In the case of any severe fault in the electrical network, the generator needs to isolate from the supply system as well as from the turbine side to protect the generator.
There are various protection schemes for the generator in use. We can classify the
generator tripping classes on the basis of the nature of the fault.
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.
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.
Class A Trip:
The stator earth fault and the differential fault in a generator are the faults that show problems in the generator winding. If the generator remains connected to the
grid, in this case, severe damage happens to the generator as well as to the electrical
system. In such a case, the generator breaker needs to be isolated immediately. In such
a situation, the protection scheme is called Class-A tripping of the generator.
The Class A can be further classified into two types of tripping, they are,
Class A1 Tripping
Class A2 Tripping
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 (87G)
b) 100% Stator Earth Fault Protection (64S2)
c) Generator Over Voltage Protection (59)
d) Dead Machine Protection (50AE)
e) 95% Stator Earth Fault Protection (64S1)
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 (99)
b) Back up Impedance Protection of Generator (21)
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:
Some of the protections for the fault that do not need quick isolation are grouped in the Class-B trip. The following protection fault comes in the category of
Class-B.
a) The problem in the turbine or in the steam process
b) Loss of generator Excitation (40)
c) Rotor Earth fault (64R)
In the case of a problem in the steam process, the turbine trips first. The generator is allowed to run to utilize the trapped steam in the system. When the power delivery of the generator reached below the forward power setting of the relay, the low forward power relay (32F) trips the Generator circuit breaker.
Class C Trip:
The generator that runs in synchronization with the Grid needs protection in the case of a fault in the Grid. The protection for the Grid faults that needs disconnection of synchronizing circuit breaker falls into Class- C trip.
The following protections come under the Class-C trip.
a) Under Frequency (81U)
b) Over frequency (81O)
c) Negative Phase Sequence or Unbalance (46)
d) Back up Impedance Protection (21)
The synchronizing breaker trips when there is a fault in the grid. The generator now operates in islanding mode, continuously delivering power to the connected load.