Study of Flashover Model on Overhead Distribution Lines

We have carried out analysis of lightning surge on overhead distribution lines by using EMTP. We made a model for electric breakdown of a 14m concrete pole with impulse voltage (rise time: 2μs) applied to the outer surface at the top of the concrete pole. We found that the transient impedance of the breakdown increased linearly before the breakdown and decreased exponentially after the breakdown. The measured voltage and current waveforms correspond to the calculated waveforms by regarding the impedance of the breakdown as resistance. Also we examined the impedance of flashover of an insulator with impulse voltage applied to the lines. The transient impedance of flashover is expressed approximately by the formula: Z(t)=A×exp(-(t-t0)/τ), (A=1500~5000, τ =3×10-8~5×10-8sec, t: time, t0: starting time of flashover).