An Improved PLL Structure to Improve Synchronization Stability of Grid-Connected Inverter Under Weak Grid

When the grid-connected inverters (GCI) connect with the weak grid, it will suffer the synchronization stability problems. The instability issues will limit the development of the renewable energy system. In this paper, an intuitive explanation of the coupling between the phase-locked loop (PLL) and weak grid is provided. Then, an improved PLL structure is adopted to increase the synchronization stability based on the coupling mechanism, which equivalently parallel a resistance with the grid impedance and modifies the qq-axis impedance. Finally, experimental results further validate the effectiveness of the improved PLL structure.