Significant Parameters in Estimating the Striking Distance of Lightning Strokes to Overhead Lines

The lightning discharge starts at the cloud end for strokes to level grounds or to low objects. In the initial stage, the leader stroke proceeds downwards without being influenced by grounded objects. As the charge of the cloud is lowered along the leader stroke, the electric field on the surface of the grounded objects increases. Finally, at a certain distance of the tip of the leader stroke from the grounded object, the critical electric field for breakdown of air at the surface of the grounded object if reached, and an upward streamer starts from the object to meet the leader stroke. This distance of the leader tip from the grounded object which produces the upward streamer is called the striking distance. The striking distance is a very significant parameter in estimating the lightning performance of overhead power lines. The longer the striking distance the higher will be the attractiveness of an overhead line to a lightning stroke; therefore, the probability of a line outage will be higher. Golde in his extensive research identified the relationship between the striking distance and the return-stroke current. Whitehead formalized this relation by a series of analytical July expressions, the most widely used being given by rs = 8 (kA)0.65 (m) (1) where / = return-stroke current in kA. Eriksson combined the effects of both the structure height H and the return-stroke current in kiloamperes on the attractive radius Ra of prominent structures, such as transmission-line towers in the following equation. Ra=0.84H0.6 (kA)0.