A New Phase-locked Loop Design Method and Stability Analysis Considering the Coupling Effect of Grid-connected Inverter Under Weak Grid

In response to the issue of mutual coupling leading to system instability among the current loop, phase-locked loop, and grid impedance of the grid-connected inverter under a weak grid, a mathematical model for the grid-connected inverter under weak grid conditions is established. The coupling principles between the phase-locked loop and the current loop are analyzed. Based on zero-pole distribution and the Nyquist stability criterion, the primary reasons for the decrease in system robustness when the grid impedance and the bandwidth of the phase-locked loop change are revealed. A new phase-locked loop control structure is proposed by constructing a pre-link to reshape the phase-locked loop transfer function and providing a specific parameter design method. Theoretical analysis indicates that the proposed new phase-locked loop can eliminate the impact of phase-locked loop bandwidth on system robustness, decoupling of phase-locked loop bandwidth and current loop, and enhance the system's adaptability to weak grids. Finally, simulations and experiments are conducted to verify the effectiveness of the proposed new phase-locked loop design method. © 2024 Power System Technology Press. All rights reserved.