Western HVDC Link

The Western HVDC Link is a high-voltage direct current (HVDC) undersea electrical link in the United Kingdom, between Hunterston in Western Scotland and Flintshire Bridge (Connah's Quay) in North Wales, routed to the west of the Isle of Man. It has a transmission capacity of 2,200 MW and was expected to cost £1bn. The link entered operational service on 7 December 2017 at a reduced capacity of 900 MW pending further work at Hunterston. It entered full use on 16 October 2018.
Description.
The link augments the existing capacity provided by double-circuit AC transmission lines which join the Scottish and English transmission grids.
It consists of of cable, of which is under water. It is the first subsea link at 600 kV. Operating at this higher voltage increases transmission capacity and reduces transmission losses in the cable, reducing power loss from transmission and the two AC/DC conversions to under 3%. 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.
Line commutated converter technology is used at each HVDC converter to maximise the amount of electrical power that can be transferred, which is predominantly exported from renewable sources in Scotland. This type of HVDC conversion technology means that large filter halls are required at each converter station. 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. The link has since suffered a number of outages due to cable faults, which typically take several weeks to locate and fix:
Engineers working on the project discovered the almost intact wreck of the World War I German submarine UB-85.