Research On Current Adaptive Control Method Based On LCL Grid-connected Inverter In Weak Grid

With the gradual depletion of fossil energy and the increasing pollution to the environment,renewable energy such as solar energy and wind energy has attracted wide attention.Regions with abundant renewable energy are mostly concentrated in remote and less populated areas.It requires multi-stage voltage converters and long-distance transmission lines to connect to the large power grid,thus introducing variable grid impedance to form a weak grid.In weak grid,on the one hand,the wide-range time-varying impedance of grid changes the performance of the original control object,on the other hand,the low-frequency background harmonics deteriorate the quality of grid-connected current.If the control mode under strong grid is continued,the stability and dynamic performance of grid-connected system may deteriorate or even fail to meet the grid-connected standard,which seriously affects the safe and stable operation of grid-connected system.This thesis takes the LCL-based grid-connected inverters system in weak grid as the research object,adopts the topology structure of double closed-loop control of grid-connected current and capacitive current,and studies the characteristics of weak grid in the following aspects: Firstly,the mathematical model of grid-connected inverters system based on LCL in weak grid is established,and the influence of grid resistance and inductance on system stability and harmonic suppression ability is analyzed by using control variable method.Secondly,in order to obtain real-time impedance information of grid,several commonly used impedance detection technologies are compared and analyzed.Finally,the dual harmonic injection method is selected,and the accuracy of the method is verified by simulation.Thirdly,according to the characteristics of the weak grid and the impedance detection technology of the grid,the adaptive control method under the weak grid is proposed,and the corresponding controller parameter design method and the implementation steps of the adaptive method are given.In order to restrain the influence of grid background harmonics on grid-connected current quality,several harmonic compensators are added on the basis of the adaptive control method proposed in this thesis,which not only avoids the influence of positive feedback loop introduced by grid impedance on system stability,but also realizes the suppression of low-frequency background harmonics in weak grid,and further improves the grid-connected current quality.Finally,the proposed control method is simulated by using MATLAB/Simulink simulation software,which verifies the correctness of the theoretical analysis.After the above theoretical analysis and simulation,this thesis builds a 3 kW single-phase grid-connected inverter experimental prototype,and introduces its hardware composition and software flow.By comparing and analyzing the experimental waveforms and data,it is proved that the adaptive control method proposed in this thesis can adapt to the wide range variation of grid impedance under weak grids and effectively suppress the background harmonics of grids,so that the grid-connected inverter system based on LCL can not only achieve good stability and dynamic performance,but also achieve high-quality grid-connected current.