M.Ing. (Electrical and Electronic Engineering) / The main purpose of the study is to propose an unique unity power factor AC-DC converter topology, with isolated output. The proposed converter is obtained by merging the unity power factor boost converter and the full bridge resonant transition converter. The converter is unique as resonant switching techniques are used to obtain a zero voltage switching boost stage, resulting in an all soft switching converter. The result is lower levels of conducted EMI, as well as lower switching losses. Conventional topologies utilize more than one converter stage to obtain the required results, whereas _ the proposed converter results in a single step solution. As the purpose of the proposed converter is power factor regulation resulting in lower levels of conducted EMI, the study is commenced with a review of the definitions and equations for both power factor and EM!. Thereafter sources and propagation of conducted EMI are discussed. To gain a better understanding of the operation of the proposed converter, the operation of the unity power factor boost converter, as well as the full bridge resonant transition converter are discussed in depth. A switching cycle of the proposed converter is divided into different intervals, and analytical equations for the relevant currents and voltages for each interval are derived seperately. Possible control schemes are introduced and where applicable, conditions for zero voltage switching are given. It is concluded that a constant frequency PWM controller is the most ideal control scheme to ensure proper operation of the converter. A simple design procedure is presented by which the size of the most important circuit parameters can be designed in terms of the desired switching frequency and permissable duty cycle variation. Finally, experimental results obtained from the practical converter, are given. Deviations from the ideal are interpreted and methods of improving the performance are suggested.

Graduate

D.Ing. / The propagation of fast disturbances along power transmission structures depends on the parameters of the excitation for the disturbance, the characteristics of the power transmission structure, and eventually on the characteristics of the electrical load connected to the structure. This thesis will therefore target the determination of the parameters of transmission structures in Chapter 2, applicable transmission-line theory in Chapter 3, modelling the load in Chapter 4, and apply this knowledge to the specific problem of over-voltages on the terminals of machines fed via cables from inverters in Chapter 5. The rest of this chapter is devoted to explaining the introductory aspects of this application problem.

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I present two algorithms which solve the one-dimensional magnetotelluric (MT) problem of finding the electrical conductivity σ(z) as a function of depth in the earth. Together, these algorithms restrict and explore the nonuniqueness of the nonlinear MT inverse problem. They accept constraints which limit the space of acceptable conductivity models and they construct diverse classes of σ(z) in order to explore this space. To avoid pitfalls during interpretation, it is essential to investigate the extent of the nonuniqueness permitted by the MT data. Algorithm 1 is a two-stage process based on the inverse scattering theory of Weidelt. The first stage uses the MT frequency-domain data to construct an impulse response analogous to a deconvolved seismogram. Since this is a linear problem (a Laplace transform), numerous impulse responses may be generated by linear inverse techniques which handle data errors robustly. I minimize four norms of the impulse response in order to construct varied classes of limited structure models. Two least-squares norms minimize the energy in the impulse response or the energy in its derivative with respect to depth. Two least absolute value norms minimize the magnitudes of the response or its derivative. It is possible to use other norms. The different classes sample the range of acceptable models and the minimum structure criterion is unlikely to allow models with spurious features. The second stage of Algorithm 1 constructs the conductivity model from the impulse response using any of four Fredholm integral equations of the second kind. I evaluate the performance of each of the four mappings and recommend the Burridge and Gopinath-Sondhi formulations. I also evaluate three approximations to the second-stage equations. One of these is equivalent to the Born approximation which assumes the impulse response has negligible multiple reflections. The approximation that includes first-order multiple reflections is the most accurate and gives conductivity models similar to those given by the integral equations. Algorithm 2 solves an integral form of a nonlinear Riccati equation relating the measured frequency-domain data to a function of the conductivity. The iterative solution scheme sacrifices the efficiency of a direct inversion process such as Algorithm 1 for the advantages of incorporating localized conductivity constraints. The linear programming formulation readily accepts a wide variety of equality and inequality constraints on σ(z). I use these constraints in two ways to combat the nonuniqueness of this nonlinear inverse problem. First, I impose physical constraints derived from external sources to restrict the nonuniqueness and construct σ(z) models that are closer to reality. Second, I impose constraints specifically designed to estimate the extent ofthe nonuniqueness and explore the range of acceptable σ(z) profiles. The first technique enhances the reliability of an interpretation and the second assesses the plausibility of particular conductivity features. The convergence of Algorithm 2 is good because Algorithm 1 provides varied initial σ(z) which already fit the data well. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

