TESTING OF CIRCUIT BREAKER

• Circuit breakers tests are Classified as follows: • Type test:

These are performed solely for the purpose of proving the correctness of general design.

Test conducted on a randomly selected breaker.

Results are applied to all other identical construction.

These tests can be performed by either by

(a) Direct testing or (b) Indirect testing.• Routine test:

These are performed for purpose of proving the correctness of assembly and material used.

These are performed on each individual c.b.

• The test procedures, methods of finding results, tests to be performed, etc. are specified by standard organizations.

• On the recommendations of the International Electrotechnical Commission (IEC) vide publication No. 56-1 on the testing of a.c. circuit breakers.

• Routine test include the following tests:A. Operational test.B. Measurement of resistance of the main circuits.C.One minute power frequency voltage dry withstand tests.

Type tests• Type tests are the test of one CB or a first few CBs of

each type made to the same specifications.• Type tests can be classified in following groups

1) Mechanical test

2) Thermal test

3) Dielectric test

4) Short circuit test

i. Making capacity test

ii. Breaking capacity test

iii. Duty cycle tests

iv. Short time current tests

1) Mechanical test (Endurance test):

• The breaker should be in a position to open and close satisfactory.

• In this test the CB is opened and closed several times (500).

• Some operations (50) are by energizing the relay, remaining are by closing the trip circuit by other means.

• Mechanical test on a.c CB are conducted without current and voltage in the main circuit.

• No adjustment or replacement of parts is permitted during the mechanical test, however lubrication is permitted as per manufacturer instructions.

• After the test there should be no distortion or wear of parts.

2) Thermal tests:• These are type tests to assign the normal current rating to

the CB.• Alternating current of rated value and rated frequency is

passed through a closed CB, continuously till a steady temp is attained.

• Readings of temperature of various conducting, insulating and structural parts are taken at an interval of one or half-an hour.

• When the steady temp is reached, the maxim temp rise of each part should be less than the permissible limit.

• The temp. rise for rated current should not exceed 40º C for current less than 800 A normal current and 50º C for normal value of current 800 A and above.

• When a CB in closed condition carries normal current, the heat is generated in carrying parts due to I2Rt loss.

• This heat is dissipated by conduction, convection and radiation.

• To maintain the temp rise within specified limits, the I2Rt loss reduced by increasing conductor cross-section, using suitable low resistively material, improving convection, conduction and radiation of heat.

• An additional requirement is the measurement of contact resistances between the isolating contacts and between the moving and fixed contacts.

• This points are main sources of excessive heat generation.

• The voltage drop across the breaker pole is measured for different values of d.c. currents.

• This drop gives a measure of resistance of current carrying parts, and hence that of contacts.

• The d.c current should be more than 100A and less than rated current of CB, the resistance of breaker pole is measured at ambient air temp, the resistance is of the order of few tens of micro-ohms.

3) Dielectric test: • These are conducted to confirm the rated insulation level of

the circuit breaker. These can be classified as 1. Power frequency tests.

Conducted on new circuit breaker .The test voltage varies with circuit breaker . rated voltage.The test voltage with a frequency between 15-100 Hz is applied as follows:

I. Between poles and earth with circuit breaker open,II. Across terminals with circuit breaker open.III. Between poles with circuit breaker closed,

The voltage gradually increased and maintained at test value for 1 minute.

2. Impulse tests.In this test impulse voltage of specified shape and magnitude is applied to the breaker.For outdoor circuit dry and wet tests are conducted.

Short circuit testing of C.B

• These tests are meant to prove the ratings of the circuit breaker.

• These are conducted in short circuit testing stations.

• There are two types of short circuit testing stations

1. The field type testing station2. Laboratory type testing station• Short circuit testing is classified in further two

types• Direct testing• Indirect testing

The field type testing station

• In these tests are conducted by taking power directly from the system.

• It is most convincing method, but it suffers form the following drawback:

That flexibility of the system available is limited. It is difficult to set the system for the specified

RRRV for breakers of high voltages. It is not possible to repeat the test again and

again without disturbing networks, hence not suitable for research and development work.

The power available in the field testing station vary according to load connected on the network and layout of the network.

Laboratory type testing station

• It has special generators to supply power for the short circuit testing.

• In this it is possible to vary the test condition as required.

• Such station is suitable for design engineer and supply engineer.

• The designer can study the behavior of arc rupture and make necessary improvements for higher voltages and higher breaking capacities.

• The test can be performed again and again in order to show the reproducibility of the results.

• The supply engineer can get satisfaction whether his breaker satisfies the requirements enunciated by specifications.

Description of a simple testing station

• In these testing stations the short-circuit power is supplied by specially designed short circuit generators, driven by induction motors.

• The magnitude of the test voltage and the short circuit can be selected within wide limits by adjusting generators excitation and connection of the transformers.

