Investigation of impulse discharges in two-layer wet and dry soil sample

Soil ionization phenomena in dry soils are considered the main cause of the observed nonlinear behaviour under impulse voltages, the very high resistivity of the dry soil falling dramatically at the instant of breakdown. In this study, a two-layer columnar test sample composed of a glass bubble material was used to investigate the associated ionisation and breakdown processes. Three voltage dividers were utilised to measure the applied lightning voltage and intermediate voltages at two probes located at fixed heights in the sample. A current transformer was used to measure the current flowing through the sample. When the voltage is high enough and after a certain delay time, current flow from the active electrode was detected by the current transformer, but the measured applied voltage did not show any indication of breakdown. The current stream in the dry material could be due to the ionisation of the air voids among the dry grains, which could support the ionisation phenomenon in dry soil, and the wet part voltage rose at the instant of current rise, while the voltage across the dry section shows occurrence of a breakdown. An equivalent circuit model of the sample with EMTP software was also proposed to simulate the behaviour of the discharge