The new guidelines are particularly applicable to overhead transmission lines in areas with a beautiful environment of national interest and in populated areas. From 2008-2016, Energinet.dk undergrounded three sections of the existing 400-kV overhead lines.
Submarine Cable Design
The choice of submarine cables was difficult to make as the maximum voltage for three-core cables in the market was 245 kV (Um = maximum voltage for equipment). The 245-kV three-core cable systems had been used for the connection of offshore wind farms, but cables for higher voltages had been based only on the use of single-core cables. Therefore, Energinet.dk faced a problem as no manufacturer had produced a 400-kV (Um = 420 kV) three-core submarine cable.
Submarine cables must satisfy a demanding specification in terms of the radial and longitudinal forces experienced on the seabed and during installation. Hence, the overall design of the cable required optimization of the mechanical characteristics and capital cost.
Therefore, Energinet.dk invited tenders for a range of different solutions. The tender gave cable manufacturers the freedom to offer 400-kV single-core or 400-kV three-core submarine cables. From an economical viewpoint, the three-core cables were the right solution, provided the specification satisfied all technical standards and operational risks. Although the unproven 400-kV three-core submarine cable joints were a concern, they were ordered from ABB for installation during the summer of 2013.
The 400-kV three-core submarine cable was designed with 1400-sq-mm (2.17-sq-inch) aluminum conductors and the 400-kV AC single-core underground cables with 1200-sq-mm (1.86-sq-inch) aluminum conductors. Aluminum was selected over copper for the conductors as a way to minimize the project cost. The cable insulation was cross-linked polyethylene (XLPE), which had been used with long-term success in cables designed for 400-kV operation.