Synchronous stability analysis and enhancement method for grid connected inverters in weak grids

Renewable energy resources (RESs) are typically integrated into the utility grid through grid-connected inverters (GCIs). Such a system may lose synchronization during grid faults, especially under weak grid conditions, resulting in instability problems or even triggering large-scale power outage accidents. This paper analyzes the influences of the system parameters, including the phase-locked loop (PLL) parameters and the grid parameters, on the GCI's synchronous stability. The results reveal that the PLL exhibits a negative damping effect during the transient process, which is bad for stabilizing the GCI in transients. An improved PLL, which is achieved by adding a feedback low-pass filter into the conventional PLL, is then proposed to address this issue, making the system more stable in transients. Analyzing results show that the improved PLL can enhance the synchronous stability of GCI during transients without changing its steady-state performance. The simulation and experimental results performed by a 2-MW GCI integrated into a weak grid verify the correctness of the theoretical analysis and the effectiveness of the proposed method.