A submarine power cable is a 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.
This section needs additional citations for verification .(May 2022) |
Since electric power is a product of electric current and voltage: P=IU, one can increase, in principle, the power transmitted by a cable by either increasing the input voltage or the input current. In practice, however, electric power transmission is more energy efficient, if high-voltage (rather than high-current) powerline are used. [2]
This can be explained by the following back-of-the-envelope calculation: [3]
Define: P=power , U=voltage , I=current, i=in , o=out then: input power Pi=Ii*Ui and the output power Po=Io*Uo . Due to the conservation of charge the current's absolute value is conserved (both in DC and AC cases), thus the output current is the same as the input current |Io| = |Ii| =I . Then the voltage drop is : Ui-Uo = I*R or Uo = Ui-I*R, the output power is Po=I*Uo = I* (Ui-I*R) and the energy efficiency = Po/Pi = I* (Ui-I*R)/ I*Ui = Ui/Ui-IR/Ui=1- IR/Ui .
The latter formula shows, that decreasing operating current and increasing input voltage improves the efficiency of electric power transmission via an electric conductor.
Most electrical power transmission systems above ground use alternating current (AC), because transformers can easily change voltages as needed (see War of the currents for historical details). High-voltage direct current transmission requires expensive and inefficient converters at each end of a direct current line to interface to an alternating current grid.
However this logic fails for below-the-ground electric powerlines, such as submarine electric cables. This is because the capacitance between the cable and its surrounding (i.e. the capacitance of capacitance of a single cable) is not negligible, when the cable is immersed into an electrically conducting salt water.
The inner and outer conductors of a cable form the plates of a capacitor, and if the cable is long (on the order of tens of kilometres), this will result in a noticeable phase shift between voltage and current, thus significantly decreasing the efficiency of the transmitted power, which is a vector product of current and voltage. [4]
An AC electric powerline under water would require larger, therefore more costly, conductors for a given quantity of usable power to be transmitted.
When the reasons for high voltage transmission , the preference for AC, and for capacitive currents are combined, one can understand why there are no underwater high electric power cables longer than 1000 km (see the table in "Operational submarine power cables" section below).
As explained in the 2 preceding sections, the purpose of submarine power cables is the transport of electric current at high voltage. The electric core is a concentric assembly of inner conductor, electric insulation, and protective layers (resembling the design of a coaxial cable). [5] Modern three-core cables (e.g. for the connection of offshore wind turbines) often carry optical fibers for data transmission or temperature measurement, in addition to the electrical conductors. The conductor is made from copper or aluminum wires, the latter material having a small but increasing market share. Conductor sizes ≤ 1200 mm2 are most common, but sizes ≥ 2400 mm2 have been made occasionally. For voltages ≥ 12 kV the conductors are round so that the insulation is exposed to a uniform electric field gradient. The conductor can be stranded from individual round wires or can be a single solid wire. In some designs, profiled wires (keystone wires) are laid up to form a round conductor with very small interstices between the wires.
Three different types of electric insulation around the conductor are mainly used today. Cross-linked polyethylene (XLPE) is used up to 420 kV system voltage. It is produced by extrusion, with an insulation thickness of up to about 30 mm; 36 kV class cables have only 5.5 – 8 mm insulation thickness. Certain formulations of XLPE insulation can also be used for DC. Low-pressure oil-filled cables have an insulation lapped from paper strips. The entire cable core is impregnated with a low-viscosity insulation fluid (mineral oil or synthetic). A central oil channel in the conductor facilitates oil flow in cables up to 525 kV for when the cable gets warm but rarely used in submarine cables due to oil pollution risk with cable damage. Mass-impregnated cables have also a paper-lapped insulation but the impregnation compound is highly viscous and does not exit when the cable is damaged. Mass-impregnated insulation can be used for massive HVDC cables up to 525 kV.
Cables ≥ 52 kV are equipped with an extruded lead sheath to prevent water intrusion. No other materials have been accepted so far. The lead alloy is extruded onto the insulation in long lengths (over 50 km is possible). In this stage the product is called cable core. In single-core cables the core is surrounded by concentric armoring. In three-core cables, three cable cores are laid-up in a spiral configuration before the armoring is applied. The armoring consists most often of steel wires, soaked in bitumen for corrosion protection. Since the alternating magnetic field in AC cables causes losses in the armoring, those cables are sometimes equipped with non-magnetic metallic materials (stainless steel, copper, brass).
