Research On The Control Of Three-phase LCL Grid-connected Inverter Under Unbalanced Grid Voltage

Nowadays,the society pays more and more attention to the harmonious development of people and the environment.At the same time,China’s ecological civilization construction has entered a critical period of taking carbon reduction as an important strategic direction and promoting the green transformation of economy and society.To achieve the goal of "double carbon" and strive to build a modern harmonious coexistence between man and nature,new energy power generation technology represented by photovoltaic has been developed rapidly,and three-phase grid connected inverter as a key interface between new energy and public power grid has also been widely concerned and applied.Due to the large-scale application of power electronic devices,the micro grid system for new energy power generation is prone to the actual working conditions such as high harmonic content of the power grid and unbalanced grid voltage.Therefore,the grid side current control technology of three-phase LCL grid connected inverter under unbalanced working conditions is studied in this paper.Firstly,the operation principle and mathematical models in different coordinate systems are analyzed according to the inverter circuit topology;Secondly,the mathematical principle of space vector pulse width modulation technology in grid connected inverter control is deduced.LCL filter has the advantages of better harmonic suppression ability and smaller volume,but its filtering characteristics are also more complex.This paper analyzes its filtering characteristics to lay the foundation for the subsequent controller design.According to the actual control index constraints,the parameters of the inductor capacitor circuit on the AC/DC side of the inverter are designed.Due to the strong coupling of LCL grid connected inverter in d-q coordinate system,the design of traditional current loop controller is often complicated due to its high dependence on the model;At the same time,the ability of traditional control to deal with external disturbances of LCL grid connected inverter is weak,especially in the case of unbalanced grid voltage.This paper proposes to use linear active disturbance rejection controller(LADRC)technology to optimize the design process,so that it not only has the advantages of traditional controllers,but also can play a good role in unbalanced conditions.Firstly,the basic principle of LADRC is introduced.Secondly,the model design based on LCL grid connected inverter is applicable to the second-order LADRC under unbalanced conditions.By introducing LTD,the transition process is arranged for the grid side current reference value of the inverter,so that the grid side current impact is reduced when the system is started,and the system can quickly have no overshoot response;The LSEF design is optimized,and the square difference of current is used as the feedback control quantity to amplify the error,so as to speed up the tracking of the grid connected current of the inverter to the given value,thus improving the system response speed.Based on Bode diagram,the controller parameters and the performance of the whole system are analyzed.In order to optimize the control performance of the system,an improved dual synchronous decoupling phase-locked loop is proposed.The simulation model is built with Matlab/Simulink software for simulation test,and the design ideas of main control algorithms are introduced.Steady state and dynamic simulation tests are carried out for the system under balanced and unbalanced power grid conditions,and important circuit characteristics such as grid connected current sinusoidal degree and grid connected current THD are analyzed.Compared with the traditional method based on proportional integral(PI)control,the proposed control strategy has better current control and harmonic suppression capabilities,and can reduce the use of unnecessary sensors and system costs.Finally,the RT-LAB semi physical verification platform is used for experimental verification and analysis,and the advantages of the control strategy are summarized by comparing with the simulation results.