在我国能源绿色低碳转型的新形势下，可再生能源发电取得了跨越式发展。鉴于我国可再生能源集约化开发与能源消费所呈现的典型逆向分布特征，柔性直流输电技术凭借其自身的经济性和稳定性优势在可再生能源大规模外送和跨区输电等方面得到了广泛应用。近年来，在国内外基于模块化多电平换流器（MMC）的柔性直流输电系统中发生了多起高频振荡事件，严重威胁电力系统的安全稳定运行并制约可再生能源的大规模消纳。针对柔性直流输电系统高频振荡这一新型电力系统稳定性问题，本文从阻抗建模、稳定分析、抑制策略等三方面开展系统性的研究，具体包括：1）宽频范围下模块化多电平换流器的三端口频率耦合阻抗模型构建方法；2）柔直系统阻抗网络模型中不同维度阻抗模型的聚合方法；3）适用于柔直系统高频振荡的稳定性分析方法；4）针对频率时变高频振荡的自适应抑制方法。在换流器建模方面，提出了适用于MMC的宽频三端口频率耦合阻抗模型。基于谐波线性化方法，考虑全阶控制环节、控制链路延时、内部桥臂动态以及频率耦合效应和交直流端口间的耦合效应建立三端口频率耦合阻抗模型，能够准确表示MMC在宽频范围下的阻抗特性。在柔直系统建模方面，提出了柔直系统阻抗网络模型的聚合方法。建立系统中各个电力设备的频域阻抗模型，根据系统拓扑将其互联起来构成柔直系统的阻抗网络模型，利用端口条件完成不同维度阻抗模型的拼接，实现柔直系统的频域等值聚合，为后续稳定性分析奠定了基础。在高频振荡稳定性分析方面，提出了基于聚合阻抗频率特性的稳定性判据，鲁西柔直系统和四端柔直系统的算例证明了该判据的正确性。根据稳定性分析结果，进一步阐明了高频振荡机理，并结合参数灵敏性分析确定了换流器高频阻抗特性的关键影响因素，便于后续开展高频振荡抑制策略的研究。在高频振荡抑制方面，提出了基于自适应陷波滤波器的高频振荡抑制方法，通过在线监测高频振荡模式、自适应调节陷波器参数，能够在不影响其它频率范围阻抗特性的同时实现对不同运行方式下高频振荡的抑制，有效解决频率时变的柔直系统高频振荡问题。
Under the new situation of green and low-carbon energy transformation in China, renewable energy generation has achieved leapfrog development. In view of the typical reverse distribution characteristics of the intensive development of renewable energy and energy consumption in China, voltage source converter-based high voltage direct current transmission has been widely used in large-scale renewable energy transmission and cross-regional transmission due to its economic and stability advantages. However, in recent years, several high-frequency resonance (HFR) events have occurred in modular multilevel converter-based high voltage direct current transmission (MMC-HVDC) systems at home and abroad, which seriously threaten the safe and stable operation of power systems and restrict the large-scale consumption of renewable energy.To address the above issues, this paper systematically study the impedance modeling, stability analysis and control strategy of MMC-HVDC systems, including 1) Three-port frequency-coupling impedance modeling method of MMC in a wide frequency range; 2) Aggregation of different dimensional impedance models in impedance networks of actual MMC-HVDC systems; 3) HFR stability analysis method suitable for MMC-HVDC systems; 4) Adaptive mitigation method of HFR with different frequencies under variable operating conditions.In the research of converter modeling, a three-port frequency-coupling impedance model of MMC is proposed. Based on the harmonic linearization method, considering the time delay of each control link, the unique internal bridge arm dynamics, the frequency-coupling effect and the coupling between AC and DC ports, a three-dimensional impedance matrix is established, which can accurately represent the impedance characteristics of MMC in a wide frequency range.In the research of MMC-HVDC system modeling, the aggregation method of impedance network models is proposed. The impedance model of each equipment in the MMC-HVDC system is established, and the impedance network model of the target system was formed by connecting them according to the system topology. The impedance models of different dimensions are aggregated using port conditions to realize the frequency domain equivalent aggregation of the MMC-HVDC system, which laid a foundation for the subsequent stability analysis.In the research of stability analysis, a stability criterion based on aggregated impedance is proposed, and the correctness of the criterion is verified by taking the Luxi MMC-HVDC system and multi-terminal MMC-HVDC system as examples. Through stability analysis, the mechanism of HFR is further clarified. Combined with the sensitivity analysis of parameters, the control links related to high-frequency impedance characteristics of MMC are found, which is convenient for the subsequent research of HFR mitigation strategy.In the research of HFR mitigation, a novel HFR mitigation method based on adaptive notch filters (ANFs) is proposed. By online monitoring the HFR modes and adaptively adjusting the parameters of ANFs, the HFR of MMC-HVDC systems under different operating conditions can be suppressed without affecting the impedance characteristics in other frequency ranges, and the HFR problem of MMC-HVDC system with time-varying frequency can be effectively solved.
