Coordinated control of parallel DR-HVDC and MMC-HVDC systems for offshore wind energy transmission

Parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and modular multilevel converter (MMC) based HVDC (MMC-HVDC) for transmitting offshore wind energy is investigated in this paper. An enhanced active power control scheme of the offshore MMC station is proposed to improve the power flow distribution between the MMC-HVDC and DR-HVDC links which are both connected to the offshore wind farm AC network. By regulating the offshore voltage, all the wind powers are transmitted via the DR-HVDC link in low wind conditions while the offshore MMC power is controlled around zero to reduce transmission losses, considering the efficiency superiority of DR-HVDC over its MMC counterpart. When the DR-HVDC is out of service, wind energy is transferred via the MMC-HVDC and the wind turbine generated power is automatically limited by slightly increasing the offshore AC voltage to avoid potential MMC-HVDC overload. A power curtailment control is also proposed which slightly increases the DC voltage of the DR-HVDC to enable autonomous reduction of the generated wind power so as to avoid DR-HVDC overload during MMC-HVDC outage. The proposed coordinated control only uses local measurements and, without the need for communication, can seamlessly handle transitions including various faults. The proposed scheme enables fault ride-through operation and provides a high efficient solution with flexible operation for integrating large offshore wind farms. Simulation results confirm the proposed control strategy