RELAYS
• Write down the relay numbers and their designation.
1 MASTER ELEMENT 51 AC TIME OVER CURRENT RELAY
2 TIME DELAY STARTING OR CLEARING 52 AC CIRCUIT BREAKER
3 CHECK OR INTERPOSING RELAY 53 EXCITER OR DC GENERATOR
4 MASTER CONTACTOR 54 SPARE
5 STOPPING DEVICE 55 POWER FACTOR RELAY
6 STARTING CIRCUIT BREAKER 56 FIELD APPLICATION RELAY
7 ANODE CIRCUIT BREAKER 57 SHORT CIRCUITING DEVICE
8 CONTROL POWER DISCONNECT DEVICE 58 RECTIFICATION FAILURE RELAY
9 REVERSING DEVICE 59 OVER VOLTAGE RELAY
10 UNIT SEQUENCE RELAY 60 VOLTAGE OR CURRENT BALANCE RELAY
11 SPARE 61 SPARE
12 OVER SPEED RELAY 62 TIME DELAY STOPPING OR OPENING DEVICE
13 SYNCHRONISING SPEED DEVICE 63 LIQUID OR GAS OR VACCUM RELAY
14 UNDER SPEED DEVICE 64 GROUND PROTECTION RELAY
15 SPEED OR FREQUENCY MATCHING DEVICE 65 GOVERNOR
16 SPARE 66 NOTCHING OR JOGGING RELAY
17 SHUNTING OR DISCHARGE SWITCH 67 AC DIRECTIONAL OVER CURRENT RELAY
18 ACCELERATING OR DE-ACCELERATING DEVICE 68 BLOCKING RELAY
19 STARTING OR RUNNING TRANSITION DEVICE 69 PERMISSIVE CONTACT DEVICE
20 ELECTRICALLY OPERATED VALVE 70 RHEOSTAT, ELECTRICALLY OPERATED
21 DISTANCE PROTECTION RELAY 71 LIQUID OR GAS LEVEL RELAY
22 EQUALIZER CIRCUIT BREAKER 72 DC CIRCUIT BREAKER
23 TEMPERATURE CONTROL DEVICE 73 LOAD RESISTOR CONTACTOR
24 SPARE 74 ALARM RELAY
25 SYNCHRONISING DEVICE 75 POSITION MECHANISM
26 APPARATUS THERMAL DEVICE 76 DC OVER CURRENT RELAY
27 UNDER VOLTAGE RELAY 77 PULSE TRANSMITTER
28 FLAME DETECTOR 78 PHASE ANGLE OR OUT OF STEP RELAY
29 ISOLATING CONTACTOR 79 AC RECLOSING RELAY
30 ANNUNCIATER RELAY 80 SUPPLY FAIL
31 SEPARATE EXCITATION DEVICE 81 FREQUENCY RELAY
32 DIRECTIONAL POWER RELAY 82 DC RECLOSING RELAY
33 POSITION SWITCH 83 AUTOMATIC SELECTION
34 MASTER SEQUENCE DEVICE 84 OPERATING MECHANISM
35 SLIP RING SHORT CIRCUIT DEVICE 85 CARRIER OR PILOT WIRE RECEIVER RELAY
36 POLARITY OR POLARIZING VOLTAGE DEVICE 86 LOCK OUT RELAY
37 UNDER CURRENT OR UNDER POWER RELAY 87 DIFFERENTIAL PROTECTION RELAY
38 BEARING PROTECTIVE DEVICE 88 AUXILIARY MOTOR OR MOTOR GENERATOR
39 MECHANICAL CONDITION MONITOR 89 LINE SWITCH
40 FIELD RELAY 90 REGULATING DEVICE
41 FIELD CIRCUIT BREAKER 91 VOLTAGE DIRECTIONAL RELAY
42 RUNNING CIRCUIT BREAKER 92 VOLTAGE & POWER DIRECTIONAL RELAY
43 MANUAL TRANSFER OR SELECTOR DEVICE 93 FIELD CHANGING RELAY
44 UNIT SEQUENCE STARTING RELAY 94 TRIPPING OR TRIP FREE RELAY
45 ATMOSPHERIC CONDITION MONITOR 95 SUPERVISION RELAY
46 CURRENT UNBALANCE RELAY 96 SPECIAL APPLICATION
47 POLE DISCREPANCY 97 FUSE FAIL RELAY
48 INCOMPLETE SEQUENCE RELAY 98 SPECIAL APPLICATION
49 THERMAL OVER LOAD RELAY 99 OVER FLUXING RELAY
50 INSTANTANEOUS OVER CURRENT RELAY 100 SPECIAL APPLICATION
General Description of Relays
NOMENCLATURE FOR ENGLISH ELECTRIC RELAY
FIRST LETTER – OPERATING QUANTITY
A- PHASE ANGLE COMPARISON SECOND LETTER – MOVEMENT
B- BALANCED CURRENT A ATTRACTED ARMATURE
C- CURRENT B BUCHHOLZ
D -DIFFERENTIAL C INDUCTION CUP
E- DIRECTION D INDUCTION DISC
F- FREQUENCY G GALVANOMETER (MOVING COIL)
I- DIRECTIONAL CURRENT T TRANSISTOR
K- RATE OF RISE OF CURRENT
N- MANUAL
O- OIL PRESSURE
P- POLY PHASE VA
R- REACTIVE VA
S- SLIP FREQUENCY
T- TEMPERATURE
V- POTENTIAL
W- WATTS (POWER)
Y- ADMITTANCE
Z- IMPEDANCE
THIRD LETTER – APPLICATION
A- AUXILIARY R RE CLOSING
B- TESTING S SYNCHRONISING
C- CARRIER (COUNTING) T TIMER OR TRANSFORMER
D- DIRECTIONAL U DEFINITE TIME
E- EARTH (GROUND) V VOLTAGE TIME
F- FLAG & ALARM INDICATOR W PILOT WIRE
G- GENERAL OR GENERATOR WA INTERPOSING
H- HARMONIC RESTRAINT WJ INTER TRIPPING
I- INTERLOCK OR INDUSTRIAL X SUPERVISORY
J- TRIPPING Y FLASH BACK (BACK FIRE)
JE- TRIPPING (ELECT. RESET) Z SPECIAL APPLICATION
JH- TRIPPING (HAND RESET) ZS ZERO SEQUENCE
JS- TRIPPING (SELF RESET)
JC- CONTROL
K- CHECK ALARM
L- LIMITING
M- SEMAPHORE OR MOTOR
N- NEGATIVE SEQUENCE
O- OUT OF STEP
P- POTENTIAL FAILURE
Q- ALARM
FOURTH LETTER
M – SPECIAL VARIATION
Sl. No. E E Relay Application
1 CTM Motor protection
2 CTU Locked rotor. Thermal alarm
3 CDG I.D.M.T. over current or earth fault of transformer
4 CAG Instantaneous over current or earth faults.
5 VAGM Under voltage
6 WDG Under /Over power for DG set
7 FTG Under frequency
8 VAPM Fuse failure
• What is Knee point voltage?
EMF applied to secondary of current transformer (CT) which, when increased by
10% voltage causes the excitation current to increase by 50%.
