Electronic Theses and Dissertations (PhDs)

Permanent URI for this collectionhttps://hdl.handle.net/10539/37970

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Action volume ratio: a method to classify the danger of lightning in any given volume
(University of the Witwatersrand, Johannesburg, 2015-05-07) Dickson, Andrew Stuart
The annual number of injuries and fatalities from lightning has, over the last century, been on a steady decline. This is primarily due to urbanisation and movement away from agriculturally intensive activities. In countries with a high urban population, the incidence of lightning fatalities is below 1 death per million people per year. However, in countries with a larger rural population, this rate is significantly higher, ranging between 8 and 15 deaths per million people per year. There has been a large drive towards educating the general public about the dangers of lightning and methods to avoid being in a dangerous situation. However, fatal lightning events still occur on a regular basis. There are currently no methods to determine the risk of lightning to living beings in open spaces. The international standard (IEC 62305-2) provides a method for the assessment of risk to living beings within a structure, and up to three metres outside of it. Considering that the majority of deaths by lightning occur outdoors, a method of determining these risks is necessary. The Action Volume Ratio (AVR) is proposed as a new method for the analysis of the danger of lightning in any volume. It considers the dangers of all lightning injury mechanisms in relation to the objects in the space, which are assumed to be the preferential points of strike. A union of the dangerous volumes is then formed, and a ratio to the total volume is created. The AVR uses accepted electrical engineering equations to determine the dangerous areas, and places no reliance on probability theory, which can, in many cases, skew the results of a lightning risk analysis process. The AVR can be combined with lightning ground flash density data to indicate the incidence and frequency of dangerous events within a given volume.
Discrimination between nearby and direct lightning strikes to a long operational medium voltage line to assist in the determination of the basic insulation level (BIL)
(University of the Witwatersrand, Johannesburg, 2022) Van Schalkwyk, Willem Jacobus Dirkse; Gomes, Chandima; Van Coller, John
The lightning performance of a Medium Voltage (MV) line needs to be divided into two categories: lightning performance due to nearby lightning and direct lightning strikes. A better nearby lightning performance requires a higher Basic Insulation Level (BIL) while the direct lightning performance requires a lower BIL to minimize equipment failure. The electromagnetic coupling models for calculating the Lightning Induced Overvoltage (LIOV) on a line are complicated and reliant on accurate input data. Therefore, short floating lines (< 3 km) were used to calculate the line’s lightning performance. The models were then verified with LIOV measurements on these short floating lines and the results were normalized to estimate the lightning performance of long operational lines. These estimations for long lines could never be verified due to the complexity of calculations and the cost and logistics of equipment to do measurements on long operational lines. A new methodology to measure the lightning performance of a long operational line was developed and verified. The obstacles such as the long line length, the changing soil resistivity, the lightning channel properties and the rapid attenuation of the LIOV along the energized line have been used as an advantage in the new method. The finite soil conductivity was used as an advantage to distinguish between nearby lightning and direct lightning strikes while the power frequency current was used to determine whether the LIOV exceeded the line BIL and caused a line fault. An existing electromagnetic coupling computer model, the ATP-EMTP, was verified with the existing ERM using the same input data. The ATP-EMTP model was then extended to include the equipped long MV line. There was no existing model or measurements to which the results could be compared to. Actual nearby lightning and direct lightning strikes of which the termination point was known were used to verify the new methodology. A significant size database (consisting of 38 675 lightning flashes and 1 155 line faults over two years) was used to evaluate the IEEE Std 1410-2010 estimated lightning performance of the long line.
Estimating Resistance and Performance of Earthing Systems Electrode in Variably Saturated Soil Conditions
(University of the Witwatersrand, Johannesburg, 2024) Nnamdi, Onyedikachi Samuel; Gomes, Chandima
The design and determination of post-installed resistance of earthing systems are significantly influenced by subsoil resistivity profiles, which are prone to seasonal variations due to environmental and climatic changes. These fluctuations can compromise operational safety and reliability of transmission systems, necessitating periodic monitoring of earthing installations as recommended by national and international standards. However, compliance with these recommendations is often impractical due to the vast number of earthing installations and associated costs. To address this challenge, this thesis proposes a novel multiphysics earthing model that integrates hydraulic and electrical properties of subsoil and earthing enhancement materials (EEMs) with climatic parameters to predict earthing resistance under varying conditions. The model, developed by coupling partial differential equations governing electric current dispersion and fluid retention in porous media, is validated through COMSOL Multiphysics® simulations of vertical earth rods in single and double subsoil layers. The results demonstrate that earthing resistance variation is dependent on subsoil texture, water content, and distribution of soil water potential, which determines subsoil resistivity. The proposed method achieves a relative error range of 2.72% to 6.53% and 1.47% compared to analytical and finite element method solutions, ensuring accuracy and validity. This innovative approach enables site-specific and climate-adaptive assessments of EEM effectiveness, facilitating informed decisions for earthing improvements in diverse conditions, and ultimately optimising material selection and recommendation for various soils and climates.
