风电场接入我国某四端环形柔直电网多次出现次同步的振荡现象。针对该工程多次发生的次同步振荡问题，本文建立直驱风电接入四端柔直电网的小信号模型，揭示了现场柔直工程的次同步振荡机理。研究发现，直驱风电场仅与直接接入的模块化多电平换流站(Modular Multilevel Converter，MMC)紧密耦合，其他MMC对次同步振荡模态影响较小，仅研究风电场与柔直相互作用的次同步振荡问题可建立风电场直接接入单端柔直系统的简化模型，忽略其他MMC的影响。同时发现，直驱风电场与直接接入的MMC相互作用系统中，有功耦合次同步振荡模态和无功耦合次同步振荡模态并存，且随着风电场出力逐渐增大，有功耦合次同步模态阻尼增大，无功耦合次同步模态阻尼减小，复现了现场风电场有功增大次同步振荡的规律。风电场定无功功率外环积分系数和锁相环比例系数越小，风电场侧换流站交流电压q轴分量外环积分系数越大，无功耦合振荡模式越稳定，通过优化以上参数可降低系统失稳的风险。最后在电磁暂态时域仿真软件(Power Systems Computer Aided Design，PSCAD)搭建直驱风电接入四端柔直电网的仿真模型，验证了上述理论分析的正确性。

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.