Assessment of earthing systems and enhancement of their performance

This thesis reports on the evaluation of performance and behaviour of earth electrode systems subjected to DC, AC and transient current injections with different rise times and shapes. The performance of the earth electrode system when injected with currents at the power frequency is now well understood, but the response of the system under high frequency and transient conditions is yet to be fully clarified. This thesis contributes to the better understanding of complex earthing systems such as earth grids and wind turbine earthing system behaviour under high frequency and transient conditions including voltage distributions along the length of the electrode. The frequency and time domain responses of earth electrodes (vertical and horizontal earth electrodes and earth grids) were quantified for soil resistivity ranged from 10Ωm to 10kΩm. Practical wind turbine earthing system models were developed to account for the additional effects of the above–ground tower structure of the wind turbine and also to consider the benefits of various enhancements to the earthing system. Simulations using 1/5μs and 8/20μs impulse currents were carried out, and a number of parameters were quantified; these include the earth potential rise and the voltage distributions at ground surface level. In particular, the contribution of mitigating techniques, such as rings and rods were derived. The computations allowed determination of touch and step voltages. The results show that the performance of an earth electrode depends on a number of factors such as soil resistivity and permittivity and electrode dimensions, and it was found that significant inductive effects dominate at high frequency. Thus, the ability of a horizontal earth electrode to reduce the earth potential rise is limited because, beyond a certain length known as the effective length, no further reduction is obtained. The effective length was determined experimentally by incrementally increasing the length of the test electrode. The experimental and simulation results show reasonably close agreement. Furthermore, reasonable prediction of the effective length may be possible using simple empirically derived equations. The thesis proposes a new method to increase the effective length of in-ground horizontal earth electrodes and the effective area of earth grids. It is proposed that an additional insulated parallel an above ground conductor is bonded to the horizontal electrode at suitable points along its length. Field tests show that the addition to such enhancement reduces inductance effects and helps dissipation of injected currents, so that a greater length of buried earth conductor is utilised, and this contributes to an additional reduction in the earth impedance. Hence, the earth potential rise at the point of current injection is reduced. Enhancing the earthing system in this way results in a significant reduction in the transient potentials developed at the base of the turbine structure. These TEPR reductions produce associated reductions in touch and step voltages