考虑频率耦合的MMC序阻抗建模及附加陷波器阻抗重塑方法

模块化多电平变流器(modular multilevel converter, MMC)与电网间的频率耦合会影响变流器的阻抗特性，常规研究未考虑频率耦合造成的阻抗模型不准确，对分析系统阻抗特性造成了困扰。针对此问题，建立考虑频率耦合下MMC的全阶阻抗模型，进而分析系统稳定性。在此基础上提出一种基于陷波滤波器的MMC阻抗优化方案，解决弱电网互联系统的稳定问题。在分析MMC传统阻抗建模不能有效发现实际低频段振荡的原因基础上，研究频率耦合在MMC建模中的产生机理和影响程度。根据频率耦合的产生机理，建立小信号模型及变量关系，推导出频率耦合下的MMC全阶阻抗模型。依据全阶模型，提出一种基于陷波器的系统阻抗优化方案，有效解决了MMC弱网互联系统的低频振荡及稳定分析问题。仿真结果验证了耦合下MMC全阶阻抗模型的准确性及低频振荡阻抗优化的可行性。

Modeling of MMC sequence impedance considering frequency coupling and impedance reshaping with additional notch filter

The frequency coupling between a modular multilevel converter (MMC) and the power grid will affect the impedance characteristics of the converter. Conventional research does not consider the frequency coupling, causing that the impedance model is inaccurate. This is a problem for the analysis of the impedance characteristics of the system. In this paper, a full-order impedance model of MMC considering frequency coupling is established, and then the stability of the system is analyzed. An optimization scheme of MMC impedance based on a notch filter is proposed to solve the stability problem of a weak grid interconnection system. Analysis is made on the reason that the traditional impedance modeling of MMC cannot find the actual low-frequency oscillation effectively, and the generation mechanism and influence degree of frequency coupling in MMC modeling are studied. From the generation mechanism, the small signal model and variable relationship are established, and the full-order impedance model of MMC with frequency coupling is derived. Based on the full-order model, a system impedance optimization scheme based on notch is proposed. This effectively solves the problem of low frequency oscillation and stability analysis of an MMC weak network interconnection system. The simulation results verify the accuracy of the full-order impedance model of the MMC under coupling and the feasibility of the low frequency oscillation impedance optimization.