Disturbance Observer-Based Robust Current Control Scheme for Three-Phase Grid-Tied Inverters With <i>LCL</i> Filter Considering Unbalanced Grid Voltages

This article presents a comprehensive study on the design, robust stability analysis, performance evaluation, and real-time implementation of a disturbance observer-based controller for grid-connected voltage-source inverters (VSIs) with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter. The proposed composite controller integrates a state-feedback controller with a disturbance observer to enhance the tracking performances of grid-tied inverter with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter. The grid-tied VSI is modeled in the stationary reference frame (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \beta$</tex-math></inline-formula>) to accommodate both balanced and unbalanced grid voltages. The main objective of the proposed approach is to achieve asymptotic tracking of sinusoidal references in the presence of unknown disturbances and parametric uncertainties. More precisely, not only does the controller ensure accurate tracking performances, but also it can maintain closed-loop stability and good transient performances in the presence of model uncertainties. Moreover, the integration of control saturation in the observer design makes it possible to mitigate the effect of control saturation during transients without the need for designing an antiwindup scheme. The effectiveness of the composite controller is validated through numerical simulation and experimental implementation. The results demonstrate high-performance characteristics in terms of tracking accuracy and robustness to parameter variations in the presence of balanced grid voltages, unbalanced grid voltages, and control saturation.