Submarine power cable

Cross section of the submarine power cable used in Wolfe Island Wind Farm.

A submarine power cable is a major 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

Most power systems use alternating current (AC). This is due mostly to the ease with which AC voltages may be stepped up and down, by means of a transformer. When the voltage is stepped up, current through the line is reduced, and since resistive losses in the line are proportional to the square of the current, stepping up the voltage significantly reduces the resistive line losses. The lack of a similarly simple and efficient system to perform the same function for DC made DC systems impractical in the late 19th and early 20th centuries. (Available devices, such as the rotary converter, were less efficient and required considerably more maintenance.) As technology improved, it became practical to step DC voltages up or down, though even today the process is much more complex than for AC systems. A DC voltage converter often consists of an inverter - essentially a high-power oscillator - to convert the DC to AC, a transformer to do the actual voltage stepping, and then a rectifier and filter stage to convert the AC back to DC.^[2]

DC switch gear is larger and more expensive to produce, since arc suppression is more difficult. When a switch or fuse first opens, current will continue to flow in an arc across the contacts. Once the contacts get far enough apart, the arc will extinguish because the electric field strength (volts per meter) is insufficient to sustain it. In AC circuits, current drops to zero twice during each AC cycle, at which time the arc extinguishes. If the distance between the contacts is still relatively small, the voltage will re-initiate an arc. Since DC is constant and these zero-crossing events do not occur, a DC switch must be designed to interrupt the full rated voltage and current, leading to larger and more expensive switching equipment.^[3] The voltage required to re-initiate an extinguished arc is much greater than the voltage required to sustain an arc.

DC power transmission does have some advantages over AC power transmission. AC transmission lines need to be designed to handle the peak voltage of the AC sine wave. However, since AC is a sine wave, the effective power that can be transmitted through the line is related to the root mean squared (RMS) value of the voltage, which for a sine wave is only $1/{\sqrt {2}}$ or about 0.7 times the peak value. This means that for the same size wire and same insulation on standoffs and other equipment, a DC line can carry ${\sqrt {2}}$ or just over 1.4 times as much power as an AC line.^[4]

AC power transmission also suffers from reactive losses, due to the natural capacitance and inductive properties of wire. DC transmission lines do not suffer reactive losses. The only losses in a DC transmission line are the resistive losses, which are present in AC lines as well.

For an overall power transmission system, this means that for a given amount of power, AC requires more expensive wire, insulators, and towers but less expensive equipment like transformers and switch gear on either end of the line. For shorter distances, the cost of the equipment outweighs the savings in the cost of the transmission line. Over longer distances, the cost differential in the line starts to become more significant, which makes high-voltage direct current (HVDC) economically advantageous.^[5]

For underwater transmission systems, the line losses due to capacitance are much greater, which makes HVDC economically advantageous at a much shorter distance than on land.^[6]

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.

Connecting	Connecting	Voltage (kV)	Notes
Mainland British Columbia to Texada Island to Nile Creek Terminal	Vancouver Island / Dunsmuir Substation	525	Reactor station at overhead crossing of Texada Island. Two 3 phase circuits - Twelve separate oil filled single phase cables. Shore section cooling facilities. Nominal rating 1200 MW (1600 MW - 2hr overload)
Tarifa, Spain (Spain-Morocco Interconnection)	Fardioua, Morocco through the Strait of Gibraltar	400	The Spain-Morocco Interconnection consists of two 400-kV, AC submarine cables operated jointly by Red Eléctrica de España (Spain) and Office National de l'Électricité (Morocco); the first began operating in 1998 (28 km long), the second in 2006 (31 km long).[7] The total underwater length of the cables through the Strait of Gibraltar is 26 km and the maximum depth is 660 meters.[8]
Mainland Sweden	Bornholm Island, Denmark, Bornholm Cable	60
Mainland Italy - Sicily	Italy-Sicily	380	Under the Strait of Messina, this submarine cable replaced an earlier, and very long overhead line crossing (the "Pylons of Messina")
Germany	Heligoland	30	[9]
Negros Island	Panay Island, in the Philippines	138
Isle of Man to England Interconnector		90	a 3 core cable over a distance of 104 km
Wolfe Island, Canada	Kingston, Canada	245	The 7.8 km cable installed in 2008 for the Wolfe Island Wind Farm was the world's first 3-core XLPE submarine cable to achieve a 245 kV voltage rating.[10]

