Evaluation of the impact of different frequency dependent soil models on lightning overvoltages

This work assesses the influence of including the wideband behaviour of grounding systems in EMT-type programs on evaluation of transients resulting from direct lightning strikes to transmission lines. The grounding frequency behaviour is determined by using an accurate electromagnetic model and included in EMTP/ATP by means of an equivalent circuit derived from Vector Fitting technique. Furthermore, the impact of the frequency dependence of soil parameters on the lightning performance of transmission lines is addressed. It was found that representing the tower-footing grounding by a simple resistance can lead to significant errors in terms of grounding potential rise. However, for the overvoltages that appear across the insulator strings, the representation of the grounding system by a simple resistance leads to results whose accuracy is similar to those obtained using more complex representations which consider the wideband behaviour. Also, it was shown that the frequency dependence of soil parameters leads to a reduction of the grounding impulse impedance and causes a decrease of the backflashover rates, improving the lightning performance of transmission lines. In addition, the effect of considering the variation of soil parameters with frequency is more intense in the ground potential rise than in the overvoltages in the insulator strings.