For the smooth functioning of the power system, grid operators need to ensure that the grid is disconnected from the faulty segments in case of a failure. Switchgear enables short circuiting without interrupting the flow of electricity, thus playing a key role in the protection of the electric system. It essentially isolates the circuits from power supply. Switchgear includes circuit breakers that are maintenance-intensive components as they comprise many moving parts. In order to ensure secure and reliable grid operations, adequate testing and maintenance of switchgear and its related components need to be undertaken. As switchgear failures can severely impact the grid, switchgear test procedures, system inspections and a solid maintenance plan need to be implemented to keep them in check.
Need for testing
Typically, the insulation system of metal-enclosed switchgear is designed to have a life expectancy of about 30 years. If switchgear is installed in a well-ventilated and clean environment, it is expected to function reliably for a long period. However, dirt, moisture and a corrosive atmosphere can reduce its design life. Regular inspection and testing of switchgear is, therefore, a critical part of electrical asset management. It can prolong equipment life and prevent unscheduled outages. Usually, the testing of switchgear and circuit breakers is more challenging than that of other parts because the short-circuit current in the former is significantly higher. While switchgear can isolate current faults, circuit breakers can only detect and interrupt short-circuit fault currents. Circuit breaker testing is, therefore, used to test both the performance of individual switching mechanisms and the timing of the overall tripping system.
Switchgear and circuit breaker tests
Testing of switchgear equipment starts at the design stage as the manufacturer is required to adhere to certain performance standards. Type tests are conducted for verifying the capabilities and rated characteristics of these components with national and international standards. These tests are typically carried out at the design stage and include mechanical performance tests, thermal tests, and dielectric or insulating tests. These tests are used to measure stipulated parameters such as rated voltage, insulation level, short-circuit making and frequency.
- Mechanical test: It involves repeated opening and closing of the circuit breaker to ensure that it can smoothly operate at both maximum and minimum allowable control circuit supply voltage within the specified time limits. The closing and opening operation of the circuit breaker also needs to be checked for rated supply voltage of control circuit and ensure that it is able to operate without any mechanical failure.
- Thermal/Temperature-rise test: These tests help monitor the thermal behaviour of the circuit breakers. Under this test, the breaker has to deal with steady state temperature rises due to the flow of rated current through its pole. The test ensures that the temperature rise of the rated current is within the prescribed limits.
- Dielectric test: This test is performed to demonstrate the ability of the insulation system of switchgear to withstand the voltages. In this test, the voltage is increased gradually from zero to the required test value within 5-10 seconds and is held at that value for a minute. The voltages are measured in accordance with the IEC 62271-102 standard (Clause 6.2) of the the International Electrotechnical Commission.
- Short-circuit tests: These include breaking capacity tests, duty cycle tests and short-time withstand tests. Short-time withstand current is the current limit that a circuit breaker can carry safely or withstand for a specified short time under prescribed use and behaviour. The specified time is known as the “rated duration of the short circuit”. The values must comply with the IEC 62271-102 standard (Clause 6.6).
- Tightness test: This test is mainly carried out for gas-insulated switchgear and involves manual testing of breaker connections. It measures the leakage rate in the component and ensures the desired lifespan of the switchgear.
In addition, various routine tests are performed for ensuring the reliability and quality of switchgear and circuit breakers. These tests are conducted to verify that each sample of the product matches the type-tested representative as part of the quality check of materials. It is crucial that these tests are performed under stable conditions at a suitable temperature so that there are no variations in the data. Certain type tests such as dielectric tests (of the main circuit) and tightness tests are also conducted as part of the routine tests. These are carried out in accordance with the IEC 60298 standard and performed as final acceptance tests before delivery. Some key routine tests are:
Visual and mechanical inspection: This involves the inspection of the physical, electrical and mechanical condition of switchgear or a switchboard, including its anchorage, alignment, grounding and required clearances. Maintenance inspection also requires inspecting for moisture or corona, or any evidence of leakage. The assembly base should also be checked for openings that could permit water to drain into the interior and damage the equipment.
Circuit-breaker trip testing: This entails analysis of the current consumed by the trip coil during the operation of circuit breakers. It allows the operators to check the circuit breaker for mechanical or electrical.
Measurement of the resistance of main circuit: The resistance of the main circuit is measured by the direct current voltage drop across the circuit. In this test, direct current is injected into the circuit and the corresponding voltage drop is measured to test the resistance of the circuit.
Routine tests also include insulation resistance test, verification of wiring, tests for auxiliary and control circuits, and design checks. Meanwhile, operational tests are performed on the breaker by simulating its tripping by artificially closing the contacts of relays. By evaluating different performance parameters, these tests identify the areas requiring maintenance.
Special tests and maintenance needs
Apart from these tests, there are tests for special applications that are specified by the user or are needed for specific equipment. At the commissioning stage, detailed tests are carried out to confirm the functionalities of the switch or circuit breaker, secondary wiring, protection, indicators, etc. The commissioning and supervision of work should be carried out by a manufacturer or a main contractor with significant experience and expertise.
Even though switchgear equipment usually requires low maintenance, various preventive and corrective maintenance measures can be taken at regular intervals (yearly basis) to reduce power outages or switchgear failure. Since switchgear equipment is highly sensitive to temperature changes, it is important to monitor temperature control panels, thermostats and actual room temperature during inspection.
Switchgear equipment is installed at the key points of power transmission and distribution systems. Going ahead, as energy demand increases and the grid becomes more complex with the influx of renewable energy, switchgear components are expected to play a crucial role in controlling faults and energy flow. Therefore, reliable and robust switchgear operations are the key to maintaining a secure and efficient power supply. Switchgear equipment will have to deal not only with variable generation but also with the decentralised nature of upcoming capacity. This requires cost-effective technological innovations and hybrid switchgear solutions, particularly for high voltage systems.