Characterisation of earthing systems and materials under DC, variable frequency and impulse conditions

This thesis is primarily concerned with experimental tests and computer simulations to evaluate the performance and behaviour of earth electrode systems subjected to DC, AC variable frequency and impulse currents. The performance of the earth electrode systems at the power frequency is now well understood. However, the response of the system under high frequency and transient conditions still need more clarification. Therefore, simulations and experimental investigations in both laboratory and full-scale field site have been performed and reported in this thesis. The results contribute to better understanding of complex earthing systems under high frequency and transient conditions. The response of the earthing system of different configurations (vertical and horizontal electrode) was conducted for soil resistivity ranged from 10Ωm to 10kΩm using numerical computational model. The effect of soil resistivity, permittivity, and the electrode length on the performance of the earthing system was investigated. Particular emphasis applied to study the effect of segmentation on the evaluation of earthing electrode response. The investigations result in some recommendations contribute to the better evaluation of the performance and the behaviour of simulated earth electrodes. New earthing system facilities were prepared. The first stage was soil resistivity survey, which resulted in 2D soil models construction showed both horizontal and vertical soil resistivity variation. In addition, step voltages and touch voltages were computed to ensure the safety of the workers. Then, high frequency and impulse characteristics of vertical test rod and horizontal electrode buried in non-uniform soil at Llanrumney were tested. DC, AC and impulse tests results show that the measured earth impedance is constant over a low-frequency range, while higher impedance values are observed in the high-frequency range due to the inductive effects. To validate the analytical approaches and computational models, a new earthing system facility was prepared at Dinorwig substation at North Wales, UK. High frequency and impulse characteristics of 5m × 5m earth grid electrodes immersed in fresh water (close to uniform medium) were tested. DC, AC and impulse test results show that the resistive behaviour dominates the performance of the earthing grid. In addition, the measured impulse resistance exhibits constant values with the increase of the injected currents. Experiments were carried out at new high voltage laboratory to investigate the frequency dependence of electrical soil parameters. The soil was prepared and mixed with a different percentage of water contents according to weight. The results showed that both the resistivity and the permittivity decreased with increasing water contents. In addition, the results compared with the developed models available in literature and exhibited close agreements with them. Moreover, experimental investigations carried out at the laboratory on high resistivity material (gravel and concrete) which are used to increase the contact resistance between the earth and workers. The resistance showed a decrease in its value with increasing the water contents.