Converter Station Design

The high voltage grid systems in the 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 via 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.

Indicative layouts of the Converter Stations in the UK and France are provided below.


The Converter Station will contain the components listed below which are designed to comply with the Grid Codes and other technical standards in the UK and France:

  • Converter equipment to change HVDC to HVAC (DC Hall, Valve Hall, Reactor Hall);
  • Transformers to change the voltage level from 525 kV to 400 kV;
  • AC Switchyard to provide connection control and protection for the HVAC cable which connects to the national grid sub-station;
  • Cooling system for valves;
  • Ancillary and balance of plant for converter station operations, including Control Room, Service Building, Workshop, Stores, Spares/Maintenance Building;
  • Utilities services, telecommunications system, diesel generator (for emergency use), fire-fighting system, domestic wastewater treatment facility, stormwater management system and HVA/C heating-ventilation-air conditioning units;
  • General site facilities, such as security fencing and CCTV, lighting, site roads, car parking, access gate and landscaped areas.

The Converter Station comprises a main building divided into sections for the DC Hall, Valve Hall and Reactor Hall. Additional buildings may be used to contain the control room, maintenance services and other site facilities or could be attached to the main building.

The buildings will be an appropriate size to hold the electrical equipment and typically they will comprise a steel structural frame with industrial cladding.  The design and finish of the cladding will be matt, non-reflective with suitable colouration to minimise visual impact. The tallest building will be the Valve Hall which will be up to 25m in height.

The sizes of the buildings in the preliminary design of the Converter Stations are illustrated below.


The DC Hall contains the connections to the two HVDC cables that transfer the power between the UK and France.  

The Valve Hall is the central section of the main building and contains multiple stacks of power electronics that are central to the conversion process from direct current to alternating current. There will typically be two rows of three stacks of converter modules, with each stack supported approximately 3 m above the floor and with a 3 m gap to the walls, ceiling and other stacks.  

The Reactor Hall contains three air cored reactors that are placed on steel structures. The reactors link to the transformers that are sited next to the building.

The Interface Transformers are an interface point between the Reactor Hall and AC Yard.  There will be three single phase transformers in use at any one time with a spare transformer located on site in reserve.  

The AC Yard will be an outdoor area containing the electrical equipment required to protect the Converter Station in the event of a fault and to enable switching, isolation and earthing for maintenance.  The AC Yard will also include the HVAC cable sealing ends of the HVAC cable that will connect the Converter Station to the national grid substation.

The Cooling Fans provide a cooling mechanism for the valves (located in the Valve Hall) which produce heat during the conversion process.  The fans cool the valves to maintain a correct operating temperature.

A Diesel Generator set will be provided to serve the Converter Station in the event of loss of either the local transmission network/interconnector or local distribution network.  The diesel generator will enable the safe start-up or shutdown of the converter station and running of critical systems if mains electricity supply is not available.

The Control Building will house the Control Room, AC and DC electrical plant, DC converter module cooling plant, Heating, Ventilation and Air Conditioning (‘HVA/C’), batteries, auxiliary power supplies, communication systems and an interface with National Grid. 

Other supporting systems and equipment that will be installed at the Converter Station comprise a fibre-optic telecommunications system, surface water/stormwater drainage system and a fire-fighting system, including fire water storage.

Internal site roads will be developed to enable access to all parts of the Converter Station site.  Low level street lighting of internal site roads and other areas will be provided for safety and security.

Any unused areas within the Converter Station will be landscaped with lawn grass and/or gravel.

A perimeter fence will enclose the Converter Station which comprises a nominal steel palisade fence (with an inner electrified fence, if required for security) approximately 3.4 m in height.