Routine tests of circuit breaker comprise power frequency overvoltage withstand test, dielectric test, contact resistance test, tightness test, visual checks, mechanical operation test, and SF6 gas leakage test.
The main purpose of the Routine tests is to ensure the proper functioning of the circuit breaker. The manufacturers perform these tests in the various stages of production.
Testing of Circuits Breakers is usually more difficult in comparison to other electrical equipment like motors and transformers because of the high magnitude of short circuit current in circuit breakers.
Therefore, circuit breaker testing ensures the operation of each switching system and the programming of the entire tripping structure. To ensure the safety and reliability of performance of this key link in the power asset chain, Circuit Breaker Testing is essential. Usually, Circuit Breakers perform three main tasks:
- In an event of a malfunction, the circuit breaker must disconnect the fault current as quickly and reliably as possible which in turn protects all subsequent equipment.
- When closed, the circuit breaker must conduct current as efficiently as achievable.
- When open, they should be able to insulate the contacts from one another as effectively as possible.
Why Testing of Circuit Breaker Important?
We know that a circuit breaker can remain idle for years but under certain conditions whenever malfunction occurs it must detach fault currents of huge kiloamps within a few milliseconds. Some of the major errors that happen on Circuit Breakers are incorrect behavior, damage/wear to mechanical connections, short circuits in coils, or the insulation material.
Therefore, circuit breakers need to be carefully and regularly tested. Circuit breakers perform a vital role in protecting expensive equipment from damage through faults which requires regular maintenance tests during its lifetime to prevent costly failures and problems that could compromise the safety of the substation. Therefore, testing the performance of Circuit Breakers regularly proves to be an essential and cost-effective part of any maintenance strategy.
Routine Testing Methods
We carry out numerous routine tests on the circuit breaker to maintain the quality and performance of circuit breakers. We will discuss the routine tests of circuit breakers one by one in the below sections.
The Indian standards of Engineering Service and Indian Standards recommend the types of tests for a circuit breaker. A circuit breaker is an electrical equipment that has paramount importance in view of electrical safety. Therefore, confirmation of the proper working of the circuit breaker is a must for the protection of the electrical network.
Routine Tests
The following routine tests of the circuit breakers are performed.
- Power Frequency Over Voltage Withstand Test
- 1 Minute Dry Power Frequency Voltage Withstand Test
- Tightness Test & Leakage Test
- Resistance measurement Test
- Visual Checks
- Mechanical Operation Test
Now, we will discuss the routine tests of circuit breaker in detail.
Power Frequency Over Voltage Withstand Test
One of the main routine tests of Circuit Breaker is Power Frequency Over Voltage Withstand Test. Normally a power system might experience different temporary overvoltage conditions due to the wrong operation of the online tap changer, insufficient shunt compensation in the system, or due to sudden cut of load from the system.
Manufacturers perform this test to verify the insulation strength sufficiency of the main circuit in order to withstand the highly abnormal overvoltage conditions of the system. In order to withstand all kinds of over-voltage and stresses, a circuit breaker undergoes different kinds of voltage or dielectric tests.
1 Minute Dry Power Frequency Voltage Withstand Test
According to the basic assumption, the overvoltage conditions at power frequencies usually don’t sustain beyond one minute time. Mostly they sustain for much below than one-minute time duration. Therefore, this test verifies whether the insulation provided in the main circuit of the breaker is capable of withstanding the power frequency of overvoltages for a long one-minute duration or not. The test is carried out during dry conditions of the breaker.
Tightness Test & Leakage Test
This routine test is mainly suitable for gas-insulated switchgear like the SF6 breaker. This test measures the leakage rate and this test ensures the desired lifespan of the switchgear. All the joining points of the gas-containing paths are covered airtightly with thin polythene sheets for more than 7- 8 hours. And, then inside these covers, we measure the gas density by inserting the gas detecting port of a gas detector through a hole created on the covers. As a standard practice, the maximum limit of gas leakage is 3ppm/8hours.
Resistance measurement of the main circuit
In this kind of routine test, we measure the DC voltage drop across the breaker contact on the injection of direct current into the circuit. The injected current would be from 100A to the maximum rated current of the circuit breaker.
R = V/I
Where,
V= Voltage drop across breaker contact
I = Magnitude Injected DC current
Visual Checks
Visual checks may not sound important but these tests can prove to be very crucial. The language and data on the templates should be visually checked as well as the proper identification mark of any auxiliary equipment, color, and quality of paint and corrosion on metallic surfaces, etc.
Mechanical Operation Test
A circuit breaker must smoothly operate at maximum as well as minimum allowable auxiliary and control circuit supply voltage. For a specified operating range of control circuit supply voltage, we must perform the closing and tripping operation at least 5 times. The minimum and maximum voltage limit for the closing and opening operation of the circuit breaker is 85 to 110% of the control voltage respectively.
During maximum and minimum control voltage operation, the operation time may be less and more respectively for the rated control voltage, but the closing and opening time must be within specified time limits.
In the case of pneumatic circuit breakers, the breaker’s allowable operating pressure may be 5 times of specified maximum allowable operating pressure, in specified minimum allowable operating pressure, and in specified rated operating pressure.
A circuit breaker is used for the rapid auto reclosing application. The operating duty for such an application is different than the normal operation of the circuit breaker. For auto reclosing circuit breakers, it is essential to check at least 5 open close operating cycles. The procedure for finding the opening and closing time of the breaker is;
- Record the open-close time for each operating cycle
- Compare the measured time with the rated time
- Adjust the breaker mechanism to get the rated open-close time
The above test is indispensable when the parts of circuit breakers are shipped and reassembled at the site. The test ensures that the breaker’s parts are properly fitted.
