Experimental Investigation of the Lightning Impulse Behavior of Wet Sandy Soil

Soil ionization, that is, electrical discharges developing in the ground, may have a significant impact on the behavior of grounding systems under lightning currents, as it causes a reduction of the impulse ground impedance. This may have several implications on insulation coordination studies for power systems. The effects of soil ionization have not been completely clarified yet due to its complexity associated with soil characteristics and conditions, ground electrode configuration, lightning current characteristics, as well as difficulties in the accurate determination of discharge activity. This study deals with the experimental investigation of the lightning impulse behavior of wet sandy soil. An experimental setup and measurement procedure are introduced for this purpose. Experiments under lightning impulse voltages of negative polarity were performed on two wet sandy soils, both as obtained from their extraction sites, as well as after drying and manual addition of moisture; thus, the influence of the soil sample preparation procedure is evaluated. Results are discussed considering soil ionization inception, electrical breakdown of soil, impulse impedance, and soil electrical properties. A simulation model is proposed for the prediction of the instantaneous impulse impedance when soil ionization occurs; this model is suitable for time domain simulations in ElectroMagnetic Transients Program (EMTP)-type software.