Substations and switchgear are key elements in power systems and perform the functions of voltage transformation, system protection, power factor correction metering, and circuit switching. Beyond the financial value, they play a major role in overall business performance. Regular inspection and testing of switchgear is critical in electrical asset management. It can prolong equipment life and prevent unscheduled outages as well as safety. It can also improve reliability, reduce maintenance costs and increase predictability. Some of the benefits of switchgear testing are an improvement in the quality and safety of products and processes, and a reduction in time, costs and risk exposure by meeting regulatory norms.
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 used to measure stipulated parameters such as rated voltage, insulation level and short-circuit frequency.
- Mechanical test: This involves repeated opening and closing of the circuit breaker to ensure that it can operate smoothly at both maximum and minimum allowable control circuit supply voltages within the specified time limit. The closing and opening operation of the circuit breaker needs to be checked for the rated supply voltage of the control circuit to ensure that it is able to operate without any mechanical failure.
- Thermal/Temperature rise test: This test helps monitor the thermal behaviour of the circuit breaker. Under this test, the breaker has to deal with steady state temperature rises due to the flow of the 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 the switchgear to withstand the voltage. 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 International Electro-technical Commission.
- Short-circuit test: This includes 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 for certain 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 circuit breaker connections. This test measures the leakage rate in the components and ensures the desired lifespan of the switchgear.
Apart from the above tests, 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 a 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 a 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, circuit breaker trip testing and measurement of the resistance of the main circuit.
Equipment condition monitoring
Equipment condition monitoring (ECM) involves the use of condition-based monitoring systems for substation assets through a combination of intelligent electronic devices, smart sensors, open protocols and head-end software. It can be seamlessly integrated in the existing substation communication and visualisation infrastructure, from simple bay controllers to high-end control centre applications. The application of ECM includes real-time monitoring of partial discharge in transformers and gas-insulated switchgear; the temperature, pressure and liquid level of oil in transformers; moisture levels, dissolved gas analysis, cooling performance, load tap control, fault location and loading on transformers; and performance of breakers, bushings, capacitor banks, surge arresters, reactors and protection system condition monitoring.
Conclusion
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 good health. Proper understanding of the condition of substation equipment helps prevent failure, maximises future operations, enables appropriate scheduling and determines the extent of inspections and maintenance, provides personnel safety, and protects the environment. In the near future, ECM can grow multifold to cover a wider range of substation equipment.