Study On Three-Level Inverter Digital Control

During recent years, multilevel converters, which have drawn tremendous interests as a high voltage high power conversion technology, have been widely used in medium to high voltage adjustable speed drives and Flexible AC Transmission Systems (FACTS) etc.The status quo of the development of the high voltage high power conversion technology is summarized in chapter 1 and two interrelating technology developments - power switching devices and digital control technology, are introduced. In chapter 2, the topologies of multilevel converters and control schemes are summarized. Moreover, their advantages, disadvantages and application fields are also analyzed and compared.In chapter 3, the development of a universal DSP digital control platform is discussed, which is the base of implementing digital control for the developed three-level NPC inverter in our lab.In chapter 4, the principle of carrier-based PWM control schemes for the three-level NPC inverter is discussed. The advantages and disadvantages of different carrier-based PWM methods are analyzed and compared. The problem of neutral-point voltage unbalance and the methods to solve this problem are discussed. A novel carrier-based PWM method utilizing control freedom degree (CFD) is proposed. The principle, simulation and DSP implementation of the method are discussed in detail. Compared with the traditional carrier-based PWM method, the novel method shows better harmonic characteristic under low modulation indices, while under a high modulation index, the harmonic characteristic of the novel method is almost the same as that of the traditional method. It also shows that the new PWM method has a higher dc-link voltage utilization ratio and better neutral-point balance capability. Simulation and experimental results verify the effectiveness of the proposed method.In chapter 5, the principle of SVPWM control scheme for the three-level NPC inverter is discussed. An optimized switching sequence is discussed. Using thissequence, the change of switching status firmly conforms to the principles of switching sequence assignment in SVPWM during the process of voltage vector rotation. The influence of the voltage vectors on neutral-point voltage is discussed. Using the information of dc-link voltages and load currents, the neutral-point voltage is balanced by introducing a time-control coefficient. The simulation and DSP implementation of SVPWM are discussed. The validity of the proposed method is verified by simulation and experimental results.In the last chapter, the research work of this thesis is summarized and the future research tasks are given.