Research On Lightning Leader Fractal Model And The Method To Estimate The Lightning Shielding Performance Of Transmission Lines

In recent years, the AC and DC UHV transmission lines are put into servicesuccessively in China, which intensifies the power developing strategy of “West-to-EastPower Transmission, North-to-South Power Exchange and Nationwide Interconnection”,so there are increasing demands in China to maintain safe and stable operation of grid.The lightning flashover fault of transmission lines is a tough problem affecting safepower supply of grids. Therefore, how to reduce lightning frequencies, and improve thelightning performance of power grid have become important measures to ensure thesafe operation of the grid. Meanwhile, 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. So, in China, lightning shielding problems of transmission lines are obviouslydifferent. The operation experience of E/UHV transmission lines in China and abroadshows that, because of high insulation level, lightning shielding failure is the main causeof transmission line flashover accidents. Therefore, to study the lightning shieldingmodel of transmission lines and the method to estimate the lightning shieldingperformance of transmission lines, and to analyze the lightning shielding performanceof transmission lines accurately, is essentially important to the lightning protectiondesign and the analysis of lightning shielding failure flashover accident of transmissionlines. Currently, although there are many models and estimation methods to analyze theshielding performance of transmission lines, some questions such as how to describe theprobability statistics of the process when lightning strikes on transmission lines, andhow to describe the insulator string flashover characteristics under non-standardlightning wave, are still difficulties of research awaiting to be solved when studying thelightning shielding failure flashover rate (SFFOR) of transmission lines. Therefore,based on the above two questions, this thesis is trying to find a method to estimate theshielding performance of transmission lines, which complies with the process whenlightning strikes on lines and flashover happens, which provides theoretical tools for lightning protection design of transmission lines and for the analysis of lightningshielding failure accidents.Based on the observation of thundercloud discharge process, it is known that thelightning discharge channel has obvious fractal features. Therefore, starting from fractalgeometry, this thesis studies the fractal characteristics of natural lightning path, andstudies the method to simulate the lightning leader fractal propagation. In order to makethe propagation characteristics of lightning leader path stimulated in the proposed modelsimilar with natural lightning leader, this thesis uses box counting method to calculatethe fractal dimensions of stimulated lightning path under different fractal parameters η,and this parameter is adjusted according to the fractal dimension variation range ofnatural lightning propagation path. Besides, by Charge Simulation Method, thecalculation model of dynamic electric field during lightning process is also set up. Themodel considers how the lightning fractal leaders, transmission line operation voltage,and the upper leader after inception influence lightning propagation space and thesurface electric field of transmission lines. On this basis, the thesis combines thelightning leader fractal propagation simulation method and the dynamic electric fieldcalculation model together, builds the fractal leader propagation model.The lightning flashover characteristics of transmission line insulators play a crucialrole in assessing the lightning shielding performance of transmission lines. And thewaveform of natural cloud-to-ground lightning varies in a large range, which means thelightning waveform parameter has non-ignorable influence on the lightning impulseflashover characteristics of transmission line insulators. Therefore, by the disruptiveeffect method, the thesis studies the volt-time characteristics variation pattern ofinsulators under different lightning surge waves, and studies its application in theanalysis of the lightning shielding performance of transmission lines. By analyzing theparameter setting method of traditional DE method, the thesis concludes the rules ofhow the parameter combinations in different models affect the volt-time characteristicsreproduction curve of insulator strings. And then a new parameter setting method isproposed to improve the reproduction curve of DE method, so that it can better reflectthe actual distribution pattern of the volt-time characteristics of insulators. And bycomparing with experiment data, the paper also verifies the effectiveness of theproposed method when it is used to reproduce the volt-time characteristics curves underdifferent lightning waves.The thesis uses the proposed lightning leader fractal model to study the lightning shielding performance of transmission lines. Research results show th at, the lateralshielding failure area of transmission lines shows a feature of probability distribution;the shielding failure rate is related with the lightning current amplitude and laterallightning stroke distance. In addition, for the first time the thesis proposes to use thetransmission line space shielding failure probability curve to analyze the lightningshielding performance of transmission lines. The research result shows: this analysismethod can better reflect the influence of space distribution on the lightning stroke ontransmission lines, so it can be used to analyze the accidents caused by lightningstriking on conductors in all directions and more exactly. Meanwhile, the calculation ofSFFOR of transmission lines is not only related to the lightning shielding performanceof transmission lines, but also related to the flashover characteristics of insulator string.Therefore, this thesis combines the lightning fractal leader progression theory with theimproved DE method, and proposes a method to assess the SFFOR of transmission lineconsidering both the dispersity of the lightning shielding failure and the fluctuation ofthe lightning wave parameter. Through the experiment simulating lightning striking ontransmission lines, and according to existing operation data of lightning accidents onE/UHV transmission lines, the thesis also verifies the proposed model and its estimationmethod. In addition, the thesis estimates the lightning shielding performance of severalon-running E/UHV transmission lines. The results show that, the SFFOR of typicalE/UHV transmission lines in China is within0.14times/100km·a, which can meet theoperation demand generally. As to the EHV double circuit transmission line in Japan,because of its high tower design and double circuit transmission, the SFFOR is1.2635times/100km·a. Therefore, it still needs other lighting shielding measures to improvelightning shielding performance and reduce shielding failure rate.