The balanced earth fault protection scheme is mainly used for protection of small generator where differential and self-balanced protection systems are not applicable. In a small generator, the neutral end of the three phase windings is connected internally to a single terminal. So the neutral end is not available, and protection against earth fault is provided by using the balanced earth protection scheme. Such scheme does not provide protection against phase-to-phase fault until and unless they develop into earth faults.
Connection of Balanced Earth Fault Protection Scheme
In this scheme, the current transformers are mounted on each phase. Their secondary is connected in parallel with that of CT mounted on a conductor joining the star point of the generator to earth. A relay is connected across the secondaries of the CTs.
The balanced protection schemes provide protection against earth fault in the limited region between the neutral and line CTs (current transformers). It provides protection against the stator winding of the earth fault in the stator and does not operate in case of an external earth fault. This scheme is also called restricted earth fault protection scheme. Such type of protection is provided in the large generator as an additional protection scheme.
Working of Balanced Earth Fault Protection Scheme
When the generator is in a normal operating condition the sum of the currents flow in the secondary of the current transformers is zero and the current flow into secondary to neutral is also zero. Thus the relay remains de-energized. When the fault occurs in the protected zone (left of the line) the fault current flow through the primary of current transformers and the corresponding secondary current flow through the relay which trips the circuit breaker.
When the fault develops external of the protective zone (right of the current transformer) the sum of the currents at the terminal of the generator is exactly equal to the current in the neutral connection. Hence, no current flows through the relay operating coil.
Drawback of Balanced Earth Protection Scheme
If the fault occurs near the neutral terminal or when grounding of the neutral is connected through a resistance or a distributing transformer then the magnitude of the fault current flow through the secondary of current transformer becomes small. This current is less than the pick-up current of the relay. Thus, the relay remains inoperative, and the fault current continues to persist in the generator winding which is highly undesirable.