针对风电场接入我国某四端环形柔性直流电网多次出现次同步振荡的问题,建立直驱风电接入四端柔直电网的小信号模型,揭示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(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.直驱风电场与直接接入的MMC相互作用系统中,有功耦合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.最后利用电磁暂态时域仿真软件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.

Multiple occurrences of sub-synchronous oscillations have been observed following the integration of a wind farm into a four-terminal ring-structured modular multilevel converter (MMC)-high-voltage direct current (HVDC) grid in China. To address repeated oscillation, this paper develops a small-signal model of a direct-drive wind farm integrated into the four-terminal MMC-HVDC system, and reveals the underlying mechanism of the sub-synchronous oscillations observed in the actual project. It is found that a direct-drive wind farm is closely coupled only with the directly connected MMC, while other MMCs have little impact on sub-synchronous oscillation modes. Therefore, when studying the sub-synchronous oscillation issues involving the interaction between the wind farm and the flexible DC system, a simplified model can be established using a single-ended flexible DC system directly connected to the wind farm, ignoring the influence of other MMCs. It is also found that in the system where a direct-drive wind farm interacts with a directly connected MMC, both active power-coupled sub-synchronous oscillation modes and reactive power-coupled sub-synchronous oscillation modes coexist. As the output power of the wind farm gradually increases, the damping of the active power-coupled sub-synchronous mode increases, while the damping of the reactive power-coupled sub-synchronous mode decreases, reproducing the onsite phenomenon where an increase in wind farm active power aggravates sub-synchronous oscillations. The smaller the integral gain of the outer loop of the wind farm’s reactive power controller and the proportional gain of the phase-locked loop (PLL), and the larger the integral gain of the q-axis component of the AC voltage outer loop at the wind farm-side converter station, the more stable the reactive power-coupled oscillation mode becomes. By optimizing these parameters, the risk of system instability can be reduced. A simulation model of a direct-drive wind farm connected to a four-terminal flexible DC grid was built using the electromagnetic transient simulation software PSCAD (power systems computer aided design), and the correctness of the above theoretical analysis was verified.