Research On Control Strategy Of LCL Photovoltaic Grid-connected Inverter

Energy has become the material basis and the driving force for the rapid development of the world economy.With the worldwide environmental problems and energy crisis,all countries in the world are striving to develop renewable energy.Solar energy has been widely recognized by the countries of the world for its green environmental protection and abundant resources,and it is the world fastest growing renewable energy source.Photovoltaic power generation is one of the most convenient ways to use solar energy.Grid-connected inverter,as the core of photovoltaic power generation systems,has become a hot topic of current research.In this paper,the three-phase LCL grid-connected inverter is taken as the main research object,and the mathematical model is established to analyze the mechanism of the resonance peak in the LCL grid-connected inverter.The resonant peak is restrained from the two aspects of passive damping control and the active damping control.Based on this,the grid-connected control technology of LCL grid-connected inverter is studied.The major work of this paper includes:(1)In this paper,the modeling of photovoltaic grid connected power generation system is discussed.The mathematical model of photovoltaic cells in normal working state is established,and the UI-? UP-output curves of photovoltaic cells under different temperature and illumination are obtained by Matlab /Simulink simulation;The mathematical models of LCL grid-connected inverter in three different coordinate systems are analyzed.The different filtering characteristics between LCL grid-connected inverter and L,LC grid-connected inverter are compared by the amplitude-frequency characteristic curve.The results show that the high-frequency harmonic suppression capability of the LCL grid-connected inverter is stronger,and it is more suitable for medium and large-scale photovoltaic grid-connected systems.(2)The related parameters of LCL grid-connected inverter are set in this paper.Aiming at the problem of resonant peak in grid-connected inverter,and to avoid the extra power loss caused by passive damping control,an active damping control method based on capacitive current feedback is proposed in this paper.The damping effect is better and has more practical value.(3)According to the working characteristics of LCL grid-connected inverter,a double-closed loop current control strategy based on quasi proportional resonant and repetitive control is proposed.The current inner loop selects capacitive current feedback control to suppress the resonant peak of grid-connected inverter.The current outer loop adopts repetitive quasi-PR control to suppress the influence of voltage fluctuation and nonlinear load on the grid-connected current.Based on the platform,the effect of the above-mentioned grid-connected strategy is verified.(4)Considering the problem that the electric energy from photovoltaic power plants in remote areas can only reach the load through long distance transportation,the method of combining the pole assignment with the current double-closed-loop control with repetitive quasi-PR control is proposed.This compound control strategy is used to suppress the influence of grid impedance on grid-connected current during long distance transmission.The simulation results show that the composite control strategy can improve the system stability and optimize the quality of grid-connected current waveforms.