Harmonic Interaction Mechanism And Influence Analysis Between Grid-connected Inverter And Grid

With the shortage of fossil energy,renewable energy power generation technology has been widely concerned because of its little harm to the environment and abundant resources.In recent years,distributed generation technology based on wind power generation and photovoltaic power generation has been widely used.Grid-connected inverter is an important energy conversion device.But at the same time,it also brings power quality problems to the grid.The harmonic interaction between inverter and the grid is becoming more and more serious,which leads to harmonic resonance and other problems.Taking three-phase LCL grid-connected inverter as the research object,this paper mainly studies the harmonic interaction between inverter and the grid,the frequency characteristics of inverter in weak current grid,and derives the theoretical equation of harmonic interaction coefficient.The main work includes:(1)Firstly,the mathematical model of three-phase LCL grid-connected inverter is established based on dq coordinate system.Secondly,the amplitude frequency characteristics of inverter in low frequency and high frequency are analyzed by frequency characteristic curve.Aiming at the resonance problem of LCL inverter,passive damping and active damping control methods are used to suppress the resonance problem.Through simulation analysis and comparison,it is found that the active damping scheme can effectively suppress the resonance problem of filter,and will not produce additional power loss.This control strategy lays the foundation for the later modeling.(2)Aiming at the problem of harmonic interaction between inverter and the grid,based on harmonic state space,the mathematical model of three-phase LCL grid-connected inverter under proportional integral control and active damping control is derived.Based on the model,the transfer function matrix of inverter system under harmonic state space theory is solved,The coupling coefficient equation of harmonic interaction between single inverter and the grid is derived.Furthermore,the output impedance model of the inverter is obtained through the harmonic transfer function,and the frequency characteristic curve in the case of weak current network is analyzed.The accuracy of the mathematical model is verified by simulation and hardware in the loop experiment.(3)At the same time,in order to study the harmonic interaction mechanism in the multi grid-connected inverter scenario,based on the single HSS inverter model,the mathematical model of multi grid-connected inverter is established,and the theoretical equation of harmonic coupling coefficient of AC / DC side in the multi grid-connected inverter scenario is derived.The accuracy of the mathematical model is verified by simulation of different cases and hardware in the loop experiments.The model established by harmonic state space method in this paper can better reflect the interaction between inverter and grid harmonics,and obtain the unstable mode of the system.