Abstract: Offshore wind power collection of fractional frequency transmission system draw more attention, because of its low investment and cost of the operation and maintenance, Core of fractional frequency transmission system is variable-frequency drives. Each transmission circuit connected with low frequency and power frequency needs one separate high voltage and large capacity cascaded converters. In order to improve the integration of the topology, decrease the numbers of IGBTs and bulk of equipment, a three-port nonagonal MMC converter was proposed in integrating two individual converters. On the basis of its mathematical model, power of bridge arms was analyzed for designing control strategies, and furtherly achieved the energy interaction among three ports. It turns out that the energy from ports is transferred from one to the other through circulating current of bridge arms, and power control of bridge arms, which restrains constant power, enables converter operate normally. Power control among ports achieve the energy from offshore wind power recycled by AC grid. Finally, the proposed topology and its corresponding control were verified in RT-LAB platform.