The Research And Design Of Single-phase Cascaded H-bridge Based On Independent DC Sources

With the rapid development of industrial technology, the quality of electric energyreally needs to be improved to meet the requirements of electric equipments. Especiallyin the applications in medium and high voltage fields,not only the higher voltage outputbut also the lower distortion is demanded.However, the current semiconductortechnology and manufacturing processes can not make power switching deviceswhich have both high withstand voltage and high switching frequency. Multilevelinverter technology is just proposed to solve this problem.Cascaded multilevel inverteris considered as the most promising kind of multilevel inverters. It receives a greatattention.Cascaded inverter can produce higher voltage output and equivalent switchingfrequency for its characteristics.This thesis focused on a multilevel inverter with several DC sources cascaded anddeveloped the corresponding control strategy according to its chacteristics.At last,asingle-phase cascade inverter system based on this topology was designed.This systemmainly consisted of DC cascade unit, power frequency H-bridge,LC low-pass filter, loadand DSP as the core of the digital control unit.The main content of this thesis are asfollows:1. Main topologies of cascaded multilevel inverters and their control strategi-es,advantanges and disadvantages are discussed in this thesis,especially the asymmetric cascaded inverter. It’s principle and control scheme were analyzed by means of theories and simulation.2. Carrier phase-shifted SPWM’s principle and harmonics was studied by means ofDouble Fourier series. The impact of changes of modulation on output voltage and thereason of the decrease of number of output voltage levels when the modulation is lowwere specially discussed.Besides,this thesis also researched the operation of cascadedinverter using cps-spwm by means of theories and simulation when its DC powersupplies were unbalanced.3. This thesis built a single-phase five-level cascade inverter model based on themultilevel inverter with DC souces cascaded. Its mathematical model based on state-space average method was built and praticable control strateties werediscussed.Then the LC low-pass filter and closed-loop control parameters weredesigned based on automatic control theory. The accuracy and reliability of the controlalgorithm and parameter design were verified by simulation.4. Hardware and software design based on DSP was finished.And an experimentalplat form was obtained. The feasibility of the control algorithm is verified byexperiment.