AC- and DC-Side Perturbation Analysis of Modular Multilevel Converter Based on Frequency Components Balance

Due to the existence of power electronic components, Modular Multilevel Converter (MMC) shows different ability to resist perturbation compared with traditional power equipment. Even it may amplify the perturbation from system. Thus, this paper focuses on the ac- and dc-side perturbation characteristics of MMC, to improve the system stability and security. Firstly, the state-space description of MMC is introduced, which specially considers the dynamic behavior of sub-module capacitor and the dc-side midpoint voltage. Furthermore, this paper analyzes the physical flow of the normal and perturbation frequency components in MMC based on the frequency components balance principle. And the multi-frequency perturbation matrix is built to describe the relationship among perturbation components. In addition, the positive, negative and zero sequence characteristics of frequency components and their operation rules are also analyzed. The proposed perturbation matrix is verified by time-domain simulation. Based on the perturbation matrix, the ac- and dc-side port-impedances of MMC in sub-synchronous and super-synchronous frequency range are calculated. According to the frequency characteristics of the port-impedance, this paper analyzes the perturbation amplification effect of MMC. It shows that MMC probably will amplify perturbation on certain frequencies in sub-synchronous and super-synchronous frequency range, and the specific frequencies are fairly influenced by the system parameters.