fault clearance.
One of the primary considerations in the design and application of a breaker failure protection
system is that it should be biased toward security. Since a true breaker failure occurrence is so
rare, the breaker failure protection scheme will be called upon to not trip many more times than it
will be called upon to trip. Also, since the failure of a breaker generally requires tripping out all
adjacent circuits, the consequences of mal-operation and over trip of the system are many times
worse than mal-operation and over trip of nearly any other protective scheme on the power
system. For this reason, there are several features that are commonly included to enhance
security from mal-operation. A separate breaker failure protection system is required for each
218
breaker. Backup tripping systems such as lockout relays can be common within a substation
depending on the circumstances.
18-2
OPERATING PRINCIPLES
At its most elemental level, a breaker failure protection system consists of a timer. The timer is
started at the same time that the trip signal is sent to the breaker and is used to precisely time the
period that is allowed for the breaker to interrupt the fault. If the breaker does not operate by the
expiration of the time delay, the breaker is determined to have failed and tripping of backup
breakers occurs. The breaker failure protection system can determine if the breaker has tripped
by monitoring a contact mounted to the breaker operating mechanism; however, it generally
includes a current detector to confirm that the current flowing in the tripped breaker has been
successfully interrupted.
Beyond this simple concept, the timers and fault detectors in a breaker failure protection system
can be combined in many different ways; but all can be simplified into a few common logic
schemes. Figure 1 shows three of these basic logic schemes. The timing diagrams associated
with these logic schemes are shown in Figure 2.
Figure 1a shows a scheme where both the BFI (breaker failure initiate) and the fault detector
must be true to start the timer. Successful interruption is indicated by either the fault detector
dropping out or the protective relays dropping out and removing the BFI signal. The breaker
failure fault detector is important here because it has a high dropout ratio and fast reset
characteristic whereas the protective relays do not have such a constraint put upon them. They
may be slow to drop out after the fault is cleared.
219
220
Fig (1)
221
Figure 1b shows a scheme where the importance of the dropout/reset characteristics of the fault
detector is minimized. The timer is initiated by the BFI signal from the protective relays. If the
timer expires before the protective relays drop out, the fault detector is then started. If the breaker
has interrupted successfully, the fault detector will not pick up at all. In this case, the fault
detector should have a fast pickup time because that will be added to the time required to trip
backup in the event of a failed breaker.
Fig (2)
222
When considering the two timing diagrams, it is important to call attention to the time marked as
the margin. The margin is the difference between normal clearing time and when the breaker
failure protection system will cause backup tripping to occur. A larger margin will improve
security from incorrect backup tripping. For Figure 2-b, it can be seen that the margin can be
improved by the difference between the fault detector’s pickup and drop-out time for a given
backup tripping time, or the backup tripping time can be reduced by this same amount for a
given margin time.
Figure 1c is a subtle variation on scheme 1a. In this case, the timer is started by the BFI signal
alone as in scheme 1b, so fault detector pickup time is not a factor in starting the timer. Breaker
failure trip will occur if the timer expires and the fault detector is still picked up. The difference
with this logic is that the effect of timer over travel (the timer continues for a short period after
the input is removed) is minimized.
These basic schemes can be modified to accommodate additional situations. The most important
modification to note is the need to also accommodate breaker failure protection with breaker
status contact supervision instead of fault detector supervision. This modification would be used
in situations where the faults being detected by the initiating relays may not involve high current.
For example, initiation by transformer differential or sudden pressure relays or remote transfer
trip.
With modern, solid state and numerical breaker failure relays, issues such as fault detector and
timer performance are minimized over breaker failure schemes built up using discrete
electromechanical components. Programmable logic in solid state and numerical relays also
223
19.
Bus-Bar Protection
224
19.1
INTRODUCTION
The protection scheme for a power system should cover the whole system against all probable
types of fault. Unrestricted forms of line protection, such as over current and distance systems,
meet this requirement, although faults in the bus bar zone are cleared only after some time delay.
But if unit protection is applied to feeders and plant, the bus bars are not inherently protected.
Bus bars have often been left without specific protection, for one or more of the following
reasons:
1. The bus bars and switchgear have a high degree of reliability, to the point of being
regarded as intrinsically safe
2. It was feared that accidental operation of bus bar protection might cause widespread
dislocation of the power system, which, if not quickly cleared, would cause more loss
than would the very infrequent actual bus faults
3. It was hoped that system protection or back-up protection would provide sufficient bus
protection if needed.
4. It is true that the risk of a fault occurring on modern metal clad gear is very small, but it
cannot be entirely ignored
19.2
BUSBAR FAULTS
The majority of bus faults involve one phase and earth, but faults arise from many causes and a
significant number are inter-phase clear of earth. In fact, a large proportion of bus bar faults
result from human error rather than the failure of switchgear components. With fully phasesegregated metal clad switchgear, only earth faults are possible, and a protection scheme need
have earth fault sensitivity only. In other cases, an ability to respond to phase faults clear of earth
is an advantage, although the phase fault sensitivity need not be very high.
19.3
BUS BAR PROTECTION REQUIREMENTS
225
Although not basically different from other circuit protection, the key position of the bus bar
intensifies the emphasis put on the essential requirements of speed and stability. The special
features of bus bar protection are discussed below.
