Admittance Modeling and Stability Enhancement of Grid-connected Inverter Considering Frequency Coupling in Weak Grids

In the renewable energy generation system, the phase-locked loop (PLL) for power grid synchronization plays a very important role, especially in weak grids. The asymmetric control of the d-axis and q-axis of PLL leads to frequency coupling, which changes the output admittance of the grid-connected inverter and causes lowfrequency oscillation. By deriving the self-admittance and coupling admittance models, this paper quantitatively analyzes the influence of coupling components on the output admittance and system stability. Then, an improved PLL based on the second-order low-pass filter (LPF) is proposed to reduce the negative impact of frequency coupling on system stability. The adopted method can effectively reduce the influence of coupling components and improve the phase margin of the system. Simulation and experimental results validate the correctness of the theoretical analysis and the effectiveness of the proposed strategy.