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Problems of protection in transformer
INDEX
Sr
No.
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4
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8
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Topics
Page No.
Project definition
Introduction to project
Construction of transformer
Problem summary
Faults in detail
Transformer protection principle
Protection in transformer
Core and Coil Assembly
Tanking
Oil Filling
Accessories
Painting
Testing
Dispatch
Material used in project
Estimation of project
Application
Outcomes
Overview of next semester project
Bibliography
1
PROJECT DEFINATION
AIM:
The aim of this course is to elaborate the idea of transformer protection and get
known to different types of faults and protective overcome these faults.
OBJECTIVES:
 To know about the different type of faults
appearing in transmission and distribution system.
 To know about fault occurs in transformer.
 Expected outcome for given faults
Working principal of Transformer:When an alternating voltage is introduced to one coil, called the primary, it creates a
fluctuating magnetic field in the iron core. This fluctuating field then induces an alternating voltage
in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio)
between the primary and secondary depends on the turns ratio of the two coils.
(FIG.1.1 Transformer core)
“EMF is induced in a closed conducting circuit when the magnetic flux linking with that circuit
changes in time.”
Transformer’s main basic is mutual induction between two circuits linked by a common magnetic
flux.
2
INTRODUCTION
If we arrange two electrically isolated coils in such a way that the time-varying flux
due to one of them causes an electromotive force (emf) to be induced in the other, they are said to
form a Transformer. In other words, a transformer is a device that involves magnetically coupled
coils. If only a fraction of the flux produced by one coil links the other, the coils are said to be
loosely coupled. In this case, the operation of the transformer is not very efficient.
3
In order to increase the coupling between the coils, the coils are wound on a common
core. When the core is made of a nonmagnetic material, the transformer is called an air-core
transformer. When the core is made of a ferromagnetic material with relatively high permeability,
the transformer is referred to as an iron-core transformer. A highly permeable magnetic core
ensures that (A) almost all the flux created by one coil links the other and (B)the reluctance of the
magnetic path is low. This results in the most efficient operation of a transformer.
In its simplest form, a transformer consists of two coils that are electrically isolated
form each other but are wound on the same magnetic core. A time-varying current in coil sets up a
time-varying flux in the magnetic core. Owing to the high permeability of the core, most of the flux
links the other coil and introduces a time varying emf (voltage)in the coil. The frequency of the
induced emf in the other coil is the same as that of the current in the first coil. if the other coil is
connected to a load, the induces emf in the coil establishes a current in it. Thus the power is
transferred from one coil to the other via the magnetic flux in the core.
The coil to which the source supplies the power is called the prima ry
winding. The coil that delivers power to the load is called the secondary winding.
Either winding may be connected to the source and/or the load.
Since the induced emf in a coil is proportional to the number of turns in a
coil, it is possible to have a higher voltage across the secondary than the applied
voltage to the primary. In this case, the transformer is called a step -up transformer. A
step-up transformer is used to connect a relatively high -voltage transmission line to a
relatively low-voltage generator. On the other hand,a step-down transformer has a
lower voltage on the secondary side. An examp le of a step-down transformer is a
welding transformer. The secondary of which is designed to de-liver a high load
current.
When the applied voltage to the primary is equal to the induced emf in the
secondary, the transformer is said to have a one -to-one ratio. A one-to-one ratio
transformer is used basically foe the purpose of electrically isolating the secondary
side from its primary side. Such a transformer is usually called an isolation
transformer. An isolation transformer can be utilized for direct current (dc) isolation.
That is, if the input voltage on the primary side consists of both dc and alternating
current (ac) components, the voltage on secondary side will be purely ac in nature.
4
Construction of Transformer:-
In order to keep the core loss to a minimum, the core of a transformer is built up of thin
laminations of highly permeable ferromagnetic material such as silicon-sheet steel. Silicon steel is
used because of its nonaging properties and low magnetic losses. The laminations thickness varies
from 0.014 inch to 0.024 inch. A thin coating of varnish is applied to both sides of the lamination in
order to provide high interlamination resistance. The process of cutting the laminations to the
proper size results in punching and shearing strains. These strains cause an increase in the core loss.
In order to remove the punching and shearing strains, the laminations are subjected to high
temperatures in a controlled environment for some time. It is known as the annealing process.
( fig-1.2 Construction of shell type Transformer)
Basically two types of construction are in common use for the transformer,: shell type
and core type. In the construction of a shell type transformer, the two windings are usually wound
over the same leg of the magnetic core, as shown in fig. In a core type transformer, shown in fig.,
each windings may be evenly split and wound on both legs of the rectangular core. The
nomenclature, shell type and core type, is derived from the fact that in a shell type transformer the
core en-circles the windings, whereas the windings envelop the core in a core type transformer.
5
For relatively low power applications with moderate voltage ratings, the windings may be
wound directly on the core of the transformer. However, for high voltage and/or high power
transformers, the coils are usually form wound and then assembled over the core.
Both the core loss (hysteresis and eddy-current loss) and the copper loss(electrical
loss) in a transformer generate heat, which in turn, increases the operating temperature of the
transformer. For low-power applications, natural air circulation may be enough to keep the
temperature of the transformer within an acceptable range. If the temperature increase cannot bee
controlled by natural air circulation, a transformer may be cooled by continuously forcing air
through its core and windings. When forced air circulation is not enough, a transformer may be
immersed in a transformer oil, which carries the heat to the walls of the containing tank. In order to
increase the radiating surface of the tank, cooling fins may be welded to the tank or the tank may be
built from corrugated sheet steel. These are some of the methods used to curb excessive
temperature in the transformer.
6
PROBLEM SUMMARY
There are also so many problems in the transformer as following..
 winding failures due to short circuits (turn-turn faults, phase-phase faults, phase-ground,
open winding)
 Open circuit fault
 Localized overheating
 Measuring hot spots temperature
 core faults (core insulation failure, shorted laminations)
 terminal failures (open leads, loose connections, short circuits)
 on-load tap changer failures (mechanical, electrical, short circuit, overheating)
 Failures in the mechanical circuit.
 Failures in the electrical circuit.
 Failures in the dielectric circuit.
 Arcing, or high current break down
 Over flux of transformer
 Heating of oil
 Moisture effect in transformer
 Fault in design of transformer
 Low energy sparking, or partial discharges
 General overheating due to inadequate cooling or sustained overloading
7
FAULTS IN DETAIL
Failures in the mechanical circuit:a) Burning of Lamination of the core
b) Vibrations.
c) Loose bolts of the core.
Burning of Lamination of the core:The magnetic core is built up of laminations of high grade silicon or other sheet steel
which are insulated from each other by varnish or through a coating of iron oxide. The core can be
constructed in different ways relative to the windings.
