Study On Harmonic Interaction And Suppression Of Multi-Inverter Grid-Connected System

With the rapid development of distributed power generation technology,a large number of inverters have been incorporated into the microgrid,increasing the penetration rate of renewable energy in the power system.The existence of distributed series inductance along the line may cause the amplification or increase in the number of resonance peaks of the system,and the voltage and current will be distorted.In addition,when multiple inverters are operated in parallel on a weak grid,due to the coupling effect of grid impedance,there are harmonic interaction phenomena between the inverter and the grid,and between the inverter and the inverter.Triggering system resonance,which threatens the stability of the system.Therefore,studying the harmonic interaction mechanism and suppression strategies of multi-inverter grid-connected systems under weak grids is of great significance to realize the safe and stable operation of the system.This article first builds the mathematical model of a single LCL grid-connected inverter in different coordinate systems,and analyzes the pros and cons of their respective control methods.In the αβ coordinate system,a single inverter Norton circuit model considering the digital control delay is established,and it is extended to the equivalent model of a multi-inverter grid-connected system,and then the stability criterion of the system is derived.Secondly,for the inherent resonance of the LCL filter,this article adopts the control strategy of grid-side current and capacitor current feedback to suppress it.The influence of the delay introduced by the digital control system on the active damping of capacitive current feedback is analyzed,combined with the Nyquist stability criterion,the stability constraint conditions of the system when the resonance frequency of the LCL filter is in different frequency bands are discussed,and give the system parameter design method to realize the stable operation of a single inverter.Thirdly,the equivalent model of the multi-inverter grid-connected system is analyzed,and the prerequisite for the harmonic interaction of the system is that the system has a harmonic source and an impedance resonance network.In order to suppress the harmonic interaction between the inverter and the grid,this paper adopts a multi-resonance control strategy with phase compensation to increase the amplitude of the inverter output impedance at a specific frequency,thereby suppressing the loworder harmonics of the grid-connected current,and this strategy can improve the robustness of the system to the grid impedance;In order to suppress the harmonic interaction between parallel inverters,a control strategy of adding a lag compensator in the capacitive current feedback link is adopted to increase the phase of the inverter output impedance,thereby suppressing the harmonic current between the parallel inverters.Finally,this article builds an experimental platform based on the parameters of the grid-connected inverter in the theoretical part,the hardware design and software design of the experimental platform are introduced in detail.On the basis of the experimental platform,the effectiveness of the harmonic interaction suppression strategy adopted in this paper is verified.