3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Inference of charge transfer from lightning flashes in South Africa(2019) Tasman, Jesse Dean; Tasman, Jesse DeanThe objective of this study is to determine the quantity of charge transferred, in Coulombs, during the continuing current phase of natural cloud-to-ground (CG) lightning flashes over an area in Johannesburg, South Africa. Continuing current is responsible for most thermal related lightning damages such as destruction of property, electrical fires and physical human trauma. The mitigation of lightningrelated risks can be better managed through improved measurement methods of naturally occurring lightning. The application of a point-charge model used to infer charge transfer from changing electric field measurements is detailed. A flatplate antenna with an integrator is set up to record the changing electric fields from lightning flashes. These measurements, along with high-speed video footage to determine continuing current durations, are processed and charge transfer quantities are inferred. From 34 negative lightning strokes with long continuing current (i.e. > 40 ms), the quantity of charge transfer ranges from 0.3 C to 145.5 C and has a mean quantity of 18.3 C. For the 5 recorded positive strokes, the quantity of charge transfer ranges from 3.7 C to 66.6 CItem 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 Lightning return stroke electromagnetics - time domain evaluation and application(2016) McAfee, Carson William IanThe work presented extends and contributes to the research of modelling lightning return stroke (RS) electromagnetic (EM) fields in the time domain. Although previous work in this area has focused on individual lightning electromagnetic pulse (LEMP) modelling techniques, there has not been an investigation into the strengths and weaknesses of different methods, as well as the implementation considerations of the models. This work critically compares three unique techniques (Finite Antenna, FDTD, and Single Cell FDTD) under the same ideal simulation parameters. The research presented will evaluate the EM fields in the range of 50m to 500m from the lightning channel. This range, often referred to as the near field distance, has a significant effect on lightning induced overvoltages on distribution lines, which are primarily created by the horizontal EM fields of the RS channel. These close distances have a significant effect on the model implementations, especially with the FDTD method. Each of these modelling methods is explained and tested through examples. The models are implemented in C++ and have been included in the Appendix to aid in future implementation. From the model simulations it is clear that the FDTD method is the most comprehensive model available. It allows for non-ideal ground planes, as well as complex simulation environments. However, FDTD has a number of numerical related errors that the Finite Antenna method does not suffer from. The Single Cell FDTD method is simple to implement and does not suffer from the same numerical errors as a full FDTD implementation, but is limited to simple simulation environments. This work contributes to the research field by comparing and evaluating three techniques and giving consideration to the implementation and the applicability to lightning EM simulations.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.Item On the sixth mechanism of lightning injury(2015-04-30) Blumenthal, RyanThe work presented in this thesis extends and contributes to research in the field of lightning injury mechanisms. Six mechanisms have been described in the literature about lightning injury. This thesis takes an in-depth look at the sixth injury mechanism. The sixth mechanism may be thought of as a ‘pressure-shock wave’ which is directly proportional to the current of the lightning discharge, and which is present immediately surrounding lightning’s luminous channel. A literature review, case studies and two novel experiments helped confirm the sixth mechanism’s existence. The medical data and the lightning data were then aligned. Two main questions were addressed, namely within what range is a human at risk; and what is the risk of lightning’s pressure shock wave. This ‘pressure-shock wave’ may explain some of the more curious lightning injury patterns seen on lightning-strike victims. Knowledge and insight into the sixth mechanism may have direct and indirect applications to those working in the fields of lightning injury and lightning protection. This thesis represents a contribution to the literature in both medicine and engineering.Item Initial steps in the development of a comprehensive lightning climatology of South Africa(2009-07-08T07:14:57Z) Gill, TraceyThe summer rainfall region of South Africa is dominated by convective thunderstorm development from October to March. The result is that lightning is a common event over most of the country during this time. The South African Weather Service (SAWS) installed a stateof- the-art Lightning Detection Network (LDN) in late 2005 in order to accurately monitor lightning across South Africa. Data from this network for 2006 was utilised in order to develop an initial climatology of lightning in South Africa. Analyses were performed of lightning ground flash density, flash median peak current and flash multiplicity on a 0.2° grid across South Africa. The highest ground flash density values were found along the eastern escarpment of the country, extending onto the high interior plateau. There is a general decrease in flash density from east to west, with almost no lightning recorded on the west coast of the country. The regions of highest flash density recorded the highest percentages of negative polarity lightning. The percentage of positive lightning was higher in the winter months, as was the median peak current of lightning of both polarities. The median peak current distribution displayed distinct bands of current values oriented in northwest to southeast bands across the country. The bands of higher median peak current correspond to the regions to the rear of the interior trough axis in areas dominated by stratiform cloud development and were more dominant in the mid summer months. The highest flash multiplicity was recorded in the regions of highest flash density. Along the southern escarpment, on the eastern side of South Africa, flash multiplicity values exceeded 3 flashes per square kilometer. The highest flash multiplicity of negative polarity lightning was recorded in the spring and early summer. Throughout the year, the percentage of single stroke flashes for positive lightning is high. Topography and the position of the surface trough have a very strong influence on the ground flash density and median peak current distributions, but not on the flash multiplicity distribution. The results from the analyses of the three lightning variables were then combined to determine risk indexes of high intensity lightning and of positive polarity lightning. The eastern part of South Africa is at extreme risk from both large amounts of lightning and from positive polarity lightning, whereas the regions in the northwest of the country that are dominated by mining are at extreme risk from mainly positive polarity lightning.