Submarine cables for classical HVDC have been used since the first HVDC link in 1954. Most HVDC cables are of a type that has a copper conductor and the insulation is made of oil-impregnated paper. A typical 450
Kilovolt (kV) HVDC cable has an outer diameter of approx.13 cm, and the cable laying is made from special ships which can carry around 100 km of cable.
This type of cable is not well suited as underground cable, since transporting it on trucks requires it to be cut off in short lengths and then joined in a fairly complicated operation. Therefore there is only one underground cable for classical HVDC of some distance.