Analysis of 6Hz SSO caused by the interaction between wind turbines and MMC-HVDC

Recently, 6Hz sub-synchronous oscillation (SSO) in wind farms connected with the modular multilevel converter (MMC) based HVDC has caused wide concern in actual projects. Existing studies mostly consider a single wind turbine connected to MMC-HVDC, which is impossible to reveal the mechanism of 6Hz SSO adequately. Besides, the assessment of the influence of different factors on SSO is qualitative, causing inconvenience to the parameter optimization of the system. Therefore, this paper fully considers the impacts of the types and positions of wind turbines and establishes the model of wind farms integrated with MMC-HVDC based on an actual project. Then the mechanism of 6Hz SSO is studied based on modal and participation factor analyses. Finally, to offer accurate information for parameter optimization of the system, quantitative research on the influence of control parameters of wind turbines and MMC on 6Hz SSO is conducted by sensitivity analysis.