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Doctoral Degrees (Electrical Engineering)

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    An investigation into the utilization of swarm intellingence for the control of the doubly fed induction generator under the influence of symmetrical and assymmetrical voltage dips.
    (2022) Reddy, Kumeshan.; Saha, Akshay Kumar.
    The rapid depletion of fossil, fuels, increase in population, and birth of various industries has put a severe strain on conventional electrical power generation systems. It is because of this, that Wind Energy Conversion Systems has recently come under intense investigation. Among all topologies, the Doubly Fed Induction Generator is the preferred choice, owing to its direct grid connection, and variable speed nature. However, this connection has disadvantages. Wind turbines are generally placed in areas where the national grid is weak. In the case of asymmetrical voltage dips, which is a common occurrence near wind farms, the operation of the DFIG is negatively affected. Further, in the case of symmetrical voltage dips, as in the case of a three-phase short circuit, this direct grid connection poses a severe threat to the health and subsequent operation of the machine. Owing to these risks, there has been various approaches which are utilized to mitigate the effect of such occurrences. Considering asymmetrical voltage dips, symmetrical component theory allows for decomposition and subsequent elimination of negative sequence components. The proportional resonant controller, which introduces an infinite gain at synchronous frequency, is another viable option. When approached with the case of symmetrical voltage dips, the crowbar is an established method to expedite the rate of decay of the rotor current and dc link voltage. However, this requires the DFIG to be disconnected from the grid, which is against the rules of recently grid codes. To overcome such, the Linear Quadratic Regulator may be utilized. As evident, there has been various approaches to these issues. However, they all require obtaining of optimized gain values. Whilst these controllers work well, poor optimization of gain quantities may result in sub-optimal performance of the controllers. This work provides an investigation into the utilization of metaheuristic optimization techniques for these purposes. This research focuses on swarm-intelligence, which have proven to provide good results. Various swarm techniques from across the timeline spectrum, beginning from the well-known Particle Swarm Optimization, to the recently proposed African Vultures Optimization Algorithm, have been applied and analysed.
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    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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    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.
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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.
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    Stability of the grid incorporating multi terminal HVDC: case study of a south African network.
    (2021) Oni, Oluwafemi Emmanuel.; Swanson, Andrew Graham.; Carpanen, Rudiren Pillay.
    Transmission lines make one of the significant parts of power systems; faults or disturbances along any of the transmission medium often transcend to both the generating ends and the loads' end. Besides, the strength of any particular grid depends solely on the impedance of the tie-lines of that grid. Therefore, in this thesis, the line commutated converter (LCC) multiterminal high voltage direct current (MTDC) system is modelled and improved for the stability of an AC network. The converter control architecture and modelling are emphasized and explained. The effective short circuits ratio (ESCR) of the interconnecting AC lines is first described and analyzed as well. The existing CIGRE control techniques for a point-to-point LCC HVDC system have been enhanced and adapted for this study. The control and the filter parameters have also been calculated to generate a better and efficient result during a steady-state and dynamic analysis of the study. The work carried out in this study is divided into four sections, with each section focusing on each of the research objectives. In the first section, dynamic modelling and control of LCC MTDC systems were carried out with consideration to the ESCR of the inverter side of the AC substation. The impact of large-disturbance at the inverter is investigated. This analysis has been proposed to study the impact of AC short circuit fault on the three substations. The results from this study, which are shown on a subplot, show that the system experienced a large transient overcurrent and non-severe commutation failures. Also, a voltage dip at the faulted inverter station was recorded; however, the efficacy of the converter controller disallowed the transfer of such voltage dip to the other two converters. The second section of this study focuses on the application of MTDC system. We have carried out a comparative analysis of MTDC and AC transmission line on a single machine infinite bus (SMIB) network. The main focus of the investigation was on the transient and rotor angle stability of the SMIB network with or without MTDC link. The study also carried out a power-angle curve with the use of equal area criterion. The third section focuses on the interarea oscillation reduction in a power system. Kundur's two-area four-machine network was adapted to suit the scenarios of this study. Different fault analysis was carried out, and the response of the generator active power, frequencies, and DC-bus voltages are recorded. The results in this study show the better performance of the MTDC implemented in this study over the other well-known method of AC transmission medium. Also, the integration of the MTDC link is constrained by the variation of the current order of the overall power controller. The result is observed in the damping rate of the interarea oscillation of the network. The final section of this study carried out dynamic modelling of the South African grid, and detailed dynamic response to different stability studies was carried out. An auxiliary controller for the MTDC system capable of reducing the active power oscillation by generating a new current order is proposed. This secondary control for the MTDC system is based upon dynamic sensitivity analysis of the oscillations, and thereby generate a DC current compensation for the reduction of active power oscillations in the MTDC converters' station. Two network configurations were considered in this section. System disturbance during the first configuration shows a loss of synchronizing effect from both the AVR and PSS, which causes the generator to lose synchronism with subsequent oscillations. A negative damping torque for the rotor angle and negative synchronizing torque for the interarea oscillations was also observed. Meanwhile, the results during the second configuration recorded quick damping of the interarea oscillations with a significant improvement to the voltage profile. Among all of these benefits, the power carrying capacity at a reduced loss and cost stood out. The conclusion from this section is that the implementation of the MTDC link on the South African grid provided a better system performance. Therefore, the adoption of this research into South African transmission network will surely help enhance the stability margin of the grid. The proposed secondary controller also provided potential mitigation of excessive active power dip of the MTDC link during the system disturbance.
