Resonance damping for an LCL filter type grid-connected inverter with active disturbance rejection control under grid impedance uncertainty

Higher-order filters, like LCL, are more commonly employed in grid-connected inverters (GCIs) as an interface element for the better attenuation of switching harmonics. However, the LCL filter has resonance poles which may degrade the performance of control without damping. Among the different damping techniques, notch filter (less expensive) and derivative filtered capacitor voltage (insensitive to parameter variation) based damping is recommended where the resonance pole is canceled with zero. However, notch filter requires prior information about grid impedance and remain sensitive to parameter variations. On other hand, derivative filtered capacitor voltage based active damping may not achieve higher stability margin depending on the filter parameters. In this paper, reduced-order extended state observer based linear active disturbance rejection control is introduced to the LCL filter type GCI. With the proposed method, the resonance poles in the LCL filter are compensated via pole-zero cancelation and it remains effective under grid impedance, and filter parameters uncertainties without control tuning. In addition, it is found that the proposed method achieves higher stability margins, dynamic performance, simple, less reliant on the system modeling, and highly decoupled current control. The stability and performance of the proposed method are compared with the single-loop PI control without/with notch filter based damping and dual loop with capacitor voltage based active damping under filter parameters and grid impedance variations via simulation and experimental results.