  Differing from synchronous generators, there are lack of physical laws
governing the synchronization dynamics of voltage-source converters (VSCs). The
widely used phase-locked loop (PLL) plays a critical role in maintaining the
synchronism of current-controlled VSCs, whose dynamics are highly affected by
the power exchange between VSCs and the grid. This paper presents a
design-oriented analysis on the transient stability of PLL-synchronized VSCs,
i.e., the synchronization stability of VSCs under large disturbances, by
employing the phase portrait approach. Insights into the stabilizing effects of
the first- and second-order PLLs are provided with the quantitative analysis.
It is revealed that simply increasing the damping ratio of the second-order PLL
may fail to stabilize VSCs during severe grid faults, while the first-order PLL
can always guarantee the transient stability of VSCs when equilibrium operation
points exist. An adaptive PLL that switches between the second-order and the
first-order PLL during the fault-occurring/-clearing transient is proposed for
preserving both the transient stability and the phase tracking accuracy.
Time-domain simulations and experimental tests, considering both the grid fault
and the fault recovery, are performed, and the obtained results validate the
theoretical findings.