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    Performance and cost benefit analyses of university campus microgrid.
    (2021) Akindeji, Kayode Timothy.; Tiako, Remy.; Davidson, Innocent Ewean.
    Affordable and clean energy is one of the sustainable development goals (SDGs) to be achieved by the year 2030. Renewable energy sources such as wind, hydro, solar are free and inexhaustible globally to produce clean, reliable and cost effective power. However, most renewable energy sources are intermittent, to overcome this barrier, the concept of microgrid has been deployed in many applications to aggregate renewable energy resources, energy storage system and energy management system for sustainable, reliable, economical and environmental - friendly power system. Furthermore, considering the continuous increase in the cost of electricity and recent load shedding in South Africa, universities can reduce cost of energy demand, avoid interruption of academic activities due to load shedding and develop a test-bed or laboratory in which students and faculty staff can conduct research to advance modern power system through a self-sustaining microgrid. The university is like a separate entity and can operate as an island with sufficient resources to meet her energy demands. This thesis analyses the performance of a university campus microgrid using the five campuses of the University of Kwa-Zulu Natal as case studies considering economical and environmental benefits. Three different studies are carried out to achieve the aim and objectives of this work. The first study considers a grid connected microgrid using the real time data from the university energy management system, the modelling and simulations are implemented in HOMER Grid®. The main objective is to determine the optimal generation mix and size of a hybrid system consisting of the utility (eThekwini Electricity), solar PV, wind turbine, diesel generator and battery system taking into consideration the cost of energy (COE), net present cost (NPC), return on investment (ROI), payback period (PBP), utility cost saving and CO2 emission reduction. The second study aims to optimize the operational cost of a hybrid power system (PV-Wind-Diesel Generator-Battery) using two campuses as case studies. The objective function is formulated as a non-linear cost function and solved using a MATLAB function, ‘quadprog’ considering daily demands during summer and winter study and vacation periods with the aim of comparing the fuel costs and assess the effectiveness of the hybrid system. The third study proposes a novel optimization algorithm, the Quantum-behaved bat algorithm (QBA) to solve combined economic and emission dispatch (CEED) problem in an off-grid microgrid with onsite thermal generators and renewable energy sources (PV and Wind). The results obtained from these studies show and validate the fact that renewable energy source (RES) can be used to meet university energy demands in an economical way and reduce carbon footprint on campuses. It is observed from the result that the annual utility bill savings range from R3.97 million to R17.42 million and directly proportional to the peak load. The average emission reduction for all campuses is 49.6% except Pietermaritzburg where it is 33.7 %. In addition, the results will help university management as well as city management to invest wisely in renewables for energy sustainability and reliability.
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    Reliability of multi-channel IEC 61850 mission-critical substation communication networks based on Markov process incorporating linear dynamical systems and calculus inferences.
    (2021) Mathebula, Vonani Clive.; Saha, Akshay Kumar.
    IEC 61850 based Substation Communication Networks (SCN) enable substation processes to be digitalised to fulfil the most sought substation monitoring, protection and control of electrical systems. The standard enables peer-to-peer communication of mission critical messages, aided by onboard diagnostic capabilities to ease the identification of system faults. The implementation of Safety-Related Systems in industrial facilities comprising sensors, logic solvers and final elements in power distribution centres necessitate compliance to IEC 61508 standard, where circuit breakers act as final elements to isolate electrical machines. In recent times, combinatorial methods such as the Reliability Block Diagram have been used to evaluate the architecture of IEC 61850 based SCN reliability and availability due to the simplicity of the approach. These methods, however, assume that all system faults are identified and fully repaired, which is not the case in practice. In this thesis, the reliability of a repairable multi-channel IEC 61850 based SCN architecture is modelled using a structure function and the Markov process while Systems Thinking integrates imperfect repair factors into the model. Thereafter, a novel eigenvalue analysis method based on Markov partitions and symbolic dynamics in the context of linear dynamical systems is used to investigate the impact of imperfect repairs on the system's reliability based on the number of mean state transitions and dynamical behaviour. The eigenvalue method is then advanced by a complimentary analysis technique based on the absorbing Markov Chain process and matrix calculus methods to determine the system's responsiveness to repair factors. The case studies results demonstrate that imperfect repairs cannot be ignored for mission-critical applications because the simplifying assumptions of combinatorial analysis methods greatly over-state the system's reliability performance. The results also indicate that common causes of failure coupled with imperfect repairs significantly negatively impact the system's performance. Moreover, system performance is highly dependent on the diagnostic coverage of the individual subsystems than their repair efficiencies for high diagnostic coverages at 90% and 99% based on ISO 13849-1. Hence, the results demonstrate that emphasis should be more on the system diagnostic coverage for the fact that it is embedded in the system design itself that cannot easily be changed once the system is commissioned and operational.
