The story of how BritNed was constructed began long before all the necessary approvals were in place in both the UK and The Netherlands. It was a great feat of engineering that required careful planning and precise execution.
We worked with our construction partners as one single team to safely design, manufacture and install the BritNed Interconnector to high standards of civil, marine and electrical engineering.
The work consisted of three parts:
Part 1: Manufacturing and installation of the subsea cable
Part 2: Civil construction of two converter stations - one on the Isle of Grain, Kent (Great Britain) and the other at Maasvlakte (The Netherlands)
Part 3: Manufacturing and installation of the electrical equipment in the converter stations
ABB High Voltage Cables produced and installed the subsea cable and a consortium of Siemens / BAM Nuttall built the converter stations and installed the equipment to operate the interconnector and to convert the electricity between direct and alternating current.
We used over 30 special cable laying vessels and a range of support vessels to bundle and bury two High Voltage Direct Current (HVDC) power cables on the seabed of the North Sea at water depths of between 30-50 metres.
The subsea cable laying works were carried out in seven sections, from 2009 to 2010. In September 2009 the first section of marine cable was laid from The Netherlands at Maasvlakte. In June 2010 the BritNed connection arrived at the UK shoreline at the Isle of Grain.
Figures BritNed Cable
Voltage: ± 450 kV DC
Cable capacity: 1000 MW
Weight: 44 kg/metre (23.000 tonnes)
Length sea cable: 250 km (two cables, bundled)
Length land cable: 7 km (NL) and 2 km (GB), two cables, laid together
Conductor: 1 x 1430mm² Cu (copper cable)
In both the UK and The Netherlands, electricity is supplied to customers as alternating current (AC). However, over long distances this is not efficient and energy is lost in transmission. To avoid this, power is carried through the interconnector as direct current (DC).
We therefore needed to construct land based converter stations at each end of the HVDC cable to convert power between AC and DC. The converter stations needed to act as rectifiers - converting alternating current (AC) into direct current (DC) – and inverters – converting DC back to AC. This meant power could be transferred through the HVDC cable in either direction.
Our converter station sites are of similar size, approximately 5 hectares, and have the same equipment.
This equipment includes 12 thyristor valve groups in a double stack configuration (two per phase). The 1200 thyristor valves are at the heart of the converter station as they convert from AC to DC (and vice versa). The valves connect to the DC cable and link to the AC connection through the converter transformers.
We installed six transformers at each site to regulate the power into the electricity transmission networks in the UK and The Netherlands:
Weight: 282 tonnes (incl. oil)
Length: 9.8 m
Width: 3.8 m
Height: 4.8 m
So that we could get power into the transmission systems of National Grid and TenneT, on behalf of our customers, we needed to construct an AC switch yards to provide the interface. To balance the input from our reactive power exchange with customer demands we needed to include large banks of filters.
One short underground AC cable system connects the converter station with the 400kV Grain substation. At Maasvlakte the converter station connects into the TenneT 380kV substation via a short overhead line.