Significant failure rate observed at short-circuit testing of large power transformers

Recent experience shows that at initial access to short-circuit tests, 48% of the transformers (with power 25 MVA and above) fail to pass short-circuit tests. When only the change of winding reactance (in accordance with IEC standard 60076-5) is taken into account, the initial failure rate drops to 28%. These results are discussed in the context of the reported much smaller failure rate in service. 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 short-circuit 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.