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Generator and GT
Classification Generator and Generating Transformer Protections

Generator, Generator Transformer and Unit Auxiliary (UAT) Transformer Protections have been classified into many classes on view of their criticality  such as Class-A protections , Class-B protections  and Class-C protections . Class-A again classified into Class-A1 and Class-A2 Protection.

Why the protections are classified ?

Ans: The classification of tripping of generator is based on the need of the isolation of generator on the basis of type of fault. For better under standing let us discuss an example, there are some faults like Generator Differential Protection which required immediate Tripping of Generator Breaker without delay, where as some other faults such as Loss of Excitation and Rotor Earth Fault which do not required immediate protection of Generator.

Class-A Protection:

Class-A1 Protection: 

The protections for the faults in the Generator which needs immediate tripping are grouped under this Class-A1. The faults are coming under this Class is listed below

  • Generator Differential Protection 
  • 100 % Stator Earth Fault Protection
  • 95 %  Stator Earth Fault Protection
  • Generator Over Voltage Protection
  • Over Current Protection
If Class-A1 protection activated Generator Circuit Breaker and Field Circuit Breaker open along with Turbine Tripping.

Class-A2 Protection: 

This protections for the faults initiated in Generator Transformer (GT), Isolated Phase Bus Duct (IPBD), and Unit Transformer (UT) which need immediate isolation are coming under this Class-A2.  The faults under this group are listed below
  • Over Fluxing Protection of Generator 
  • Back up impedance Protection of Generator 
  • Differential Protection of GT
  • Buchcholz Relay of GT
  • Tripping from OTI and WTI of GT
  • Fire Protection of GT
  • Differential Protection of UAT
  • Trip from OTI and WTI of UAT
  • Fire Protection UAT
These protection when initiated tripping of Generator Circuit Breaker, Field Circuit Breaker, Generator Transformer Circuit Breaker and Unit Auxiliary Transformer LV Circuit Breaker and Turbine.

Class-B Protection: 

These protection 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 tripped first while Generator will continue to run utilizing trapped steam till reverse power relay operate. Generator Circuit Breaker is tripped on initiation of reverse power. Normally Loss of Excitation and Rotor Earth Fault of Generator are coming under this Class. When this Protection initiated it will Tripped Generator Circuit Breaker , Field Circuit Breaker and Turbine.

Class- C Protection: 

This Protection for the Faults/abnormal condition in the Grid side which disconnect the Generator from Grid. In this class of protection Generator is isolated from Grid by opening the line Breaker i.e Generating Tranasfomer HV side Breaker. Keep in mind that Only Generator only isolated from Grid. So Generator continue to feed Station loads( also known as house load). The faults coming under the class are listed below
  • Unbalance or Negative Sequence Protection 
  • Back up Impedance  Protection
  • Under Frequency Protection
  • Over Frequency Protection
  • Pole Slip Protection.

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