In recent years

several failures of double-circuit power transmission line on same tower due to lighting have happened. Although the transmission line can be reclosed successfully

the lighting strike is still a threat against the power grid security. In this paper

taking the Xi-Long 110 kV double-circuit power transmission line on same tower as an example

the relay protection action and recorded fault waveforms are analyzed

and the over-voltage level on the insulator strings is calculated. Based on the analysis and the calculation

the generation and development of the accident are deduced

and its cause is described. Conclusion is drawn that the lighting accident concerned terrain feature

large corner towers

strong thunderstorm weather and poor grounding contact of the tower.