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.

Picture taken from http://universalerectors.com/steam_turbine_generator.html


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

Comments

Post a Comment

Popular posts from this blog

Concept of Subtransient, Transient & Steady State

Image
The concept of Subtransient, Transient and Steady State arises in case of fault in an Alternator. Let us assume a sudden short circuit in three phase of alternator. The fault current will flow in all the three phases of alternator and its waveform will be as shown in figure below. When the alternator is short-circuited, the currents in all the three-phases rise rapidly to a high value of about 10 to 18 times of full load current, during the first quarter cycle. The flux crossing the air gap is large during a first couple of cycles. The reactance during these first two or three cycle is least and the short circuit current is high. This reactance is called subtransient reactance and is denoted by X". The first few cycles come under sub-transient state. After a first few cycles, the decrement in the r.m.s. value of short circuit current is less rapid than the decrements during the first few cycles. This state is called the Transient  State and the reactance in th...
Read more

Open Circuit and Short Circuit Characteristics of Synchronous Machine

Image
Open Circuit Test and Short Circuit Test are performed on a Synchronous Machine to find out the parameters of Synchronous Machine and hence to have an idea of their performance.  Open Circuit Test of Synchronous Machine is also called No Load, Saturation or Magnetizing Characteristics for the reason which will be clear after going through the post. For getting the Open Circuit Characteristics of Synchronous Machine, the alternator is first driven at its rated speed and the open terminal voltage i.e. voltage across the armature terminal is noted by varying the field current. Thus Open Circuit Characteristic or OCC is basically the plot between the armature terminal voltage E f versus field current I f while keeping the speed of rotor at rated value. It shall be noted that for OCC, the final value of E f shall be 125% of the rated voltage. Figure below shows the connection diagram for performing the Open Circuit Test of Alternator. As clear from the figure a...
Read more

Transformer Cooling Classes

Image
Losses in the transformer are of the order of 1% of its full load kW rating. There are mainly two types of losses in a Transformer i.e. Core Loss and Ohmic Loss. Because of these losses the winding of transformer as well as Core gets heat up and we need to provide sufficient cooling to remove heat and maintain a constant winding and Core temperature. Various types of cooling methodologies are used for Transformer but before going to each methodology, we will have a look at the General Method of designating a particular cooling method i.e. class. In the nameplate of Transformer, four letter represents the cooling method of the Transformer. The above table is self explanatory, therefore if you see ONAN on the Name Plate of Transformer then it means that Oil with flash point less than 300° is used as cooling media and Natural convection is used for removing heat. In secondary side of cooling mechanism, Air with Natural convection is used by employing Radiator. ...
Read more