A submarine power cable is a transmission cable for carrying electric power below the surface of the water.Underwater Cable an Alternative to Electrical Towers, Matthew L. Wald, New York Times, 2010-03-16, accessed 2010-03-18. 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
As key tools in bulk power transmission, submarine power cables tend to operate at high voltage in order to minimize resistive losses between the endpoints. https://www.google.com/books/edition/Electric_Power_Transmission_and_Distribu/KpY1hpKKwdQC?hl=en&gbpv=1&bsq=high%20voltage%20power%20transmission page 436: "The possibility for a reduction in current for an increase in voltage has an important economic aspect of power transmission. In the case of a transmission system the load, which the conductors can carry, will depend on the heating effects of the current. Hence, of the current can be reduced by using a high voltage, the resistance can be increased without incurring additional losses and causing a greater temperature rise. Therefore, we can use smaller conductors, thus, saving cost. Alternatively, with the same conductor the losses and voltage drops are reduced and the efficiency of transmission is increased." Unlike overhead powerlines, many submarine power cables tend to operate with DC current. Electrical phases must endure close proximity inside the cable, increasing parasitic capacitance. It is more economical to use AC only with lines shorter than 100 km in length, in which case losses at the landing point grid interfaces dominate.Ardelean, M.; Minnebo, P. The suitability of seas and shores for building submarine power interconnections. Renewable Sustainable Energy Rev 2023, 176, 10.1016/j.rser.2023.113210.
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 indicated above, submarine power cables transport 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)."Submarine Power Cables - Design, Installation, Repair, Environmental aspects", by T Worzyk, Springer, Berlin Heidelberg 2009 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.
ConnectingConnectingVoltage (kV)Length(km)YearNotes Peloponnese, Greece Crete, Greece 150 135 2021 Two 3-core XLPE cables with total capacity of 2x200 MVA. 174 km total length including the underground segments. Maximum depth 1000 m. Total cost 380 million EUR. It is the longest submarine/underground AC cable interconnection in the world.Mainland British Columbia to Gulf Islands Galiano Island, Parker Island, and Saltspring Island thence to North Cowichan Vancouver Island 138 33 1956 "The cable became operational on 25 September 1956" Mainland British Columbia to Texada Island to Nile Creek TerminalVancouver Island / Dunsmuir Substation 525 35 1985 Twelve, separate, oil filled single-phase cables. Nominal rating 1200 MW.Tarifa, Spain (Spain-Morocco interconnection)Fardioua, Morocco through the Strait of Gibraltar400261998 A second one from 2006"A Bridge Between Two Continents", Ramón Granadino and Fatima Mansouri, Transmission & Distribution World, May 1, 2007. Consulted March 28, 2014. Maximum depth: ."Energy Infrastructures in the Mediterranean: Fine Accomplishments but No Global Vision", Abdelnour Keramane, IEMed Yearbook 2014 (European Institute of the Mediterranean), under publication. Consulted 28 March 2014.Norwalk, CT, USANorthport, NY, USA138 18 A 3 core, XLPE insulated cableSicilyMalta220 95 2015 The Malta–Sicily interconnectorMainland SwedenBornholm Island, Denmark60 43.5 The Bornholm CableMainland ItalySicily380381985 Messina Strait submarine cable replacing the "Pylons of Messina". A second 380 kV cable began operation in 2016GermanyHeligoland30 53 Negros IslandPanay Island, the Philippines138 Douglas Head, Isle of Man,Bispham, Blackpool, England90104 1999The Isle of Man to England Interconnector, a 3 core cable Wolfe Island, Canada for the Wolfe Island Wind FarmKingston, Canada245 7.8 2008The first three-core XLPE submarine cable for 245 kV Cape Tormentine, New BrunswickBorden-Carleton, PEI13817 2017Prince Edward Island CablesTaman Peninsula, Mainland RussiaKerch Peninsula, Crimea22057 2015The corresponding page on Russian Wikipedia cites the June 15, 2015 changes (in Russian) to Russian federal program "Socio-economic development of the Republic of Crimea and the city of Sevastopol until 2020 [Социально-экономическое развитие Республики Крыми г. Севастополя до 2020 года]".
