Transient Stability Analysis for Grid-Connected PLL-Based VSC during Asymmetric Grid Faults

The transient stability of grid-connected phase-locked loop (PLL)-based voltage source converter (VSC) has a high risk of transient synchronous instability under asymmetric grid faults. The nonlinear large signal model of VSC during asymmetrical faults is established in this paper. Then, the coupling control characteristics among positive and negative sequence (PNS) PLL during asymmetrical grid faults are analyzed. The interaction mechanism of positive and negative sequence PLL is revealed by phase portraits of PNS equivalent power angles. Finally, the correctness of the analysis results is verified by simulation.