Thesis (Ph. D.)--Michigan State University. Dept. of Electrical and Computer Engineering, 2006. / Title from PDF t.p. (viewed on Nov. 17, 2008) Includes bibliographical references (p. 168-175). Also issued in print.

Thesis (Ph. D.)--Oregon State University, 2004. / Printout. Includes bibliographical references (leaves 75-79). Also available on the World Wide Web.

Thesis (M.S.)--California Polytechnic State University, 2010. / Title from PDF title page; viewed on June 10, 2010. Major professor: Taufik, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Electrical Engineering." "May 2010." Includes bibliographical references (p. 67-69).

An optically pumped far-infrared laser and superconducting magnet have been used to perform high resolution studies of the energy levels of neutral and negatively charged shallow donors in high purity n-GaAs and n-InP in magnetic fields where the dimensionless magnetic field gamma is approximately one (where gamma=hoc/(2R*), hoc is the cyclotron energy and R* is the Coulomb binding energy). The central cell structure caused by the presence of different shallow donor species has been studied on the 1s-2p+1,0 transitions of neutral shallow donors in undoped GaAs samples grown by molecular beam epitaxy, liquid phase epitaxy and vapour phase epitaxy (VPE). VPE material showed two new shallow donor species with negative central cell shifts. The ls-2p-1 transition at magnetic fields where gamma > 1 shows exceptionally well resolved central cell structure. Detailed structure at magnetic fields below the 1s-2p+2 transition is due to transitions from the is to higher excited states. Samples of undoped high purity InP grown by the VPE, metal organic chemical vapour deposition and bulk growth techniques have been studied. VPE samples always show a strong component related to sulphur though some also show a strong silicon related component, and some show up to 7 components. A bulk, sample showed two strong components shallower than silicon which may have negative central cell shifts. Transitions between the excited states of neutral shallow donors in GaAs have been studied. Recent theoretical work by Makado (1982) describes the transition energies very well. Clearly resolved central cell structure is observed on inter-excited state transitions involving the 2s state. The first unambiguous observation of negatively charged shallow donors (D-states) in GaAs is reported. Simultaneous observations of transitions involving D-states, the cyclotron resonance and inter-excited state transitions of neutral donors over a wide magnetic field range, 0.03 < gamma < 3.5, highlight the differences between the transitions and the relative effects of optical excitation, temperature, magnetic field and electric field bias.

M.Ing. (Electrical & Electronic Engineering) / Induction heating is a relatively old and established process. New switching devices and ideas are, however, resulting in much improved performance at reduced cost. Some of these devices are investigated in this thesis, and some new ideas presented. From a literature study a general review of the current state of the technology is given. The fundamental principles and limits of induction heating are discussed. Power electronic topologies for induction heating, as well as switching devices which can be used in these topologies, are compared and evaluated. From the knowledge gained, possible fields for improvement are identified. One such field is that of semiconductor switch drives, and a high-performance gate drive was developed. Using such fast drives, special attention must be given to the electromagnetic layout of the circuit. Another new field is that of integrated resonant elements. These elements find application in most resonant converters, and offer reduced weight and losses compared to conventional technologies. They are evaluated both experimentally and theoretically. A prototype induction heater was built, using a newly released switching device named a zero turn-off thyristor. The high-performance gate drive developed earlier was tested in the inverter and gave excellent results.

The purpose of this investigation is to compare high frequency to low frequency (50Hz) technologies. To accomplish this, two highly optimized candidate converters are built to champion each of the inverter classes. Both candidates are constrained to produce the same quasi-sinusoidal output waveforms in identical operating conditions. The low frequency inverter is designed with optimization of its output power quality and accurate loss characterization taken in mind. A new iron core design procedure is proposed that may predict core losses when using non-sinusoidal excitation. Experimental results indicate that it has a promising degree of accuracy. The high frequency candidate is designed using planar integration technology. A topology selection determines that a critical-conduction mode flyback is the most suitable option. Components are added the topology to allow resonant switching for reduced switching losses. Common-mode current arising from good capacitive coupling between windings in the planar coupled inductor is reduced using charge balancing technology. The novelty of the converter lies is in the fact that all passive components, barring the bulk of the input capacitance and the output capacitance, are incorporated into the single planar structure. Finally, a comparison of the inverters is made in the categories of power quality, regulation, efficiency, robustness and applicability to the application. The high frequency inverter is found to promise substantial advantages over the low frequency variety in most of these elements. / Dr. I.W. Hofsajer

M.Ing. (Electrical & Electronic Engineering) / Please refer to full text to view abstract

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The main focus of this PhD thesis is fundamental investigations into control techniques of inverter-based microgrids. It aims to develop new and improved control techniques to enhance performance and reliability. It focuses on the modelling, stability analysis and control design of parallel inverters in a microgrid. In inverter-based microgrids, the paralleled inverters need to work in both grid-connected mode and stand-alone mode and should be able to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using droop control. Inverters can have different power set-points during grid-connected mode but in stand-alone mode they all need their power set-points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set-points to inverters with low power set-points, which can raise the DC link voltage of the inverters causing them to shut down. This thesis investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the DC link voltage and adjusts the power set-point in proportion to the rise in the voltage. A small signal model of an island microgrid has been developed and used to design the controller. The model and the controller design have been validated by simulation and practical experimentation. The results confirmed the performance of the proposed controller for limiting the DC link voltage and supporting a seamless mode transfer. The limitation of the droop controller, that is utilized to achieve load sharing between parallel-operated inverters in island mode, has also been addressed. Unequal output impedances among the distribution generation (DG) units lead to the droop control being inaccurate, particularly in terms of reactive power sharing. Many methods reported in the literature adopt low speed communications to achieve efficient sharing. However, the loss of this communication could lead to inaccuracy or even instability. An improved reactive power-sharing controller is proposed in this thesis. It uses the voltage at the point of common coupling (PCC) to estimate the inductance value of the output impedance including the impedance of the interconnecting power cables and to readjust the voltage droop controller gain accordingly. In an island microgrid consisting of parallel-connected inverters, the interaction between an inverter’s output impedance (dominated by the inverter’s filter and voltage controller) and the impedance of the distribution network (dominated by the other paralleled inverters’ output impedances and the interconnecting power cables) might lead to instability. This thesis studies this phenomenon using root locus analysis. A controller based on the second derivative of the output capacitor voltage is proposed to enhance the stability of the system. Matlab simulation results are presented to confirm the validity of the theoretical analysis and the robustness of the proposed controller. A laboratory-scale microgrid consisting of two inverters and local load has been built for the experimental phase of the research work. A controller for a voltage source inverter is designed and implemented. A dSPACE unit has been used to realize the controller and monitor the system in real time with the aid of a host computer. Experimental results of the two voltage source inverters outputs are presented.

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