Balanced Earth-fault Protection of Alternator

Balanced Earth-fault Protection of Alternator

This scheme provides protection against earth-faults only (in small-size alternators where Differential protection is not possible to apply). It provides no protection against phase-to-phase faults, unless and until they develop into earth-faults.

It consists of three line current transformers, one mounted in each phase, and a single current transformer in the conductor joining the star point of the alternator to earth. Secondaries of all the CTs are connected in parallel. A relay is connected across the secondaries of CT. The protection against earth faults is limited to the region between the neutral and the line current transformers.

Operation

Under normal operating conditions, the currents flowing in the alternator leads and hence the currents flowing in secondaries of the line current transformers add to zero and no current flows through the relay.  Also under these conditions, the current in the neutral wire is zero and the secondary of  neutral current transformer supplies no current to the relay. If an earth-fault develops at F₂ external to the protected zone, the sum of the currents at the terminals of the alternator is exactly equal to the current in the neutral connection and hence no current flows through the relay. When an earth-fault occurs at F₁ or within the protected zone, these currents will not be equal and the differential current flows through the operating coil of the relay. The relay then closes its contacts and trip the circuit breaker.