Multi-Level Inverters exploiting an Open-end Winding configuration

Multilevel converters are becoming more and more popular, overcoming some key limitations of conventional two-level structures in handling medium voltages and high voltage gradients. Today they provide the ground for the realization of high efficiency energy conversion systems for medium voltage applications, such as pumps, compressors, extruders, fans, grinding mills, rolling mills, conveyors, crushers, blast furnace blowers, gas turbine starters, mixers, mine hoists, reactive power compensation, marine propulsion, wind energy conversion, and railway traction. A detailed overview of multilevel converters is provided in Chapter 1, while, the state of the art of Open-end Winding Systems is described in Chapter 2. The last systems can be considered as special multilevel inverter structures, tailored around an electrical machine fed from both the ends of the stator, or primary, winding. Overvoltage phenomena generated in industrial motor drives at motor terminals by long feeding cables are investigated in Chapter 3 and an Open-end Winding configuration approach is presented to actively mitigate them. Moreover, an adaptive algorithm is described to make independent the active overvoltage mitigation from system parameters. The main contribution of this work is the development of a new multilevel inverter topology, the Asymmetrical Hybrid Multilevel Inverter (AHMLI), which is introduced in Chapter 4. According to the AHMLI structure, an open-end winding machine (motor, generator or transformer) is supplied on one end by a main multilevel converter, fully managing the active power stream, and, on the other end by an auxiliary two level inverter. This acts as an active power filter, suitably shaping the electrical machine phase current. A mathematical analysis of the proposed structure is first provided, followed by an exhaustive comparison between AHMLI and conventional multilevel structures, emphasizing advantages in terms of efficiency and output current THD. Voltage and current control systems, optimally coping with key characteristics of the AHMLI structure are carried out and an original input capacitors voltage equalization technique is also presented. The application of the AHMLI concept to industrial induction motor drives is then evaluated by simulation and experimental test. A possible exploitation of the AHMLI approach in the realization of photovoltaic and wind plants, as well as STATCOM devices is also assessed. Moreover, a high efficiency three phase rectifier for high speed generation systems exploiting the AHMLI configuration is carried out. Finally, the application of the AHMLI approach to Multiple Motor Drive systems is proposed in Chapter 5. Two new topologies are presented, namely: Open-end Winding Multi Motor Single Converter (MMSC) and Open-end Winding Multi Motor Multi Converter (MMMC). Both configurations exploit the AHMLI structure but the MMMC exploits a five-leg two level inverter to independently control the stator currents of two induction motors.