In the case of differential protection of the generator, current transformers are provided at both ends of generator stator windings.
Under normal operating conditions and for through faults, the currents in the pilot wires fed from CT connections are equal.
The differential current flowing through the operating coil of the relay is zero (I1-I2 = 0).
When the fault occurs inside the protected zone, the balance is disturbed. The differential current. (I1-I2) flows through the operating coil of relays which causes the operation of the relay. The relay sends the signal to the circuit breaker, thereby the generator circuit is tripped. The field is disconnected and discharged through a suitable impedance.
Connection of C. T.'s for differential protection of generator :
The percentage differential relay consists of two coils; an operating coil and a restraining coil. A separate set of coils are used for each phase.
The restraining coils are connected at the center of the pilot wires. The operating coil is connected between the midpoints of the restraining coil and the neutral pilot wire.
Referring to Fig.(a), Suppose an earth fault occurs on phase 'R' due to break down of its insulation to earth then current in the affected phase winding will flow through the core and frame of the machine to earth the circuit being completed through the neutral earthing resistance.
The secondary current of two CTs connected in phase 'R' will become unequal. The difference between the two currents will flow through the corresponding relay coil, and complete its path via the neutral pilot. Consequently, the relay operates and the circuit breaker trips the circuit.
currents in the secondaries of the two CTs in the phase 'R' will become unequal and Consider now a short circuit fault occurs between the phases Y and B as shown in Fig. (a). The short circuit current circulates via the neutral end connection through the two windings and through the fault as shown by dotted arrows.
The currents in the secondaries of two CTs in each affected phase will become unequal and the differential current will flow through the operating coils of the relays (R2 and R3) connected in these phases. The relay then closes its contacts to trip the circuit breaker.
Differential relay provides fast protection to the stator winding against phase to phase faults and phase to ground faults. The operating time is about 15 ms. Also, it provides full stability through fault conditions.