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Home / Technical Articles / Protection and Relays Used In Main Circuit Board at a Power Grid Substation
Differential relay
Differential relay for transformer applications. Protection features include over-excitation (V/Hz), under/over-voltage, phase overcurrent, negative-sequence overcurrent, and breaker fail-to-trip/fail-to-close indicators. (pic by

1. High speed biased differential relay

The DMH type relay provides high speed biased differential protection for two or three winding transformers. The relay is immune to high inrush current and has a high degree of stability against through faults. It requires a max of two cycles operating time for current above twice relay rated current. Instantaneous overcurrent protection clears heavy internal faults immediately. This relay is available in two forms.

Firstly for use with time Cts, the ratios of line which are matched to the load current to give zero differential current under normal working conditions. Secondly with tapped interposing transformers for use with standard line current transformers of any ratio.

2. Directional inverse time overcurrent and earth fault relays

The CDD type relays are applied for directional or earth fault protection of ring mains, parallel transformers or parallel feeders with the time graded principle. It is induction disc type relay with induction cup used to add directional feature.

3. Instantaneous voltage relay

The type VAG relay is an instantaneous protection against abnormal voltage conditions such as over voltage, under voltage or no voltage in AC and DC circuits and for definite time operation when used with a timer. It is an attracted armature type relay.

4. Auxiliary relays

The VAA/CAA type auxiliary relays are applied for control alarm, indication and other auxiliary duties in AC or DC systems. CAA is a current operated and VAA is a voltage operated relay.. it is attracted armature type.

 5. High speed tripping relays

This VAJH type relay is employed with a high speed tripping duties where a number of simultaneous switching operations are required. This is a fast operating multi contact attracted armature relay.

6. Definite time delay relay

This VAT type relay is used in auto reclosing and control schemes and to provide a definite time feature for instantaneous protective relay. It is an Electro mechanical definite time relay. It has two pair of contacts. The shorter time setting is provided by a passing contact and longer time setting by the final contact.

7. Trip circuit supervision relay

This VAX relay is applied for after closing or continuous supervision of the trip circuit of circuit breakers.

They detect the following conditions:

  1. Failure of trip relay
  2. Open circuit of trip coil
  3. Failure of mechanism to complete the tripping operation

8. Instantaneous over current and earth fault relay

An instantaneous phase or earth fault protection and for definite time operation when used with a timer. It is a CAG 12/12G standard attracted armature relay with adjustable settings. It may be a single pole or triple pole relay.

9. Inverse time over current and earth fault relay

This CDG 11-type relay is applied for selective phase and earth fault protection in time graded systems for AC machines. Transformers, feeders etc. this is a non-directional relay with a definite minimum time which has an adjustable inverse time/current characteristics. It may be a single pole or triple pole relay.

10.  Fuse failure relay

This VAP type relay is used to detect the failure or inadvertent removal of voltage transformer sec. fuses and to prevent incorrect tripping of circuit breaker. It is three units, instantaneous attracted armature type relay the coil of each unit connected across one of the VTs.

The secondary fuses under healthy conditions, the coil is SC by fuses and can’t be energized. But one or more fuses blow the coil is energized and relay operates.

11. Instantaneous high stability circulating current relay

It is used to serve the following three purposes

  1. Differential protection of Ac machines , reactors auto transformers and bus bars
  2. Balanced and restricted earth fault protection of generator of generator and transformer windings
  3. Transverse differential protection of generators and parallel feeders.

This CAG type relay is a standard attracted armature relay. In circulating current protection schemes, the sudden and often asymmetrical growth of the system current during external fault conditions can cause the protection current transformers to go into saturation, resulting in high unbalance current to insure stability under these conditions.

The modern practice is to use a voltage operated high impedance relay, set to operate at a voltage slightly higher than that developed by CT under max fault conditions. Hence this type of relay is used with a stabilizing resistor.

12.   Local breaker back up relay

this is a CTIG type three phase or two phase earth fault instantaneous over current unit intended for use with a time delay unit to give back up protection in the event of a circuit breaker failure.

