GREAT FEAT OF ENGINEERING
Construction of this subsea interconnector required careful planning and precise execution. BritNed worked with its 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 construction comprises three major components:
1) manufacturing of the subsea cable and corresponding cable installation
2) civil construction of two converter stations
3) manufacturing and installation of the required electrical equipment and components in the converter stations
ABB High Voltage Cables manufactured and installed the cable on behalf of BritNed. The Siemens / BAM Nuttall consortium was responsible for the construction of the converter stations and for manufacturing and installing the converter equipment.
BritNed cable
The BritNed cable is a bi-pole high voltage direct current interconnector. Due to the distance involved direct current (DC) electricity is the only option to transmit power through the marine and land cables which connect the converter stations at each end.
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)
Cable laying
The marine works involved bundling and burying two HVDC power cables on the seabed of the North Sea by using over 30 special cable laying vessels and a range of support vessels. The cables are buried at least a metre below the bottom of the North Sea. Water depths vary between 30 -50 metres.
The subsea cable laying works were carried out in 7 sections from 2009 to 2010. In September 2009 the first section of marine cable was laid at Maasvlakte (The Netherlands). In June 2010 the BritNed connection hits the UK shoreline at the Isle of Grain
Converter stations
There are land based converter stations built at each end of the HVDC cable, one at the Isle of Grain in Kent (Great Britain) and the other at Maasvlakte (The Netherlands). The sites are approximately 5 hectares in size.
The converter stations each can act either as rectifiers – converting alternating current (AC) into direct current (DC) – or as inverters – converting DC back to AC. This enables power transfer in either direction.
Both the main land transmission networks in The Netherlands and Great Britain transmit at alternating current. The BritNed cable is a Direct Current connection, therefore the converter stations are needed to convert the power
Figures BritNed Converter Transformer
Weight: 282 tonnes (incl. oil)
Length: 9.8 m
Width: 3.8 m
Height: 4.8 m
Thyristor valve
The valve hall houses the 12-pulse optically triggered thyristor valve groups in a double stack configuration (two per phase). The 1200 thyristor valves converts from AC to DC (and vice versa) and is thus the heart of the converter station. The valves are connected to each pole of the DC cable through each of the DC halls with the AC side of the valve connecting to the converter transformers
AC switch yard
The AC switch yards provide the interface between BritNed and the transmission systems of National Grid and TenneT. 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.
Converter transformers
BritNed installed a total of 14 transformers, six transformers plus one spare at each AC/DC converter station. There are three 201MVA single phase transformers for each pole. The converter transformers regulate the AC power system voltage.