Research On Resonance Damping Method For The LCL-Filtered Three-Level Grid-Connected Inverter In Weak Grid

The LCL-filtered neutral point clamped(NPC)three-level grid-connected inverter has better harmonic suppression and higher power conversion efficiency,which is widely used in distributed power generation systems based on renewable energies.However,the varation of grid impedances has a innegligible impact on the resonance dampling performance and stability of the system in weak grid.In this paper,the active damping(AD)with grid current feedback is studied in weak grid.Furthermore,an improved AD method is proposed based on phase-lead compensation,which can ensure the inverter operate stably against grid impedance varation and improve the grid currents quality.The mathematical model of the LCL-filtered grid-connected inverter is established,in which active damping and closed-loop control are achevived with grid currents feedback only.Besides,this paper analyes the influence of control parameters on the resonance damping performance,and the parameters are further optimized.Based on the impedance characteristic of the AD loop considering the digital control delay,the reasons for the failure of the active damping is analyzed in weak grid.An improved active damping method with phase-lead compensation is proposed,which reduces the influence of the digital control delay on the positive damping interval and extends the positive damping interval of the system.By applying this methoed,the inverter can maintain the effectiveness of resonance damping and good stability in weak grid.An experimental prototype of the LCL-filtered NPC three-level grid-connected inverter is built to verify the effectiveness of the method proposed in this paper.Simulation and experimental results show that the positive damping interval of the system has been extended,which can ensure the inverter operate stably against grid impedance varation and improve the grid currents quality.