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 Converter Stations are connected to the national grids in the UK and France by underground HVAC cables. The cable routes will be approximately 1.5 km in length in the UK and 3 km in length in France.
The connection point in the UK is a spare bay at the existing NGESO Kingsnorth 400 kV sub-station. No extension to the sub-station is necessary to accommodate the connection. The underground HVAC cable to the substation is entirely within the former E.on Kingsnorth Power Station site (now demolished). The cable route is shown below.
The connection point in France is a new substation that will be constructed on the south-eastern side of the existing RTE Warande sub-station. The underground HVAC cable route to the new substation is currently under investigation and consultation by RTE in accordance with the concertation fontaine procedure. After completion of the procedure, the cable route will be defined. The Converter Station site and new sub-station are shown below.
Each circuit will consist of three conductor cables, with each cable providing one of the three phases of an electrical circuit. The diameter of each cable will be up to 140 mm, with a central conductor made of copper. The cables will be insulated and protected by an outer covering.
The cross-section of the underground HVAC cable is 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, except where it crosses existing public network systems or where there are environmental constraints, in which case manual excavation methods will be used. The excavated material will be stored temporarily before reuse as backfill. A protective layer of concrete and a warning layer will be placed above the circuits to indicate the presence of the cables.
An indicative cross-sectional profile of the cable trench is provided below.