Multi-level converters that provide more than two levels of voltage to achieve an output waveform closer to sinusoidal waveform with less distortion are very attractive to power applications. This thesis investigates several multi-level converter topologies and different modulation strategies such as pulse-width modulation and space vector modulation. Attention is paid in particular to SVM strategy. Although SVM strategy is applicable for N-level converter, this thesis only focuses on five-level and three-level diode clamped converter (DCC). Despite their appealing harmonic spectrum and low losses, multi-level converters are known to suffer from inherent voltage imbalance on their dc side. The thesis presents a method in order to balance the dc side capacitor voltages for an N-level converter. The presented balancing method is based on minimizing a cost function which is related to voltage divergence of the dc capacitors. This method is used for a three-level SVM to overcome the voltage drifting problem.

Multi-level converters that provide more than two levels of voltage to achieve an output waveform closer to sinusoidal waveform with less distortion are very attractive to power applications. This thesis investigates several multi-level converter topologies and different modulation strategies such as pulse-width modulation and space vector modulation. Attention is paid in particular to SVM strategy. Although SVM strategy is applicable for N-level converter, this thesis only focuses on five-level and three-level diode clamped converter (DCC). Despite their appealing harmonic spectrum and low losses, multi-level converters are known to suffer from inherent voltage imbalance on their dc side. The thesis presents a method in order to balance the dc side capacitor voltages for an N-level converter. The presented balancing method is based on minimizing a cost function which is related to voltage divergence of the dc capacitors. This method is used for a three-level SVM to overcome the voltage drifting problem.

Over the past ten years, there has been increased use of electronic power processing in alternative, sustainable, and distributed energy sources, as well as energy storage systems, transportation systems, and the power grid. 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, such as high efficiency and fast response. For transportation applications, high power density is the key design target, since increasing power density can reduce fuel consumption and increase the total system efficiency. While power electronics devices have greatly improved the efficiency, overall performance and power density of power converters, using power electronic devices also introduces EMI issues to the system, which means filters are inevitable in those systems, and they make up a significant portion of the total system size and cost. Thus, designing for high power density for both power converters and passive components, especially filters, becomes the key issue for three-phase converters. This dissertation explores two different approaches to reducing the EMI filter size. One approach focuses on the EMI filters itself, including using advanced EMI filter structures to improve filter performance and modifying the EMI filter design method to avoid overdesign. The second approach focuses on reducing the EMI noise generated from the converter using a three-level and/or interleaving topology and changing the modulation and control methods to reduce the noise source and reduce the weight and size of the filters. This dissertation is divided into five chapters. Chapter 1 describes the motivations and objectives of this research. After an examination of the surveyed results from the literature, the challenges in this research area are addressed. Chapter 2 studies system-level EMI modeling and EMI filter design methods for voltage source converters. Filter-design-oriented EMI modeling methods are proposed to predict the EMI noise analytically. Based on these models, filter design procedures are improved to avoid overdesign using in-circuit attenuation (ICA) of the filters. The noise propagation path impedance is taken into consideration as part of a detailed discussion of the interaction between EMI filters, and the key design constraints of inductor implementation are presented. Based on the modeling, design and implementation methods, the impact of the switching frequency on EMI filter weight design is also examined. A two-level dc-fed motor drive system is used as an example, but the modeling and design methods can also be applied to other power converter systems. Chapter 3 presents the impact of the interleaving technique on reducing the system passive weight. Taking into consideration the system propagation path impedance, small-angle interleaving is studied, and an analytical calculation method is proposed to minimize the inductor value for interleaved systems. The design and integration of interphase inductors are also analyzed, and the analysis and design methods are verified on a 2 kW interleaved two-level (2L) motor drive system. Chapter 4 studies noise reduction techniques in multi-level converters. Nearest three space vector (NTSV) modulation, common-mode reduction (CMR) modulation, and common-mode elimination (CME) modulation are studied and compared in terms of EMI performance, neutral point voltage balancing, and semiconductor losses. In order to reduce the impact of dead time on CME modulation, the two solutions of improving CME modulation and compensating dead time are proposed. To verify the validity of the proposed methods for high-power applications, a 100 kW dc-fed motor drive system with EMI filters for both the AC and DC sides is designed, implemented and tested. This topology gains benefits from both interleaving and multilevel topologies, which can reduce the noise and filter size significantly. The trade-offs of system passive component design are discussed, and a detailed implementation method and real system full-power test results are presented to verify the validity of this study in higher-power converter systems. Finally, Chapter 5 summarizes the contributions of this dissertation and discusses some potential improvements for future work. / Ph. D.