• What is I.D.M.T?
Inverse time relay with definite minimum time is called IDMT.
• What is Negative sequence reactance?
Negative sequence can arise whenever there is any unbalance present in the system.
Their effect is to setup a field rotating in opposite direction to the main field.
• What is Zero sequence reactance?
If a machine is operating with an earthed neutral, a system earth fault will give rise to
zero sequence current in the machine.
• Purpose of over current relay (Inverse); type- CDG
It is a self powered inverse time over current and earth fault relay, used for selective
phase and earth fault protection in time graded systems for A.C. machines,
transformers, feeders etc. A non-directional heavily damped induction disc relay,
which has an adjustable inverse time/current characteristic with a definite minimum
time. The relay has a high torque movement combined with low burden and low
overshoot. The relay disc is so shaped that as it rotates the driving torque increases
and offsets the changing restraining torque of the control spring.
• Purpose of Directional inverse Over current & earth fault relay; type- CDD
Directional phase or earth fault protection of ring-mains, parallel transformers,
transformer feeders, parallel feeders etc., employing the time graded principle.; This
relay comprises an inductive disc over current unit with wound shading coils and a
directional high speed induction cup unit. The cup-unit contact is wired across the
shading coils so that no torque is exerted on the disc of the over current unit until the
cup unit contact closes. The inductive disc unit is thus directionally controlled and it
operates only when the current flows in the tripping direction. The directional unit is
a high speed, low inertia four pole induction cup movement designed to give a high,
steady and non-vibrating torque. its current coil is connected in series with the
operating coil of the induction disc unit. The directional unit is normally provided
with voltage polarising coils.
• Purpose of Over current & earth fault relay; type- CAG
This relays are designed for instantaneous phase or earth fault protection and
instantaneous high set over current protection.; A standard hinged-armature unit
forms the basic movement for this relay. It consists an operating coil mounted on a
cylindrical iron core bolted to a frame at one end. This frame extends along the side
of the coil, with its end forming a knife-edge on which the armature is pivotally
mounted. The armature is 'L' shaped and pivoted at its corner so that one arm can be
attracted to the end of the core while the other arm to operate a set of contacts.
• Purpose of Local breaker back-up relay; type -CTIG
To safe guard against the drastic consequences of failure to clear faults rapidly, many
power supply authorities install 2 independent systems of protection for major
transmission lines. There remains however the possibility of the circuit breaker itself
failing to operate and this hazard is traditionally covered by remote breaker back-up.;
CTIG relay is a 3 phase or 2 phase and earth fault instantaneous over current unit
intended for use with a time delay to give back-up protection in the event of a circuit
breaker failure. A particular feature of the CTIG relay is a fast reset, which enables
the time delay to be set closer to the breaker trip-time.
• Purpose of Battery earth fault relay; type- CAEM-21
The battery earth fault relay is used to detect earth faults and deterioration of wiring
insulation in either pole of battery. The scheme consists of a centre tapped resistor, a
measuring relay, plug setting bridge, auxiliary relay and rectifier bridge to provide
unidirectional supply to the measuring relay coil. For different battery voltages
different values of centre tapped resistors are used. Variable sensitivities are
provided by means of the tapped coil whose taps are connected to the plug setting
bridge. The centre tap of resistor is brought to one terminals of the relay and this
terminal is either directly earthed or earthed through a centre zero milli
ammeter. Under healthy condition no current flows through the measuring relay coil
and in any pole of the battery or wiring insulation failure, current flows through the
measuring relay coil and the relay operates.
