Continuing from Part 1 of our HVAC vs HVDC article, which examined the key differences in
the fabrication of these two cables, Part 2 looks at the important differences in performance, application, and deployment.
This article concludes our HVAC vs HDVC series and will provide you with a fundamental understanding of the differences between offshore HVAC and HVDC cables, allowing you to ensure the reliability and efficiency of offshore wind and energy transmission systems.
Mechanical Performance and Durability
HVAC cables are generally heavier and more complex due to the need for three-phase conductors and additional shielding. The twisting (layup) of the three conductors reduces flexibility, making installation and handling more challenging, particularly in offshore environments. Additionally, HVAC cables require larger bend radius monitoring to prevent insulation stress and maintain proper phase balance. Their heavier, less flexible design is also more vulnerable to mechanical damage in deep-sea environments, where water pressure, strong currents, and shifting seabeds exert greater forces on the cable’s significant mass, increasing the risk of insulation fatigue or conductor strain.
HVDC cables (when stranded), are lighter and more flexible as they typically consist of only one or two conductors, simplifying transport, handling, and installation. They also feature stronger mechanical protection, with thicker sheathing and enhanced water-blocking layers, making them highly durable in subsea environments. Their streamlined design allows for easier deployment along long-distance routes, especially in deep-water applications where cable integrity is critical, as repairs are logistically challenging and costly, and any disruption to power transmission can lead to significant downtime. Additionally, HVDC cables are more resistant to mechanical fatigue, as DC transmission does not create the oscillating electrical stress that HVAC systems experience.
Overall, HVDC cables offer superior mechanical durability and easier installation, particularly for long-distance offshore applications, while HVAC cables require more precise handling and are more challenging to deploy due to their three-phase structure.
Testing During Manufacturing
During manufacturing, HVAC and HVDC cables undergo many of the same tests to verify electrical and mechanical integrity. These typically include AC routine and sample tests, such as conductor resistance checks, partial discharge evaluations, and mechanical stress assessments. Leading up to the factory acceptance test (FAT), HVDC cables follow the same procedures as HVAC cables, including power-frequency (AC) testing and material checks. At the FAT stage, HVAC cables undergo an AC high-voltage test, while HVDC cables receive their first DC high-voltage test just before delivery.
A key distinction in the manufacturing test setup for HVAC vs. HVDC cables lies in the terminations used for high-voltage testing. During factory acceptance testing (FAT), HVAC and HVDC cables each require high-voltage stress, (AC for HVAC and DC for HVDC) to verify insulation integrity. HVAC cables typically use water-cooled terminations because the oscillating AC voltage generates substantial heat at the cable interface, which must be dissipated to prevent partial discharges or insulation breakdown. In contrast, HVDC cables use gas-cooled terminations, often pressurized with SF₆ or nitrogen, because constant DC voltage produces less heat yet imposes a continuous electric field that demands a stable, dry environment. It should be noted that the HVDC termination is only used during the DC Voltage testing during FAT.
While HVAC cables rapidly lose their charge once the AC high-voltage source is switched off and grounded, HVDC cables can retain a significant charge for an extended period due to the absence of a cyclical current that naturally helps dissipate energy. As a result, strict grounding procedures are critical following HVDC testing to ensure the cable is fully discharged and safe to handle. Personnel must use various methods and proper earthing systems to gradually bleed off the stored charge, reducing the risk of discharge or equipment damage. This additional discharge process makes post-test procedures for HVDC cables more significant than for HVAC cables, which lose their charge more rapidly.
Loadout and storage
HVAC cables, with their three-phase cores and added armoring, are significantly heavier and often have a larger allowable minimum bend radius. This makes loadout more complex and time-consuming, as operators must handle the cable with greater caution to avoid excessive bending or mechanical stress on the cable. HVDC cables, however, are generally lighter and can be wound on larger continuous lengths for loadout due to the weight constraints on the turntables. HVAC cables may also necessitate larger vessels, heavier tensioning and burial equipment to manage the additional weight and maintain a safe bend radius.
In summary, while HVAC cables require larger vessels, heavier-duty turntables, and longer loadout times due to their weight and bend radius constraints, HVDC cables benefit from lighter weight, longer lengths, and less demanding equipment requirements, making them easier to load and store.
Conclusion
Offshore HVAC and HVDC cables each have distinctive advantages, driven by their underlying electrical principles and application requirements. A major deciding factor in choosing between them is the transmission distance.
HVAC cables, designed for three-phase alternating current, are typically preferred for shorter routes (under 80–100 km) thanks to existing grid compatibility and lower upfront costs. However, they face higher reactive power losses over longer stretches, making them less efficient and sometimes more expensive to operate if transmission needs to extend far offshore. The heavier, three-core structure, combined with additional armoring and a higher minimum bend radius, also makes HVAC cables more challenging to manufacture, install, and handle. This added complexity is reflected in the layup process, longer loadout times, and heavier vessel and equipment requirements.
HVDC cables excel at transmitting power efficiently over long distances by eliminating reactive power losses, the skin effect, and EMI concerns. Their simpler single- or dual-core design, combined with lighter weight and fewer conductors, allows for easier handling, larger continuous cable lengths, and reduced losses, all highly desirable for far-offshore wind farms or interconnections exceeding 100 km. These unique features help make HVDC cables a compelling choice for high-capacity links, where reducing losses and simplifying installation is critical to project success, despite higher initial converter station costs.
Return to Part 1: HVAC vs HVDC Offshore Cable Fabrication here…
