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The development of high power density three-phase ac converter has been a hot topic in power electronics area due to the increasing needs in applications like electric vehicle, aircraft and aerospace, where light weight and/or low volume is usually a must. Many challenges exist due to the complicated correlations in a three-phase power converter system. In addition, with the emerging SiC device technology the operating frequency of the converter can be potentially pushed to the range from tens of kHz to hundreds of kHz at higher voltage and higher power conditions. The extended frequency range brings opportunities to further improve the power density of the converter. Technologies based on existing devices need to be revisited. In this dissertation, a systematic methodology to analyze and design the high power density three-phase ac converter is developed. All the key factors of the converter design are explored from the high density standpoint. Firstly, the criteria for the passive filter selection are derived and the relationship between the switching frequency and the size of the EMI filter is investigated. A function integration concept as well as the physical design approach is proposed. Secondly, a topology evaluation method is presented, which provides the insight into the relationships between the system constraints, operating conditions and design variables. Four topologies are then compared with the proposed approach culminating with a favored topology under the given conditions. Thirdly, a novel average model is developed for the selected topology, and used for devising a carrier-based control approach with simple calculation and good regulation performance. Fourthly, the converter failure mode operation and corresponding protection approaches are discussed and developed. Finally, a 10 kW three-phase ac/ac converter is built with the SiC devices. All the key concepts and ideas developed in this work are implemented in this hardware system and then verified by the experimental results. / Ph. D.

Three-phase voltage source converters(VSCs) have become the converter of choice in many ac medium and high power applications due to their many advantages, including low harmonics, high power factor, and high efficiency. Modular VSCs have also been a popular choice as building blocks to achieve even higher power, primarily through converter paralleling. In addition to high power ratings, paralleling converters can also provide system redundancy through the so-called (N+1) configuration for improved availability, as well as allow easy implementation of converter power management. Interleaving can further improve the benefit of paralleling VSCs by reducing system harmonic currents, which potentially can increase system power density. There are many challenges to implement interleaving in paralleled VSCs system due to the complicated relationships in a three-phase power converter system. In addition, to maximize the benefit of interleaving, current knowledge of symmetric interleaving is not enough. More insightful understanding of this PWM technology is necessary before implement interleaving in a real paralleled VSCs system. In this dissertation, a systematic methodology to analyze and design a paralleled three-phase voltage source converters with interleaving is developed. All the analysis and proposed control methods are investigated with the goal of maximizing the benefit of interleaving based on system requirement. The dissertation is divided into five sections. Firstly, a complete analysis studying the impact of interleaving on harmonic currents in ac and dc side passive components for paralleled VSCs is presented. The analysis performed considers the effects of modulation index, pulse-width-modulation (PWM) schemes, interleaving angle and displacement angle. Based on the analysis the method to optimize interleaving angle is proposed. Secondly, the control methods for the common mode (CM) circulating current of paralleled three-phase VSCs with discontinuous space-vector modulation (DPWM) and interleaving are proposed. With the control methods, DPWM and interleaving, which is a desirable combination, but not considered possible, can be implemented together. In addition, the total flux of integrated inter-phase inductor to limit circulating current can be minimized. Thirdly, a 15 kW three phase ac-dc rectifier is built with SiC devices. With the technologies presented in this dissertation, the specific power density can be pushed more than 2kW/lb. Fourthly, the converter system with low switching frequency is studied. Special issues such as beat phenomenon and system unbalance due to non-triplen carrier ratio is explained and solved by control methods. Other than that, an improved asymmetric space vector modulation is proposed, which can significantly reduce output current total harmonic distortion (THD) for single and interleaved VSCs system. Finally, the method to protect a system with paralleled VSCs under the occurrence of internal faults is studied. After the internal fault is detected and isolated, the paralleled VSCs system can continue work. So system reliability can be increased. / Ph. D.

Blood products are essential components of any healthcare system, and their safety, in terms of being free of transfusion-transmittable infections, is crucial. While the Food and Drug Administration (FDA) in the United States requires all blood donations to be tested for a set of infections, it does not dictate which particular tests should be used by blood collection centers. Multiple FDA-licensed blood screening tests are available for each infection, but all screening tests are imperfectly reliable and have different costs. In addition, infection prevalence rates and several donor characteristics are uncertain, while surveillance methods are highly resource- and time-intensive. Therefore, only limited information is available to budget-constrained blood collection centers that need to devise a post-donation blood screening scheme so as to minimize the risk of an infectious donation being released into the blood supply. Our focus is on "robust" screening schemes under limited information. Toward this goal, we consider various objectives, and characterize structural properties of the optimal solutions under each objective. This allows us to gain insight and to develop efficient algorithms. Our research shows that using the proposed optimization-based approaches provides robust solutions with significantly lower expected infection risk compared to other testing schemes that satisfy the FDA requirements. Our findings have important public policy implications. / Ph. D.

