Transformer Internal Over-Voltages Caused by Remote Energisation

There have been several cases of dielectric faults and failures in power transformers identified internationally that have been associated with system operation. This paper discusses UK examples and concludes the evidence indicates internal over- voltages arising from part winding resonance. These resonances are initiated by remote energisation, either intentionally or by the action of automatic re-energisation schemes after line trips. Some designs are more prone to the problem, and it is here that diagnostics such as sweep frequency response analysis may give some indication as to designs more at risk.