Research On Grid Connected Control Of New Quasi-Z Source Inverter In Weak Grid

With the rapid depletion of non-renewable energy sources,distributed power generation technologies that utilize renewable energy sources such as solar energy and wind energy have received widespread attention.Due to the remote location of the distributed power generation system,the long line and the multi-level transformer device connecting it to the grid introduce a non-negligible grid impedance to the grid.The grid impedance makes the grid appear weak grid characteristics.In order to meet the weak grid connection requirements,the traditional inverter needs to add an additional booster circuit and insert the switching signal into the dead time.These factors further complicate the structure and control strategy of the grid-connected system and seriously affect the stable,safe and efficient operation of the system.The Quasi-Z source inverter overcomes the defects of traditional inverters due to its impedance network characteristics,and is gradually applied to distributed generation systems.However,the Quasi-Z source inverter itself also has shortcomings such as limited boosting capacity,large capacitor voltage stress and large inductor current ripple.Therefore,this thesis focuses on the improvement of the topology of the Quasi-Z source inverter and its grid-connected control strategy in weak grid.The main research contents are as follows:(1)Two new switched-type Quasi-Z source inverter topologies are proposed.The switch tube embedded structure is used to change the charging and discharging circuit of the energy storage element during operation.The new topologies has high boosting capacity,low capacitance voltage stress and inductor current ripple.The working principles of the two new Quasi-Z source inverters are analyzed in detail,and compared with the performance of traditional Quasi-Z source and switched-inductor quasi-Z source inverters.A new Switch type I Quasi-Z source inverter quasi-Z source inverter with superior performance is selected as the grid-connected inverter for grid-connected control research.(2)The transient and filtering characteristics of the grid-connected system are determined by establishing and analyzing the mathematical model of the new Quasi-Z source grid-connected inverter based on LCL filter in ideal grid.The capacitive current feedback coefficient and the quasi-PR controller control scheme are used to suppress the resonance peak of the LCL filter in the stationary coordinate.The integrated tuning steps of the capacitor current feedback coefficient and the quasi-PR controller parameters are given at the same time.This method improves the dynamic performance of the grid-connected system and the quality of the grid-connected current.(3)A current adaptive control method based on grid impedance matching is proposed in the weak grid.The adverse effects of the grid impedance on the stability and dynamic performance of the grid-connected system in the weak grid are analyzed in detail.Utilize the integrated parameter setting method in the ideal grid and combine the grid impedance online detection technology to update the grid-connected control parameters in real time.This method realizes the current adaptive control of the grid impedance matching and improves the robustness of the grid-connected system to the grid impedance.The current adaptive control strategy under weak power grid is simulated to verify the correctness of theoretical analysis in MATLAB / Simulink.This thesis builds an experimental platform of 3kW new Quasi-Z source gridconnected inverter for experimental research after completing the above theoretical analysis and simulation.The experimental results verify the effectiveness and superiority of the new switched-type Quasi-Z source inverter topology and its gridconnected control strategy in ideal and weak grids.