Research On Grid Impedance Detection Of Grid Connection Inverter In Distributed Generation System

With the large-scale integration of distributed generation systems such as photo voltaics and wind power generation,the penetration rate of new energy generation grids has gradually increased.Since most of the distributed power generation system installation sites are located in remote areas,long transmission lines and a large number of transformer devices cause the impedance of the point of common coupling to be not negligible.At this time,the power grid presents weak grid characteristics.Under weak grid conditions,dynamic interaction between the inverter and the grid will occur.At this time,the influence of the grid impedance on the control performance of the inverter's grid connection stability cannot be ignored.Therefore,obtaining grid impedance information quickly and accurately is the basis for analyzing and studying the dynamic interaction between inverters and grids,and improving the stability of grid-connected inverters.This paper-first studies the grid-connected inverter mathematical model in the case of weak grid,and analyzes the influence of the grid impedance on the dynamic performance of the grid-connected inverter and the control performance of the grid-connected inverter.Then,the basic principle of grid impedance detection is introduced in this paper.On the basis of the existing literature research,the disturbance injection method and disturbance extraction method of grid impedance detection are analyzed and studied.The disturbance injection method mainly studied the single harmonic injection method and the wide pulse injection method,and compared the advantages and disadvantages of the two injection methods and the scope of application.Harmonic disturbance extraction is mainly studied discrete fourier transform(DFT)extraction method and complex coefficient filter(CCF)extraction method.Aiming at the problem of large inherent delays in DFT calculations,an SDFT-based grid impedance detection method was proposed,which effectively shortened the program running time and reduced the amount of DSP computation.Aiming at the problem that the detection accuracy of the grid impedance detection method based on the single modular coefficient filter(SCCF)is degraded when the harmonic content in the power grid is large,a multiple complex coefficient filter(MCCF)is proposed.Thus,the accuracy of grid impedance detection can be guaranteed under unbalanced and harmonic grids.Finally,this paper compares and analyzes the grid impedance detection based on SDFT extraction and MCCF extraction from the aspects of extraction accuracy and dynamic performance.Through Matlab/Simulink simulation analysis and experimental platform research,the verification of the influence of the grid impedance on the grid-connected inverter stability and the validity and practicality of the proposed grid impedance detection method are verified.