The inner array cables are the “veins” of the wind farm, transporting the electricity from the 72 wind power plants to the farm's offshore substation. Nine wind turbines are each connected to the offshore substation by a cable string. Each two strings can be connected to one another so as to ensure the operation of the turbines even in the case of cable damage.
All in all, approximately 96 km of cable are to be installed within the wind farm. In this process, two different cross-sections are employed: 630 mm2 and 185 mm2. The cables are produced by JDR Cables in Hartlepool, Great Britain. The main contractor of the inner park cabling is VBMS, based in the Netherlands. Further, the cables contain an optical fibre that not only allows for the data exchange between each wind turbine and the offshore substation, but also the remote control and surveillance of the wind farm by the Vattenfall Control Center in Esbjerg, Denmark.
To begin with, the cables are placed on the seabed and later they are injected at a minimum depth of 0.6 meters up to more than a meter into the bottom of the North Sea. This work is implemented by the cable-laying vessel Stemat Spirit. The vessel further undertakes the transport of the cable from Great Britain to the sea construction site at Sandbank.
One of the central elements of the wind farm is its offshore substation, which is currently being built in Denmark, Poland, and Germany, the responsible main contractor being Bladt Industries A/S of Denmark. Within the offshore substation the power produced by the wind turbines is transformed from 33 kilovolt to a voltage level of 155 kilovolt. Then it is passed on to the converter station SylWin alpha. This is where the transformation from alternating to continuous current takes places, a process which ensures a low-loss power transmission to the mainland.
The investment costs for the offshore wind farm Sandbank are around EUR 1.2 billion. Vattenfall holds a 51% stake in Sandbank Offshore Wind GmbH, which was set up to implement the project, while SWM holds a 49% stake. 72 Siemens wind power plants in the 4-megawatt (MW) class will be constructed, providing a total installed capacity of 288 MW. Sandbank is planned to be fully commissioned in 2017. This will give them a combined portfolio of 576 MW of installed wind capacity, making Vattenfall and SWM some of the largest operators of offshore wind farms in Germany.
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