• Purpose of Rotor earth fault relay (type- CAEM-33)
When a single E/F is detected in the DC field circuit of a machine, the machine has
to be taken out of service at the first opportunity. This is because, if allowed to run
with an E/F on the rotor, a subsequent second E/F can cause severe damage to the
machine. However, a relay like CAEM-33 which can detect such a second E/F and
trip out the machine can make it possible to run the machine even with a single E/F,
without any such risks, thus helping to preserve the generation capacity. The start of
the second rotor earth fault detection scheme is a very sensitivity transductor
element. The AC winding of the transductor is connected in series with a rectified
AC voltage relay. The Dc winding of the transductor on the other hand is connected
in series with the rotor E/F circuit. Under normal conditions- i.e. with no DC
flowing, the AC wining of the transductor presents a high impedance, and the AC
voltage applied is mostly dropped across this winding. Hence the relay remains deenergised.
When a second rotor E/F occurs, a DC current flows through the
transductor dc winding which causes the impedance of the AC winding to reduce
considerably by driving the transductor core into saturation. Hence, the applied
voltage is fully available across the relay and it operates.
• Purpose of Sensitive earth fault relay (type - CTUM-15 & CTIGM-15
It may not be always possible to detect high resistance faults by convectional earth
fault relaying. In such cases a very sensitive current relay will be required for this
purpose. It can be connected residually since it has an adjustable definite time delay
provided to take care of transient spills in the residual circuit due to CT mismatch.
Also, its low burden enables it to be used with existing CT's/ Relays without
affecting the performance.; The incoming current is stepped down by an internal
current transformer and converted to a voltage by a variable resistor network. The
signal is compared with an internal reference. When this reference level is exceeded,
a time delay is initiated, after the time delay has elapsed, a relay operates.
• Purpose of Negative phase sequence current relay; Type- CTN/CTNM
Negative phase sequence current in the stator of a generator, caused due to
unbalanced loads or faults, it induces double frequency eddy current in the rotor.
These currents, if allowed to persist, can cause serious overheating and the purpose
of this relay is to disconnect the machine before such excess temperature is reached.
The inputs from the current transformers, which are connected in each phase of the
generator supply, are fed to a negative sequence filter which gives an AC output
voltage proportional to the negative sequence current. This voltage is rectified and
smoothed and fed into the squaring circuit of the main measuring element, the
definite time delay circuit and the alarm element. The output from the squaring
circuit is proportional to the square of the input voltage and is applied directly to the
main timing circuit to give the required relationship between I2
2t and the relay
operates time t.
• Purpose of definite time Over current & earth fault relay: Type-CTU
This relay can be used for definite time over current protection against phase and
Earth faults on medium and low voltage distribution systems. The definite time relay
offers a considerable advantage over inverse time relays in instances where there ia a
wide variation in line impedance. Another application is in the field of stalling
protection of motors. When the thermal overload relay does not provide protection
against stalling, separate definite time O/C relay type CTU can be used to provide
the same. This relay comes in following nomenclature: CTU-12/22/32/52/62/15.
CTU relay combines the advantage of complete static measurements with
characteristic of the robust, well proved attracted armature unit. These relays
measure current and time accurately, imposes low burden on CT's. Each phase
comprises a static overload detector and timer, which is accurate over a 10:1 time
setting range. When the positive peak of the input signal exceeds the reference level,
the time delay circuit starts and after a preset time, drives the output relay.
Instantaneous high set unit when fitted uses alternate half cycle for measurement and
through a separate level detector drives a separate output relay.
• Purpose of Motor protection relay: Type- CTMM/CTMFM
This relay contains all the protection factors to protect the motor, from Thermal
overload (Ith), Instantaneous over current (I1), Instantaneous or time delayed
unbalance element, Earth fault Element (I0) & Stalling protection (I1(t))
• Purpose of Overfluxing Relay: Type-GTTM
Transformers need protection against the risk of damage, which may be caused when
the transformers are operated at flux density levels significantly greater than the
design values. The overfluxing withstand time is generally found to be varying
inversely with the working flux density in the core, having higher withstand times
during extreme overfluxing conditions.
The overfluxing condition can occur during system over voltage or under frequency
conditions.
The basic operating principle is to produce an ac voltage, which depends upon the
ratio between AC input voltage and the frequency. The AC input voltage is fed to a
step-down transformer, which also provides isolation and the stepped down voltage
is fed to a V/F ratio detector circuit. This circuit is a simple operational amplifier
integrator with the provision for V/F pickup adjustment. The AC voltage is rectified
by true RMS. to dc converter. This circuit gives a frequency output and this
frequency increases rapidly with the increase in voltage. The frequency output is
given to a curve shaping circuit, which involves counter and comparators. The
counter counts the frequency output and the number of counts required for final trip
condition is changed by the comparator circuits to get the required timing
characteristic.
• Purpose of Biased Differential Relay: Type-MBCH
This relay is suitable for protection of two or three winding power transformers, auto
transformers or generators transformer units.
