PLL and Additional Frequency Control Constituting an Adaptive Synchronization Mechanism for VSCs

Phase-locked loop (PLL) control provides a typical synchronization mechanism for voltage-source converters (VSCs) integrated into strong grids. However, such a synchronization mechanism is hardly adaptive to the high-impedance weak grid neither to the islanded grid. In the scope of low-inertia weak grid issues, additional frequency control is conceived as an effective way for VSC-based devices to improve the frequency dynamics. The involved two controls are studied in two separate areas. In fact, additional frequency control combined with PLL is an adaptive synchronization mechanism with the ability to adapt to impedance changes in the power grid, which allows VSCs to remain synchronization stability under either a strong, weak or islanded grid, as proved in this letter. Recognition of this synchronization mechanism offers a helpful insight into the stable operation of PLL-synchronized VSCs in different grid conditions.