Peak-to-average power ratio reduction in optical-OFDM systems using lexicographical permutations
(University of the Witwatersrand, Johannesburg, 2023) Niwareeba, Roland; Cox, Mitchell A.; Cheng, Ling
The work presented in this thesis extends and contributes to the research in reducing the high Peak-to-Average Power Ratio (PAPR) in optical-Orthogonal Frequency Division Multiplexing (OFDM) systems using probabilistic-based and hybrid techniques. Whereas the high PAPR problem has been extensively studied and a number of solutions provided for the conventional Radio Frequency (RF)-OFDM systems, there are only a few solutions proposed specifically for PAPR reduction in optical-OFDM systems. Although the probabilistic-based techniques such as Conventional Selected Mapping (CSLM) and Data Position Permutation (DPP) result into significant PAPR reduction performance with negligible Bit Error Rate (BER) degradation, the resulting increase in both hardware and computational complexity as a result of a large number of Inverse Fast Fourier Transform (IFFT) operations that have to be performed to generate the candidate signals is still a major drawback. In order to reduce the complexity, in this research, two techniques which are applied in opticalOFDM systems are proposed. The first technique is the hybrid method composed of a modified CSLM and µ-law companding techniques called Low Complexity Hybrid Selected Mapping (LCHSLM). The proposed method achieves almost 50% reduction in complexity compared to CSLM with less BER degradation. The second technique based on lexicographical permutations called Lexicographical Symbol Position Permutation (LSPP) works by dividing the optical-OFDM symbol into a number of sub-blocks and performing lexicographical permutations to obtain the candidate signals after the IFFT operations. In the proposed LSPP, all the candidate permutation sequences are not obtained at once unlike in the DPP where the number of candidate permutation sequences increases at a factorial rate of growth as the number of sub-blocks increases resulting in a more complex system. Additionally, the research proposes an algorithm where a threshold PAPR value is introduced and the candidate signals are generated until a candidate with a PAPR value less or equal to the threshold is obtained. The results show that the complexity in terms of IFFT operations can be reduced substantially depending on the selected threshold and the number of candidate signals. Furthermore, the research introduces a new algorithm based on the global gain (net gain) to determine the most suitable number of permutation candidate sequences to achieve a reasonable PAPR reduction performance without increasing the time and hardware complexity to levels that the systems cannot tolerate.
Physical layer of Security with NOMA in mixed RF/FSO systems
(University of the Witwatersrand, Johannesburg, 2024) Osman, Amani Alged Fetaha; Takawira, Fambirai; Moualeu, Jules
The ever-increasing demand for spectrum resources, driven by the surge in high data tra c poses a signi cant challenge to the e ciency of wireless communication systems. In response, non-orthogonal multiple access (NOMA) has been proposed as a pivotal technique designed to address the spectral challenges inherent to tra- ditional multiple access schemes. NOMA, through its ability to allow multiple users to share the same time-frequency resources non-orthogonally, represents a promising avenue for improving spectral e ciency (SE). To further improve connectivity and enhance e ciency, NOMA can seamlessly be integrated with multiple-input-multiple-output (MIMO) systems. MIMO, a cor- nerstone technology in modern wireless communication, utilizes multiple antennas at both the transmitter and receiver to exploit spatial diversity, thereby achieving substantial gains in terms of capacity and reliability. The integration of MIMO with NOMA not only addresses the escalating demand for connectivity but also unlocks the potential for massive connectivity and enhanced spectral e ciency in wireless networks. However, the distribution of radio resources within the MIMO- NOMA framework brings forth security challenges owing to the intrinsic charac- teristics of wireless transmissions. In this regards, this thesis tackles the security aspect of MIMO-NOMA systems from an information theoretic viewpoint. The information theoretic security or physical layer security (PLS) e ectively mitigates vulnerabilities stemming from eavesdropping and interception, thereby fortifying the overall security of wireless communication in the present case of MIMO-NOMA systems. Another challenge that requires some attention is ensuring reliability over long distances. Although the radio frequency (RF) spectrum band is recognized for its reliability in diverse weather conditions, its