1 - Differential relay:

Differential relay is a protection device which is used for special selected zone. According principle of DR if a fault is occurred in a selected zone than input and output supply will be different then relay will provide a tripping signal to the breaker and system will safe. So it is called as differential relay.

It is installed in transformer, Generator, line and feeders etc. Selected zone input and output supply is compared through both side CT. If the compared voltage is zero then fault is not available and system is normal working. If the differential net value is not zero then fault is available and relay will be activate.

2 - Working principle of differential relay.

The main function of deferential relay monitors the difference in between input and output CTs current value in any selected zone. Selected zone input side and output side CTs are installed and both side CTs is connected to the differential relay. Both side CTs difference current is monitored by differential relay.

If the value of current both side is equal then difference is zero in this condition the relay will not activate. If the value of current both side is not equal then difference is not zero in this condition, the relay will activate and circuit breaker will tripped. It is used in transformer HT and LT side, Generator output supply and feeders.

3 - Connection diagram of differential relay.

4 - Relay setting parameters:

Sr.No.	Parameters	Setting range
1	Current	: 1A to 5A
2	frequency	: 50, +/- 2.5 %
3	Auxiliary supply	: 20V to 110V AC/DC 88V to 264V AC/DC
4	Over current	: 0.2 - 2.0 times (in steps of 0.05 In)
5	Time	: 0.1 to 1.6 in steps of 0.05
6	Pickup Current	: Same as set current
7	Reset Current	: 95% - 90% of pickup current
8	Reset time	: 0.1- 10Sec in steps of 0.1Sec or Less than 50ms
9	Current Transformer	: Less than 0.25 VA / phase

5 - Why it is used?.

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