Series-MMC hybrid for enhancing grid-forming control of offshore wind turbines connected to Diode based HVdc transmission

Diode-based connection of offshore wind farms is a promising substitute for the traditional voltage source converter-based connection, due to its lower cost, size and higher reliability. A P / f and Q / V drooped based grid-forming control has been proposed to establish the offshore ac grid by wind turbines, which has a decoupling and stabilizer feature. However, it relies on dc voltage adjustment of receiving-end MMC through communication. This paper proposes a series auxiliary modular multilevel converter (aux-MMC) of merely 10% of SM number and power rating to connect to the DR for enhancing grid-forming control of the system. The dc voltage of aux-MMC varies to coordinate with active power control, while its ac side supplies reactive power and active power filtering and fully exploits its capability. The benefits of this configuration are attractive. First, the active power control is achieved locally, thus omitting communication and receiving-end coordination. Second, it can be extended to multi terminal dc transmission as studied in this paper. Third, ac passive harmonic filtering and capacitor banks are also not needed. Dimensioning and comparison to previous method are conducted and simulation results verify the proposed control of the system.