Second-Order Repetitive ADRC with Frequency Robustness and Harmonic Suppression for Single-Phase Grid-Connected Inverters

Conventional active disturbance rejection control (ADRC) schemes for grid-connected inverters (GCIs) suffer from performance degradation under grid frequency fluctuations and voltage distortions, primarily due to the limited bandwidth of conventional extended state observers (ESOs) and their reliance on fixed-frequency internal models. To address these issues, this paper proposes a second-order repetitive ADRC (SRC-ADRC) strategy for LCL-type single-phase GCIs. The proposed method integrates a second-order repetitive extended state observer (SRC-ESO) and a frequency-adaptive proportional-quasi resonant (FPQ) controller. By cascading an additional internal model into the repetitive control-based ESO, the SRC-ESO achieves higher internal model gain and wider resonant bandwidth, significantly improving periodic disturbance estimation accuracy under off-nominal grid frequencies. The FPQ controller ensures reference current tracking without phase error and amplitude attenuation, while the control law and observer are decoupled for independent parameter design. The robustness and frequency adaptability of the SRC-ADRC are verified through theoretical stability analysis and frequency-domain evaluation. Experiments on a 1.5 kW LCL-type single-phase GCI platform show that SRC-ADRC provides better frequency robustness, stronger harmonic mitigation, and improved current-tracking performance than the comparison methods.