The study of the oscillatory lightning surge in transformers and the calculation method of its frequency

When the surge into the transformer becomes a consecutive oscillation waveform and its oscillation frequency equals the resonance frequency of the transformer winding, resonance overvoltage that threatens the insulation of the transformer may be generated. We studied the condition that the lightning surge in the transformer becomes an oscillation waveform and the calculation method of its oscillation frequency, targeting the all-GIS 500 kV substation. As a result, we found out that, when the number of connecting circuits of the substation is more than "two transmission line and two transformers", the lightning surge in the transformer becomes an oscillation waveform. But in case the 500 kV substation's transformers are directly connected to GIS, we could confirm that, its oscillation frequency is 400-600 kHz, and ten times as high as the resonance frequency of the transformer winding. Furthermore, we found out its oscillation frequency equals the resonance frequency that could be calculated from the L-C circuit between two transformers.