Separate-Structure UDE-Based Current Resonant Control Strategy on $LCL$-Type Grid-Tied Inverters With Weighted Average Current Method for Improved Injected Current Quality and Robustness

Due to its effective high-frequency suppression ability, the LCL filter has been widely used between inverters and the grid. However, its resonance causes system instability. The weighted-average current (WAC) strategy has been extensively studied to suppress resonance. By selecting a proper weight factor of the inductor and injected current, the order of the LCL can be reduced to first without resonance in the control loop, which can significantly simplify the controller design. Nevertheless, disturbances and uncertainties in the system will affect the control performance of the inverter. In this article, for a WAC-form LCL grid-tied inverter, a separate-structure uncertainty and disturbance estimator (SUDE) inner-loop control strategy with a zero-phase, low-pass, time-delay FIR filter is designed in the discrete domain to eliminate the influence of disturbances and parameter uncertainties on the system, and a proportional resonant controller is adopted as an outer-loop controller. By using the proposed FIR filter, the performance in the rejection of high-frequency harmonics is improved. Moreover, the stability of the proposed two-degree-of-freedom compound controller is analyzed in detail, and the superiority and effectiveness of its robustness to grid impedance and harmonic rejection are shown. Finally, the proposed strategies are validated on a 2-kW experimental platform.