Technology & Type-Test Progress of ±525 kV and ±640 kV Ultra-High-Voltage HVDC Submarine Cables

September 5, 2026

Introduction

Global demand for large-capacity offshore wind power and cross-border grid interconnection pushes HVDC submarine cable technology to higher voltage levels. In 2026, the industry has witnessed continuous type-test breakthroughs for ±525 kV commercial-grade submarine cables, while ±640 kV HVDC cable systems are moving from laboratory research into formal pre-qualification verification. Strict testing standards defined by CIGRE TB 852 govern the full qualification workflow, covering electrical endurance, thermal stability, mechanical bending and deep-water simulation conditions for new-generation transmission projects.

Latest Qualification Milestones for ±525 kV HVDC Submarine Cables

Leading cable manufacturers have completed a series of authoritative type-tests this summer.

• Nexans finished the full qualification of the ±525 kV mass-impregnated HVDC submarine cable at the CIGRE Paris Session. The cable system including flexible repair joints passed verification for an installation depth up to 3,000 metres, setting a new world benchmark for ultra-deep-sea power transmission, tailor-made for the Crete-Cyprus Great Sea Interconnector project.
• NKT launched a fully qualified ±525 kV XLPE-insulated submarine cable allowing continuous conductor operation at 90 °C. The upgraded thermal performance increases power transmission capacity and optimises economic design for North-Sea offshore wind clusters managed by TenneT.
• In China, the complete thermal stability test for domestic ±525 kV HVDC submarine cables has been successfully witnessed by third-party certification organisations, confirming 30-year service reliability under cyclic thermal-electric load conditions.
• ENTSO-E also opened public consultation on unified assessment frameworks for ±525 kV HVDC land and submarine cable systems across Europe, aiming to harmonise qualification rules for future tender projects.

R&D and Pre-Qualification Progress of ±640 kV HVDC Cable Systems

While ±525 kV products enter mass-production bidding cycles, the industry is focusing its next-generation development on ±640 kV ultra-high-voltage HVDC submarine cables.

Advanced test platforms that support simultaneous double-circuit verification for ±640 kV-class submarine cables have been built in Asia, capable of simulating extreme working conditions with ultra-high DC voltage and large-ampere cyclic loading.

Test procedures for ±640 kV systems follow the latest CIGRE specifications, including:

• Long-duration DC voltage loading
• Load-cycle tests
• Lightning-impulse superposition tests
• Tension-bending mechanical verification
• Temporary overvoltage (TOV) assessment

These procedures validate insulation stability under complex offshore grid faults. Multiple cable suppliers are constructing dedicated production lines and test facilities for future ±640 kV commercial projects, with pre-qualification testing scheduled from 2026 to 2028. Once fully certified, ±640 kV HVDC submarine cables will deliver significantly higher transmission capacity over longer sea routes for large-scale deep-sea offshore wind farms and cross-continental power interconnections.

Higher Testing Requirements for Ultra-High-Voltage Cable Projects

The upgrade from ±525 kV to ±640 kV raises stricter technical barriers across the whole industry. Test laboratories and field-test teams must complete full-sequence withstand tests, partial-discharge inspection, thermal cycling evaluation and fault-location verification for complete cable systems including joints and terminations. Reliable high-voltage testing equipment and professional field diagnostic solutions become essential for factory acceptance, site commissioning and long-term asset maintenance of new-generation HVDC submarine cables.

Conclusion

The global HVDC submarine-cable market is experiencing a critical technical upgrade phase. Proven ±525 kV systems are widely adopted in current offshore wind and island-interconnection schemes, while ±640 kV technology moves steadily toward commercial qualification. Standardised, repeatable high-voltage testing and cable-fault-diagnostic solutions will play a vital role throughout manufacturing, installation and operational lifecycles for ultra-high-voltage power-transmission infrastructure.