Testing of circuit breaker is more difficult than the testing of other electrical equipment like transformer or machine because the short-circuit currents are very large. Also there is no satisfactory method of testing circuit breaker at reduced power.
Classification of Tests:
Testing of circuit breakers can be classified into two main groups, viz.:
(I) Type tests and
(II) Routine tests.
(I) Type Tests:
These tests are conducted on first few prototype circuit breakers of each type for the purpose of proving the capabilities and confirming the rated characteristics of the circuit breaker of that design. Such tests are conducted in specially built testing laboratories. Type tests are performed as per recommendations of standards (IEC) or (IS).
Type tests can be broadly classified as:
(i) Mechanical performance tests
(ii) Thermal tests
(iii) Dielectric or insulation tests and
(iv) Short-circuit tests in order to check making capacity, breaking capacity, short-time rating current and operating duty.
(i) Mechanical Tests:
These are mechanical endurance type tests involving repeated opening and closing of the breaker. A circuit breaker must open and close at the correct speed and perform its designated duty and operation without mechanical failure.
(ii) Thermal Tests:
Thermal tests are carried out to check the thermal behaviour of the breakers. The breaker under test is subjected to steady-state temperature rise due to flow of its rated current through its poles in closed condition. The temperature rise for rated current should not exceed 40° C for current less than 800 A normal current and 50° C for normal value of current 800 A and above.
In such tests the contact drops or the contact resistances are also measured as these contact surfaces are responsible for generation of heat and subsequent temperature rise.
(iii) Dielectric Tests:
These tests are performed to check power frequency and impulse voltage withstand capacity.
Power frequency tests are conducted on a clean 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 with circuit breaker closed,
(ii) Between poles and earth with circuit breaker open, and
(iii) Across terminals with circuit breaker open.
The voltage is gradually increased and maintained at test value for one minute.
In impulse tests impulse voltage of specified shape and magnitude is applied to the breaker. For outdoor circuit dry and wet tests are conducted.
(iv) Short-Circuit Tests:
Circuit breakers are subjected to sudden short-circuits in short-circuit test laboratories and oscillograms are taken to know the behaviour of the circuit breakers at the time of switching-in, during contact breaking, and after arc extinction. The oscillograms are studied with particular reference to the making and breaking currents, both symmetrical and asymmetrical-re-striking voltages, and switchgears are tested a number of times at rated conditions.
(II) Routine Tests:
Once type tests are conducted and a particular design is found to be satisfactory the product becomes prototype and a large number of circuit breakers of similar design are manufactured. However, each and every circuit breaker is still subjected to a few more tests before commissioning. These tests are called routine tests.
Routine tests are also performed as per recommendations of the standards (IEC/IS). These tests are performed in the manufacturer’s premises. Routine tests confirm the proper functioning of the circuit breaker.
The routine tests include (i) power frequency voltage test (ii) millivolt-drop test and (iii) operational tests.
Millivolt drop test is performed in order to determine the voltage drops within the current path of the breaker mechanism. Operational test is performed on the breaker by simulating its tripping by artificially closing the contacts of the relay.
Testing Station and Equipments of Circuit Breaker:
There are three types of testing stations:
(i) Field type testing station,
(ii) Laboratory type testing station;
(iii) Composite testing station.
(i) Field Type Testing Station:
In the field type of testing the power required for testing is directly drawn from a large power system, the breaker being tested is connected in the system. Though this type of testing provides the most convincing method of testing hv circuit breakers but it suffers from the drawback of limited available flexibility of the system. It is difficult to set the system for the specified RRRV for hv breakers.
(ii) Laboratory Type Testing Station:
In the laboratory type testing station, special generators, called the short-circuit generators provide the power for testing. In this type of testing station, it is possible to vary the test conditions at will. Establishment of short-circuit testing plant particularly of the laboratory type is an exceedingly costly project, and it is not possible for all switchgear manufacturers to have such a facility of their own.
(iii) Composite Testing Station:
A composite testing station is a combination of the field type testing station and laboratory type testing station.
Description of a Simple Test Plant:
In such stations, the short-circuit power is supplied by specially built short-circuit generators. The short-circuit generators are driven by three-phase induction motor and special type of excitation, known as impulse excitation is provided. Series resistors and reactors are provided for adjustment of magnitude of short-circuits current and pf.
