Onshore AC Grid Low Voltage Ride-Through (LVRT) of Diode-Rectifier Units based HVDC Transmission Systems for Offshore Wind Farms

Diode-rectifier units based HVDC (DRU-HVDC) transmission systems, which consist of multiple distributed offshore DRUs and an onshore modular multilevel converter (MMC), are promising candidates for delivering offshore wind energy to shores in an efficient and cost-effective way. In case of short-circuit faults in interconnected onshore ac grids, a voltage sag occurs, and it leads to a mismatch between the power delivered from the offshore wind farm and the instantaneous power capacity of the onshore MMC. However, the MMC is required to stay connected riding through the voltage sag and support reactive currents into the faulty ac grid in order to restore the voltage rapidly. In this paper, a communication-free low voltage ride-through (LVRT) strategy is proposed for DRU-HVDC systems for offshore wind farms. With the proposed strategy, the onshore MMC can support reactive currents in accordance with the grid code and track the pre-fault active power as far as possible during the LVRT within the current limit of the MMC. Thereby, the DRU-HVDC system can contribute to not only the voltage restoration but also the rotor angle stability of the onshore ac grid. The validity of the proposed strategy is verified by simulations of a +/- 200 kV, 400MW DRU-HVDC based offshore wind farm.