Transient Stability Impact of the Phase-Locked Loop on Grid-Connected Voltage Source Converters

The phase-locked loop (PLL) is widely used in the grid-connected voltage source converter (VSC) for the purpose of grid synchronization. The impact of the PLL on the small-signal stability of VSC has recently been revealed, yet its influence on the transient stability of the VSC has seldom been addressed. This paper thus presents a comprehensive analysis on the transient stability effect of the PLL. It points out that the conventional equal-area criterion (EAC), which is used to analyze the transient stability of the synchronous generator (SG), cannot be extended to analyze the PLL effect, even though a dynamic analogy between the PLL and the swing equation of the SG can be found. The phase portrait is thus used in this work, and it shows that the long settling time and a high damping ratio of the PLL can enhance the transient stability of the VSC. Finally, the experimental tests are performed to verify the effectiveness of the theoretical analysis.