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Title: Aspect of earthing systems under variable frequency and impulse conditions
Author: Griffiths, huw
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2008
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The performance of earthing systems under variable frequency and transient conditions has been investigated in this research. The work has involved an extensive review of the published literature, theoretical and computational studies of earth electrode systems, laboratory experiments and outdoor tests at the University's earthing test site and at operational power installations. A critique was carried out of the classical fall-of-potential measurement method for determining the earth resistance of earth electrode systems. It was found that when there is insufficient spacing between the earth electrode under test and the auxiliary current return electrode, there can be a significant error when applying the 61.8% rule. This error is negative for single electrodes such as rings or square grids, while, for earth systems involving extended earthing systems such as earthed overhead towerlines, the error is positive. The performance of a variable frequency impedance measurement system (IMS) developed at Cardiff University was tested in the laboratory and in the field. Using a controlled source of noise mixed with a pure sinusoidal waveform, optimal test frequencies of 48Hz and 52Hz were identified to obtain high noise rejection and accurate prediction of the power frequency impedance. The IMS system was applied successfully to measure earth impedance at a range of high voltage operational installations. Calculations based on equivalent circuit models and computer simulations using a commercial numerical code were used to quantify the frequency dependence of the earth impedance of rod, horizontal and grid electrodes in the range DC to IOMHz. It was shown that inductive effects become significant for all electrode systems above a particular frequency referred to as the upturn frequency which is related to resistivity. In high resistivity media, the earth impedance of a rod falls above a particular threshold frequency which may be attributed to capacitive effects. New analytical expressions were derived to describe these features. The effective length and impedance at effective length were calculated for a horizontal earth electrode. It was demonstrated that these quantities are dependent on resistivity and frequency. Simulations and impulse tests on a 275kV transmission tower base show that a significant earth potential rise was found at the tower base, which highlights the importance of assessing transient step and touch voltages under surge conditions. The impulse resistance was found to be equal to the value corresponding to the steady state conditions, for the range of input current shapes considered. However, the impulse response of full tower models was shown to be dependent not only on soil resistivity, but also on the shape of the impulse current. The surge impedance of the tower was determined using a circuit model representation of the tower and frequency analysis of the tower top and tower base potentials. This model gives values higher than those calculated using an analytical expression for a conical tower, which was based on the assumption of zero soil resistivity. High voltage tests results showed that the measured impulse resistance decreases with increasing surge current. This behaviour may be due to non-linear conduction phenomena occurring in the soil (thermal conduction and soil ionisation).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available