The differential transformer protection measuring circuit is based on the well-known
Merz-price circulating principle.
• Purpose of Digital frequency relay: Type-MFVUM
This relay is used to monitor the frequency of an electrical system. The relay are
suitable for any application in industrial plants and to generators where definite time
under or over frequency protection is required.
The operating principle of the relay is the comparison of the time interval of the
incoming frequency with that of a preset time derived from an accurate oscillator
within the relay. The incoming frequency signal is connected to an INPUT
CIRCUIT, which then drives an IMPULSE GENERATOR to produce a pulse at the
beginning of each period of the input voltage. The preset time interval is obtained
from an OSCILLATOR and COUNTER and adjustment is achieved using
SELECTOR switches, which drive a DECODER circuit. A COMPARATOR
compares the two-time interval and this triggers an adjustable TIMER which then
operates the output relay and latched light emitting diode (LED) glows.
• Purpose of Stator Earthfault Relay: Type-PVMM
A 100% stator earthfault protection is designed to detect earthfault occurring in the
regions of machine winding close to the neutral end. This relay is a composite
modular relay that gives 100% stator earthfault protection for machines, whose
neutral are not directly earthed. It works on the principle involving monitoring of the
neutral side and line-side components of the third harmonic voltages produced by
AC generators in service.
• Purpose of Voltage regulating relay and line drop compensator: Type-VTJCM & CIJC.
This relay is used with on load transformer tap changers and induction regulators to
provide close and accurate automatic voltage regulation on power systems of any
voltage.
When the regulated voltage moves outside a dead band, set by the sensitivity control,
the volts high or volts low circuits are initiated and after a time delay, determined by
the response characteristic, the appropriate tap changer control auxiliary relay closes
its contact to initiate a tap change.
• Purpose of Directional power relay: Type-MWTU.
This relay setting ranges from 0.25% to 18.56% of rated power. This makes the relay
suitable for sensitive reverse power applications. For example with turbo-generator,
where the detection of 1% or 2% reverse power is necessary to prevent the
synchronous machine from motoring in the event of the power from the prime mover
becoming too low. It is also suitable for low forward power interlock and under
power protection.
• Purpose of Check synchronising relay: Type-SKD/SKE.
This relay is used to prevent interconnection of badly synchronised supplies. Type
SKD relay are used for auto reclosing sequence, type SKE relay are used to
safeguard manual synchronising of generators. Phase measurement is achieved by
algebraically subtracting the 2 supply voltage waveforms and comparing the
resultant modulated beat waveform envelope with a Dc reference voltage. The DC
reference is proportional to the sum of the peaks of the 2 supply voltages to provide
phase measurement independent of supply voltage variation.
• Purpose of Static distance protection: Type-SHPM.
This relay (QUADRAMHO) is a static distance protection relay specially designed
for comprehensive high-speed protection of HV & EHV distribution/transmission
lines. 3 zones of protection are included, each employing separate measuring
elements, one element each for 3 phase-to-phase and 3 phase-to-earth faults. Thus a
total; of 18 elements are provided thereby increasing the reliability of the protection.
Poly phase measuring elements are not used in QUADRAMHO as in some of the
contemporary schemes. The relay is suitable for both three poles & single-and-threepole
tripping of the circuit breaker.
• Purpose of Static offset MHO relay: Type- YTGM.
This relay is a static single phase, single step, and distance protection with MHO
offset MHO characteristic. With suitable current/voltage input selection, the relay
can be made to have the required characteristic in the R-X plane for various
applications such as Generator Field failure protection, Generator backup impedance
protection and as offset MHO relay for use in conjunction with generator pole
slipping protection.
• Purpose of sensitive power relay: Type-WCD.
This power relay is a sensitive Poly phase induction cup unit, providing under power,
reverse power and over power protection. This relay detects a reversal of current
flow, caused by insufficient driving power from the prime mover, preventing the
generator operating as a synchronous motor. The electrical quantities energize
windings on an eight pole laminated stator. The moving contact is operated by a cup
shaped Aluminium rotor, which turns on jewelled bearings in an air gap between the
stator and a fixed center core. Only a small arc of rotation is needed to cause contact
closer. Low rotor inertia and very high driving torque ensures a high speed
operations.
• Purpose of pole slipping relay: Type-ZTO.
This pole slipping relay has been designed to protect synchronous Generators against
the possibility of the machine running in the unstable region of the power angle
curve which would result in power oscillations and pole slip. The relay consists
basically of one directional relay and one blinder relay operating in conjunction with
a 40-80 milli seconds static timer. Intended primarily for installation between the
generator and associated transformer (preferably on the generator terminals)
• Purpose of fuse failure relay: Type - VAPM
This relay detects a failure or inadvertent removal of voltage transformer secondary
fuses and prevention of incorrect tripping of circuit breakers. This relay consists of a
rectified AC voltage operated hinged armature unit. Three coils for the three phases
are wound over a single core producing in effect a common relay for the three
phases. Each coil is connected across one of the voltage transformer secondary fuses
and under healthy conditions, this coil is short circuited by the fuse and cannot be
energized. When one or more fuses or are removed the appropriate coil(s) is
energized under relay operates immediately to open the trip circuit.
