Influence of corona on the voltages induced by nearby lightning on overhead distribution lines

A model for the calculation of lightning-induced voltages is presented with the aim of assessing the effect of corona when power distribution lines are illuminated by lightning electromagnetic fields. Corona is taken into account by means of dynamic capacitance describing a charge-voltage diagram. Such an equivalent capacitance is then introduced in a model which describes a line illuminated by a lightning electromagnetic field. It is first concluded that the influence of corona on lightning induced overvoltages is of importance only for particularly severe strokes. It is also found that corona acts to increase the magnitude of these overvoltages, contrary to the case of voltages due to direct strokes, which are attenuated by corona. Atheoretical explanation of such an amplitude increase is presented. The effect of the ground resistivity is also taken into account in the calculations. The results showthat such an effect is in general as important as the effect of corona. This is different from the direct-strike behavior, where corona, when present, affects the surge propagation more than the ground resistivity. This is due to the fact that for the case of induced-voltages, the ground resistivity may affect more strongly the lightning-radiated fields rather than the surge propagation along the line, while corona affects only surge propagation. The need for experimental results to test the theoretical finding of the paper is stressed and some indication is given. Finally, a sensitivity analysis of the induced voltages as a function of the charge-voltage diagram adopted to model corona is also performed.