Doctoral Degrees (Electrical Engineering)

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

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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.
An alternative approach to impulsive noise characterisation and statistical modelling for broadband powerline communication networks.
(2024) Chelangat, Florence.; Afullo, Thomas Joachim Odhiambo.
Research into the modelling of powerline communication impulsive noise – which exhibits unpredictable behaviour in its time domain characteristics of amplitude, inter-arrival time, and service time - is still in progress. As a result, in order to propose an appropriate mitigation strategy to address the interference generated by powerline communication impulsive noise, an appropriate characterization of its time domain parameters is crucial. Given the complex structure of the powerline communication network that includes a heavy wiring system, the models proposed for the various noise characteristics are stochastic in nature. In this work, extensive noise measurements were carried out over various indoor networks in the School of Engineering, University of KwaZulu-Natal, Durban, South Africa. The measurements were conducted at the following sites: the Computer Laboratory, the Machines Laboratory, the Electronic Laboratory, the Second-year Laboratory, the Post-graduate office, as well as at an adjacent apartment. This campaign was undertaken to adequately capture the behaviour of powerline communication noise, which varies randomly depending on location, time, and the devices linked to the electrical network. To begin with, the amplitude distribution of the powerline communication impulsive noise was examined. The Gaussian mixture model was used to analyse the amplitude distribution of powerline communication noise, which is essential in estimating the level of noise reaching the receiver. Gaussian mixture models are commonly employed in modelling the powerline communication impulsive noise amplitude distribution. However, the weights of the Gaussian mixture components are derived using statistical distributions, with the most common models employing the Bernoulli and Poisson distributions. These models, however, have been found to be insufficient in describing powerline communication noise. This thesis contributes to the modelling of the amplitude distribution of powerline communication impulsive noise by using unsupervised learning to determine the parameters of the Gaussian mixture. Regression analysis is also proposed to solve the issue of singularity in the likelihood function of this model as well as to determine the optimum number of Gaussian components. Further analysis of the amplitude distribution is performed using a fully Bayesian treatment referred to as the Variational Bayesian model, where the parameters of the Gaussian mixture model are assumed to be random variables, such that prior distributions over the parameters are introduced. Moreover, the optimal number of components is determined from the measurement data through the Variational Bayesian criterion. This ensures that improved accuracy due to the increased number of components in modelling the powerline communication impulsive noise amplitude distribution is eliminated thus reducing the model complexity while adequately describing the data. The variational-expectation algorithm, analogous to the expectation-maximisation algorithm employed in the Gaussian mixture model, is used to determine the model parameters. Measurements have shown that the powerline communication impulsive noise can be modelled as a superposition of several exponential distributions. Consequently, most of the research models proposed for modelling the inter-arrival and service time distribution are based on the Markov chain. There is still no defined method of evaluating the number of states, with existing models employing various curve-fitting techniques to find the optimum model for the measurement data. This work provides an alternative approach based on the queueing theory technique, where the impulsive noise occurrence in the powerline communication channel is modelled as an Erlangian queue. A straightforward method for obtaining the optimum number of exponential phases by employing the mean and the variance of the Erlang-k distribution is presented. The proposed model assumes that the impulsive noise passes through k arrival stages before entering the powerline communication network and another k service stages before leaving the powerline communication network. In all of the measurement data under consideration, impulsive noise events are observed to achieve steady-state in the inter-arrival and service time distributions. In this work, the measurements indicate that the powerline communication noise can occur as a single-impulse noise or a burst-impulse noise. The burst-impulse noise is caused by the overlap of three or more high-amplitude single-impulse noise events that occur successively in an impulse train. The amplitude of the noise, as well as the interarrival and service time distribution, vary depending on the location and time. As a result, the impulsive noise is categorised as low, medium, or highly impulsive, depending on the noise levels. The probability density function of the noise amplitude exhibits heavy tails comparable to the Gaussian mixtures. The performance of the maximum likelihood estimate and the Variational Bayesian model in finding the parameters of the Gaussian mixture are validated through measurements, where the maximum likelihood estimate yields better accuracy. However, cases of singularity are encountered in addition to an increase in performance as the number of impulsive noise components is increased. Therefore, the implementation of the Variational Bayesian approach in modelling the parameters of the Gaussian mixture enables the determination of the appropriate number of Gaussian mixture components and no singularity case is found. Although the Variational Bayesian model provides a good generalization to the measured data, the maximum likelihood technique gives better accuracy since the Variational Bayesian model provides an approximate solution, as it is based on maximising the lower bound. Both models are observed to have a high level of significance as well as a good correlation to the measured data and thus either can be used in modelling the amplitude distribution of the powerline communication noise. In modelling the inter-arrival and service time distributions, the Erlang-k distribution is observed to be more appropriate for modelling the burst-impulse noise events with a high level of significance to the measured data. The exponential distribution, which is a special case of the Erlang-k distribution, is determined to be appropriate in estimating the inter-arrival time of the single-impulse noise events, indicating high variance in the measurement data. The models proposed in this thesis can be used as simulation tools to assist the development of physical layers of powerline communication systems.
