PLL Synchronization Transient Stability Analysis of a Weak-Grid Connected VSC During Asymmetric Faults

During asymmetric faults, the coupling between positive and negative (PN) sequences significantly affects the phase-locked-loop (PLL) synchronization transient stability (PSTS) of a weak-grid connected voltage source converter (WG-VSC). However, little literature is on analyzing the PSTS while considering PN sequence coupling. This article has analyzed the PSTS of the WG-VSC system considering PN sequence coupling under asymmetric faults for the first time. First, the Takagi-Sugeno method is adopted to construct the largest estimated domain of attraction (LEDA) at an equilibrium point of the system during asymmetric faults. This LEDA enables quantitative analysis of PLL synchronization transient instability phenomena. Furthermore, from the point of view of ensuring the PSTS, the feasible region of PN currents (FR-PNC) of the system during asymmetric faults has been obtained. In addition, an algorithm is proposed to visualize this 4-D FR-PNC in 2-D space. Finally, the theoretical analysis is validated by time domain experiments conducted on the RT-Box hardware-in-the-loop experimental platform.