This study presents a cost comparison between commercially available high-voltage DC (HVDC) and extra highvoltage AC shore connection (±320 kV voltage source converter and 420 kV-50 Hz single-core and three-core cables), for a 1 GW offshore wind farm cluster, considering transmission distances up to 400 km. The HVDC system is a point-to-point connection whereas multiple AC intermediate compensating stations are envisaged for AC when needed. Capital costs are evaluated from recently awarded contracts, operating costs include energy losses and missed revenues due to transmission system unavailability, both estimated using North Sea wind production curves. Optimal AC intermediate compensation, if any, and reactive profiles are also taken into account. Resultsshow that HVDC has lower transmission losses at distances in excess of 130 km; however, due to the combined effect of lower AC capital cost and unavailability, using three-core aluminium cables can be more convenient up to 360 km distance.
The paper deals with the steady-state operation of a radial EHVAC cable transmission system with multiple power injection points, e.g. offshore wind farms, and variable shunt compensation. Generator voltages and variable compensation settings are determined by means of a power flow-based algorithm, aiming at symmetrical current magnitude profiles along individual cable lines (CLs): the proposed operation minimizes Joule losses and maximizes cable line (CL) exploitation. Network code constraints on the reactive power exchanged with the bulk power system are also considered. Successful application to a hypothetical 199 km long, 400 kV-50 Hz cable system collecting up to 2 GW from several OWFs is presented, showing that the proposed operation ensures at least 97% transmission efficiency at full load, with near-unity generator power factor values. Moreover, possible constraints due to sending end voltage rise at full load suggest the implementation of voltage control at the network connection point
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