Modelling of long ground electrodes for lightning studies

For evaluating the lightning performance of transmission lines by computer simulation, the accurate modelling of tower footing is very important. Considering the high frequencies and magnitudes characteristics of lightning currents, an adequate representation of earth electrodes must take into account the distributed nature of their parameters, as well as its nonlinear behaviour, due to soil ionisation phenomena. The purpose of this paper is to study the nonlinear behaviour of long ground electrodes, based on a finite differences transmission line model able to take nonlinearities into account. Results obtained show the influence of soil ionisation process at the voltage and current distributions along the electrode and the important role played by the lightning current rise time.