Modeling of Grounding Electrodes Under Lightning Currents

More precise modeling of the dynamic performance of grounding electrodes under lightning currents must include both the time-dependent nonlinear soil ionization and the frequency-dependent phenomena. These phenomena might have mutually opposing effects since the soil ionization effectively improves, while frequency-dependent inductive behavior impairs, the grounding performance. Modern approaches that take into account both phenomena are based on circuit theory that does not allow for accurate analysis of high-frequency behavior. This paper aims to further improve the understanding of the dynamic behavior of grounding electrodes under lightning currents by focusing on the following aspects: analyzing the validity domains of popular modeling approaches, based on circuit, transmission line, and electromagnetic theory; providing parametric analysis that takes into account both the propagation and soil ionization effects; analyzing simple formulas for surge characteristics; and comparing the modeling with experimental data. A model and a simple formula that combine the electromagnetic approach, suitable for high-frequency analysis, with the method that accounts for the soil ionization effects, recommended by the International Council on Large Electric Systems (CIGRE) and the IEEE Working Groups, are used for the parametric analysis. Both the model and the simple formula are verified by comparison with experimental results available in the literature.