Introduction of Balanced Earth-fault Protection
In small size alternators, the neutral ends of the 3-phase windings are often connected internally to a single terminal. In these cases it is not possible to use the Merz-Prize circulating current principle since there are no facilities for accommodating the necessary current transformer in the neutral connection for each phase winding. Under these circumstances we will provide only protection against earth fault by the use of balanced earth fault protection scheme. This protection will not be able to provide protection from phase to phase faults unless and until they developed into earth fault..
Construction of Balanced Earth-fault Protection
The fig shows the schematic arrangement of a balanced earth fault protection for a 3-phase alternators. In this , 3 current transformer each one mounted in each phase having their secondaries connected in parallel with that of a single current transformer in the conductor joining the star point of the alternator to earth. A relay is connected across the secondary of transformer. The protection against faults is limited to the region between the neutral and the line current transformer.
Operation of Balanced Earth-fault Protection
When there is normal operating condition, the currents flowing in the alternators leads and hence the current flowing in the secondary of neutral current transformer supplies no current to the relay.
If an earth fault occurs at F2 outside to the protection zone, the sum of the currents at the terminals of the alternators is exactly equal to the current in the neutral connection and hence no current flows through the relay coil.
When an earth-fault occurs at F1 or within the protected zone, the currents are no longer equal and the differential current flows through the operating coil of the relay. The relay is thus energised and closes the trip coil circuit and our system is protected..
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