TRIPPING CLASSIFICATION OF GENERATOR
The tripping classification of generator is based on the need of isolation of generator on the basis of type of fault.
Trips are mainly classified as Class A, Class U, Class B, Class C etc.
Class A trip (Simultaneous tripping):
Provides the fastest means of isolating the generator. This tripping mode is used for all internal generator faults and severe abnormalities in the generator protection zone. Isolation is accomplished by simultaneously tripping the generator breakers and field breaker, shutting down the prime mover by closing the turbine valves and transferring unit auxiliary loads to standby power.
Class-A1 trip:-
The protections for the faults in the generator which need immediate isolation are grouped under this class-A1. There is list of faults which are kept under this class.
They are as follows:-
1. Generator differential protection
2. Stator earth fault protection
3. Generator over voltage protection
4. Starting over current protection
5. Generator inter-turn protection
6. Generator negative phase sequence
Class-A2 trip:- The protections for the faults in generator transformer, isolated phase bus duct, and unit transformer which need immediate isolation are grouped under this class-A2. Normally following protections are kept under this class:
1. B/up Z protection of generator
2. Differential protection of GT
3. Buchholz relay of GT
4. Over fluxing protection of generator
5. Trip of GT
6. Fire protection of GT
7. Differential protection of UT
8. Restricted earth fault
Class-B Trip:-
The purpose of sequentially tripping a turbine generator is to minimize the possibility of damaging the unit as the result of an over speed condition following the opening of the main generator breaker. The protections for the faults in the generator which do not need immediate isolation are grouped under this class-B. This results in tripping of turbine first. Let us suppose that there is some fault in the process side i.e. in steam cycle, under that condition also turbine will be tripped first while generator will continue to run utilizing trapped steam till reverse power relay operates. Generator circuit breaker is tripped on initiation of reverse power. Normally,
1. Stator water flow very low
2. Hp heater level very high
3. Turbine hand trip
4. Loss of excitation
5. Generator 32R
Class-C Trip:–
The protections for the faults / abnormal condition in the grid which call for disconnection of the generator from the grid are grouped under this class-C. In this case, generator is isolated from the grid by opening the suitable breaker i.e. generator transformer HV side breaker. Mind that in this case only generator is isolated from the grid. Thus generator continues to feed station loads (also known as house load).
Such scheme where generator is operated on house load at reduced power is known as generator islanding.
1. NPS protection
2. B/up Z protection
3. u/f
4. o/f
