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 Realisation of AD hoc renewable rural power systems with decentralised active power dispatch techniques(2019) Lange, Jarren HiltonAd hoc power systems offer a promising opportunity to provide affordable reliable renewable energy to rural areas. Classical grid solutions are impeded by low population densities and poor economic conditions that perpetuate energy poverty in large areas of rural Africa. Existing islanded renewable energy based solutions can not typically be expanded at will as the needs of its users increase. Scalable power systems, that can lower the engineering costs of commissioning and modifying the system represents a potential solution to energy poverty. Existing solutions rely on the ability to tightly model and control all elements a result of the stringent requirements imposed on these systems. Meaning scalable power systems are theoretically unrealisable. Thus, this is a system architecture and control issue, not a generation or storage issue. This thesis explores ways to realise low cost scalable power systems for low (> 1 kW) to medium (< 1 MW) power requirements. Allowing power system parameters to indefinitely deviate from nominal values, which discards a 140 year old assumption, is achievable in new electronic generation based power systems. This allows all system elements to contribute towards system operation without additional communication. Modelling is presented which simplifies complex power interactions in AC systems to passive circuit components. The desired characteristics of each element while utilising existing technologies can be identified from this modelling. These techniques, which are demonstrated and verified on a hardware based power system simulator, enable scalable economically feasible renewable power systems. This provides a novel, flexible and robust alternative to existing power systems that enables the affordable decentralised ownership and operation of renewable power systems at a household level.Item Investigation into the impact of wind generation on the inter-area and local oscillation modes of power systems(2018) Chidzikwe, CliffThe operation and dynamic characteristics of power system grids with mixed generation technologies in particular synchronous generators and wind turbine generators are receiving great attention towards the better understanding of the modern power system stability. Wind power generation introduces new types of non-conventional generators that are being operated in parallel with conventional generation. However, as the penetration of wind energy increases, the power system dynamics and behaviour are modified. It is thus important to understand and quantify the impact of wind energy technologies on the performance of the interconnected power systems. The investigation in this dissertation focuses on the impact of wind power generation on the power system small-signal stability. The focus is on the variable speed Type 4 Wind Turbine Generators (WTGs) and the Wind Power Plants (WPPs) impact on the inter-area and local oscillation modes of the power systems. An aggregated WPP model based on the generic IEC 61400-27-1 (2015) Type 4 WTG of varying capacities was used in this investigation together with a multi-machine small-scale power system comprising conventional synchronous generators and their associated controls. The Power System Stabilizer (PSS), an additional generator control, is widely used to resolve the power system small-signal stability problems by providing additional damping to the inter-area and local oscillation modes. This investigation considered the impact of wind power on the inter-area and local oscillation modes without and with PSSs installed on the synchronous generators. The procedure involved employing small-signal stability analysis through eigenvalue analysis and tracking the oscillation modes after the integration of wind power generation sources. The investigation has shown that the introduction of wind power in parallel with synchronous generators alter the generator operating conditions and the power flows causing the inter-area and local oscillation mode characteristics to change. As the wind power penetration increases, the inter-area and local mode frequencies decrease. The decrease in frequencies has been attributed to the change in the operating conditions of synchronous generators when operated in parallel with wind turbine generators. It was further found that wind power can have negative or positive impact on the damping of inter-area and local modes. However, for the particular case studies examined, this investigation suggests that PSSs already installed in the absences of wind power generation sources do not require retuning. The work recommends that power system small-signal stability requirements in the presence of wind power should be investigated as this is a unique characteristic for a given network.Item A general solution to optimising the DC-bus energy storage requirements in single phase inverers(2018) Du Toit, Francois PaulusPower electronic converters that convert DC to AC, or vice versa, require an energy buffer between the AC and DC ports of the converter to compensate for the instantaneous power mismatch. Electrolytic capacitors are mostly used for these buffering applications because of the high energy density when compared to other capacitors, but unfortunately this type of capacitor also has low reliability. This dissertation proposes a general solution from a fundamental approach to solve the required capacitor power requirements on the DC-bus of an inverter. From the resulting model, an alternative active filter design technique to reduce the required capacitance of the DC-bus capacitor of a single phase inverter is presented. In this model, the minimum and maximum voltages of the capacitor can be chosen and the corresponding waveforms are calculated. An optimum region for the choice of capacitor voltage is shown to visually illustrate the trade-offs between the capacitor voltage, capacitance and converter losses. In this optimum area the reduction in capacitance is enough to allow the elimination of electrolytic capacitors, while maintaining comparable volume. In this technique, the DC-bus capacitor is decoupled from the DC-bus to allow wide voltage variation and the power processed by the capacitor is directly controlled, instead of the bus voltage. The allowable voltage variation of the capacitor can also be selected to fit the application or traded off in favour of capacitance as chosen by the designer. This general solution is applicable to any bi-directional converter used to decouple the capacitor from the DC-BusItem Use of Thyristor Controlled Series Capacitors (TCSCs) to enhance power system transient stability and their possible application on the South African Grid(2015) Gumunyu, TonderayiThyristor Controlled Series Capacitors (TCSCs) are FACTS devices which incorporate power-electronic-based and other static controllers to enhance controllability and increase power transfer capability. This research investigated the possibility of applying TCSCs on the South African transmission network, in particular application on long transmission lines connecting bulk thermal generators in the northern part of South Africa to load centres located hundreds of kilometres elsewhere in the country. The investigation, conducted using PSS/E (a power system analysis software) demonstrated that application of TCSCs on this part of South African transmission network results in improved transient stability margins. The resulting improvement in transient stability is comparable to other transient stability enhancement options like addition of transmission lines, thus the use of TCSCs can be considered as an alternative. Further studies would be vital to understand the interaction between Power System Stabilizers (PSSs) and TCSCs in order to ensure proper tuning and interaction amongst the devices.Item Modelling integrated passive structures for power converters(2015-01-23) Floor, AdrianIntegrated architectures for power electronic circuits have been a subject of recent interest. Integration offers several benefits such as reliability, control on parasitic elements related to discrete components, and ease of manufacture. The main objective of this particular research has been to contribute towards effective modelling of integrated passive circuits operating in power electronic circuits. Integrating passive components in one distributed space can be difficult to understand, and hence to design. Field electromagnetics is often unwieldy for a power electronics circuit designer, so a SPICE-like circuit simulator is often an effective design environment. This dissertation closely examines both lumped and distributed SPICE-compatible models. Four SPICE-compatible models have been investigated by comparing them with an analytical distributed solution. This analytical solution is used to thoroughly derive the causes of all resonance points, as well as impedances at low/high frequencies; which are the important factors that characterize the integrated passive. This analytical solution is only implemented in a narrow range of boundary conditions; hence the SPICE-compatible methods must be developed, since SPICE then handles the algorithmic work of handling the more complicated boundary conditions found in power electronics.