Grid Impedance Estimation During Large SCR Drop Events With Grid-Forming Converters

Grid-connected converters face significant synchronization challenges when suddenly exposed to ultraweak grid conditions, primarily due to the large and unknown grid impedance. This article proposes a passive method for efficiently estimating grid impedance during large short-circuit ratio (SCR) drop disturbances. The proposed method relies solely on local real-time measurements—specifically, voltage magnitude, active power, and reactive power at the point of common coupling (PCC)—to estimate fundamental grid impedance during transients. By leveraging the superior voltage control capability of grid-forming (GFM) converters, PCC voltage magnitude remains stable throughout the estimation process, thereby satisfying a requirement of the proposed approach. Furthermore, this method is inherently robust to grid frequency variations and phase angle jumps, further enhancing its practical applicability. With efficient impedance estimation, the converter can promptly adjust its active power reference to maintain synchronization. The effectiveness of this method has been validated through experimental results.