SPEED
Bus bar protection is primarily concerned with:
1. Limitations of consequential damage
2. Removal of bus bar faults in less time than could be achieved by back-up line protection,
with the object of maintaining system stability
Some early bus bar protection schemes used a low impedance differential system having a
relatively long operation time, of up to 0.5 seconds. The basis of most modern schemes is a
differential system using either low impedance biased or high impedance unbiased relays capable
of operating in a time of the order of one cycle at a very moderate multiple of fault setting. To
this must be added the operating time of any tripping relays, but an overall tripping time of less
than two cycles can be achieved. With high-speed circuit breakers, complete fault clearance may
be obtained in approximately 0.1 seconds. When a frame-earth system is used, the operating
speed is comparable.
STABILITY
The stability of bus protection is of paramount importance. Bearing in mind the low rate of fault
incidence, amounting to no more than an average of one fault per bus bar in twenty years, it is
clear that unless the stability of the protection is absolute, the degree of disturbance to which the
power system is likely to be subjected may be increased by the installation of bus protection. The
possibility of incorrect operation has, in the past, led to hesitation in applying bus protection and
has also resulted in application of some very complex systems. Increased understanding of the
response of differential systems to transient currents enables such systems to be applied with
confidence in their fundamental stability. The theory of differential protection is given later in
section 15.7. Notwithstanding the complete stability of a correctly applied protection system,
dangers exist in practice for a number of reasons. These are:
1. Interruption of the secondary circuit of a current transformer will produce an unbalance,
which might cause tripping on load depending on the relative values of circuit load and
effective setting. It would certainly do so during a through fault, producing substantial
fault current in the circuit in question
2. A mechanical shock of sufficient severity may cause operation, although the likelihood of
this occurring with modern numerical schemes is reduced
3. Accidental interference with the relay, arising from a mistake during maintenance testing,
may lead to operation
226
In order to maintain the high order of integrity needed for bus bar protection, it is an almost
invariable practice to make tripping depend on two independent measurements of fault
quantities. Moreover, if the tripping of all the breakers within a zone is derived from common
measuring relays, two separate elements must be operated at each stage to complete a tripping
operation. The two measurements may be made by two similar differential systems, or one
differential system may be checked by a frame-earth system, by earth fault relays energized by
current transformers in the transformer neutral-earth conductors or by voltage or over current
relays.
Alternatively, a frame-earth system may be checked by earth fault relays. If two systems of the
unit or other similar type are used, they should be energized by separate current transformers in
the case of high impedance unbiased differential schemes. The duplicate ring CT cores may be
mounted on a common primary conductor but independence must be maintained throughout the
secondary circuit. In the case of low impedance, biased differential schemes that cater for
unequal ratio CTs, the scheme can be energized from either one or two separate sets of main
current transformers. The criteria of double feature operation before tripping can be maintained
by the provision of two sets of ratio matching interposing CTs per circuit. When multi-contact
tripping relays are used, these are also duplicated, one being energized from each discriminating
relay; the contacts of the tripping relay are then series-connected in pairs to provide tripping
outputs.
Separate tripping relays, each controlling one breaker only, are usually preferred. The
importance of such relays is then no more than that of normal circuit protection, so no
duplication is required at this stage. Not least among the advantages of using individual tripping
relays is the simplification of trip circuit wiring, compared with taking all trip circuits associated
with a given bus section through a common multi-contact tripping relay.
In double bus bar installations, a separate protection system is applied to each section of each bus
bar. An overall check system is also provided, covering all sections of both bus bars. The
separate zones are arranged to overlap the bus bar section switches, so that a fault on the section
switch trips both the adjacent zones. This has sometimes been avoided in the past by giving the
section switch a time advantage; the section switch is tripped first and the remaining breakers
delayed by 0.5 seconds. Only the zone on the faulty side of the section switch will remain
operated and trip, the other zone resetting and retaining that section in service. This gain,
applicable only to very infrequent section switch faults, is obtained at the expense of seriously
delaying the bus protection for all other faults. This practice is therefore not generally favored.
Some variations are dealt with later under the more detailed scheme descriptions. There are
many combinations possible, but the essential principle is that no single accidental incident of a
secondary nature shall be capable of causing an unnecessary trip of a bus section.
Security against mal-operation is only achieved by increasing the amount of equipment that is
required to function to complete an operation; and this inevitably increases the statistical risk that
a tripping operation due to a fault may fail. Such a failure, leaving aside the question of
consequential damage, may result in disruption of the power system to an extent as great, or
greater, than would be caused by an unwanted trip. The relative risk of failure of this kind may
227
be slight, but it has been thought worthwhile in some instances to provide a guard in this respect
as well.
Security of both stability and operation is obtained by providing three independent channels (say
X, Y and Z) whose outputs are arranged in a ‘two-out-of three’ voting arrangement, as shown in
Fig (1).
Fig (1)
19.4
TYPES OF BUS BAR PROTECTION SYSTEMS
A number of bus bar protection systems have been devised:
1.
2.
3.
4.
5.
System protection used to cover bus bars
Frame-earth protection
Differential protection
Phase comparison protection
Directional blocking protection.
228