Submarine power cable

A submarine power cable is a transmission cable for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water. Examples of the latter exist that connect the mainland with large islands in the St. Lawrence River.

Design technologies

As key tools in bulk power transmission, submarine power tables tend to operate at high voltage in order to minimize resistive losses between the endpoints. Unlike overhead powerlines, many submarine power cables tend to operate with DC current. Electrical phases must endure close proximity inside the cable, increasing parasitic capacitance. It is more economical to use AC only with lines shorter than 100 km in length, in which case losses at the landing point grid interfaces dominate.
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.

Conductor

As indicated above, submarine power cables transport electric current at high voltage. The electric core is a concentric assembly of inner conductor, electric insulation, and protective layers. Modern three-core cables 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 are laid up to form a round conductor with very small interstices between the wires.

Insulation

Three different types of electric insulation around the conductor are mainly used today.
Cross-linked polyethylene 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. 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.

Armoring

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.
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.

Operational submarine power cables

Alternating current cables

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	Length	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.
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"
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.
Tarifa, Spain	Fardioua, Morocco through the Strait of Gibraltar	400	26	1998	A second one from 2006 Maximum depth:.
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
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
Cape Tormentine, New Brunswick	Borden-Carleton, PEI	138	17	2017	Prince Edward Island Cables
Taman Peninsula, Mainland Russia	Kerch Peninsula, Crimea	220	57	2015

Direct current cables

Name	Connecting	Body of water	Connecting	kilovolts	Undersea distance	Year	Notes
Baltic Cable	Germany	Baltic Sea	Sweden	450		1994
Basslink	mainland State of Victoria	Bass Strait	island State of Tasmania, Australia	500		2005
BritNed	Netherlands	North Sea	Great Britain	450		2010
COBRAcable	Netherlands	North Sea	Denmark	320		2019
Cross Sound Cable	Long Island, New York	Long Island Sound	State of Connecticut	150		2003
East–West Interconnector	Dublin, Ireland	Irish Sea	North Wales and thus the British grid	200		2012
Estlink	northern Estonia	Gulf of Finland	southern Finland	330		2006
Fenno-Skan	Sweden	Baltic Sea	Finland	400		1989
HVDC Cross-Channel	French mainland	English Channel	England	270		1986	very high power cable
HVDC Gotland	Swedish mainland	Baltic Sea	Swedish island of Gotland	150		1954	1954, the first HVDC submarine power cable Gotland 2 and 3 installed in 1983 and 1987.
HVDC Inter-Island	South Island	Cook Strait	North Island	350		1965	between the power-rich South Island of New Zealand and the more-populous North Island.
HVDC Italy-Corsica-Sardinia	Italian mainland	Mediterranean Sea	the Italian island of Sardinia, and its neighboring French island of Corsica	200		1967	3 cables, 1967, 1988, 1992
HVDC Italy-Greece	Italian mainland - Galatina HVDC Static Inverter	Adriatic Sea	Greek mainland - Arachthos HVDC Static Inverter	400		2001	Total length of the line is 313 km
HVDC Leyte - Luzon	Leyte Island	Pacific Ocean	Luzon in the Philippines			1998
HVDC Moyle	Scotland	Irish Sea	Northern Ireland within the United Kingdom, and thence to the Republic of Ireland	250		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		2000	in 2010 the world's highest-capacity long-distance submarine power cable. This power cable connects two large islands in the Japanese Home Islands
Kontek	Germany	Baltic Sea	Denmark			1995
Konti-Skan	Sweden	Kattegat	Denmark	400		1965	Commissioned:1965 ;1988 Decommissioned:2006
Maritime Link	Newfoundland	Atlantic Ocean	Nova Scotia	200		2017	500 MW link went online in 2017 with two subsea HVdc cables spanning the Cabot Strait.
Nemo-Link	Belgium	North Sea	United Kingdom	400		2019
Neptune Cable	State of New Jersey	Atlantic Ocean	Long Island, New York	500		2003
NordBalt	Sweden	Baltic Sea	Lithuania	300		2015	Operations started on February 1, 2016 with an initial power transmission at 30 MW.
NordLink	Ertsmyra, Norway	North Sea	Büsum, Germany	500		2021	Operational May 2021
NorNed	Eemshaven, Netherlands		Feda, Norway	450		2012	700 MW in 2012 previously the longest undersea power cable
North Sea Link	Kvilldal, Suldal, Norway,	North Sea	Cambois near Blyth, United Kingdom	515		2021	1.4 GW the longest undersea power cable
Shetland HVDC Connection	Shetland islands	North Sea	Scotland	600		2024
Skagerrak 1-4	Norway	Skagerrak	Denmark	500		1977	4 cables - 1700 MW in all
SwePol	Poland	Baltic Sea	Sweden	450		2000
Western HVDC Link	Scotland	Irish Sea	Wales	600		2019	Longest 2200 MW cable, first 600kV undersea cable