Enhanced Active Power Transfer and Stability for Grid-Following Inverters Within an Ultraweak Grid

Grid-following (GFL) inverters face both severe largesignal destabilization (LSD) and small-signal destabilization (SSD) risks in ultra-weak grids. To address the LSD risk, this paper reveals the effect of reactive current injection (RCI) on the powerangle property, terminal voltage, and maximum transferable active power (MTAP) in the presence of different short-circuit ratio (SCR) conditions. Then, the feasible ranges for active power and RCI are developed to prevent LSD and terminal voltage violation. By excluding the possibility of LSD with an enhanced MTAP, the small-signal stability of the GFL inverter is ensured by applying the impedance stability criteria. In this article, an optimization problem with constraints of internal and external stabilities is established to redesign the gains of the current controller, without requiring real-time detection during the solution process. The redesigned gains effectively ensure the smallsignal stability of the GFL inverter while keeping a relatively high PLL bandwidth in an ultra-weak grid with SCR = 1.1, which is lower than that achieved by state-of-the-art methods. Finally, the simulation and experimental results confirm the effectiveness of the proposed guideline.