不对称交流电压下MMC及柔性直流电网基于小信号方法的稳定性研究

When the modular multilevel converter (MMC) station is connected to weak ac grid or remote ac faults occur, the unbalanced ac voltages at the ac bus may cause the instability of MMC or even the entire dc grid. Eigenvalue analysis and impedance analysis are the most typical methods for stability study of converters and both are based on the small-signal model (SSM). Therefore, this project will first investigate the SSMs of MMC and dc lines which distributed parameters are actually dependent on frequency under unbalanced ac voltages. Based on that, the SSM of the entire dc grid will be constructed in a modular way. Next, the accurate and efficient reduced-order models of the flexible dc grid will be proposed to facilitate the eigenvalue analysis for large scale of the flexible dc grid. Meanwhile, the ac and dc equivalent impedances of MMC under unbalanced ac voltages will be derived in the full-frequency domain. Based on that, this project will further investigate the corresponding impedance analysis methods based on the general Nyquist stability criterion and the equivalent admittance matrix of dc grid for ac side and dc side of MMC, respectively. Following that, the stable operating area of MMC under unbalanced ac voltages will be analyzed and then be further constrained by control parameters to obtain the safety area in the eigenvalue and impedance analysis methods. Finally, the autonomous control strategy of MMC after remote ac faults occur will be presented to satisfy the constraints of the safety area. This project will provide the theoretical support for the stability study of the flexible dc grid under unbalanced ac voltages.

当模块化多电平换流器(MMC)接入弱交流电网或发生远端交流故障时，不对称交流电压可能引起MMC乃至整个直流电网的不稳定。特征值和阻抗分析法是研究换流器稳定性最常用的方法，二者都是以小信号模型（SSM）为基础。为此，本项目将首先研究不对称交流电压下MMC和直流线路考虑参数频变特性的SSM，并按照模块化思想建立柔性直流电网的SSM；然后，发展兼顾准确性和高效性、适合大规模柔性直流电网稳定性分析的降阶模型；同时，推导不对称交流电压下MMC的全频域交直流等效阻抗模型，研究交流侧基于广义Nyquist稳定判据和直流侧基于直流电网等效导纳矩阵的稳定性分析方法；最后，研究不对称交流电压下MMC的稳态运行域，在此基础上利用特征值和阻抗分析法确定考虑控制参数约束的安全域，提出交流故障后MMC满足安全域约束的自主控制策略。本项目将为不对称交流电压下柔性直流电网的稳定性研究提供理论支撑。

当模块化多电平换流器(MMC)接入弱交流电网或发生远端交流故障时，不对称交流电压可能引起MMC乃至整个直流电网的不稳定。特征值和阻抗分析法是研究换流器稳定性最常用的方法，二者都是以小信号模型（SSM）为基础。为此，本项目研究了：不对称交流电压下柔性直流电网模块化小信号模型、柔性直流电网稳态参数的模块化求解算法、适用于大规模柔性直流电网稳定性分析的直流电网降阶模型、基于等效阻抗的MMC及直流电网稳定性分析方法以及不对称交流电压下MMC换流站的稳定性分析及控制方法。项目建立了考虑直流线路频变特性的柔直电网模块化小信号模型，建立了适用于稳定性分析的大规模柔直电网的降阶小信号模型，提出了MMC换流站工作域的刻画方法与自主边界控制方法，实现了基于MMC换流站全频域等效阻抗模型的稳定性分析，为大规模柔性直流电网的稳定性研究奠定理论基础。项目完成了预定目标，所产生的研究成果更适用于不对称交流电压下柔直电网的小信号稳定性分析与控制，为柔直电网的安全稳定运行提供了理论支持。