Understanding the differences between offshore HVAC and HVDC cables is critical for project planning, cost optimization, and ensuring the reliability and efficiency of offshore wind and energy transmission systems. This article explores the key differences between these two cables which should be considered when undertaking a new offshore wind project.

Offshore HVAC (High-Voltage Alternating Current) Cables

Offshore HVAC cables are power transmission cables designed to transport electricity
using alternating current (AC) from offshore wind farms, oil & gas platforms, or other offshore installations to onshore grids or between offshore substations. They are commonly used for shorter distances (typically less than 70–80 km). As reactive power losses in AC cables grow with cable length, this makes HVAC transmission a less attractive option for
longer routes. 

Common Applications

·        Interconnecting offshore wind turbines and substations.

·        Transmitting power from offshore platforms to shore.

·        Grid interconnections between islands or coastal regions.

Key Features

·        Typically operate at voltages of 110–285 kV but can be higher.

·        Comprise three-core cables (for three-phase power).

·        Require reactive power compensation beyond certain distances (approximately 60-80km).

Offshore HVDC (High-Voltage Direct Current) Cables

Offshore HVDC cables are used to transmit direct current (DC) electricity over long distances (>100 km) with minimal power losses, making them more efficient than HVAC cables for far offshore wind farms and intercontinental grid connections. HVDC transmission is preferred for distances beyond 80km, where HVAC becomes inefficient due to reactive power limitations. HVDC offshore systems also require costly converter stations at both ends to convert AC power to DC offshore and back to AC onshore. At shorter transmission distances, these added expenses often outweigh the benefits of
lower losses in DC cables.

Common Applications

·        Long-distance power transmission from offshore wind farms to onshore grids.

·        Interconnecting different power grids (e.g., Europe-North America interconnections).

·        Reducing transmission losses in deep-sea or remote offshore projects.

Key Features

·        Typically operate at voltages ranging from ±320 kV to ±525 kV or higher.

·        Require converter stations to convert AC power to DC offshore and back to AC onshore.

·        Offer lower power losses over long distances compared to HVAC.

Material Composition

Offshore HVAC and HVDC cables share many similarities in material composition, as both
require high-conductivity metals like copper or aluminium for efficient power transmission. Their insulation systems commonly use cross-linked polyethylene (XLPE) due to its thermal and electrical properties, though HVDC cables may also incorporate mass-impregnated (MI) paper for added stability in high-voltage applications. Both types of cables feature galvanized steel, stainless, or polymer armoring for mechanical protection, and lead/polyethylene sheathing to protect against corrosion and external damage. While their designs may vary slightly to optimize performance for AC or DC transmission, the fundamental materials used are largely the same, prioritizing durability, conductivity, and resistance to harsh offshore environments.

While MI paper-insulated HVDC cables remain in use for specific deep-sea applications, the industry is shifting towards XLPE-insulated HVDC cables due to their lower losses, easier installation, and improved thermal performance. The XLPE between the HVAC and HVDC cables most likely will have some minor differences in composition, however, most new HVDC projects now favour XLPE over MI paper.

Cable Design and Structure

Electromagnetic interference (EMI) shielding is essential for HVAC cables because AC generates fluctuating electromagnetic fields that can interfere with nearby cables and electronic systems. To mitigate this, HVAC cables incorporate metallic shielding layers that safely redirect interference to the ground. In contrast, HVDC cables do not require EMI shielding because DC produces a steady electric field, eliminating electromagnetic oscillations that cause interference.

The skin effect, another phenomenon affecting HVAC cables, occurs when AC concentrates near the conductor’s surface due to induced eddy currents. This reduces the usable cross-sectional area and increases resistance. This effect worsens at higher frequencies, often leading to power losses.

Figure 1 Skin Effect

To counteract this, HVAC cables use stranded conductors to distribute AC more evenly and reduce resistance. HVDC cables, however, do not experience skin effect because the DC current flows uniformly throughout the conductor, allowing for solid or larger cross-section conductors without efficiency loss. HVDC cables generally also use stranded conductors to enhance flexibility and mechanical durability.

Figure 2 Stranded Conductor

Additionally, fiber optic cables are incorporated differently in HVAC and HVDC systems. In HVAC cables, fiber optics are embedded within the cable during the layup process, ensuring seamless integration. This arrangement leverages the unused space around the laid-up cable cores, allowing fiber optics to be securely seated before the armoring is applied, simplifying one-step manufacturing and ensuring the fiber is well-protected against mechanical stresses.

For HVDC cables, fiber optic cables are bundled onto the power cable during installation as HVDC cables are typically single- or bipole designs, leaving less inherent space to embed a fiber within the cable structure. However, some HVDC cables are designed with fiber optic integration for specific applications.

Key Design Differences

·        HVAC cables are designed for three-phase power, requiring multiple cores and EMI shielding.

·        HVDC cables use a single-core structure (or bipolar for redundancy) and focus more on voltage endurance and water-blocking mechanisms.

·        XLPE insulation is common in both, but MI paper is still used for ultra-high voltage HVDC subsea applications.

·        HVAC requires stranded conductors to mitigate skin effect, while HVDC can use solid or larger-stranded conductors for efficiency, although stranded conductors are preferred on both designs.

·        HVAC cables have fiber optics embedded during the layup process, whereas HVDC cables typically have fiber optics bundled externally during installation, though some designs allow for embedded fiber.

Manufacturing Processes

The manufacturing process for HVDC cables is faster compared to HVAC cables, primarily due to the simpler core configuration. While HVAC cables require three conductor cores, each with their own insulation, shielding, and sheathing, HVDC cables typically have only one or two conductors (monopolar or bipolar), significantly reducing the number of cable lengths that need to be produced. 

Additionally, HVAC cables involve a layup process, where the three insulated conductor cores are twisted together before jointing and armoring. This extra step ensures balanced
electromagnetic properties but adds complexity and production time.

Figure 3 HVAC Manufacturing Process

In contrast, HVDC cables can bypass the layup stage altogether. Once the conductor is insulated and sheathed, it moves directly into the jointing process before the final armoring step. By eliminating the layup stage, HVDC cable production is more streamlined, leading to a shorter overall manufacturing duration. 

In HVAC cables, each joint must integrate three conductor segments, including sheathing and insulation, for each phase, necessitating multiple verification procedures throughout the jointing sequence. During jointing, technicians align and splice each phase conductor, verifying the insulation and sheathing integrity through mechanical and electrical checks before finalizing the joint.

Consequently, this multi-core configuration extends the overall jointing duration and increases the testing frequency relative to single-conductor HVDC systems. In contrast, HVDC cables include fewer conductors, which reduces both the number of joints and associated tests, thereby enabling a more expedited and efficient jointing process.

Since HVDC cables are typically produced in much longer lengths due to the nature of
high-voltage DC transmission, the material and time savings are somewhat offset by the longer cable lengths required to meet project specifications.

Check out Part 2: HVAC vs HVDC Cable Performance and Deployment here…