The high voltage grid systems in UK and France operate using high voltage alternating current (HVAC). To reduce energy losses during transport of electricity, HVAC is converted to high voltage direct current (HVDC) at a Converter Station in the exporting country before transmission by means of a submarine HVDC cable. It is then converted back from HVDC to HVAC for connection to the high voltage network in the receiving country. The electricity can flow in either direction through the submarine HVDC cable, therefore each Converter Station is able to convert HVDC to HVAC and vice versa.
The route will be approximately 140 km long, with 32 km in French territorial waters and 108 km in UK territorial waters. The subsea cable route is shown below.
The subsea cable route has been selected following detailed research and consultation with navigation authorities. It is the most direct and shortest route that is possible between the required connection points in the UK and France. Feasibility studies have included desk-top research of published information, bathymetric, geophysical, geotechnical and environmental surveys, consultations with navigation authorities (Grand Port Maritime de Dunkerque, Port of London Authority, Peel Ports) and coordination with third party cable owners.
An overview of the current marine uses determined by the studies, and the resulting subsea cable route, is provided for the UK and France territorial waters below.
In France, the subsea cable will cross the coastline by horizontal direction drilling at the Port of Dunkerque, and from this point an underground cable approximately 13.5 km in length will cross industrial and agricultural areas owned by the Grand Maritime Port of Dunkerque (GPMD) to the Converter Station site. No overhead electricity transmission lines will be constructed.
The underground cable route in France is shown below:
The onshore cable route has been defined to fit within the development plans of the GPMD for new industrial developments in the area. The cable route is within an easement corridor assigned by GPMD for the construction of underground public utilities and associated infrastructure.
In the UK, the subsea cable will cross the coastline by horizontal directional drilling (HDD) at the former E.on coal-fired power station (now demolished) at Kingsnorth on the Isle of Grain, and emerge directly at the Converter Station site. Therefore, no underground cable is required outside the Converter Station site.
The subsea cable will consist of two high voltage direct current (HVDC), mass impregnated (MI) cables. The diameter of each cable will be up to 150 mm, with a central conductor made of copper. The cables will be insulated and protected by additional layers, steel armouring and an outer covering. A smaller fibre-optic cable will be included with the bundled HVDC cables for monitoring and control purposes.
An indicative cross-sectional profile of the cable trench is provided below.
The cables will be buried below the seabed to protect the cable from natural events and activities such as anchoring of vessels or fishing nets.
An illustration of the buried cable is provided below.
The burial depth to protect the cable may vary along the cable route to take into account different seabed geology, sediment mobility and the risks from ships (anchoring) and fishing (trawling). The typical burial depth will be approximately 2 m.
If the subsea cable has to cross another buried cable, it will need to be laid on the seabed and protected by rock placement over the top. The crossing can also include measures to encourage natural sedimentation and incorporation into the existing environment, such as the placement of additional mattresses.
A typical cross-section of the cable covered by rock placement is shown below
The coastline at Kingsnorth in the UK and Dunkerque in France will be crossed by horizontal directional drilling (HDD). The drilling will pass approximately 10 m below the ground so that the surface and any sensitive environmental habitats are not disturbed.
The steps in implementing the HDD are shown below.
The underground cables will be installed in a trench at a depth of approximately 1 m. The trench will be dug by a mechanical excavator or via manual excavation methods. The excavated material will be stored temporarily before reuse as backfill. Where the underground cable route crosses major roads, railways and large water drainage ditches, horizontal directional drilling will be used to pass underneath the infrastructure without disturbing the surface.
An indicative cross-sectional profile of the cable trench is provided below.