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.
Analysis of the impact of a facts-based power flow controller on subsynchronous resonance.
(2012) Carpanen, Rudiren Pillay.; Rigby, Bruce S.
Electric power utilities are faced with the challenge of meeting increasing demand for electric power whilst many factors prevent traditional remedies such as the expansion of transmission networks and the construction of new generating facilities. Due to issues of environment, health and rights-of-way, the construction of new generating plants and transmission lines were either excessively delayed or prevented in many parts of the world in past years. An alternative resides in loading the existing transmission network beyond its present operating region but below its thermal limit, which would ensure no degradation of the system. This alternative approach has been possible with the emergence of Flexible AC Transmission Systems (FACTS) technology. The FACTS concept involves the incorporation of power-electronic controlled devices into AC power transmission systems in order to safely extend the power-transfer capability closer of these systems to their stability limits. One member of the family of FACTS series compensators is the Static Synchronous Series Compensator (SSSC), and this thesis considers the use of the SSSC to carry out closed-loop control of AC power flow in a transmission system. Although the SSSC has the potential to enhance the operation of power systems, the introduction of such a device can cause adverse interactions with other power system equipment or existing network resonances. This thesis examines the interaction between high-level power flow controllers implemented around the SSSC and a particular form of system resonance, namely subsynchronous resonance (SSR) between a generator turbine shaft and the electrical transmission network. The thesis initially presents a review of the background theory on SSR and then presents a review of the theory and operation of two categories of SSSC, namely the reactance-controlled SSSC and the quadrature voltage-controlled SSSC. The two categories of SSSC are known to have different SSR characteristics, and hence this thesis considers the impact on the damping of subsynchronous torsional modes of additional controllers introduced around both categories of SSSC to implement AC power flow control. The thesis presents the development of the mathematical models of a representative study system, which is an adaptation of the IEEE First Benchmark system for the study of SSR to allow it to be used to analyse the effect of closed-loop power flow control on SSR stability. The mathematical models of the study system are benchmarked against proven and accepted dynamic models of the study system. The investigations begin by examining the effect of a reactance-controlled SSSC-based power flow controller on the damping of torsional modes with an initial approach to the design of the control gains of the power flow controller which had been proposed by others. The results show how the nature and extent of the effects on the damping of the electromechanical modes depend on both the mode in which the power flow controller is operated and its controller response times, even for the relatively-slow responding controllers that are obtained using the initial controller design approach. The thesis then examines the impact of a reactance-controlled SSSC-based power flow controller on the damping of torsional modes when an improved approach is used to design the gains of the power flow controller, an approach which allows much faster controller bandwidths to be realised (comparable to those considered by others). The results demonstrate that for both of the modes in which the power flow controller can be operated, there is a change in the nature and extent of the power flow controller’s impact on the damping of some the torsional modes when very fast controller response times are used. Finally, the thesis investigates the impact of a quadrature voltage-controlled SSSC-based power flow controller on the damping of torsional modes in order to compare the influence of the design of both Vsssc-controlled and Xsssc-controlled SSSC-based power flow controllers on torsional mode damping for different power flow controller response times. The results obtained indicate that a Vsssc-controlled SSSC-based power flow controller allows a larger range of SSR stable operating points as compared to a Xsssc-controlled SSSC-based power flow controller.
Compound codes based on irregular graphs and their iterative decoding.
(2004) Nkouatchah, Telex Magloire Ngatched.; Takawira, Fambirai.
