Winterbach, Germany - November 7, 2023

The DolWin kappa offshore platform was successfully installed off the coast of Lower Saxony as part of TenneT's DolWin6 grid connection project. In future, it will transform the Alternating current (AC) from three wind farms from 155 kV to 362 kV and subsequently to 320 kV direct current (DC) and then transmits it to land with virtually no losses. The platform is the first to have two gas-insulated switchgears (GIS) for this purpose. This was preceded by the successful high-voltage testing of the HV installation by PFISTERER in Cádiz, Spain Cádiz. The transmission system operator TenneT also relies on pluggable indoor CONNEX high-voltage fittings from the PFISTERER CONNEX product family for DolWin kappa.

In total, DolWin6 is already the 13th offshore grid connection from TenneT in Germany. The transmission system operator is a leader in the offshore sector in the European Union. The new 900 MW connection will supply renewable wind power from the North Sea for theoretically over one million households and thus further strengthen the security of supply in Germany and Europe. PFISTERER is supporting the project and the general contractor SIEMENS Energy commissioned with the construction of the platform - from comprehensive engineering and planning services to installation supervision of the cables. PFISTERER then took over the installation of the high-voltage fittings and the final high-voltage testing of the entire HV cabling. Among other things, around 200 CONNEX connectors of size 6-S, epoxy resin sleeves and link boxes from PFISTERER were installed for permanent safe energy transmission.


First gas-insulated switchgear for direct current transmission
For the first time, the DolWin kappa converter platform has two GIS systems - one for the alternating current (AC) side and one for the direct current (DC) side of the submarine cable connection on land. This offers considerable space advantages within the platform. PFISTERER initially calculated the current carrying capacity and the energy load of various cable connections - from the wind farm connection within the platform via the GIS on the 155 kV level to the transformer and from there to the second GIS on the 362 kV DC side. PFISTERER was thus able to provide support in the design and selection of the internal cabling, as well as in the time-optimized planning of all installation processes. PFISTERER supervised and documented the cable installation on site, both in Cadiz and offshore in the North Sea, and then coordinated and carried out the installation of the HV connectors and epoxy resin joints. Based on the existing project experience in the offshore platform segment, the cable laying workflows were optimized and adapted to the challenges of the project.
This enabled the complex logistics and tight project schedule to be mastered PFISTERER supports TenneT with DC connection - offshore platform DolWin kappa successfully installed.

HV test overnight
A final high-voltage test ensures the integrity of the systems at voltage levels 155 kV and
362 kV after completion of all onshore installations. In order not to interfere with other work on the platform, these tests were carried out at night. To feed in the test voltage, a
mobile test trailer with a PFISTERER bushing and gas-insulated elbow joint was connected to the platform via a test cable. The PFISTERER team also created a suitable connection for the 362 kV GIS in the converter room of the platform. The joints and cables also had to be repositioned several times to test all three phases. The platform was finally handed over in March 2022. In August 2022, it began its journey via Rotterdam to the North Sea, where it was installed 45 km off the coast.

Successful offshore cable connection on the high seas
To increase redundancy and grid security, DolWin kappa will be connected to its sister platform DolWin beta at sea. As in the DolWin gamma project, PFISTERER realized this with a 155 kV Feltoflex bridge connection. Around 2 km of cable was pulled between the two platforms and connected with epoxy resin joints and CONNEX connectors . In addition to the installation at sea, PFISTERER was also involved in the final testing of the HV cable connections.

TenneT has long relied on the safe and proven CONNEX HV fittings from PFISTERER for its offshore projects. Among other things, they are used in the DolWin alpha, DolWin beta, DolWin gamma and DolWin epsilon grid connections and offshore platforms, as well as several satellite platforms. Siemens Energy is the general contractor responsible for the construction of the DolWin kappa platform. The PFISTERER Group provided support with its local teams and branches in Spain, Germany and Switzerland.