Abstract: Lightning strike led to the fault tripping of an outlet line on 110 kV Ⅱ bus of the splitting 220 kV substation, resulting in the fluctuation of 10 kV Ⅱ bus voltage of the 220 kV substation, and the automatical switch-device of power supply of the 10 kV station (powered by 10 kV Ⅱ bus) act. In the process of the power switchover, the substation lost the AC power, at this time the cooling control protection of a forced oil circulation air-cooled transformer had a mal-operation in the 220 kV substation, resulting in three breakers of the transformer tripped. According to the monitoring record and the SOE record of field protection device, this paper analyzed the reasons of tripping, pointed out the incident exposed problems, and put forward preventive measures.