On Transient Instability Mechanism of PLL-Based VSC Connected to a Weak Grid

This article analyzes the transient process of phase-locked loop (PLL) based grid-connected voltage source converters (VSCs) under large disturbances by modeling the PLL dynamic model and active power loop model. With a focus on the effect of transient grid voltage changes on the stability of the PLL-based VSCs, it reveals that the mechanism of transient instability of PLL-based VSCs is different under various disturbance levels. It can be concluded that prior to the PLL losing synchronization, the power loop has already become unstable. In addition, the influence of the PLL prefilter used for asymmetrical faults on the stability criterion is analyzed. Moreover, based on the transient instability mechanism, two solutions are proposed to deal with different instability occasions, and the adaptability of these solutions is analyzed. Finally, hardware-in-the-loop experimental results validate the proposed transient instability mechanism and the proposed control strategies.