FDTD Surge Simulation of Grounding Electrode Considering Frequency Characteristics of Soil Electrical Parameters

The characteristics of a grounding electrode against lightning currents cannot necessarily be expressed by its grounding resistance or impedance measured at a low frequency since soil conductivity and permittivity depend on frequency. The frequency dependences of soil conductivity and permittivity have been ignored frequently. It is desirable to accurately represent the frequency characteristics of soil when analyzing lightning over-voltages in power and telecommunication systems. However, there have not been sufficient studies of soil characteristics in the dominant frequency band of lightning currents. In this paper, the frequency dependences of the conductivity and permittivity of a soil sample, which is composed of 1-, 2-, 3-, 4-, or 5-mm-diameter cement particles, are measured, and they are approximated with the Debye dispersion model. The approximated frequency characteristics of soil conductivity and permittivity are introduced into finite-difference time-domain computations using the recursive convolution method. The effects of the frequency dependences on transient responses of a 1.5-m-long vertical grounding electrode are investigated. © 2025 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.