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Submarine power cable

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Cross section of the submarine power cable used in Wolfe Island Wind Farm.
HVDC connections around Europe
Red=in operation
Green=decided/under construction
Blue=planned

A submarine power cable is a transmission cable for carrying electric power below the surface of the water.[1] 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

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High voltage or high current

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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.[2]

This can be explained by the following back-of-the-envelope calculation:[3] 

Define: P=power , U=voltage , I=current, i=in , o=out
then: input power Pi=Ii*Ui  and the output power Po=Io*Uo . 
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 = I*R or Uo = Ui-I*R,
the output power is Po=I*Uo  = I* (Ui-I*R) and
the energy efficiency = Po/Pi = I* (Ui-I*R)/ I*Ui = 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

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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.[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).

Conductor

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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).[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. 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

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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

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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

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Alternating current cables

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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.

Connecting Connecting Voltage (kV) Length(km) Year Notes
Peloponnese, Greece Crete, Greece 150 135 2021 Two 3-core XLPE cables with total capacity of 2x200MVA. 174 km total length including the underground segments. Maximum depth 1000m. Total cost 380 million EUR. It is the longest submarine/underground AC cable interconnection in the world.[6][7][8]
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" [9]
Mainland British Columbia to Texada Island to Nile Creek Terminal Vancouver Island / Dunsmuir Substation 525 35 1985 Twelve, separate, oil filled single-phase cables. Nominal rating 1200 MW.[10]
Tarifa, Spain
(Spain-Morocco interconnection)		 Fardioua, Morocco
through the Strait of Gibraltar		 400 26 1998 A second one from 2006[11] Maximum depth: 660 m (2,170 ft).[12]
Norwalk, CT, USA Northport, NY, USA 138 18 A 3 core, XLPE insulated cable
Sicily Malta 220 95 2015 The Malta–Sicily interconnector
Mainland Sweden Bornholm Island, Denmark 60 43.5 The Bornholm Cable
Mainland Italy Sicily 380 38 1985 Messina Strait submarine cable replacing the "Pylons of Messina". A second 380 kV cable began operation in 2016
Germany Heligoland 30 53 [13]
Negros Island Panay Island, the Philippines 138
Douglas Head, Isle of Man, Bispham, Blackpool, England 90 104 1999 The Isle of Man to England Interconnector, a 3 core cable
Wolfe Island, Canada
for the Wolfe Island Wind Farm		 Kingston, Canada 245 7.8 2008 The first three-core XLPE submarine cable for 245 kV[14]
Cape Tormentine, New Brunswick Borden-Carleton, PEI 138 17 2017 Prince Edward Island Cables[15]
Taman Peninsula, Mainland Russia Kerch Peninsula, Crimea 220 57 2015 [16]

