Western HVDC Link

It consists of 422 kilometres (262 mi) of cable, of which 385 kilometres (239 mi) is underwater.[1] It is the first subsea link at 600 kV.[4] Operating at this higher voltage increases transmission capacity and reduces transmission losses in the cable, reducing power loss from transmission and two AC/DC conversions to under 3%.[5]

There are two cables, which in shallow water are laid in a paired bundle, and in deep water are laid about 30 metres apart, generally buried about 1.5 metres below the seabed. The cables are jointed in 120 km sections.[4]

Line commutated converter technology is used at each HVDC converter to maximise the amount of electrical power, generated predominantly by renewable sources in Scotland, which can be transferred across the B6 boundary[notes 1] in parallel to two existing double-circuit AC transmission lines. This type of HVDC conversion technology means that large filter halls are required at each converter station.[6] Gas-insulated switchgear is used at the Hunterston converter station to save space.

Routing the connection through the Irish Sea, as opposed to a land-based route, reduced the impact of the link on the visual environment and prevented the rising cost of land affecting the cost of the project. For the given distance, it was necessary to use a DC transmission technology for a subsea route, as the capacitive current required for an AC connection would be too high. Due to the proximity of the converter stations to the coastline, a decision was taken to host electrical infrastructure within built structures in order to reduce the effects of the coastal air on the equipment.

Power can be transferred in either direction (which might become more likely due to the closure of Longannet power station), but it is necessary for the link to be offline for a sufficient time prior to reversing the direction. Mass impregnated non-draining (MIND) cables are used in a bipolar arrangement, but no sea- or earth-return path is permitted for environmental reasons, meaning that both cables must be in service for the link to be operational. The cables are spaced apart to minimise thermal interference, but not so far as to materially impact any marine life which navigates using the magnetic field of the Earth.

Completion was delayed a year due to cable manufacturing problems.[7] The link has since suffered a number of outages due to cable faults, which typically take several weeks to locate and fix. A fault in the southern land cable on 19 February 2019 resulted in an outage until 22 March.[8][9] A further extended outage from 6 April 2019 was caused by a fault in the undersea cable.[10] A third outage commenced on 10 January 2020,[11] leading to Ofgem investigating.[12]

Engineers working on the project discovered the almost intact wreck of the World War I German submarine UB-85.[13]

• Official website