Research On Cacaded Multilevel Inverters And The Modulation Methods

Compared with the two-level inverters, multilevel inverters have the following advantages:control flexibly, the phase and amplitude of the output voltage are convenient to regulate and control, low harmonic output, high inverter efficiency, fit to the high-voltage and high-power output, etc. With so many advantages, multilevel inverters can be widely used in these fields:high-voltage and high-power AC motor variable frequency speed regulation, hvdc, power comprehensive governance and so on. Cascaded multilevel inverters have unique advantages such as modular and extended easily in the structure, no need to balance the capacitor voltage, easily to realize soft-switching technology besides the advantages of multilevel inverters. Research on the cascaded multilevel inverters is helpful to find new main circuit topology, and the research on the modulation methods can simplify the control of the cascaded multilevel inverters.This thesis research on the main circuit topology and modulation methods of cascaded multilevel inverters. The main work of this thesis is as follows.(1) The advantages and disadvantages of presently existed main circuit topologies of cascaded multilevel inverters are compared and analyzed, the main circuit topology of the two-H-bridge cascaded multilevel inverters are deeply analyzed, and its mathematical model is established for the next research.(2) The modulation methods fit to cascaded multilevel inverters are researched. The multi-carrier PWM method and reference vector phase-shifted SVM method are analyzed, the principles and characteristics of the two modulation methods are elaborated and compared, pointed out the essential relationship between the two modulation methods.(3) The multi-carrier PWM method and reference vector phase-shifted SVM method are simulated and analyzed by Matlab in this paper. Traditional carrier phase-shifted PWM method is optimized by three new methods:third harmonic injection method, zero sequence component injection method, and the third method which put forward based on the previous two methods named mixed-signal injection method. Simulation verifies the correctness of the three optimization methods. The reference vector phase-shifted SVM method is simulated and the results demonstrate the superiority of the method and the correctness of the theory.(4) In order to solve the problem that the output of cascaded multilevel inverters contain large high harmonic, a LRC filter is designed to clear up the high harmonic. The simulation proved the correctness of this method.(5) The hardware and software design of cascaded multilevel inverter system are simply introduced in this paper. Three-level of three-phase two-H-bridge cascaded inverter system is designed.A large number of simulation waves are given. The harmonics of the optimized multi-carrier PWM methods are compared and analyzed, the results verify the correctness and validity of the theory. The forword study is the foundation to research the control strategy of cascaded multilevel inverters.