虚拟同步直驱风电场经MMC-HVDC并网的低频振荡特性分析

虚拟同步直驱风电场经功率同步环与模块化多电平换流器柔性直流(MMC-HVDC)输电互联，将存在低频振荡风险。考虑MMC-HVDC和直驱风机网侧换流器以及转子侧换流器内部的动态过程，首先建立虚拟同步直驱风电场经MMC-HVDC并网的小信号模型，并通过精细化电磁暂态仿真验证其准确性。随后，利用根轨迹方法，分析风电功率波动和交流系统强度变化对互联系统稳定性的影响，设计功率变化时虚拟同步直驱风电场的参数整定方法。结果表明，由于功率外环和MMC-HVDC送端整流站电压环作用，在风电场输出功率增大和交流系统强度降低的过程中，互联系统存在低频振荡现象。通过合理调整锁相环、虚拟同步机(VSG)有功环和MMC-HVDC送端整流站电压环的控制器参数、改变VSG阻尼项形式，可以抑制振荡并实现稳定运行。

Low-frequency oscillation characteristic analysis of grid-connected VSG-PMSG via MMC-HVDC system

The virtual synchronous generator of permanent magnet synchronous generator(VSG-PMSG) is interconnected with the modular multilevel converter based high voltage direct current(MMC-HVDC) through the power synchronization loop, and there will be a risk of low-frequency oscillation. Considering the inner dynamic process of the MMC-HVDC and the rotor-side converter and grid-side converter of direct-drive wind turbine, the small-signal model of grid-connected VSG-PMSG via MMC-HVDC system is firstly established, and its accuracy is verified by the refined electromagnetic transient simulation. Then, using the root locus method, by analyzing the influence of wind power fluctuation and AC system strength variation on the stability of the interconnected system, a parameter tuning method for VSG-PMSG with power variation is designed. The results show that due to the power outer loop and the voltage loop of MMC-HVDC sending-end rectifier, the interconnected system has low-frequency oscillation during the process of wind farm output power increasing and AC system strength decreasing. By adjusting the controller parameters of the phase locked loop, the VSG active power loop and the voltage loop of MMC-HVDC sending-end rectifier, and changing the form of VSG damping loop, the oscillation can be suppressed and the stable operation can be achieved.