If the Varnish or Iron oxide is remove from the core lamination so it Touches to the Core to core or
Core to Conductor Therefore Short circuit is occurs.
Vibrations:Due to Vibration It affect the insulation and causes to Failure of insulation.
So core to conductor or conductor to conductor is occurred.
Loose bolts of the core:Loose connection of the core causes in to the Vibration take place.
And produced vibration is not damped by this.
So more vibration produced when bolts are loose
8
OPEN CIRCUIT FAULT:-
Current will only flow in a CIRCUIT. That is, around a continuous path (or
multiple paths) from and back to the source of EMF. Any interruption in the circuit, such as an
open switch, a break in the wiring, or a component such as a resistor that has changed its resistance
to an extremely high value will cause current to cease. The EMF will still be present, but voltages
and currents around the circuit wil have changed or ceased altogether. The open switch or the fault
has caused what is commonly called an OPEN CIRCUIT.
Remember that wherever an open circuit exists, although voltage may be present there
will be no current flow through the open circuit section of the circuit. Also, as Power(P) is V x I
and the current (I) = 0, no power will be dissipated.
Looking further at the simple circuit used in Labelling Voltages and Currents let´s put
some actual voltages and currents in and see what happens under "Open Circuit" conditions.
Use the drop down box below the following diagram to select a number of open
circuit conditions that might occur in different parts of the circuit. Notice how the voltages and
currents around the circuit change depending on where the break in the circuit (the open circuit)
occurs. Checking the voltages around a circuit with a voltmeter, and noticing where they differ
from what would be expected in a correctly working circuit, is one of the main techniques used
for tracing a fault in any circuit.
9
Measuring Hot-Spot Temperature:In general, the goal of temperature-based transformer protection is to limit the impact
of the hot test-spot temperature on transformer winding insulation. Therefore, using measured hotspot temperature provides the most accurate protection against transformer over-temperature
conditions, and may be the only measurement required for protection purposes. The biggest
disadvantage to this method is the hot-spot temperature sensor. Practically, the sensor must be
installed during manufacture of the transformer, as the sensor must be physically installed in the
transformer winding at a point calculated by the transformer designer to be the location of the hotspot. As the temperature sensors must also be electrically isolated from the transformer tanks and
windings, hot-spot temperature sensors are typically fibres-optic sensors. In practice, it is rare to
measure the hot-spot temperature, except for large power transformers.
Transformer Heating:No-load losses and load losses are the two significant sources of heating considered in
thermal modelling of power transformers. No-load losses are made up of hysteresis and eddy loss in
the transformer core, and these losses are present whenever the transformer is energized. Hysteresis
loss is due to the elementary magnets in the material aligning with the alternating magnetic field.
Eddy currents are induced in the core by the alternating magnetic field. The amount of
hysteresis and eddy loss is dependent on the exciting voltage of the transformer. Load losses are the
more significant source of transformer heating, consisting of copper loss due to the winding
resistance and stray load loss due to eddy currents in other structural parts of the transformer. The
copper loss consists of both DC resistance loss, and winding eddy current loss. The amount of loss
is dependent on transformer load current, as well as oil temperature. DC resistance loss increases
with increasing temperature, while other load losses decrease with increasing oil temperature. All of
these factors are considered in calculations of thermal transformer performance.
The basic method for cooling transformers is transferring heat from the core and
windings to the insulating oil. Natural circulation of the oil transfers the heat to external radiators.
The radiators increase the cooling surface area of the transformer tank. Pumps may be used to
increase the flow of oil, increasing the efficiency of the radiators. In non-directed flow
transformers, the pumped oil flows freely through the tank. In directed flow transformers, the
pumped oil is forced to flow through the windings. Forced air cooling is commonly applied on
large power transformers, using fans to blow air over the surface of the radiators, which can double
the efficiency of the radiators. For some large power transformers, water cooling may replace large
radiators. Large power transformers may also have additional ratings for multiple stages of forced
cooling. Normally, only two stages are applied, providing transformer ratings equivalent to 133%
and 167% of the self-cooled rating.
Both the IEEE and the IEC established standard designations for the various cooling modes
of transformers. The IEEE has adopted the IEC designations. The designation completely describes
the cooling method for the transformer, and the cooling method impacts the response of the
transformer insulating oil to overload conditions.
10
Moisture in Transformer Oil:-
Water in oil appears as an unwanted substance, it is generally accepted that water in
microscopic amounts - not gallons- is the cause of more electrical breakdowns than any other
impurity. Moisture constitutes a hazard not only to the insulating qualities of the oil but also to the
insulations that are immersed in the oil. Water may be introduced to the oil by leaking gaskets, poor
handling techniques or from the product of natural insulating paper and oil degradation. As the
paper degrades, it produces Carbon Dioxide and Water and as the insulating oil ages, water, acids,
sludge and other polar compounds are formed. So its presence is inevitable during the normal
service life of a transformer. Water is a polar liquid having a high permittivity or dielectric constant
it is therefore attracted to areas of strong electric fields. This sees the internal moisture distributed
not uniformly, but in fact potentially concentrating in the most dangerous parts of the system. It is
important to note that water is in a continuous state of movement between the oil and paper
insulating system, caused by internal temperature variations due to load and ambient conditions.
Water may be present in four possible forms, they are:
• Free water – That is water that has settled out of the oil in a separate layer. It is this water which is
indicated by a low dielectric breakdown voltage.
• Emulsified water – Or water that is suspended and has not yet settled out into free water (indicated
by “caramel” coloured oil).
Nb: A high Power Factor value indicates the possible presence of this suspended water trapped in
oil decay products.
• Water in solution – or dissolved in the oil.
• Chemically bound water – Water which is chemically attached to the insulating paper and which
is released when oxidized.
The destructive effects of water include:
• Expansion of the paper insulation, altering the mechanical pressure of the transformer clamping
system.
• Loss of insulating ability (Dielectric Breakdown Voltage)
• Accelerating paper aging i.e. triggering decomposition of the fibres in the paper
• Increased corrosion of the core and tank
11
• Progressive consumption of oil additives
The most dangerous and destructive of these effects is the loss of the oils insulating
ability.
This may occur from the following events:
• During periods of high load and at high ambient temperatures, dielectric breakdowns can result
from the reduced oil strength with high absolute amounts of water.
• With sudden high loads, water can boil off conductor surfaces and the vapour bubbles can cause
dielectric failures as they rise to the top.
• During the cool-down period after high load, the relative saturation of oil will increase. At its
extreme at 100% relative saturation, water will precipitate out and greatly reduce the dielectric
strength of the oil.
