33 kV Cable Remedial Works Successfully Completed at Merkur Offshore Wind Farm

We’re proud to share that we’ve successfully completed an eight-month remedial works programme at the Merkur Offshore Wind Farm, Germany – one of the leading offshore wind providers in the German North Sea.

The project focused on strengthening the 33 kV cable system at the offshore substation, improving insulation coordination and transient overvoltage protection — all delivered without any modifications to existing MV GIS or backup generator set transformers.

Scope

Flashovers were observed during switching operations at the 33 kV backup generator set transformers on the Merkur Offshore Substation.

These events indicated an elevated risk of transient over voltages affecting the medium voltage cable system, particularly at the interfaces with the 33 kV shunt reactors located in Rooms L201 and L202.

If left unaddressed, the condition presented a risk to system integrity, operational reliability, and equipment safety, with potential implications for unplanned outages and asset damage in a challenging offshore environment.

Approach & Challenges 

A targeted remedial solution was developed to improve insulation coordination and overvoltage protection without requiring modifications to the existing MV GIS or backup generator set transformers.

The scope of works included:

• Replacement of 33 kV cable connectors at the shunt reactor interfaces
• Installation of additional surge arresters on individual cable cores
• Preparation of cable sheaths for indirect earthing via sheath voltage limiters (SVLs)
• Post installation very-low-frequency (VLF) cable testing

All works were executed offshore using CTV and helicopter access, carefully planned around weather constraints. Activities were delivered in full compliance with German electrical safety regulations, GWO certification requirements, and site-specific RAMS.

Strict safety controls were applied throughout, including full LOTO procedures, verification of dead systems, and earthing and short circuiting prior to work execution.

Following termination, the 33 kV cables were pressure tested using VLF at 3 × U₀ (approximately 57 kV DC) for 60 minutes, with surge arresters temporarily disconnected. Testing confirmed the electrical integrity of the cable system.

Outcomes

The remedial works significantly reduced the risk of future flashovers, providing enhanced protection against transient over voltages at the shunt reactor interfaces and improving overall system resilience.

• Verified integrity of the 33 kV cable system through successful VLF testing
• Improved long-term reliability without the need for GIS or transformer modifications
• Safe, compliant delivery in a complex offshore environment
• Clear pathway defined for final SVL connection as a second stage activity once link boxes become available
• Delivered a cost‑effective solution that avoided intrusive GIS or transformer modifications.
• Provided full assurance of asset condition through successful VLF integrity testing.
• Ensured safe, compliant delivery using GWO‑certified teams, with all work completed offshore without disrupting operations.
• Implemented a scalable, repeatable remediation approach that can be applied across other offshore substations.