NR Electric Co., Ltd., Nanjing 211102, China

Due to the characteristics of low cost, small size and weight, and high reliability, the transmission scheme using the diode rectifier unit (DRU) at the sending end and the modular multilevel converter (MMC) at the receiving end has attracted much attention in the field of offshore wind power transmission. Since DRU cannot form an AC power grid and provide reverse active power, this paper proposes a remote offshore wind power transmission scheme based on DRU-MMC with black-start and grid-forming capability. The proposed scheme adds conventional small-capacity voltage-source auxiliary converters, energy-dissipating units, and necessary fast switches on the basis of DRU-MMC transmission technology. With the multiplexing of DC marine cables and the topology reconstruction by the fast switch switching operations, the low-voltage circuit for the black-start power supply and the high-voltage circuit for wind power transmission are constructed respectively, and the online-switching between the black-start condition and the power transmission condition is realized. The grid-forming control strategies of the auxiliary converter in different operation stages are proposed to solve the formation of the offshore AC power grid, the reactive power compensation and harmonics suppression of the DRU. Moreover, the restriction of the DRU-MMC transmission scheme on the type of wind farm is eliminated by the proposed scheme. Finally, the superiority and the effectiveness of the proposed scheme are verified by means of economic analysis and case simulation.

This work is supported by National Key R&D Program of China (No. 2021YFB2401100).