If oil is oxidized to any extent, any water coming into the transformer will partially
be absorbed into the oil decay products (It is this fact which causes old or highly oxidised oil to
dissolve more water than new oil). As the decay products build up in the oil, the surface tension of
the water or the interfacial tension between the oil and the water is lowered dramatically. This
heavier decay molecule will then re-circulate through out the entire transformer and will find its
way into the paper insulation, or into areas of high electrical intensity thus reducing the insulation
resistance. The water saturated oil decay molecule has a preference for the coolest part of the
transformer (bottom and fins, leading to corrosion) and areas of highest electrical stress (leading to
arcing). It has been proven that insulating paper with 2% moisture content ages three times faster
than one with 1% moisture and thirty times faster with 3% moisture. It is thus easy to see the
importance of maintaining low moisture levels with in a transformer to ensure a long and trouble
free service life.
Design of the Transformer Fault Diagnosis Expert System Based
on Fuzzy Reasoning
12
Transformer fault is the long-term accumulation complex result of transformer itself and
its application environment, so the symptoms of transformer fault is also diversified and the
connection between fault symptoms and fault mechanism is complex. Lot of work staff members at
the scene lacked of knowledge of fault diagnosis, and they can not frequently asked experts in the
field, so carrying out the research and applications of transformer fault diagnosis expert system, to
ensure the transformer long-term, reliable operation, which has an important meaning and
application prospects. This paper analyzes the current research status of transformer fault diagnosis
expert system, based on specific application, to study the structure and key technologies of power
transformer further. The most important issues of fault diagnosis expert system are knowledge
representation and reasoning mechanism. According to the characteristics that human experts
diagnose the fault of transformer, on the basis of learning the process that human experts diagnose
the fault of transformer, the paper analyzes and discusses in detail the system structure, knowledge
representation and reasoning mechanism to build fault diagnosis expert system of transformer.
Location 1: Faults on Bus Between Transformer Protective Device and Transformer,
and Transformer Primary Bushing-to-Ground Faults
The short length of bus and greater line-to-ground clearance make these types of faults
rare. Fault current magnitudes in this area can be high because they are only limited by the
upstream impedance of the system. This location is typically not part of the relaying for a primary
side protective device due to the location of the primary side current transformers. It is important to
remember that the current transformers for the transformer differential protection are typically
located on the bushings of the transformer. Therefore, the primary-side protective device will not
receive a trip signal for events on the primary bus, and this device will not get called on to clear full
primary-side fault currents on the primary side. For
example, primary arrester failures will be cleared by the line terminal breakers, Faults in this area
cannot usually be detected by the relay schemes used with a local device. If current transformers are
available at the primary-side protective device, a relay scheme could be used to cover this small
section of the high-side bus but, due to the 3-cycle speed of operation of line-terminal circuit
breakers, both devices would respond to the event unless an expensive pilot-wire blocking scheme
is implemented. On more complex substation bus arrangements like ring-bus or breaker-and-onehalf schemes, the primary-side protective device usually has current transformers and is used to
clear faults in this short section.
Location 2: Transformer Primary Winding Faults
Faults at the transformer primary winding are uncommon in a well-protected and
well-maintained transformer. The fault current magnitude can be either high or low depending on
the location of the fault within the winding. Typical winding faults are turn-to-turn or multiple-turn
and are low magnitude, on the order of load current. Less likely winding-to-ground faults are
limited by the winding impedance to low to moderate magnitude. A winding-to ground fault near
the high-voltage bushing results in high currents limited by the system impedance. These faults are
detected by differential, over current, or ground fault relaying, or by sudden pressure, gas analysis,
or other overpressure detection methods. Depending on where they occur and the magnitude of the
fault current, they will either be cleared by the line breakers or the primary side protective device.
13
Location 3: Transformer Secondary Winding Faults
Secondary winding faults are also uncommon in a well protected and wellmaintained transformer. The fault current magnitude can be either low or moderate depending on
the location of the fault with in the winding. Turn-to-turn and multiple-turn faults are typically low
magnitude, limited by the winding impedance. Winding to ground faults are of moderate magnitude
and always less than the available secondary fault current limited by the transformer impedance.
These faults are detected by differential, overcurrent, or ground-fault relaying, or by sudden
pressure, gas analysis, or other overpressure detection methods, and are cleared by the primary-side
transformer protective device without disturbing the transmission line.
Location 4: Transformer Secondary Bus Faults and Secondary Bushing-to-Ground
Faults
Secondary bus and transformer secondary bushing-to ground faults represent the
majority of faults that a transformer protective device will have to interrupt. With its typically
greater length and smaller phase-to-ground distance, it more likely that wildlife intrusion or nearby
equipment problems will cause a fault on the secondary bus. The fault current magnitude is
moderate because it is limited by the transformer impedance. Faults in this location are detected by
secondary differential or over current protective schemes, and are cleared by the primary-side
transformer protective device without disturbing the transmission line.
Secondary-Side Fault Interruption:Secondary-side faults, found in Location , are difficult to interrupt. Special
attention must be paid to selecting a device that can interrupt these low current transformer-limited
faults. Such faults are limited by the impedance of the transformer, so they have modest
magnitudes, but the transient recovery voltage (TRV) frequency seen by the interrupting device
after clearing the current is high because of the small bushing and winding capacitance of the
transformer, compared to its large inductance. To illustrate, a fault in Location 4 would be detected
by the transformer’s differential relaying, which would signal the primary-side device to open. In a
puffer-type SF6 circuit-switcher or circuit breaker, during the opening operation, the SF6 gas is
compressed by moving the cylinder supporting the contact system, or by a piston which forces the
SF6 through the interrupting nozzle. This causes a rapid gas flow across the arc, which serves to
cool the arc while the arc is lengthened as the contacts move farther apart. By controlling and
cooling the arc, its conductivity can be rapidly decreased as the current passes through zero, such
that the arc is extinguished and the circuit is interrupted. At that time, the transient recovery voltage
across the gap between the contacts is impinged, as a race starts between the cooling processes
within the transformer protective device due to increasing dielectric strength and rising recovery
voltage across the contacts. If the cooling processes are successful, the interruption is successful
and there is no resumption in current. When interrupting a secondary-side fault across a
transformer, the reactance-to resistance ratio (X/R) is high, so fault current will significantly lag the
voltage. When current is at zero, the voltage is at or near peak. In general, the impedance of the
transformer is high compared to the source, so there is a significant voltage (assumed to be .9 per
unit. by C37.016) [3] trapped in the stray capacitance of the transformer. As this energy resonates
back and forth between the electric field of the stray capacitance and the magnetic field of the
14
transformer inductance, a substantial voltage excursion occurs on the terminals of the transformer
protective device, which highly stresses the contact gap due to the high-frequency nature of this
excursion—typically on the order of 10 kHz to 15 kHz. If the insulation between the contacts
recovers more quickly than the TRV, the interruption is successful. If the TRV creates too
excessive a rise in voltage, re-ignition of the arc can occur.