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• Write down the relay numbers and their designation.
1 MASTER ELEMENT 51 AC TIME OVER CURRENT RELAY
2 TIME DELAY STARTING OR CLEARING 52 AC CIRCUIT BREAKER
3 CHECK OR INTERPOSING RELAY 53 EXCITER OR DC GENERATOR
4 MASTER CONTACTOR 54 SPARE
5 STOPPING DEVICE 55 POWER FACTOR RELAY
6 STARTING CIRCUIT BREAKER 56 FIELD APPLICATION RELAY
7 ANODE CIRCUIT BREAKER 57 SHORT CIRCUITING DEVICE
8 CONTROL POWER DISCONNECT DEVICE 58 RECTIFICATION FAILURE RELAY
9 REVERSING DEVICE 59 OVER VOLTAGE RELAY
10 UNIT SEQUENCE RELAY 60 VOLTAGE OR CURRENT BALANCE RELAY
11 SPARE 61 SPARE
12 OVER SPEED RELAY 62 TIME DELAY STOPPING OR OPENING DEVICE
13 SYNCHRONISING SPEED DEVICE 63 LIQUID OR GAS OR VACCUM RELAY
14 UNDER SPEED DEVICE 64 GROUND PROTECTION RELAY
15 SPEED OR FREQUENCY MATCHING DEVICE 65 GOVERNOR
16 SPARE 66 NOTCHING OR JOGGING RELAY
17 SHUNTING OR DISCHARGE SWITCH 67 AC DIRECTIONAL OVER CURRENT RELAY
18 ACCELERATING OR DE-ACCELERATING DEVICE 68 BLOCKING RELAY
19 STARTING OR RUNNING TRANSITION DEVICE 69 PERMISSIVE CONTACT DEVICE
20 ELECTRICALLY OPERATED VALVE 70 RHEOSTAT, ELECTRICALLY OPERATED
21 DISTANCE PROTECTION RELAY 71 LIQUID OR GAS LEVEL RELAY
22 EQUALIZER CIRCUIT BREAKER 72 DC CIRCUIT BREAKER
23 TEMPERATURE CONTROL DEVICE 73 LOAD RESISTOR CONTACTOR
24 SPARE 74 ALARM RELAY
25 SYNCHRONISING DEVICE 75 POSITION MECHANISM
26 APPARATUS THERMAL DEVICE 76 DC OVER CURRENT RELAY
27 UNDER VOLTAGE RELAY 77 PULSE TRANSMITTER
28 FLAME DETECTOR 78 PHASE ANGLE OR OUT OF STEP RELAY
29 ISOLATING CONTACTOR 79 AC RECLOSING RELAY
30 ANNUNCIATER RELAY 80 SUPPLY FAIL
31 SEPARATE EXCITATION DEVICE 81 FREQUENCY RELAY
32 DIRECTIONAL POWER RELAY 82 DC RECLOSING RELAY
33 POSITION SWITCH 83 AUTOMATIC SELECTION
34 MASTER SEQUENCE DEVICE 84 OPERATING MECHANISM
35 SLIP RING SHORT CIRCUIT DEVICE 85 CARRIER OR PILOT WIRE RECEIVER RELAY
36 POLARITY OR POLARIZING VOLTAGE DEVICE 86 LOCK OUT RELAY
37 UNDER CURRENT OR UNDER POWER RELAY 87 DIFFERENTIAL PROTECTION RELAY
38 BEARING PROTECTIVE DEVICE 88 AUXILIARY MOTOR OR MOTOR GENERATOR
39 MECHANICAL CONDITION MONITOR 89 LINE SWITCH
40 FIELD RELAY 90 REGULATING DEVICE
41 FIELD CIRCUIT BREAKER 91 VOLTAGE DIRECTIONAL RELAY
42 RUNNING CIRCUIT BREAKER 92 VOLTAGE & POWER DIRECTIONAL RELAY
43 MANUAL TRANSFER OR SELECTOR DEVICE 93 FIELD CHANGING RELAY
44 UNIT SEQUENCE STARTING RELAY 94 TRIPPING OR TRIP FREE RELAY
45 ATMOSPHERIC CONDITION MONITOR 95 SUPERVISION RELAY
46 CURRENT UNBALANCE RELAY 96 SPECIAL APPLICATION
47 POLE DISCREPANCY 97 FUSE FAIL RELAY
48 INCOMPLETE SEQUENCE RELAY 98 SPECIAL APPLICATION
49 THERMAL OVER LOAD RELAY 99 OVER FLUXING RELAY
50 INSTANTANEOUS OVER CURRENT RELAY 100 SPECIAL APPLICATION
General Description of Relays
NOMENCLATURE FOR ENGLISH ELECTRIC RELAY
FIRST LETTER – OPERATING QUANTITY
A- PHASE ANGLE COMPARISON SECOND LETTER – MOVEMENT
B- BALANCED CURRENT A ATTRACTED ARMATURE
C- CURRENT B BUCHHOLZ
D -DIFFERENTIAL C INDUCTION CUP
E- DIRECTION D INDUCTION DISC
F- FREQUENCY G GALVANOMETER (MOVING COIL)
I- DIRECTIONAL CURRENT T TRANSISTOR
K- RATE OF RISE OF CURRENT
N- MANUAL
O- OIL PRESSURE
P- POLY PHASE VA
R- REACTIVE VA
S- SLIP FREQUENCY
T- TEMPERATURE
V- POTENTIAL
W- WATTS (POWER)
Y- ADMITTANCE
Z- IMPEDANCE
THIRD LETTER – APPLICATION
A- AUXILIARY R RE CLOSING
B- TESTING S SYNCHRONISING
C- CARRIER (COUNTING) T TIMER OR TRANSFORMER
D- DIRECTIONAL U DEFINITE TIME
E- EARTH (GROUND) V VOLTAGE TIME
F- FLAG & ALARM INDICATOR W PILOT WIRE
G- GENERAL OR GENERATOR WA INTERPOSING
H- HARMONIC RESTRAINT WJ INTER TRIPPING
I- INTERLOCK OR INDUSTRIAL X SUPERVISORY
J- TRIPPING Y FLASH BACK (BACK FIRE)
JE- TRIPPING (ELECT. RESET) Z SPECIAL APPLICATION
JH- TRIPPING (HAND RESET) ZS ZERO SEQUENCE
JS- TRIPPING (SELF RESET)
JC- CONTROL
K- CHECK ALARM
L- LIMITING
M- SEMAPHORE OR MOTOR
N- NEGATIVE SEQUENCE
O- OUT OF STEP
P- POTENTIAL FAILURE
Q- ALARM
FOURTH LETTER
M – SPECIAL VARIATION
Sl. No. E E Relay Application
1 CTM Motor protection
2 CTU Locked rotor. Thermal alarm
3 CDG I.D.M.T. over current or earth fault of transformer
4 CAG Instantaneous over current or earth faults.
5 VAGM Under voltage
6 WDG Under /Over power for DG set
7 FTG Under frequency
8 VAPM Fuse failure
• What is Knee point voltage?
EMF applied to secondary of current transformer (CT) which, when increased by
10% voltage causes the excitation current to increase by 50%.
