| According to the results of operation that lightning is an important factor causing operation fault of transmission lines. And in all trip accident, the proportion of lightning trip rises with the improvement of lines' voltage level. There are strong lightning activities in most regions, which pose a threat to the operation of grid. At present our country strives to develop UHV grid which have higher overhead lines' height, more complex circuit parameter and adverse environment. More lightning threats are posed to UHV lines than to low voltage grade lines. So on the foundation of lightning protection work in the past, strengthening the research of UHV grid lines' lightning protection performance and design of lightning protection is of great significance. Due to the random processes of lightning transmission line are numerous, random factors have impact on the tripped rate of transmission lines. Traditional analysis methods can not specifically reflect the randomness of these processes.Firstly, the background and research significance of transmission lines' lightning shielding problem are elaborated. The present research methods about lightning problem and unresolved issue are introduced respectively.Secondly, the effects of lightning incidence angle, lightning current parameters, ground obliquity and power frequency voltage are taken into account. Shielding failure probability of transmission lines is analyzed based on the electro-geometric model(EGM).Meanwhile, according to the theory of Monte Carlo method, Matlab is used to simulate the parameters from random process. For the randomness of the lightning current parameters, it can be verified that the amplitude probability formula IEEE Recommended is more in line with the objective situation. At the same time, based on Monte Carlo method, the calculation of trip-out rate is simplified to an easy-solve form. Tripping rate calculation formula of universality is deduced and the method is confirmed can provide objective and specific assessment of lightning performance for transmission lines.Finally, combining with measures' characteristics and requirements of line's transformation, a multi-level model is established. Based on the judgment standard of criterion factors, the weight of evaluation index is determined and super matrix is structured. The super matrix is solved by Super Decisions software. The priority ranking of all alternative measures can be obtained from the calculation of comprehensive assessment. In addition, the sensitivity of criterion is also being analyzed to know how much effect would it have on the overall goal by changing the important evaluation factors' weights. The analysis results can provide reference for revamping scheme. |
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