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

High voltage or high current

Since electric power is a product of electric current and voltage: P=IU, one can increase, in principle, the power transmitted by a cable by either increasing the input voltage or the input current. In practice, however, electric power transmission is more energy efficient, if high-voltage (rather than high-current) powerline are used. This can be explained by the following back-of-the-envelope calculation: Define: P=power, U=voltage , I=current, i=in , o=out then: input power Pi=IiUi and the output power Po=IoUo. Due to the conservation of charge the current's absolute value is conserved (both in DC and AC cases), thus the output current is the same as the input current |Io| = |Ii| =I. Then the voltage drop is : Ui-Uo = IR or Uo = Ui-IR, the output power is Po=IUo = I (Ui-IR) and the energy efficiency = Po/Pi = I (Ui-IR)/ IUi = Ui/Ui-IR/Ui=1- IR/Ui. The latter formula shows, that decreasing operating current and increasing input voltage improves the efficiency of electric power transmission via an electric conductor.

AC or DC

Most electrical power transmission systems above ground use alternating current (AC), because transformers can easily change voltages as needed (see War of the currents for historical details). High-voltage direct current transmission requires expensive and inefficient converters at each end of a direct current line to interface to an alternating current grid. However this logic fails for below-the-ground electric powerlines, such as submarine electric cables. This is because the capacitance between the cable and its surrounding (i.e. the capacitance of capacitance of a single cable) is not negligible, when the cable is immersed into an electrically conducting salt water. 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. 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).

Conductor

As explained in the 2 preceding sections, 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. 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 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).

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.

Direct current cables

Submarine power cables under construction

Proposed submarine power cables