Due to low loss, low cost, and the capacity to connect to the passive network of the modular multilevel converter (MMC), it has been widely applied for the deep-sea offshore wind power transmission. However, there will be a risk of sub-synchronous oscillation in the sending-end system, due to the interaction between the MMC, the wind turbines (WTs), and the AC network. Besides, the positive and negative sequence separation module (PNSSM) of the MMC and WTs introduced for the imbalance control can also affect the small signal stability. To help the small signal stability analysis during asymmetric faults, the paper is aimed to establish the overall linearized mathematical model. In this model, the state-space method is adopted and the phase-locked loop (PLL) of the WTs, circulation controller of the MMC, and the PNSSM of both the MMC and the WTs are considered. Finally, the correctness of the small signal model is verified through simulation in the MATLAB/Simulink.

Modular multilevel converter,wind turbines,positive and negative sequence separation module,sub-synchronous oscillation,small signal stability