Fig. 18 shows the arrangement of voltage balance protection. In this scheme of protection, two similar current transformers are connected at either end of the element to be protected (e.g. an alternator winding) by means of pilot of wires. The secondaries of current transformers are connected in series with a relay in such a way that under normal conditions, their induced e.m.f’s are in opposition

Under healthy conditions, equal currents will flow in both primary windings. Therefore, the secondary voltages of the two transformers are balanced against each other and no current will flow through the relay-operating coil. When a fault occurs in they protected zone, the currents in the two primaries will differ from one another and their secondary voltages will no longer be in balance. This voltage difference will cause a current to flow through the operating coil of the relay, which closes the trip circuit.

Disadvantages

The voltage balance system suffers from the following drawbacks

(i) A multi-gap transformer construction is required to achieve the accurate balance between current transformer pairs.

(i) The system is suitable for protection of cables of relatively short, lengths due to the capacitance of pilot wires.

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