Transformers are subject to problems. Problems or faults occur due to poor design, assembly, handling, poor environments, overloading or poor maintenance. Measuring the resistance of the windings assures that the connections are correct and the resistance measurements indicate that there are no severe mismatches or opens. Many transformers have taps built into them. These taps allow ratio to be increased or decreased. Any of the ratio changes involve a mechanical movement of a contact from one position to another. These tap changes should also be checked during a winding resistance test.
Regardless of the configuration, either wye or delta, the measurements are normally made phase-to-phase and comparisons are made to determine if the readings are comparable. If all readings are within one percent of each other, then they are acceptable. Keep in mind that the purpose of the test is to check for gross differences between the windings and for opens in the connections. The tests are not made to duplicate the readings of the manufactured device which was tested in the factory under controlled conditions and perhaps at other temperatures.
Transformer winding resistances are measured in order to check for abnormalities due to loose connections, broken strands, and high-contact resistance in tap changers. Interpretation of results is usually based on a comparison of measurements made separately on each phase in the case of a wye-connected winding or between pairs of terminals on a delta-connected winding. Comparisons may also be made with original data measured in the factory.
Protec Equipment Resources stocks a variety of Winding Resistance Test Sets from Vanguard test, Raytech and Adwel. To compliment your transformer testing Protec also offer TTR and Power Factor test sets from Vanguard, Raytech, Megger Power Factor and Doble Power Factor. All products are backed with the best application support available 24/7.