Visualization of the ionization phenomenon in porous materials under lightning impulse

the electric discharge and soil ionization phenomena have a great effect on the performance of earthing systems, especially under lightning currents. These phenomena create a nonlinear behaviour in the soil around the earthing electrode, where the resistivity of the soil drops to a lower value allowing the current to increase to a higher magnitude. Very limited studies have been conducted to image the soil ionization discharge developments from the earthing electrode in the soil. However, the imaging process of the electric discharge in opaque porous materials such as soil is extremely difficult. Therefore, in this study, a photographic investigation of the electric discharge in a new dielectric glass bubble material was conducted. Also the expansion of the ionization zone in this material was investigated with a specially-adapted sample configuration. A rod-plane electrode configuration was utilized in the test rig. The sample was placed in a Perspex vertical tube, so that the discharge light can be visible and recorded by a fast camera.Voltage probes were installed in the tube to measure the voltage at different places in the sample. Standard lightning and switching impulse voltages were used in these experiments. Synchronized frames of the recorded video with voltage and current waveforms allow visualization of the dynamic change of the discharge at various times during the impulse. It was observed that the expansion of the ionization zone does not have a constant speed and the resistance of the ionization zone is not the same throughout the zone