Low-density parity-check (LDPC) codes form a Shannon limit approaching class of linear block codes. With iterative decoding based on their Tanner graphs, they can achieve outstanding performance. Since their rediscovery in late 1990's, the design, construction, and decoding of LDPC codes as well as their generalization have become one of the focal research points. This thesis takes a few more steps in these directions. The first significant contribution of this thesis is the introduction of a new class of codes called Generalized Irregular Low-Density (GILD) parity-check codes, which are adapted from the previously known class of Generalized Low-Density (GLD) codes. GILD codes are generalization of irregular LDPC codes, and are shown to outperform GLD codes. In addition, GILD codes have a significant advantage over GLD codes in terms of encoding and decoding complexity. They are also able to match and even beat LDPC codes for small block lengths. The second significant contribution of this thesis is the proposition of several decoding algorithms. Two new decoding algolithms for LDPC codes are introduced. In principle and complexity these algorithms can be grouped with bit flipping algorithms. Two soft-input soft-output (SISO) decoding algorithms for linear block codes are also proposed. The first algorithm is based on Maximum a Posteriori Probability (MAP) decoding of low-weight subtrellis centered around a generated candidate codeword. The second algorithm modifies and utilizes the improved Kaneko's decoding algorithm for soft-input hard-output decoding. These hard outputs are converted to soft-decisions using reliability calculations. Simulation results indicate that the proposed algorithms provide a significant improvement in error performance over Chase-based algorithm and achieve practically optimal performance with a significant reduction in decoding complexity. An analytical expression for the union bound on the bit error probability of linear codes on the Gilbert-Elliott (GE) channel model is also derived. This analytical result is shown to be accurate in establishing the decoder performance in the range where obtaining sufficient data from simulation is impractical.
The Coriolis effect and travelling waves in porous media convection subject to rotation.
(2000) Patrascoiu, Mihail Radu.; Vadasz, Peter.
This study intends to recover and expand the analytical work of Vadasz (1998) for linear and weak non-linear stability of a rotating porous media heated form below and subject to gravity and Conolis forces. It is shown that the viscosity has a destabilising effect at high rotation rate. It has been established that the critical wave number in a plane containing the streamlines is dependent on rotation. Finite amplitude calculations provide a set of differential equations for the amplitude and phase, corresponding to the stationary and over-stable convection, identifying the post-transient conditions that a fluid is subject to, i.e. a pitchfork bifurcation for the stationary case, or a Hopf bifurcation in the case of over-stable convection. The previous model (Vadasz [1998]) was extended with an additional time scale in order to represent amplitude fluctuations and a short space scale to include horizontal modes of oscillations. When the complete solution for the stream function or temperature is analysed, where left and right travelling waves are considered, we obtain a set of differential equations for the amplitude and phase. The solutions are discussed in this context.
DC coronation electroporation.
(2015) Chetty, Nevendra Krishniah.; Davidson, Innocent Ewean.; Chetty, Leon.; Govender, T.; Ijumba, Nelson Mutatina.
Cells are surrounded by a semi-permeable bilayer lipid membrane that acts as a barrier against the entry of foreign molecules. In the fields of molecular biology, biotechnology, and medicine, the ability to breach the cell membrane and introduce molecules into cells for therapeutic purposes is often necessary. Molecules, which are considered foreign to the cell like drugs and extraneous genetic materials, are administered to cells for numerous applications including the treatment and prevention of diseases. There are many accepted methods of facilitating the delivery of molecules to cells. Of all these methods, one important and well-established physical method is electroporation which has been utilised for decades. Electroporation is a widely adopted procedure for the temporary permeabilization of cell membranes due to the application of short electrical pulses. It is a phenomenon resulting from the effects of pulsed electric fields, which induces biochemical and physiological changes to a cell membrane. As a result, some of the molecules that are ordinarily unable to pass through the membrane are thereafter able to gain access to the cell interior via pores that are formed in the membrane. Even though electroporation is fairly safe, there are some drawbacks associated with this method. The traditional method of electroporation requires direct contact of high voltage electrodes and fairly high currents are involved. As a result, the procedure can cause pain, muscle spasms, discomfort, burning and cell and tissue damage. Alternative methods of molecular delivery are therefore being researched, especially non-contact methods such as the use of high voltage plasma and high voltage corona discharge. Successful cell permeabilization with corona discharge ions and plasma has been previously