3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Relationship between lightning impulse current injected and the degree of electrical degradation of the MOV(2017) Mashaba, Mathews MusaSurge Protective Devices (SPDs) are used to protect electrical and electronic equipment against damages and operational malfunctions caused by surges. The commonly employed SPDs are composed of Metal Oxide Varistors (MOVs). MOVs are typically known for being inexpensive and for withstanding reasonable high values of current transients; they degrade over time depending on the severity and frequency of the surges they are subjected to. When MOVs are not properly monitored, the equipment they are meant to protect may remain unprotected and susceptible to surges. Reference voltage and leakage current of the MOV are commonly measured to deduce the operational status of the MOV. However, measuring the reference voltage only provides a pass or fail status of the MOV and not the degree of degradation, and measuring the leakage current and other dielectric parameters of the MOV (such as return voltage, decay voltage, etc.) is not always practical depending on the manner in which the MOVs are connected, especially the low voltage MOVs. This results in limited preventative maintenance techniques since the degree of degradation of the MOV is not known and other parameters of the MOV cannot always be measured. Therefore, the preventative maintenance mechanisms are sought. Extensive studies have been done to investigate the electrical and microstructural degradation of MOVs. However, the relationship between the current impulse injected and the degree of electrical degradation of the MOV have not been clearly defined. Therefore, defining this relationship can help to achieve preventative maintenance on lightning protection composed of MOVs by characterising and quantifying the degree of degradation of the MOV caused by a lightning impulse current without physically measuring dielectric parameters of the MOV. Thus, the study done in this dissertation answers the research question entitled: What is the relationship between an 8/20 µs lightning impulse current applied and the degree of degradation of the MOV? This question is answered through proposing a mathematical model that characterises the percentage of degradation of the MOV caused by a lightning impulse current. The mathematical model is specifically for MOVs with sizes ranging from 5 mm to 40 mm. The proposed model is tested against experimental test results and is found to match them by at least 75%. The discrepancy in matching is due to the assumption used in the matching process that all the MOVs exhibit the same response when subjected to the same impulse current. Nonetheless, the proposed model provides a minimum possible degradation level caused by a particular impulse current. The proposed model is thus deemed suitable to describe the relationship between the lightning impulse current injected and the degradation of the MOV. This dissertation is dedicated to Hlengiwe Mnisi Thank you for your love, support and encouragement. Most of it all, thank you for believing in meItem Investigating the effects of altitude (air density) on the HVDC breakdown voltage of small rod-plane air gaps(2016) Gora, TatendaThe validity of the atmospheric correction method presented in the IEC 60060-1 (2010) standard is analysed and evaluated by means of theoretical and laboratory work. In order to understand the problem, the evolution of the atmospheric correction methods, from as early as 1914, has been presented. A procedure (Calva prediction method) for predicting the direct current (DC) breakdown voltage for an air gap at any altitude was discovered and was also analysed along with the IEC 60060-1 (2010). A critique of some of the atmospheric correction methods commonly used standards was also done. Experiments were carried out at altitudes of 1 740 m (Wits University), 130 m (UKZN HVDC centre) and at less than 2 m above sea level (Scottburgh beach, Clansthal). More tests were conducted using a pressure vessel where high altitude relative air density was simulated. All tests were conducted on rod-plane air gaps using a 15 mm diameter at tip rod. Test results from Scottburgh beach were used as the standard breakdown voltages of the air gaps tested since the environmental conditions were the closest to the conventional standard conditions (stp). The test results obtained were compared with predictions using the Calva method in order to validate the method. The test results were also corrected according to IEC 60060-1 (2010) and compared to the standard breakdown voltages obtained at Scottburgh beach. It was shown that the IEC 60060-1 (2010) is quite suitable for atmospheric correction for data obtained at low altitudes (about 130 m). When applied to high altitude (1 740 m) data, the correction method is accurate and suitable for very small air gaps less than 0.1 m. As the air gap length increased, the corrected results began to deviate from the expected standard voltage. The same trend was shown with the corrected results from the pressure chamber tests. The prediction method by Calva was accurate when compared to the experimental data from the high altitude and low altitude test results. When compared to the data from the pressure chamber, the prediction method had a linear error factor which was di erent for each gap length. It was concluded that the IEC 60060-1 (2010) is not only unsuitable for atmospheric correction for data at relative air densities below 0.8, but also that the correction method is prone to an increase in error as the air gap length increases when the relative air density is higher than 0.8. The Calva prediction method was found to be suitable to use after additional factors are added when applied to high altitude conditions.Item Comparison of the lightning performance between the poles of the Cahora-Bassa ±533 kV HVDC lines(2016) Strelec, Gavin JasonThis work contributes toward research in the field of lightning performance of High Voltage Direct Current (HVDC) transmission lines, focusing on the impact of the line polarity on the incidence of line faults. Although there has been some recent research into the influence of polarity, there appears to be no confirmed effect that might influence the design of new lines. The research presents an investigation into the lightning performance of the two poles of the Cahora-Bassa HVDC transmission line. In order to compare the performance of the two polarities, the average lightning exposure over an 8-year period was confirmed to be very similar for both lines. Lightning stroke data from the South African Lightning Detection Network was correlated with fault times from the transmission-line protection scheme. The classification of the lightning related faults was used to determine the relative performance of the two poles, particularly in relation to polarity, and to infer if there was any influence of polarity on the lightning attachment process. This investigation for the Cahora-Bassa scheme shows that twenty-three out of twenty-five lightning related faults occurred on the positive pole. The results concur with performance experience on several HVDC lines from China and Canada, which indicate that lightning related faults favour the positive pole by a ratio of between 8:1 and 10:1. This represents a valuable contribution, which substantiates that HVDC line polarity has an influence on the lightning attachment process, and indicates that there is a need to re-examine the lightning shielding design for HVDC transmission lines.