Abstract：Lightning shielding failure is an important issue in addressing lightning performance of an UHVAC transmission lines. It is shown from the operating experience of UHVAC lines in Japan and Russia that the shielding failure rate predicted using the existing Electro-Geometric Model (EGM) does not match with that obtained from operation data. In this paper, an improved EGM is proposed to evaluate shielding failure rate of the UHVAC transmission line. In the improved model, the striking distances to phase conductors, shielding wires and ground are differentiated. Furthermore, the influences of ground obliquity, wind deflection and operating voltage of phase conductors are taken into account. The proposed improved EGM is applied to calculate the lightning shielding failure rate of a UHVAC transmission line to be built in a demonstration project in China. The simulation results show that the maximal difference of shielding failure rates due to the phase angle of conductor voltage in a period can reach 0.16 times/100km-year. The shielding failure rate accelerates with the increase of the ground obliquity and the wind speed. The shielding failure rate of the transmission line with ZBS2 tower is free of shielding failure outage in the investigated range. However, the shielding failure rate of the transmission line with ZMP2 tower is less than the tolerable limit of 0.1 times/100km-year only if the ground obliquity is smaller than 10°.
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2009, Vol. 24 
