1096

For grid-connected inverter, phase-locked loop (PLL) is a common method to synchronize the output current with the grid voltage. However, in weak grid, as the grid impedance cannot be ignored, the inverter tends to be unstable. Besides, the grid impedance can vary in the actual application, using fixed PLL parameters designed for specific grid condition cannot guarantee the inverter robustness against the grid impedance varying. Hence, this paper aims to propose an improved method to improve the typical single-phase delay-based PLL. At first, the impedance model of the inverter is established and the reason for the system stability deterioration is analyzed in detail. Then, an adaptive PLL is proposed based on the quasi-passive method for grid-impedance measurement. The PI parameters of the PLL have been adaptively adjusted online according to the measured grid impedance so as to achieve the high phase margin (PM, not less than 40°) in the weak grid. Simulation results have proved that the proposed adaptive PLL can ensure the system stability, maintain the high quality of grid current and robustness in the weak grid.