Breakdown characteristics and gap distance standardization of neutral point gap in 110kV and 220kV transformers

When the local ungrounded system is formed, and the low and medium voltage power supply of 110kV or 220kV transformer cannot be cut off by relay protection device within a certain period of time, or the ferromagnetic resonance over-voltage caused by open phase operation of circuit breaker is empirically calculated, which endangers the insulation of neutral point of 110kV and 220kV neutral point ungrounded transformer, and no phase loss protection is installed, the neutral point of transformer should be installed with gap. However, at present, there are various forms of gaps, and there is lack of unified specifications for material, size, structure, etc., which leads to a series of system failures caused by improper configuration of transformer neutral point gap. So it is urgent to standardize the gap configuration. In this paper, the configuration of neutral point gap of 110kV and 220kV transformers in China was given. The accidents caused by improper gap configuration were analyzed. The power frequency, operation, lightning over-voltage stress and over-voltage value borne by neutral point gap were given through simulation calculation. The discharge characteristics of transformer neutral point gap were obtained through test, and the gap distance was accurately calculated. On this basis, the standardized configuration of transformer neutral point gap was given. The research results have important reference value for 110kV and 220kV project design and transformer protection.