Two offshore network solutions have been studied under different assumptions (Fig. 1). A radial solution has been considered in conjunction with some new interconnectors: Scotland-England (East and West), England-Belgium, England-Norway, and Germany-Norway. For these new interconnectors, there are three overall sizing assumptions: 0 (without new interconnectors), 1 and 3 GW. An HVDC multi-terminal grid solution has been studied as well without new interconnectors. Two different sizes of corridors have been taken: one with “abundant” and one with “moderate” transport capacity (i.e. reducing redundancies to the minimum).
Fig. 1 – Radial “point to point” connections (in blue) and future interconnectors (in red) (left); mixed offshore grid: MTDC (in orange) and radial “point to point” (in blue), without future interconnectors (right)
It was quantitatively established that DCGs use HVDC underwater cable capacities more effectively than radial DC connection schemes to feed offshore wind power back to the continent, along with the additional benefit of interconnecting energy production areas at the European scale.
Global cost-benefit comparisons between radial DC connection and DCG schemes were carried out while varying parameters like the cost of CO2 emissions, of new HVDC solutions (including the HVDC Circuit Breaker) and cable capacities. The results highlight the difficulty to take a choice among the several available technological solutions: the convenience or not to deploy HVDC multi-terminal grids depends on the costs for its realization (not yet clear especially with reference to DC circuit breakers) and on the future evolution of the Voltage Source Converter (VSC) technology. However, the DCG can also implement very beneficial functions for the operation of the onshore AC grids connected to it, which were not assessed in the framework of this study: improved ACsecurity margins through appropriate power injections via the onshore DCG terminals; ancillary services like voltage control, frequency support, synthetic inertia or damping of inter-area oscillations; black start capability restoration of the AC system from the offshore grid. Therefore, grid schemes are more costly in terms of investment but provide added benefits for operation and remain thus competitive overall. At the 2020 or 2030 horizon, other uncertainties like regulatory framework on structural adequacy of the European generation mix could also play a significant role in the balance.