Submarine power cable

A submarine power cable is a 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.
Design technologies.
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 (resembling the design of a coaxial cable). Modern three-core cables (e.g. for the connection of offshore wind turbines) often carry optical fibers for data transmission or temperature measurement, in addition to the electrical conductors.
Conductor.
The conductor is made from copper or aluminum wires, the latter material having a small but increasing market share. Conductor sizes ≤ 1200 mm2 are most common, but sizes ≥ 2400 mm2 have been made occasionally. For voltages ≥ 12 kV the conductors are round, so that the insulation is exposed to a uniform electric field gradient. The conductor can be stranded from individual round wires, or can be a single solid wire. In some designs, profiled wires (keystone wires) are laid up to form a round conductor with very small interstices between the wires.
Insulation.
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, with an 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 in cables up to 525 kV for when the cable gets warm but rarely used in submarine cables due to oil pollution risk with cable damage.
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. Mass-impregnated insulation can be used for massive HVDC cables up to 525 kV.
Armoring.
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 three-core cables, three cable cores are laid-up in a spiral configuration before the armoring is applied.
The armoring 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).
AC or DC.
Most electrical power transmission systems use alternating current (AC), because transformers can easily change voltages as needed. High-voltage direct current transmission requires a converter at each end of a direct current line to interface to an alternating current grid. A system using submarine power cables may be less costly overall if using high-voltage direct current transmission, especially on a long link where the capacitance of the cable would require too much additional charging current. The inner and outer conductors of a cable form the plates of a capacitor, and if the cable is long (on the order of tens of kilometres), the current that flows through this capacitance may be significant compared to the load current. This would require larger, therefore more costly, conductors for a given quantity of usable power to be transmitted.
Operational submarine power cables.
Alternating current cables.
Alternating-current (AC) submarine cable systems for transmitting lower amounts of three-phase electric power can be constructed with three-core cables in which all three insulated conductors are placed into a single underwater cable. Most offshore-to-shore wind-farm cables are constructed this way.
For larger amounts of transmitted power, the AC systems are composed of three separate single-core underwater cables, each containing just one insulated conductor and carrying one phase of the three phase electric current. A fourth identical cable is often added in parallel with the other three, simply as a spare in case one of the three primary cables is damaged and needs to be replaced. This damage can happen, for example, from a ship's anchor carelessly dropped onto it. The fourth cable can substitute for any one of the other three, given the proper electrical switching system.