Analysis of Grid Faults in Offshore Wind Farm with HVDC Connection

Future offshore wind farms are expected to be built farther away from shore and have larger capacities than today. This leads to new challenges related to grid connection. At distances longer than roughly 100 km, HVDC transmission is preferred over AC transmission due to large charging currents in AC-cables. Conventional LCC (line commutated converter) HVDC is not suited for connection to weak grids like offshore wind farms, and the less mature VSC (voltage source converter) HVDC technology is preferred instead. A future large offshore wind farm with full power converter turbines and three-terminal VSC HVDC grid connection has been modelled in PSCAD. With three terminals the HVDC link can be used for direct transmission between the onshore terminals in addition to transmission of wind power. Due to the power electronics interfaces, the system has low short circuit capacity and is missing inertia. Also, DC-cables are discharged very fast during faults. This leads to different fault responses than in conventional grids. This work focuses on fault responses and protection of a HVDC-connected wind farm, both within the wind farm itself and in the HVDC-link.