Transformer Testing

Measures to maintain equipment health

The quality and reliability of power supplied to consumers is largely dependent on how efficiently it is transmitted from power plants. As per a Power Finance Corporation report, transmission losses stood at 21.81 per cent in 2015-16. In the transmission and distribution (T&D) system, the transformer plays an instrumental role since it scales down high voltage energy, making it usable at the customer level. Hence, maintaining transformer operations is crucial to ensure transmission system efficiency. Moreover, transformer failures are costly and time-consuming problems.

While a wide range of operations and maintenance measures are taken to maintain transformer health, transformer testing is done to identify faults. It gives an insight into the transformer’s functioning and helps reduce the chances of failure. Various testing techniques are employed in order to gauge the efficacy and reliability of transformers. Power Line takes a look at some of the widely used testing measures…

Routine tests

Routine tests are conducted to evaluate the operational performance of individual units in a large production lot. There are different types of routine tests such as ratio test, winding resistance test, polarity/vector group test and load test.

The ratio test is one of the most vital tests for maintaining transformer performance. It measures the induced voltages at the high voltage and low voltage terminals of transformers and then calculates the actual transformer voltage. Ratio measurements are conducted on all taps and are calculated by dividing the induced voltage reading by the applied voltage value. When ratio tests are being conducted on three-phase transformers, the ratio is calculated for one phase at a time.

However, several factors like physical damage, deteriorated insulation, shipping damage and contamination can change the transformer turns ratio. If the deviation in the transformer turns ratio is more than 0.5 per cent from the rated voltage ratio, the working of the transformer could get severely impacted.

The winding resistance test measures the resistance of windings by applying a small direct current voltage to the windings and measuring the current through it. The test is dependent on several conditions. To begin with, the measured resistance should be corrected to a common temperature such as 75 °C or 80 °C. Further, the transformer is shut down for three to four hours prior to the test, in order to ensure that the winding temperature becomes equal to the oil temperature. To minimise observation errors, the polarity of the core magnetisation has to be kept constant in all resistance readings. On the manufacturing premises, the computation of winding resistance measurements in transformers is of fundamental importance for the computation of the I2R losses; computation of winding temperature at the end of the temperature rise test of transformers; and for assessing possible damages in the field. On site, the test is employed to check for faults caused by loose connections, broken strands of the conductor, high contact resistance in tap changers, high voltage leads, bushings, etc.

In order to ensure the successful parallel operation of transformers, the polarity/ vector group test is conducted. Polarity primarily refers to the direction of induced voltages in the primary and secondary winding of the transformer. For the test, the primary and secondary windings are connected together at one point and the low voltage three-phase supply is applied to the high voltage terminals. Voltage measurements are then taken between various pairs of terminals.

The load test helps determine the total loss that takes place when the transformer is loaded. The test is done to determine the rated load of the machine, and determine the voltage regulation and efficiency of the transformer. Rated load is determined by loading the transformer on a continuous basis and noting the steady rise in temperature. The losses generated inside the transformer due to loading appear as heat.

Pre-commissioning tests

The tests that are conducted before the commissioning of the transformer are called pre-commissioning tests. The objective of these tests is to assess the condition of the transformer after it has been set up, and compare the test results with the factory test reports. All transformers are subjected to pre-commissioning tests such as protective relay testing, transformer oil testing and magnetic balance test.

The protective relay test involves the testing of the communication path between the relays to ensure that it is not broken and the signal strength is within the specified limits. The test, as the name suggests, is a predictive maintenance measure to check transformer health.

Transformer oil testing is a widely used loss prevention technique. Transformer oil not only serves as a heat transfer medium, but is also a part of the transformer’s insulation system. Hence, these tests serve the larger objective of monitoring the transformer’s dielectric strength. The test starts with collecting the oil from the transformer and connecting the equipment with grounding wire. The next step is to set the air gap between the electrodes and dip these in transformer oil. The associated equipment is then switched on and readings recording the voltage increase are noted. The average of six readings is considered for the analysis.

In order to check the imbalance in the magnetic circuit of three-phase transformers, the magnetic balance test is used. The test begins by disconnecting the transformer neutral from the ground and applying 230V AC supply across one of the HV winding terminals and the neutral terminal. The test is repeated individually for each of the three phases before arriving at a conclusion.

Type tests

Type tests are conducted to demonstrate that the transformers comply with the specified requirements, which is not covered by routine tests. There are two type tests that are widely used temperature rise test and dielectric type test.

The temperature rise test primarily determines whether the temperature rising limit of the transformer winding and oil is as per the defined specifications. For testing the temperature of oil, a thermometer is placed in a pocket in the transformer top cover while two other thermometers are placed at the inlet and outlet of the cooler bank. Hourly readings of the three thermometers are taken and the ambient temperature is measured by means of a thermometer that is placed around the transformer at three or four points.

The dielectric type test is performed in two separate steps, which involve 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 together and a single-phase power frequency voltage is applied for 60 seconds. The test is successful if no breakdown occurs in the dielectric of the insulation. This test is primarily 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, three-phase voltage is applied to the secondary winding. The test starts with a low voltage, which is gradually increased up to the desired level. The test is declared to be successful if no break occurs at the full voltage level.

Special tests

Special tests are those that are conducted for specific parameters. The short circuit withstand ability test is a kind of special test.

The short circuit test measures the ability of the transformer to withstand the mechanical and thermal stresses caused by an external short circuit. The test involves connecting the high voltage terminals to the supply bus of the testing plant. After this, the low voltage is short circuited and the testing plant parameters are adjusted to provide the rated short circuit current. After a specified duration of the short circuit, the supply is closed. The test is considered to be successful if no mechanical or waveform distortion occurs and there is no fire in the machine during the test. The transformer should be able to withstand the routine tests after the short circuit test.

Other special tests include dielectric special tests, zero-sequence impedance on three-phase transformers and the harmonic analysis of no-load current.

Conclusion

There are a host of tests that enable utilities to identify and measure the different performance aspects of a transformer. This highlights a significant opportunity for testing equipment manufacturers. As utilities strive to improve power quality and lower loss levels, testing standards and requirements are expected to become more stringent. This can provide a boost to the equipment manufacturing segment. Moreover, given the ongoing digitisation initiatives in the sector, testing procedures are also likely to get automated, thereby opening up the testing equipment segment to a broader range of products.

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