Submarine power cable

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.

Design Technologies

Most power systems use alternating-current. This is due mostly to the simplicity of the AC transformer, which allows AC voltages to be easily stepped up and down. 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 simple DC transformer made DC systems impractical in the late 19th and early 20th centuries. As technology improved, DC transformers became possible, though even today they are much more complex than AC transformers. A DC transformer often consists of an oscillator or inverter to convert the DC to AC, an AC 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.

Connecting	Connecting	Voltage (kV)	Notes
Mainland British Columbia to Texada Island to Nile Creek Terminal	Vancouver Island	525	Reactor station at overhead crossing of Texada Island.
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")
Germany	Heligoland	30
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

Direct current cables

Name	Connecting	Body of water	Connecting	kilovolts (kV)	Undersea distance	Notes
Baltic-Cable	Germany	Baltic Sea	Sweden		250 kilometres (160 mi)
Basslink	mainland State of Victoria	Bass Strait	island State of Tasmania, Australia	500	290 kilometres (180 mi)
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			due operational 2012
Estlink	northern Estonia	Gulf of Finland	southern Finland	330	105 kilometres (65 mi)
Fenno-Skan	Sweden	Baltic Sea	Finland
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	Italy	Adriatic Sea	Greece
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			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	Sweden	Baltic Sea	Denmark
Neptune Cable	State of New Jersey	Atlantic Ocean	Long Island, New York		64 miles (103 km)
Skagerrak 1-3	Norway		Denmark (Jutland)			3 cables - 1000 MW in all
Swepol	Poland	Baltic Sea	Sweden
NorNed	Eemshaven, Netherlands		Feda, Norway	450	580 kilometres (360 mi)	700 MW in 2012 the longest undersea power cable

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.

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

External links

• 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.