The first project that involves direct current technology was the grid connection for the BARD Offshore 1 wind farm. The project name is based on the Borkum 2 wind farm cluster and is called BorWin1. A transformer station was provided by transpower on a platform for transporting the wind energy to land. From there, the electricity produced by the wind power plants will be transformed into direct current, and conveyed through the sea and then overland by what is known as a high voltage direct current (HVDC) transmission system to the nearest network supply entry point, a transformer station. Direct current is particularly suitable when long distances have to be covered and large amounts of electricity transmitted. For the connection of offshore wind farms to the network, both are the case.
The transformer station will transform the DC energy into alternating current and feed it into the grid

Several kilometres of cable were already laid in 2008 for the grid connection on shore. The rest of the total of 200 kilometres followed in 2009 from the high seas around the island of Norderney down to the substation at Diele near Papenburg. This long stretch is the longest connection in the world to be built for a grid connection for an offshore wind farm. In order to carry out the construction of a high voltage DC transmission line with a capacity of 400 MW, transpower (formerly E.ON Netz) contracted the company, ABB (also see the ABB-Website).
The land route from Hilgenriedersiel to Diele is around 75 kilometres long. In May 2008, E.ON Netz started working on the northern part of the land ducting connection from Hilgenriedersiel to the Diele substation. Almost 38 kilometres of cable were laid up to the A31 road. In the summer of 2008, eight subterranean horizontal bores were made with a total length of 742 metres under the dunes and dykes of Norderney and by Hilgenriedersiel for the passage of the first direct current cable. To protect the Lower Saxony Wadden Sea National Park, the cable was passed through these subterranean bores and offshore cable was connected to the onshore cable. In autumn 2008 a length of 1,330 meters passed under the Ems. The second part of the onshore cable connection to the transformer station followed in 2009. Around 125 kilometres of cable was also laid through the tidal flats and offshore in the summer and fall of 2009. The connection to the voltage transformation substation platform of BARD is also upcoming.

The technical installations carried out by transpower in the voltage transformation substation in Diele were also more or less completed at the end of November 2009. Test and trial operation was started successively. There the direct current will then again be transformed into three-phase alternating current and fed into the 380 kilovolt grid.

The final start-up of the grid connection will first take place following the erection of the wind park in the spring/summer of 2010. A high-performance grid connection via the world's longest direct current connection between an offshore wind park and the extra high voltage grid can then be used to feed large amounts of wind energy into the power grid.

Some offshore wind farms that are quite close to the coast can be connected by means of individual cable connections that are laid through the sea to the nearest network supply entry point.

The first wind farm for which transpower realised such a grid connection is "alpha ventus". The offshore test field is 45 kilometres from the coast of Borkum and is a joint venture between E.ON Climate & Renewables, EWE and Vattenfall Europe New Energy.
Already in 2007, four subterranean horizontal bores were carried out under the dunes and in the mudflats in front of Norderney for the alpha ventus connection. In the spring of 2008, a 1.5-kilometre long hollow ducting structure was completed across Norderney. Simultaneously, the new substation at Hagermarsch was erected to which alpha ventus was connected.

At the end of May 2008, the cabling was laid for the connection from alpha ventus to the hollow ducting construction. In summer 2008, both the onshore as well as the offshore cables were laid.

As one can imagine, the laying of cable in the North Sea places very different demands on technology and equipment compared to laying land-bound cabling into cable trenches. Cables to the wind farm weighing many tonnes – and many kilometres in length – must be aligned and laid on schedule, whilst taking the characteristics of the sea bed into account using varied procedures.

Special ships are required to transport the pre-rolled cable on large rotating disks and unroll them into the sea. There are only a few of these special ships and one example is the Giulio Verne: the ship can transport up to 7,000 tonnes of cable, it is 128.5 metres long and 35 metres wide. As a comparison, a normal football field in Germany is 68 by 105 metres.

The grid connection went into operation in May 2009.

www.alpha-ventus.de