Transient Stability Analysis of the Phase-Locked Loop in Grid-Connected Converters

∗ The nonlinearity of phase-locked loops (PLLs) is a main reason whichcauses unstableoperationin grid-connectedconverters. How-ever, linearized methods or transient analysis cannot reveal the nonlinear characteristic of the PLL system. Furthermore, the stability criterion of the PLL is hard to be acquired under large-signal perturbations. In this paper, the LaSalle’s theory is employed to derive the stability boundaries of the PLLs. Once the grid parameters, the controller proportional-integral (PI) gains of the PLL, and the initial system states are given, the stability of the PLL system can be determined. The effectiveness of the proposed method is verified by detailed circuit simulations and experiments.