With large capacity addition and growing deployment of advanced technology in the power sector, the demand for testing and measurement (T&M) of critical equipment like switchgear, substations and circuit breakers is expected to rise. Further, increasing concerns of reliability and safety call for new maintenance strategies and T&M techniques. Therefore, there is a need to keep abreast with the latest technologies and practices in the T&M space to ensure testing accuracy and compliance of the equipment with all applicable standards and regulations.
Switchgear enables short circuiting without interrupting the flow of electricity. 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 good health.
Types of 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 the stipulated parameters such as rated voltage, insulation level and short-circuit 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 needs to be checked for rated supply voltage of control circuit to 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 switchgears 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 International Electro-technical 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 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, there are certain 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 (on a yearly basis) to reduce power outages or switchgear failure. Moreover, these checks are quick and easy to perform on site, and can be tested on or off load. They test the performance of the entire tripping cycle as well as the overall timing of the tripping system. Circuit breaker (CB) testing is able to identify the need for maintenance and find early indications of possible problems. It is further able to test the record database for trending and picking out repetitive failures or bad actors.
According to the Central Electricity Authority (CEA) data, it is observed that seven cases of CB failure were reported to it during the period from September 2015 to December 2016 by four utilities. Of the seven CBs, two CBs are of 400 kV class and the rest are of 220 kV class. In most of the cases, the cause of the failure of CB was internal insulation failure. In all these cases of, failed pole or component such as pre-Insertion resistor or grading capacitor had to be replaced with a new one. Repair of the failed CB was not possible in any of the cases. While formulating specifications for the procurement of CBs for new substations, a provision for the procurement of operational analyser along with the dynamic contact resistance measurement test kit should be included for one substation or a group of nearby substations, depending upon the requirement.