Direct current cables

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See also: List of HVDC projects
Name Connecting Body of water Connecting kilovolts (kV) Undersea distance Notes
Baltic Cable Germany Baltic Sea Sweden 450 250 km (160 mi)
Basslink mainland State of Victoria Bass Strait island State of Tasmania, Australia 500 290 km (180 mi)[17]
BritNed Netherlands North Sea Great Britain 450 260 km (160 mi)
COBRAcable Netherlands North Sea Denmark 320 325 km (202 mi) Commissioned September 2019
Cross Sound Cable Long Island, New York Long Island Sound State of Connecticut 150 [citation needed]
East–West Interconnector Dublin, Ireland Irish Sea North Wales and thus the British grid 200 186 km (116 mi) Inaugurated 20 September 2012
Estlink northern Estonia Gulf of Finland southern Finland 330 105 km (65 mi)
Fenno-Skan Sweden Baltic Sea Finland 400 233 km (145 mi)
HVDC Cross-Channel French mainland English Channel England 270 73 km (45 mi) very high power cable (2000 MW)[citation needed]
HVDC Gotland Swedish mainland Baltic Sea Swedish island of Gotland 150 98 km (61 mi) 1954, the first HVDC submarine power cable (non-experimental)[18] Gotland 2 and 3 installed in 1983 and 1987.
HVDC Inter-Island South Island Cook Strait North Island 350 40 km (25 mi) between the power-rich South Island (much hydroelectric power) of New Zealand and the more-populous North Island. Commissioned in 1965.
HVDC Italy-Corsica-Sardinia (SACOI) Italian mainland Mediterranean Sea the Italian island of Sardinia, and its neighboring French island of Corsica 200 385 km (239 mi) 3 cables, 1967, 1988, 1992[19]
HVDC Italy-Greece Italian mainland - Galatina HVDC Static Inverter Adriatic Sea Greek mainland - Arachthos HVDC Static Inverter 400 160 km (99 mi) Total length of the line is 313 km (194 mi)
HVDC Leyte - Luzon Leyte Island Pacific Ocean Luzon in the Philippines[citation needed]
HVDC Moyle Scotland Irish Sea Northern Ireland within the United Kingdom, and thence to the Republic of Ireland 250 63.5 km (39.5 mi) 500MW
HVDC Vancouver Island Vancouver Island Strait of Georgia mainland of the Province of British Columbia 280 33 km In operation in 1968 and was extended in 1977
Kii Channel HVDC system Honshu Kii Channel Shikoku 250 50 km (31 mi) in 2010 the world's highest-capacity[citation needed] long-distance submarine power cable[inconsistent] (rated at 1400 megawatts). This power cable connects two large islands in the Japanese Home Islands
Kontek Germany Baltic Sea Denmark
Konti-Skan[20] Sweden Kattegat Denmark 400 149 km (93 mi)
Maritime Link Newfoundland Atlantic Ocean Nova Scotia 200 170 km (110 mi) 500 MW link went online in 2017 with two subsea HVdc cables spanning the Cabot Strait.[21]
Nemo-Link[22] Belgium North Sea United Kingdom 400 140 km (87 mi)
Neptune Cable State of New Jersey Atlantic Ocean Long Island, New York 500 104.6 km (65.0 mi)[23]
NordBalt Sweden Baltic Sea Lithuania 300 400 km (250 mi) Operations started on February 1, 2016 with an initial power transmission at 30 MW.[24]
NordLink Ertsmyra, Norway North Sea Büsum, Germany 500 623 km (387 mi) Operational May 2021[25]
NorNed Eemshaven, Netherlands Feda, Norway 450 580 km (360 mi) 700 MW in 2012 previously the longest undersea power cable[26]
North Sea Link Kvilldal, Suldal, in Norway, Cambois near Blyth North Sea United Kingdom, Norway 515 720 km (450 mi) 1.4 GW the longest undersea power cable
Shetland HVDC Connection Shetland islands North Sea Scotland 600 260 km (160 mi)
Skagerrak 1-4 Norway Skagerrak Denmark (Jutland) 500 240 km (150 mi) 4 cables - 1700 MW in all[27]
SwePol Poland Baltic Sea Sweden 450
Western HVDC Link Scotland Irish Sea Wales 600 422 km (262 mi) Longest 2200 MW cable, first 600kV undersea cable[28]

Submarine power cables under construction

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  • 500 MW capacity, 165 km DC Maritime Transmission Link between the Canadian province of Newfoundland and Labrador and the province of Nova Scotia.[29]
  • 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.[30][31]
  • Black Sea submarine electric cable with a capacity of 1 GW and voltage of 500 kV will transfer green electricity from Azerbaijan through Georgia, Romania, Moldova to the EU. It is estimated to be approximately 1100 km in length and to be built in late 2029.[32]

