Unrestricted earth fault relay:
The unrestricted earth fault protection is used to protect the alternator winding from internal earth fault. It uses a residual connected earth fault relay. Unrestricted earth fault relay consists of three Current Transformer one in each phase. The primary of the current transformer is considered as line. The earth fault relay is connected across the secondary’s which carries a residual current.
Principle of Unrestricted earth fault relay:
The vector sum of three-line current is equal to zero under normal condition. If the fault occurs in the generator, the relay reads the unbalance current in the system.
Under normal conditions the vector sum of the all current Īrs+ Īys + Ībs = 0. 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 the trip circuit. In the scheme shown in the figure, 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.
