Time- and Frequency-Dependent Lightning Surge Characteristics of Grounding Electrodes

Two phenomena dominantly influence the dynamic performance of grounding electrodes during lightning discharge: 1) the time-dependent nonlinear behavior related to soil ionization during high-current pulses and 2) the frequency-dependent electromagnetic (EM) effects related to fast rise-time current pulses. The first phenomenon improves the grounding performance, while the second might have the opposite effect of impairing the grounding characteristics. It is important to simultaneously analyze these opposing effects; however, modern approaches that take into account both phenomena are mostly based on circuit theory, which does not allow for accurate analysis of fast rise-time pulses. This paper proposes a procedure that combines a rigorous EM approach based on the method of moments with an approximation method for assessing soil ionization effects as recently recommended by the CIGRE and IEEE Working Groups. Based on this procedure, we derive a simple new formula for approximating the surge characteristics that include time-dependent ionization and frequency-dependent inductive effects. We verify the model and formula by comparing our data with published experimental results. We also describe a parametric analysis of the opposing ionization and inductive effects.