# What is Differential Relays?

The relay which is used to checks the difference between the output and input currents for power system current in known as differential relay. The difference amongst the currents may also be in phase angle or in magnitude or in each. For hale and energetic operation, angle and magnitude variations must be zero. In case there's a difference which difference go beyond some value, the relay can work and interconnected electrical fuse can disconnect.

### Principle Operation of differential relay

Allow us to assume an easy example of an influence power transformer with transformation magnitude (ratio) relation 1:1 and (Y/Y) connection and therefore the CT1 and CT2 ensure a similar transformation magnitude relation as shown. The current flows within the primary side and secondary side of power transformer are equal, presumptuous ideal power transformer. The secondary current I1 and I2 are same in magnitude and reverse in direction. Therefore, the net current within the differential coil is nil at load situation (without any fault), and therefore the relay won't operate.

### External Fault Condition in Differential Relay

Assigning the previous one the power transformer with an external fault F is shown in figure. During this case the 2 currents I1, and I2 can increase to terribly high magnitudes values however there's no modification in phase angle. Hence, net current within the differential coil continues to be zero and therefore the relay won't operate.

### Internal Fault Condition in Differential Relay

An internal fault F is shown in this figure. Now, there are 2 anticipated conditions:

There’s other supply to feed the fault thus I2P includes a nonzero value Idiff = I1S + I2S which can be terribly high and sufficient to function the differential relay.

Radial system, I2P = 0. So, Idiff = I1S and additionally the relay can work and disconnect the breaker.

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