PREPARED BY

NIHAL SHIROYA

Why "Testing of Circuit Breaker" is Necessary?

A Circuit Breaker should be capable of carrying, making, and breaking under normal and abnormal conditions. In any power system circuit breaker has to withstand power frequency over voltages and transient over voltages due to switching and lightning.

The performance of a circuit breaker under normal and abnormal conditions can be verified by performing different type of tests on circuit breakers. The main purpose of testing of circuit breakers is to confirm if circuit breaker is able to work on particular voltage and current ratings or not.

There are mainly two tests classified:1) Type test2) Routine Test

1) Type Tests:The purpose of type tests is to prove design

features and the quality of circuit breaker. Type tests are not conducted on each circuit breaker. This is done to prove the capabilities and to confirm the rated characteristics of the circuit breakers.

2)Routine Tests:

Routine test is performed before circuit breaker dispatch to ensure the product. This gives result about defects in materials and construction of circuit breaker. We can check quality of material of circuit breaker by performing Routine Test.

Mechanical endurance tests

Thermal tests

Dielectric tests

Measurement of resistance of the main circuits

Short Circuit tests

In this test, the C.B.. is open and closed 500 times or other value as agreed to between the purchaser and the supplier.the test are carried out without current through the main circuit of the C.B.Out of the total number of tests, 10% should be closed-open operation,that is with the tripping,mechanism energized by the closing of main contacts.During the tests,occasional lubrication,but no mechanical adjustments are permissible.after the tests,all parts including contacts should be in good condition and there should be no permanent distortion and undue wear of the parts.

This test determines the maximum normal current that the circuit breaker can carry without exceeding the maximum allowable temperature rise.In this test the rated normal current of normal frequency is passed through the current carrying parts of circuit breaker.

Method are recognized by Indian standards for measuring temperature rise of parts:-

1) Thermometer method2) Thermocouple method3) Self resistance method

1) Breaking capacity Test:-

• Sequence of performing this tests is as follows:- First of all,the master circuit breaker (MB)and the breaker under test (TB)are closed. The s.c.current is passed by closing the make switch. The circuit breaker under test(TB) is opened to interrupt the s.c.current at desired

moment.

• The following measurements related to the breaking capacity performance are taken from the oscillogram during the test:-

Symmetrical breaking current Asymmetrical breaking current Amplitude factor Natural frequency of oscillations and RRRV(RATE OF RISE OF RISTRIKING

VOLTAGE)

Sequence of Performing this test :-

First of all,the master circuit breaker (MB)and the make switch(MS) are closed.Then,the short circuit current is initiated by closing the test breaker (TB).The rated short circuit making current i.e.the peak value of the first major loop of the short circuit current envelope is measured from the oscillorgram.

3 )Short Time Withstand Current Capacity

In this test,the rated short-time withstand current is applied to the circuit breaker under test for the specified duration of the time.

The rated short time withstand current is equal to be rated short circuit breaking current and standarad value of rated duration of short circuit current is 1 second or 3 seconds.

The current is measured by taking an oscillograph of the short circuit current wave.

After the test,there should be no mechanical or insulation damage and any contact welding.

Routine Test Listed Below-:

1)Mechanical Operational test 2)One-Minute power frequency

voltage dry withstand test 3 )Measurement of resistance of main

circuits

This test should be made before temperature rise test.The measurement of resistance is made by measuring dc voltage drop or resistance across the terminals of each pole of circuit breaker.The current during the test shall have any convenient value between 100 A and the rated current of the circuit breaker.The d.c. resistance of main contact of each pole of a circuit breaker is of the order of a few tens of micro-ohms.

Operational tests are made to ensure that the circuit breakers comply with prescribed operating conditions within the specified voltage and supply pressure limits of their opening and closing devices.

During routine test five opening and five closing operations should carry out at

a. Minimum supply voltage and pressure

b. Maximum supply voltage and pressure

After the test,all parts should be in good condition and should not undue wear.

 

The preliminary preparations of circuit breaker testing include the following:

Connect the equipment. Adjust the correct values of resistance and reactors to

set the required magnitude of short circuit. Connect and set the transformer to get desired test

voltage. The contacts on sequence switch are adjusted to get

desired timings. Oscillographs are adjusted and calibrated.

• 1) Short Circuit generator:- It is a specially designed generator having very low

leakage reactance to provide short power for testing c.b. It is driven by 3-phse induction motor.A flywheel is provided on shaft of generator to supply kinetic energy during short circuit and helps in spped regulation.

• 2)Short Circuit Transformer:- It is connected between TB and power source.It is used

to allow for flexibility to test at different voltage levels and also provide isolation between the power source and test breaker.

It is designed to have low leakage reactance and to withstand repeated short circuit.

It is used as a back up c.b. to interrupt the short circuit current in event of c.b.under test(TB) fails to operate.It has higher capacity than the circuit breaker under test.After every test,it is also isolates the test circuit breaker(TB) from the supply source.It is set to operate at a predetermined time after the initiation of short circuit

4) Make Switch(MS):- It is very high speed switch used for initiation of short

circuit test current at desired moment.Recently,SF6 gas make switch are used in short circuit testing station.

5) Reactors(L):-It is connected in series.It is used to limit or control the

magnitude of the short circuit or test current to its required value.

6) Resistors and Capacitors (R and C):-TRV shaping circuit(R and C) is connected across the circuit breaker under test(TB).This circuit is used to control transient recovery voltage(TRV).Capacitors are also used for voltage measurement.

7) Measuring Equipments:- For the measurements of voltage,voltage dividers are used and for cureent measurements shunts or CTs are used in the short circuit testing stations.

Photo Of Testing Of Circuit Breaker