The master CB has capacity higher than that of CB being tested and pf. In case, the circuit breaker being tested fails, the master CB opens and protects the circuit. Making switch is a specially designed circuit closing device which can close at the desired moment and can withstand the making currents. Transformers are provided to give test voltages other than the generator voltage.
In addition to above equipment there is equipment for:
(i) Measurement, record, control;
(ii) Sequence switch to provide sequential operation;
(iii) Auxiliaries etc.
Equipment Used in Testing Station:
1. Short-Circuit Generator and Drive Motor:
Short-circuit generators provide power to the CBs under test. Short-circuit generators must be capable of withstanding extremely high reactive power surges lasting for a short duration. Therefore, they are of different design from that of the conventional alternators.
Short-circuit generators must have low reactance in order to provide maximum short-circuit output. Short-circuit generator is a 3-phase alternator. Each winding is made in two or more parts which can be connected in series, parallel compensations of star or delta to provide different terminal voltages.
The generator is driven by a 3-phase induction motor connected through a resilient shaft. The generator is equipped with a built-in flywheel which provides the kinetic energy during short circuit and speed regulation of the set.
2. Impulse Excitation:
A separate dc converter set with a high peak output provides the “impulse excitation.” Impulse excitation or super excitation is provided to counteract the demagnetizing effect of armature reaction. The SC currents which are at lagging pf have demagnetizing effect. This results in a reduction of total field, hence in the reduced emf. As a result recovery voltage is less than the voltage before the short circuit.
The effect is reduced by boosting the generator field current by means of impulse excitation. The converter set employed for providing impulse excitation is fitted with a large flywheel. The motor is disconnected from the supply before the application of excitation. The field current is increased shortly to about 10 times its normal value at the instant of short circuit. This takes care of the demagnetizing effect of short circuit and provides the desired recovery voltage.
3. Pilot Generator:
It is a small 3-phase alternator directly coupled to the main shaft of the short- circuit generator and synchronised in phase with the latter. Any present voltage is maintained constant by an automatic voltage regulator during SC tests. This dependable voltage is required to supply control power to the sequence timer, electromagnetic oscillograph and various actuating circuits for conduction of the test.
4. Short-Circuit Transformers:
Short-circuit transformers are employed when test voltages are different from the generator voltage. For stepping down the voltage a three-phase transformer is normally used, whereas for stepping up the voltage, usually banks of single phase transformers are used. Such transformers are designed to withstand repeated short circuit and their windings are often arranged in sections for voltage adjustment in series and parallel combinations. The leakage reactance of the short- circuit transformer is kept low. Transformer winding is mechanically strong and provided with extra-turn insulation. For 3-phase tests, the transformers are connected in delta on alternator side. The four winding of each phase on secondary side can be connected in series/parallel combinations and the three phases can be connected in star or delta.
Depending on the type of test to be carried out in the test cells, different types and ratings of transformers are connected in different test cells, e.g. a hv testing transformer in one and a low voltage high current transformer in the ether.
5. Resistors and Reactors:
For controlling the short-circuit test current, three-phase banks of resistors and reactors are used. The reactors being used for adjustment of magnitude and resistors for control of rate of decay of the dc component of the current. The short-circuit pf is also controlled by these means. There are a number of coils per phase and so many variations can be had by connecting them in series/parallel combinations.
6. Master Circuit Breaker:
This is normally an air-blast type of capacity more than that of the breakers under test. It is provided mainly as a backup circuit breaker. In case of failure of the test circuit breakers, the master circuit breaker opens. In addition after every test, it isolates the specimen under test from the supply source and must be capable of handling the full SC power of the test circuit.
7. Make Switch:
The making switch or making device, as the name indicates, is employed to ensure that the short-circuit currents are applied correctly at the desired moment. The equipment is characterised by close making time and high making capacity. However, the breaking capacity is negligible as the making device is not used for circuit interruption. At the moment of making, the backup CB and the test CB are already closed. As such on closure if there is even a slight contact rebound the contacts might get burnt or even welded together. In order to ensure, that the contact separation is minimum on closure, a high air pressure is maintained in the chamber. The closing speed is also so high that the contacts are fully closed before the short-circuit current attains its peak value.
