Lightning Shielding Failure Analysis of UHVAC Transmission Lines with an Improved EGM

Lightning shielding failure is an important issue in addressing the lightning performance of HVAC transmission lines. In this paper, an improved electro-geometric model (EGM) is proposed to evaluate the shielding failure outage rate of UHVAC transmission lines. In the improved model, the striking distances to phase conductors, shielding wires and the ground are differentiated, respectively. Furthermore, the influences of ground obliquity, wind deflection and instantaneous operating voltage of phase conductors are taken into account. The proposed improved EGM is applied to calculate the lightning shielding failure outage 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 outage rate due to the phase angle change of the conductor voltage in a period can reach 0.12 times/100 km·a. The shielding failure outage rate increases monotonously with the increment of ground obliquity and wind speed. The shielding failure outage rate of the transmission line with the ‘cup’ tower ZBS2 is 0 due to its lower tower height and negative protection angle. The shielding failure outage rate of the transmission line with the ‘cathead’ tower ZMP2 is less than the standard limit of 0.1 times/100 km·a if the ground obliquity is smaller than 10°.