Current Stress Optimization of Nonagonal MMC for FFTS-based Offshore Wind Power Application

Nonagonal Modular Multilevel Converter (MMC) is a new type of three-port AC-AC converter suited for applications in FFTS-based offshore wind power. It can accommodate three AC ports with only nine branches, greatly reducing the cost and size of energy conversion. However, the multiplexing structure of nonagonal MMC inevitably increases the peak branch current resulting in high semiconductor ratings, which weakens its advantage in cost and size. Since the control degrees of freedom are reduced, traditional current attenuation methods are not applicable. To address this issue, in this paper, a parameter adjustment method is proposed for nonagonal MMC to reduce the current stress. By establishing the mathematical model of nonagonal MMC, phase angle difference between ports of nonagonal MMC is proved to be a key influencing factor of peak branch current. By adjusting phase angle difference, the average current stress of nonagonal MMC can be reduced by as great as 66.31%. Finally, the correctness of the conclusions of this paper is verified by RT-LAB results. A parameter adjustment method is proposed for nonagonal MMC to reduce the current stress. Phase angle difference between ports is proved to be a key influencing factor of peak branch current. By adjusting phase angle difference, the average current stress can be reduced by as great as 66.31%. image