In recent years

several failures of double circuit transmission line on the same tower due to lightning were happened in Beijing power grid.Although it can be reclosed successful

the lightning strike caused a grave threat to the power grid security.The cause of the accident and the accident process were studied for the sake of further understanding of the impact of lightning on power grid.As an example

110 kV double circuit transmission line(Xilong-line) was analyzed.At first

the system topology was given.Through the analysis on relay protection actions and the fault recorder data

over voltage on the insulator strings was calculated.Based on the analysis and the calculation

accident cause and the process were presented respectively.Secondly

it comes to the conclusion that the lightning failure was caused by counterattack.The wave of the lightning over voltage would spread to the not grounded neutral point of the transformers

and make the neutral protective gap breakdown

then cause freewheeling with the frequency of 50 Hz.As results of the relay protection

the double circuit transmission line all tripped out.Finally

the causes of the accident were proposed that included terrain features

large corner towers

strong thunderstorm weather and poor grounded contact of the tower.