Abstract: The wind farm connected to a four-terminal MMC-HVDC grid in China, and subsynchronous oscillations has occurred many times. In order to reveal the subsynchronous oscillation mechanism of real MMC-HVDC project, this paper establishes a small-signal model of direct-drive wind power connected to a four-terminal MMC-HVDC grid It is found that the direct-drive wind farm is only tightly coupled with the direct-access MMC converter station, and the other MMC converter stations have less influence on the subsynchronous oscillation modes. When only study subsynchronous oscillations of wind farms interacting with MMC-HVDC grid, it can be established as a simplified model of wind farms directly connected to single-ended MMC converter station, and ignore the influence of other MMC converter stations. The active coupled subsynchronous oscillation mode and reactive coupled subsynchronous oscillation mode coexist in the systems where direct-drive wind farms interact only with direct-access V/F-controlled MMC converter stations, and the damping of the active coupled subsynchronous mode increases and the damping of the reactive coupled mode subsynchronous coupled mode decreases with the increase of the output of the wind farm, which reappears the law of the subsynchronous oscillations of the real project. At the same time, the smaller the wind farm fixed reactive power outer loop integration coefficient and the phaselocked loop scale factor, the larger the WFMMC ac voltage q-axis component outer loop integration coefficient, and adjusting the above parameters reduces the risk of system instability. The correctness of the above theoretical analyses is verified by building a simulation model of direct-drive wind power connected to a four-terminal MMC-HVDC grid in PSCAD.

Key words: modular multilevel converter-based HVDC gird, direct-drive permanent magnet synchronous generator, subsynchronous oscillation