Accurate modelling of rod driven tower footing

For evaluating the lightning performance of transmission lines by computer simulation, the accurate modelling of tower footing is very important. In particular, the decrease of the earth resistance observed for high values of the current flowing from the tower to earth has to be considered. In this paper, different modelling approaches allowing to take into account the nonlinear behaviour of the tower footing are overviewed, and a new model to describe the soil ionization process is presented. The proposed model corresponds to considering the ionized soil region as an equifield shell. In order to represent the ionization phenomena, the values of the resistivity on the ionized region are decreased according to the local current density, and the electric field is kept at a critical value. Deionization of the soil is also taken into account. Simulation results are presented, and compared to the published results of experimental tests.