Design and implementation of Quasi-proportional-resonant controller of LCL filter inverter with grid current feedback active damping in weak grid to enhance stability

Abstract

This paper proposes a systematic design and tuning methodology for a Quasi-Proportional-Resonant (Q-PR) controller combined with grid-current-feedback active damping (GCF-AD) for grid-connected inverters equipped with LCL filters. Unlike previous works, the proposed approach explicitly considers digital sampling delays and provides clear stability margins, ensuring robust operation even under weak grid conditions. The Q-PR controller design is examined in two scenarios: first, a single-loop grid-side current control without active damping, suitable when the filter’s resonance frequency exceeds one-sixth of the sampling frequency ($f_s/6$). A discrete-domain stability analysis based on the Nyquist Criterion is conducted to determine the theoretical limit of the proportional gain for stable operation. In the second scenario, active damping is introduced, and a coordinated tuning procedure is developed using the Routh criterion to jointly select the Q-PR and AD parameters, ensuring adequate gain and phase margins. Experimental validation confirms that the proposed method effectively mitigates resonance, reduces total harmonic distortion (THD), and improves dynamic robustness compared to traditional approaches, demonstrating its suitability for grid-connected applications under varying grid conditions.