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    Adaptive model predictive control of renewable energy-based micro-grid.
    (2021) Gbadega, Peter Anuoluwapo.; Saha, Akshay Kumar.
    Energy sector is facing a shift from a fossil-fuel energy system to a modern energy system focused on renewable energy and electric transport systems. New control algorithms are required to deal with the intermittent, stochastic, and distributed nature of the generation and with the new patterns of consumption. Firstly, this study proposes an adaptive model-based receding horizon control technique to address the issues associated with the energy management system (EMS) in micro-grid operations. The essential objective of the EMS is to balance power generation and demand through energy storage for optimal operation of the renewable energy-based micro-grid. At each sampling point, the proposed control system compares the expected power produced by the renewable generators with the expected load demand and determines the scheduling of the different energy storage devices and generators for the next few hours. The control technique solves the optimization problem in order to minimize or determines the minimum running cost of the overall micro-grid operations, while satisfying the demand and taking into account technical and physical constraints. Micro-grid, as any other systems are subject to disturbances during their normal operation. Hence, the power generated by the renewable energy sources (RESs) and the demanded power are the main disturbances acting on the micro-grid. As renewable sources are used for the generation, their time-varying nature, their difficulty in predicting, and their lack of ability to manipulate make them a problem for the control system to solve. In view of this, the study investigates the impacts of considering the prediction of disturbances on the performance of the energy management system (EMS) based on the adaptive model predictive control (AMPC) algorithm in order to improve the operating costs of the micro-grid with hybrid-energy storage systems. Furthermore, adequate management of loads and electric vehicle (EV) charging can help enhance the micro-grid operation. This study also introduced the concept of demand-side management (DSM), which allows the customers to make decisions regarding their energy consumption and also help to reduce the peak load demand and to reshape the load profile so as to improve the efficiency of the system, environmental impacts, and reduction in the overall operational costs. More so, the intermittent nature of renewable energy and consumer random behavior introduces a stochastic component to the problem of control. Therefore, in order to solve this problem, this study utilizes an AMPC control technique, which provides some robustness to the control of systems with uncertainties. Lastly, the performances of the micro-grids used as a case study are evaluated through simulation modeling, implemented in MATLAB/Simulink environment, and the simulation results show the accuracy and efficiency of the proposed control technique. More so, the results also show how the AMPC can adapt to various generation scenarios, providing an optimal solution to power-sharing among the distributed energy resources (DERs) and taking into consideration both the physical and operational constraints and similarly, the optimization of the imposed operational criteria.
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    Power quality improvement in low voltage distribution network utilizing improved unified power quality conditioner.
    (2020) Osaloni, Oluwafunso Oluwole.; Saha, Akshay Kumar.
    The upgrade of the power system, network, and as it attained some complexity level, the voltage related problems and power loss has become frequently pronounced. The power quality challenges load at extreme end of the feeder like voltage sag and swell, and power loss at load centre due to peak load as not received adequate attention. Therefore, this research proposes a Power Angle Control PAC approach for enhancing voltage profile and mitigating voltage sag, voltage swell, and reduced power loss in low voltage radial distribution system (RDS). The amelioration of voltage sag, voltage swell, weak voltage profile, and power loss with a capable power electronics-based power controller device known as Improve Unified Power Quality Conditioner I-UPQC was conceived. Also, the same controller was optimally implemented using hybrid of genetic algorithm and improved particle swarm optimization GA-IPSO in RDS to mitigate the voltage issues, and power loss experienced at peak loading. A new control design-model of Power Angle Control (PAC) of the UPQC has been designed and established using direct, quadrature, and zero components dq0 and proportional integral (PI) controller method. The simulation was implemented in MATLAB/Simulink environment. The results obtained at steady-state condition and when the new I-UPQC was connected show that series inverter can participate actively in ameliorating in the process of mitigating sag and swell by maintaining a PAC of 25% improvement. It was observed that power loss reduced from 1.7% to 1.5% and the feeder is within the standard limit of ±5%. Furthermore, the interconnection of I-UPQC with photovoltaic solar power through the DC link shows a better voltage profile while the load voltage within the allowable range of ±5% all through the disturbance and power loss reduction is 1.3%. Lastly, results obtained by optimal allocation of I-UPQC in RDS using analytical and GA-IPSO show that reactive power injection improved the voltage related issues from 0.952 to 0.9989 p.u., and power loss was further reduced to 1.2% from 3.4%. Also, the minimum bus voltage profile, voltage sag, and power loss are within statutory limits of ±5 % and less than 2 %, respectively. The major contributions of this research are the reduction of sag impact and power loss on the sensitive load in RDS feeder.
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    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.
