In contrast to high-temperature superconductors, so-called high-current cables have a significantly larger cross-sections compared to standard conductors (see ‘High-temperature superconductor’) and are preferred for technical and economic reasons when constructing new lines along existing or new routes. As a general rule, high-current cabling has a continuous current-carrying capacity of 3,600 or 4,000 A per circuit with a permissible conductor cable end temperature of 80°C. Due to its larger cross-section, high-current cabling results in lower levels of network losses when transporting an identical amount of current and in lower noise levels compared to standard conductors and the above-mentioned HTS lines. Since high-current cables have investment-cost advantages and operators have many years of experience working with this technology, it is generally preferred to HTS-cables.
High voltage direct current transmission (HVDC) is a process for transmitting large volumes of electrical power at very high voltages (100 – 1000 kV) over very long distances. The abbreviation ‘DC’ (direct current) is also often used to refer to this. High voltage inverters are required in order to feed the electricity into the conventional energy grid; the conversion takes place in tansformer facilities and substations.
High-temperature superconductors (HT superconductors or HTS) are cables that use certain materials allowing them to operate at higher temperatures than standard aluminium or steel cables. Standard cables have a maximum permissible temperature of 80°C, whereas high-temperature superconductors can reach operating temperatures of 150°C to 210°C. Thanks to this level of temperature resistance, HT superconductors offer a greater current-carrying capacity than standard conductors with a comparable cross-section.There are different types of HT superconductors – TAL (thermal resistant aluminium) conductors, which are already in use, and the latest generation of conductor cables, HTLS conductors (high temperature, low sag). TAL conductors have a maximum operating temperature of 150°C, whilst HTLS conductors can operate at up to 210°C. Due to the special core material used in HTLS conductors, they sag less compared to other types of conductor cables under higher levels of load. Provided that it is technically feasible, recabling routes from standard conductors to HT superconductors is one option for grid development in accordance with the NOVA principle.
An undersea cable runs from every offshore wind farm to a platform with a rectifier station on it (usually referred to as a converter station). There, the three-phase alternating current produced by the wind turbines is converted to to direct current (DC) and transported along high-voltage direct current transmission lines (HVDC) under the sea and over land to the nearest infeed point – a converter station – on land. This technology is currently only used for connecting offshore wind farms in the North Sea to the grid.
The energy generated at offshore wind farms is transported to a point in or close to the respective offshore wind farm cluster. If a DC grid connection system is being used, the energy is then transmitted from here through a HVDC transmission line to an onshore grid connection point. One or more AC connections and a HVDC transmission line form a DC grid connection system.