Features of Testing Distribution Transformers for Short Circuit Withstand Strength

Short circuit (SC) withstand strength is one of the key characteristics that define the reliability of power transformers. The SC withstand strength test methods are stated in State Standards GOST (State Standard) 20243 and 55188 (IEC 60076-5). This work considers problems related to the SC withstand strength test of distribution transformers that are not indicated in these standards. The dependence of SC peak current on average temperature of windings is considered. The possibility of heating of the windings of distribution transformers during the SC withstand strength test and related to it change of peak factor are revealed. The requirements of standards related to the winding temperature when the SC withstand strength test are considered. It is revealed that during the test of distribution transformers, one can expect the deviation of actual peak factor from its designed value by about 5% that should be taken into account when testing and analyzing the test results. The specific of test of the transformers with δ-connected winding is that, as a rule, merely the line currents of this winding can be recorded during SC experiments, at that, there are differences in peak currents for line currents and phase currents of windings. The requirements presented in GOST (State Standard) 20243 and R 55188 (IEC 60076-5) for SC peak current were stated as applied to the phase currents of windings, as well as to the line currents of star-connected windings. The difference of the peak factor for the line currents when testing the transformers with star- and δ-connected windings was revealed in the work, and it was concluded that the abovementioned standards for the requirements and test methods to SC withstand strength of the transformers need to be refines.