Cross-Skagerak
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Cross-Skagerak is the name of an HVDC transmission facility between Tjele (Denmark) and Kristiansand (Norway). Cross-Skagerrak went in service in 1977 as a bipolar HVDC scheme.
This facility was built with thyristor valves. In its initial configuration it had a capacity of 1000 megawatts across a distance of 250 kilometres. The 230 kilometre line of Cross-Skagerak consists of a 100 kilometre overhead line and a 130 kilometre underwater cable. When installed this underwater cable was the world's longest underwater HVDC power cable. The cable is laid in a water depth of 500 metres.
In 1993 the scheme was extended by HVDC Cross-Skagerak 3. Cross-Skagerak 3 is a monopolar line for a voltage of 350 kV with a capacity of 500 MW. In installing Cross-Skagerak 3, the old poles Cross-Skagerak 1 and Cross-Skagerak 2 were converted to monopolar HVDC schemes, which run with opposite polarity to Cross-Skagerak 3.
HVDC Cross-Skagerak uses electricity pylons with three conductors, mounted on pylons with two crossbars. In Denmark these pylons are designed for carrying a fourth conductor. Near Aggesund HVDC Cross-Skagerak crosses Aggesund strait overhead on tall pylons with a 470 metres long span.
The use of three underwater cables allows the interchange of hydroelectric power, thermal generation, and wind generation between Norway and Denmark, allowing overall lower cost of electricity production. For such a long underwater cable, an AC transmission scheme would not be feasible since too much of the cable's capacity would be consumed by the capacitance of the cable itself. High-voltage DC makes long underwater power transmission cables economically feasible.
