Research On Robust Current Control Of Photovoltaic Grid Connected Inverter Based On LCL Filter In Weak Grid Environment

In recent years,with the demand for green,efficient and low-carbon energy,the distributed new energy grid generation industry has made great progress.Due to grid connected inverter is the necessary interface between distributed generation and large power grid,the research on the stability and control method of grid connected inverter has attracted more and more attention.Considering that the actual geographical distribution of distributed new energy,such as photovoltaic and wind power,often deviates from the load center,in this case,the leakage reactance of long-distance transmission lines and transformers will make the unity grid appear the characteristics of weak grid.In the weak grid,the time-varying characteristics of the grid impedance will affect the quality of the grid connected current injected into the grid by the grid connected inverter,which also puts forward higher requirements for the control method of the grid connected inverter in the weak grid.In order to meet the high requirements of actual grid connected systems for grid connected current quality,modern control algorithms based on sliding mode variable structure theory are widely used in grid connected inverter systems.This thesis takes the single-phase grid connected inverter system based on LCL filter as the research object.In order to improve the quality of grid connected current output by the inverter and enhance the adaptability of the system to the perturbation of filter parameters and the change of grid impedance,two grid connected current control strategies are proposed.The two control strategies are a novel compound sliding mode current control(CSMCC)strategy based on sliding mode observer(SMO)with embedded active damping method and a super twisting sliding mode control(STSMC)strategy with active damping method based on super twisting algorithm.The specific research contents of this thesis are as follows:(1)The filtering characteristics and structures of common grid connected filters are compared.After synthesizing various factors,LCL filter is selected as the device to filter the switching frequency on the output side of the inverter,and the detailed design process of LCL filter parameters is given.In order to suppress the adverse effect of the inherent resonance peak of LCL filter on the grid connected current quality,the advantages and disadvantages of passive and active damping methods are analyzed,and the mechanism of improving the weighted average current(WAC)active damping method considering the grid voltage feedforward is studied.And it is introduced into the improvement of grid connected current quality of the whole system.(2)For the single-phase grid connected inverter system under weak grid,a novel SMO-based CSMCC strategy with active damping is proposed in this thesis.Firstly,in order to estimate the inverter side current and capacitor voltage of LCL filter,an SMO with adaptive switching function term is designed.The designed SMO can reduce the number of sensors and increase the reliability of the system.Compared with the conventional SMO,the designed SMO can weaken the chattering.Then,based on the theory of sliding mode variable structure,a novel CSMCC strategy is proposed to enhance the robustness to parameter changes.Finally,active damping based on WAC is used to provide feedback current for CSMCC.In addition,the stability of SMO and CSMCC are analyzed respectively.The results of Matlab simulation platform and Star Sim real-time simulation platform show that the proposed SMO-based CSMCC strategy with active damping can make the single-phase grid connected inverter system in weak grid have satisfactory dynamic and steady-state performance and strong robustness.(3)Due to discontinuous switching control,SMC method has chattering.In order to weaken this problem while maintaining strong robustness,another simple and efficient grid connected current control strategy is proposed in this thesis.Firstly,the mathematical model of single-phase LCL grid connected inverter with damping resistance based on WAC damping method is established.Then,the STSMC strategy is designed by introducing super twisting(ST)algorithm.Finally,the designed STSMC strategy and PR strategy are compared in the Matlab simulation platform to verify the effectiveness of STSMC strategy.