Capacitor banks serve two purposes:
i. Provide leading current for testing the performance of CB in interruption of charging currents.
ii. Control the rate of rise of re-striking voltage given by –
fn = 1/2√LC
In synthetic testing and other indirect tests, capacitors are vital items in the test circuit. These are single- phase banks and can be connected in series or parallel as desired, both individually and in any combination of the three.
9. Test Cells (or Cubical):
These are constructions of RCC or strong brick work. There is a provision for observation. Supply of compressed air and oil purifying system is given to the test cells for facilitating testing of air blast circuit breaker and oil circuit breakers respectively. Separate test cubicles are provided for testing LV; HV; and EHV circuit breakers. Each test cell has got an end box where control and measuring cables are terminated.
10. Test Control Room:
It is situated nearly at a distance of 20 m from test cubicles and is provided with a control desk for remote control operation of the excitation of the SC generators, etc. There are also oscillographs, distribution panel, an electromagnetic oscillograph; a cathode ray oscilloscope (CRO), an electric sequence switch and an electronic sequence switch. The control room is designed to provide facilities for the observation of tests by both the station staff as well as visitors.
11. Safety and Signalling System:
A foolproof interlocking system is provided for the safety of the working personnel, as enormous electro-dynamic forces are brought into play during short-circuit testing. This system makes the excitation of the machines impossible, if any of door leading to machine hall basement reactor room, etc. remains open. A system of audio visual alarms is also provided as a warning before SC tests.
Testing Procedure of Circuit Breaker:
A preliminary checkup of various equipments is made before the actual commencement of tests. The correct values of resistance and reactance coils are inserted as per the magnitude of the short-circuit current. Transformers are properly set and the measuring circuits are connected and oscillograph loops are calibrated.
The sequential switching of equipment measurement and control circuits is accomplished by a sequence switch. Sequence switch is a drum switch with several pairs of contacts. The drum is rotated by a motor. Once the drum is rotated, it closes and opens several control circuits according to a certain sequence.
For example, the sequences for breaking capacity test in one test were as follows:
1. Drive motor of SC generator and the exciting machines is switched off.
2. Impulse excitation is switched on.
3. Master circuit breaker is closed.
4. Oscillograph circuit is connected.
5. Make switch is closed.
6. Circuit breaker under test is opened.
7. Master circuit breaker is switched off.
8. Exciter is switched off and its field is suppressed.
The above operations take a very short time of the order of 0.2 second. After the completion of short circuit, the starters of the drive motors of the SC generator and the exciting machines are automatically reset to a lower degree. The motors are then switched on and are allowed to run at full speed, so that in a short time a further short circuit can be initiated. A schematic representation of test circuit.
Direct and Indirect Testing of Circuit Breaker:
a. Direct Testing:
The direct testing of CBs in a plant enables us to test it under conditions largely representing those in actual network as well as tests of greater severity. The circuit breaker under test is subjected to the value of transient re-striking voltage to which it is expected to be put in practice rather than testing it under most severe conditions.
The preliminary preparation for testing of a CB includes connecting the equipment adjusting the magnitude of reactors, connecting transformers to give the desired test voltages etc. The contacts on sequence switch are adjusted to have desired timings. The oscillographs are adjusted and calibrated. The operations of test follow automatically by means of sequence switch.
The various tests carried out for circuit breaker testing are described below:
1. Making Capacity Test:
Making capacity test is necessary type test. All CBs are tested for their ability to make on to a short circuit. The master circuit breaker and the make switch are closed first, then the breaker under test is closed on a 3-phase short-circuit test.
2. Braking Capacity Test:
For testing breaking capacity, Master CB and circuit breaker under test are closed first and then the short circuit is applied by closing the main switch. The breaker under test is opened at desired moment.
3. Duty Cycle Tests:
Unless the rated operating duty as marked on the name plate differs from that specified, it shall consist of the following test duties.
Test duty (1) B-3′-B-3′-B at 10% of rated symmetrical breaking capacity.
Test duty (2) B-3′-B-3′-B at 30% of rated symmetrical breaking capacity.
Test duty (3) B-3′-B-3′-B at 60% of rated symmetrical breaking capacity.
Test duty (4) B-3′-MB-3′-MB at not less than 100% of rated symmetrical breaking capacity and not less than 100% of rated making capacity.
Test duty (4) may be carried out as two separate duties as follows:
Test duty (4a) M-3′-M (make test)
Test duty (4b) B-3′-B-3′-B (break test)
Test duty (5) B-3′-B-3′-B at not less than 100% rated asymmetrical breaking capacity.
