Routine Tests of Circuit Breakers
Various routine tests are performed for ensuring the quality, performance of a circuit breaker and these are
Power frequency over voltage withstand test
Dielectric test on auxiliary circuit and control circuit
Measurement of resistance of main circuit or contact resistance test
Tightness test or SF6 gas leakage test
Design and visual checks
Mechanical operation tests.
Let us discuss them one by one.
Power Frequency Over Voltage Withstand Test
The power system may experience different temporary power over voltage conditions may be due to sudden cut of load from the system, wrong operation of online tap changer, insufficient shunt compensation in the system, etc. Power frequency over voltage withstand test of circuit breaker is performed to verify sufficiency of insulation strength of the main circuit to withstand this kind of abnormal over voltage conditions of the system. The circuit breaker should also be designed to be capable of withstanding over voltages due to lightning and switching impulses. A circuit breaker like other costly engineering equipment, are designed for safely facing all kind of abnormal situations, but at the same time, the designers cannot sacrifice economical aspects.
To verify the capability of withstanding all kinds of over voltages conditions without sacrificing the economical aspects of manufacturing, a circuit breaker has to go through and pass different dielectric tests. But only power frequency over voltage withstand test comes under the category of routine test of circuit breakers.
One Minute Dry Power Frequency Voltage Withstand Test
It is assumed that over voltage conditions, at power frequency cannot be sustained beyond one minute time moreover it is actually sustained for much less time than one minute duration. This test is carried out to verify whether the insulation provided in the main circuit of breaker is capable of withstanding power frequency over voltages for long one minute duration or not.
The test is performed in dry conditions of the breaker. The power frequency voltages, applied to the breaker during the test, are specified in the standard as per the nominal voltage level of the system.
Let us discuss one common example of One Minute Dry Power Frequency Voltage Withstand Test of SF6 Circuit Breaker. Here normally top of all poles of all circuit breakers of same voltage rating to be tested, are connected together preferably by copper conductor. This connection then is earthed properly. Similarly base of all circuit breakers under test is to be properly connected to earth.The buttom of all poles of all circuit breakers under test, are connected together preferably by copper conductor.
This connection then is connected to the phase terminal of single phase high voltage cascaded transformer. High voltage transformer used here is a cascaded auto transformer where input voltage can be varied from zero to several hundred volts and corresponding secondary voltage would be zero to several hundred kilo volts. During test the voltage is applied at the button terminal of breakers by high voltage cascaded transformer, and varied from 0 to specified value slowly and gently then stay there for 60 second and then slowly decreased to zero. During test the leakage current to the ground to be measured and the leakage current should not cross the specified maximum allowed limit. Any failure of insulation during test indicates the insufficiency of insulation used in breaker.
Dielectric test on auxiliary and control circuit
There may be abnormal over voltage condition in the auxiliary and control supply circuits, too. Hence, the auxiliary and control circuits of breakers should also be gone through short duration power frequency voltage withstand test. Here test voltage of 2000 V is applied for duration of one minute. The insulation of auxiliary and control circuit should pass this test, and there should not be any destructive discharge during the test.
Measurement of the resistance of main circuit
The resistance of main circuit is measured from DC voltage drop across the circuit. In this test, direct current is injected to the circuit and corresponding voltage drop is measured and from this resistance of the circuit is measured. The injected current would be from 100 A to maximum rated current of the circuit breaker. The maximum measured value can be 1.2 times the value obtained at the temperature rise test.
Tightness Test
This test is carried out on mainly gas insulated switch-gear. In this test, leakage rate is measured. This test ensures the desired lifespan of the switchgear. Here all the jointing points in the gas containing paths are covered air tightly with thin sheets of polythene (preferably transparent) for more than 8 hours and then the gas density inside this covers is measured by inserting gas detecting port of a gas detector through a hole now created on the covers. The measurement is taken in ppm unit and should be within the specified limit. Maximum limit of gas leakage 3 ppm / 8 hours, is taken as standard.
Visual Checks
The circuit breaker should be visually checked for language and data on the templates, proper identification mark of any auxiliary equipment, color and quality of paint and corrosion on metallic surface, etc.
Mechanical Operation Test
The circuit breaker must be smoothly operated at maximum as well as minimum allowable auxiliary and control circuit supply voltage. Closing and tripping operation should be performed at least 5 times for specified maximum allowable control circuit supply voltage as well as specified minimum allowable control circuit supply voltage. The closing and opening operation of circuit breaker are also to be checked for rated supply voltage of control circuit.
110% of control voltage is taken as maximum limit for closing and opening operation of circuit breaker. 85% of control voltage is taken as minimum limit for closing operation of circuit breaker and 70% of control voltage is taken as minimum limit for opening or tripping operation of circuit breaker. During maximum and minimum control voltage operation it would be found that operating times are less and more respectively than that in rated control voltage but all the times should be within specified times limits. If it is applicable, such as in the case of pneumatic circuit breakers, the breaker should also be operated at least 5 times in specified maximum allowable operating pressure, in specified minimum allowable operating pressure and in specified rated operating pressure. A circuit breaker is also intended for rapid auto reclosing; at least 5 open close operating cycles should be checked against specification given on the rating plate.
The actual time interval between open and close operations should be tallied with the time interval given in the operating cycle specification. When the circuit breakers are shipped as separate units and reassembled at site, the manufacturer should participate in commissioning test to give confirmation about compatibility of such separate units and components when assembled as a complete circuit breaker. For all required operation sequences, the test should be conducted and all closing and opening operating times along with intervals between two conjugative operations are recorded. Where applicable, the measurements of fluid compression (pressure difference) during circuit breaker operation are also recorded.
No load operating cycle can be performed on the circuit breaker to draw the no-load travel curve. The curve should be within the prescribed envelope of the reference mechanical travel characteristics.
N.B: The parameters should be measured and recorded during operation test of circuit breaker, are given below
Closing time of each pole
Closing time difference between poles or closing mismatch time
Opening time of each pole
Opening time difference between poles or opening mismatch time
Close-Open time of each pole
Time difference between two conjugative opening operation (O-C-O)
Maximum bounce of moving contact during closing operation
Total bounce of moving contact during closing operation
Over travel of moving contact
Contact speed for closing in deg/ms (as transducer is of rotary type)
Contact speed for opening in deg/ms (as transducer is of rotary type)
Damping time during opening
Spring charging time
When sub assemblies of circuit breaker are fitted together at site, the mechanical travel characteristics of the breaker should confirm the correctness at the end of the commissioning test on site. If this is done on-site, the manufacturer should prescribe the exact procedure of doing that otherwise the result may be different and the comparison of the instantaneous contact stroke may be impossible to achieve. The mechanical travel characteristics of circuit breaker contacts are produced by using travel transducer or similar device connected to the circuit breaker contacts mechanism.
In addition to this, each connection in the control and auxiliary circuit in the circuit breaker kiosk should be checked. It should also be checked whether control and/or auxiliary switches correctly indicate the open and close position of circuit breaker. All auxiliary equipments should also be operated correctly and smoothly for specified maximum and minimum allowable control voltage supply.
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