A novel topology for HVDC link connecting to offshore wind farms

Offshore wind power has developed rapidly and will play an important role in China power market in the near future due to its advantages in effectiveness and convenience. HVDC transmission technology is believed to be an effective way for integration of offshore wind power. To push the connection to grid at a low price to be achieved sooner, this paper proposes a novel topology for transmission of offshore wind power with integration of permanent magnet direct-driven wind turbines. HVDC diodes on the rectifier side and a modular multilevel converter (MMC) on the inverter side is adopted in the HVDC link. The HVDC diodes are used instead of IGBTs for rectification in sending end convertor station. An integrated control strategy based on wind converter and MMC is investigated. The presented results show a stable operation of the offshore ac-grid voltage and frequency. Moreover, the proposed system shows good behavior in the case of cable fault. It is concluded that the proposed topology is a technically feasible solution for the connection of offshore wind farms. By virtue of the HVDC diodes instead of the voltage-source converter on the offshore platform, the cost, occupied area and power loss of HVDC converters are reduced compared with the case of the conventional fully rated VSC. The system performance, meanwhile, is kept stable.