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    Enhancement of deregulated and restructured power network performance with flexible alternating current transmission systems devices.
    (2020) Adewolu, Babatunde Olusegun.; Saha, Akshay Kumar.
    The increase in power transactions, consequent open access created by deregulation and restructuring has resulted into network operation challenges including determination as well as enhancement of available transfer capability (ATC), and congestion management among others. In this study, repeated alternating current power flow (RACPF) approach was implemented for determination of ATC. ATCs for inter-area line outage and generator outage contingency conditions were obtained and analyzed. Analyses of most severe line outage contingencies resulting from evaluation of different performance index (PI) ranking methods were carried out for severe line outage contingency identification. A comprehensive review of FACTS controllers with their various background, topological structures, deployment techniques and cutting-edge applications was carried out for network performance enhancement. In addition, different placement methods were investigated for optimal performance evaluation of FACTS devices. Following this, comparative performance of static var compensator (SVC) and thyristor-controlled series compensator (TCSC) models for enhancement of ATC, bus voltage profile improvement and real power loss minimization was investigated. In addition, particle swarm optimization (PSO) and brain-storm optimization algorithms (BSOA) were engaged for optimum setting of FACTS devices through multi-objective problem formulation and allocation purposes. Thereafter, sensitivity-based technique involving incorporation of proposed FACTS device loss with the general loss equation for the determination of optimum location with same objectives was developed and TCSC location was established based on this sensitivity factors analyses, obtained from partial derivatives of the resultant loss equations with respect to control parameters. Subsequently, investigation and analyses of capability of an optimized VSC-HVDC transmission system in enhancing power network performance were conducted. Furthermore, this optimized VSC-HVDC transmission system was applied for mitigation of bus voltage and line thermal limit violation as a result of n-1-line outage contingency. All these investigations and analyses were implemented for bilateral, simultaneous and multilateral transactions as characterized by network liberalization and IEEE 5 and 30 bus networks were used for implementation in MATLAB environment. RACPF method found to be more accurate especially when compared with other methods with 11.574 MW above and 29.014 MW below recorded ATC values. Voltage and real power PI have also been proven to be distinctly dissimilar in severe contingency identification. In placement method comparison however, disparities in ATC enhancement ranges between 2% and 85% were achieved while real power loss minimization of up to 25% was obtained for different methods. Real power loss minimization of up to 0.06 MW and voltage improvement of bus 21 to 30 were achieved with SVC, while ATC enhancement of up to 14% were recorded for both devices. However, BSO behaved much like PSO throughout the achievements of other set objectives but performed better in ATC enhancement with 27.12 MW and 5.24 MW increase above enhanced ATC values achieved by the latter. The comparison of set objectives values relative to that obtained with PSO methods depict suitability and advantages of BSOA technique. Sensitivity based placement technique resulted into ATC enhancement of more than 60% well above the values obtained when TCSC was placed with thermal limit method. In addition, a substantial bus voltage improvement and active power loss reduction were recorded with this placement method. With incorporation of a VSC-HVDC based transmission system into ac network however, there was an improvement in power flow up to 15.66% corresponding to 46 MW for various transactions, transmission line power loss minimization up to 0.38 MW and bus voltage profile deviation minimization. Besides, automatic alleviation of violated thermal and voltage limits during contingency present VSC-HVDC system as a solution for network performance optimization especially during various transactions occasioned by unbundling power processes. Therefore, ATCs were properly enhanced, bus voltage profile improved, and system real power loss minimized. Likewise, HVDC system enhanced network performance and automatically alleviated violated thermal and voltage limits during contingency.
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    Utilisation of line surge arrestors to improve overhead HVAC and EHVDC line performance under lightning conditions.
    (2020) Singh, Hariram.; Davidson, Innocent Ewean.
    In high lightning areas, lightning strokes play an important role in the performance of overhead EHV AC and DC lines. A single lightning stroke, that terminate on the earth wire and/or tower can lead to back flashovers. This flashover depends on factors such as conductor type, tower, soil resistivity and magnitude of the stroke. The flashover across the insulator and the resultant fault current surge will propagate along the line, until it is extinguished or the breaker operates. This movement of the surge currents tend to damage and reduce the life span of associated equipment such and circuits breakers, insulators, transformers and impact network performance adversely. Furthermore, this operation of the protective devices leads to power interruption to consumers on that network, and loss of production, thus negatively impacting the economy. This thesis investigates the incidences of network failure due to lightining strokes occuring on Eskom HVAC network as well as HVDC networks, considering soil resistivity, tower footing resistance and factors that influence the earthing resistances. Tower footing resistance needs to be kept uniform and as low as possible to extinguish the surge across the tower and hence reducing the back flashovers across the insulator under lightning conditions. Theoretical simulations were conducted on the different methods that are available to improve the tower footing resistance values. A case study was undertaken to ascertain the tower footing resistance of an 88kV Eskom line. The crows earthing configuration was then utilized to reduce the footing resistance to a value less than 30 ohms, using line surge arrestors (LSA) which are devices that can drain power surges to ground, if placed adequately and in sufficient numbers. Furthermore the thesis determines the relationship between the magnitude of the lightning stroke, the tower top voltage, tower footing resistance and hence the back flashover voltage that would appear on the line, which would lead to power interruptions. Surge arrestors were modelled using MATLAB software. The required number of surge arrestors per phase is thus determined that is required to drain the surge current down to earth., thus preventing power interruptions. EHV AC and DC cases studies are simulated and results are presented snd discussed.
