# TenneT and Siemens Energy Finalize 525 kV DC Transformer and GIS Architecture for 2 GW North Sea Hubs

> Transmission operator TenneT and Siemens Energy have reached a milestone technical design freeze on the standardized electrical transmission architecture for the LanWin and BalWin 2 GW offshore grid connection systems, incorporating single-phase 525 kV DC converter transformers and fluoronitrile-based SF6-free gas-insulated switchgear.

- **Publish Date:** 2026-09-27
- **Category:** renewable-integration
- **Reading Time:** 5 min
- **Original Source Citation:** [TenneT TSO & Siemens Energy](https://www.tennet.eu/news/tennet-siemens-energy-2gw-offshore-platforms-transformer-architecture)
- **Tags:** TenneT, Siemens Energy, HVDC 525kV, Offshore Wind, North Sea, Converter Transformers, SF6-Free GIS, LanWin BalWin, Germany
- **Canonical URL:** https://voltformer.com/news/tennet-siemens-energy-advance-2gw-offshore-hvdc-lanwin-balwin-platforms
- **Markdown Source:** https://voltformer.com/news/tennet-siemens-energy-advance-2gw-offshore-hvdc-lanwin-balwin-platforms.md

## Standardized 2 GW Voltage-Sourced Converter Architecture

As part of the offshore grid expansion in the German North Sea, TenneT’s 2 GW Program establishes a uniform engineering standard replacing historical 900 MW and 1.4 GW systems. By stepping up transmission direct current to ±525 kV, the system doubles power transmission capacity across subsea cable routes while slashing transmission losses by over 30%.

The offshore topsides, being constructed in Cadiz by partner Dragados Offshore, will each house six single-phase converter transformers (plus one hot spare) rated at 370 MVA each, stepping up offshore wind farm collector voltages directly to the modular multilevel converter (MMC) valve halls.

## Fluoronitrile Clean-Air GIS and Low-Weight Transformer Design

Weight and environmental impact are central engineering parameters for offshore topsides operating in harsh marine environments. The 525 kV switchgear installed on the LanWin and BalWin platforms utilizes Siemens Energy’s Clean Air and fluoronitrile gas mixtures, eliminating sulfur hexafluoride (SF6) and cutting global warming potential (GWP) by over 99.9%.

The converter transformers utilize specialized high-strength core clamps and vibration-damping elastomer mountings that withstand continuous mechanical stresses caused by North Sea wave action while maintaining acoustic emissions within strict underwater ecological thresholds.

## Onshore Grid Integration at Continental Hub Substations

On the mainland, the corresponding onshore converter stations will interface directly with the German 400 kV extra-high-voltage AC transmission grid. Synchronous condensers and dynamic reactive power compensation units will operate in tandem with the converter transformers to supply artificial inertia and short-circuit capability to the northern German network.

The design freeze clears the path for comprehensive factory acceptance testing (FAT) of the primary windings and bushings starting in early 2027.

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