DIFFERENT KINDS OF RELAYS

DIFFERENT KINDS OF RELAYS

1	OCR	Over current relay	50 / 51
2	EF	Earth fault	50 N / 51 N
3	DR	Different relay	87
4	RPR	Reverse power relay	67
5	REF	Restricted earth fault relay	87TG
6	ELR	Earth leakage relay	50G
7	UV	Under voltage relay	27

All these relay will have instantaneous tripping as well as IDMT (Inverse Definite Minimum Time) delay tripping or only IDMT or instantaneous.

OVERCURRENT RELAY: 

This relay operates when a over current of short circuit noticed in the load.

Normally these relays will be having plug setting for current selection.

IDMT is normally used for over current. Instantaneous will be actuated only during short circuit and for this normally the setting will be 10 times the normal current.

Whatever the current fall in the secondary of C. T. will flow through over current relays 50 / 51.

EARTH FAULT RELAY: 

When any fault occurs between phases & earth, this relay will operate.

This relay connected to the zero balanced (residually connected) C. T. connection as shown in the diagram. Normal over current will not pass through this relay, at point 0 the current will be zero. When a fault occurs between phase and earth, there will be unbalance and that unbalance current will pass through relay (50 N / 51 N).

RESTRICTED EARTH FAULT RELAY:  

This relay protects only the zone connected in between the neutral C. T. and residually connected  C. T. in the phase side i.e., it is protecting star winding of the transformer. When any unbalance or earth fault occurs after the phase C. T. there will be unbalance currents neutral C. T. as well as residually connected phase CT and both currents nullified at point ‘O’. So there won’t be any current flow in the relay and the relay won’t operate.

When fault occurs in the winding there will be a current flow in the neutral CT only. The current will operate the 87TG relay.

DIFFERENTIAL RELAY

This relay protects the Zone connected between the CTs – i.e. it is protecting only the transformer. The CT connection is in winding side and in winding side, this is because to compensate vector difference. The CT ration is also to be selected according to the transformation ratio.

When there is fault after the transformer secondary (i.e. after the secondary CT) there won’t be any difference of primary and secondary currents, are balanced in the Restraining coil.

When a fault occurs in the transformer, that current will be transformed to primary side only. So that there will be different in the currents of primary and secondary. That unbalanced current will pass through the operating coil and the relay will operate.

In generator the differential relay connections will be as shown.

COORDINATION OF RELAY

If any fault occurs in motor, the neares relay has to trip ‘D’ breaker itself, so that it won’t affect the other system. Similarly if any fault occurs after ‘A’ breaker, the nearest is A breaker’s relay. So ‘A’ should trip and other system not affected.

Normally the operating and tripping of breaker time will be around 350 msec.

If after ‘A’ breaker fault occurs ‘A’ will trip at 350msec if it fails ‘B’ will trip at 700msec, I ‘B’ also fails ‘C’ will trip 1 sec. This is called the co-ordination of Relays.

UNDER VLTAGE RELAY: 

This relay operates when under voltage occurs in a connected system, normally this relay setting be 80% of the applied voltage. This relay is used to trip the equipments connected, during under voltage condition. These relay having two types, instantaneous and IDMT (Inverse Definite Minimum Time)

IDMT relay are all dis type and the setting can be done through plug setting arrangement.

AC – Alternating current where the voltage is alternating from peak voltage (on both +ve –ve side) to ZERO voltage periodically.

DC – Direct current where the voltage in constant.

 FREQUENCY – 

 In AC, the voltage which completes one cycle of Sin wave form is called frequency and the unit is ‘HERTZ’ (Hz)

Normal supply source will be 50Hz.