With such model the effect of tower structure,lightning current amplitude,angle of the lightning incidence,as well as the ground angle on the shielding failure rate are analyzed.

Shielding failure rate is an important parameter to calculate lightning withstands level.

It is indicated that the risk of shielding failure in the protection range of lightning rods and overhead ground wires should be acquired through theoretical analysis,laboratory test and research as well as sta.

To evaluate the shielding failure trip-out rate of the transmission lines accurately,a new model combining electromagnetic transient program(EMTP) simulation with Monte Carlo method is presented,where all random factors are taken into account and the higher accuracy for electromagnetic transient analysis of EMTP is kept.

This paper presents an algorithm of shielding failure trip-out rate (SFTOR) of overhead transmission line based on Monte Carlo method.

The projection of exposed surface on the ground is adopted to calculate shielding failure trip-out rate.

Experimental methods and models were analyzed,such as standard method,electrical geometric model(EGM),leader progression model(LPM) and shielding failure probability model.

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.