Due to increasing penetration of solar power generation, distribution grids are facing a number of challenges. Frequent reverse active power flows can result in rapid fluctuations in voltage magnitudes. However, with the revised IEEE 1547 standard, smart inverters can actively control their reactive power injection to minimize voltage deviations and power losses in the grid. Reactive power control and globally optimal inverter coordination in real-time is computationally and communication-wise demanding, whereas the local Volt-VAR or Watt-VAR control rules are subpar for enhanced grid services. This thesis uses machine learning tools and poses reactive power control as a kernel-based regression task to learn policies and evaluate the reactive power injections in real-time. This novel approach performs inverter coordination through non-linear control policies centrally designed by the operator on a slower timescale using anticipated scenarios for load and generation. In real-time, the inverters feed locally and/or globally collected grid data to the customized control rules. The developed models are highly adjustable to the available computation and communication resources. The developed control scheme is tested on the IEEE 123-bus system and is seen to efficiently minimize losses and regulate voltage within the permissible limits. / Master of Science / The increasing integration of solar photovoltaic (PV) systems poses both opportunities and technical challenges for the electrical distribution grid. Although PV systems provide more power to the grid but, can also lead to problems in the operation of the grid like overvoltages and voltage fluctuations. These variations can lead to overheating and burning of electrical devices and equipment malfunction. Since the solar generation is highly dependent on weather and geographical location, they are uncertain in their output. The uncertainity in the solar irradiance can not be handled with the existing voltage control devices as they need to operate more frequently than usual which can cause recurring maintenance needs for these devices. Thus, to make solar PV more flexible and grid-friendly, smart inverters are being developed. Smart inverters have the capability of advanced sensing, communication, and controllability which can be utilized for voltage control. The research discusses how the inverters can be used to improve the grid profile by providing reactive power support to reduce the power losses and maintain voltages in their limits for a safer operation.

Growing municipal waste mismanagement and associated environmental impacts is an enormous environmental concern in developing countries such as South Africa. Hence, this study explored current waste management and minimisation patterns and practices in the Ekurhuleni Metropolitan Municipality (EMM), located east of the Gauteng province. The study was undertaken using a mixed method design, particularly the concurrent triangulated design where the quantitative and qualitative data were collected at the same time. The methods employed were desktop surveys, interviews with the participants and use of questionnaires which were designed based on the objectives of the study. The questionnaires were designed for different types of participants (namely, households, informal reclaimers, municipal officials and landfill officials). All the data collected were stored in Microsoft Excel (2010) spread sheet for statistical analyses. The study has revealed some patterns, practices as well as trends regarding waste management and minimisation within the EMM municipality. At household level, there was some environmental awareness on waste management practices provided by the municipality as well as local recycling options although there are numerous challenges to be resolved before these functions can become effective. With informal recycling, a number of waste materials are being reclaimed at various landfill sites. However, current informal waste picking activities by the so-called scavengers are not sustainable as waste is not separated prior to disposal at various point sources. In addition, informal reclaimers have to travel long distances to reach waste sources. Another concerning constraint hampering the effectiveness of informal waste recovery, has to do with their daily exposure to several environmental and health risks. Furthermore, the study has found out that the EMM is predominantly focused on providing better waste management services rather than balancing this activity with waste minimisation through reclaiming and recycling operations. Thus, the municipality lacks adequate infrastructure to undertake waste minimisation effectively. Also, waste minimisation and awareness campaigns were found to be inadequate and at an infant stage, unlike those carried out by private companies. In view of these findings, a number of recommendations have been made. / Environmental Sciences / M. Sc. (Environmental Science)

