Doctoral Degrees (Electrical Engineering)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6855

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Analysis and utilization of reverse power flow of wind energy source using multi-port power electronic transformer.
(2020) Aladesanmi, Ereola Johnson.; Dorrell, David George.
The recent liberalization of the electricity market and increased environmental concerns as well as an increase in energy demand across the globe have brought the use of renewable energy sources such as wind energy to the fore. Some of the potential benets of renewable energy sources (RESs) are: localized generation, environmental-friendliness, generation of clean energy, reduction in greenhouse gas (GHG) emissions, increase in energy generation for increasing demand, and reduction in transmission losses. However, high penetration of RESs exposes power grids to several challenges. Some of these challenges for RESs are: increases in voltage prole level, high power losses, reverse power ow (RPF), protection and control issues. The main concern of this research work is RPF. RPF is a situation whereby excess power generated on a grid as a result of high integration or penetration of RES is fed back to the source of generation. RPF exposes power grids to various challenges; aside from causing grid instability. RPF incurs additional losses on the grid, causing over-voltage and overloading of the connecting elements such as conductors and transformers. In recent times, various control strategies have been deployed to mitigate these effcts on the grid. Energy management systems (EMSs) with energy storage devices (ESDs) are the most commonly applied strategies. However, intrusion into consumers' privacy and the high cost of energy storage devices poses a challenge to this approach. Voltage rise (VR) is one of the consequences of RPF. Line impedance reduction and reactive power compensation using exible AC transmission system (FACTS) devices are some of the methods use for voltage rise control. On-load tap changer transformers (OLTCs), generation curtailment and reverse power relay are also deployed to control RPF. However, reactive power compensation and generation curtailment approaches lead to power losses and voltage instability respectively. This thesis proposes a more secure method for utilising reverse power to supply power to modern electric vehicle (EV) charging stations through a multi-port power electronic transformer (MPPET). The proposed method consists of a RPF detection stage (RPFDS) electrically coupled to the point of common coupling (PCC), which discriminates between the total power generated on the grid and the actual load demand. A smart circuit breaker operates as soon as it picks up signal from RPFDS. The MPPET receives power from RPF utilization substation which is then used for electric vehicle (EV) charging. The method was validated experimentally in the laboratory. The results of the research work proved the ectiveness of the MPPET involtage regulation and in RPF utilisation.
Frequency stability study of interconnected power systems with high penetration of renewable energy in the restructured environment: emulation and control of virtual inertia using intelligent techniques.
(2021) Aluko, Anuoluwapo Oluwatobiloba.; Carpanen, Rudiren Pillay.; Dorrell, David George.; Ojo, Evans.
The main aim of power system operations and control is to ensure reliability and quality of power supply, a key action that helps in achieving this aim is frequency control. Frequency control in power systems is the ability to maintain the system frequency within specified operating limits, i.e., proper coordination between generation and load. The task of frequency control, more importantly, load frequency control (LFC) is becoming a complex control problem in the design and operation of modern electric power systems due to its growing size, changing market structure, newly emerging distributed renewable energy sources with little or no inertia support, evolving regulatory requirements and the increasing interconnectedness of power systems. These developments can lead to a reduction in the active overall inertia in the power system which reduces its frequency response capability by increasing the amplitude of frequency deviation, continuous frequency oscillations and increased settling time after a power mismatch in the system. The potential role of virtual inertia in the task of frequency control has been identified as an integral part of modern power systems. Therefore, in this thesis, novel methods for implementing virtual inertia using intelligent control techniques are proposed in the LFC framework of a multi-area interconnected system with high penetration of renewable energy in the deregulated environment. The first method proposes the novel application of the artificial bee colony (ABC) optimization algorithm in the design of the virtual inertial control in a grid-connected wind energy conversion system (WECS). The WECS operates below the maximum power point to reserve a fraction of active power for frequency response. The proposed ABC-based control method minimizes the first frequency undershoot and active power transients compared to the classical optimization method. Due to the non-storable and variable nature of renewable energy sources, the first method may not be accessible when needed. To tackle this challenge, the second method proposes the application of an energy storage system (ESS) and the type-II fuzzy logic control (FLC) in the development of the virtual inertia control strategy. The proposed type-II FLC method gives a better performance than the type-I FLC and derivative-based control methods with adaptive inertia gain, faster response time for active power injection/discharge, and damped frequency oscillations. Lastly, a novel hybrid LFC scheme is developed to further improve the dynamic response and stability of the system. The hybrid LFC scheme consists of a robust unknown input observer (UIO) for state estimation of the system in the presence of unknown inputs/disturbances, and the interval type-II FLC for the LFC loop. The robust UIO relays the true state of the system frequency to the LFC block in each control area to maintain its frequency and net tie line power flow at scheduled values. The proposed methods are designed and implemented using the MATLAB/Simulink Software.