• What is I.D.M.T?
Inverse time relay with definite minimum time is called IDMT.
• What is Negative sequence reactance?
Negative sequence can arise whenever there is any unbalance present in the system.
Their effect is to setup a field rotating in opposite direction to the main field.
• What is Zero sequence reactance?
If a machine is operating with an earthed neutral, a system earth fault will give rise to
zero sequence current in the machine.
• Purpose of over current relay (Inverse); type- CDG
It is a self powered inverse time over current and earth fault relay, used for selective
phase and earth fault protection in time graded systems for A.C. machines,
transformers, feeders etc. A non-directional heavily damped induction disc relay,
which has an adjustable inverse time/current characteristic with a definite minimum
time. The relay has a high torque movement combined with low burden and low
overshoot. The relay disc is so shaped that as it rotates the driving torque increases
and offsets the changing restraining torque of the control spring.
• Purpose of Directional inverse Over current & earth fault relay; type- CDD
Directional phase or earth fault protection of ring-mains, parallel transformers,
transformer feeders, parallel feeders etc., employing the time graded principle.; This
relay comprises an inductive disc over current unit with wound shading coils and a
directional high speed induction cup unit. The cup-unit contact is wired across the
shading coils so that no torque is exerted on the disc of the over current unit until the
cup unit contact closes. The inductive disc unit is thus directionally controlled and it
operates only when the current flows in the tripping direction. The directional unit is
a high speed, low inertia four pole induction cup movement designed to give a high,
steady and non-vibrating torque. its current coil is connected in series with the
operating coil of the induction disc unit. The directional unit is normally provided
with voltage polarising coils.
• Purpose of Over current & earth fault relay; type- CAG
This relays are designed for instantaneous phase or earth fault protection and
instantaneous high set over current protection.; A standard hinged-armature unit
forms the basic movement for this relay. It consists an operating coil mounted on a
cylindrical iron core bolted to a frame at one end. This frame extends along the side
of the coil, with its end forming a knife-edge on which the armature is pivotally
mounted. The armature is 'L' shaped and pivoted at its corner so that one arm can be
attracted to the end of the core while the other arm to operate a set of contacts.
• Purpose of Local breaker back-up relay; type -CTIG
To safe guard against the drastic consequences of failure to clear faults rapidly, many
power supply authorities install 2 independent systems of protection for major
transmission lines. There remains however the possibility of the circuit breaker itself
failing to operate and this hazard is traditionally covered by remote breaker back-up.;
CTIG relay is a 3 phase or 2 phase and earth fault instantaneous over current unit
intended for use with a time delay to give back-up protection in the event of a circuit
breaker failure. A particular feature of the CTIG relay is a fast reset, which enables
the time delay to be set closer to the breaker trip-time.
• Purpose of Battery earth fault relay; type- CAEM-21
The battery earth fault relay is used to detect earth faults and deterioration of wiring
insulation in either pole of battery. The scheme consists of a centre tapped resistor, a
measuring relay, plug setting bridge, auxiliary relay and rectifier bridge to provide
unidirectional supply to the measuring relay coil. For different battery voltages
different values of centre tapped resistors are used. Variable sensitivities are
provided by means of the tapped coil whose taps are connected to the plug setting
bridge. The centre tap of resistor is brought to one terminals of the relay and this
terminal is either directly earthed or earthed through a centre zero milli
ammeter. Under healthy condition no current flows through the measuring relay coil
and in any pole of the battery or wiring insulation failure, current flows through the
measuring relay coil and the relay operates.