demonstrated. These methods offer the advantage of contact-free treatment with low associated current. In this thesis, the research investigates the delivery of tracer molecules, SYTOX Green, into HeLa cells and the consequential cell destruction by the phenomenon of corona discharge. A high voltage DC, multipoint-to-plane atmospheric-air corona discharge apparatus was designed and constructed to investigate the conditions as well as the characteristics of the corona discharge current pulses that resulted in an acceptable balance between high cell permeabilization and low cell destruction. Firstly, the salient variables that affect molecular delivery and cell destruction were established. Secondly, the variables were optimized to allow for reliable molecular delivery to cells with acceptable levels of cell destruction. Thirdly, the nature and variation of the corona discharge current pulses and its effect on molecular delivery and cell destruction were investigated. Finally, a new method of assessing cell destruction, which combined the measurements of cell viability and cell lysis were used. The variables that were identified, over the course of many experiments, were exposure time to corona discharge, incubation time with SYTOX Green, volume of liquid during exposure, and inter-electrode distance. Further experiments show that when the variables of the experiment are set at optimal values, cell permeabilization is reliable with minimal damage to cells. Once these conditions were obtained and optimised, the effect of different applied voltages on the level of cell permeabilization and the short-term destructive effects on cells were investigated. The general trend is an increase in fluorescence and therefore, molecular delivery, with an increase in applied voltage. Cell destruction also tends to increase with increasing applied voltage. The characteristics of the corona current pulses that were analyzed include amplitudes, repetition rates, widths, and rise-times. The characteristic frequencies of single pulses, obtained from the application of a discrete fast Fourier transform, were also analyzed. For the corona-generating device constructed and the voltages tested, it was found that the only characteristic that varies appreciably with voltage is the pulse repetition rate. A higher pulse repetition rate relates to a greater number of pulses per unit time and therefore, a greater exposure of the cells to the applied electric field. This would, therefore, translate to a higher extent of molecular delivery and a higher accompanying level of cell destruction. This study shows that permeabilization of HeLa cells due to corona discharge can be reliably achieved and the results provide a greater understanding of cell permeabilization due to the influence of corona discharge. It therefore forms an important basis for future research on practical applications that would promote the establishment and acceptance of corona discharge as a procedure for molecular delivery to cells.
Design and investigation on substrate integrated waveguide fed dielectric resonator antennas for d-band applications.
(2023) Chemweno, Emmanuel Kiprop.; Kumar, Pradeep.; Afullo, Thomas Joachim Odhiambo.
This thesis is devoted to the design of substrate integrated waveguide (SIW) fed dielectric resonator antennas (DRAs) for D-band applications. These antennas are necessary to establish high-speed communication links in wireless communication networks so as to support the emerging broadband applications such as the internet of things (IoTs), smart cities, virtual reality among others. Some of these applications demand for networks that can support high data rates and low latencies that exceed the capabilities of 5G networks. Consequently, communication systems are projected to operate in the millimeter wave (mm-wave) and terahertz (THz) frequencies. The D-band frequency spectrum is attractive for utilization in 6G communication systems owing to its vast bandwidths and is capable of very high data rates up to terabits per second (Tbps). However, this band suffers the limitations of short propagation distances due to the increased path losses at high frequencies. These effects can be mitigated by efficient antenna designs characterized by broad bandwidths and high-gains with low-loss interconnect networks. In addition, antenna structures with low profiles are attractive for ease of integration with other front-end devices. In this work, the substrate integrated waveguide fed dielectric resonator antenna is the preferred topology for the antenna design. Three different wideband antenna designs are presented and analyzed. The wideband operation of the presented designs is achieved through the higher-order mode (HOM) excitation of the dielectric resonator (DR) element to achieve multiple resonances in this frequency band. Adjacent resonances are merged together using various impedance matching techniques to enhance the antenna bandwidth. These designs investigate on different impedance matching techniques such as the use of defected ground structures (DGS), inductive metal vias (posts), cross-slots among others for antenna bandwidth enhancement and gain improvement. This work extends further the design of single element antenna to multiple-input multiple-output (MIMO) antenna design. A technique for mutual coupling reduction in closely packed antenna elements, which is based on metamaterial polarization rotation is presented and investigated. The