Analysis and Enhancement of Static and Dynamic Power Transfer Limits of Grid-Forming Energy Storage Converter Under Ultra-Weak Grids

With the large-scale integration of renewable energy sources and power electronic converters, grid-forming energy storage converter (GFM-ESC) with voltage and frequency support capabilities is emerging as an important component in power-electronic-dominated systems. Although GFM-ESCs are generally stable under weak grids, this paper reveals that under ultra-weak grids, limitations in power transfer capability, specifically the static power transfer limit (SPTL) and dynamic power transfer limit (DPTL), can still cause stability issues and under-utilization of converter capacity. The SPTL is determined by grid impedance, while the DPTL further considers the control dynamics. It is first demonstrated that the SPTL exhibits asymmetry between discharging and charging modes, primarily attributed to grid resistance, which limits the power transfer in charging mode. A third-order nonlinear dynamic model is then developed, with phase trajectory analysis used to evaluate the impact of various control parameter configurations on the DPTL. To address the instability induced by DPTL under ultra-weak grids, an adaptive voltage compensation strategy based on reactive power deviation is proposed, which improves both power transfer capability and capacity utilization. Finally, experimental results validate the theoretical analysis and the effectiveness of the proposed strategy.