Research On π Type AC/AC Converter With Variable Structure Control Strategy

This thesis researches on aπtype AC/AC converter and its variable structure control strategy. The converter comprises of a Buck type AC/AC unit and a Boost type AC/AC unit, and reduces a LC filter comparing with cascaded Buck-Boost type AC/AC converter. It works in Buck mode, Boost mode or Filter mode under the variable structure control strategy with dual carrier and dual output reference, according to working conditions of the front and back stage. It employs two stage duty-ratio modulations, but there is in fact no more than one stage power conversion. Then it has the advantages of easy control and high efficiency. According to the theoretical analysis and parameter calculation, an experimental prototype was designed and the experimental waveforms and datas were given in the thesis.The circuit structure of Buck type AC/AC converter was introduced at first, and analysis of its operating principle and control method are given in details. According to the input voltage and inductor current polarity, Buck type AC/AC converter works in three different switch patterns (power transmission pattern, freewheeling pattern and dead-time pattern) in all four sessions of one input voltage cycle. Simulation and experimental results were given correspondingly. Then Boost Type AC/AC converter was analyzed in the same way.A novel single-phaseπtype AC/AC converter was obtained by optimizing off a LC filter from cascaded Buck-Boost type AC/AC converter, which comprises of a single-phase Buck type AC/AC converter and a single-phase Boost type AC/AC converter. Three different implementation of variable structure control strategy with dual-carrier modulation dual output reference was proposed for its different voltage regulating applications.An experimental prototype of single-phaseπ-type AC/AC converter was designed according to the key design parameters determined by theoretical calculation and simulation testing. Principle experiments were carried out, and the results verify the feasibility of control strategy and the correction of theoretical analysis.Then, the circuit structure and working principle of three-phase Buck type and Boost Type AC/AC converter were introduced, and their single-phase feedback and three-phase feedback control implementations were analyzed in details. Related simulation and experiment were carried out respectively. On this basis, three-phaseπType AC/AC converter comprises of a three-phase Buck type AC/AC unit and a three-phase Boost type AC/AC unit. Its working principle was introduced, and then simulation results were given correspondingly.Theoretical analysis and experimental results verify the correction of the theory and the feasibility of theπtype AC/AC converter. The converter can be used for wide range voltage regulating, with the advantages of simple circuit structure, easy control method, and high efficiency.