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) | Length(km) | Year | Notes |
---|---|---|---|---|---|
Peloponnese, Greece | Crete, Greece | 150 | 135 | 2021 | Two 3-core XLPE cables with total capacity of 2x200MVA. 174 km total length including the underground segments. Maximum depth 1000m. Total cost 380 million EUR. It is the longest submarine/underground AC cable interconnection in the world. [6] [7] [8] |
Mainland British Columbia to Gulf Islands Galiano Island, Parker Island, and Saltspring Island thence to North Cowichan | Vancouver Island | 138 | 33 | 1956 | "The cable became operational on 25 September 1956" [9] |
Mainland British Columbia to Texada Island to Nile Creek Terminal | Vancouver Island / Dunsmuir Substation | 525 | 35 | 1985 | Twelve, separate, oil filled single-phase cables. Nominal rating 1200 MW. [10] |
Tarifa, Spain (Spain-Morocco interconnection) | Fardioua, Morocco through the Strait of Gibraltar | 400 | 26 | 1998 | A second one from 2006 [11] Maximum depth: 660 m (2,170 ft). [12] |
Norwalk, CT, USA | Northport, NY, USA | 138 | 18 | A 3 core, XLPE insulated cable | |
Sicily | Malta | 220 | 95 | 2015 | The Malta–Sicily interconnector |
Mainland Sweden | Bornholm Island, Denmark | 60 | 43.5 | The Bornholm Cable | |
Mainland Italy | Sicily | 380 | 38 | 1985 | Messina Strait submarine cable replacing the "Pylons of Messina". A second 380 kV cable began operation in 2016 |
Germany | Heligoland | 30 | 53 | [13] | |
Negros Island | Panay Island, the Philippines | 138 | |||
Douglas Head, Isle of Man, | Bispham, Blackpool, England | 90 | 104 | 1999 | The Isle of Man to England Interconnector, a 3 core cable |
Wolfe Island, Canada for the Wolfe Island Wind Farm | Kingston, Canada | 245 | 7.8 | 2008 | The first three-core XLPE submarine cable for 245 kV [14] |
Cape Tormentine, New Brunswick | Borden-Carleton, PEI | 138 | 17 | 2017 | Prince Edward Island Cables [15] |
Taman Peninsula, Mainland Russia | Kerch Peninsula, Crimea | 220 | 57 | 2015 | [16] |
Name | Connecting | Body of water | Connecting | kilovolts (kV) | Undersea distance | Year | Notes |
---|---|---|---|---|---|---|---|
Baltic Cable | Germany | Baltic Sea | Sweden | 450 | 250 km (160 mi) | 1994 | |
Basslink | mainland State of Victoria | Bass Strait | island State of Tasmania, Australia | 500 | 290 km (180 mi) [17] | 2005 | |
BritNed | Netherlands | North Sea | Great Britain | 450 | 260 km (160 mi) | 2010 | |
COBRAcable | Netherlands | North Sea | Denmark | 320 | 325 km (202 mi) | 2019 | |
Cross Sound Cable | Long Island, New York | Long Island Sound | State of Connecticut | 150 | 2003 | [ citation needed ] | |
East–West Interconnector | Dublin, Ireland | Irish Sea | North Wales and thus the British grid | 200 | 186 km (116 mi) | 2012 | |
Estlink | northern Estonia | Gulf of Finland | southern Finland | 330 | 105 km (65 mi) | 2006 | |
Fenno-Skan | Sweden | Baltic Sea | Finland | 400 | 233 km (145 mi) | 1989 | |
HVDC Cross-Channel | French mainland | English Channel | England | 270 | 73 km (45 mi) | 1986 | very high power cable (2000 MW)[ citation needed ] |
HVDC Gotland | Swedish mainland | Baltic Sea | Swedish island of Gotland | 150 | 98 km (61 mi) | 1954 | 1954, the first HVDC submarine power cable (non-experimental) [18] Gotland 2 and 3 installed in 1983 and 1987. |
HVDC Inter-Island | South Island | Cook Strait | North Island | 350 | 40 km (25 mi) | 1965 | 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 | 200 | 385 km (239 mi) | 1967 | 3 cables, 1967, 1988, 1992 [19] |
HVDC Italy-Greece | Italian mainland - Galatina HVDC Static Inverter | Adriatic Sea | Greek mainland - Arachthos HVDC Static Inverter | 400 | 160 km (99 mi) | 2001 | Total length of the line is 313 km (194 mi) |
HVDC Leyte - Luzon | Leyte Island | Pacific Ocean | Luzon in the Philippines [ citation needed ] | 1998 | |||