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    Security and privacy in smart grid advanced metering infrastructure network.
    (2018) Diovu, Remigius Chidiebere.; Agee, John Terhile.
    a group of consumers are sufficient for data recipients to perform their duties. Unfortunately, this approach usually relies on cryptographic algorithms which increases the overhead on computations and communication for the network. This often drives the network to a congestive state, thereby resulting in delays which can minimize the task of a cyber-criminal targeting the availability of data in the network. The congestion problem was tackled in this work by designing a robust state-of-the-art communication architecture herein referred to in this thesis as Ring Triangulation Communication Architecture (RTCA). Congestive scenarios in Wireless Fidelity (Wi-Fi) and ZigBee wireless communication technology standards were modeled. The designed architecture was then applied to those networks and its performance was analyzed through extensive simulations. The results of the simulations show that notwithstanding the congestive effects of data aggregation, the designed architecture provides a good QoS guarantee for the two considered networks.
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    Energy efficiency and interference management in long term evolution-advanced networks.
    (2019) Mafuta, Armeline Dembo.; Walingo, Tom Mmbasu.
    Cellular networks are continuously undergoing fast extraordinary evolution to overcome technological challenges. The fourth generation (4G) or Long Term Evolution-Advanced (LTE-Advanced) networks offer improvements in performance through increase in network density, while allowing self-organisation and self-healing. The LTE-Advanced architecture is heterogeneous, consisting of different radio access technologies (RATs), such as macrocell, smallcells, cooperative relay nodes (RNs), having various capabilities, and coexisting in the same geographical coverage area. These network improvements come with different challenges that affect users’ quality of service (QoS) and network performance. These challenges include; interference management, high energy consumption and poor coverage of marginal users. Hence, developing mitigation schemes for these identified challenges is the focus of this thesis. The exponential growth of mobile broadband data usage and poor networks’ performance along the cell edges, result in a large increase of the energy consumption for both base stations (BSs) and users. This due to improper RN placement or deployment that creates severe inter-cell and intracell interferences in the networks. It is therefore, necessary to investigate appropriate RN placement techniques which offer efficient coverage extension while reducing energy consumption and mitigating interference in LTE-Advanced femtocell networks. This work proposes energy efficient and optimal RN placement (EEORNP) algorithm based on greedy algorithm to assure improved and effective coverage extension. The performance of the proposed algorithm is investigated in terms of coverage percentage and number of RN needed to cover marginalised users and found to outperform other RN placement schemes. Transceiver design has gained importance as one of the effective tools of interference management. Centralised transceiver design techniques have been used to improve network performance for LTE-Advanced networks in terms of mean square error (MSE), bit error rate (BER) and sum-rate. The centralised transceiver design techniques are not effective and computationally feasible for distributed cooperative heterogeneous networks, the systems considered in this thesis. This work proposes decentralised transceivers design based on the least-square (LS) and minimum MSE (MMSE) pilot-aided channel estimations for interference management in uplink LTE-Advanced femtocell networks. The decentralised transceiver algorithms are designed for the femtocells, the macrocell user equipments (MUEs), RNs and the cell edge macrocell UEs (CUEs) in the half-duplex cooperative relaying systems. The BER performances of the proposed algorithms with the effect of channel estimation are investigated. Finally, the EE optimisation is investigated in half-duplex multi-user multiple-input multiple-output (MU-MIMO) relay systems. The EE optimisation is divided into sub-optimal EE problems due to the distributed architecture of the MU-MIMO relay systems. The decentralised approach is employed to design the transceivers such as MUEs, CUEs, RN and femtocells for the different sub-optimal EE problems. The EE objective functions are formulated as convex optimisation problems subject to the QoS and transmit powers constraints in case of perfect channel state information (CSI). The non-convexity of the formulated EE optimisation problems is surmounted by introducing the EE parameter substractive function into each proposed algorithms. These EE parameters are updated using the Dinkelbach’s algorithm. The EE optimisation of the proposed algorithms is achieved after finding the optimal transceivers where the unknown interference terms in the transmit signals are designed with the zero-forcing (ZF) assumption and estimation errors are added to improve the EE performances. With the aid of simulation results, the performance of the proposed decentralised schemes are derived in terms of average EE evaluation and found to be better than existing algorithms.
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    Microwave and millimetre radio wave propagation modelling for terrestrial line-of-sight links in Central Africa.
    (2018) Djuma, Sumbiri.; Afullo, Thomas Joachim Odhiambo.
