DIFFERENTIAL PROTECTION / MERZ PRICE PROTECTION
This type of protection provides a continuous check on faults within the points, where the C.T.s are used as in shown in fig 2. There are two sets of C.T.s; each set is mounted on either end of the stator phase. The secondaries of these current transformer sets are connected in star and their ends are connected together through pilot wires. The over current inverse time relay is connected across the equipotential points in the pilot wires, which will naturally be in center of these wires as shown in fig 2. But it’s convenient to locate these relays adjacent to the main circuit breaker; this can be achieved by inserting balancing resistance in series with the pilot wires to achieve equipotential point near the main circuit breaker as shown in fig 3. If there is no fault in the generator, same current will pass through the C.T.s at both the ends and there will be no current spilling into the relay.
Now if earth fault occurs as shown in fig 2, there will be an increase in current through set C.T.1 as compared to current through C.T.2. Thus there is no current balance any more resulting into spilling of current through the relay R hence tripping the circuit. If there is short circuit between the two phases as in fig3 it will also cause operation of relay R and tripping of the circuit breaker. This scheme does not provide protection for the turns of same phase