HVDC Moyle | Scotland | Irish Sea | Northern Ireland within the United Kingdom, and thence to the Republic of Ireland | 250 | 63.5 km (39.5 mi) | 2001 | 500MW |
HVDC Vancouver Island | Vancouver Island | Strait of Georgia | mainland of the Province of British Columbia | 280 | 33 km | 1968 | In operation in 1968 and was extended in 1977 |
Kii Channel HVDC system | Honshu | Kii Channel | Shikoku | 250 | 50 km (31 mi) | 2000 | in 2010 the world's highest-capacity[ citation needed ] long-distance submarine power cable[ inconsistent ] (rated at 1400 megawatts). This power cable connects two large islands in the Japanese Home Islands |
Kontek | Germany | Baltic Sea | Denmark | 1995 | |||
Konti-Skan [20] | Sweden | Kattegat | Denmark | 400 | 149 km (93 mi) | 1965 | Commissioned:1965 (Kontiskan 1);1988 (Kontiskan 2) Decommissioned:2006 (Kontiskan 1) |
Maritime Link | Newfoundland | Atlantic Ocean | Nova Scotia | 200 | 170 km (110 mi) | 2017 | 500 MW link went online in 2017 with two subsea HVdc cables spanning the Cabot Strait. [21] |
Nemo-Link [22] | Belgium | North Sea | United Kingdom | 400 | 140 km (87 mi) | 2019 | |
Neptune Cable | State of New Jersey | Atlantic Ocean | Long Island, New York | 500 | 104.6 km (65.0 mi) [23] | 2003 | |
NordBalt | Sweden | Baltic Sea | Lithuania | 300 | 400 km (250 mi) | 2015 | Operations started on February 1, 2016 with an initial power transmission at 30 MW. [24] |
NordLink | Ertsmyra, Norway | North Sea | Büsum, Germany | 500 | 623 km (387 mi) | 2021 | Operational May 2021 [25] |
NorNed | Eemshaven, Netherlands | Feda, Norway | 450 | 580 km (360 mi) | 2012 | 700 MW in 2012 previously the longest undersea power cable [26] | |
North Sea Link | Kvilldal, Suldal, in Norway, Cambois near Blyth | North Sea | United Kingdom, Norway | 515 | 720 km (450 mi) | 2021 | 1.4 GW the longest undersea power cable |
Shetland HVDC Connection | Shetland islands | North Sea | Scotland | 600 | 260 km (160 mi) | 2024 | |
Skagerrak 1-4 | Norway | Skagerrak | Denmark (Jutland) | 500 | 240 km (150 mi) | 1977 | 4 cables - 1700 MW in all [27] |
SwePol | Poland | Baltic Sea | Sweden | 450 | 2000 | ||
Western HVDC Link | Scotland | Irish Sea | Wales | 600 | 422 km (262 mi) | 2019 | Longest 2200 MW cable, first 600kV undersea cable [28] |
This section needs to be updated.(April 2020) |
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is part of electricity delivery, known as the electrical grid.
A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages between 100 kV and 800 kV.
The HVDC Cross-Channel is the 73-kilometre-long (45 mi) high-voltage direct current (HVDC) interconnector that has operated since 1986 under the English Channel between the continental European grid at Bonningues-lès-Calais and the British electricity grid at Sellindge. The cable is also known as IFA, and should not be confused with the new IFA-2, another interconnect with France that is three times as long but only half as powerful.
The HVDC Inter-Island link is a 610 km (380 mi) long, 1200 MW high-voltage direct current (HVDC) transmission system connecting the electricity networks of the North Island and South Island of New Zealand together. It is commonly referred to as the Cook Strait cable in the media and in press releases, although the link is much longer than its Cook Strait section. The link is owned and operated by state-owned transmission company Transpower New Zealand.
The National Grid is the high-voltage electric power transmission network serving Great Britain, connecting power stations and major substations, and ensuring that electricity generated anywhere on the grid can be used to satisfy demand elsewhere. The network serves the majority of Great Britain and some of the surrounding islands. It does not cover Northern Ireland, which is part of the Irish single electricity market.
