TenneT award contracts for 900 MW offshore grid connection

Power grid operator Tennet has awarded contracts for its planned 900-MW DolWin6 offshore grid link in the German North Sea.

The company says it has done so at 15 per cent lower costs than similar projects currently under construction.

Tennet installed the 900-MW DolWin3 converter platform last week bringing total offshore transmission capacity in the German part of the North Sea above 6 GW, it said, with two more links already under construction boosting transmission capacity above 7 GW by 2019.
According to Tennet, total project costs dropped below Eur1 billion ($1.1 billion) with engineering firm Siemens, which will provide the main offshore converter platform, putting the contract value for its part into the high triple-digit million euros area.

French cable maker Nexans was awarded the contract for the 90 kilometre cable, of which 45 kilometres will be below the sea, it said.

DolWin6 is expected to link German offshore wind farms in the North Sea cluster 3 to the grid from 2023.

Siemens will supply the technology for efficient direct-current transmission for the ” grid connection. Spanish partner, Dragados Offshore S.A., is a leading general contractor in the energy sector and is responsible for construction and offshore installation of the associated platform.

After it is completed, DolWin6 will be able to transmit enough electricity to supply just under one million German homes. Commercial commissioning is scheduled for 2023.

Ralf Christian, CEO of Siemens Energy Management Division said, “This makes Siemens the technology partner in six out of ten DC projects. After BorWin3 and DolWin6 are commissioned, approximately 4.7 GW of electrical power will be converted and transmitted to shore using Siemens technology alone. This is enough to supply just under five million homes.”

Siemens’ gas-insulated switchgear for 320 kilovolts (kV) requires as much as 95 per cent less space compared to the current air-insulated solutions. If they are used on an offshore platform, the size of the platform can thus be reduced by approximately 10 per cent. As a result, Siemens is significantly helping to lower the cost of grid connections for offshore wind farms and thus to the cost parity of offshore wind energy in general.

The 155 kV alternating current supplied by the wind turbines will be converted into 320 kV direct current on the DolWin6 offshore platform and transported to Hilgenriedersiel on the mainland via a 45-kilometer subsea cable. On the ocean, the cable will pass underneath the island of Norderney by means of horizontal bore holes. Once on land, the electricity will be transported by another 45-kilometer underground cable to Emden, East Frisia, where Siemens is building another converter station for converting the direct current back into three-phase current and feeding it into the high voltage grid.

Siemens is now implementing a total of six out of ten DC North Sea grid connections for TenneT. The earlier projects were HelWin1 (576 MW) and HelWin2 (690 MW) off the coast of Helgoland, BorWin2 (800 MW) off the coast of Borkum and SylWin1 (864 MW) off the coast of Sylt

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