Generator Protection - Generator Inter-turn Fault Protection

Generator Inter-turn Fault Protection

Inter-turn fault have commonly been disregarded on the basis that if they occur they will quickly develop into earth faults. This is probably true if the fault is in the slot portion but will take a little longer in the region of the end connection. An approach of this kind is never attractive and may be entirely unjustified. There is a possibility of the machine being very seriously damaged before the fault evolves to a condition that can be detected by the longitudinal system.

Modern medium size and large size turbo generators have the stator winding designed with only one turn per phase per slot. For these machines Inter-turn faults can only occur in case of double ground faults or as a result of severe mechanical damage of the stator end winding. The latter is considered rather unlikely to occur.

It is generally considered difficult to obtain a reliable protection against short-circuiting of one turn if the stator winding has a large number of turn per phase.

For generators with split neutrals, the conventional Interturn fault protective scheme comprises a time delayed low set over-current relay which senses the current flowing in the connection between the neutral of the stator winding. The fault current can be extensively large in case of Interturn fault, hence the time delay must be short, 0.2 to 0.4 sec and the over current relay must be set higher than the maximum unbalanced current in case of external faults and the maximum unbalanced current for single turn short circuit have to be obtained from the manufacturer of the machine.

Due to the difficulties in obtaining a reliable and secure Interturn protection, it is in most cases omitted. It is assumed that the Interturn fault will lead to a single phase ground fault at the faulty spot, and the machine will then be tripped by ground fault relay within 0.3-0.4 sec.

Relay is for one phase only. Similar connection are for other two phase. Time delay of 200 second is provide to avoid operation of relay in system disturbance condition.

Fig H: Generator Interturn Fault Protection

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