Analysis on tripping accident caused by improper protection measures of transformer neutral point on 110kV double circuit transmission lines

Paper foucused on the tripping accident of double circuit lines on the same tower caused by lightning flashover of 110 kV transmission lines. Analyses were made for the flashover and fault recording of the protection gap of 110 kV transformer's ungrounded neutral point. Paper determined that the fundamental cause of the accident was that the gap protection mode adopted by transformer's neutral point caused the malfunction of relay protection under lightning overvoltage. Taking 110 kV substation as an example, the lightning overvoltage, switching overvoltage, single-phase grounding and ground-fault overvoltage, non-full-phase switching overvoltage that the neutral point of 110 kV transformer may withstand during operation were calculated. The results showed that the lightning overvoltage, single-phase grounding and ground-fault overvoltage and non-full-phase switching overvoltage may exceed the insulation level of the neutral point of transformer. Protective measures should be taken. The configuration and parameter setting of protective measures must be reasonable. Otherwise tripping accidents or equipment damages would occur and affect the reliability of power supply. The research results have important reference value for 110kV engineering design and transformer protection.