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.

• Mainland British Columbia to Nelson Island to Texada Island to Vancouver Island, the destination of the power. This is a high-capacity 500 kilovolt (kV) three-phase system.
• Mainland Sweden to Bornholm Island, Denmark, Bornholm Cable ( 60 kilovolts).
• Under the Strait of Messina, connecting southern tip of the mainland of Italy with the large island of Sicily (380 kV). This submarine cable replaced an earlier, and very long overhead line crossing (the "Pylons of Messina")
• Negros Island to Panay Island, in the Philippines (138 kV)
• Isle of Man to England Interconnector, a 3 core cable (90kV) over a distance of 104km

Direct current cables

• Baltic-Cable - between Germany and Sweden beneath the Baltic Sea
• Basslink - between the mainland State of Victoria and the island of Tasmania, Australia, 500 kilovolts (kV), with a length of 290 kilometers beneath the Bass Strait
• BritNed - between the Netherlands and Great Britain beneath the North Sea
• Cross Sound Cable - between Long Island, New York, and the State of Connecticut beneath Long Island Sound^{[citation needed]}
• East–West Interconnector - (due operational 2012) connecting Ireland with Wales/England and thus the GB grid
• Estlink - between northern Estonia and southern Finland beneath the Gulf of Finland
• Fenno-Skan - between Sweden and Finland beneath the Baltic Sea
• HVDC Cross-Channel - very high power cable between the French mainland and the island of Great Britain beneath the English Channel
• HVDC Gotland - the first HVDC submarine power cable (non-experimental) - between the Swedish mainland and the Swedish island of Gotland beneath the Baltic Sea
• HVDC Inter-Island - between the power-rich South Island (much hydroelectric power) of New Zealand and the more-populous North Island beneath the Cook Strait
• HVDC Italy-Corsica-Sardinia (SACOI) - between the Italian mainland, the Italian island of Sardinia, and its neighboring French island of Corsica beneath the Mediterranean Sea^{[citation needed]}
• HVDC Italy-Greece - between Italy and Greece beneath the Adriatic Sea^{[citation needed]}
• HVDC Leyte - Luzon - between Leyte Island and Luzon in the Philippines, beneath the Pacific Ocean^{[citation needed]}
• HVDC Moyle - connecting Scotland with Northern Ireland within the United Kingdom, and thence to the Republic of Ireland, beneath the Irish Sea
• HVDC Vancouver Island - between Vancouver Island and the mainland of the Province of British Columbia, beneath the Strait of Georgia
• Kii Channel HVDC system - now (2010) the world's highest-capacity long-distance submarine power cable (rated at 1400 megawatts). This power cable connects the large islands of Honshu and Shikoku beneath the Kii Channel in the Japanese Home Islands
• Kontek - between Germany and Denmark beneath the Baltic Sea
• Konti-Skan - between Sweden and Denmark beneath the Baltic Sea^{[citation needed]}
• Neptune Cable - between the State of New Jersey and Long Island, New York - 64 miles beneath the Atlantic Ocean
• Skagerrak 1-3, 3 cables between Norway and Denmark (Jutland) - 1000 MW in all
• Swepol - between Poland and Sweden beneath the Baltic Sea

Longest

• NorNed (between Eemshaven, Netherlands and Feda, Norway), HVDC, 700 MW, 580 km (360 mi)[3]

Proposed submarine power cables

• EuroAsia Interconnector, a 1,000Km 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
• 100 megawatts 165 km Canadian province of Newfoundland and Labrador and province of Nova Scotia
• 200 megawatts 95 km Magħtab (Malta) and Marina the 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 eletricity 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.

See also

• High-voltage direct current
• Electric power transmission
• Single-wire earth return
• 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.