Comments on the high-frequency response of hemispherical grounding electrodes with emphasis on magnetic induction effects

Abstract

Accurate calculation of electrodes’ impedance is of major importance for the operation and protection of overhead power lines whose towers are periodically bonded to earth via grounding electrodes; particularly when the towers are hit by lightning discharges. A recent paper has proposed a simple RC-circuit to model the frequency response of hemispherical electrodes which predicts a continuous decrease to zero in electrode impedance as the frequency increases. The prediction agrees with the model, but the model is not sound and, therefore, the prediction is disputable. We show that the model is inadequate because it is founded on the conviction that the magnetic field is zero everywhere inside the soil ^___which we claim is a mistake. We employ a full-wave FEM solver to recalculate the numerical results presented in that paper and observed a notable disagreement for high frequencies. In addition, we also show how the problem of calculating the impedance of hemispherical grounding systems can be solved, encompassing displacement-current effects and magnetic-field effects.