Purpose/Aim
Direct lightning strikes to overhead lines cause overvoltages, which stress line insulators. When these exceed the insulation level, a flashover may occur. Two flashover mechanisms are distinguished: shielding failure flashover and backflashover. The former is caused by strikes to phase conductors, whereas the latter to shield wires and towers. Lightning-related flashover is one of the main causes of line outages. Hence, the computation of lightning overvoltages is an important task, necessary for the estimation of the lightning flashover rate of overhead lines. The overvoltages and the associated minimum lightning currents causing flashover are affected by lightning current waveform characteristics; these are investigated in this work.
Modeling methods
This work investigates via ATP-EMTP simulations the effect of lightning current waveform on the overvoltages and critical currents to a typical 150 kV overhead line. Both negative and positive downward lightning flashes are considered for shielding failure flashover and backflashover cases. Waveform characteristics are varied taking into account statistical distributions. Recorded lightning return-stroke current waveforms reported in literature are used as well.
Results/discussion
Preliminary results for the widely used CIGRE waveform and recorded waveforms show that there are evident differences in the waveshape and peak value of the computed overvoltages. These would affect the critical lightning current and flashover rate of the line.
Conclusions
The representation of lightning return-stroke current affects considerably the computed lightning overvoltages and the minimum lightning currents causing flashover to overhead lines. The effects of several waveform parameters are quantified for the 150 kV overhead line under study.

Backflashover,EMTP,Lightning,Shielding failure flashover,Overhead transmission lines