±660 kV HVDC transmission line is the world’s first line in this voltage grade.The lightning protection performance of long span line is the relatively weaker part in the whole transmission line.So it is necessary to carry out a more detailed investigation.In this paper

an improved EGM is proposed to evaluate the shielding failure tripout rate of ±660 kV HVDC transmission line.The striking distances to conductors

shield wires and ground are differentiated.Furthermore

the influences of ground obliquity

wind deflection and operating voltage of phase conductors are taken into account.The simulation results show that the impacts of conductor voltage to the shielding failure trip-out rate decrease with the increasing ground obliquity.But the impacts should not be ignored and the conductor should be considered.Also

the shielding failure rate accelerates with the increase of the ground obliquity and the wind speed.When the ground obliquity is more than 20° and the wind speed is more than 20 m/s

the shielding trip-out failure rate of long span transmission line exceeds the indicator 0.1 times/（100km.a）.Furthermore

given the long span transmission lines generally lie in bad geographical and climatic conditions

thus it is critical to take effective lightning protection measures to improve the shielding failure lightning protection performance.