Power transformers are costly assets and they play a key role in every power distribution system. It's not surprising that the Megger technical support team receives many questions about transformer test techniques. Here we are looking at some of those questions with an emphasis on winding resistance testing.
Q: Is it necessary to use a dedicated transformer ohmmeter to make accurate winding resistance measurements on power transformers?
A: The use of an instrument that has been designed with transformer resistance testing in mind is strongly recommended, as it will give more dependable results more quickly and more safely, especially with large transformers. It will also include provision for demagnetising the transformer core after testing has been completed. This is important as, if a transformer is returned to service with a magnetised core, a large and potentially damaging inrush current may flow. It may not, however, be necessary to use a dedicated transformer ohmmeter. A new type of multifunction tester has recently reached the market which provides an extensive range of tests for transformers, including not only winding resistance tests but also, for example, turns ratio tests, leakage reactance measurement, and tan delta measurement. These new testers can also perform basic measurements on circuit breakers, protection relays and many other items of equipment used in power distribution networks. For many users they are, therefore, a better and more useful investment than a dedicated single-function transformer ohmmeter.
Q: When measuring winding resistance in a transformer, how do I decide on the right test current to use?
A: With DC resistance measurements, the objective is always to try to saturate the transformer core as this reduces the effective inductance of the winding and allows the test current to stabilise more quickly. Saturation typically occurs when the test current is around 1% of the rated current for the winding. It is, however, usually advantageous to use a somewhat higher test current than this to minimise the effect of noise on the measurements. If the test current is too low, it will often be found that successive measurements give inconsistent results. Nevertheless, test currents of more than 10% of the rated current should be avoided, as they are likely to lead to erroneous results because of heating of the winding. In most cases, the optimum test current is between 1% and 10% of the rated current.
Q: When should winding resistance measurements be carried out?
A: Winding resistance tests are usually carried out on new transformers after they are delivered to site and before they are put into service as a way of detecting transit damage. Such tests will also provide baseline results for comparison with measurements made later in the life of the transformer. Winding resistance tests can also be performed as part of scheduled maintenance as an aid to detecting incipient faults. Finally, winding resistance tests are invaluable when fault finding transformers, as many transformer faults or problems will cause a change in the DC resistance measured from the bushings.
Q: What types of fault can be found with winding resistance tests?
A: Faults that can be detected using resistance tests include shorted and open turns, defective internal connections, and problems with tap-changer contacts, including open circuits during on-load tap changer operations. Information derived from the resistance tests usefully complements information from other transformer diagnostic tests, such as frequency response analysis (FRA), dielectric frequency response (DFR) and power factor measurements. Winding resistance measurements can also help in the determination of winding losses.