Design-Oriented Transient Stability Analysis of PLL-Synchronized Voltage-Source Converters
For power system engineers designing VSC-based renewable energy systems, this work provides design-oriented insights into synchronization stability under large disturbances.
This paper analyzes the transient stability of PLL-synchronized voltage-source converters (VSCs) using phase portraits, revealing that first-order PLLs guarantee stability when equilibrium exists, while second-order PLLs may fail despite high damping. An adaptive PLL switching between modes is proposed and validated via simulations and experiments.
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.