    The rapid expansion of the global telecommunication has led to an exponential growth in the demand of wireless services. This has led to the migration to higher frequency bands in the microwave and millimeter wave spectrum. Research has shown that rainfall is the most dominant factor affecting the provision of network services in these bands. Rainfall attenuation is among the major factors often considered in the design of wireless networks operating at higher bands within microwave and millimeter wave spectrum. At tropical and equatorial locations, not only is the occurrence frequency of rainfall events of serious concern to terrestrial and satellite communication systems, but also the high intensity of rain rates and drop size distribution result in extreme fading of line of sight (LOS) system during such events. In this work, daily rainfall measurements from the Rwanda Meteorology Agency (Meteo Rwanda) are obtained for 60 locations within equatorial Rwanda (between latitudes of 1o2'S and 2o45'S and longitudes of 280 45'E and 30052'E), in Central Africa, to develop rain rate and rain attenuation maps for wireless radio links. From these long term annual rainfall measurements spanning a minimum of 10 years at these locations, rainfall rate statistics and drop size distribution result in extreme fading of line of sight (LOS) system during such events. In this work, daily rainfall measurements from the Rwanda Meteorology Agency (Meteo Rwanda) are obtained for 60 locations within equatorial Rwanda (between latitudes of 1o2'S and 2o45'S and longitudes of 280 45'E and 30052'E), in Central Africa, to develop rain rate and rain attenuation maps for wireless radio links. From these long term annual rainfall measurements spanning a minimum of 10 years at these locations, rainfall rate statistics estimated from appropriate models are applied to determine fade margin for radio link availabilities between 99% and 99.999%. Furthermore, specific attenuation estimates due to rainfall are proposed from International Telecommunication Union (ITU) recommendations at selected frequencies of the microwave and millimeter bands, for the design of wireless networks. Results obtained from this approach incorporating both rainfall rate zones and specific attenuation over Rwanda are presented as spatial contour maps representations for different ranges of link availability. Further, disdrometer data collected in Butare, Rwanda (20 35' 53.88” S and 290 44' 31.5” E) for a period of 32 months between 2012 and 2015 have been use to develop a suitable model on drop size distribution in the region. Rainfall data was classified into four different regimes, namely, drizzle, widespread, shower and thunderstorm. Different raindrop size distribution (DSD) models such as Lognormal, Gamma, Marshall-Palmer and Weibull distributions are selected and the method of moment technique is applied for estimating input DSD fit-parameters for those DSD models. From the results, it is observed that different models have varying performances as the rainfall regime varies from drizzle to widespread, shower and later as thunderstorm, except the Marshall- Palmer model which shows the inadequacy for the region. It is found that neither the Lognormal nor other models match perfectly wel I with the measured DSD, particularly at high rainfall rates. Therefore, a new rainfall DSD model or Central Africa is developed and found to be an improvement over the existing models. The Mie Scattering technique (spherical method) is employed to derive the scattering parameters. Therefore, the derived scattering parameters with DSD models are used for the estimation of rainfall attenuation in the region of Central Africa. Finally, the synthetic storm techniques (SST) is applied for comparison with other rainfall attenuation models.
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    The study of the self-damping properties of overhead transmission line conductors subjected to wind-induced oscillations.
    (2017) Ojo, Evans Eshiemogie.; Ijumba, Nelson Mutatina.
    Conductors are flexible, elastic structural components of power lines. The relatively high flexibility of the conductors, coupled with the long spans and the axial tension, makes conductors to be highly prone to dynamic excitation such as wind loading. The problem of the dynamic behavior of overhead power transmission line conductors under the action of wind and other forms of excitations is very important, since it proffers the optimal design of the line in terms of its dynamic characteristics. Thus, mechanical vibration of power lines needs to be mitigated, especially from aeolian vibration as they can lead to damage of the lines causing power interruptions. The dynamic behaviour of conductors can be influenced by its damping. However, available tools for the analysis of this phenomenon is scarce. The objective of this study is to evaluate the conductor self-damping. The goal is to characterize and ascertain the influence of various conductors’ parameters on the amount of energy dissipation. In this study, a numerically based investigation of the response of conductors was carried out i.e. finite element analysis (FEA or FEM). This was used to model the conductor using a new modeling approach, in which the layers of its discrete structure of helical strands were modelled as a composite structure. Due to the helical structure of the conductor strands, this give rise to inter-strands contacts. During bending caused by external loading, the stick-slip phenomenon does occur around the contact region resulting in damping of energy out of the system. Characterizing the damping mechanism as hysteresis phenomenon, this resulted from coulomb’s dry-friction with the stick-slip regime at contacts points between the conductor strands. Employing contact mechanics to characterize and the use of FEM to discretize these contact regions, parameters such as the contact forces, strain and stress were established. When the conductor experiences a dynamic excitation in a sinusoidal form, a hysteresis loop is formed. The use of contact region parameters, to evaluate the area of the hysteresis loop and the area of the loop determines the amount of self-damping. Experimental studies were conducted to validate the FEM model. Two forms of experiment were done. The first was the sweep test, done at a specified axial tension i.e. as a function of its ultimate tensile strength. This was used to determine the resonance frequencies for the conductors. In the second test, using the determined resonance frequencies from the first test were used to vibrate the conductors at these frequencies to establish the hysteresis loop at the same specified axial tension. The experiment was conducted with four different conductors with different number of layers. This was used to establish the relation between the numbers of layer and the amount of damping from the conductor. The conductors’ vibration experimental results obtained at a defined axial tension (as percentage of its UTS) correlate with that of FEM model. The results obtained showed a general increase in the resonance frequencies of vibration and a decrease in damping as the axial tension of the conductor is increased. The establishment of the hysteretic constitutive behaviour of strands under specific loading conditions as described in the thesis, using this FEM model, an algorithm was developed to evaluate the conductor self-damping. Based on this algorithm, computer programs have been developed to evaluate the conductor’s dynamic behaviour and implemented in MATLAB environment. Due to the very close relation between damping and conductor fatigue, this model can also be extended to investigate fatigue failure of conductors.
