Submarine power cable

A submarine power cable is a major transmission cable for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water (arms of the ocean, seas, straits, etc.) but it is also possible to use submarine power cables beneath fresh water (large lakes and rivers). Examples of the latter exist that connect the mainland with large islands in the St. Lawrence River.

The purpose of submarine power cables is the transport of electric current at high voltage. The electric core is a concentric assembly of inner conductor, electric insulation and protective layers. The conductor is made from copper or aluminum wires, the latter material having a small but increasing market share. Conductor sizes ≤ 1200 are most common, but sizes ≥ 2400 mm2 have been made occasionally. For voltages ≥ 12 kV the conductors are round. Three different types of electric insulation around the conductor are mainly used today. Cross-linked polyethylene (XLPE) is used up to 420 kV system voltage. It is produced by extrusion in insulation thickness of up to about 30 mm. 36 kV class cables have only 5.5 – 8 mm insulation thickness. Certain formulations of XLPE insulation can also be used for DC. Low-pressure oil-filled cables have an insulation lapped from paper strips. The entire cable core is impregnated with a low-viscosity insulation fluid (mineral oil or synthetic). A central oil channel in the conductor facilitates oil flow when the cable gets warm. Rarely used in submarine cables due to oil pollution risk at cable damage. Is used up to 525 kV. Mass-impregnated cables have also a paper-lapped insulation but the impregnation compound is highly viscous and does not exit when the cable is damaged. MI insulation can be used for massive HVDC cables up to 525 kV. Cables ≥ 52 kV are equipped with an extruded lead sheath to prevent water intrusion. No other materials have been accepted so far. The lead alloy is extruded onto the insulation in long lengths (over 50 km is possible). In this stage the product is called cable core. In single-core cables the core is surrounded by a concentric armoring. In thre-core cables, three cable cores are laid-up in a spiral configuration before the armoring is applied. The armouring consists most often of steel wires, soaked in bitumen for corrosion protection. Since the alternating magnetic field in ac cables causes losses in the armoring those cables are sometimes equipped with non-magnetic metallic materials (stainless steel, copper, brass). Modern three-core cables, e.g. for the interconnection of offshore wind turbines) carry often optical fibers for data transmission or temperature measurement.

...
Wikipedia