抄録
Cloud-to-ground lightning flashes in Niigata area have been observed for a few years by a magnetic direction-finder network, Data obtained so far are carefully analyzed taking into account the operating characteristics of the system. As a result, frequency distributions of return-stroke current for each polarity are produced, and their seasonal variations and the occurrence probabilities of high current exceeding 200kA are clarified.
Based on the estimated current distributions, back-flashover rates of 500kV 2cct transmission lines are calculated. It is known that the back-flashover caused by downward flashes can account for most of the line outages in summer for both single and multiple line faults, however, this mechanism cannot explain the high outage rate in winter experienced in winter thunderstorm area in late 1970s. In spite of the low lightning flash density in winter, the multiple-line fault rate in winter caused by back-flashovers is predicted to be comparable to that in summer, because of the seasonal variation of the frequency distributions of return-stroke current.
