Research On Resonance Suppression Strategy Of LCL Grid-connected Inverter In Weak Grid

In recent years,with the increasing shortage of traditional energy,the importance of new clean energy such as photovoltaics has become increasingly prominent.With the continuous advancement of high frequency power electronic devices and digital processing signal technology,digital control has been widely used in LCL grid-connected inverters.However,the inherent delay of digital control will have a negative impact on the safe and stable operation of photovoltaic grid-connected inverters in weak grids.In addition,the LCL filter,as a third-order underdamped system,exhibits inherent resonance peaks that need to be effectively damped.The above two problems bring great challenges to the resonance suppression of LCL gridconnected inverters.Based on this,this paper proposes two improvement strategies from the perspectives of optimizing the damping control structure and adding a delay compensation link.Starting from the optimization of the damping control structure,the paper first analyzes the mathematical model of the grid-connected inverter under the traditional capacitive current active damping control,and designs the control parameters to stabilize the system without considering the grid impedance.The cause of resonance is analyzed in detail,and the analysis shows that: due to the digital control delay,the equivalent virtual impedance will have negative damping in a certain frequency range,resulting in a non-minimum phase system.Based on the above analysis,the paper proposes a new active damping strategy with positive feedback of capacitor voltage,which is optimized by bilinear proportional active damping.That is,a positive feedback linear damping link is connected in parallel at both ends of the capacitor current and the capacitor voltage.Under this control strategy,the positive damping range is extended to almost the entire Nyquist frequency,ensuring high robustness against a wide range of grid impedance changes.Secondly,from the perspective of delay compensation,the paper takes the equivalent virtual impedance of the system as the research object,deduces the basic compensation principle of extending the effective damping area of the system in the Nyquist frequency range,and proposes a method based on negative proportionality.-The lead delay compensation strategy of the integral link,that is,adding a negative proportional-integral lead compensator in the damping link to offset the negative impact of the digital delay.The analysis results show that this strategy can theoretically extend the positive resistance range to the entire Nyquist frequency,and can still make the control system have sufficient stability margin when the grid impedance changes in a wide range,and improve the grid-connected inverter’s performance.robustness.Finally,the paper uses MATLAB/Simulink software for simulation verification,and uses real-time digital controller RTU-BOX204 control platform for experimental verification.The simulation and experimental results correlate with the theoretical derivation,which verifies the correctness and effectiveness of the novel capacitor-voltage positive feedback active damping strategy and the lead-delay compensation strategy of the negative proportional-integral link proposed in this paper.This article contains 66 figures,5 tables,and 77 references.