Transformer overvoltage problems and countermeasures at black start

A system of Kyushu Electric Power Company Inc. or KEPCO (about 19 GW) belongs to the western Japan system (110 GW) and has a 500-kV and 220-kV transmission line in a loop configuration. Japan´s power system has high reliability and experienced no blackout for the past several decades. However, there is a possibility, though very slight, of a blackout in the event of a big earthquake or unforeseen accident. Considering the risk of blackout, KEPCO has been conducting studies on system restoration. This paper discusses a mechanism of an overvoltage phenomenon during a restoration period in which, about 0.2 seconds or more after a transformer is energized, a voltage becomes maximum in excess of 200% of the rated voltage. The paper also discusses countermeasures against this phenomenon.