investigation on the performance of the antennas is carried out using the commercial CST Microwave Studio full-wave electromagnetic simulator. The simulation results are analyzed in terms of the bandwidth, gain, radiation efficiency and radiation characteristics. The first design presents a SIW-DRA operating between 122.58 GHz and 139.51 GHz. The antenna exhibits multiple resonances at 123.64 GHz, 125.76 GHz, 127.4 GHz, 129.9 GHz, 134.9 GHz and 137.7 GHz. The design investigates the use of DGS and iristype discontinuity techniques for impedance matching improvement of a multi-resonant antenna. Simulation results show that the applied techniques are effective in merging together the adjacent bands of a multiband antenna for bandwidth enhancement. The antenna achieves a -10 dB impedance bandwidth of 13.4%, a peak gain of 12.3 dBi maximum directivity of 13.14 dBi and a high radiation efficiency of 84%. In addition, the antenna possesses stable broadband radiation patterns across the frequency band of operation. The second design presents a novel approach for the transformation of a dual-band SIW-DRA to a broadband antenna design. The design proposes systematic and sequential application of impedance matching techniques to improve the impedance matching in the stop band of the dual-band antenna without deterioration of its performance in the passbands. In the development of this approach, two preliminary designs are presented and investigated. The first preliminary design investigates the use of inductive metal vias in the SIW feeding structure for bandwidth enhancement. The effects of incorporating inductive via and DGS on the SIW feed structure of a dual-band antenna operating between 140.82 GHz - 145.24 GHz and 147.27 GHz - 151.02 GHz with a relatively narrow stop band of 2.03 GHz are investigated. The simulated results show that the inductive via and DGS effectively merge together the passbands of a dual-band antenna to achieve a wideband response of 138.27 GHz - 150.95 GHz. The second preliminary design presents a dual-band antenna operating between 126.95 GHz - 136.5 GHz and 139.67 GHz - 149.48 GHz in the lower and upper-frequency bands. The antenna is optimized for dual-band operation through the modification of the feeding slot to an I-shape to achieve a bandwidth 9.55 GHz and 9.81 GHz, representing fractional bandwidths of 7.2% and 6.78% in the lower and upper-frequency bands respectively. The impedance matching in the relatively wide stop band can be improved to merge together the passbands of the dual-band antenna to achieve a wideband response. This transformation is achieved through the use of inductive via, matching stub and DRA offset techniques in the dual-band antenna design. These techniques are systematically applied to the dual-band design and the parameters optimized to achieve a high-gain and wide bandwidth SIW-DRA. The antenna operates between 123.97 GHz and 152.13 GHz, with a -10 dB impedance bandwidth of 20.39% at the center frequency of 138.05 GHz, exhibiting a high gain of 11.67 dBi, directivity of 13.36 dBi and maximum total radiation efficiency of 79%. The radiation characteristics of the higher-order resonances excited at 124.38 GHz, 125.22 GHz, 130.28 GHz, 140.72 GHz, 141.59 GHz, 143.47 GHz, 149.7 GHz and 151.17 GHz show broadside radiation patterns. The research also focuses on the design of a wideband and low-isolation 2 × 2 SIWDRA based MIMO antenna. Initially, a single-element SIW-DRA is designed based on higher-order mode excitation with the antenna exhibiting multiple resonances. Different impedance matching techniques involving the embedding of inductive metal vias in the SIW feed, cross-slot feed and stepped-impedance sections are employed to minimize reflections and to achieve a good impedance matching between 136.68 GHz – 166.28 GHz. A -10 dB bandwidth performance of 19.5%, a gain of 11.06 dBi and a high radiation efficiency of 84% is achieved. Two antenna elements are linearly arranged with zero interelement distance in a 2 × 2 MIMO configuration to achieve high integration of antenna elements in a given space. A metamaterial-based decoupling network is investigated for isolation enhancement in the 2 × 2 MIMO configuration. The decoupling network is designed as a metamaterial polarization rotator (MTMPR) wall to rotate the polarization state of the electromagnetic wave transmitting through it. The MTMPR wall is integrated with the MIMO antenna for mutual coupling reduction. The Simulation results show that the MTMPR wall does not degrade the bandwidth performance of the MIMO antenna, and the presented design achieves an isolation performance greater than 21.16 dB across the entire bandwidth of operation. Besides the isolation, the antenna is analyzed by evaluating its diversity metrics. MIMO diversity metrics such as the envelope correlation coefficient (ECC), diversity gain (DG), mean effective gain (MEG), channel capacity loss (CCL) and the total active reflection coefficient (TARC) are evaluated. The MIMO design achieves an ECC of 0.008, DG > 9.9 between the two ports, MEG < 3 dB, TARC close to -10 dB for different phase angles of the input excitation and a CCL below 0.4 bits/s/Hz. The performances of the presented SIW-DRA designs are compared with published results and confirmed to meet the established minimum criteria for the appraisal of the presented designs. The results show that the presented designs are suitable for application in future 6G wireless systems.