Direct current cables
NameConnectingBody of waterConnectingkilovolts (kV)Undersea distanceYearNotesBaltic CableGermanyBaltic SeaSweden450 1994Basslinkmainland State of VictoriaBass Straitisland State of Tasmania, Australia5002005BritNedNetherlandsNorth SeaGreat Britain4502010COBRAcableNetherlandsNorth SeaDenmark3202019Cross Sound CableLong Island, New YorkLong Island SoundState of Connecticut150 2003East–West InterconnectorDublin, IrelandIrish SeaNorth Wales and thus the British grid200 2012Estlinknorthern EstoniaGulf of Finlandsouthern Finland 330 2006Fenno-SkanSwedenBaltic SeaFinland400 1989HVDC Cross-ChannelFrench mainlandEnglish ChannelEngland270 1986 very high power cable (2000 MW)HVDC GotlandSwedish mainlandBaltic SeaSwedish island of Gotland150 1954 1954, the first HVDC submarine power cable (non-experimental) Gotland 2 and 3 installed in 1983 and 1987.HVDC Inter-IslandSouth IslandCook StraitNorth Island 3501965 between the power-rich South Island (much hydroelectric power) of New Zealand and the more-populous North Island.HVDC Italy-Corsica-Sardinia (SACOI)Italian mainlandMediterranean Seathe Italian island of Sardinia, and its neighboring French island of Corsica200 1967 3 cables, 1967, 1988, 1992HVDC Italy-GreeceItalian mainland - Galatina HVDC Static InverterAdriatic SeaGreek mainland - Arachthos HVDC Static Inverter4002001 Total length of the line is 313 km (194 mi)HVDC Leyte - LuzonLeyte IslandPacific OceanLuzon in the Philippines 1998HVDC MoyleScotlandIrish SeaNorthern Ireland within the United Kingdom, and thence to the Republic of Ireland250 2001 500MWHVDC Vancouver IslandVancouver IslandStrait of Georgiamainland of the Province of British Columbia280 33 km 1968 In operation in 1968 and was extended in 1977Kii Channel HVDC systemHonshuKii ChannelShikoku250 2000 in 2010 the world's highest-capacity long-distance submarine power cable (rated at 1400 megawatts). This power cable connects two large islands in the Japanese Home IslandsKontekGermanyBaltic SeaDenmark 1995Konti-SkanSwedenKattegatDenmark400 1965Commissioned:1965 (Kontiskan 1);1988 (Kontiskan 2)
Decommissioned:2006 (Kontiskan 1)Maritime LinkNewfoundlandAtlantic OceanNova Scotia200 2017 500 MW link went online in 2017 with two subsea HVdc cables spanning the Cabot Strait.Nemo-LinkBelgiumNorth SeaUnited Kingdom400 2019Neptune CableState of New JerseyAtlantic OceanLong Island, New York5002003NordBaltSwedenBaltic SeaLithuania3002015Operations started on February 1, 2016 with an initial power transmission at 30 MW.NordLink Ertsmyra, NorwayNorth SeaBüsum, Germany5002021Operational May 2021NorNedEemshaven, Netherlands Feda, Norway4502012 700 MW in 2012 previously the longest undersea power cableNorth Sea Link Kvilldal, Suldal Municipality, Norway, North Sea Cambois near Blyth, United Kingdom5152021 1.4 GW the longest undersea power cableShetland HVDC ConnectionShetland islandsNorth SeaScotland600 2024Skagerrak 1-4NorwaySkagerrakDenmark (Jutland)500 1977 4 cables - 1700 MW in all SwePolPolandBaltic SeaSweden 450 2000Western HVDC LinkScotlandIrish SeaWales 600 2019 Longest 2200 MW cable, first 600kV undersea cable
Submarine power cables under construction 500 MW capacity, 165 km DC Maritime Transmission Link between the Canadian province of Newfoundland and Labrador and the province of Nova Scotia. British and Danish power companies (National Grid and Energinet.dk, respectively) are building Viking Link, a 740 km cable to provide the two countries with 1,400 MW transmission by 2022.