Synchronization Stability Analysis and Parameter Design of Grid-Following Inverters Considering the Interactions of Current Control and Phase-Locked Loop

For grid-following inverters (GFLIs), grid voltage sag can induce the loss of synchronization (LOS) under weak grids. Numerous studies have discussed the synchronization instability dominated by phase-locked loops (PLLs). However, since the inverter is a high-order nonlinear system with the coupling of multiple control loops, its transient process is hard to be accurately described by merely considering PLL dynamics. Hence, this study establishes a nonlinear model, which is suitable for not only the small-signal stability analysis at equilibrium points (EPs) but also transient stability evaluation under various large-disturbance scenarios. Then, the synchronization stability considering the multiple-loop coupling is assessed with the aid of phase portrait. Unlike existing studies by ignoring inner control loops or applying model reductions, it is proved that inner control loop parameters are expected to be neither too large nor too small. Parameters of multiple control loops that jointly affect the synchronization stability deserve a careful design. Consequently, approaches for acquiring stability boundaries are depicted and a graphical stability-oriented design of multiple-loop parameters is proposed, which can provide an accurate guidance for engineers and researchers to achieve the small-signal and transient synchronization stabilities under large grid impedance and grid voltage sag. Finally, experiments verify the correctness of the analysis.