The various protections associated with the generator, Generator transformer and Unit transformer are connected to a trip unit through trip relays 86A, 86B and 86C. The protective levels of generator are in three classified groups Class A, Class B and Class C protections which involve fault in the generator, Generator transformer and requires high speed clearance are grouped under Class A. These are routed through trip relays 86A. This trips generator transformer HV side breaker, generator field breaker, and LV side breaker of UT and Turbine simultaneously.
Certain protections such as loss of excitation, negative sequence protection, overfluxing etc., can tolerate sequential tripping of turbine followed by the generator such that the entrapped steam in the turbine is fully spent before generator is tripped and reduces the risk of over speeding of the turbine. These protections are classified as Class B. These are connected to operate on trip relay 86BG. This relay initiates the tripping of turbine (closure of stop valves) and also the LV side breaker of UT through trip relay 86B1 and 86B2. After turbine stop valves are closed and the entrapped steam is spent, the output power of the generator will come down and is sensed by under power relay 32A and 32B. These interlocks are wired in series with the Class B trip relay 86B, which is wired to trip the generator breaker, generator field. Obtaining better security, the Under Power interlock circuits are duplicated. Some protections such as Bus bar differential, generator under frequency etc, requires tripping only of the 220 kV side of the generator transformer to isolate the external fault. These are classified as Class C. These protections are wired to trip relay 86C, which initiate only the tripping of the generator transformer HV side breaker. During Class C trip, the generator will come on House load mode of operation.