DolWin6, TenneT’s 13th offshore grid connection in Germany, will help in delivering renewable wind power from the North Sea
DolWin kappa installation. (Credit: TenneT)
In the TenneT DolWin6 network connection project, the offshore platform known as DolWin kappa has been effectively installed off the coast of Lower Saxony. Its primary purpose is to convert the alternating current (AC) generated by three wind farms, initially at 155 kV, into 362 kV AC, and then further transform it into 320 kV direct current (DC) for efficient transmission to the mainland with minimal energy loss. Notably, the DolWin kappa platform boasts the distinction of being the first to incorporate gas-insulated switchgear (GIS) for this purpose.
This milestone achievement followed the successful high voltage testing of the high-voltage installation, conducted by Pfisterer in Cádiz, Spain. TenneT, the transmission network operator, once again relied on Pfisterer’s CONNEX inner cone high voltage fittings for this groundbreaking project.
DolWin6 marks a significant achievement for TenneT, representing their 13th offshore grid connection in Germany. As a transmission network operator, TenneT holds a prominent position in the European Union’s offshore sector. This new 900 MW connection plays a crucial role in delivering renewable wind power from the North Sea, theoretically capable of providing energy for over a million households. Consequently, it contributes to enhancing supply security not only in Germany but also across Europe.
Pfisterer has been a valuable partner throughout the project’s lifecycle. They provided extensive engineering and planning services right from the project’s inception and offered oversight during the cable installation process. Subsequently, Pfisterer assumed responsibility for the assembly of high voltage fittings and conducted the final high voltage testing of the entire high-voltage cabling. Approximately 200 size 6 CONNEX connectors, epoxy resin joints, and link boxes were expertly installed by PFISTERER, ensuring a safe and reliable long-term power transmission solution.
The DolWin kappa converter platform represents a pioneering development by featuring two Gas-Insulated Switchgear (GIS) systems for the first time: one for the alternating current (AC) side and another for the direct current (DC) submarine cable connection to the mainland. This innovation offers significant space advantages within the platform’s design.
Pfisterer played a crucial role in this achievement by initially taking charge of calculating the current-carrying capacity and power loading for various cable connections. These connections spanned from the wind farm connection within the platform, through the 155 kV GIS, to the transformer, and onwards to the second GIS on the 362 kV DC side. This analysis enabled Pfisterer to provide support in designing and selecting the internal cabling while optimizing the time-efficient planning of all installation works.
Pfisterer further ensured seamless project execution by monitoring and documenting the cable installation both in Cádiz and offshore in the North Sea. Subsequently, their team took on the coordination and execution of fitting the CONNEX HV plug connectors and epoxy resin joints. Leveraging their extensive experience in offshore platform projects, they fine-tuned work sequences for cable laying, adapting them to the specific challenges of this project. This meticulous planning allowed the team to effectively manage the complex logistics and adhere to the tight project schedule.
Remarkably, the construction and outfitting of the platform, which stands at approximately 36m in height and covers an area equivalent to eleven tennis courts, was accomplished within a remarkable three-year timeframe.
Following the completion of all onshore installations, a final high voltage testing was conducted to verify the integrity of the 155 kV and 362 kV voltage level systems. To avoid disruption to other platform activities, these tests were carried out during nighttime hours. To supply the required test voltage, a mobile test trailer equipped with PFISTERER bushing and a gas-insulated elbow joint was connected to the platform via a test cable. Additionally, the PFISTERER team established a suitable connection in the platform’s converter room for the 362 kV GIS. As part of the testing process, joints and cables had to be relocated multiple times to test all three phases thoroughly. Ultimately, the platform was officially handed over in March 2022. In August of the same year, it embarked on its journey via Rotterdam to the North Sea, where it was installed 45 km off the coast.
In a significant move to enhance redundancy and network security, DolWin kappa is interconnected at sea with its sister platform, DolWin beta. Pfisterer employed a 155 kV Feltoflex bridge connection, similar to the DolWin gamma project, requiring the pulling of approximately 2 km of cable between the two platforms. This cable connection was secured with epoxy resin joints and CONNEX plug connectors. Concurrently with the offshore installation work, PFISTERER also conducted the final testing of the high-voltage cable connection to ensure its reliability and performance.
TenneT has a longstanding track record of relying on Pfisterer’s dependable CONNEX HV fittings for its offshore projects. These fittings have been used in various network connections and offshore platforms, including DolWin alpha, DolWin beta, DolWin gamma, and DolWin epsilon, as well as in several satellite platforms. Siemens Energy, serving as the general contractor for the construction of the DolWin kappa platform, received support from the PFISTERER Group, leveraging its local teams and branches in Spain, Germany, and Switzerland.