inherent nature renders it sus- ceptible to security concerns. To address the imperative for secure long-distance communications, it is essential to explore innovative solutions that harness the inherent reliability of the RF spectrum while concurrently mitigating its vulner- abilities through enhanced security measures. Integrating technologies such as free-space optical (FSO) communication, acknowledged for its secure transmission capabilities, along with the implementation of advanced security protocols stands as a promising solution to achieving reliability and security in long-distance com- munications. In this work, PLS of a MIMO-NOMA based system in a mixed RF-FSO com- munications is investigated. Additionally, this thesis delves into the injection of vi arti cial noise (AN) in an e ort to explore other features of the proposed sys- tem from a PLS perspective. The last part of the work investigates the secrecy performance of the proposed system in the presence of multiple colluding and non- colluding eavesdroppers. To this end, the statistical properties are examined by deriving the statistical characteristics of the underlying system model, which will subsequently be used to establish both the analytical and asymptotic frameworks for evaluating secrecy performance measures. Finally, Monte Carlo simulations are adopted to validate the accuracy of our proposed mathematical derivations.
Reducing water absorption characteristics of kraft paper reinforced with modified nanoparticles
(University of the Witwatersrand, Johannesburg, 2023) Katun, Mohammed Mahmood; Nyamupangedengu, Cuthbert; Gomes, Chandima
The inherent hydrophilic characteristics of cellulose wood fibre compromise the dielectric properties of kraft paper insulation that is used mainly in oil-insulated power transformers. This thesis, therefore, presents a novel material design model of nanocomposite kraft paper with improved hydrophobic properties for power transformer insulation applications. The concept of nanodielectric kraft paper design was used. Rutile-titanium dioxide nanoparticles (rutile-TiO2 NPs) were selected as the nanofiller. Compared with other metal-oxides, rutileTiO2 NPs are stable in chemical reactions, have good thermal stability and also have high electrical resistivity. Rutile-TiO2 of 19.72 nm diameter were fabricated using the sol-gel method and then used in reinforcing the kraft paper to produce a nanocomposite kraft paper with improved dielectric properties. Since the nanoparticles are inherently hydrophilic, and the intention is to produce a hydrophilic nanocomposite kraft paper, a technique was devised to make the NPs hydrophobic. The rutile-TiO2 NPs were surface conditioned with two alternative surfactants; Alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). Various quantities of the two surfactants were investigated to determine the optimal amount. The resultant surface-modified rutile-TiO2 NPs were studied to understand their hydrophilicity and thermal stability properties. It was found that the unmodified rutile-TiO2 NPs absorbed more moisture, compared with the surface-modified nanofiller, the mass increased by about 4% due to moisture absorption. The surface-modified rutile-TiO2 NPs with 5% AKD had 45% more thermal resilience than the unmodified rutile-TiO2 NPs surface and this is a significant knowledge contribution of this thesis. Using an unbleached kraft pulp, nanocomposite kraft paper specimens were fabricated with nanoparticles having varying surfactant loading. The specimens were then characterized to reveal various critical properties such as hydrophobicity, thermal, dielectric losses and dielectric strength. The version of the nanocomposite kraft paper that gave the most improved water and moisture absorption properties was that with 5vol/vol% ASA surface-modified NPs. The moisture absorption rate dropped by 74% compared to the unfilled kraft paper, and the water vapor transmission rate decreased by 30%. The contact angle of water droplets improved by 12%, and water absorption rate improved by being 4 times slower. The dielectric loss measurements showed that the nanocomposite kraft paper containing rutile-TiO2 NPs (5 vol/vol% ASA) also had 40% lower dielectric losses than the reference (unfilled) samples. The breakdown voltage of the nanocomposite kraft paper increased by about 15% while thermal withstand was improved by 5.4%. This research, therefore, has successfully improved the hydrophobic properties of kraft paper by filling it with surface-modified rutileTiO2 NP. It can be argued from the results that for power transformer application, the novel nanocomposite kraft paper developed in this thesis will improve power transformer insulation reliability design by mitigating the main agents of insulation degradation which are water and thermal stress.