Different relay is that relay which checks the different between the input and output currents for a power system current. The difference between the currents may be in magnitude or in phase angle or in both. For healthy operation, magnitude and angle differences must be zero. If there is a difference and that difference exceeds some value (setting valued, the relay will operate and associated circuit breaker will trip .
Principles of differential relay
Let us assume a simple example of a power transformer with transformation ratio 1:1 and (Y / Y) connection and the CT1 and CT2 have the same transformation ratio as shown in figure (35).
The current flow in the primary and secondary sides of power transformer are identical, assuming ideal transformer. The secondary current I1, and I2 are equal in magnitude and opposite in direction. So, the net current in the differential coil is zero at load condition (without fault), and the relay will not operate .
External Fault Condition
Assuming the previous power transformer with an external fault F as shown in figure (36). In this case the two currents I1, and I2 will increase to very high magnitudes values but there is no change in phase angle. So, the net current in the differential coil is still zero and the relay will not operate.
Internal Fault Condition
For an internal fault F as shown in figure (37). Here, there are two expected situation :
- There is another source to feed the fault so I2P has a nonzero value Idiff = I1S + I2S which will be very high and sufficient to operate the diff relay.
- Redial system , I2P = 0. So, Idiff = I1S and also the relay will operate and trip the circuit breaker.
