Method of calculating the lightning outage rate of large-sized transmission lines

To evaluate the lightning outage risk to large-sized transmission lines, the lightning outage rate is predicted by calculating the probable lightning stroke rate. Conventionally, the increased potential caused by lightning strokes is calculated based on distributed constant circuit theory, and the calculated results are compared with the air gap withstand voltage between the arc horns of phase conductors, usually having six phases, to evaluate whether flashover will occur, and thus calculate its frequency of occurrence. However, the lightning outage rate of large-sized transmission lines calculated by this conventional method provides a poor match with reality. Consequently, the present study was conducted to improve the calculated prediction of the lightning outage rate. The lightning outage rate is a composite figure that includes back flashover caused by lightning strokes to ground wires and flashover caused by lightning strokes to phase conductors. The calculation of each of these cases has been improved. The main improvements include corrections to the withstand voltage to match the differences at larger and smaller facilities, as well as an improved method of calculating the development from a one-phase ground fault (1LG) of a phase conductor to a two-phase ground fault (2LG). Thanks to these improvements, the calculated lightning outage rate was closer to the actuality than that calculated by the conventional method. A trial calculation of the lightning outage rate with the voltage increased from 500 kV to UHV was also performed using the improved calculation method. Even though both 1LG and 2LG due to flashover caused by direct lightning strokes and back flashover increased to a certain extent, the lightning outage rate was about 30% lower than that of the present 500 kV transmission lines.