Direct current cables

Name	Connecting	Body of water	Connecting	kilovolts (kV)	Undersea distance	Notes
Baltic-Cable	Germany	Baltic Sea	Sweden	450	250 kilometres (160 mi)
Basslink	mainland State of Victoria	Bass Strait	island State of Tasmania, Australia	500	290 kilometres (180 mi)[11]
BritNed	Netherlands	North Sea	Great Britain	450	260 kilometres (160 mi)
Cross Sound Cable	Long Island, New York	Long Island Sound	State of Connecticut
East–West Interconnector	Ireland	Irish Sea	Wales/England and thus the GB grid		186 kilometres (116 mi)	Inaugurated 20 September 2012
Estlink	northern Estonia	Gulf of Finland	southern Finland	330	105 kilometres (65 mi)
Fenno-Skan	Sweden	Baltic Sea	Finland	400	233 kilometres (145 mi)
HVDC Cross-Channel	French mainland	English Channel	England			very high power cable (2000 MW)
HVDC Gotland	Swedish mainland	Baltic Sea	Swedish island of Gotland			the first HVDC submarine power cable (non-experimental)
HVDC Inter-Island	South Island	Cook Strait	North Island		40 kilometres (25 mi)	between the power-rich South Island (much hydroelectric power) of New Zealand and the more-populous North Island
HVDC Italy-Corsica-Sardinia (SACOI)	Italian mainland	Mediterranean Sea	the Italian island of Sardinia, and its neighboring French island of Corsica
HVDC Italy-Greece	Italian mainland - Galatina HVDC Static Inverter	Adriatic Sea	Greek mainland - Arachthos HVDC Static Inverter	400	160 kilometers (100 miles)	Total length of the line is 313 km (194 mi)
HVDC Leyte - Luzon	Leyte Island	Pacific Ocean	Luzon in the Philippines
HVDC Moyle	Scotland	Irish Sea	Northern Ireland within the United Kingdom, and thence to the Republic of Ireland
HVDC Vancouver Island	Vancouver Island	Strait of Georgia	mainland of the Province of British Columbia
Kii Channel HVDC system	Honshu	Kii Channel	Shikoku	250	50 kilometres (31 mi)	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 Islands
Kontek	Germany	Baltic Sea	Denmark
Konti-Skan[12]	Sweden	Baltic Sea	Denmark	400	149 kilometres (93 mi)
Neptune Cable	State of New Jersey	Atlantic Ocean	Long Island, New York	345	64 miles (103 km)[13]
NordBalt	Sweden	Baltic Sea	Lithuania	300	400 kilometres (250 mi)	Operations started on February 1, 2016 with an initial power transmission at 30 MW.[14]
Skagerrak 1-4	Norway		Denmark (Jutland)	500	240 kilometres (150 mi)	4 cables - 1700 MW in all[15]
SwePol	Poland	Baltic Sea	Sweden	450
NorNed	Eemshaven, Netherlands		Feda, Norway	450	580 kilometres (360 mi)	700 MW in 2012 the longest undersea power cable[16]

Proposed submarine power cables

• EuroAsia Interconnector, a 1,000 km submarine power cable, reaching depths of up to 2,000 meters under sea level, with the capacity to transmit 2,000 megawatts of electricity connecting Asia and Europe (Israel-Cyprus-Greece)^[17]
• Champlain Hudson Power Express, 335-mile line. The Transmission Developers Company of Toronto, Ontario, is proposing "to use the Hudson River for the most ambitious underwater transmission project yet. Beginning south of Montreal, a 335-mile line would run along the bottom of Lake Champlain, and then down the bed of the Hudson all the way to New York City."^[18]
• Power Bridge, Hawaii^[1]
• Power Bridge, State of Maine^[1]
• Puerto Rico to the Virgin Islands^[19]
• 400 kV HVDC India to Sri Lanka^[20]
• Atlantic Wind Connection between Delaware and New Jersey, potentially between Virginia and New York^[21]
• 500 MW capacity, 165 km DC Maritime Transmission Link between the Canadian province of Newfoundland and Labrador and the province of Nova Scotia.^[22]
• 220 kV HVAC, 225 megawatts, 117 km Magħtab (Malta) and Ragusa (Sicily)^[23]
• The 58.9-km, 161-kV Taiwan PengHu submarine power cable system (T-P-Cable), the first submarine project of the Taiwan Power Company (Taipower) in this level, will be commercially operated in 2012.
• Skagerrak 4, addition to the 3 DC cables between Norway and Denmark, 700 MW, 140 km, ready 2014
• The British and Icelandic Governments are in "active discussion" to build a cable between Scotland and Iceland powered by geothermal energy.^[24]
• Norwegian and German operators have agreed to build a cable transmitting up to 1,400 MW between the two countries by 2018.^[25]
• British and Danish operators (National Grid and Energinet.dk respectively) have agreed to study Viking Link, a 740 km cable to provide the two countries with 1,400 MW transmission by 2022.^[26][27]
• British and Norwegian operators (National Grid and Statnett) have agreed to jointly construct NSN Link, a 730 km cable to provide the two countries with 1,400 MW transmission by 2021. Such a cable would be one of the longest in the world and cost between 1.5 and 2 billion Euro.^[28]
• Western Link, a 2,200 MW HVDC transmission line connecting Hunterson on the West coast of Scotland to Connah's Quay in North Wales through a 385 km submarine cable across the Irish sea. Construction commenced in 2013, with completion anticipated in 2016.^[29]
• Eastern Link, a proposed 2,000 MW HVDC line connecting Peterhead on the East coast of Scotland to Sunderland via an approximately 320 km submarine cable under the North Sea. The system is planned for operation before 2018.^[30]
• On February 1, 2016 Danish and Dutch operators (Energinet.dk and TenneT) awarded construction contracts to Siemens and Prysmian for COBRAcable, a 294 km submarine cable to provide the two countries with 700 MW transmission at 320 kV DC from 2019.^[31][32]

