Simulation of neutral point overvoltage and research on new protection measures for 220kV transformer

In order to limit single-phase grounding short-circuit current, prevent communication interference and satisfy relay protection setting and configuration, some transformer neutral points often adopt non-grounding operation mode on 220 kV neutral point effective grounding system. During operation, transformer neutral point may withstand the lightning overvoltage, switching overvoltage, single-phase grounding and ground-fault overvoltage, non-full-phase switching overvoltage. Such overvoltage may damage the insulation of transformer neutral point. At present, the neutral point of ungrounded transformer in our country is usually protected by gap and/or arrester. The overvoltage mentioned above may cause misoperation of protection gap or thermal collapse of arrester, which would affect the safe and reliable operation of the system. For this reason, the typical 220 kV system was selected and the typical lightning overvoltage, switching overvoltage, single-phase grounding and ground-fault overvoltage, non-full-phase switching overvoltage applied to transformer neutral point under various operation modes were simulated by electromagnetic transient calculation program (EMTP). Based on the calculated overvoltage values, the insulation protection measures of transformer neutral point were studied and made be reasonable. The parameters configuration and the research results were of great significance to ensure the reliable operation of 220 kV system and the safety of equipment.