Comparison of PLL and without PLL control for grid-connected VSCs under large disturbances

Grid-connected voltage source converters (VSCs) typically utilize phase-locked loops (PLL) for synchronization. However, this approach may lead to transient synchronization instability under weak grid conditions, especially during significant disturbances such as large power steps or asymmetric faults, etc. The transient synchronization instability issues resulting from PLL and weak grid interactions can be avoided by using a natural abc frame without PLL control structure. In this paper, the transient characteristics of VSC with and without PLL control strategies are compared by considering two common large disturbance scenarios: 1) large power step and 2) asymmetric faults. The PSCAD/EMTDC time-domain simulation results are also provided.