| Multilevel converters are emerging as a new breed of power converter options to accommodate the commercially available semiconductor devices for Medium Voltage (MV) high power applications. So far, there is no consolidation of the power converter topologies at the MV network as the widely-accepted two-level Voltage Source Converter (VSC) does at the Low Voltage (LV) network. At present, the most frequently used multilevel converters are the neutral point clamped converter, flying capacitor converter, and the cascaded H-bridge converter, among which the cascaded H-bridge converter has gained significant interest due to a modular structure with easy construction and maintenance, isolated dc buses with no voltage unbalance problem, and easy extension. However, since single-phase converter modules are employed, large number of components is required and the dc energy storage requirement is high.; The dissertation is dedicated to develop new multilevel converters with better perfomunces. A synthesis of the multilevel converters based on the Converter Building Blocks (CBBs) is conducted. The unique and systemization of the CBB-based multilevel converters re-categorized the existing converters including the successful cascaded H-bridge converter, and more important, discovered new converters with some superior characteristics. Three promising multilevel converters based on the three-phase VSC modules are explored, among which the Hexagram converter has the best characteristics. Except for the common features owed by the converters in this family, the Hexagram converter has the advantages of symmetrical structure with even component stress and automatic voltage and current sharing, much reduced voltage stress, low dc energy storage requirement, and easy control with well-developed two-level control techniques.; Five application examples of the Hexagram converter are given, including the Hexagram inverter for MV Variable Speed Drive (VSD) systems, Hexagram rectifier as the Active Front End (AFE) of the Hexagram inverter for VSD systems, Hexagram converter for static VAR compensation, Hexagram converter for active power filtering, and Hexagram converter for both static VAR compensation and active power filtering. The analysis is all supported with the simulation and experimental results. |
