FDTD simulation of grounding electrodes considering soil ionization

If an impulse or surge high current flows in a grounding electrode, the soil in the vicinity of the grounding electrode would be ionized and the voltage generated at the top of the grounding electrode would be reduced. Recently, Ala et al. (2008) have proposed a soil ionization model, on the basis of the dynamic soil-resistivity model of Liew and Darveniza (1974), for finite-difference time-domain (FDTD) computations. In the model, the resistivity of each soil-representing cell is controlled by the instantaneous value of the electric field there and time. Ala et al. have tested the validity of the model against experiments on a single vertical grounding rod having two different lengths: 0.61 m and 3.05 m, into which a simple-shape unipolar high current is injected. In this paper, the model is applied to analyzing the surge responses of four parallel vertical grounding rods of length 3.05 m and a single horizontal grounding conductor having two different lengths: 8.1 m and 34 m to a single-peak unipolar high current, and the FDTD-computed responses are compared with the corresponding ones measured by Bellaschi et al. (1942) and Sekioka et al. (1998). Furthermore, it is applied to analyzing the surge responses of a 1-m single vertical grounding rod and a 5-m single horizontal grounding conductor to a double-peak unipolar high current. The FDTD-computed responses are then compared with the corresponding ones measured by Geri et al. (1992).