Laboratory characterisation of soil ionisation under impulse voltages

Since the discovery of the soil ionisation phenomenon in the earthing systems under high lightning current, there have been tremendous investigations studying this ambiguous phenomenon. Some aspects are still not yet fully addressed, which some of them were considered in this thesis, such as visualizing the phenomenon, soil ionisation delay times, propagation velocity, and breakdown stages in dry porous materials, the initiation and propagation of the soil ionisation in two-layer soil with different moisture contents and thicknesses. Therefore, a special test cell has been developed to facilitate the various laboratory tests conducted in this study. The rig has a rod-plane electrode configuration. A transparent dielectric tube installed between the electrodes, as the samples were placed in this tube. Glass bubble material and sand were used as tests media throughout the study. In this thesis, a new methodology was used to visualise the phenomenon in a new dielectric porous material under fast and slow impulse voltages. The correlation between the recorded videos and the discharge waveforms was a major achievement. It provided significant results, exhibiting the dynamic developments of the discharge throughout the applied impulse. Various discharge scenarios were visualised with a new sample configuration, which was prepared especially for this investigation, and showed great outcomes. Due to the high performance of new sample configuration, and in order to understand the ionisation process in the dry layer, multi-point voltage measurement technique was utilised to acquire the potential in the ionised zone in the dry layer. The measured voltages tracked the initiation and propagation of the soil ionisation in the dry layer. The velocity of the propagation was also investigated. Two delay times were obtained, which represented the various initiation and propagation stages of the ionisation until the dry layer breakdown. The new sample arrangement was also simulated with a proposed equivalent circuit that presented a satisfactory agreement with the real test performance. Several scenarios were examined to study the initiation and propagation of the soil ionisation in two-layer sand under lightning surges. Multi-point voltage measurement technique was also used to trace the ionisation in both layers. Similar breakdown stages to those in glass bubble material were found. The soil ionisation was found to initiate and propagate in the lower wet sand layer after the breakdown of the upper dry sand layer. The amount of water and the thickness of both layers were found to have a great impact on the initiation and propagation of the ionisation phenomenon in both layers of the sample

Visualization of electric discharge in porous materials

Very few studies have been conducted to visualize the dynamic discharge development from ground electrode in the soil. The imaging process of the electric discharge in opaque porous materials is extremely difficult. Therefore, in this study, a photographic investigation of the electric discharge in a new dielectric glass bubble material was conducted. A Photron FASTCAM SA5 high speed camera was utilized to record the discharge light emitted from the sample. A special test rig was developed to facilitate these tests with a rod - plane electrode configuration. The material was placed in a transparent acrylic tube to allow the discharge light to be transmitted so that the camera can record the discharge. 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 discharge phenomenon and monitoring the dynamic change of the discharge at various times during the impulse

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

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

Investigation of soil ionization propagation in two- layer soil samples

High current lightning strikes into earthing systems can result in ionization in the soil surrounding the earthing electrode. Most of the published studies investigating this phenomenon have assumed uniform one-layer soil, but soil ionization propagation in a multilayered soil sample has not been extensively addressed. Practical soils may consist of several layers with different water contents, and hence soil resistivity will vary continuously with depth. This investigation considers several sand samples, consisting of two layers with different water contents subjected to standard lightning impulse voltages. A rod-plane electrode configuration was constructed inside a cylindrical plastic test rig, in order to house both wet and dry soil test samples. In order to quantify the propagation of ionization inside the test sample, voltage probes were installed along the tube at specific positions. Localized changes in the ionization zone potential could, therefore, be monitored in real time

Investigation of soil ionization propagation in two-layer soil samples

High current lightning strikes into earthing systems can result in ionization in the soil surrounding the earthing electrode. Most of the published studies investigating this phenomenon have assumed uniform one-layer soil, but soil ionization propagation in a multilayered soil sample has not been extensively addressed. Practical soils may consist of several layers with different water contents, and hence soil resistivity will vary continuously with depth. This investigation considers several sand samples, consisting of two layers with different water contents subjected to standard lightning impulse voltages. A rod-plane electrode configuration was constructed inside a cylindrical plastic test rig, in order to house both wet and dry soil test samples. In order to quantify the propagation of ionization inside the test sample, voltage probes were installed along the tube at specific positions. Localized changes in the ionization zone potential could, therefore, be monitored in real time