Estimation of Flashover Rates of Overhead Power Distribution Lines by Lightning Strokes to Nearby Ground

Most outages on overhead power distribution lines are caused by lightning strokes to nearby ground. The electrostatic and the magnetic fields of the return stroke of a lightning stroke interact with the overhead line to induce transient voltages on the line. This interacting process is complex. However, the designer of an electric distribution system needs to know the outage rate of an overhead power line for a selected BIL (basic impulse insulation level) to meet the reliability criteria set for the system. A method is proposed which will facilitate estimating the outage rate of an overhead power distribution line caused by lightning strokes to nearby ground. In the analysis, the return-stroke current is varied from 1 kA to 200 kA, and the time to crest from 0.5 μs to 10.5 μs. The least perpendicular distance from the line to cause a strike to nearby ground is then computed from the striking distance of the stroke for the given current. Starting at this distance, the voltage induced on the line is calculated for the given current and the time to crest. If the induced voltage exceeds the BIL of the line, the strike point is receded from the line in steps until the induced voltage is below the BIL. A bivariate lognormal statistical analysis is made for each current level and for each given front time to estimate the probability of occurrence of that combination.