Soil ionization effects on surge characteristics of grounding electrodes

Abstract

For finite‐difference time‐domain (FDTD) simulations, Ala et al. (2008) proposed a simplified engineering soil ionization model on the basis of the dynamic soil‐resistivity model of Liew and Darveniza (1974), and tested its validity against experiments on a 0.61‐m single vertical grounding conductor. In the model, the resistivity of each soil‐representing cell is controlled by the instantaneous value of the electric field there and time. In this article, to test the validity of this model, it was applied to FDTD simulations of the surge responses of different grounding electrodes: an 8.1‐m single horizontal grounding cylindrical electrode buried at a depth of 1 m in a homogeneous soil, and a 1.5‐m single vertical grounding cylindrical electrode in a homogeneous soil. The FDTD‐computed responses agree reasonably well with the corresponding ones measured by Sekioka et al. (1998) and Asaoka et al. (2005). Also, influences of ground‐surface soil ionization, direction‐dependent or direction‐independent resistivity, and three parameters (ionization constant, deionization constant, and critical electric field) of the model were investigated. Surface ionization has a non‐negligible influence only on a short grounding electrode buried in a relatively high‐resistivity soil. Direction‐dependent resistivity yields a little higher voltage and an acute ionization zone near the tip of electrode. Both ionization constant and critical electric field have significant effects: the voltage generated decreases with decreasing ionization constant and/or critical electric field. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.