Case Studies

33kV Cable Remedial Works Successfully completed at Merkur Offshore Windfarm

Challenge

• Flashover events occurring during switching operations at the 33kV backup generator transformers, indicating elevated transient overvoltage risk
• Increased risk to system integrity, asset reliability and equipment safety if left unaddressed in a complex offshore environment
• Managing critical interfaces between cable systems and shunt reactors, where overvoltage exposure was highest
• Delivering a robust solution without modifying existing MV GIS or transformer infrastructure
• Executing all works offshore, requiring careful coordination of logistics, weather windows and access via CTV and helicopter
• Maintaining strict compliance with German electrical safety regulations, GWO requirements and site-specific procedures

Solution

• Developed a targeted remedial approach to improve insulation coordination and overvoltage protection without requiring intrusive asset modifications
• Replaced 33kV cable connectors at key shunt reactor interfaces to enhance system performance and reliability
• Installed additional surge arresters on individual cable cores to mitigate transient overvoltages
• Prepared cable sheaths for indirect earthing through sheath voltage limiters (SVLs), supporting improved electrical control
• Conducted very-low-frequency (VLF) testing at 3 × U₀ (approximately 57kV DC) for 60 minutes to verify cable integrity following installation
• Delivered all works in line with strict safety protocols, including full lockout/tagout (LOTO), verification of dead systems, and earthing and short-circuiting procedures

Outcome

• Successfully reduced the risk of future flashovers, improving protection against transient overvoltages at critical system interfaces
• Verified the electrical integrity of the 33kV cable system through successful VLF testing
• Improved long-term resilience and operational reliability without requiring modifications to GIS or transformer assets
• Delivered all works safely and in full compliance with German regulations and offshore safety standards
• Provided a clear pathway for future system enhancement through planned SVL integration
• Implemented a cost-effective, minimally intrusive solution that can be replicated across similar offshore assets