Digital Control Technique Research Of Mutilevel Inverter Based On Carrier Phase Shift PWM

With the continuous development of power electronic device and high-speed micro controller, power electronic technology is used more and more widely in the high power applications, and the researches of topology structure and control strategy of high power converter are also increasingly in-depth. In order to solve the inherent contradiction between power handling capability and switching frequency of switching devices, the carrier phase shift pulse width modulation(CPS-PWM) arises at the historic moment as a new type of multilevel modulation strategy.Although the traditional CPS-PWM technique has good harmonic characteristics, it has disadvantage of low utilization ratio of DC voltage. To solve this problem, the carrier phase shifted third harmonic injection PWM(THI-PWM) and carrier phase shift trapezoidal PWM(CPS-TPWM) are proposed as two kinds of improved modulation strategy in this thesis. At the same time, in order to improve the real-time performance and precision of the control system of multilevel inverter, lots of researches have been carried out on the digital realization of the CPS-PWM modulation strategy.The main research contents include the following aspects:(1) The working principles of cascaded H-bridge multilevel inverter and carrier phase shift sinusoidal PWM(CPS-SPWM) strategy are inquired based on the analysis of common multilevel inverter topologies and modulation strategies. The essence of improving equivalent switch frequency by offsetting the lower switching harmonics is derived by double fourier transform from the mathematical view, and a three-phase five-level inverter model is built in Matlab/Simulink, reasearches are carried out on the aspects of its output multilevel voltage waveform and harmonic characteristic.(2) For the issue of the low utilization ratio of DC voltage, the carrier phase shift THI-PWM and CPS-TPWM are proposed as improved modulation strategies. The concepts of waveform factor and triangulation rate are introduced to analysis the harmonic characteristic and utilization ratio of DC voltage of these two modulation strategies mathematically. A three-phase nine-level inverter model is built in Matlab/Simulink, and the conclusion that the CPS-TPWM modulation strategy has the best overall performance is made through the comparation of a series of simulations.(3) A digital control system of the cascade five-level inverter is designed with digital signal processor(DSP) and field programmable gate array(FPGA) as the control core, the sampling calculation and control are completed by DSP using the irregular symmetrical sampling method, and FPGA is responsible for the generation of multi-channel PWM trigger pulses. The software design of DSP and FPGA is carried out respectively, and the carrier phase shift module and PWM comparison output module are mainly constructed. The simulation results in QuartusII show that the design functions of each module and its sub module in the digital control system have been effectively acquired, and the digital realization of CPS-PWM modulation strategy is achieved successfully.