F =
1
2π√lc
15
Transformer Protection Principles
Transformers are a critical and expensive component of the power system. Due to
the long lead time for repair of and replacement of transformers, a major goal of transformer
protection is limiting the damage to a faulted transformer. Some protection functions, such
as over excitation protection and temperature-based protection may aid this goal by
identifying operating conditions that may cause transformer failure. The comprehensive
transformer protection provided by multiple function protective relays is appropriate for
critical transformers of all application.
This chapter describes the protection practices for transformers of the following
types
whose three-phase bank rating is 510 KVA and higher:







Power transformers
Power autotransformers
Regulating transformers
Step voltage regulators
Grounding transformers
Electric arc-furnace transformers
Power-rectifier transformers
Contrasted with generators, in which many abnormal circumstances may arise,
transformers may suffer only from winding short circuits, open circuits, or overheating. In practice
relay protection is not provided against open circuits because they are not harmful in themselves.
Nor in general practice, even for unattended transformers, is overheating or overload protection
provided; there may be thermal accessories to sound an alarm or to control banks of fans, but, with
only a few exceptions, automatic tripping of the transformer breakers is not generally practiced. An
exception is when the transformer supplies a definite predictable load. External-fault back-up
protection may be considered by some a form of overload protection, but the pickup of such
relaying equipment is protection except for prolonged short usually too high to provide effective
transformer circuits. There remains, then, only the protection against short circuits in the
transformers or their connections, and external-fault back-up protection. Moreover, the practices are
the same whether the transformers are attended or not.
16
PROTECTION IN TRANSFORMER
Primary-Side Transformer Protection:-
Distribution substation transformers, covering a wide range There are many different
protection schemes used today for breaker-and-one-half schemes, to low-end flash bus and of
expense and complexity—from high-end ring bus and the continuity of service, more advanced
protective devices grounding switch schemes. Given the pressure to in crease are called for than a
motor-operated disconnect switch to protect each transformer with a local protective device. initiate
a fault. Today, the best practice is to individually Doing so eliminates the need to take off-line all
transformers. Transformer has experienced a fault . . . unnecessary connected to the transmission
line, when only one interruptions to service are avoided. Equipment event that a primary-side
protective device must interrupt.
But they may be difficult for some devices due the transient recovery voltage
(TRV). One might high frequency such as a 40-kA circuit breaker—would be able to handle a think
that a device with a robust fault interrupting rating—relatively low-magnitude fault on the
secondary-side of the transformer. But the secondary-fault interrupting rating of a device is
dependant on the device’s ability to withstand a fast-rise transient voltage—much faster than that
seen during high-current fault interrupting. Therefore, the device must be specifically tested to
determine its ability to with stand and interrupt fast TRVs. A device with a 40-kA primary-fault
17
interrupting rating may not necessarily be able to interrupt 4-kA secondary faults with a fast-rise
transient voltage.
Standards for testing a device for appropriate secondary-side fault interrupting
capabilities—which take into account high-frequency TRVs—are currently being reviewed. The
draft standard, PC37.016, “Standard for AC High Voltage Circuit Switchers rated 15kV through
245kV,” [1] provides the testing requirements to verify that a protective device can interrupt a fast
TRV. C37.06.1-1997, “Trial Use Guide for High-Voltage Circuit-Breakers Rated on a Symmetrical
Current Basis Designated Definite Purpose for Fast Transient Recovery Voltage Rise Times,” [2]
does so as well, although the main C37.06 standard, “AC High-Voltage Circuit Breakers Rated on a
Symmetrical Current Basis,” [3] breaker’s ability to interrupt faults with associated fast-rise does
not currently incorporate requirements for testing a TRV, or to require a secondary-side fault
interrupting rating. protection, including the author’s company, have Manufacturers of purposebuilt equipment for transformer equipment with a secondary-side fault interrupting rating.
traditionally tested for fast-transient TRVs and provide their Given the position of the primary-side
device and that most fault activity is on the secondary bus, this secondary-side interrupting rating is
critical for a primary-side protective device. Before discussing how to select a primary-side
protective device, let’s examine the possible locations of faults and the subsequent response of the
protective Equipment.
18
Power Transformer And Power Autotransformer,
The Choice Of Percentage-Differential Relaying For Short-Circuit Protection:It is the practice of manufacturers to recommend percentage-differential relaying
for short circuit protection of all power-transformer banks whose three-higher.1 A survey of a large
number of representative phase rating is 1000 KVA and power companies showed that a KVA
banks, but that they were practically unanimous in approving differential relaying minority favored
differential relaying for as low as 1000- higher.2 To apply these recommendations to power for
banks rated 5000 KVA and autotransformers, the foregoing ratings should be taken as the
“equivalent physical size” of autotransformer banks, where times [1 – (VL/VH)], and where VL the
equivalent physical size equals the rated capacity and VH are the voltage ratings on the low-voltage
and The report of an earlier survey3 included a high-voltage sides, respectively. recommendation
that circuit breakers be installed in the connections to all windings when banks in parallel. The
more recent report is not very clear on larger than 5000 KVA are connected this subject, but
nothing has transpired that would change the earlier recommendation. The protection of parallel
banks without separate breakers and the protection of a transmission line terminates without a highvoltage single bank in which a breaker will be considered later.
The differential relay should operate a hand-reset auxiliary that will trip all transformer
breakers. The hand-reset feature is to minimize the likelihood of a transformer breaker transformer
to further damage being reclosed inadvertently, thereby subjecting the unnecessarily. Where
transmission lines with high-speed distance relaying terminate on the same bus as a transformer
bank, the bank should have high speed relaying. Not only is this required for the same reason that
the lines require it, but also it distance relays “looking” toward the bus to be set lower permits the
second-zone time of the and still be selective.
19
CURRENT-TRANSFORMER CONNECTIONS FOR DIFFERENTIAL
RELAYS
A simple rule of thumb is that the CT’s on any way be connected in delta, and the
CT’s on any delta winding of a power transformer should winding should be connected in way.
This rule may be broken, but it rarely is; for the moment let us assume that it is inviolate. Later, we
shall learn the basis for this rule. The remaining problem is how to make the required
interconnection between the CT’s and the differential relay.
Connections must satisfy are:
(1) The Two basic requirements that the differential-relay differential relay must not operate for
load or external faults; and
(2) The relay must If one does not know what the proper connections are, operate for severe enough
internal faults.
