Hybrid bipolar VSC-DRU HVdc connection for offshore wind farms

This paper presents a hybrid bipolar high voltage direct current (HVdc) link for offshore wind farms (OWFs) connection. Voltage source converter (VSC) technology is used in one of the offshore poles, using a diode rectifier unit (DRU) in the other one. Therefore, economic and power-loss advantages of the DRU over the VSC are leveraged by changing one of the HVdc poles conventional technology. In the proposed control strategy, VSC reactive power is used to control OWF ac grid frequency, allowing the DRU commutation and active power transfer. VSC active power is used for OWF ac grid start-up and hybrid bipolar HVdc link power flow control. OWF ac grid voltage magnitude is controlled while the DRU pole is blocked, and it is automatically accommodated to the required value when the DRU is conducting. Moreover, VSC is also used as an active filter for the main DRU current harmonic components, thus removing the huge passive ac filters from the offshore platform. Poles balanced and unbalanced power flow control strategies are proposed for the hybrid bipolar HVdc, considering both bipolar with neutral return and rigid bipolar configurations. Simulation results validate the proposal which aims reducing cost and size of the OWF HVdc connection.