Transformer overvoltages and countermeasures during blackstart

A system of Kyushu Electric Power Company Inc. or KEPCO (about 19 GW) belongs to the Western Japan system (110GW) and has 500-kV and 220-kV transmission lines in a loop configuration. Considering a risk of blackout, KEPCO has been conducting studies on system restoration. A power system, when it recovers from a blackout, is very small in scale, incomparable to that of a normally operational system. Hence, in a system undergoing a restoration process, overvoltages of a magnitude not conceivable in normal power systems may occur causing damages to power system equipment. This work discusses a mechanism of an overvoltage phenomenon during a power system restoration, in which after the transformer is first energized, a repetition of saturated and unsaturated states of transformer core flux causes a transformer terminal voltage to increase excessively. The work also describes countermeasures for this phenomenon.