1.School of Electric Power, South China University of Technology, Guangzhou510640, China;2.Guangdong Key Laboratory of Clean Energy Technology (South China University of Technology), Guangzhou510640, China;3.College of Electrical Engineering, Zhejiang University, Hangzhou310027, China

Among numerous offshore wind power transmission schemes, the scheme based on parallel connection of diode rectifier unit (DRU) and modular multilevel converter (MMC) has significant economic benefits, which has gained widespread attention. Compared to flexible DC transmission scheme, the DRU-MMC parallel transmission scheme has smaller volume, lighter weight, and less cost of the offshore converter platform. In this scheme, the DRU transmits most of the power, while the small-capacity MMC provides the power required for the wind turbine start-up, supports voltage and frequency and compensates for harmonic currents introduced by the DRU. An active power distribution strategy and a harmonic compensation strategy are proposed for the DRU-MMC parallel transmission topology, so that the power is reasonably distributed between DRU and MMC. Under the premise of realizing main functions of the MMC, the selection of the minimum capacity of MMC is analyzed. From two aspects of investment and operation, the cost of the offshore converter platform of MMC and DRU-MMC is compared. Finally, simulations are carried out on PSCAD/EMTDC platform to verify the effectiveness of the proposed strategy.