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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