M.Ing. / Pulse Compression is a technique that may be employed for the generation of extremely high amplitude current and voltage pulses. These pulses can be as short as 50 to 100 ns, and may have amplitudes in the kiloampere and kilovolt ranges. Pulse Compression entails the compression of relatively "flat" pulses in the time domain, to pulses of very high amplitudes and extremely short duration. The pulse amplitudes and durations necessary to be achieved in this research, lie in the range where the switching speeds and other parameters of semiconductors are inadequate and where even the working life of conventional gas discharge apparatus are drastically reduced by the extreme switching demands. The burden of excessively high current densities and unmanageable current rise-rates can be transferred from the semiconductor switches to electromagnetic switches, by making use of pulse compression. Pulse compression can be carried out simultaneously or separately for the compression of the current or voltage content of pulses derived from slowly switched sources, to obtain pulses of extremely short duration and very high amplitudes. The main theme of this dissertation is Current compression. Current compression is accomplished through series-resonance in capacitors and saturable inductors connected in a transmission-line configuration. Energy is transferred in this process from one stage to the next, with reduction in pulse-time in each successive stage and a commensurate increase in amplitude. The generated pulses can attain gigawatt amplitudes and nanosecond durations, whilst loading on the semiconducting switches remains low. In addition to design of the pulse-compressor proper, the work also includes design and development of a voltage-controlled pulse power supply, suitable for generating the initial pulses which are to be compressed. Multistage pulse compression is based on the non-linear characteristics of saturable inductors. Dynamic analogue-time simulation is indispensable in a study thereof, as new theory has to be validated and because non-linear analysis is complex and capable only of being executed by employing approximation methods. Because of the difficulties involved, a considerable amount of attention has been devoted to the development of suitable analogue-dynamic simulation programs for execution on a digital computer. A numerical technique has been developed to express non-linear parameters in differential form. This technique makes it possible to model and simulate virtually any non-linear, physically realizable lumped parameter system with ease. The program is based on State Space techniques and has been developed for its versatility, to accomplish the simulation of a wide variety of circuit configurations.

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<p> This thesis contributes significantly to the advanced electromagnetic (EM) modeling of the interaction of the microwave field with human tissues. The proposed EM models achieve unprecedented computational efficiency, accuracy, and reliability. </p> <p> The EM modeling is crucial in (a) multiphysics and (b) EM analysis m support of optimization procedures with applications in design optimization and inverse problem solutions. The challenge in such applications stems from the fact that EM modeling requires extensive computational resources. Therefore, the reduction of these computational requirements is necessary in order to handle the complexity of multi physics modeling and microwave imaging. </p> <p> In this thesis, an efficient EM/thermal analysis of the interaction of the radiofield (RF) fields of mobile phones with the human eyes is presented. Another advanced application developed here is the solution of inverse problem in microwave imaging and detection by making use of response sensitivity analysis. </p> <p> Two methods are proposed for the evaluation of the maximum specific absorption rate (SAR) in the human eyes due to RF exposure from handheld devices. They account for the existence of resonance in the eye and are applied to the case of near-field exposure. The first method is semi-analytical. As an input. it requires the measured or simulated open-space near field of the device under test in the absence of the eye. As an output. depending on the mutual position and orientation of the eye and the device, it produces the maximum SAR value in the eye averaged over I and I 0 grams of tissue. The second method is experimental. It requires the fabrication of a simple eye phantom and relies on a measurement with an SAR robot. The proposed methods allow for the fast and reliable SAR evaluation of newly developed handheld devices in an industrial environment. Results concerning the temperature rise in the eyes are also presented. They are based on detailed simulation eye models. </p> <p> A conceptually new detection algorithm is proposed in this thesis for the localization of electrically small scatterers in a known background medium. The algorithm requires the knowledge of the electric field distribution inside the known background medium where no scatterers are present. It is based on a self-adjoint response sensitivity computation which can be performed in real time. Using the Efield distribution in the background medium. it provides three-dimensional maps of the Frechet derivative within the imaged volume. The peaks and dips in these maps identify the locations where the permittivity and conductivity of the measured medium differ from those in the background medium. The background medium can be heterogeneous. The performance of the detection algorithm is studied in terms of the number of transmission/reception points, the dielectric contrast of the scatterer compared to the background medium, and the size of the scatterer. Its resolution is also addressed. The proposed detection algorithm is successfully applied in breast cancer detection. </p> / Thesis / Doctor of Philosophy (PhD)