According to the operation data abroad,the tripped-out accidents caused by shielding failure amount to a large percentage of the tripped-out accidents caused by lightning strike on the UHV AC transmission lines.

The performance of shielding failure of 500 kV quadruple-circuit overhead AC transmission lines on the same tower with the revised electric geometry model (EGM) method is analyzed, and the problem of shielding for transmission lines by ground wires is discussed.

With the increase of voltage level of transmission line,the proportion of outages because of shielding failure increases,therefore,exactly evaluating lightning-withstanding level of transmission line is especially important for system service and stable operation.