Research On Resonance Suppression Strategy Of LCL Grid-connected Inverter Considering Transmission Line

In new energy grid-connected systems,inverters often need to be connected to the grid via transmission lines.At this time,the power grid will show the characteristics of a weak grid.The characteristic of a weak grid is to consider the impact of grid impedance and background harmonics on the grid side.This article analyzes two situations: the inverter is connected to the grid via the inductive grid impedance,and connected to the grid via RLC-type transmission line.Both situations will cause resonance problems in the inverter output current,and the background harmonics on the grid side will also act as an excitation source to aggravate the harmonic resonance phenomenon,thereby worsening the grid-connected current and degrading the power quality of the system.Firstly,this paper analyzes the frequency characteristics of the L-type and LCL-type inverters,and illustrates the advantages and disadvantages of the LCL-type grid-connected inverter.Aiming at the problem of resonance spikes in the frequency characteristics of the LCL filter,detailed analysis of several suppression strategies,and the simulation verifies the effectiveness of the active damping strategy in suppressing resonance spikes.Secondly,analyze the impedance interaction between the inverter and the grid by establishing the Norton equivalent model of the grid-connected system,and use the Bode diagram to analyze the single resonance problem caused by the interaction between the inverter output impedance and the grid impedance when the grid impedance is purely inductive.And the multi-resonance problem caused by the interaction with the RLC structure transmission line impedance.Finally,not only the existence of resonance phenomenon is proved through simulation analysis,but also the background harmonics are added to the grid side to prove that the harmonic source near the resonance point will stimulate the harmonic components in the grid-connected current.This will seriously affect the quality of grid-connected current.Thirdly,for the two kinds of resonance problems analyzed,this paper designs control strategies to suppress them from two optimization angles.Aiming at the single resonance problem when the inverter output impedance interacts with the inductive grid impedance,a phase compensation link is connected after the current controller to improve the phase angle margin of the inverter output impedance,so as to avoid the appearance of a single resonance point.Aiming at the multi-resonance problem caused by the interaction between the inverter output impedance and the RLC structure transmission line impedance,an improved grid voltage feedforward control strategy is proposed,which can avoid the interaction with the inverter by greatly increasing the amplitude of the inverter’s equivalent output impedance to avoid the resonance interaction point between it and the transmission line impedance,the occurrence of multi-resonance is suppressed.The background harmonics on the power grid side can be suppressed,and the influence of the background harmonics on the quality of the grid current can be solved from the source.Simulation analysis verifies the effectiveness of the two resonance suppression strategies.Finally,a τ-shaped transmission line model and an inverter platform based on DSPTMS28335 control were built to verify the feasibility of the two resonance suppression strategies.