| In recent years, lightning stroke is one of the important causes of the accidents that occur on transmission lines. With the increase of voltage grade of transmission line, the proportion of outages because of shielding failure also increases. Therefore, exactly evaluating lightning-withstanding level and shielding failure flashover rate (SFFOR) of transmission line, is especially important for system service and stable operation. At present, the main methods to evaluate the characteristic of shielding failure for transmission line are the standard method , leader progression model (LPM) and electric geometry model (EGM). The electric geometric method relates the lightning discharging characteristic with structural dimension of transmission line together, and "striking distance"is introduced to describe the object's lightning attracting ability. It is much more advanced than the traditional method. With the development of EGM, many scholars proposed different calculation expressions and methods for "striking distance"and shielding failure flashover rate. Based on the principle of EGM, the expression for "striking distance"of IEEE working group commended is adopted, the upper limit and lower limit of lightning current magnitude for the integral calculation is discussed, and the coordinates of intersecting points for objects'protecting area is calculated in this paper. Different kinds of methods for SFFOR calculation are compared, and the calculation results of examples indicate that those calculated by "exposure distance"is much more exact. Different transmission line structure has different range of "exposure distance". Most situations are discussed and shielding failure flashover rates of Xunqing line with the voltage of 220kV in Jiangxi province are calculated. The calculation results accord with real practical experience data. The influence of many aspects to shielding failure flashover rate is discussed. That is, SFFOR will increase with the increase of protect angle of ground line, obliquity ground and the decrease of atmospheric temperature. Some measures are suggested to reduce shielding failure of transmission line. The striking distance factor is introduced to describe the difference of striking electric intensity between lightning leader to phase conductor and the ground. Based on the progression of lightning leader, a model of striking distance factor was established. From this model, the relation between striking distance factor k and height of tower H is : k=1.18-H/108.7. According to the comparability of lightning discharge and rod-plane gap discharge, a simulated experiment was carried out. The results of experiment indicated that the striking distance coefficient k decreased with the increase of the height of tower H. It is the same to the results concluded from practical experienced data. The relation of factor k with height H concluded from experiment is : k=1.066-H/240.5. In the striking distance factor model, randomicity of progression for up leader and down leader is ignored, and the critical striking electric intensity of lightning leader to phase conductor and ground are both decided to 500kV/m. This suppose is not proper. The range of factor k from experiment is in the suggested range from 0.6 to 1.0 which is commended by IEEE working group, and the results describe the different lightning attracting ability of transmission line with different height. The SFFOR calculated by experimental conclusion accord with the real situation. So, the expression which received by experiment can be used in the shielding failure calculation.
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