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    Power quality enhancement in secondary electric power distr[i]bution networks using dynamic voltage restorer.
    (2018) Ogunboyo, Patrick Taiwo.; Tiako, Remy.; Davidson, Innocent Ewean.
    This research study investigates and proposes an effective and efficient method for improving voltage profile and mitigating unbalance voltage, voltage variation disturbances in rural and urban secondary distribution networks. It also proffers solutions for improving the performance of future distribution networks in order to increase the optimum functioning, security and quality of electricity supply to end users, thus making the power grid smarter. This study involves the compensation of power quality disturbance in balanced and unbalanced, short and long distribution networks. The mitigation of result of this voltage variation, poor voltage profile and voltage unbalance with an effective power electronics based custom power controller known as Dynamic Voltage Restorer (DVR) conceived. DVR is usually connected between the source voltage and customer load. An innovative new design-model of the DVR has been proposed and developed using a dq0 controller and proportional integral (PI) controller method. Model simulation was carried out using MATLAB/Simulink in Sim Power System tool box. An analysis of the results obtained when the new DVR is not connected to and tested on LV networks shows that the voltage profile, percentage voltage deviation and percentage voltage unbalance for 0.5 km for balanced and unbalanced distribution networks are within standards and acceptable limits, hence, the voltages are admissible for customers’ use. It was further established that the voltage profile, percentage voltage unbalance, voltage drop and percentage voltage deviation for distribution networks of 0.8 km to 5 km range from the beginning to the end of the feeder are less than the statutory voltage limits of -5%, 2 %, 5 % and ± 5 % respectively, hence, voltages are inadmissible for customers’ use. Others results obtained when DVR was connected recognized that for distribution feeder lengths of 0.5 km to 5 km range for balanced and unbalanced, short and long distribution networks the voltage profile, voltage variation, voltage drop and percentage voltage unbalance are within statutory voltage limits of 0.95 p.u and 1.05 p.u, -5 %, and less than 2 % respectively. Based on this investigation, and in order to achieve efficient, reliable and cost-effective techniques for improving voltage profiles, decreasing voltage variations and reducing voltage unbalances, the new DVR model is recommended for enhancing optimal performances of secondary distribution networks.
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    Survey and assessment of the impact of embedded generation on the eThekwini electricity distribution grid.
    (2016) Sewchurran, Sanjeeth.; Davidson, Innocent Ewean.; Olorunfemi, Stephen Ojo.
    Under frequency load shedding, rising electricity tariffs, environmental concerns, reducing costs of renewable energy technology and delays in constructing new power stations has lead consumers and producers alike to explore various energy generation options to reduce their own electricity needs whilst assisting the sector. Embedded Generation (EG), Distributed Energy Resources (DER) or Distributed Generation (DG) is further predicted to play a substantial role in the electrical power system in the near future. Various EG technologies are entering a period of major growth and commercialization. Small scale Embedded Generation (SSEG) projects are quickly becoming a popular sight locally within the city of Durban and numerous projects are been connected to the eThekwini Electricity distribution grids. In these projects, there seems to be a reluctance to couple expensive energy storage technologies to these systems but rather synchronize and utilise the municipal grid as a virtual battery. Whilst the advantages make synchronization to the grid a logical choice, the municipal network architecture and framework was not designed to facilitate this. The municipal mandated core function is to procure electricity from Eskom (the national energy generator), transform it and distribute it to its customers. Power flow is from generation (Eskom), transmission (Eskom) and distribution to the end customer. This includes all technical, regulatory, administrative and legal aspects that have been structured to support this unidirectional power flow. The introduction of DER/DG/EG introduces bi-directional power flow on the existing distribution network. An analysis of the impact of this phenomenon is required as it affects fault level, protection selectivity and sensitivity, network losses, quality of supply, network planning, metering and control of power flow on the existing distribution grid. In order to address and understand the impacts of EG on the existing eThekwini Electricity distribution grid, an investigation was first carried out to identify the drivers and available renewable energy resources in Durban. 