Connections that will satisfy the requirement of not the procedure is first to make the
tripping for external faults. Then, one tripping for internal faults. can test the connections for their
ability to provide.
20
TRANSFORMER WITH PROTECTING A THREE-WINDING A TWOWINDING PERCENTAGE-DIFFERENTIAL RELAY:-
Unless there is a source of generation back of only one protect a three-winding side of a
power transformer, a two winding percentage-differential relay should not be used to relay is used, the
CT secondary’s on transformer. When a Two-winding Three-winding transformer is that, A further
advantage of a three-winding relay with a where relay types are involved having taps for matching the
CT secondary currents, it is often unnecessary to use any auxiliary CT’s. Thus, a with advantage where
a two-winding relay might suffice. There is no disadvantage, other than a slight increase in cost, in
using a three-winding relay on a two-winding transformer. No harm is done if one of the restraint
circuits is left unconnected.
21
SHORT-CIRCUIT PROTECTION WITH OVERCURRENT RELAYS:Over current relaying is used for fault protection of only when the cost of differential
relaying cannot be transformers having circuit breakers justified. Over current relaying cannot
begin to compare with differential relaying in sensitivity. Three CT’s, one in each phase, and at
least two Over current ground relay should be provided on each Over current phase relays and one
side of the transformer bank that is connected through a circuit breaker to a source of short-circuit
current. The over current pickup can be adjusted to somewhat relays should have an inverse-time
element whose above maximum rated load current, say about 150% of maximum, and with
sufficient time delay so as to be selective with the relaying equipment of adjacent system elements
during instantaneous element whose pickup can be external faults. The relays should also have an
made slightly higher than either the maximum short-circuit current for an external fault. it may be
necessary for at least some of the over current .
When the transformer bank is connected to more than or the magnetizing-current inrush.
one source of short-circuit current, obtain good protection as well as selectivity for external relays
to be directional in order to faults.
The over current relays for short-circuit protection of transformers provide also the externalfault back-up protection discussed elsewhere.
22
GROUNDING PROTECTIVE RELAY:-
On grounded-neutral systems, protection can be provided by insulating a
transformer tank from ground except for a connection to ground through a CT whose secondary
energizes an over current relay. Such an arrangement will give sensitive protection for arc-over to
the faults in the leads to the tank or to the core, but it will not respond to turn faults or transformer.
23
REMOTE TRIPPING
When a transmission line terminates in a single transformer bank, the practice is
frequently to omit the high-voltage breaker and there by avoid considerable expense. Such
practice is made possible by what is called “transferred tripping” or, preferably, “remote
tripping.’’16Remote tripping is the tripping of the circuit breaker at the line for faults in the power
transformer. The protective relays at that other end of the line transformer bank. Consequently, are
not sensitive enough to detect turn faults inside the transformer bank’s own differential-relaying
equipment trips the bank’s low-voltage breaker and initiates tripping of the breaker at the other end
of the line in one of two basic ways.
One way to cause the distant relays to operate and trip their breaker is to throw a short
circuit on the line at the high-voltage terminals of the power transformer.16,17 This is done by
arranging the transformer-differential relays to trip the latch of a spring-closed air break-type
disconnecting switch that grounds one or three end of the line; this is to protect switch is used if
there is automatic reclosing at the other the transformer against further damage by preventing the
reapplication of voltage to the station is attended, a single phase transformer. If automatic reclosing
is not used, and if the switch is sufficient. Method of remote tripping is the principal disadvantage
of the grounding-disconnect that it is relatively slow. To the closing time of the switch must be
added the operating time tripping time of the breaker there; this total time may amount to about a
half second or more, which is long for transformer protection. Of course, if a three-phase grounding
switch is used, the transformer is de-energized reclosing is used, the system is subjected to the
shock of one or more re-closings on a short circuit. It may be necessary to delay reclosing to be
sure first when high-voltage transformer-bushing flashovers that the grounding switch is closed
occur. That these disadvantages about half of the installations in this are not always too serious is
shown by the fact that country use this method. The other way to trip the distant breaker is with a
pilot.l6,l8 Any of the types of pilot (wire, on the circumstances. In any carrier-current, or
microwave) may be used, depending undesired tripping because of event, the equipment must be
free of the possibility of tripping signal that is not apt to be extraneous causes; this is achieved by
transmitting a duplicated otherwise. One of the most successful system;l8 not only is this system
most reliable but it is methods is the so-called “frequency-shift” also high speed, requiring only
about 3 cycles to energize the trip coil of the distant breaker relay has closed its tripping contacts.
By using two after the transformer-differential frequency-shift channels, the equipment can be
tested without removing it from service. An inherent advantage of remote tripping over a pilot is
that the received tripping signal can also block automatic reclosing. It may be necessary, however,
to delay reclosing a few cycles to be sure that reclosing is blocked when high-voltage transformerbushing flashovers occur.
24
PROTECTION OF PHASE-SHIFTING TYPE:Wherever possible, the phase-shifting type of regulating same manner as the inphase type. However, with transformer is protected in the conventional percentage-differential
relaying, a 10° phase shift is about all that can be tolerated; such a phase shift requires that relays in
two phases operate before the differential relays have about a 40% slope and that tripping is
permitted, in order not to trip undesirably for external faults. When phase shifts of more than about
10° are involved, special forms of relaying equipment are necessary. Certain modifications to
conventional differential relaying may sometimes be possible, but the basis for such modifications
is too complicated to consider more importance where over-all here. Gas-accumulator and pressure
relaying take on differential relaying is not completely adequate. Complete percentage-differential
protection can often be provided for way windings if CT’s of each winding,19 or differential
protection against are made available at both end ground faults only can be provided if CT’s at the
neutral ends are lacking. Over current relaying can protect against ground faults in a delta winding
connected to a grounded-neutral source.
EXTERNAL-FAULT BACK-UP PROTECTION:The external-fault back-up relays of the power transformer or circuit associated
with the protection. Regulating transformer will provide the necessary backup protection.
25
GROUNDING TRANSFORMERS
Two types of grounding transformer are in general use:
(1) The way-delta transformer, and
(2) The zigzag transformer.
The neutral of either type may be grounded directly or through current-limiting
impedance. It is assumed here that neither load nor a source of generation is connected to the delta
winding of the way-delta transformer and that the zigzag transformer does not have another
winding connected to load or generation; should either type have such connections, it would be
treated as an ordinary power transformer. The recommended way to protect either type of bank. For
external ground faults, only zero-phase-sequence currents flow through the primaries of the delta
connected CT’s. Therefore, current will flow only in the external-fault back-up over current
relay, and its time delay should be long enough to be selective with other relays that should operate
for external faults. The other three relays will provide protection for short circuits on the groundingtransformer side of the CT’s. These relays may be sensitive and quite fast because, except for
magnetizing current and small currents that may flow through the relays because of CT errors,
current will flow only when short circuits requiring tripping occur. The pickup of the over current
relays should be 25% to 50% of the grounding transformer’s continuous-current rating, and the
primary-current rating of the CT’s should be about the continuous-current rating of transformer .
