Download Chapter 6: Relay test and commisioning

TESTING AND COMMISIONING
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CHAPTER 6
PROTECTIVE RELAY
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6.1 INTRODUCTION
-Power systems and their components need protection from natural hazards as
well as human error.
-Lightning, wind, ice, switching surges, resonance, trees, animals and humans
are some of the causes of faults.
-These faults produce overcurrents and/or overvoltages at various locations in
a power system and must be cleared before they cause before they damage
any machines, transformers, lines etc.
-This is generally accomplished by isolating the faulted portion (as small a
portion as possible) of the system so that the remainder of the system can
serve without interruption.
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-continue:
-In low-voltage distribution systems, lightning (surge) arresters are used for
overvoltage protection and fuses and slow-acting circuit breakers are employed
for overcurrent protection.
-In high-voltage transmission systems reliable sensors, fast-acting relays and
circuit breakers are needed to clear the fault quickly so that the stability of the
remaining system is secured.
- In short, the protection, stability and security of a power system are affected by
the ability of the protection devices to detect and respond to system
abnormalities like overvoltages and overcurrents
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6.1 Protection Components
Protection systems have three basic components:
-Sensors (transducers, detectors) to detect system abnormalities
-Relays (activators) to provide signals to activate the protection
devices.
-Circuit breakers (interrupters) to open (disconnect) the circuits
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Figure 6.0
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Continue:
In order to perform its functions properly, the protection system must
have the following characteristic:
a) Reliability: The reliability of the protection system is its ability to
operate upon the occurrence of any fault for which it was designed
to protect. In other words, the protection system should operate
when it is supposed to and not operate when it is not required.
b) Selectivity: Selectivity is the ability of the protection system to
detect a fault, identify the point at which the fault occurred and
isolate the faulted circuit element by tripping the minimum number
of circuit breakers. Selectivity of the protection system is obtained
by proper coordination of the operating currents and time delays of
the protective relays.
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6.2 CLASSIFICATIONS OF RELAYS
Relays may be classified according to the technology
used:
a. electromechanical
b. static
c. digital
d. numerical
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Continue:
c) Speed: The speed of the protection system refers to the operating
times of the protective relay. The potential damage to the faulted
element depends on the length of time the short-circuit currents are
allowed to flow. The speed of clearing or isolating the faulted system
component also affects the stability of the whole system.
d) Sensitivity: Sensitivity refers to the characteristic of a protective
relay that it operates reliably, when required, in response to a fault that
produces the minimum short-circuit current flowing through the relay.
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6.2.1 ELECTROMECHANICAL RELAYS
Electromechanical relays can be classified into several
different types as follows:
a. attracted armature – comprises of iron cored electromagnet which attracts
an armature which is pivoted, hinged or supported.
b. moving coil- light coil which can be energised moves in a strong permanent
magnet field.
c. Induction – operated on the same principle as the induction motor
d. Thermal – operates based on the generated heats in a resistance winding
and temperature sensitive component
e. Timing relays- are used in conjunction with protection relays
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6.2.2 Static Relays
-The term ‘static’ implies that the relay has no moving parts.
-In a protection relay, the term ‘static’ refers to the absence of
moving parts to create the relay characteristic.
-Their design is based on the use of analogue electronic devices
instead of coils and magnets to create the relay characteristic.
-Early versions used discrete devices such as transistors and
diodes in conjunction with resistors,capacitors, inductors,
- Advance versions enabled the use of linear and digital integrated
circuits in later versions for signal processing and implementation of
logic functions.
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6.2.3 Digital Relays
-Digital protection relays introduced a step change in technology.
Microprocessors and microcontrollers replaced analogue circuits used
in static relays to implement relay functions.
-Compared to static relays, digital relays introduce A/D conversion of
all measured analogue quantities and use a microprocessor to
implement the protection algorithm.
-The microprocessor may use some kind of counting technique, or use
the Discrete Fourier Transform (DFT) to implement the algorithm.
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6.2.4 Numerical Relays
-Typically, they use a specialised digital signal processor (DSP) as the
computational hardware, together with the associated software tools.
- The input analogue signals are converted into a digital representation
and processed according to the appropriate mathematical algorithm.
- Processing is carried out using a specialised microprocessor that is
optimised for signal processing applications, known as a digital signal
processor
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6.3 Zone of Protection
-To limit the extent of the power system that is disconnected when a
fault occurs, protection is arranged in zones.(Figure 6.1).
- Ideally, the zones of protection should overlap, so that no part of the
power system is left unprotected. (Figure 6.2).
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Figure 6.1
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Figure 6.2
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6.4 Protection principles
The best and common protection techniques can be classified into
2 categories
a) Overcurrent/earth fault-distance protection
b) Differential protection
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6.4.1 Overcurrent and Earth-fault Protection
-Protection against excess current was naturally the earliest protection
system to evolve.
- The actuating quantity of an overcurrent relay is a current.
- The relay is designed to operate when the actuating quantity equals
or exceeds its pickup value.
- An overcurrent relay can either be an instantaneous type or a timedelay type.
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6.4.1.1 IDMTL relay
-Most commonly used type of relay.
-Characteristics of relay is the higher the current, the shorter the
operating time.
-The current/time tripping characteristics of IDMT relays may need to
be varied according to the tripping time required and the
characteristics of other protection devices used in the network.
-IEC 60255 defines a number of standard characteristics as follows:
a) Standard Inverse (SI)
b) Very Inverse (VI)
c) Extremely Inverse (EI)
d) Definite Time (DT)
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The tripping
characteristics for
different TMS settings
using the SI curve
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6.4.1.2 Setting of IDMTL relays
-In order to adjust the current setting, the relay coil is arranged to have
a tapped winding which is connected to a plug bridge.
(electromechanical relay)
-In modern relays, it is known as PLUG SETTING MULTIPLIER.
-Example: if Tap is set at 80% of 5A, then the current into the relay is
0.8 x 5A = 4A.
-TIME MULTIPLIER SETTING (TMS) the time the relay disc to move
through 180 degrees or relay operates to trip
- The setting is in term of percentage such as 10 %(0.1), 20%(0.2),
30% (0.3) etc.
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Continue:
Tripping time of standard overcurrent relay:
Where: TMS = time multiplier setting
Ifault
I (
)
r
Iprimary
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Example:
Calculate tripping time of an overcurrent relay at 150% with the
The PSM setting is at 80%. CT ratio connected to the CT is 1000/5A
The TMS setting is 0.1
Solutions:
Time,
0.14
t  0.1x
(1200 )0.02  1
800
t = 1.72 s
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6.5 RELAY TESTING AND COMMISIONING
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Continue:
Tripping time of standard overcurrent relay:
Where: TMS = time multiplier setting
Ifault
I (
)
r
Iprimary
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Continue:
Tripping time of standard overcurrent relay:
Where: TMS = time multiplier setting
Ifault
I (
)
r
Iprimary