B and M in the above duty cycle indicate break and make operations respectively. MB means a making operation followed by a breaking operation without any intentional time lag. 3′ denotes the time duration in minutes between successive operations of an operating duty.
4. Short Time Current Tests:
The CB is tested for this by means of a separate high current and low voltage transformer. The current is passed through the breaker for a short time (1 second or 3 seconds) and the oscillograph is taken. Figure 7.4 shows an example of oscillograph taken during the short time current test.
OT is duration of short-circuit (say 1 second)
I0, I1, I2,…, I10 rms value of asymmetrical current at each instant. The rms value of current during the time interval 0 to T of such a wave is given by the expression –
Where, i is the variable current, t is the variable time in seconds, and T is duration of current in seconds.
Equivalent rms value of short-time current is determined as follows:
The time interval 0T is divided into 10 equal parts marked by 0, 1, 2, … , 10. The rms values at these instants are I0, I1, I2 , … , I10.
Where, Isym is rms value of ac component at this instant and Idc is dc component at this instant.
It is rms value of current at this instant. Thus I0, I1, I2, … etc., up to I10 are computed. From these values, the rms value of short-time current is determined by Simson formula i.e.
Temperature rise limits are not specified for short-time current tests. It is very difficult to record the transient temperature during 1 second duration.
Good contact is required to be maintained in spite of forces developed by the short-circuit current so as to avoid welding. The heat produced by the short-circuit current over the period of test (1 second or 3 seconds) should not cause damage to any insulation in contact with the current carrying parts. When the breaker has cooled down to the ambient temperature, it should be capable of carrying full-load current continuously without excessive temperature rise.
b. Indirect Testing:
The short-circuit power available in earlier testing stations (of the order of 4,000 MVA in laboratory type station) is not sufficient for testing a complete circuit breaker (which is of rated breaking capacity of the order of 10,000 MVA at 245 V). Even single pole of an EHV circuit breaker cannot be tested by direct means. It is, therefore, necessary to utilize some form of indirect testing.
The important indirect methods of testing are:
1. Unit testing and
2. Synthetic testing.
1. Unit Testing:
Almost all modern EHV circuit breakers, minimum oil, air blast SF6 etc. consists of two or more identical units (or interrupters) per pole. These interrupters operate simultaneously and share the voltage across the pole almost equally. The breaking capacity in MVA is also shared equally. Hence a test on one unit can be accepted as proof for all units. Such tests are called the unit tests. Unit testing is an internationally accepted method.
While applying unit test the voltage is to be reduced by a factor k and all the impedances should be reduced by the same factor k in order to have test voltage across the unit same as that following expressions:
k = 1/n when one unit is tested together
= m/n when m units are tested together
Where, n is the number of units per pole
For instance, consider a 3-pole 220 kV circuit breaker with three units per pole. Test is to be performed at normal voltage of 230 kV. Voltage across each pole is 230/√3 i.e. 132.8 kV.
k = 1/n = 1/3 ... n = 3
Voltage required for testing one unit = k x 132.8 kV = 1/3 x 132.8 kV = 44.3 kV
Also, L and C test circuit should be reduced to have the same natural frequency as that direct testing i.e.
fn = 1/2√LC in direct testing
The natural frequency of the transient re-striking voltage remains the same. Time scale also remains the same.
With circuit breakers in which the voltage distribution across the pole is not evenly distributed amongst the units, some units will be stressed more and the others less. The test should be performed so as to test the highest stress coming over the unit. Statically, unit test has been established as a reliable testing method.
2. Synthetic Testing:
Synthetic testing is a practical and economical solution for testing of CBs of high rupturing capacities, without actually using the corresponding short-circuit capacity of the testing station. The synthetic circuit is designed to simulate as accurately as possible the electrical stresses impressed on the circuit breaker during the interruption of fault current under system conditions.
Figure 7.5 depicts the principle of synthetic testing. Synthetic testing makes use of two sources viz.:
(1) Current source (of relatively low voltage) and
(2) Voltage source (of relatively low current).
The current source is to provide the short-circuit current while the voltage source is to provide re-striking voltage in addition to recovery voltage. Others r, L, C etc., are used for having the desired conditions. The switch S1 is closed for supplying short-circuits current iG. At near final current zero switches S2, usually a spark gap, are closed and Vs is applied to the CB under test at an appropriate moment. The voltage will have transient because of L and C of the circuit.