• Further limitation and variation of current can be effected by means of tapped resistors and reactors.

• This also serves control on power factor.

• The circuit is closed by specially designed make switch, designed for closing on very heavy currents, but never break currents.

• Synchronized closing is controlled by means of a small pilot generator coupled to the generator shaft and can be very accurately set to occur at any instant within the voltage wave.

• With this the phase opposition at the commencement of short circuit can be selected.

• And short circuit current either fully symmetrical or with any degree of symmetry can be produced.

• Master c.b. is provided as a backup protection, in case of failure of c.b. under test to open the circuit.

• By adding capacitor, any desired high frequency can be obtained.

• The breaker under test is enclosed in a test cell made of reinforced concrete having a provision of observation while test is in progress.

• The recording equipment is located in a control room.

• The control operations are carried out from the control room.

• Direct testing involves subjecting a complete breaker or breaker pole to full power or stress during the test.

• The necessary preparation of C.B testing include connecting the equipment, adjusting the magnitude of reactors, connecting transformers to get desired test voltage etc.

• The contacts on sequence switch are adjusted to get desired timings.

• The oscillographs are adjusted and calibrated.

• While testing breaking capacity; Master circuit-breaker and C.B under test are closed first, Short circuit is applied by closing the making switch.

• The breaker under test is opened at desired moment

• Making capacity test is necessary type test, all C.Bs are tested for their ability to make on to a Short-circuit.

• The master C.B and the make switch are closed first, the breaker under test is closed on a three phase short-circuit

• Rules for type test1) Breaking current: The short-circuit current

broken by the CB should be measured at the instant of contact separation.

2) Breaking capacity: The braking capacity test should be performed with specified TRV of test Circuit

3) Peak making current: The peak making current made by the circuit breaker during the test should be expressed by maximum current in any pole.

4) Condition of severity for making capacity and braking capacity tests are specified as under the following clauses:

1) Conditions of breaker before test

2) Conditions during the test

3) Conditions of breaker after test

4) Applied voltage before test

5) Transient recovery voltage

6) Short-circuit power factor

7) Test frequency

8) Earthing of test circuit

9) Test duties

Indirect testing• The short-circuit power available in testing stations

(which is of the order of 4000 MVA in lab type testing station) is no longer sufficient to test a complete breaker ( which is of rated breaking capacity of the order of 10,000 MVA at 245 KV)

• Even single pole of a EHV CB cannot be tested by direct means, as all EHV CB are with several arc interrupter units per pole, each unit can be separately tested. This is called unit testing.

• From tests on one unit, the capacity of the complete pole and breaker is determined.

• Synthetic testing is another popular method which permits testing of breaker of capacity 5, times that of the plant.

The important indirect Methods include the following:1. Unit testing: which means testing one or more units

separately.2. Synthetic testing: In which the current source providing

short circuit current and voltage source supplying restriking and recovery voltage are different

3. Substitution test: These are conducted for oil CB, the characteristics of current versus time are obtained for different voltages, the performance beyond the tested value is determined by approximation

4. Compensation tests: which are conducted on oil CB in critical range of low current by suitable compensation such as increased frequency, increase restriking voltages etc.

5. Capacitance tests: the capacitor which is charged by a voltage source is discharged through the breaker, an oscillatory circuit provides restriking voltage.

SYNTHETIC TESTING

• The synthetic test employs two sources namely

1) Current source (of relatively low voltage)

2) Voltage source (of relatively low current)• The current source provides short-circuit current, the

voltage source provides restriking voltage plus recovery voltage.

• Other L,r,C etc are used to get desired test conditions, the switch S1 is closed to supply short-circuit current IG.

• At near final current zero switch S2 ( which is usually a spark gap) is closed and V3 to applied to the breaker at an appropriate moment, the voltage will have transient because of L and C of the circuit

• Advantages of this method• The breaker can be tested for desired TRV and

R.R.R.V• The short-circuit generator has to supply

currents at a relatively less voltage ( as compared to direct testing)

• Both test current and test voltage can be independently varied, so test become flexible

• This is a simple method and can be applied to unit testing

• With this method a breaker of capacity of five time that of the capacity of the test plan can be tested.

Types of Synthetic Test circuits: There are two types of synthetic circuit

Parallel current injection method

• In parallel current injection method, the voltage circuit (2) is effectively connected in parallel with current circuit (1) and the test breaker before the main current IG in test breaker current is properly simulated.

Series current injection Method

• In series current injection method, the voltage circuit (2) is connected to current circuit in series before main current zero, as a result IH and IG are in opposition to breaker circuit

Several factors influence the stress during the test like

1) high current mode

2) Instant of applying voltage

3) Frequencies of TRV etc

4) High voltage mode

5) t1,t2,t3,t4 etc