Phase-Locked Loop Structure Optimization Method for Grid-Following Inverters Based on Higher-Order Orthogonal Signal Generators

In the context of the “dual-high” power grid, the study of grid-connected inverters that enhance power quality while maintaining fault tolerance under unbalanced conditions carries significant theoretical and practical value. Grid-following(GFL) inverters, as a widely adopted type of grid-connected inverter, continue to be a focal point of research. This paper proposes a quasi-proportional resonant (QPR) control method in the aB-reference frame. An impedance model of the GFL system is established through impedance analysis, and the influence of control parameters on system stability is evaluated using impedance stability criteria. Optimal control parameters are identified to enhance dynamic response and stability. Additionally, an improved phase-locked loop (PLL) control strategy is introduced, incorporating an orthogonal signal generator (OSG) with pre-filtering. The frequency-domain response characteristics of second-, third-, and fourth-order OSGs are compared, with the fourth-order OSG selected as the pre-filter for the PLL. This PLL strategy demonstrates robust adaptability to unbalanced grid conditions. Simulation results indicate that the proposed inverter control method achieves a total harmonic distortion (THD) of 0.47% for the grid-injected current under normal operation and 0.50% THD after fault clearance. The PLL method accurately tracks the grid phase during normal operation, fault conditions, and post-fault recovery. These findings validate the effectiveness of the proposed approach, providing valuable insights for optimizing the stability and power quality of distributed active distribution networks and advancing the autonomy of distributed generation systems.