5 cases studies were selected based on the investigation which showed that there will be growth and projects in these sectors in Durban. These cases studies were selected to address growth in residential rooftop PV, commercial/industrial rooftop solar PV, PV farm potential on closed landfill sites, wind farm potential at identified sites around the city and landfill gas to electricity projects from existing landfill sites in the city. Accurate models of these sources and their interaction with the grid were then studies. Studies were also carried out on the recently published NRS guidelines for SSEG and the South African Renewable Energy Grid Code to understand how this will provide operational flexibility to the System Operator and assist with mitigating the negative impacts to the distribution network. The 5 case studies provide excellent results and greater understanding of the impacts of increased penetration of EG onto the existing eThekwini Electricity distribution grid. The impacts of increased penetration of EG on the existing eThekwini Electricity distribution network included impacts to the network voltage, fault level rise, losses, power flow, network planning and revenue loss. Based on the results and studies from the case studies methods were then derived to mitigate the impacts of increased penetration of EG on the existing distribution network. The following outcomes and key contributions, were achieved in this research investigation, namely:  An understanding of the drivers of EG in eThekwini Municipality.  Evaluation of the available renewable energy resources within eThekwini Municipality.  The feasibility of residential rooftop solar PV in Durban.  Identified factors affecting residential rooftop solar PV feasibility in Durban.  Assessed the feasibility of municipal landfill gas to electricity EG projects.  Developed and propose methods to improve operational and financial viability of landfill gas to electricity projects in Durban.  Provides results showing the impacts of increasing EG on the eThekwini Municipality distribution network design and performance.  Developed methods to assist and enable distribution network designers when designing distribution networks with increasing EG.  Developed a methodology for selecting EG size on an existing eThekwini Electricity distribution network.  Provide methods to minimise the impacts of preselected size of EG given that the municipality has no control over the size selection which may be dictated by the IPP.  An understanding of the local South African guidelines on small scale EG, and the South African Renewable Energy Grid code requirements.  Provide controllability options to assist manage EG plants on the existing distribution network in eThekwini Municipality. vii  Understand the operation and effects of different EG sources available within eThekwini Municipality. These have been accomplished using the 5 case-studies, modelling and simulation, field tests and measurements; as well as extensive research investigation and analysis.
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    PMSG-based wind power integration-modelling and analysis of impacts on the dynamic performances of a power system and mitigation under stochastic wind disturbances.
    (2017) Legesse, Ayele Nigussie.; Saha, Akshay Kumar.; Carpanen, Rudiren Pillay.
    Because of the ever-growing demand for electrical energy and environmental challenges of fossil fuel consumption, a priority has been given to the development of wind energy systems, among which, currently, permanent magnet synchronous generator (PMSG)-based wind power is receiving much attention from researchers, engineers, and turbine manufacturers. However, high PMSG-based wind power integration into a power system brings several challenges to transmission system operators. One of the challenges is its impacts on the dynamic performances of a power system due to the presence of stochastic wind disturbances. Thus, for a thorough investigation of the influences of stochastic wind speed disturbances, a proper wind speed model should be adopted. Therefore, this thesis proposes the use of Markov chain model for modelling wind speed series in dynamic simulations of wind turbines. In this regard, comparison of statistical quantities of measured wind speed data from Durban and Markov model generated ones confirms the accuracy of the model adopted. The results have shown that the dynamic performances of a power system deteriorate with the presence of stochastic wind speed disturbances, and thus the need for improving poor dynamic performances. Wind gusts cause stress, over currents, over voltages and instability in a power system. This thesis, therefore, introduces novel mitigation techniques based on virtual controls stemming from real resistors, compensators, and damper windings, and supplementary controllers to enhance the dynamic performances of a wind turbine directdriven PMSG, the main component of a PMSG-based wind farm. In the proposed schemes, the virtual controllers adjust the terminal d- and q-axis reference voltages in the generator side converter controller and their influences on the dynamic performances of the wind turbine are investigated. MATLAB/Simulink simulations on a wind turbine connected to an infinite bus show that virtual controls are effective in enhancing the dynamic performances of the PMSG. Local oscillations caused by wind disturbances are efficiently suppressed. Overall, the proposed mitigation techniques smooth the rotor speed and power of a PMSG, and hence reducing the influences of the stochastic wind speed disturbances. Furthermore, the results have demonstrated that stochastic wind speed disturbances affect the dynamic performances of a power system containing a PMSG-based wind farm as the dynamics of synchronous machines within the system depend on power balance, which is influenced by the power response of the wind farm. Finally, investigations in this thesis have confirmed that virtual controls and FACTS devices such as STATCOM and SVC are efficient in improving the dynamic performances of a power system containing PMSG-based wind farms under stochastic wind disturbances.
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    Smart real-time scheduling of generation units in an electricity market considering environmental aspects and physical constraints of generators.
    (2017) Goudarzi, Arman.; Swanson, Andrew Graham.; Van Coller, John.
    Abstract available in PDF file.