Class-A, Class-B and Class-C Tripping Classification of Generator

Generator, Generator Transformer and Unit Transformer protections have been classified into Class-A, Class-B and Class-C. Class-A tripping is further classified into Class-A1 and Class-A2. In this post we will discuss each type of tripping classes and their significance.

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Basis of Tripping Classification: The tipping classification of Generator is based on the need of isolation of Generator on the basis of type of fault. For example, there are some faults like Generator Differential Protection which calls for immediate tripping of Generator Breaker without delay whereas there are some fault like Loss of Excitation, Rotor Earth Fault etc. which do not call for immediate tripping of Generator.

Class-A1 Trip: The protections for the faults in the Generator which need immediate isolation are grouped under this Class-A1.  There are a list of faults which are kept under this class. They are as follows:

a)    Generator Differential Protection

b)    100% Stator Earth Fault Protection

c)    Generator Over Voltage Protection

d)    Dead Machine Protection

e)    95% Stator Earth Fault Protection

f)     Starting Over Current Protection

In case of actuation of Class-A1 protection, Generator Circuit Breaker and Filed Circuit Breaker are opened along with turbine tripping.

Class-A2 Trip: The protections for the faults in Generator Transformer (GT), Isolated Phase Bus Duct (IPBD), and Unit Transformer (UT) which need immediate isolation are grouped under this Class-A2. Normally following protections are kept under Class-A2:

a)   Over fluxing Protection of Generator

b)   Back up Impedance Protection of Generator

c)   Differential Protection of GT

d)   Buchcholz Relay of GT

e)   PRD of GT

f)    Trip from OTI & WTI of GT

g)   Fire protection of GT

h)  Differential Protection of UT

i)    Buchcholz Relay & PRD of Main Tank of UT

j)     Trip from OTI & WTI of UT

k)   Fire protection of UT

These protection when operated initiate tripping of Generator Circuit Breaker, Field Circuit Breaker, Generator Transformer Circuit Breakers & Unit Transformer LV Circuit Breakers and turbine.

Class-B Trip: The protections for the faults in the Generator which do not need immediate isolation are grouped under this Class-B. The turbine is tripped first and Generator is allowed to run utilizing trapped steam in turbine. 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, Loss of Excitation and Rotor Earth Fault of Generator are kept under this class. These protection when operated initiate tripping of Generator Circuit Breaker, Field Circuit Breaker and turbine.

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. Normally following protections of Generator are kept under this class:

a)  Unbalance or Negative Sequence Protection

b)  Back up Impedance Protection

c)   Under Frequency

d)  Over Frequency

e)  Pole Slipping Protection