Assessment of lightning shielding performance of double-circuit UHV overhead transmission lines

Field study indicates that more than 90% lightning stroke trip-out accidents for UHV transmission lines are caused by lightning shielding failure flashover. The large scale of the tower and the high operation AC voltage increase the probability of direct lightning strokes attaching to the phase conductor of double circuit UHV transmission line. The unexpected high lightning shielding failure flashover rate of double circuit UHV transmission line in Japan is obtained which has been in 500kV-operation for 10 years. Moreover the characteristics of direct lightning strokes attaching to phase conductor conflicts to the prediction result of classical EGM method. A new double circuit UHV transmission line is planned to be under construction in eastern China and the lightning shielding design is one of the key aspects to ensure a well reliability of this double circuit UHV system. Firstly, a new 2-D analysis model based on leader propagation concept for lightning shielding performance assessment is proposed in this paper. The extrapolation utility of existing upward leader inception criterion is discussed and the critical corona radius method is used in this study. A new upward leader continuous progression criterion is developed based on the classical long air gap discharge theory. The typical terrain including plain area, mountain top, valley and side slope is established in this model by using Schwarz-Christoffel mapping. Secondly, the lightning shielding performance of double circuit UHV transmission line in Japan is analyzed by employing this model, and the calculation result is in a good agreement with the field observation by TEPCO which verifies the utility of this model. Finally, the lightning shielding performance assessment of double circuit UHV transmission line in China is carried out. The lightning shielding performance of designed double circuit UHV transmission line in China is better than that of Japan in typical terrain corresponding to ground flash densit- - y is 2.8 flashes/km²/year which is recommended in Chinese standard of design technology for overvoltage Protection of power apparatus. The lightning shielding failure flashover rate will rise taking into account the lightning activity frequency is much stronger in the partial line segments.