There are three type of generator protection 1.Class A: When any severe fault occur in generator/generator transformer/Unit auxiliary transformer than generator protection system give the trip command to generator circuit breaker,GT breaker and field breaker simultaneously. 2.Class B In class B trip protectionWhen the fault occur in generator which not need to required immediate isolation of generator from the grid. In this class the turbine trip first than after some time delay the generator circuit breaker,GT breaker and field breaker trip.so that the trap steam in turbine properly used. 3.Class C When the fault occur outside the GT unit ,only Generator transformer circuit breaker trip and unit not synchronised with grid but generator provide the power for house load of plant.
There is no zero sequence current in delta winding line of transformer.so for protect star delta three phase Transformers from external fault zero sequence current compansation must we used in differential protection scheme (87T).
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