Thirteen years test experience with short-circuit withstand capability of large power transformers

The ability to withstand a short circuit is recognised more and more as an essential characteristic of power transformers. IEC and IEEE Standards, as well as other national standards specify short-circuit testing and how to check the withstand capability. Unfortunately, however, there is extensive evidence that the matter is not as simple as the standards make it sound. Failures caused by short circuits are still a major cause of transformer outages, though failure rates vary widely in different countries and systems, depending on various circumstances, network characteristics and the equipment installed. Experience is reported of short-circuit testing of large power transformers during the past 13 years by KEMA. In total, 102 transformers >= 25 MVA participated in the survey. Test statistics show that at first access to standard IEC 60076-5 short-circuit tests, 28% failed initially. Failures were observed throughout the complete range of voltage (20 - 500 kV) and power (25 - 440 MVA) tested.