3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Investigation into energy manipulation in reactive components using transmission line modelling(2019) Mofu, MubangaThe principle objectives of this research are to seek alternative avenues to enhance qualitative and quantitative understanding at a fundamental level of the internal electromagnetic environment in reactive components during energy flow, conversion and transfer processes. That can be seen, for example in power electronic converters. The study of these fundamental energy processes is necessary, as it has been documented in literature that the adoption of wide-bandgap (WBG) semiconductors and the increased switching frequencies they enable, are facilitating the miniaturisation of components which leads to increased power densities. Consequently, this means that multiple components or converter functions can be packaged in discrete or integrated power modules. The spatial arrangement of the components in these densely populated power modules means that greater emphasis needs to be placed on the internal electromagnetic structure and environment of the various components. The ramifications of these environmental conditions are seen as fundamental switching limits or bottlenecks to the maximum realisable switching frequency. The implications of these limits are seen as increases in switching frequency passed a certain threshold provide diminishing returns in terms of power density as well as negatively affecting converter efficiency. From these observations and the ones made in contemporary literature, it has been established that a more wholistic approach to improving power electronic converters that is not solely dependent on the switching function is necessary for improvements in contemporary as well as future converters. Consensus from various sources in literature suggest that a fundamental breakthrough is required in the power passives i.e. reactive components of the converter before appreciable performance gains are established. In this work the fundamental energy manipulation interactions between electric and magnetic energies in these power passives i.e. reactive components will be analysed as a potential means of providing this paradigm shift. Increased insight and understanding of the energy manipulations processes in reactive components may provide the foundational framework for improved power passives to be developed in future These energy manipulation processes are examined by considering the common LC Resonant tank and Twin Capacitor topologies. These topologies are selected because they are fundamental examples of electric to magnetic and electric to electric energy manipulation processes respectively. It is shown in this work that analysing the Twin Capacitor Paradox with conventional circuit theory leads to the violation of conservation of energy, highlighting the limitations in circuit theory in modelling energy manipulations. In this research, the reactive components are modelled using lossless 1-D TLM Link-Line equivalent transmission line models. The implications of this mean that the interconnection of two components effectively forms a cascaded transmission line with three principle nodes. The Energy Conversion Efficacy (ECE) quantification metric is coined to establish the quality of an energy manipulation process. ECE is principally concerned with establishing the maximum amount of energy that can be transferred from a primary energy store to a secondary energy store and the maximum amount of energy that can be reconstituted back into the primary energy store in the original form. Additionally, the term Fragmentation is coined to better explain some of the diffusion and scattering effects that may take place at the interface or junction of the transmission lines segments with arbitrary lengths. Consequently, the implications this imposes on ECE are subsequently analysed. The nature of this research is purely theoretical and as a direct consequence; the contributions arising as a result are purely theoretical. A major contribution arising from this work is the derivation of an alternative solution to the Twin Capacitor Paradox in which the perceived loss or deficit in energy is found to be present in the self-inductors of the respective capacitors. It is also shown in this work that a condition for maximum ECE can only be achieved provided both transmission line segments have identical impedances and propagation delays. If the ratio of propagation delays in the cascaded transmission line structure is increased an accompanying increase in generated noise will also be seen.Item An autonomous smart battery for an isolated 12V distributed DC grid(2018) Aswat, MuhammedAn autonomous smart battery is developed to incorporate storage on an isolated 12V DC grid called the PeCo grid. The PeCo grid is a personal consumer grid aimed at electrification of rural areas with no existing electrical infrastructure. In order to develop the smart battery a 12V lead-acid battery is combined with a Dual Active Bridge (DAB) bi-directional DC-DC converter. Using this combination with the relevant control algorithms the smart device achieves grid stabilisation, black starting and fault protection without the use of a dedicated communication network. These features are achieved through the usage of a decentralised control architecture that is solely based upon the grid voltage. The smart battery is able to seamlessly integrate into an existing system due to its plug and play nature. A working prototype of the smart battery is developed and experimentally validated on the distributed energy system.Item Feasibility study of the dual active bridge as a low-frequency sine wave inverter(2018) Ortlepp, Kerren MarkThe conventional Sinusoidal PulseWidth Modulation (SPWM) inverter is limited by the fact that it does not allow for Zero Voltage Switching. This means that the switching frequency is kept low to reduce the switching losses. As a consequence of holding these switching frequencies low, the distribution of power over the frequency spectrum is kept closer to the fundamental frequency (compared to higher switching frequencies) leading to larger reactive components to filter out these harmonics. The use of high-frequency switching, Zero Voltage Switching, and different modulation schemes can lead to higher power densities. This research investigates under what conditions the use of these techniques in a Dual Active Bridge (DAB) inverter might lead to a higher power density than the SPWM. Volumetric approximations for the different circuit components in the investigated inverter topologies are demonstrated. These approximations are used to design circuits using physical volume as the cost function where possible. Additionally, a loss model is derived to determine the expected efficiency of each topology being investigated. This model is related to the power density since it is directly proportional to the size of heat sink required to cool the inverter. The techniques for improving power density mentioned above are presented, and the impact that they have on power density is shown using the volumetric approximation function. From this approximation, the volumes between the DAB and the SPWM are compared and investigations into where the DAB may have a higher power density have been performed. It was found that the DAB was not smaller than the SPWM for frequencies less than 72kHz. When simulating the converters operating at different frequencies, the general trend is that the SPWM increases in volume as the frequency increases, whereas, the DAB decreases in volume as the frequency increases. An exact frequency at which the DAB would be smaller than the SPWM was not found in this research. However, many conclusions have been drawn around the use of a DAB as an inverter and the strengths and shortcomings it provides. The modulation scheme would need to be modified to reduce the losses and provide a more competitive volume. Additionally, multi-level and multi-stage techniques could be used to reduce the volume further.Item Voltage uprating of existing high voltage substations when transient voltage stress and available withstand strength are coordinated(2017) Schutte, PeetServitude availability in space-constrained built-up areas within the Johannesburg or Central Load Network (CLN) poses every-day challenges for power system engineers. Strengthening the backbone 88/275 kV transmission system within the CLN becomes even more difficult when multi-circuit transmission lines are required for increased power transfer capabilities. When uprating is considered to increase the power transfer capability, the withstand levels of existing external insulation demands an optimisation to find a new stress versus strength balance that allows reliable operation of substations at higher voltages. The research includes primarily an investigative simulation study to evaluate the current Eskom available design clearances in terms of their withstand capability when subjected to over-voltage transients. Two voltage range classes were evaluated and the results are discussed. For voltage range 1, it was found that the over-voltage stress was low enough to allow for a higher nominal operating voltage while maintaining the existing clearances. For voltage range 2, existing clearances are also found to be conservative and smaller safety margins will most likely be acceptable. From a transient analysis evaluation, voltage uprating is considered as a very attractive option to increase the power transfer capability of existing substations. Current Eskom clearances for 88 kV and 275 kV are expected to perform well during transients generated in uprated systems. Electrode grading to improve the field gradients in the substation will require attention to increase gap factors. Additional surge arresters are considered to be a cost effective solution to control over-voltages throughout the whole uprated substation. The physical modification of substations to replace strung conductors with tubular conductors, ensuring sufficient outage time to refurbish and rebuild with new equipment will be the most challenging part of uprating existing substations.Item The use of steady-state level combinations and signal event edge correlations in the disaggregation of total power measurements(2015) Penn, Joseph JThe work presented extends and contributes to research in Non-Intrusive Load Monitoring (NILM), focussing on steady-state and transient power measurement disaggregation techniques for circuits containing household ap- pliances. Although previous work in this area has produced and evaluated a wide range of NILM approaches, much of it has involved the use of datasets captured from real-world household implementations. In such cases, the lack of accurate ground truth data makes it di cult to assess disaggregation tech- niques. In the research presented, three NILM techniques are comparatively evaluated using measurements from typical household appliances assembled within a laboratory environment, where accurate ground truth data could be compiled to complement the measurements. This allows for the accu- racy of the various disaggregation approaches to be precisely evaluated. It is demonstrated that the correlation of transient event edges in aggregated power measurements to individual appliance transient exemplars performs better than the matching of steady-state power levels against individual ap- pliance state combinations. Furthermore, the transient approach is shown to be the most appropriate technique for further development.