Transient characteristics of wind turbine grounding

The goal of the analysis in this paper is twofold. First, we analyze the influence of different grounding electrode arrangements on the transient ground potential rise in case of direct lightning strokes. Particularly, we analyze the influence of the integration of the metallic foundation reinforcement with the grounding system. In addition we analyze the influence of long (30 m) vertical electrodes and additional ring electrode. Next, we analyze soil ionization effects. We compute the distribution of the electric field in the earth in time domain (for typical lightning current pulses) looking for the volume within which the magnitude of the electric field is above the critical strength (300 kV/m). Then we look for the possibility to apply a simple formula to account for the soil ionization effects. The parameters in the analysis are varied for low and highly resistive earth and for lightning current pulses related to the first and subsequent return strokes. We apply a rigorous electromagnetic model for frequency dependent analysis and an approximate model to account for soil ionization effects. Both models have been previously validated by comparison with experiments.