Research On Flexible Grid-connected Inverter And On-line Efficiency Optimization

Nowadays, the renewable energy has attracted increasingly attention all aroundthe world. Considering the features of wide-distribution, unstability and sensitivityto the enrironment, the grid-connected inverter for renewable energy should havebetter performance. One hand, the renewable energy should be converted to theelectric energy meeting the standard of power grid, meanwhile the DC power rangeshould be broaden and the efficiency should be improved. On the other hand, theinverter should be controlled properly to improve power quality and reduce theharmonic and reactive power pollution. Therefore, the research on topology, controlstrategy, efficiency and harmonic analysis has important theoretical and practicalsignificance.In view of problems and features of renewable energy above, this thesisproposed a novel flexible inverter topology, in order to abroad the DC power range,improve the inverter system efficiency and guarantee the electric energy meet thepower grid standard. Based on the proposed inverter topology, the thesis focused onthe analysis of the performance of the flexible inverter grid-connected system,mainly including operation principle, control strategy, the efficiency and currentTHD model.Firstly, the basic operation principle of flexible inverter was analyzed. Theinverter could operate in two modes: cascaded three-level mode and two-level mode,then the operation performance and SPWM control strategy of inverter operating inboth modes are analyzed respectively. Further more, the Carrier-MultiplexingSPWM strategy for flexible inverter was proposed.The loss (power device loss and filter inductor loss) model of inverter operatingin two modes was built and compared in condition of DC voltage variation. Then themodel of current THD in relation with the DC voltage and inverter parameters wasbuilt. Based on loss and current THD models, the on-line topology-switchingefficiency optimization technique which meets the harmonic constraintsrequirements was studied.The principle of grid-connected inverter current loop control strategy wasanalyzed, and the grid-connected inverter system control proposal was designed.Based on the proposal above, the MATLAB model was built, to verify the feasibilityof system performance and control strategy.The proposed flexible grid-connected inverter system platform based on DSPand FPGA was built and the implementation proposal based on DSP+FPGA wasdesigned, to verify the basic principle, carrier-multiplexing SPWM strategy, loss and current THD model, and efficiency optimization proposal of the proposed flexibleinverter. The experiment results verify the feasibility and validity of proposedflexible grid-connected inverter and control strategy.