Methods to reduce the starting current of an induction motor.
(2022) Habyarimana, Mathew.; Carpanen, Rudiren Pillay.; Dorrell, David George.
Power system loads that have high starting currents are a serious source of concern in smaller grids or remote locations on the main grid. This problem is envisaged to be exacerbated by the rollout of smart microgrids. When a high power induction motor is turned on in such a power system, its inrush current can be up to about ten times the full-load current. This transient current can cause problems when attached to weak grids. The increased current is due to the power required to start the load and the increased reactive power demand during the starting process. To protect the grid connection as well as the load, energy storage units can be used to compensate for the increased power requirement. A more pragmatic approach is to reduce the reactive power requirement using tuned compensation capacitors in order to reduce the inrush current. The aim of this research is to address the selection, calculation and switching of the capacitor bank for reactive power compensation. The capacitors are calculated and switched on to compensate the starting transient and disconnected when the machine has run up to speed using a point-on switching approach that reduces the switching transient.
Reliability study under the smart grid paradigm using computational intelligent techniques and renewable energy sources.
(2022) Onaolapo, Adeniyi Kehinde.; Carpanen, Rudiren Pillay.; Dorrell, David George.; Ojo, Evans Eshiomogie.
The increase in the demand for a reliable electricity supply by the utilities and consumers has necessitated the evaluation of the reliability of power systems. A reliable electricity supply is characterized by no or minimal duration and frequency of supply outages. Current power systems are changing due to increasing power demand and depletion of fossil fuel deposits. These changes are related to smart grids which are intelligent electric networks that are capable of using demand management methods, supporting communication devices and monitoring of consumer energy consumption. They can also integrate renewable energy sources thereby reducing reliance on fossils fuel sources. The main objective of this study is to optimize power systems operations and improve reliability. Different optimization methods are proposed in this study to address the issues of power systems operations. These optimization problems consider different constraints for maximum operations of the power systems. Case studies are used to confirm the proposed methods using the historical and climatic data for the City of Pietermaritzburg (29.37°S and 30.23°E), and Newcastle (27.71°S, 29.99°E) South Africa. Firstly, the implementation of the back-propagation algorithm method of the artificial neural networks (ANNs) for designing a predictive model for power system outage is proposed. The results obtained are found to be satisfactory. In situations where there is the problem of accessibility to large system data and presence of multiple system constraints, another method is proposed. This second technique proposes the application of a maximum entropy function-based multi-constrained event-driven outage prediction model, using the collaborative neural network (CONN) algorithm. The outcome is better than the conventional event-driven methods. Lastly, an adaptive model predictive control (AMPC) method with the integration of renewable energy sources (RESs) and a battery energy storage system (BESS) is proposed to further improve the reliability of the power system. The developed method uses a modified Roy Billinton Test System (RBTS) to implement the reliability improvement of the power system. The proposed computational intelligent techniques fulfil the necessities of operation robustness, implementation simplicity and reliability improvement of the power systems.

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Browsing Doctoral Degrees (Electrical Engineering) by Author "Dorrell, David George."