Multifunctional Grid-Forming Converter Control Based on a Disturbance Observer

A multifunctional grid-forming control method based on a disturbance observer structure is proposed for grid converters. The proposed method has several different control modes, including the grid-forming, grid-following, and current control modes. The proposed method does not require any separate synchronization loop as it is incorporated in the disturbance observer. Links to existing control methods are shown and tuning recommendations are provided based on small-signal stability analysis. Furthermore, robustness against the inductance error in the grid-forming mode is analytically shown. Experiments with a 12.5-kVA converter are used to assess power-tracking performance. The method is shown to have excellent dynamic performance regardless of grid strength and to provide seamless switching between grid-forming and grid-following modes. In addition, the effect of grid voltage sags and grid-frequency deviations is studied.