26
HIGH VOLTAGE IN TRANSFORMER
In the insulating material is used for the long period, the insulation resistance.
Proper maintenance has to be carried material decomposes slowly so there is decreases in its life of
insulation. Steps should be taken so that the dust after studying the causes of deterioration so as to
increase the particles, oil or grease do not stick to the surface of insulation. Steps should also be
taken to prevent over voltage and maintain the temperature of the equipment within the limits.
Application
1) It is used to step up voltage for transmission of electrical power.
2) It is used to step up voltage from the Generating Station.
3) It is used to step down voltage for the distribution.
4) It is used to step down voltage for the Electronic circuit.
5) It is used in Circuit for the Impedance matching.
6) It is used in measuring equipment of temperature, pressure, Weight, etc. by Linearly
Voltage Differential Transformer.
7) It used in uninterrupted power supply for Voltage stabilizer.
8) It used for welding.
9) It is used for Heating.
10) It is used for Protection System.
(like CT, PT)
27
Transformer tap changer:A tap changer is a device fitted to power transformers for regulation of the output
voltage to required levels. This is normally achieved by changing the ratios of the transformers on
the system by altering the number of turns in one winding of the appropriate transformer/s. Supply
authorities are under obligation to their customers to maintain the supply voltage between certain
limits. Tap changers offer variable control to keep the supply voltage within these limits. About
96% of all power transformers today above 10MVA incorporate on load tap changers as a means of
voltage regulation.
Tap changers can be on load or off load. On load tap changers generally consist of a
diverter switch and a selector switch operating as a unit to effect transfer current from one voltage
tap to the next. It was more than 60 years ago on load tap changers were introduced to power
transformers as a means of on load voltage control.
28
Tap changers possess two fundamental features:
(a) Some form of impedance is present to prevent short circuiting of the tapped section,
(b) A duplicate circuit is provided so that the load current can be carried by one circuit whilst
switching is being carried out on the other.
The impedance mentioned above can either be resistive or reactive. The tap changer with a resistive
type of impedance uses high speed switching, whereas the reactive type uses slow moving
switching. High speed resistor switching is now the most popular method used worldwide, and
hence it is the method that is reviewed in this report.
The tapped portion of the winding may be located at one of the following locations, depending
upon the type of winding:
(a) At the line end of the winding;
(b) In the middle of the winding;
(c) At the star point.
The most common type of arrangements is the last two. This is because they give the
least electrical stress between the tap changer and earth; along with subjecting the tapings to less
physical and electrical stress from fault currents entering the line terminals. At lower voltages the
tap changer may be located at either the low voltage or high voltage windings.
Tap changers can be connected to the primary or secondary side windings of the transformer
depending on:
- Current rating of the transformer
- Insulation levels present
- Type of winding within the transformer (eg. Star, delta or autotransformer)
- Position of tap changer in the winding
- Losses associated with different tap changer configurations eg. Coarse tap or reverse winding
- Step voltage and circulating currents
-Cost
- Physical size
-on-load tap changer
29
Transformer protective relay:-
Protect and monitor most transformer applications with the powerful SEL-487E
Transformer Protection Relay. Apply up to 5 three-phase restraint inputs, three independent
restricted earth fault protection elements, and 2 three-phase voltage inputs, all with synchrophasors
. Limit transformer damage by responding to internal fault conditions in less than 1.5 cycles. Avoid
catastrophic transformer failure by detecting turn-to-turn faults involving as little as 2 percent of the
total winding. Minimize commissioning time and eliminate costly errors with the first relay
software that recommends matrix compensation settings. And, track transformer wear with
through-fault and thermal monitoring to reduce inefficient and costly breaker maintenance.
Synchrophasors:synchrophasors messages over serial or Ethernet communications to easily detect
reactive loop flows, turn state estimation into state measurement, and provide early warning of
potential system instability. Receive synchrophasor messages from up to two phasor
measurement units. Use built-in time correlation, and take control actions based on combined
local and remote messages. Apply control functions based on phase angles, currents, and
voltages for basic or advanced applications. Multicast synchrophasor data to simplify system
architecture and improve system operations. For applications requiring a dedicated phasor
measurement unit (PMU), choose the Station Phasor Measurement Unit.
30
Current Differential Protection With Two to Five Restraints:


Achieve fast, sensitive, dependable, and secure differential protection. The SEL-487E Relay
uses a two-stage slope that adapts to internal or external fault conditions automatically, even
with CT saturation and heavily distorted waveforms.
Additional Protection
Apply two 3-phase voltage inputs for over- and under voltage, frequency, and volts-per-hertz
protection. Make any over current element directional using voltage-polarized directional
elements as torque control inputs to the over current elements. Implement transformer and
feeder backup protection using adaptive time-over current (IDMT) elements with selectable
operating quantity, programmable pickup, and time delay settings. Apply three independent
restricted earth fault (REF) elements for sensitive ground fault detection in grounded wye
transformer applications.
Security and Dependability:
Provide maximum security during external faults and transformer magnetizing inrush
conditions. Detect internal faults quickly, during energization or normal operating conditions,
using combined harmonic blocking and restraint differential elements. Detect turn-to-turn
winding faults for as little as 2 percent of the total transformer winding with the negativesequence differential element.
Rugged Design:-
Trust the industry’s widest ambient operating temperature range, –40° to +85°C (–40° to
+185°F).
Application:
Protect large transformers with breaker-and-a-half high- and low-side
connections.
31





Configure the SEL-487E in a typical two-winding transformer application, and
use the remaining three-phase current inputs for feeder backup protection.
Save time, money, and improve power system quality with SEL
synchrophasors (IEEE C37.118) from all 24 analog channels simultaneously (6
voltage and 18 current sources) available in your SEL-487E transformer relay.
With synchrophasors over serial or Ethernet communications, you will easily
detect reactive loop flows, turn state estimation into state measurement, and
provide early warning of potential system instability.
Provide backup protection with phase-, negative-, and zero-sequence overcurrent
elements. Set up breaker failure protection with subsidence detection to rapidly
detect breaker failure and minimize system coordination times.
Apply over- and under voltage and frequency elements, along with volts-perhertz elements. Provide accurate transformer protection for off-frequency events
and over excitation conditions.
Configure the SEL-487E for transformer differential protection in transformer
applications using up to five restraint currents. This includes single transformers
with tertiary windings. Use three independent REF elements for protection of
grounded wye windings.
