Submarine power cable
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (March 2010)|
Submarine power cables are major transmission cables 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.
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 (such devices as did exist, for instance the rotary converter, being less efficient and requiring considerably more maintenance) made DC systems impractical in the late 19th and early 20th centuries. 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.
DC switch gear is also more expensive to produce, since arc suppression is more difficult. When a high voltage AC line is switched off, the voltage will arc across the switch contacts. Once the contacts get far enough apart, the arc will naturally extinguish itself since the voltage drops to zero twice during the AC sine wave cycle. Since DC is constant and doesn't cycle to zero, a DC switch will draw a much longer arc, and suppressing this arc requires more expensive switching equipment.
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.
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.
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.
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 (and complicated) electrical switching system.
|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)|
|Mainland Sweden||Bornholm Island, Denmark, Bornholm Cable||60|
|Italy||Sicily||380||Under the Strait of Messina, this submarine cable replaced an earlier, and very long overhead line crossing (the "Pylons of Messina")|
|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.|
Direct current cables
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)
- 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."
- Power Bridge, Hawaii
- Power Bridge, State of Maine
- Puerto Rico to the Virgin Islands
- 400 kV HVDC India to Sri Lanka
- Atlantic Wind Connection between Delaware and New Jersey, potentially between Virginia and New York
- 500 MW capacity, 165 km DC Maritime Transmission Link between the Canadian province of Newfoundland and Labrador and the province of Nova Scotia.
- 220 kV HVAC, 225 megawatts, 117 km Magħtab (Malta) and Ragusa (Sicily)
- 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
- Petrobras (Brazil) is evaluating the possibility to generate electricity in pre-salt layer Oil Rigs and connect them to national power grid via submarine cables.
- NordBalt, 400 km 700 MW DC submarine cable under the Baltic Sea, connecting Klaipėda, Lithuania and Nybro, Sweden. Operation expected in 2015-2016.
- The British and Icelandic Governments are in "active discussion" to build a cable between the UK and Iceland powered by geothermal energy.
- Norwegian and German operators have agreed to build a cable transmitting up to 1,400 MW between the two countries by 2018.
- British and Norwegian operators (National Grid and Statnett respectively) have agreed to study a cable up to 1,400 MegaWatt operational by 2020 connecting the two countries. Such a cable would be the longest in the world.
- High-voltage direct current
- Electric power transmission
- Single-wire earth return
- List of HVDC projects
- List of high voltage underground and submarine cables
- Underwater Cable an Alternative to Electrical Towers, Matthew L. Wald, New York Times, 2010-03-16, accessed 2010-03-18.
- "Introduction to Modern Power Electronics" By Andrzej M. Trzynadlowski
- "The electric power engineering handbook" By Leonard L. Grigsby
- "Advances in high voltage engineering" By D. F. Warne, Institution of Electrical Engineers
- "High voltage direct current transmission" By J. Arrillaga
- "AC/DC: the savage tale of the first standards war" By Tom McNichol
- "Mit der Zukunft Geschichte schreiben". Dithmarscher Kreiszeitung (in German).
- "Wolfe Island Wind Project". Canadian Copper CCBDA (156). 2008. Retrieved 3 September 2013.
- Bright Future for Long Island
- The Norned HVDC Cable Link
- "Cyprus group plans Greece-Israel electricity link". Reuters. 2012-01-23.
- 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
- Territory study linking power grid between Puerto Rico and Virgin Islands
- "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
- "Lower Churchill Project". Nalcor Energy.
- Carrington, Damian (2012-04-11). "Iceland's volcanoes may power UK". The Guardian (London).
- "Agreement to realize electricity interconnector between Germany and Norway", Statnett 21 June 2012. Retrieved: 22 June 2012.
- "Statnett and National Grid confirm plans to move forward with the world’s longest electricity interconnector", Statnett 21 June 2012. Retrieved: 22 June 2012.
- Open Electrical has a detailed technical overview of AC subsea power cables covering construction, design and installation
- In-depth info: Textbook on submarine power cables.
- Google Map of Submarine HVDC Projects across the World. Maintained by refabrica.com.