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Laboratory characterisation of soil ionisation under impulse voltages

Elzowawi, Alseddig 2016. Laboratory characterisation of soil ionisation under impulse voltages. PhD Thesis, Cardiff University.
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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.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Uncontrolled Keywords: Visualisation of soil ionisation ; Glass bubbles; Soil ionisation propagation; Soil breakdown; Two-layer soil; Multi-point voltage measurement.
Date of First Compliant Deposit: 23 May 2016
Last Modified: 10 Aug 2021 13:57

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