The Effect of Frequency Dependence of Soil Electrical Parameters on the Lightning Performance of Grounding Systems

A rigorous full-wave approach based upon the finite-element method (FEM) is used for the analysis of grounding systems taking the frequency dependence of soil electrical parameters into account. The method uses minimum approximations to model the transient behavior of grounding systems. In this analysis, frequency dependence of the soil conductivity and relative permittivity is represented using available analytical formulae obtained from experimental data. It is shown that there are cases in which the frequency dependence of the soil conductivity and relative permittivity can affect the transient behavior of grounding systems. Our simulation results show that the effect of frequency dependence is more pronounced for grounding systems buried in low conductive soils in contrast with those buried in highly conductive soils and especially when these systems are subjected to lightning subsequent return stroke currents. In practice, it seems legitimate to disregard the frequency dependence of the soil conductivity and relative permittivity for grounding systems buried in highly conductive soils subjected to lightning first return stroke currents. Within this context, it is shown that the soil conductivity, length of grounding electrode, and the frequency content of the injected current are the main factors contributing to the beneficial effect of the frequency dependence of soil electrical parameters.