Flashover Rate of 6.6-kV Distribution Line Due to Direct Negative Lightning Return Strokes

The overvoltage associated with direct lightning hits has been the important factor for the insulation design of a 6.6-kV distribution line in Japan. In this paper, the flashover rate of the distribution line associated with direct lightning hits is investigated based on numerical calculations and statistical analysis by taking into account the correlation between the peak and the front duration of negative return-stroke current waveforms. When the line is equipped with surge arresters at the interval of 100 or 200 m in addition to the overhead ground wire, the flashover rate associated with subsequent strokes is higher than that associated with first strokes. This demonstrates the importance of the study of current parameters for subsequent strokes. To obtain a conservative estimate of the flashover rate of a line with surge arresters installed every 100 m, calculated for the variable front duration, it is necessary to assume a constant front duration of less than 4 mu s and 0.9 mu s in the cases of first and subsequent strokes, respectively. With the increase of the installing interval of surge arresters, a constant front duration, leading to a conservative estimate, increases.