Multilevel converters have attracted a great deal of interest in recent years since they offer a number of advantages in many high voltage and high power applications, such as adjustable speed electric motor drives and power systems through Flexible Alternating Current Transmission Systems (FACTS) controllers and active harmonic filters. They can reach high voltages with low harmonics without the use of transformers or series-connected synchronised switching devices by their unique structures. Along with proper Pulse-Width Modulation (PWM) control scheme, they can also provide lower cost, higher performance, lower Electro-Magnetic Interference (EMI), and higher efficiency than the traditional PWM converters. However, switching losses become a serious issue in high power applications. In order to improve the efficiency and reliability of the system, and reduce the size of the output filter, the stresses on the semiconductors and the development and manufacturing costs, reducing the switching frequency and associated losses of multilevel PWM converters and systems needs to be properly addressed. The thesis gives an overview on multilevel converter topologies and control schemes. It then presents mathematical analysis towards further understanding of the Neutral-Point-Clamped (NPC) and the Flying Capacitor (FC) converters. The Fundamental Frequency Sinusoidal PWM (FF-SPWM) control method is examined as a potential "carrier" based approach in reducing the converter switching frequency and associated losses. The performance of multi-modular parallel connected systems based on the NPC and FC converters as a building block is reported along with the influence of the multicarrier PWM techniques. The voltage-unbalancing problem of the FC converter is addressed and a solution is provided. DSP based controllers for the three-level and the five-level FC converters have been developed and experimentally verified. Results taken from the laboratory prototype are presented to support the theoretical part of the project.
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