Generator Protection Part 7
The unrestricted earth fault protection uses a residually connected earth fault relay. It consists of three C.T.s, one in each phase. The secondary windings of three C.T.s are connected in parallel. The earth fault relay is connected across the secondaries which carries a residual current. The scheme is shown in the Fig. 1.
Where there is no fault, under normal conditions, vector sum of the three line currents is zero. Hence the vector sum of the three secondary currents is also zero.
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| Fig. 1 Unrestricted earth fault protection |
So if Irs, Iys and Ibs C.T. secondary currents then under normal conditions we can write,
Īrs+ Īys + Ībs = 0
The sum of the three currents is residual current IRs which is zero under normal conditions.
The earth fault relay is connected in such a way that the residual current flows through the relay operating coil. Under normal condition, residual current is zero so relay does not carry any current and is inoperative. However in presence of earth fault condition, the balance gets disturbed and the residual current IRs is no more zero. If this current is more than the pickup value of the earth fault relay, the relay operates and opens the circuit breaker through tripping of of the trip circuit.
In the scheme shown in the Fig. 1, the earth fault at any location near or away from the location of C.T.s can cause the residual current. Hence the protected zone is not definite. Such a scheme is hence called unrestricted earth fault protection.
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