Current Differential Relay:-
The Current Differential/Over current Relay provides protection for any two-input
apparatus such as transformers, motors, generators, and reactors. Apply it for differential and over
current protection and use event reports for quick post-event analysis.
Two-Winding Current Differential Protection:Sensitive current differential protection with programmable single- or
dual-slope percentage restraint, supervised by the second- and fifth-harmonic elements for
secure differential protection on two-winding apparatus. Choose from common or independent
harmonic blocking or restraint. Unrestrained differential elements provide fast operation for
high-magnitude internal faults. Automatic calculations of relay tap settings simplify application.
32
Individual Winding Over current Protection:Individual winding phase, residual, and negative-sequence over current elements,
including instantaneous, definite-time, and inverse time-over current elements for
comprehensive over current protection.
Monitoring and Reporting:Oscillgraphic event reports (up to ten 15-cycle reports) and metering
functions eliminate or reduce external recorder and metering requirements.
Application:




Protect any two-winding power transformer.
Protect reactors, generators, large motors, and other two-terminal power apparatus.
Apply with any combination of delta- or wye-connected CT secondary circuits.
Use built-in winding over current elements for backup protection.
Detect ground faults through delta-wye transformer banks, or provide sensitive
phase-to-phase protection, independent of load current, with negative-sequence over
current element.
 Use low-side over current elements for backup distribution bus or feeder protection.
Feeder Protection Relay:-
The Feeder Protection Relay is the right solution for industrial and utility feeder protection,
with flexible I/O options, easy mounting, and fast settings. Provides complete feeder protection,
with over current, overvoltage, under voltage, and frequency elements. Easily upgrade protection
without cutting or drilling existing cut outs with a small form factor and multiple mounting
adapters. Quickly integrate into serial- or Ethernet-based communications and other protocols.
33

Complete Feeder Protection:Maximize control scheme flexibility using time- and instantaneous- over current,
over voltage, under voltage, and frequency elements with breaker failure protection for one
three-pole breaker.

Optional Protection Features:Use the SEL-751A with one of the voltage input options to provide over- and
under frequency, rate of change of frequency, over- and under voltage, synchronism check,
dc station battery monitoring, arc-flash detection, power and demand metering elements.

Convenient Controls:Use the four programmable pushbuttons on the front panel for quick, personalized
control.

Reclosing Control:Programmable four-shot re closer with optional synchronism check to match various
reclosing practices.

Rugged Design:Rely on the industry’s widest ambient temperature operating range, –40° to +85°C
(–40° to +185°F).

Automatic Notification:Alert key personnel to problems automatically with direct support.

Easy Installation:Easily install into existing locations using available retrofit kits and mounting
adapters, with no cutting or drilling.

Flexible Design:Choose from many installation and integration options with a small form factor and slide-in
expansion cards.
Application:


Customize front-panel pushbutton operation and LEDs, or use default breaker trip/close
functions.
Personalize LCD messages using event-driven display point settings.
Create an integrated control system with a variety of I/O and communications options.
34






Use the programmable control logic and integration features with a communications link for
control and protection of remote substations.
Use comprehensive reporting to understand events, schedule maintenance, detect unfavorable
trends, modify loads, and satisfy information requirements of supervisory systems.
Include RTD inputs as part of system integration or to bias protection.
Remediate arc-flash hazards with arc-flash detection.
Analyze over current protection system performance using built-in Sequential Events
Recorder (SER).
Install protection where it is needed without special enclosures or ventilation systems. Class
1, Division 2 certification allows the SEL-751A in locations that may be adjacent to
hazardous gasses, vapors, or liquids.
Earth fault protection:-
In a 11KV switchgear incomer the earth fault protection has been provided by using
the sumamtion of three out puts of the mounted current transformers in three phases.
Some times it is observed that during starting of a largest motor feeder in the switchgear the
protection mal faucntions causing outage of the switchgear.( Earth fault relay opeartion may
35
be due to saturation of Current transformers resulting unbalance current outputs in CT
secondary’s).
The current setting is 8% & cannot be increased further as the 11KV system is resistance
earthed.
The time setting also cannot be increased for the in comer from fault level point of view.
Now my question is whether is there any method available to make the above protection
reliable.
Buchholz Relay
A Buchholz Relay is also called a gas detection relay. It is a safety device generally
mounted at the middle of the pipe connecting the transformer tank to the conservator. A Buchholz
Relay may be used to detect both minor and major faults in the transformer.
This device functions by detecting the volume of gas produced in the transformer
tank. Minor faults produce gas that accumulates over time within the relay chamber. Once the
volume of gas produced exceeds a certain level, the float will lower and close the contact, setting
off an alarm.
36
Major faults can cause the sudden production of a large quantity of gas. In this case, the
abrupt rise in pressure within the tank will cause oil to flow into the conservator. Once this is
detected the float will lower to close the contact, which causes the circuit breaker to trip or sets off
the alarm.
In the field of electric power distribution and transmission, a Buchholz relay is a safety
device mounted on some oil-filled power transformers and reactors, equipped with an external
overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device
sensitive to the effects of dielectric failure inside the equipment.
Depending on the model, the relay has multiple methods to detect a failing transformer. On
a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of
insulating oil accumulates in the top of the relay and forces the oil level down. A float switch in the
relay is used to initiate an alarm signal. Depending on design, a second float may also serve to
detect slow oil leaks.
37
If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This
flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch
normally will operate a circuit breaker to isolate the apparatus before the fault causes additional
damage. Buchholz relays have a test port to allow the accumulated gas to be withdrawn for testing.
Flammable gas found in the relay indicates some internal fault such as overheating or arcing,
whereas air found in the relay may only indicate low oil level or a leak.
38
DO’S AND DONT’S
(1) DO’S FOR POWER TRANSFORMER:01. Check and thoroughly investigate the transformer whenever any alarm or protection operated.
02. Check air cell in conservator.
03. Attend the leakages on the bushing immediately.
04. Examine the bushings for dirt deposits and coats, and clean them periodically.
05. Check the oil in transformer and OLTC for dielectric strength and moisture content and take
suitable action for restoring the quality.
06. Check the oil level in oil cup and ensure air passages are free in the breather. If oil is less, make
up the oil.
07. Check the oil for acidity and sludge as per IS : 1866
08. If inspection covers are opened or any gasket joint tightened, then tighten the bolts evenly to
avoid uneven pressure.
09. Check and clean the relay and alarm contacts. Check also their operation , and accuracy and if
required change the setting.
