UDE-Based Current Controller With Enhanced Grid Frequency Fluctuation Adaptability for LCL-Type Grid-Tied Inverters

The uncertainty and disturbance estimator (UDE) with a time delay filter has been widely used for inverter systems due to its satisfactory harmonic rejection performance. However, the control performance may be affected when the grid frequency fluctuates. In this article, a modified UDE (FUDE) is proposed to improve the grid frequency fluctuation adaptability of the $LCL$ grid-tied inverter systems, which contains a compound filter consisting of time delay, zero-phase low-pass, and high-pass filters. Compared with the traditional time delay UDE, it can offer a wider notch bandwidth at the selected frequencies. A dual-loop controller is built in the separate-structure-type UDE-based controller, containing an outer loop PR controller and an inner loop FUDE. The sufficient conditions for the system stability are obtained by the small-gain theorem. The comparison experiments of the dual-loop controller with the proposed FUDE and the conventional time delay zero-phase low-pass UDE are performed on a 2-kW experimental platform, and the results demonstrate the effectiveness of the proposed FUDE.