Design synthesis of LCC HVDC control systems.
(2011) Chetty, Leon.; Ijumba, Nelson Mutatina.
From the early days of HVDC system applications, the importance of mathematical modelling of the dynamics of Line Commutated Converter (LCC) HVDC systems has been appreciated. There are essentially two methodologies used to develop mathematical models of dynamic systems. One methodology is to define the properties of the system by the “laws of nature” and other well-established relationships. Basic techniques of this methodology involve describing the system’s processes using differential equations. This methodology is called “Deductive Modelling”. The other methodology used to derive mathematical models of a dynamic system is based on experimentation. Input and output signals from the original system are recorded to infer a mathematical model of the system. This methodology is known as “Inductive Modelling”. A review of the current state of the art of modelling LCC HVDC systems indicates that majority of the techniques utilized to develop mathematical models of LCC HVDC systems have used the “Deductive Modelling” approach. This methodology requires accurate knowledge of the ac systems and the dc system and involves complicated mathematics. In practice, it is nearly impossible to obtain accurate knowledge of the ac systems connected to LCC HVDC systems. The main aim of this thesis is to present an “Inductive Modelling” methodology to calculate the plant transfer functions of LCC HVDC systems. Due to the uncertain nature of the effective short circuit ratio of rectifier and inverter converter stations, generic ranges of parametric uncertainties of the developed plant transfer functions were determined. Based on the determined range of HVDC plant parametric uncertainty, Quantitative Feedback Theory (QFT) methodology was used to design the parameters of the LCC HVDC control system. The stability of the start-up and step responses for varying ac system conditions validated the “Inductive Modelling” technique and the QFT design methodology. The thesis presents the following, which are considered to be scientific advancements and contributions to the body of knowledge: · Novel LCC HVDC Step Response (HSR) equations were developed using an “Inductive Modeling” technique. · The range of parametric variations of the LCC HSR equations were determined for various rectifier and inverter ac system effective short circuit ratios. · The LCC HSR equations were used to develop the LCC HVDC plant transfer functions for various rectifier and inverter effective short circuit ratios. · The LCC HVDC plant transfer functions were utilized to design an LCC HVDC control system for varying ac system conditions using Quantitative Feedback Theory (QFT) methodology. The main contributions of this thesis relate to LCC HVDC systems. This thesis does not attempt to advance control theory however this thesis does apply existing classical control theory to LCC HVDC control systems. Index Terms: Line Commutated Converter, HVDC, inductive modelling, power system, transient analysis.
Development of neuro-fuzzy strategies for prediction and management of hybrid PV-PEMFC-battery systems.
(2017) Mutombo, Ntumba Marc-Alain.; Inambao, Freddie Liswaniso.; Tiako, Remy.
Abstract available in PDF file.
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.
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.
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.
A gridless, variable perveance Pierce electron gun.
(1994) Foulis, Bruce David.; Nattrass, Henry Lee.
This thesis covers the design and development of a modulated Pierce electron gun used in the construction of experimental travelling wave tube (TWT) amplifiers. The gun incorporated an open aperture switching electrode, positioned mid-way between anode and cathode, to pulse the beam. This method of modulation did not have the same adverse effects on electron trajectories as in the case of a conventional mesh grid, but rather the electrode could be used to alter the focus conditions within the gun and subsequently improve certain beam characteristics. Ion focusing effects could also be eliminated with the electrode, allowing dual mode operation of the guns without the complications normally associated with such a practice. The switching electrode was simulated to ascertain its effect on electron trajectories within the gun, using finite element analysis as well as an electron optics design program. A test gun was constructed in a glass envelope in order to investigate the performance of the new design. The glass gun allowed a beam analysis to be performed, as well as thermal measurements to be made. Results from this gun compared favourably with earlier simulations. The results of two metal/ceramic construction TWTs are presented, showing the beneficial effects of the switching electrode on the performance of the tubes as a whole, and the electrode's potential to compensate for constructional anomalies. The joining of metals to ceramic using active brazing techniques is also an important aspect tackled by the thesis, with several innovative ideas being implemented in the construction of the devices. A simple yet reliable electrical feed-through was developed for those guns having a ceramic envelope. Extensive work was also performed on the manufacture of impregnated tungsten cathodes for use in the electron guns. Several test diodes, including a water-cooled demountable test vehicle, were constructed to test the performance of the cathodes. An analysis was performed on the patchy behaviour of some of the initial cathodes to improve the preparation methods used in the laboratory. The emission results obtained from the cathodes are documented, as is the successful incorporation of several of them into the new modulated gun design.
