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. However, just as the gap becomes to wide for the arc to continue while opening the switch, the gap will become narrow enough for arcing to occur when the switch is closed. When current arcs across the gap between two contacts, the current is flowing across far less contact surface area than when closed, and the arc will quickly burn and erode the contact surfaces. Ideally, current flow in a DC circuit is halted by opening a switch downstream of the load (ground or negative side), where the current flow is lowest. By doing so, the main switch can either remain closed, or will be subjected to less current and arcing when opened. If a ground side switch isn't feasible, current flow through the main switch should be minimized by first opening branch circuit switches, to reduce main circuit current flow as much as possible. This minimizes arcing both when opening and closing the main switch. After closing the main switch to reconnect power, the individual circuit switches or ground switch can be closed to complete the circuit.

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 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 1.4 times as much power as an AC line.[4]

However, since the DC current flow is continuous, DC current flowing through a conductor of a specific resistance will produce more heat. Heating a conductor increases resistance, and the increased resistance due to heating results in a further increase in resistance. If the DC current flow through the conductor is not carefully controlled, the conductor will overheat and be melted, causing and open circuit. Fuses and circuit-breakers protect the circuit in case of an excessive current condition, but circuits are usually designed to keep actual current flow sufficiently below fuse or breaker current ratings to prevent blown fuses or tripped breakers during a momentary overload.

Comparable electrical systems which have been built using both AC and DC power have shown AC power to be more efficient in converting electrical energy into mechanical energy, despite the supposed disadvantages of AC current, such as the need for more complex equipment. Modern diesel-electric locomotives can be found with both DC and AC electrical systems, and system and component life is significantly increased when AC current is used to power the traction motors. Locomotives with AC power produce more tractive effort at any given engine horsepower level, despite the more complex systems, because there is less electrical power loss to heat. Components such as the main generator and traction motors stay cooler, less internal resistance is present, and auxiliary systems that would normally be DC can be operated on AC from the propulsion system. The use of AC power for things like cooling fans allows them to operate at several speed levels, rather than simply being either on or off, and drawing full current whenever on.

The advantages of AC current generation are also present in modern automotive alternators, which are occasionally referred to as AC generators. Alternators produce AC current that is then rectified by passing the current through a rectifier, also known as a diode trio. A voltage regulator, either mechanical or electronic, controls current flow to the alternator field coils, to accurately and constantly regulate output voltage. As additional load is added and voltage is reduced, the regulator increases current flow to the fields and more electricity is produced. Whereas a typical DC generator found on a vintage automobile may only produce 15 amperes of current at 12 volts (nominal), the early alternators that replaced DC generators are capable of producing 45 amps at 12 volts (nominal). Alternators have the additional advantage of lower rotating resistance, compact size, reduced maintenance and longer service life.

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]

Ultimately, because the systems are significantly different, direct and accurate comparison is impossible. Nikola Tesla and Thomas Edison engaged in what were at the time called "The Current Wars". Each inventor believed in his specific technology, with Tesla promoting AC power and Edison promoting DC power. Eventually, the inherent advantages of AC power for long-distance transmission became clear, and the "war" was won by Nikola Tesla. Later, when existing distribution systems were expanded and additional end-users were added to existing AC systems, the superiority of AC power became even more apparent.

The continuous current flow of DC, and the need for relatively large conductors (wires) to carry a given current flow at the nominal voltage, make it expensive to build extra current flow capacity into DC circuits. And if extra capacity is built in to the original system but is never required for upgrades, the additional expense is wasted. However, AC systems deliver a surplus of electrical power to the end user. The additional capacity is essentially built into the system, and upgrades to end-user service to increase current capacity and/or add circuits with higher voltage are relatively cheap and simple.

It's worth mentioning that despite the eternal and theoretical comparisons between the two current types, each has advantages and disadvantages. A modern automotive electrical system is an example of a well-engineered system that uses both AC and DC current to maximize the efficiency of the system as a whole. It can take advantages of the benefits of each, while minimizing the penalties that would exist by using either individually.

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)Notes
Mainland British Columbia to Texada Island to Nile Creek TerminalVancouver 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		400The 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 SwedenBornholm Island, Denmark, Bornholm Cable60
Mainland Italy - SicilyItaly-Sicily380Under the Strait of Messina, this submarine cable replaced an earlier, and very long overhead line crossing (the "Pylons of Messina")
GermanyHeligoland30[9]
Negros IslandPanay Island, in the Philippines138
Isle of Man to England Interconnector90 a 3 core cable over a distance of 104 km
Wolfe Island, CanadaKingston, Canada245The 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

NameConnectingBody of waterConnectingkilovolts (kV)Undersea distanceNotes
Baltic-CableGermanyBaltic SeaSweden450 250 kilometres (160 mi)
Basslinkmainland State of VictoriaBass Straitisland State of Tasmania, Australia500290 kilometres (180 mi)[11]
BritNedNetherlandsNorth SeaGreat Britain450260 kilometres (160 mi)
Cross Sound CableLong Island, New YorkLong Island SoundState of Connecticut
East–West InterconnectorIrelandIrish SeaWales/England and thus the GB grid Inaugurated 20 September 2012
Estlinknorthern EstoniaGulf of Finlandsouthern Finland 330 105 kilometres (65 mi)
Fenno-SkanSwedenBaltic SeaFinland400 233 kilometres (145 mi)
HVDC Cross-ChannelFrench mainlandEnglish ChannelEngland very high power cable (2000 MW)
HVDC GotlandSwedish mainlandBaltic SeaSwedish island of Gotland the first HVDC submarine power cable (non-experimental)
HVDC Inter-IslandSouth IslandCook StraitNorth 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 mainlandMediterranean Seathe Italian island of Sardinia, and its neighboring French island of Corsica
HVDC Italy-GreeceItalian mainland - Galatina HVDC Static InverterAdriatic SeaGreek mainland - Arachthos HVDC Static Inverter400160 kilometers (100 miles) Total length of the line is 313 km (194 mi)
HVDC Leyte - LuzonLeyte IslandPacific OceanLuzon in the Philippines
HVDC MoyleScotlandIrish SeaNorthern Ireland within the United Kingdom, and thence to the Republic of Ireland
HVDC Vancouver IslandVancouver IslandStrait of Georgiamainland of the Province of British Columbia
Kii Channel HVDC systemHonshuKii ChannelShikoku250 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
KontekGermanyBaltic SeaDenmark
Konti-Skan[12]SwedenBaltic SeaDenmark400 149 kilometres (93 mi)
Neptune CableState of New JerseyAtlantic OceanLong Island, New York 64 miles (103 km)[13]
NordBaltSwedenBaltic SeaLithuania300400 kilometres (250 mi)Operations started on February 1, 2016 with an initial power transmission at 30 MW.[14]
Skagerrak 1-4Norway Denmark (Jutland)500 240 kilometres (150 mi) 4 cables - 1700 MW in all[15]
SwePolPolandBaltic SeaSweden 450
NorNedEemshaven, Netherlands Feda, Norway450580 kilometres (360 mi) 700 MW in 2012 the longest undersea power cable[16]

Proposed submarine power cables

See also

References

  1. 1 2 3 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
  20. ↑
  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.
  23. ↑
  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

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