Online identification of single-phase grounding fault at low-voltage side of excitation transformer

When single-phase grounding fault occurs at the low-voltage side of excitation transformer,the ping-pong type rotor grounding fault protection may detect the reduction of grounding insulation resistance and misconstrue it as the single-point grounding fault of excitation winding with location percentage of 50 %.A method to online identify the single-phase grounding fault at the low-voltage side of excitation transformer is proposed. The criteria are:the calculated rotor winding grounding resistance is smaller than the setting value;the grounding location percentage is about 50 %;and the ratio of the fundamental voltage at any terminal of excitation winding to the excitation voltage is larger than the setting value. Theoretical analysis and field waveform records prove its correctness.