To transport the electricity from offshore wind farms to land wind power produced off the German North Sea coast is transmitted from two wind farms to one TenneT converter platform each (HelWin beta and DolWin kappa). There the three-phase current is converted into direct current and transported to the converter station on land, where it’s then converted back into three-phase current and fed into the extra-high voltage grid via the neighbouring transformer station. The produced offshore electricity is collected in a transformer station platform before TenneT takes over at the offshore converter station with a 155-kilovolt three-phase cable. Global subsea cable manufacturer Zhongtian Technology Submarine Cable Co., Ltd [ZTT for short] has been commissioned for both wind farm connections of five and 16 kilometres respectively. The cable systems will be produced at one of its manufacturing base in Nantong/ China and then transported to Europe. The laying itself will be carried out by the subcontractor asso.subsea (Greece). The first cable laying is planned for summer 2022 to connect Kaskasi II wind farm.

Two cable systems with a conductor cross-section of 630 square millimetres will be used to transmit 342 megawatts on the five-kilometre long route between the wind farm’s transformer station and HelWin beta. Using the HelWin2 grid connection, the electricity is transported over 130 kilometres to the land station in Büttel (Schleswig-Holstein) and fed into the extra-high voltage grid. For the cable laying that will follow in the summer of 2023, TenneT uses the first time the advanced three-phase cable with a conductor cross-section of 1,200 square millimetres, which will transport the almost 242 megawatts from the Gode Wind 3 wind farm via just one cable system. Various cables and pipelines to be crossed pose a particular challenge, and the route, which is over 16 kilometres long, also runs through the Gode Wind 1 wind farm, which is already in operation. Gode Wind 3 is connected to the extra-high-voltage grid via DolWin kappa and the associated DolWin6 grid connection in Emden/East (Lower Saxony).

With its operational grid connection projects in Germany and the Netherlands, TenneT provides offshore wind capacities of 8,532 megawatts. Currently, TenneT is implementing seven further offshore grid connection projects in both countries.

Facts and figures HelWin2

  • in operation since June 2015
  • 130-kilometer high-voltage direct current (HVDC) transmission connection with a maximum capacity of 690 megawatts
  • 45 kilometers of land cable, 85 kilometers of sea cable
  • Network connection point: Büttel

Facts and figures DolWin6

  • 90-kilometre high-voltage direct current (HVDC) transmission connection with a maximum capacity of 900 megawatts
  • 45 kilometres of land cable, 45 kilometres of sea cable
  • Network connection point: Emden/East
  • Completion of the network connection planned for 2023

Image source: Courtesy of TenneT

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