Effect of shielding by trees on the frequency of lightning strokes to power lines

The effect of trees is analyzed using a three-dimensional electrogeometric model, with emphasis on the case of 500 kV lines. The effects of the height of the trees, the width of the right-of-way, and the voltage class of the power line on the number of strokes collected by the line and their median amplitude are discussed. The number of outages experienced on transmission lines running through forests is known to be less than the values predicted using the methods published in the literature. The present analysis indicate that the phenomenon is due to large reductions in the number of strokes to the line and the median amplitude of the collected strokes. The shielding effect of the forest is significantly reduced for higher voltage lines. Thus the low outage rate of unshielded 500 kV lines does not necessarily mean that unshielded higher voltage lines (765 kV or 1000 kV). >