SISO admittance reshaping and enhanced robustness of single-phase grid-connected inverter based on frequency-decoupling in weak grid

• A PLL-based frequency decoupling method is proposed to suppress frequency coupling in single-phase grid-connected systems. • The multi-frequency admittance model is simplified to a SISO form for easier analysis and design. • The proposed admittance shaping method eliminates PLL-induced negative resistance and enhances stability in weak grids. Asymmetric control often introduces frequency coupling in single-phase phase-locked loop (PLL) under weak grid condition, which deteriorates the stability for the grid-connected system. In this paper, in order to address the infinite characteristics of frequency coupling in the single-phase system, an admittance model based on harmonic transfer function matrix is established. Then, by designing a decoupling factor in the current loop of the inverter, a reduced-order single-input single-output (SISO) admittance model is established. Furthermore, based on the improved model, an effective admittance shaping method is proposed to realize the passivity of output admittance, which ensures the stable operation of grid-connected inverter under weak grid condition. Finally, experimental results validate the effectiveness of the proposed method in suppressing frequency coupling and stability enhancement.