| According to foreign statistical data, with the increase of operating voltage, the proportion of trips accidents caused by lightning strike on transmission lines also increases among a total number of trip incidents. As to EHV transmission lines, the percentage even amounts to 20%～35%. In Russia, the lightning activities of a 494km and 1150kV transmission line which covers from Ekibastuz to Kokchetav were studied. Researching statistics on the line from 1985~1992 show that lighting trips account for 94% of total trip accidents on the whole line. As the Chinese government is implementing the power developing strategy of"West-to-East Power Transmission, North-to-South Power Exchange and Nationwide Interconnection", and the±800 kV UHV DC and 1000 kV UHV AC power transmission systems are being constructed, how to guarantee the safe operation of the EHV/UHV transmission lines becomes a key problem to be solved in the operation of Chinese power system. Besides, lightning strikes happen frequently and strongly in China, most of the EHV/UHV transmission lines have to cross the areas in which lightning strikes happen actively. And in these areas, the high soil resistivity would cause problems to the grounding of towers. Therefore, how to design the towers of the EHV/UHV transmission lines so as to meet the requirements of the lines'lightning protection and grounding is a key problem to improve the reliability of EHV/UHV power transmission system. In this thesis, by analyzing the physical process of the back-flash and lightning shielding failure on transmission lines, the issues of the lightning back-flash calculation model including the impulse corona and the back-flash protection performance, the way to improve the current distribution characteristics of the tower grounding system, and the lightning shielding performance based on fractal leader progression model, will be researched respectively. Form theoretical researches and experiment analysis, the following conclusions are drawn in this thesis:(1) Based on the ATP-EMTP software, the calculation model for the lightning back-flash protection performance of UHV DC transmission lines is developed. The model considers several factors, such as, the tower's multi-wave impedance, the impulse grounding resistance, and the leader progression model for the lightning flashover of insulators. Moreover, based on the analysis of the impulse corona's coupling effect on conductors, the evaluation method on the lighning back-flash protection performance considering the impulse corona is established, which makes the proposed calculation model accord better with the physical process when lightning strikes on the tower and back-flash is generated. Bases on this model, the back-flash protection performance of the typical UHV DC transmission line towers under different conditions are calculated in this thesis. The results show that, with the increase of the insulation level and the decrease of gounding resistance and tower height, the lightning back-flashover performance of UHV DC transmission line can be improved.(2) In order to solve the tower grounding problems in high soil resistivity environment and improve the lightning back-flash protection performance, based on dimensional similarity between the impulse resistance and the conductor resistance, the experiments on the impulse current divergent characteristics of different tower grounding conductors are carried out. By measuring the current distribution in different positions and axial directions of the grounding electrode, the surface current divergence rules of grounding conductors with different structures is analyzed under the effects of impulse current. And according to the rules, the tower grounding measures for UHV DC transmission lines in regions with high soil resistance are proposed.(3) According to the similarity between the cloud-to-ground lightning and the long air gap, and by analyzing the physical process when lightning strikes on transmission lines, it is proposed that the fractal theory can be used to describe the deterministic and stochastic factors in the leader's progression. Based on the simulation analysis on the fractal characteristics of leader progression, the calculation on the space field in the leader's progression, the fractal theory, and the initial criterion of the UHV DC transmission line's upward leader, the leader's fractal-characterized progression is realized and the fractal leader progression model is established for the UHV DC transmission line with high height.(4) Based on the fractal leader progression model, the shielding failure space distribution of different lightning currents is analyzed. Then the method to calculate the shielding failure rate and the lightning insulator flashover rate is established. The shielding performance of UHV DC transmission lines is analyzed and the results show that the shielding angle, the ground obliquity and the tower height all have the effects on the lighning shielding performance. And it is concluded that the negative shielding angle can be used to prevent the lightning shielding failure on UHV transmission lines.
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