Optimization and analysis of phase-locked loop for grid-following inverters with weak grid connection

Purpose This study aims to analyze and enhance the stability of three-phase grid-connected inverters in weak grid environments, with a particular focus on the negative impact of the phase-locked loop (PLL) on system stability. Design/methodology/approach A small-signal transfer function model of a three-phase LC grid-connected inverter has been constructed to explore the inherent mechanism by which the PLL affects system stability. Based on this model, an optimized PLL structure and its corresponding parameter design method are proposed. Moreover, system stability is evaluated using methods such as Bode analysis, pole plot drawing and simulation. Findings When the impedance of the power grid increases, the traditional PLL can weaken system stability, leading to oscillations in the output current. However, an optimized PLL can significantly enhance the phase margin and expand the stable range of the system in weak grid environments. Originality/value In contrast to conventional analysis approaches, this study directly integrates both the PLL and grid impedance into the system model, enabling a straightforward analysis of their influence on system stability and the determination of stability boundaries. When compared to conventional PLLs, the optimization method proposed in this study improves the stability of inverters operating under weak-grid environments.