Thesis (MDF)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Johannesburg generates in excess of 1 500 000 tonnes of general waste annually and has on average 10 years’ lifespan remaining on its four landfill sites. As a signatory to the Polokwane Declaration, the City of Johannesburg has recognised the need for new interventions to divert waste from landfills by various methods, such as separation at source; alternative treatment methods and the conversion of waste to energy. Progress has, however, been slow and this research aims to investigate alternative waste management techniques that can be applied in the City of Johannesburg to fast track the realisation of these targets. Using scenario planning techniques, the research considers implications for policy and management decisions in realising the best possible future in the area of waste management in Johannesburg. The scenario process was used to develop the following scenarios for waste management in Johannesburg: Long walk to freedom. Waste collection coverage includes pockets of the community where waste collections services are less than optimal. The residents of the city, particularly in these underserviced areas, are also not very knowledgeable of the impact that the waste generated within their communities can have on the environment and what alternatives there are to manage this. Pick it up. The City provides full services to a society that functions in relative oblivion of the implications of their behaviour on the environment. It is assumed to be the role of government to “pick up” after communities and dispose of waste. This scenario is oblivious of the waste hierarchy and the role communities could play in minimising waste. Wishing on a star. The city continues to have under-serviced areas, public awareness is high and this fuels correct behaviour and a mind-set shift with regard to waste management. Working together we can do more. The City optimises its service provision to cover all areas while also ensuring maximum public awareness and behaviour change with regard to waste management.

Thesis (PhD (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2008. / ENGLISH ABSTRACT: The power conversion unit of a large solar chimney power plant converts the fluid power, first into mechanical power, and then into electrical power. In this dissertation a tool is developed to determine the layout and the number of turbines of the solar chimney power conversion unit providing the lowest cost of electricity. First, the history of the solar chimney concept and the related fields of research are presented. Potential features and configurations of the power conversion unit are introduced, and it is shown how the solar chimney power conversion unit compares to those of other applications. An outline of the dissertation is given, and its potential impact is discussed. An analytical turbine model is developed. Several modelling approaches and the performance of single rotor and counter rotating turbine layouts are compared. Preliminary turbine designs are investigated, experimentally and numerically. The main aim of the experimental investigation is to verify the applicability of the loss model used in the analytical turbine model. The aim of the numerical investigation is to evaluate a commercial software package as a tool in context with solar chimney turbines. For each component of the power conversion unit an analytical performance model is introduced. Using these models, the single vertical axis, multiple vertical axis and multiple horizontal axis turbine configurations are compared from an efficiency and energy yield point of view, and the impact of the various losses on the overall performance is highlighted. A detailed cost model for the power conversion unit is also presented. To optimize for cost of electricity this cost model is then linked to the performance models, and the resulting optimization scheme is applied to several plant configurations. It is shown that for a large solar chimney power plant the power conversion unit providing minimal cost of electricity consists of multiple horizontal axis turbines using a single rotor layout including inlet guide vanes. / AFRIKAANSE OPSOMMING: Die drywingsomsettingseenheid van ’n groot sonskoorsteenaanleg sit die vloeidrywing om, eers in meganiese drywing en dan in elektriese drywing. In hierdie proefskrif word ’n gereedskapstuk ontwikkel om die uitleg en aantal turbines van die sonskoorsteen-drywingsomsettingseenheid te bepaal wat die laagste koste van elektrisiteit lewer. Eerstens word die geskiedenis van die sonskoorsteen en verwante navorsingsvelde behandel. Moontlike eienskappe en konfigurasies vir die drywingsomsettingseenheid word voorgestel, en daar word aangetoon hoe die sonskoorsteendrywingsomsettings- eenheid vergelyk met ander toepassings. ’n Raamwerk van die proefskrif word gegee, en die potensiële trefkrag daarvan word bespreek. ’n Analitiese turbine-model word ontwikkel. Verskeie nabootsingsbenaderings en die vertoning van ’n enkelrotor en teenroterende turbine-uitlegte word vergelyk. Voorlopige turbine-ontwerpe word ondersoek, eksperimenteel en numeries. Die hoofdoel van die eksperimentele ondersoek is om die toepaslikheid van die verliesmodel in die analitiese turbine-model te bevestig. Die doel van die numeriese ondersoek is om kommersiële sagteware op te weeg as ’n gereedskapstuk in die konteks van sonskoorsteenturbines. Vir elke onderdeel van die drywingsomsettingseenheid word ’n analitiese model voorgestel. Met gebruik van hierdie modelle word die enkele vertikale-as, die veelvoudige vertikale-as an die veelvoudige horisontale-as turbinekonfigurasies vergelyk vanuit ’n benuttingsgraad- en energie-opbrengsoogpunt,en die uitwerking van die verskillende verliese op die algehele gedrag word uitgewys. ’n Kostemodel in besonderhede word vir die drywingsomsettingseenheid aangebied. Om vir die koste van elektrisiteit te optimeer word hierdie kostemodel dan gekoppel aan die vertoningsmodelle, en die gevolglike optimeringskema word toegepas op verskeie aanlegkonfigurasies. Daar word aangetoon dat vir ’n groot sonskoorsteenaanleg die drywingsomsettingseenheid wat die minimumkoste van elektrisiteit gee, bestaan uit veelvoudige horisontale-as turbines met enkelrotoruitleg en inlaatleilemme. / Centre for Renewable and Sustainable Energy Studies