• Purpose of Rotor earth fault relay (type- CAEM-33)
When a single E/F is detected in the DC field circuit of a machine, the machine has
to be taken out of service at the first opportunity. This is because, if allowed to run
with an E/F on the rotor, a subsequent second E/F can cause severe damage to the
machine. However, a relay like CAEM-33 which can detect such a second E/F and
trip out the machine can make it possible to run the machine even with a single E/F,
without any such risks, thus helping to preserve the generation capacity. The start of
the second rotor earth fault detection scheme is a very sensitivity transductor
element. The AC winding of the transductor is connected in series with a rectified
AC voltage relay. The Dc winding of the transductor on the other hand is connected
in series with the rotor E/F circuit. Under normal conditions- i.e. with no DC
flowing, the AC wining of the transductor presents a high impedance, and the AC
voltage applied is mostly dropped across this winding. Hence the relay remains deenergised.
When a second rotor E/F occurs, a DC current flows through the
transductor dc winding which causes the impedance of the AC winding to reduce
considerably by driving the transductor core into saturation. Hence, the applied
voltage is fully available across the relay and it operates.
• Purpose of Sensitive earth fault relay (type - CTUM-15 & CTIGM-15
It may not be always possible to detect high resistance faults by convectional earth
fault relaying. In such cases a very sensitive current relay will be required for this
purpose. It can be connected residually since it has an adjustable definite time delay
provided to take care of transient spills in the residual circuit due to CT mismatch.
Also, its low burden enables it to be used with existing CT's/ Relays without
affecting the performance.; The incoming current is stepped down by an internal
current transformer and converted to a voltage by a variable resistor network. The
signal is compared with an internal reference. When this reference level is exceeded,
a time delay is initiated, after the time delay has elapsed, a relay operates.
• Purpose of Negative phase sequence current relay; Type- CTN/CTNM
Negative phase sequence current in the stator of a generator, caused due to
unbalanced loads or faults, it induces double frequency eddy current in the rotor.
These currents, if allowed to persist, can cause serious overheating and the purpose
of this relay is to disconnect the machine before such excess temperature is reached.
The inputs from the current transformers, which are connected in each phase of the
generator supply, are fed to a negative sequence filter which gives an AC output
voltage proportional to the negative sequence current. This voltage is rectified and
smoothed and fed into the squaring circuit of the main measuring element, the
definite time delay circuit and the alarm element. The output from the squaring
circuit is proportional to the square of the input voltage and is applied directly to the
main timing circuit to give the required relationship between I2
2t and the relay
operates time t.
• Purpose of definite time Over current & earth fault relay: Type-CTU
This relay can be used for definite time over current protection against phase and
Earth faults on medium and low voltage distribution systems. The definite time relay
offers a considerable advantage over inverse time relays in instances where there ia a
wide variation in line impedance. Another application is in the field of stalling
protection of motors. When the thermal overload relay does not provide protection
against stalling, separate definite time O/C relay type CTU can be used to provide
the same. This relay comes in following nomenclature: CTU-12/22/32/52/62/15.
CTU relay combines the advantage of complete static measurements with
characteristic of the robust, well proved attracted armature unit. These relays
measure current and time accurately, imposes low burden on CT's. Each phase
comprises a static overload detector and timer, which is accurate over a 10:1 time
setting range. When the positive peak of the input signal exceeds the reference level,
the time delay circuit starts and after a preset time, drives the output relay.
Instantaneous high set unit when fitted uses alternate half cycle for measurement and
through a separate level detector drives a separate output relay.
• Purpose of Motor protection relay: Type- CTMM/CTMFM
This relay contains all the protection factors to protect the motor, from Thermal
overload (Ith), Instantaneous over current (I1), Instantaneous or time delayed
unbalance element, Earth fault Element (I0) & Stalling protection (I1(t))
• Purpose of Overfluxing Relay: Type-GTTM
Transformers need protection against the risk of damage, which may be caused when
the transformers are operated at flux density levels significantly greater than the
design values. The overfluxing withstand time is generally found to be varying
inversely with the working flux density in the core, having higher withstand times
during extreme overfluxing conditions.
The overfluxing condition can occur during system over voltage or under frequency
conditions.
The basic operating principle is to produce an ac voltage, which depends upon the
ratio between AC input voltage and the frequency. The AC input voltage is fed to a
step-down transformer, which also provides isolation and the stepped down voltage
is fed to a V/F ratio detector circuit. This circuit is a simple operational amplifier
integrator with the provision for V/F pickup adjustment. The AC voltage is rectified
by true RMS. to dc converter. This circuit gives a frequency output and this
frequency increases rapidly with the increase in voltage. The frequency output is
given to a curve shaping circuit, which involves counter and comparators. The
counter counts the frequency output and the number of counts required for final trip
condition is changed by the comparator circuits to get the required timing
characteristic.
• Purpose of Biased Differential Relay: Type-MBCH
This relay is suitable for protection of two or three winding power transformers, auto
transformers or generators transformer units.
