This scheme is based on the principle that the input power to the power transformer under
normal conditions is equal to the output power.Under normal conditions, no current will flow
into the differential relay current coil. Whenever a fault occurs, within the protected zone, the
current balance will no longer exist, and relay contacts will close and release a trip signal to
cause the certain circuit breakers (CBs) to operate in order to disconnect the faulty
equipment/part. The differential relay compares the primary and secondary side currents of
the power transformer. Current transformers (CTs) are used to reduce the amount of currents
in such a way their secondary side currents are equal. Fig. 1 shows the differential relay in its
simplest form. The polarity of CTs is such as to make the current circulate normally without
going through the relay, during normal load conditions and external faults.
Current transformers ratings are selected carefully to be matched with the power
transformer current ratings to which they are connected so as the CTs secondary side
currents are equal. However, the problem is thatthe CTs ratios available in the market have
standard ratings. They are not available exactly as the desired ratings. Therefore, the