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.