Proposed submarine power cables

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See also

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References

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  1. ^ a b c Underwater Cable an Alternative to Electrical Towers, Matthew L. Wald, New York Times, 2010-03-16, accessed 2010-03-18.
  2. ^ 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."
  3. ^ see the derivation at https://www.google.com/books/edition/Electric_Power_Transmission_and_Distribu/KpY1hpKKwdQC?hl=en&gbpv=1&bsq=high%20voltage%20power%20transmission , page. 436.
  4. ^ 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.
  5. ^ "Submarine Power Cables - Design, Installation, Repair, Environmental aspects", by T Worzyk, Springer, Berlin Heidelberg 2009
  6. ^ "Crete-Peloponnese: The record-breaking interconnection is completed". IPTO.
  7. ^ "Crete – Peloponnese Interconnection. Selection of tenderers for the cables of one of the most important submarine interconnection projects globally". admieholding.gr. Archived from the original on 2020-10-18. Retrieved 2020-03-05.
  8. ^ "Crete – Peloponnese 150kV AC Interconnection" – via www.researchgate.net.
  9. ^ "The 132,000 volt submarine cable in the Mainland - Vancouver Island interconnection : part 3, cable laying - RBCM Archives". search-bcarchives.royalbcmuseum.bc.ca.
  10. ^ "British Columbia Transmission Corporation Application for Certificate of Public Convenience and Necessity For Vancouver Island Transmission Reinforcement Project" (PDF). Archived (PDF) from the original on 2021-05-26.
  11. ^ "A Bridge Between Two Continents", Ramón Granadino and Fatima Mansouri, Transmission & Distribution World, May 1, 2007. Consulted March 28, 2014.
  12. ^ "Energy Infrastructures in the Mediterranean: Fine Accomplishments but No Global Vision", Abdelnour Keramane, IEMed Yearbook Archived 2020-10-20 at the Wayback Machine 2014 (European Institute of the Mediterranean), under publication. Consulted 28 March 2014.
  13. ^ "Mit der Zukunft Geschichte schreiben". Dithmarscher Kreiszeitung (in German). Archived from the original on 19 July 2011.
  14. ^ "Wolfe Island Wind Project" (PDF). Canadian Copper CCBDA (156). 2008. Retrieved 3 September 2013.
  15. ^ "P.E.I.'s underwater electric cable project officially plugged in - New underwater cables supply about 75% of the Island's electricity". CBC News. Aug 29, 2017. Retrieved 1 August 2020.
  16. ^ The 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]".
  17. ^ "Basslink - About". www.basslink.com.au. Retrieved 11 February 2018.
  18. ^ "European Subsea Cables Association - Submarine Power Cables". www.escaeu.org.
  19. ^ "Sardinia's electricity transmission network". 2009.
  20. ^ "THE KONTI-SKAN HVDC SCHEME". www.transmission.bpa.gov. Archived from the original on 2005-09-02.
  21. ^ "Maritime Link Infrastructure". Emera Newfoundland and Labrador.
  22. ^ Chestney, Nina (January 14, 2019). "New UK-Belgium power link to start operating on Jan. 31". Reuters – via www.reuters.com.
  23. ^ "Home". Neptune Regional Transmission System.
  24. ^ "Power successfully transmitted through NordBalt cable". litgrid.eu. 2016-02-01. Retrieved 2016-02-02.
  25. ^ "NordLink - TenneT". www.tennet.eu. Retrieved 2021-10-17.
  26. ^ "The Norned HVDC Cable Link" (PDF). www05.abb.com.
  27. ^ "Skagerrak An excellent example of the benefits that can be achieved through interconnections". new.abb.com. Archived from the original on 2016-01-20. Retrieved 2016-01-21.
  28. ^ "None". www.westernhvdclink.co.uk.
  29. ^ "Lower Churchill Project". Nalcor Energy. Archived from the original on 2016-11-29. Retrieved 2013-06-08.
  30. ^ "Kabel til England - Viking Link". energinet.dk. Archived from the original on 2017-03-23. Retrieved 2015-11-12.
  31. ^ "Denmark - National Grid". nationalgrid.com. Archived from the original on 2016-03-03. Retrieved 2016-02-03.
  32. ^ "Quadrilateral agreement inked on Black Sea electric cable Link". Archived from the original on 2022-12-17. Retrieved 2022-12-17.
  33. ^ "Australia Fast Tracks Approval Process for $16 Billion Solar Power Export Project". Reuters. 2020-07-30. ISSN 0362-4331. Retrieved 2020-11-03.
  34. ^ The EuroAsia Interconnector document, www.euroasia-interconnector.com October 2017.
  35. ^ "ENERGY: End to electricity isolation a step closer". Financial Mirror. 2017-10-19. Retrieved 2017-01-04.
  36. ^ "Cyprus group plans Greece-Israel electricity link". Reuters. 2012-01-23. Archived from the original on 2012-01-26.
  37. ^ Transmission Developers Inc. (2010-05-03), Application for Authority to Sell Transmission Rights at Negotiated Rates and Request for Expedited Action, Federal Energy Regulatory Commission, p. 7, retrieved 2010-08-02
  38. ^ "Territory to Study Linking Power Grid to Puerto Rico". stcroixsource.com. June 29, 2010. Archived from the original on July 16, 2011.
  39. ^ HVDC Transmission & India-Sri Lanka Power Link www.geni.org 2010
  40. ^ "Malta signs €182 million interconnector contract". Times of Malta. 15 December 2010.
  41. ^ Čavčić, Melisa (July 31, 2024). "World's 'most ambitious' subsea interconnector igniting zest for clean power superhubs: Embracing NATO-L to reinforce energy security bonds between America and Europe". Offshore Energy. Retrieved October 28, 2024.
  42. ^ "Taiwan power company-Taipower Events". www.taipower.com.tw. Archived from the original on 2014-05-17.
  43. ^ Carrington, Damian (2012-04-11). "Iceland's volcanoes may power UK". The Guardian. London.
  44. ^ FAB website fablink.net, as well as (fr) Interconnexion France Aurigny Grand-Bretagne website rte-france.com, site of Réseau de Transport d'Électricité.
  45. ^ "EuroAfrica Interconnector". www.euroafrica-interconnector.com.
  46. ^ Electricity Cable Aims to Link Cyprus, Egypt, Greece Bloomberg, February 8, 2017
  47. ^ "ENERGY: EuroAfrica 2,000MW cable boosts Egypt-Cyprus ties". Financial Mirror. February 8, 2017.
  48. ^ "EEHC, Euro Africa Company sign MoU to conduct a feasibility study to link Egypt, Cyprus, Greece". dailynewsegypt.com. February 6, 2017.
  49. ^ "Proposed 11kV Submarine Cables Replacement Connecting Liu Ko Ngam and Pak Sha Tau Tsui at Kat O" (PDF). Government of Hong Kong. 22 January 2016. Archived (PDF) from the original on 13 March 2022. Retrieved 13 March 2022.
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