Study On Control Strategy Of A New Cascade Multilevel Converter With Single DC Source

This thesis mainly focuses on the control strategy of a new cascade multilevel converter with single DC source. In Chapter 1, the status quo of the development of high voltage high power conversion is reviewed. Main multilevel converter topologies and control strategies of multilevel converter are discussed. Their advantages, disadvantages, and applications are compared. In Chapter 2, the character and operating principle of the new topology are discussed. Design of a three phase five-level converter prototype and a digital control scheme with DSP and CPLD are proposed.In Chapter 3, the SPWM method of multilevel converters is investigated in the new topology platform. Some typical SPWM methods are analyzed and compared. The carrier phase-shifted SPWM (CPS-SPWM) method can make voltages of different cells balanced, and it shows good harmonic characteristics. The merits of Carrier phase-shifted PWM methods are proven in experiments. These methods can make use of redundant switching status in order to make capacitors of different cells connect in parallel and to make energy transfer between adjacent cells in different switching status.Carrier phase-shifted PWM methods are further studied. In Chapter 4, the carrier phase-shifted space voltage PWM (CPS-SVPWM) method is analyzed. This method is suitable for the new topology, and it is proven by simulation and experiments.CPS-SVPWM has good performance on voltage-balancing and harmonic characteristics, further more, it has a higher DC voltage utilization ratio. Carrier phase-shifted selective harmonic elimination (CPS-SHEPWM) is studied and verified by simulation and experiments in chapter 5. CPS-SHEPWM is suitable for the application of low frequency switching with good harmonic characteristics.In Chapter 6, two new topologies based this new cascade multilevel topology are proposed. The operating principle and control strategy of these new topologies are researched and proved by simulation.In the last chapter, the research work of this thesis is summarized and the future research tasks are discussed.