Directlink (Terranora)Interconnector is a mixed buried and above ground 59 kilometre (37 mi) High Voltage Direct Current (HVDC) electricity transmission cable route from near Lavertys Gap (28°34′15″S153°27′8″E), 5 kilometres (3.1 mi) Southwest of Mullumbimby, New South Wales and Bungalora (28°15′20″S153°28′20″E) & connected via a 3.5 km (2.2 mi) AC Overhead Transmission Line to the NorthEast to the Terranora Electrical Substation (28°14′28.3″S153°30′12.7″E) @ Terranora, New South Wales in Eastern Australia. The DC cables alternate between above ground in a galvanised steel trough and below ground with depths up to 1 metre.
The East–West Interconnector is a 500 MW high-voltage direct current submarine and subsoil power cable from 2012 which connects the Irish and British electricity markets, between Dublin and the Wales/England border. The project was developed by the Irish national grid operator EirGrid.
NordBalt is a submarine power cable between Klaipėda in Lithuania and Nybro in Sweden. The purpose of the cable is to facilitate the trading of power between the Baltic and Nordic electricity markets, and to increase the supply and energy security in both markets.
The synchronous grid of Continental Europe is the second largest synchronous electrical grid in the world. It is interconnected as a single phase-locked 50 Hz mains frequency electricity grid that supplies over 400 million customers in 24 countries, including most of the European Union. In 2009, 667 GW of production capacity was connected to the grid, providing approximately 80 GW of operating reserve margin. The transmission system operators operating this grid formed the Union for the Coordination of Transmission of Electricity (UCTE), now part of the European Network of Transmission System Operators for Electricity (ENTSO-E).
TenneT is a transmission system operator in the Netherlands and in a large part of Germany.
A super grid or supergrid is a wide-area transmission network, generally trans-continental or multinational, that is intended to make possible the trade of high volumes of electricity across great distances. It is sometimes also referred to as a "mega grid". Super grids typically are proposed to use high-voltage direct current (HVDC) to transmit electricity long distances. The latest generation of HVDC power lines can transmit energy with losses of only 1.6% per 1,000 km.
BritNed is a 1,000 MW high-voltage direct-current (HVDC) submarine power cable between the Isle of Grain in Kent, the United Kingdom; and Maasvlakte in Rotterdam, the Netherlands.
The North Sea Link is a 1,400 MW high-voltage direct current submarine power cable between Norway and the United Kingdom.
The Great Sea Interconnector, formerly known as the EuroAsia Interconnector is a planned HVDC interconnector between the Greek, Cypriot, and Israeli power grids via the world's longest submarine power cable, with a length of 310 kilometres (190 mi) from Israel to Cyprus and 898 kilometres (558 mi) from Cyprus to Greece for a total of 1,208 kilometres (751 mi).
Shetland HVDC Connection is a high-voltage direct current submarine power cable connecting Shetland to the British mainland.
An interconnector is a structure which enables high voltage DC electricity to flow between electrical grids. An electrical interconnector allows electricity to flow between separate AC networks, or to link synchronous grids. They can be formed of submarine power cables or underground power cables or overhead power lines.
Viking Link is a 1,400 MW HVDC submarine power cable between the United Kingdom and Denmark, which was completed in 2023. As of 2024, it is the longest land and subsea HVDC interconnector in the world. The project is a cooperation between British National Grid plc and Danish Energinet.
FAB Link is a proposed HVDC Interconnector, spanning the 220 kilometres (140 mi) between France and Great Britain, running close to the island of Alderney.
EuroAfrica Interconnector is a planned HVDC interconnector and submarine power cable between the Greek, Cypriot, and Egypt power grids. The Interconnector is an energy highway bridging Africa and Europe. It will have a capacity to transmit 2,000 megawatts of electricity in either direction. Annual transmission capacity will be rated at 17.5 TWh, much more than the annual production at the Aswan Dam power stations. President of Egypt Abdel Fattah el-Sisi, President of Cyprus Nicos Anastasiades and Prime Minister of Greece Kyriakos Mitsotakis, issued a joint declaration at the conclusion of the 7th Trilateral Summit, held in Cairo on October 8, 2019, in which they expressed their desire to continue strengthening their cooperation in matters of energy. In particular, the joint declaration by the three leaders stated they recognised the importance of establishing an electrical grid between Egypt, Cyprus and Greece, building on the framework agreement between the Egyptian Electricity Holding Company and the Euro Africa Interconnector Company on 22 May 2019.