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

Experience is reported of short-circuit testing of large power transformers during the past 14 years by KEMA in the Netherlands. In total, 119 transformers > 25 MVA participated in the survey. KEMA shows that at initial access to standard IEC short-circuit tests, 28% failed initially in a wide range of power (119 transformers 25 - 440 MVA, 20 – 500 kV tested). No clear single cause for major failures could be identified; instead, a variety of defects was observed, mostly because of the dynamics associated with passage of short-circuit current. It is the author's observation that change in winding impedance due to short-circuit current passage is an excellent indicator for the short-circuit withstand capability. The authors comment on often heard considerations to replace short-circuit testing by design review procedures, mainly because of costs: ?? "There is no necessity to test because calculation methods are sufficiently adequate". ?? "There is no necessity to test because service experience with transformers is so good". ?? "The costs of testing have become prohibitively high and testing takes too much time". ?? "Short-circuit testing may have a deteriorating influence on the transformer under test". The authors conclude that design review, as the only component of quality assessment of power transformers, is not sufficient since 1/3 of the transformers that failed in service passed design review. No transformer failed in service had been short-circuit tested prior to installation. In addition, it is demonstrated that a major leap in quality improvement could be realised through full-power testing. It is concluded that although statistically every transformer faces several full short-circuits during its life, the current that really occurs at such a full shortcircuit current is smaller than the rated short-circuit for which the transformer is designed. In the future, however, this gap will narrow due to more efficient utilisation of the networks and growth of energy consumption. A brief overview is given on test methods and test installations of the author's laboratory.