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Offshore: Transmission - Contracts issued for offshore HVDC lines

Use of high-voltage direct current (HVDC) technology is becoming increasingly common around the world, because the transmission efficiency of the standard, cheaper alternating current (AC) high-voltage technology is insufficient over distances greater than about 80 kilometres.

HVDC is indispensable for bringing the power generated at most of Germany's offshore wind stations onshore since the wind farms have to be sited far out to sea to avoid coastal nature protection areas. Transmission system operator Transpower, responsible for cables for German offshore wind stations in the North Sea, has signed up suppliers for five offshore cable systems for a total 2.6GW of capacity. Four of these are more than 80 kilometres and, therefore, require the use of HVDC technology.

The latest contracts, announced by Transpower in July, went to a consortium of Siemens and cable supplier Prysmien PowerLink for the HelWin 1 project, and to ABB, which manufactures its own cables, for the DolWin 1 project. The two HVDC links will be ready to operate in 2013. The total investment is EUR1.5 billion.

The Helwin 1 cable will be 145 kilometres and will initially connect the 295MW Nordsee Ost offshore project to shore. DolWin 1, at 140 kilometres, will connect the 400MW Borkum West 2 project to shore.

The Siemens/Prysmien consortium also won the contract in June to build the BorWin 2 HVDC link to connect the Global Tech 1 and Veja Mate wind stations totalling 800MW. Cable system investment is about EUR800 million and completion is due in spring 2013. The DC cable system is 215-225 kilometres long.

ABB won the BorWin 1 HVDC cable project and completed it in November 2009. It will transmit electricity from the 400MW Bard Offshore 1 project, currently in construction. Investment totalled EUR400 million. The cable system has a total length of 205 kilometres.

The 60-kilometre AC cable connecting the 60MW Alpha Ventus offshore wind station has been in operation since last year.

The tender procedure for a fifth 210-kilometre HVDC project - Sylwin 1 - is under way. It may be ready for commissioning in 2013, according to Transpower.

The investment costs of marine cable links are rolled over to German electricity consumers through network usage charges.

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