Offshore wind

HVDC provides the platform for German North Sea connections

1 August 2010

The infrastructure for connecting offshore wind farms in the North Sea to the German grid, via 300 kV HVDC links, is now beginning to take shape, thanks to major investments by TSO, Transpower.

June and July saw Transpower the German TSO (transmission system operator) – formerly E.On Netz, but now a subsidiary of Dutch TSO TenneT* – place three big contracts for HVDC connections to offshore wind farm clusters in the North Sea.

ABB won a £700 million contract – the largest transmission order in the company’s history – to supply the 800 MW DolWin1 connection, while Siemens Energy, in consortium with cable maker Prysmian Powerlink, won contracts to provide the 800 MW BorWin2 connection and the 576 MW HelWin1 connection, valued at about 500 million euro each.

When other costs are taken into account the three projects amount to a total investment by Transpower – which since 2006 has been legally mandated to construct and operate grid connections for offshore wind in its area – of around 2.4 billion euro.

Transpower’s existing BorWin 1 HVDC connection, supplied by ABB, is already essentially completed, at a cost of about 40 million euros, with a capacity of 400 MW, and is being readied to transmit power from Bard Offshore 1 – the first commercial scale offshore wind project in the German North Sea. Bard Offshore 1 is located about 90 km north west of the island of Borkum, in a water depth of about 40 km. Turbine installation started in March and the hope is to have at least half of the eighty 5 MW turbines (which are of the Bard 5.0 type) installed by the end of this year.

The Bard Offshore 1 wind farm – now 70% owned by SuedWestStrom Windpark GmbH, representing a group of small community based utilities – will provide power to Transpower’s offshore converter platform (BorWin Alpha) at 155 kV AC, via its own 33 kV/155 kV step-up transformer platform.

Transpower has also provided the link – all-AC rather than DC – for the 12 x 5 MW Alpha Ventus test field, which is currently in operation. This 60 MW AC link cost around 90 million euros.

The DolWin1 connection, due to be completed in 2013, will initially serve the planned 400 MW Borkum West II wind farm (being developed by Prokon Nord), and subsequently other facilities to be developed nearby.

BorWin 2, again due to be fully commissioned by 2013, will transmit power from the Veja Mate wind farm (also being developed by Bard) and Stadtwerke Munchen’s Global Tech 1 wind farm.

HelWin 1, also due to enter service in 2013, will provide a connection for RWE Innogy’s Nordsee Ost wind farm, which looks like being the next German North Sea project to enter the construction phase.

The names BorWin, DolWin and HelWin refer to grid connection clusters of wind farms in the North Sea (see map, opposite page), Bor standing for Borkum, Dol for Dollart, Hel for Helgoland and Win for wind. The number denotes the connection project within each cluster, eg BorWin 1 and 2, while, to confuse matters further, the greek alphabet is used to name converter platforms at sea, eg BorWin Alpha and BorWin Beta.

The BorWin2 HVDC line will be about 200 km long, with an onshore converter substation at Diele, near Papenburg, which is also the connecting substation for BorWin 1.

The 165 km long DolWin1 HVDC line will have its onshore converter and grid connection at Dörpen/West, about 90 km inland.

The HelWin1 line, about 130 km long, will be the first grid connection in the eastern North Sea, connecting to a new converter substation at Büttel, with 45 km of the link onshore and 85 km undersea.

ABB is employing HVDC Light for its offshore connection projects, while Siemens is using its competing technology, HVDC Plus – so the German offshore environment should provide some interesting comparisons between the two technologies.

ABB makes its own cable for HVDC Light applications and, at 320 kV, says BorWin2 will employ “the highest voltage level of extruded cable ever used for HVDC.”

Siemens says its offshore converter substation will be designed as a floating, “self-lifting” platform. This platform will be towed by tugs to its destination at sea, where the water is about 40 meters deep, and “a large heavy-duty crane vessel will not be needed to lift the topside onto its foundation.”

* As Europe’s “first cross-border TSO”, Transpower and TenneT will operate under the common name of TenneT as from October.

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