Impact of three-dimensional photovoltaic structure on solar power generation.
(2016) Mafimidiwo, Olufunmilayo Alice.; Saha, Akshay Kumar.
Abstract available in PDF file.
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.
An investigation of the influence of silver doping on the intergranular 'weak-link' properties of the superconducting system Y1Ba2Cu307-x.
(2006) Jarvis, Alan Lawrence Leigh.; Broadhurst, Anthony D.; Doyle, Terence Brian.
High-temperature superconducting materials have found considerable technological application and still have a largely unrealised potential. The key to unlocking this potential depends on a better understanding of their properties; in particular, the maximum 'critical current density' which these materials, in the form of wires, tapes, thin-films and bulk monolithic forms, are able to support for high-current applications. The 'critical current density' in a polycrystalline high-temperature oxide superconductor system is determined by a percolation process of the super current through a three dimensional grain-boundary network. Grain-boundaries in these systems behave as Josephson junction 'weak-links' and they severely limit the critical current density in the presence of even moderate self or applied magnetic fields. In the present work, isothermal quasi-static magnetisation measurements on the polycrystalline YIBa2Cu307-x system are presented and analysed. An effective granular penetration depth in conjunction with a critical state model, which includes an approximate treatment for the percolation process, is used to obtain many of the salient physical parameters of the grain-boundary Josephson junctions and of the three-dimensional grain boundary junction network. Determination of the temperature and magnetic field dependence of several of these parameters, in particular a magnetic field-independent critical current which depends on the micro structure of the grain-boundary junction network, allows for testing and verification of models of the weak-link and network behaviour. This treatment has been carried out specifically on various silver doped polycrystalline Y1Ba2Cu30 7-x specimens in order to determine and quantify the effects of silver doping. An improvement in the critical current density with silver doping is explained in terms of silver scavenging and ridding grain-boundaries of impurities, and a proximity effect where trace amounts of silver residing in the grain-boundaries decreases the normal resistance of the grain-boundary Josephson junction. The insight gained from silver doping experimentation led to a macroscopic investigation into the joining of large single-domain YIBa2Cu307-x specimens for large-scale applications.
The lightning ground flash : an engineering study.
(1979) Eriksson, Andrew John.
The thesis is concerned with a study of the electrical engineering parameters of the lightning ground flash - i.e. the statistical distributions of peak current amplitudes, discharge current waveform characteristics, and flash striking distances - in the event of flashes to practical engineering structures. In view of its predominating frequency of occurrence in practical situations, the discharge of primary concern is the downward progressing and negatively charged ground flash. A central feature of this work is the establishment of a lightning research station (incorporating a 60 m instrumented mast) in the Transvaal highveld region of South Africa. The design of this station and the related measurement techniques are fully described. Preliminary results accumulated over a 6-year period of observation are presented, and include recordings obtained during direct strikes to the mast, as well as data from associated measurements of additional thunderstorm and lightning parameters. The latter studies include the use of closed circuit television video recordings, together with electrostatic field mills and lightning flash counters. Analysis of the resultant data serves to provide a comprehensive characterisation of the thunderstorm and lightning climatology in the region - on the basis of electrical activity. With only few exceptions, it is concluded that the characteristics of lightning observed in the. Transvaal region are generally consistent with the trends of data from other regions of the world. A unique aspect of the project is a study of lightning striking distances. An attempt to estimate these distances using bi-directional photography of flashes to the research mast is described, and several preliminary results are also presented - in conjunction with the associated measurements of discharge current amplitude. These results are compared with previously used relationships between striking distance and peak current.
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.
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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