When the operating point of the transformer moves to the nonlinear saturation region due to DC bias, the excitation inductance of the transformer will no longer be constant, which will directly distort the system voltage and cause voltage fluctuation. The specific distortion is related to the winding connection and magnetic circuit structure of the transformer. In China, transformers with 1 10kV and above generally adopt Y0/△ wiring. Y0/△/Y0 connection is generally adopted for EHV large capacity transformers, especially autotransformer. When the magnetic core works in the severe saturation region, the leakage flux will increase, which will flatten the voltage peak to some extent.