See also

• High-voltage direct current
• Electric power transmission
• Single-wire earth return
• List of HVDC projects
• List of high voltage underground and submarine cables

References

1. Underwater Cable an Alternative to Electrical Towers, Matthew L. Wald, New York Times, 2010-03-16, accessed 2010-03-18.
2. "Introduction to Modern Power Electronics" By Andrzej M. Trzynadlowski
3. "The electric power engineering handbook" By Leonard L. Grigsby
4. "Advances in high voltage engineering" By D. F. Warne, Institution of Electrical Engineers
5. "High voltage direct current transmission" By J. Arrillaga
6. "AC/DC: the savage tale of the first standards war" By Tom McNichol
7. "A Bridge Between Two Continents", Ramón Granadino and Fatima Mansouri, Transmission & Distribution World, May 1, 2007. Consulted March 28, 2014.
8. "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.
9. "Mit der Zukunft Geschichte schreiben". Dithmarscher Kreiszeitung (in German).
10. "Wolfe Island Wind Project" (PDF). Canadian Copper CCBDA (156). 2008. Retrieved 3 September 2013.
11. http://www.basslink.com.au/index.php?option=com_content&view=article&id=58&Itemid=82
12. http://web.archive.org/web/20050902175957/http://www.transmission.bpa.gov/cigresc14/Compendium/KONTI.htm
13. Bright Future for Long Island
14. "Power successfully transmitted through NordBalt cable". litgrid.eu. 2016-02-01. Retrieved 2016-02-02.
15. http://new.abb.com/systems/hvdc/references/skagerrak
16. The Norned HVDC Cable Link
17. "Cyprus group plans Greece-Israel electricity link". Reuters. 2012-01-23.
18. 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
19. Territory study linking power grid between Puerto Rico and Virgin Islands
21. "Offshore Wind Power Line Wins Praise, and Backing" article by Matthew L. Wald in The New York Times October 12, 2010, Accessed October 12, 2010
22. "Lower Churchill Project". Nalcor Energy.
24. Carrington, Damian (2012-04-11). "Iceland's volcanoes may power UK". The Guardian. London.
25. "Agreement to realize electricity interconnector between Germany and Norway", Statnett 21 June 2012. Retrieved: 22 June 2012.
26. "Kabel til England - Viking Link". energinet.dk. Retrieved 2015-11-12.
27. "Denmark - National Grid". nationalgrid.com. Retrieved 2016-02-03.
28. "The world's longest interconnector gets underway". statnett.no. Retrieved 2016-02-03.
29. , Western HVDC Link. Retrieved 23 November 2014.
30. , Scottish and Southern Energy. Retrieved 23 November 2014.
31. "Cable to the Netherlands - COBRAcable". energinet.dk. 2015-06-10. Retrieved 2016-01-28.
32. "Siemens and Prysmian will build the COBRA interconnection between Denmark and the Netherlands". Energinet.dk. 2016-02-01. Retrieved 2016-02-02.

External links

• In-depth info: Textbook on submarine power cables.
• Subsea Cables UK - An organisation of submarine cable owners, operators and suppliers aimed at promoting marine safety and protecting cable installations on the UK continental shelf
• The International Cable Protection Committee
• Subsea Cables UK article on Submarine Power Cables
• Export cables from Offshore Wind farms to Offshore substations
• Transmission cables from Offshore converter to shore