Evaluation of the Impact on Transformer Insulation by Lightning Surge Voltage Analysis on Wide‐Area Power Network

When lightning surges simultaneously enter three phases of the primary circuit of a transformer, specific frequency components within the lightning surge may match the resonance frequency of the transformer. This can lead to overvoltages at the neutral point of the transformer windings, potentially causing insulation breakdown. In this study, an analysis simulating the entry of the lightning surge into a transformer due to indirect lightning was conducted, and the insulation impact at the transformer neutral point was investigated. As a result of simulations modeling a wide‐area power network, voltages containing frequency components corresponding to the resonance frequency of the transformer neutral point in the range from several to several tens of kHz were confirmed. While it is self‐evident that a direct lightning strike to a transmission tower can lead to transformer failures, it has been suggested that lightning surges occurring in transformers as a result of indirect lightning may also cause transformer failures owing to overvoltages at the transformer neutral point.