The differential transformer protection measuring circuit is based on the well-known
Merz-price circulating principle.
• Purpose of Digital frequency relay: Type-MFVUM
This relay is used to monitor the frequency of an electrical system. The relay are
suitable for any application in industrial plants and to generators where definite time
under or over frequency protection is required.
The operating principle of the relay is the comparison of the time interval of the
incoming frequency with that of a preset time derived from an accurate oscillator
within the relay. The incoming frequency signal is connected to an INPUT
CIRCUIT, which then drives an IMPULSE GENERATOR to produce a pulse at the
beginning of each period of the input voltage. The preset time interval is obtained
from an OSCILLATOR and COUNTER and adjustment is achieved using
SELECTOR switches, which drive a DECODER circuit. A COMPARATOR
compares the two-time interval and this triggers an adjustable TIMER which then
operates the output relay and latched light emitting diode (LED) glows.
• Purpose of Stator Earthfault Relay: Type-PVMM
A 100% stator earthfault protection is designed to detect earthfault occurring in the
regions of machine winding close to the neutral end. This relay is a composite
modular relay that gives 100% stator earthfault protection for machines, whose
neutral are not directly earthed. It works on the principle involving monitoring of the
neutral side and line-side components of the third harmonic voltages produced by
AC generators in service.
• Purpose of Voltage regulating relay and line drop compensator: Type-VTJCM & CIJC.
This relay is used with on load transformer tap changers and induction regulators to
provide close and accurate automatic voltage regulation on power systems of any
voltage.
When the regulated voltage moves outside a dead band, set by the sensitivity control,
the volts high or volts low circuits are initiated and after a time delay, determined by
the response characteristic, the appropriate tap changer control auxiliary relay closes
its contact to initiate a tap change.
• Purpose of Directional power relay: Type-MWTU.
This relay setting ranges from 0.25% to 18.56% of rated power. This makes the relay
suitable for sensitive reverse power applications. For example with turbo-generator,
where the detection of 1% or 2% reverse power is necessary to prevent the
synchronous machine from motoring in the event of the power from the prime mover
becoming too low. It is also suitable for low forward power interlock and under
power protection.
• Purpose of Check synchronising relay: Type-SKD/SKE.
This relay is used to prevent interconnection of badly synchronised supplies. Type
SKD relay are used for auto reclosing sequence, type SKE relay are used to
safeguard manual synchronising of generators. Phase measurement is achieved by
algebraically subtracting the 2 supply voltage waveforms and comparing the
resultant modulated beat waveform envelope with a Dc reference voltage. The DC
reference is proportional to the sum of the peaks of the 2 supply voltages to provide
phase measurement independent of supply voltage variation.
• Purpose of Static distance protection: Type-SHPM.
This relay (QUADRAMHO) is a static distance protection relay specially designed
for comprehensive high-speed protection of HV & EHV distribution/transmission
lines. 3 zones of protection are included, each employing separate measuring
elements, one element each for 3 phase-to-phase and 3 phase-to-earth faults. Thus a
total; of 18 elements are provided thereby increasing the reliability of the protection.
Poly phase measuring elements are not used in QUADRAMHO as in some of the
contemporary schemes. The relay is suitable for both three poles & single-and-threepole
tripping of the circuit breaker.
• Purpose of Static offset MHO relay: Type- YTGM.
This relay is a static single phase, single step, and distance protection with MHO
offset MHO characteristic. With suitable current/voltage input selection, the relay
can be made to have the required characteristic in the R-X plane for various
applications such as Generator Field failure protection, Generator backup impedance
protection and as offset MHO relay for use in conjunction with generator pole
slipping protection.
• Purpose of sensitive power relay: Type-WCD.
This power relay is a sensitive Poly phase induction cup unit, providing under power,
reverse power and over power protection. This relay detects a reversal of current
flow, caused by insufficient driving power from the prime mover, preventing the
generator operating as a synchronous motor. The electrical quantities energize
windings on an eight pole laminated stator. The moving contact is operated by a cup
shaped Aluminium rotor, which turns on jewelled bearings in an air gap between the
stator and a fixed center core. Only a small arc of rotation is needed to cause contact
closer. Low rotor inertia and very high driving torque ensures a high speed
operations.
• Purpose of pole slipping relay: Type-ZTO.
This pole slipping relay has been designed to protect synchronous Generators against
the possibility of the machine running in the unstable region of the power angle
curve which would result in power oscillations and pole slip. The relay consists
basically of one directional relay and one blinder relay operating in conjunction with
a 40-80 milli seconds static timer. Intended primarily for installation between the
generator and associated transformer (preferably on the generator terminals)
• Purpose of fuse failure relay: Type - VAPM
This relay detects a failure or inadvertent removal of voltage transformer secondary
fuses and prevention of incorrect tripping of circuit breakers. This relay consists of a
rectified AC voltage operated hinged armature unit. Three coils for the three phases
are wound over a single core producing in effect a common relay for the three
phases. Each coil is connected across one of the voltage transformer secondary fuses
and under healthy conditions, this coil is short circuited by the fuse and cannot be
energized. When one or more fuses or are removed the appropriate coil(s) is
energized under relay operates immediately to open the trip circuit.
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