Explain the “Differential Protection Scheme” used for alternators with neat labeled diagram.
Differential Protection Scheme for Alternator:
Figure shows the differential protection provided to 3 phase alternator. Here identical CTs are placed on both sides of 3 phase alternator and their secondary‟s are connected in star formation. Protective relays are connected as shown. Under normal operating conditions, the currents in the pilot wires fed from CT connections are equal. The differential current flowing through operating coil of relay is zero (I1-I2=0). When fault occurs in the protected zone balance is disturbed, the differential current flows through the operating coil of relay causing its operation. Relay sends trip signal to the CB, thereby alternator circuit is tripped.