PLL Synchronization Stability Analysis of MMC-Connected Wind Farms under High-Impedance AC Faults

This paper analyzes the transient stability of an offshore wind farm connected via modular multilevel converter (MMC)-based high-voltage direct-current transmission system. The wind farm is composed by full-scale type-4 wind turbines, whereas short-circuit faults occurring in between the MMC and wind farm are considered. The MMC tends to be switched to the current limiting mode or the overmodulation mode during the ac-side fault, which further interacts with the control systems of wind turbines, complicating the transient stability problem of the system. Hence, in this work the parametric impacts of the phase-locked loop of wind turbine converters and over-current/-modulation limiters of the MMC are revealed analytically, and further corroborated by time-domain simulations.