Performance Check

Key tests to identify and measure transformer health

For the smooth functioning of the power system, it is important to ensure that the quality and reliability of power supplied to consumers is transmitted efficiently from power plants. High transmission losses are a cause of major revenue losses for distribution utilities. In the transmission and distribution (T&D) system, transformers play an instrumental role since they scale down high voltage energy, making it usable at the customer level. Hence, maintaining transformer operations is crucial to ensuring transmission system efficiency. Moreover, transformer failures are costly and time-consuming to deal with.

Unscheduled outages of transformers due to unexpected failures can have serious implications. Diagnosis and proper monitoring are, therefore, important to increase the life expectancy of a transformer. To gauge the efficacy and reliability of transformers, and ensure they conform to specifications, a number of testing procedures are undertaken. Transformer testing methods are broadly divided into two based on where the test is conducted. Some of the tests are done at the factory during the manufacturing process; others are done at site at the time of installation. A look at some of the widely used testing measures…

Tests undertaken at the factory

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 tests, winding resistance tests, polarity/vector group tests and load tests.

The ratio test is one of the key 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 such as physical damage, deteriorated insulation, shipping damage and contamination can change the transformer turns ratio (TTR). The TTR test confirms that the transformer has the correct ratio of primary turns to secondary turns. Using this test correctly can help identify tap changer performance, shorted turns, open windings, incorrect winding connections and other faults inside the transformer.

Transformer bushing CTs can be tested using the current ratio test method before the transformer has been completely assembled. The CTs should be tested before they are mounted on the transformer. In some cases, the CTs may have to be tested by connecting test leads to both ends of an installed bushing. Occasionally, it is not possible to perform a current ratio test. CT tap ratios can be verified by applying a voltage across the full CT winding, that is, a tap voltage ratio test, and then measuring the voltage drop across each individual tap.

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. 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 the winding temperature at the end of the temperature rise test of the transformer, and the assessment of 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 the 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 at one point and 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 conducted to determine the rated load of the machine and the voltage regulation and efficiency of the transformer. The 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.

Type tests

Type tests are done mainly in a prototype unit, not in all manufactured units in a lot. Type tests of transformers confirm the main and basic design criteria of a production lot. Type tests are conducted to demonstrate that the transformer complies with the specified requirements that are not covered by routine tests.

There are two type tests that are widely used: the temperature rise test and the 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 the oil, a thermometer is placed in a pocket in the transformer top cover and 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 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 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 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. This 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 harmonic analysis of no-load current.

Tests done at site 

Pre-commissioning tests

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. Along with oil sample quality tests, performing a dissolved gas analysis (DGA) of the insulating oil is useful in evaluating transformer health. A breakdown of electrical insulating materials and related components inside a transformer generates gases within the transformer. The identity of the gases being generated is very useful information in any preventive maintenance programme. There are several techniques for detecting those gases. DGA is recognised as the most informative method. This method involves sampling the oil and testing the sample to measure the concentration of the dissolved gases. Where DGA results include a sharp increase in key gas concentration levels and/ or normal limits have been exceeded, it is suggested that an additional sample and analysis be performed to confirm the previous evaluation and determine if the key gas concentrations are increasing.

Conclusion

A wide range of tests exist, which enable utilities to identify and measure a transformer’s operational aspects. To improve their operational efficiency, utilities are striving to reduce their loss levels. This highlights a significant opportunity for testing equipment manufacturers. Moving ahead, more stringent testing standards will give a boost to the equipment market. Moreover, given the ongoing digitalisation 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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