13. Poly-phase directional relay

The PGD relay is a high speed induction cup unit used to give directional properties to three phase IDMT over-current relays, for the protection of parallel feeders, inter connected networks and parallel transformers against phase to phase and three phase faults. Owing to low sensitivity on phase to earth faults the relay is used with discretion on solidly earthed systems.

14.  Auto reclose relay

Five types of auto reclose relays are available:

a) VAR21 giving one reclosure. The dead time and reclaim time are adjustable form 5 to 25 secs. If the circuit breaker reopens during reclaim time, it remains open and locked out.

b) VAR41B is a single shot scheme for air blast circuit breakers. Reclaim time is fixed at between 15 to 20 secs. Dead time adjustment is from 0.1 to 1.0 sec of which first 300 millisec will be circuit breaker opening time.

c) VAR 42 giving four reclosure. It is precision timed from 0 to 60 sec. it can be set for max four enclosures at min intervals of 10 sec and instantaneous protection can be suppressed after the first reclosure so that persistent faults are referred to time graded protection.

d) VAR 71 giving single shot medium speed reclosure with alarm and lockout for circuit breaker. This allows up to 10 faults clearance before initiating an alarm. The alarm is followed by lockout if selected no. of faults clearances exceed. If the circuit breaker reopens during reclaim time, it remains open and locked out. It offers delay in reclosing sequence. Instantaneous lockout on low current earth fault and suppressing instantaneous protection during reclamation time.

e) Var81 is a single shot high-speed reclosure with alarm and lockout for circuit breaker This allows up to 10 faults clearance before initiating an alarm.

Reactance distance scheme

this scheme consists of the following relays, XCG22-3 for phase to phase and 3 for phase to ground, YCG17, mho starting unit one in each place, VAT51 along with timing unit for zone 2 and 3, 86-X aux. tripping relay and 30G, H, and J for 1st, 2nd and 3rd. Zone indication VAA51, CAG12 and VAA31. These schemes provide three zone phase and earth fault protection using reactance relays type XCG22 and also starting relays YCG17.

They are applicable to important line sections where high values of arc resistance would otherwise affect the accuracy of measurement and where high speed tripping is essential. High-speed protection is provided for phase and earth faults on 80-90% of the line section and faults on the remaining section are cleared in second zone, time. The third zone provides backup protection after further time interval.

Each mho starting unit Y3 and its auxiliary Y3 X is associated with one phase and operates for all faults involving this phase. Each reactance unit X is connected to measure phase or earth fault distance, but is prevented from operating by short circuit across the polarizing coils. Under the phase fault conditions, the Y3 X units unblock the appropriate X1 reactance units, which initiate tripping immediately for faults within their setting.

Operation of the earth auxiliary relay 64 in conjunction with the Y3 X units selects the appropriate reactance units for measurement of earth faults. The reach of reactance units is extended by the timer, 2 after successive intervals to cover faults in zone 2 and 3.

The principle of distance scheme - Block diagram
The principle of distance scheme - Block diagram

Discrimination is not affected by changing faults, for example a zone 2 earth fault which develops into a double phase to earth fault will be cleared correctly by the X1 (phase fault) units in zone 2 time. In the rare event of two faults occurring simultaneously at different points on the line; the scheme will measure to a distance approximately half way.

Source: Internet and several books of Electrical Engineering

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Bipul Raman

Bipul Raman (@BipulRaman) is a Technology Enthusiast, Programmer and Blogger. Read more at :


  1. sanjay goyal
    Nov 12, 2014

    We want a aux. relay for transformer protection. The relay details are as under. Auxiliary relay, Type: VAA. Model no: SPECM2ZG76B. Volts: 110VDC. Make: E.E. So kindly provide the dealer or shop from where we can purchase this relay.

  2. jobert moleno
    Oct 20, 2014

    Nice article very informative.

  3. jeewangarg
    May 03, 2013

    how to calculate the capacity of a contactor (high/low voltage cutoff).

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