Synchronization Stability of the MMC-Connected Wind Farm Under Severe Asymmetrical Faults

This paper analyzes the phase-locked loop (PLL) synchronization stability of an offshore wind farm connected via the modular multilevel converter (MMC)-based high-voltage direct-current (HVDC) transmission system. The investigated wind farm is composed of full-scale type-4 wind turbines, and severe asymmetrical faults between these turbines and the MMC are considered. The MMC has dual-sequence control requirements under such faults, which interacts with the control systems of the wind turbines and further complicates the transient stability problem for the system. Thus, we thoroughly investigate how the interaction caused by the MMC affects the synchronization stability of wind turbines. Under severe faults, the MMC would be switched into a current limitation mode and act as a current source. Therefore, the influences of the MMC's positive- and negative-sequence currents as well as their assignments between dq axes, on transient synchronization stability of wind turbines, are studied according to the phase portrait method. Furthermore, the optimal assignment region (OAR) is proposed for the stability-oriented assignment of MMC's positive- and negative-sequence currents. This theoretical analysis is validated through extensive simulation results obtained from MATLAB/Simulink.