TenneT connects offshore wind farm Veja Mate to grid connection BorWin2

TenneT connects offshore wind farm Veja Mate to grid connection BorWin2

In mid-November, transmission system operator TenneT connected the offshore wind farm (OWF) Veja Mate to BorWin2. TenneT’s offshore grid connection system BorWin2 has been in operation since January 2015. It consists of the converter station BorWin beta, where the alternating current (AC) of the connected wind farms is transformed into direct current (DC). From BorWin beta the direct current flows via 125 kilometres of sea cables and 75 kilometres of land cables to the converter station in Diele, where it is transformed in AC again and fed into the high voltage grid. BorWin2 has a capacity of 800 MW and is already connecting the OWF Global Tech I to the high voltage grid.

The connection between BorWin beta and Veja Mate’s substation is realised by two 11,4 km AC cables with a voltage of 155 kV. TenneT and Veja Mate carried out the so called Soak Test, a 24 hours high voltage cable quality test, from 16 to 17 November and completed it as planned.

“We had a very ambitious schedule”, says an executive manager at TenneT, “but thanks to the good team work between TenneT and Veja Mate and the continuous support and confidence which our shareholder in this project, Mitsubishi Corporation, placed in our project team we managed to connect Veja Mate’s substation in time to the grid.

For TenneT this step marks the successful completion of Veja Mate’s connection to the grid. As Veja Mate’s turbines are still to be erected, the wind farm will use the grid connection in the construction phase to obtain power for their construction works from the grid. .

Veja Mate's Electrical Package Manager said “Throughout the whole project, TenneT and Veja Mate managed to keep an atmosphere of good cooperation which helped both parties achieve their goals. Even in the most difficult situations, TenneT proved to be a solution and customer oriented provider to our satisfaction.”

TenneT is a leading European electricity transmission system operator (TSO) with its main activities in the Netherlands and Germany. With approximately 22,000 kilometres of high-voltage connections we ensure a secure supply of electricity to 41 million end-users. We employ approximately 3,000 people, have a turnover of EUR 3.3 billion and an asset value totalling EUR 15.4 billion. TenneT is one of Europe’s major investors in national and cross-border grid connections on land and at sea, bringing together the Northwest European energy markets and enabling the energy transition. We take every effort to meet the needs of society by being responsible, engaged and connected.

Veja Mate is a 402 MW, 1,9 bn EUR offshore wind project owned by the Highland Group Holdings Ltd, Siemens Financial Services and CI II managed by Copenhagen Infrastructure Partners. Bank financing was completed on the 1st of July 2015. Construction started on the 4th of April 2016 and the project is expected to be fully operational by the end of 2017. It will produce over 1,6 TWh of green, renewable electricity annually, equivalent to the annual consumption of 400.000 German households.

Technical Specifications and Grid Integration

The BorWin2 grid connection system is a pioneering example of high-voltage direct current (HVDC) technology in offshore wind energy. By converting alternating current to direct current at the BorWin beta offshore converter platform, TenneT significantly reduces transmission losses over the 200-kilometre route to the Diele onshore converter station. This efficiency is critical for integrating large-scale offshore wind farms like Veja Mate into the German high-voltage grid, ensuring that the renewable energy generated far out at sea reaches consumers with minimal waste.

Veja Mate's connection to BorWin2 involves two 155 kV alternating current export cables, each approximately 11.4 kilometres long, linking the wind farm's offshore substation to the BorWin beta platform. The successful completion of the 24-hour soak test on 17 November 2016 validated the integrity and reliability of these cables under high-voltage conditions. This test is a standard industry practice to ensure that the cable system can withstand operational stresses before full energisation.

The integration of Veja Mate into BorWin2 also highlights the shared use of offshore grid infrastructure. BorWin2, with its 800 MW capacity, already connects the Global Tech I wind farm, and the addition of Veja Mate optimises the utilisation of this asset. This approach aligns with TenneT's strategy to develop efficient, multi-user offshore grid connections, reducing costs and environmental impact compared to individual point-to-point links.

Construction and Commissioning Milestones

The connection of Veja Mate to BorWin2 marked a significant milestone in the project's construction timeline. Following the installation of the offshore substation in September 2016, the focus shifted to the electrical infrastructure necessary to export power. The timely completion of the cable laying and testing activities allowed Veja Mate to use the grid connection for construction power, a practical advantage that supported the subsequent installation of the wind turbines.

This milestone was achieved through close collaboration between TenneT and Veja Mate, as emphasised by the project leads on both sides. The ambitious schedule required meticulous planning and execution, particularly given the challenging offshore conditions in the German Bight. The successful connection underscored the effectiveness of the partnership and the commitment of all stakeholders, including the project's shareholder Mitsubishi Corporation.

For more details on earlier construction phases, see Groundwork launched for Veja Mate and Veja Mate Offshore Project Installs One Third of its monopiles. The installation of the offshore substation is covered in Veja Mate Offshore Project Installs OSS.

Role of the Soak Test in Ensuring Reliability

The soak test, conducted from 16 to 17 November 2016, is a critical quality assurance procedure for high-voltage cable systems. During the test, the cables are energised at their rated voltage for a continuous 24-hour period while monitoring for any signs of insulation weakness or thermal instability. A successful soak test provides confidence that the cable can handle the electrical load and environmental conditions it will face during decades of operation.

For Veja Mate, the completion of the soak test as planned was a testament to the quality of the cable installation and the robustness of the connection design. It also allowed the project to proceed with confidence to the next stages of commissioning, knowing that the export system was ready to deliver power once the turbines were operational.

Impact on Germany's Energy Transition

The connection of Veja Mate to the grid via BorWin2 is a concrete step forward in Germany's Energiewende, the transition to a low-carbon, renewable-based energy system. With a capacity of 402 MW, Veja Mate will generate over 1.6 TWh of green electricity annually, enough to power approximately 400,000 German households. This contribution is vital for meeting national renewable energy targets and reducing dependence on fossil fuels.

Moreover, the efficient use of HVDC technology in BorWin2 demonstrates how offshore wind can be integrated at scale without overwhelming the onshore grid. By transmitting power as direct current, TenneT minimises losses and enhances grid stability, making it possible to harness the strong and consistent winds of the North Sea effectively. The success of projects like Veja Mate reinforces the viability of offshore wind as a cornerstone of Germany's energy future.

Future Outlook and Project Completion

Following the grid connection, Veja Mate proceeded with the installation of its 67 Siemens SWT-6.0-154 wind turbines, each with a capacity of 6 MW. The project remained on track to be fully operational by the end of 2017, as stated in the original announcement. The use of the grid connection for construction power during turbine installation was a strategic advantage, reducing the need for diesel generators and lowering the project's carbon footprint.

The completion of Veja Mate added significant renewable capacity to the German grid and demonstrated the effectiveness of collaborative offshore wind development. The project's success also provided valuable experience for future offshore wind farms, particularly in terms of grid connection strategies and the use of shared infrastructure. For further updates on project certification and performance, see Veja Mate receives SGS Project Certificate.

For more information on TenneT's offshore grid connections and other projects, visit the Media section or browse all Press Releases.