Control of an MMC-based HVDC link for offshore wind farms to enable reliable ancillary service provision via wind turbines

This paper describes the modelling and controls for an HVDC-connected offshore wind power plant using MMCs as HVDC converters. The important controls and operational strategy from the research project "Offshore Wind Farms as Power Plants" ("OffWiPP") are highlighted within this paper. A new approach for controlling an MMC-based HVDC link is introduced, mainly leveraging the wind turbine rotational energy for onshore grid response. Using the energies stored in the HVDC or wind farm converters for onshore grid events is omitted. The high-and low-level controls for the MMCs are described within this paper, including the energy balancing control, the GFM control concept with an additional power limitation, and a current limitation scheme allowing the limitation of symmetrical and asymmetrical faults. Time domain simulations are used to investigate the overall system behaviour during onshore grid phase angle jumps, phase angle jumps combined with a RoCoF, and symmetrical and asymmetrical fault scenarios.