Transmission Systems for Grid Connection of Offshore Wind Farms : HVAC vs HVDC Breaking Point

Abstract: Offshore wind is rapidly growing and optimised grid connections are crucial for its success. Generally, costs and losses are higher for HVDC at short distances due to the converters, while HVAC costs and losses increase more rapidly with distance due to the ac cables. Hence, there is a breaking point over which HVDC becomes beneficial, which is important knowledge for grid connection design. Recent research and practice indicate increasing distances for the breaking point, enabled by the introduction of offshore reactive compensation substations (RCS) for HVAC. In the study, steady-state models of HVAC and modular-multilevel converter (MMC) based VSC HVDC systems up to 260 km have been simulated in the Matlab/Simulink based program EeFarm-II. For base case assumptions, the average loss breaking point is 80 km and the levelised cost breaking point is 229 km. The resulting breaking point with respect to levelised cost of energy (LCOE) is 205 km and with respect to net present value (NPV) 186 km, agreeing with the trend of increasing breaking points. Given the range of distances in literature, it is of interest to also investigate how the breaking point depends on assumptions on technical, practical and economic parameters. For the NPV breaking point: lifetime and interest rate have no impact, availability and cost of RCSs have low impact, electricity price has moderate impact, operation and maintenance (O&M) cost has high impact while investment cost and lead time have very high impact. This could be taken into consideration in offshore projects and in future research.