10. Check the protection circuits periodically.
11. Check the pointers of all gauges for their free movement.
12. Clean the oil conservator thoroughly before erecting.
13. Check the buchholz relay and readjust the flats , switches etc.
14. Inspection the painting and if necessary retouching should be done.
15. Check the OTI and WTI pockets and replenish the oil if required.
16. Remove the air through vent plug of the diverter switch before you energies the transformer.
17. Check the oil level in the diverter switch and if found less, top up with fresh oil conforming to
IS: 335.
18. Check the gear box oil level, if less top up with specified oil.
19. Examine and replace the burnt or worn out contacts as per An…….Of maintenance schedule.
20. Check all bearings and operating mechanism and lubricate them as per schedule.
21. Open the equalizing valve between tank and OLTC wherever provided at the time of filling the
oil tank.
22. Connect gas cylinder with automatic regulator if transformer is to be stored for long, in order to
maintain positive pressure.
23. Fill the oil in the transformer at the earliest opportunity at site and follow storage instruction.
24. Check the door seals of marshalling box change the rubber lining if required.
25. Equalize the diverter compartment of the OLTC by connecting equalizing pipe between flange
joints provided on the tap changer head.
39
(2) DON’TS FOR POWER TRANSFORMER:-
01. Do not energies without thorough investigation of the transformer, whenever any alarm of
protection has operated.
02. Do not re-energies the transformer, unless the Buchholz gas in analyzed.
03. Do not re-energies the transformer or without conducting all pre-commissioning checks. The
results must be comparable with results at works.
04. Do not handle the off circuit tap switch when the transformer is energized.
05. Do not energies the transformer, unless the off-circuit tap switch handle is in locked position.
06. Do not leave –off circuit tap switch handle unlocked.
07. Do not leave tertiary terminals unprotected outside the tank connect them to tertiary lightning
arrestors protection scheme when connected to load.
08. Do not allow WTI/OTI temperature to exceed 55 0C during dry out of
transformer , and filter machine temperature beyond 600C
09. Do not parallel transformers which do not fulfill the condition.
10. Do not use low capacity lifting jack on transformer for jacking.
11. Do not move the transformer with bushings mounted.
12. Do not overload the transformer other than the specified limits as per
IS : 6600
13. Do not change the settings of WTI and OTI alarm and trip frequently. The setting should be
done as per the site condition.
14. Do not leave red pointer behind the black pointer in OTI and WTI.
15. Do not leave any connection loose
16. Do not meddle with the protection circuits
17. Do not allow conservator oil level to fall below ¼ level
18. Do not allow oil level to fall in the bushings, they must immediately to be topped up.
19. Do not leave marshalling box doors open, they must be locked.
20. Do not switch off the heater in marshalling box except to be periodically cleaned.
21. Do not allow dirt and deposits on bushings, they should be periodically cleaned.
22. Do not allow unauthorized entry near the transformer
23. Do not leave ladder unlocked, when the transformer is “ON” in service, in case it is provided.
24. Do not change the sequence of valve opening for taking stand by pump and motor into circuit.
25. Do not switch on water pump unless oil pump is switched on.
26. Do not allow water pressure more than oil pressure in differential pressure gauge.
27. Do not mix the oil, unless it conforms fully to IS :335
28. Do not allow inferior oil to continue in transformer. The oil should be immediately processed
and to be used only when BDV/ppm conforms to 1S :1866.
29. Do not continue with pink silica gel, this should immediately be changed or regenerated.
30. Do not leave secondary terminal of an unloaded CT Open.
31. Do not store transformer for long after reaching site, commission at
the earliest.
32. Do not keep the transformer gas filled at site for a longer period
40
POWER TRANSFORMER CHECK POINTS
Following points should be check for a transformer before Commissioning / charging
after Overhauling, Filtration or annual maintenance.
(1) Perform all electrical test on winding. Like voltage ratio, short circuit, magnetizing current etc.
and confirm that this OK.
(2) IR value measurement of Transformer.
(3) Check BDV of oil of main tank and OLTC.
(4) Check oil level in main tank & OLTC tank.
(5) Oil level in bushing.
(6) Check tightness of tan-delta cap of bushing.
(7) Check condition of silica & oil cup of breather .
(8) Check all protection of Transformer like buchhloz, PRV OSR, WTI & OTI.
(9) Put all bushing CT, either in service or short secondary winding if not in used.
(10) Check position of all valves for their correct opening and closing sequence.
(11) Confirm correct direction of cooling pump and fan.
(12) Check arc horn gap.
(13) Check diaphragm of exploration vent.
(14) Measure earth resistance of X’mer neutral & body earth pit.
(15) Check tightness of all external connection.
(16) Check oil in pot of WTI & OTI.
(17) Check earthing of neutral, and body earth.
(18) Release air form all plugs, wherever necessary.
(19) Check oil leakage for 24 hrs.
41
Bibliography
[1] IEEE Guide for Loading Mineral-Oil Immersed Power Transformers, IEEE Standard
C57.91, Institute of Electrical and Electronic Engineers, New York NY, 1995.
[2] L. L. Grigsby, editor, The Electric Power Engineering Handbook, CRC Press, Boca Raton,
FL, 2001.
[3] Adaptive Transformer Thermal Overload Protection, Final Report of IEEE Power System
Relaying Committee Working Group K3, IEEE Power Engineering Society Power System
Relay Committee Report, January, 1999.
[4] IEEE Guide for Protective Relay Applications to Power Transformers, IEEE Standard
C57.91, Institute of Electrical and Electronic Engineers, New York NY, 2000.
[5] G. Swift, D. Fedirchuk, Z. Zhang, A New Relaying Principle for Transformer Overload
Protection, 52nd Annual Georgia Tech Protective Relaying Conference, May 6-8, 1998.
[6] G. Swift, T. S. Molinski, W. Lehn, A Fundamental Approach to Transformer Thermal
Modeling – Part I: Theory and Equivalent Circuit, IEEE Transactions On Power Delivery,
Vol. 16, No. 2, April 2001, pp. 171 – 175.
[7] G. Swift, T. S. Molinski, R. Bray, R. Menzies, A Fundamental Approach to Transformer
Thermal Modeling – Part II: Field Verification, IEEE Transactions On Power Delivery, Vol.
16, No. 2, April 2001, pp. 171 – 180.
[8] SIPROTEC 7UT612 Differential Protection Relay Instruction Manual, Siemens AG,
Nuremburg, Germany, 2002.
[9] T-PRO Transformer Protection Relay User Manual Version 3.3 Rev 1, NxtPhase T&D
Corporation, Vancouver, BC, 2003.
[10] Functional Diagramming of Instrument and Control Systems, The Measurement, Control &
Automation Association, Williamsburg, VA, 1981.
Special thanks to Wayne Hartmann as the original author of some of the drawings.
42