Power Imbalance Analysis and Neutral Offset Voltage Decoupling Control of Nonagonal MMC Applied in FFTS

Nonagonal modular multilevel converter (NMMC) is a three-port ac/ac/ac conversion converter, and it can achieve the connection of two fractional frequency transmission systems (FFTSs) as wind power collection to ac grid. However, the connection of ac grid to low-frequency systems brings challenges to branch power balance, which may cause dc voltage fluctuation of the submodules (SMs). According to the model construction, the relation between power imbalance and dc voltage fluctuation is quantified in this article. Based on branch power balance condition, a neutral offset voltage decoupling control is proposed in this article to address the dc voltage imbalance among the branches and decrease dc voltage fluctuation. The scheme that NMMC is applied in wind power collection and the proposed control is verified by MATLAB/Simulink platform and hardware-in-the-loop (HIL) experiment.