Multilevel Inverter PWM Methods And Relative Techniques

Recently, with the rapid development in the electrotechnics fields, the equipments of power electronics are expected to handle the higher ratings of voltage and capacity, so multilevel inverters have drawn the tremendous interests under this background. The PWM methods of multilevel inverters are systemically researched to find their relationship and the law in this dissertation.The thesis presents the idea of control degrees of freedom combination. The Carrier-Overlapping PWM method is presented, which utilizes vertical offsets among carriers of the control freedom degree (CFD). This method shows better harmonic characteristics of output voltage than Subharmonic PWM method under low modulation index region. The Carrier-Overlapping-SFO PWM method is presented in the thesis, which utilizes vertical offsets among carriers of the CFD and injecting zero sequence signals to the reference wave of the CFD. This method has advantages of the Switching Frequency Optimal PWM method (SFOPWM) and the Carrier-Overlapping PWM method: under low modulation index, this method can show better characteristics of the output voltage; and the DC voltage utilization is improved.In order to improve the harmonic performance of output voltage under low modulation index region, a novel carrier-based PWM method for flying capacitor multilevel inverter is presented in the thesis, and the flying capacitors can keep balance in Ts under this PWM method. The relationship between the carrier-shifted PWM and the space vector PWM in flying capacitor multilevel inverters is established, and according to the relationship, the switching loss minimized PWM for flying capacitor multilevel inverters is presented, the switching loss can be greatly reduced and efficiency of the system can be improved.Three kinds of novel PWM methods are researched theoretically in the thesis, which utilizes vertical offsets among carriers of the CFD and horizontal offsets among carriers of the CFD. Harmonic components of the COPWM-A, the COPWM-B and the COPWM-C can be calculated with the double Fourier analysis, the detailed analysis and calculation steps are given in the thesis. According to the calculation results, analytical spectrum can be obtained, and the harmonic characteristics of the COPWM-A, the COPWM-B and the COPWM-C are given in the thesis. Through the comparison among the COPWM-A, the COPWM-B and the COPWM-C method, the COPWM-A method shows the best harmonic characteristics. According to the double summation Fourier series of three kinds of PWM methods, the relationship among the harmonic performance, vertical offsets and modulation index is obtained in the thesis. The advantage of the above Carrier-Overlapping PWM method can be proved again by the theory results.With the topology of flying capacitor multilevel inverters, the multilevel PFC isresearched in the thesis. The topology, operation modes and control method of the four level PFC converter are analyzed in the thesis. In the four level PFC converter, voltage stresses of switches and diodes is one third of that in the conventional boost PFC converter, therefore, the conduction loss and switching loss are reduced, and the efficiency of system is improved. With the decrease of voltage stresses of diodes, the reverse recovery performance of diodes is improved. In the four-level PFC converter, the boost inductance is one ninth of that in conventional PFC converter. The presented four-level PFC can be operated in high voltage applications with low rated switches and diodes.The work in the thesis provides the theoretical basis and technical basis for the optimal design of multilevel inverter PWM methods.