Under such circumstances, an earth fault relay connected in a residual circuit of three CT's on the primary side operates on internal earth faults in primary winding only. Because earth faults on the secondary side do not produce zero-sequence current on the primary side.
Therefore for faults on star connected earthed secondary winding of power transformer restricted earth fault protection is used.
Such protection provides high-speed tripping under fault conditions.
The Fig. shows the star connected side is protected by restricted earth fault protection.
When an earth fault occurs beyond the transformer (point f1), the currents I1 and I2 flow in CT secondaries. Therefore the resultant current in the earth fault relay is negligible.
But when earth fault occurs within the transformer star-connected winding (point f2), only current I2 flow and I1 is negligible. Hence only current I2 flows through the earth fault relay.
Thus restricted earth fault-relay does not operate for earth fault beyond the protected zone of the transformer.
When a fault occurs very near to the neutral point of the transformer, the voltage available for driving earth fault current is small. Hence fault current would be low.
If the relay is set too sensitive to sense such faults, then it may operate for external faults and switching surges also.
Therefore relay setting is adjusted in such that it operates for earth fault the current of the order 15% of rated winding current.
Such a setting protects a restricted portion of the winding. Hence the name restricted earth fault protection.
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