The supply of quality and reliable power depends on the efficient functioning of transmission and distribution (T&D) assets. Since transformers play a critical role in the transmission of power, it is important to maintain their health through regular testing and maintenance. This not only ensures transmission system efficiency, but also lowers the T&D losses in the system. Transformer testing techniques help identify faults and provide an insight into the transformer’s functioning to reduce the chances of failure. Type tests, routine tests and special tests are performed at the manufacturing facility itself while certain pre-commissioning tests such as periodic/ condition monitoring tests and emergency tests are carried out at the consumer’s site.
Type and routine tests
The objective of type tests is to ensure that the transformers comply with the specified standards. The various kinds of type tests include winding resistance tests, transformer ratio tests, transformer vector group tests, short circuit tests, open circuit tests, dielectric tests, temperature rise tests, tests on on-load tap-changer, and vacuum tests on tank and radiators. Routine tests are conducted to check the operational performance of individual units in a production lot.
The two type tests that are widely used are the temperature rise test and the dielectric type test. The former is used to assess whether the temperature rising limit of the transformer winding and oil is as per the specified provisions. To test the oil temperature, a thermometer is placed in a pocket in the transformer top cover. Two more thermometers are placed at the inlet and outlet of the cooler bank. Temperature readings are noted until the top oil temperature reaches a steady value and this steady value of the top oil is determined as the final temperature rise of the transformer insulating oil.
The dielectric test is divided into two steps – the source voltage withstand test and the induced voltage withstand test. For the source voltage withstand test, all three-line terminals of the winding are connected and a single-phase power frequency voltage is applied for 60 seconds. The test is a success if no breakdown occurs in the dielectric of the insulation. This test is used to check the main insulation source. The induced voltage withstand test checks the insulation at the line end and the main insulation between windings. For this, the secondary winding is applied with a three-phase voltage. The test starts with a low voltage, which is gradually increased up to the desired level. The test is a success if no break occurs at the full voltage level.
The winding resistance test is carried out as both a type and routine test. It calculates the I2R losses to check loose connections, high contact resistance in tap changers, broken strands of conductor, high voltage leads and bushings, etc. The resistance of transformer windings is measured to ensure that each circuit is wired properly and that all connections are tight.
The ratio test is carried out for ensuring the proper performance of the electrical power transformer. A three-phase voltage power is supplied to high voltage winding, keeping the low voltage winding open. The induced voltages at the transformer’s HV and LV terminals are measured to find out the actual voltage ratio of the transformer.
Onsite Transformer Testing
Several utilities prefer to get power transformers tested by NABL approved independent third-party test agencies to ensure their trouble-free operation. One such organisation is the Electrical Research and Development Association (ERDA) which is accredited by NABL and has commenced onsite testing services for the transformers. ERDA’s new portable loss measurement system has stepped up their existing on-site testing scope from 100 MVA (220 kV) class to 500 MVA (400 kV) class.
The portable loss measurement system is an advanced measuring system that uses passive two-stage compensated technology. The main advantage of a two-stage compensated technology is that the effective burden is zero when feeding into another compensated current transformer (CT). As a result, the errors of the CT are effectively reduced to zero in both magnitude and phase. For voltage measurement, three shielded gas filled (SF6) high voltage standard capacitors and three model voltage dividers are used for measuring the phase to ground voltage in each phase.
Some tests that can performed at manufacturer’s works include measurement of winding resistance, turns ration, no load losses, measurement of load losses and percentage impedance, zero-sequence impedance as well as temperature rise test, magnetic balance test, induce over voltage withstand test and separate source withstand voltage test.
Conclusion
There are several established transformer testing techniques to enable utilities to identify and measure the different performance aspects of a transformer. Besides testing, proper handling, loading, unloading and storage are necessary for the efficient operation of transformers.
Going forward, the demand for transformers is expected to increase, led by T&D network expansion efforts. This will, in turn, increase the need for transformer testing, creating a significant market opportunity for testing equipment manufacturers in the coming years.