Research On The Method To Estimate Lightning Shielding Performance Of EHV/UHV Transmission Line

The lightning flashover fault of transmission lines is a tough problem affecting safepower supply of grids. In recent years, the AC and DC UHV transmission lines are putinto service successively in China, since current power grid develops very fast, thetransmission line accidents caused by lightning have new features. On one hand,compared with traditional transmission lines, E/UHV transmission lines is characterizedby high operation voltage, large tower size, and long distance and long span powertransmission, which brings more lightning risks to transmission lines. On the other hand,in different areas, the transmission line corridor, the terrain landform, the lightningactivity parameters, the voltage operation grade and every section of the line aredifferent.Therefore, to study the lightning shielding model of transmission lines and themethod to estimate the lightning shielding performance of transmission lines, and toanalyze the lightning shielding performance of transmission lines accurately, isessentially important to the lightning protection design and the analysis of lightningshielding failure flashover accident of transmission lines.The physical approach of electric geometry model (EGM) is discussed first,analyzing its physical basis,focusing on the concept and development process ofstriking distance,meanwhile, EGM concerning the operation voltage，ground obliquityand wind deflection is introduced based on the argumentation. Applying this model, thelightning shielding performance of±500kV EHV DC transmission lines tower arecalculated here, And summarized the variation of maximum shielding failure lightningcurrent and shielding failure trip rater.The existing leader propagation models (LPM) are analyzed here next, and basedon the observed lightning and spark discharge data, a new numerical model for thelightning shielding performance of the E/UHV transmission lines is presented, whichconsidering the operating voltage, sag and ground obliquity. This model considered themain approaches of the earth flash, such as: the propagation of descend leader, theinception of upward leader, the relative development of the descend leader, upwardleader and the final jump. This model also introduced numerical method on electricfield of transmission lines, which taking into account the coupling between theconductors and simulating with the actual-to-ground high of transmission lines instead of average height. Then programmed Mat lab interface program, by which the shieldingfailure trip rate (SFTR) of the500kV EHV AC transmission lines in Guangdongprovince was calculated to verify the accuracy and reasonableness of the model.Meanwhile, the SFTR of the1000kV UHV AC demonstration line was also calculated.The result shows that: the SFTR of1000kV UHV AC demonstration lines in ourcountry are less than.0575times/100km a, which is acceptable for safe operation forUHV transmission lines in China.Several influencing factors which would impact on transmission lines’ shieldingfailure performance, such as protection angle, ground angle, tower height, wind speedand operating voltage were discussed at the end of this article using the two improvedmodels,and regular patterns were also be summarized.The shielding failure trip rate offour transmission lines with different voltage level were calculated by two moldels, andcompared with actual operation data, the application scope of the two models wassuggested based on the comparison result.