International studies have indicated that the failure rate of transformers is around 0.6%, with converter transformers failing up to 5%. In depth study reveals that a major portion of these failures (up to 20%) is directly related to short-circuits. During a short-circuit, the large currents involved lead to severe mechanical forces and stresses in the transformer windings, which may become deformed when the structural design of the transformer is not adequate.
Two methods in order to verify whether a transformer survives a short-circuit are practiced today. The first one is “design review”, in which third-party consultants check calculation results of forces and stresses and compare these with critical values based on tests or based on internal manufacturer’s rules. Design review is based on calculation results of idealized, homogeneous structures, it does not cover transient phenomena, it excludes a number of key subcomponents and it is not embedded in a strict quality surveillance system.
The second verification method is “short-circuit testing”, in which the complete transformer is subjected to real short-circuit current and thus to the same stresses as would occur in service.
Short-circuit testing is the only complete verification method of short-circuit withstand capability of power transformers. In the last few years, KEMA laboratories was extended significantly, among others to deal with the increasing request for transformer short-circuit tests. Having now six 2250 MVA short-circuit generators and ten short-circuit test transformers (making in total 550 kV), we are now ready to test power transformers with rated voltage up to 800 kV and power up to 1000 MVA (based on three-phase transformer banks). By the time of writing, already several 800 kV class power transformers have been tested.
In spite of the wide application of advanced calculation methods, still around 20 – 30% of the transformers submitted to a short-circuit test, fail to pass the standardized short-circuit tests. The results suggest a tendency of the highest initial failure rates for the highest ratings: the failure rate of the largest transformers (> 300 kV or > 200 MVA), around 100 tested, is in the range of 30%.
These statistics are based on a population of 320 power transformers of 25 MVA and above in a 21 year time span. Mostly, the reason for failure is a reactance increase beyond the limit set by the IEC 60076-5 standard, which indicates an unacceptable internal deformation.
In a number of cases, however, unexpected events are triggered by short-circuit current which are outside of the “usual” failure modes, like breaking of a bushing, oil spill, internal flashover etc. The rate of failure to pass a short-circuit tests hardly changes over time, which may be related to the high pressure on costs that lead to design close to the margin.