This testing method has the following advantages:
1. This method is simple and can be applied to unit testing also.
2. This method makes possible testing of CB of capacity (MVA) of five times that of the capacity of the test plant.
3. The CB can be tested for desired TRV and RRRV.
4. The short-circuit generator has to supply currents at a relatively lower voltage (as compared to direct testing).
5. Both test current and test voltage can be independently varied. This provides flexibility to the test.
Synthetic test circuits are of two types viz. parallel current injection method and series current injection method.
Parallel Current Injection Method is widely employed for testing CBs because it can provide high frequency transient voltages as needed by standards. In parallel current injection method [Fig. 7.6 (a)], the voltage circuit (2) is effectively connected in parallel with current circuit (1) and the test circuit breaker before the main current iG in test breaker current is properly simulated.
Initially the short-circuit generator G is excited and current limiting reactor LG set to provide the desired test current, make switch S1 open and auxiliary breaker and test circuit breaker are closed.
In the voltage circuit CH is charged to provide the required voltage and the spark gap S2 is set ready to be fired. LH is so chosen that when iH flows consequent to the triggering of the gap, the rate of change of current through zero is the same, as that of test current. CH is selected to provide the required transient re-striking voltage frequency.
The make switch S1 is closed and the normal frequency 50 Hz current iG flows in the circuit breaker. At time t0, the auxiliary breaker and test circuit breaker begin to open so that they are fully open at the following current zero (t2) in Fig. 7.6 (b). At time t1 the spark gap S2 is fired and the current flows in the test circuit breaker. The frequency of this current is determined by LH and CH and timed so that its peak approximately coincides with zero of current iG.
At time t2 when the current iG becomes zero it is interrupted by the auxiliary breaker and the test circuit breaker carries current iH from the voltage circuit. When this current becomes zero at t3 the re-striking voltage transient appears across the circuit breaker. The magnitude and frequency of this re-striking voltage transient depend on voltage to which the capacitor CH is charged and the parameters CH and iH. This enables the supply of current and voltage at the moment of zero current from one and the same source.
Thus breaking capacities up to ten or more times the short-circuit capacity of the short-circuit generator can be achieved and the test circuit breaker can be subjected to exactly the same stresses as in an actual system.
Series Current Injection Method:
Figure 7.7 (a) represents series current injection method. This circuit differs from the parallel current injection circuit in a way in which the small loop of current from the voltage source is introduced. The voltage circuit is connected across auxiliary breaker, instead of test circuit breaker and the source capacitance CH is charged to the opposite polarity, with the result that the injected current flows in the direction opposite to that of 50 Hz current, and thus subtracts form it (Fig. 7.7).
As the voltage and current circuits are series connected, it is rather difficult to choose circuit parameters to suit both the current and voltage circuit which would at the same time provide required re-striking voltage transient. This arrangement is particularly well suited for low-frequency circuits.
Brown Boveri Synthetic Testing Circuit:
Synthetic test circuit shown in Fig. 7.8 is used by Brown Boveri, Switzerland. In this circuit, the short-circuit current is supplied from a circuit at a relatively low voltage while the re-striking and recovery voltage is provided by a separate HV circuit.
The circuit on the left side of the test circuit breaker is high current circuit which consists of short-circuit generator G, short-circuit transformer and also capacitor CG and resistor RG. CG and RG control the natural frequency of high current circuit. The short-circuit power is supplied at voltage VG which corresponds to about 30 kV, this voltage is smaller than recovery voltage Vs required for testing the specimen. The recovery voltage Vs is supplied from a separate voltage circuit on the right side of the test circuit breaker.
The auxiliary breaker S1 is opened simultaneously with the circuit breaker under test and a few microseconds before the current interruption (iG) in the test circuit breaker. The spark gap is triggered by control S1 and the voltage Vs is applied to the test circuit breaker.
The current iH has a natural frequency of 500 Hz and amplitude of one-tenth of that of current iG. The currents are superimposed in current zero zones in such a way that during last 100 µs only current iH is flowing through test circuit breaker. The auxiliary circuit breaker S1 interrupts high current circuit from HV circuit before current is = iG + iH has to interrupt only current iH. The re-striking voltage across test circuit breaker is, therefore, given by that of HV circuit.