Foundations laid for DolWin5 onshore converter station

4C Offshore | Tom Russell
By: 14/10/2021 TenneT

Yesterday (13th October 2021), TenneT celebrated the laying of the foundation stone for the converter station for its
DolWin5 offshore grid connection project in Emden / East.

A time capsule - containing coins, a regional daily newspaper and construction plans - was placed in the ground. TenneT, Hitachi Energy, Heitkamp, and the consortium of Aibel and Keppel FELS walled up the time capsule together.

The converter station is being built in several construction phases on an area of ​​25,000 square metres. In the halls, there are 'valves', which are made of semiconductors and are responsible for converting direct current into three-phase current. Due to the soil conditions, a deep foundation was necessary to support the project. 600 bored pile, each with a length of 24 metres, were used, with the help of which the structural load is transferred to deeper, load-bearing soil layers. The foundation work for the onshore converter station in Emden / East started in spring 2021.

DolWin5 (Epsilon) grid connection project will facilitate the transfer of 900 MW of electricity from three connected offshore wind farms in the North Sea 100 km off the German coast to the onshore grid. In addition to an onshore converter platform, TenneT will install an offshore converter station located in Emden/Ost in Germany.

The wind power produced at sea off the coast of Lower Saxony is fed as three-phase current to TenneT's offshore converter platform, where it is converted into direct current and transported across the 130-kilometer cable route to the Emden / Ost onshore converter station. Here the electricity is converted back into three-phase current and fed into the high-voltage network via the substation.

Due to the distance and the power to be transmitted, direct current is used for low-loss transport. The onshore converter station will convert direct current into three-phase current.

A consortium of Aibel and Keppel FELS have been commissioned to build the offshore platform and the onshore converter station. The HVDC technology is to be supplied by Hitachi Energy, the construction company is Heitkamp. Prysmian Group was commissioned by TenneT to lay the cables that connect the platform at sea with the station on land.

Production work on the
DolWin5 offshore converter platform in Singapore started in December 2020 with the first steel cut. The installation of onshore cables started in May of this year.

TenneT is also utilising innovative technologies to make network connections even more efficient and cost-effective. One of these innovations includes the 66 kV direct connection, which is being used for the first time in the
DolWin5. According to TenneT this technology enables significant cost savings, as the wind turbines are connected directly to TenneT's offshore platform via 66 kV three-phase cables. Until now it was necessary to build a separate transformer station for each offshore wind farm and to connect it to the converter station with 155 kV three-phase cables.

DolWin5 will connect the
Borkum Riffgrund 3 offshore wind farm with the onshore grid.

DolWin5 is one of a total of three offshore grid connection projects that TenneT is currently realising in Germany. It currently has twelve grid connections in the German North Sea with a total transmission capacity of 7,132 MW are already in operation.