Experimental investigation of enhanced earth electrode systems under high frequency and transient conditions

Abstract

This thesis is primarily concerned with experimental tests and computer simulations to determine the high frequency and transient performance of earth electrode systems. The work has involved an extensive review of published literature, theoretical and analytical investigations of earth electrode systems. The experimental investigations on earth electrodes were carried out an outdoor site prior to electrode testing the test site soil resistivity were undertaken. Specifically, the soil resistivity was obtained at the location of the vertical test rod. In order to obtain better understanding of the soil stratification, fourteen profiles of soil resistivity were measured at the field site. From the measurements 2D soil models were constructed to visualise both horizontal and vertical resistivity variation. High frequency and impulse characteristics of vertical test rods up to 6m length and horizontal electrodes up to 88m buried in a non-uniform soil outdoor test site were tested. DC, AC and impulse test results show that increasing the length of electrode reduces the earthing resistance but not impedance. It was shown that, the earth resistance/impedance is constant over a low frequency range, while higher or lower impedance values are observed in the high-frequency range due to inductive or capacitive effects, depending on the length of earth electrode. Improved high- frequency and transient response of earth rods was determined experimentally by connecting horizontal electrode enhancements in star or cross formation at the top of vertical rods. Using these additional enhancements, a reduction in both resistance and impedance has been demonstrated. The addition of horizontal enhancements to the vertical rod can reduce the earth potential rise (EPR) by approximately 70% and 48% for 1.2m and 6m rods respectively. Voltage and current distributions of earth electrode systems under low/high frequency and impulse conditions, for different lengths of vertical rods with horizontal electrode enhancements and along a horizontal electrode with and without insulated conductor, were investigated experimentally and verified by computer simulation. In the case of the of the rods with an added ‘4-cross’ horizontal conductor enhancement, it was shown that the rods carry the majority of the current at low frequency, but this proportion decreases significantly as frequency increases The field test results show that current distribution in earth conductor systems is significantly different under high-frequency and impulse energisation compared with power frequency conditions. Close agreement was obtained between the measured and computed current and voltage High voltage tests in the ground around the vertical electrodes were investigated experimentally. It was observed that when a sufficiently high current magnitude is injected through vertical electrodes, a significant reduction in the impulse resistance by increase in current with a sudden fall of voltage is observed which is called soil ionisation. Such phenomenon does not occur when the vertical electrodes with horizontal enhancements is tested, where the current through all earth electrodes is small