All 48 foundations for the wind turbines and the foundation of the substation were connected with each other by so-called inter-array cables. The 33-kV underwater cables have a diameter of up to 16 cm. A particularly robust sheathing protects the cables from the extreme stresses caused in particular by the installation process out at sea. At the same time, they guarantee maximum power transmission across large distances. Special-purpose cable laying vessels were used to install these high-capacity power lines.
Two of these special installation vessels, the "Normand Flower" (in 2013) and the "Stemat Spirit " (in 2014) were in operation at the Nordsee Ost wind farm. At the port of Landskrona, Sweden, they took up to 30 km of cable on board before taking off to the wind farm’s construction field. Upon arrival, they connected twelve foundations in a so-called loop each time. For this purpose, the cables first had to be cut to the right length, which was done on board. Subsequently, the cable ends were pulled into the cable tunnels provided in the foundation where they were fastened. The cables connecting the individual foundations were buried at a depth of around 1 ½ m in the seabed. Subsequently, diving robots, so-called remotely operated vehicles (ROVs), surveyed the final cable position and checked the proper functioning of the cables laid.
In August the inter-array cables will be connected with the offshore substation, the nerve center of the wind farm. The substation transforms the electricity generated by the wind turbines at 33 kV to a transmission voltage of 155 kV so that the electricity can be transmitted via high-voltage undersea cables to the coast. The substation of the Nordsee Ost wind farm is around 15 m high and weighs approx. 2,000 tons. Like the wind turbines, it is mounted on a steel structure foundation in the seabed and will tower around 35 m above sea level after final assembly. The steel foundation which weighs around 700 tons was already installed in July 2013. The actual substation platform (topside) is to follow in mid-July.
