Submarine power cable

[1] These are called "submarine" because they usually carry electric power beneath salt water (arms of the ocean, seas, straits, etc.)

As a result of this the point where it is more economical to use AC falls below 100 km in line length [4] 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), this will result in a noticeable phase shift between voltage and current, thus significantly decreasing the efficiency of the transmitted power, which is a vector product of current and voltage.

[4] An AC electric powerline under water would require larger, therefore more costly, conductors for a given quantity of usable power to be transmitted.

When the reasons for high voltage transmission , the preference for AC, and for capacitive currents are combined, one can understand why there are no underwater high electric power cables longer than 1000 km (see the table in "Operational submarine power cables" section below).

As explained in the 2 preceding sections, the purpose of submarine power cables is the transport of electric current at high voltage.

[5] 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.

The conductor is made from copper or aluminum wires, the latter material having a small but increasing market share.

For voltages ≥ 12 kV the conductors are round so that the insulation is exposed to a uniform electric field gradient.

The entire cable core is impregnated with a low-viscosity insulation fluid (mineral oil or synthetic).