This thesis aims at contributing to the study of the valuation of insurance liabilities and the management of the assets backing these liabilities. It consists of four parts, each devoted to a specific topic. In the first part, we study the pricing of a classical single premium life insurance contract with profit, in terms of a guaranteed rate on the premium and a participation rate on the (terminal) financial surplus. We argue that, given the asset allocation of the insurer, these technical parameters should be determined by taking explicitly into account the risk management policy of the insurance company, in terms of a risk measure such as the value-at-risk or the conditional value-at-risk. We then design a methodology that allows us to fix both parameters in such a way that the contract is fairly priced and simultaneously exhibits a risk consistent with the risk management policy. In the second part, we focus on the management of the surrender option embedded in most life insurance contracts. In Chapter 2, we argue that we should model the surrender time as a random time not adapted to the filtration generated by the financial assets prices, instead of assuming that the surrender time is an optimal stopping time as it is usual in the actuarial literature. We then study the valuation of insurance contracts with a surrender option in such a model. We here follow the financial literature on the default risk and in particular, the reduced-form models. In Chapter 3 and 4, we study the hedging strategies of such insurance contracts. In Chapter 3, we study their risk-minimizing strategies and in Chapter 4, we focus on their ``locally risk-minimizing' strategies. As a by-product, we study the impact of a progressive enlargement of filtration on the so-called ``minimal martingale measure'. The third part is devoted to the systematic mortality risk. Due to its systematic nature, this risk cannot be diversified through increasing the size of the portfolio. It is thus also important to study the hedging strategies an insurer should follow to mitigate its exposure to this risk. In Chapter 5, we study the risk-minimizing strategies for a life insurance contract when no mortality-linked financial assets are traded on the financial market. We here extend Dahl and Moller’s results and show that the risk-minimizing strategy of a life insurance contract is given by a weighted average of risk-minimizing strategies of purely financial claims, where the weights are given by the (stochastic) survival probabilities. In Chapter 6, we first study the application of the HJM methodology to the modelling of a longevity bonds market and describe a coherent theoretical setting in which we can properly define the longevity bond prices. Then, we study the risk-minimizing strategies for pure endowments and annuities portfolios when these longevity bonds are traded. Finally, the fourth part deals with the design of ALM strategies for a non-life insurance portfolio. In particular, this chapter aims at studying the risk-minimizing strategies for a non life insurance company when inflation risk and interest rate risk are taken into account. We derive the general form of these strategies when the cumulative payments of the insurer are described by an arbitrary increasing process adapted to the natural filtration of a general marked point process and when the inflation and the term structure of interest rates are simultaneously described by the HJM model of Jarrow and Yildirim. We then systematically apply this result to four specific models of insurance claims. We first study two ``collective' models. We then study two ``individual' models where the claims are notified at a random time and settled through time.

Approved for public release; distribution is unlimited / The contribution of this thesis is the development of a CAD (computer aided design) tool for current mode multiple-valued logic (MVL) CMOS circuits. It is only the second known MVL CAD tool and the first CAD tool for MVL CMOS. The tool accepts a specification of the function to be realized by the user, produces a minimal or near-minimal realization (if such a realization is possible), and produces a layout of a programmable logic array (PLA) integrated circuit that realizes the given function. The layout is in MAGIC format, suitable for submission to a chip manufacturer. The CAD tool also allows the user to simulate the realized function so that he/she can verify correctness of design. The CAD tool is designed also to be an analysis tool for heuristic minimization algorithms. As part of this thesis, a random function generator and statistics gathering package were developed. In the present tool, two heuristics are provided and the user can choose one or both. In the latter case, the better realization is output to the user. The CAD tool is designed to be flexible, so that future improvements can be made in the heuristic algorithms, as well as the layout generator. Thus, the tool can be used to accommodate new technologies, for example, a voltage mode CMOS PLA rather than the current mode CMOS currently implemented. / http://archive.org/details/cadtoolforcurren00leeh / Lieutenant, Republic of Korea Navy