Why Subsea AC Cables Hit a Physical Limit at 80 Kilometres

Developers often assume they can just run standard alternating current (AC) cables further out to sea as offshore wind expands, but physics dictates otherwise. The limitation is not the strength of the wind or the size of the turbines, but the electrical properties of the subsea cables themselves.[2]

At a distance of roughly 80 to 100 kilometres, a standard AC cable hits a hard physical limit where it can no longer transmit useful power to the shore. This barrier forces a costly transition to High Voltage Direct Current (HVDC) technology, fundamentally altering the economics of remote offshore wind projects.[3]

The Anatomy of a Subsea Cable

To understand this 80-kilometre limit, one must look inside the infrastructure. A subsea power cable consists of a central copper or aluminium conductor surrounded by a thick layer of polymer insulation, designed to keep the electricity contained.[1]

This insulation, known as the dielectric, is then encased in a metallic sheath and heavy steel armouring. These outer layers are essential to protect the delicate internal components from the crushing pressures and corrosive environment of the ocean floor.[1]

In an electrical circuit, any two conductors separated by an insulator form a capacitor, which is a device that naturally stores electrical charge. Because the cable runs for dozens of kilometres across the seabed, this capacitor effect is distributed along its entire length.

"Medium and high voltage power cable consists of center conductor and an outer metallic shield separated by insulation which has the characteristic of a capacitor," explains a 2022 engineering analysis by Voltage Disturbance. This creates a massive capacity to store electrical charge across the ocean floor.

The electrical phases must endure close proximity inside the subsea cable, which severely increases this parasitic capacitance. Unlike overhead power lines, which use air as an insulator and space conductors far apart, subsea cables pack everything into a tight, highly capacitive bundle.[1]

The Dielectric Charging Penalty

The problem arises directly from the nature of alternating current. In an AC system, the voltage reverses direction 50 or 60 times every second, depending on the regional grid standard, creating a constant state of electrical flux.

With every single reversal, the massive capacitor formed by the subsea cable must be fully charged, discharged, and then charged again in the opposite direction. This continuous cycle creates a flow of electricity known as the capacitive charging current, or dielectric charging current.

This charging current flows back and forth between the cable and the onshore grid continuously. Crucially, it flows even if no actual wind power is being transmitted to the shore, acting as a permanent electrical burden on the system.

This reactive current grows linearly with the length of the cable. A 40-kilometre cable requires exactly twice the charging current of a 20-kilometre cable, steadily eating into the system's overall capacity as the wind farm moves further offshore.

"The longer the cable, the more reactive power it produces, limiting how much real power you can transmit," notes power systems engineer Shuvangkar Das in a 2025 technical review. This dynamic makes AC transmission increasingly inefficient over long distances.

Exhausting the Thermal Ampacity

Every power cable has a maximum thermal ampacity, which is a strict limit on the total amount of electrical current it can carry. Pushing past this limit causes the copper to overheat, which would eventually melt the surrounding polymer insulation.[2]

The total current flowing through the cable is a combination of two things. It includes the active power generated by the wind turbines, plus the reactive charging current demanded by the cable's own capacitance.

Because the thermal ampacity is a fixed physical limit, the growing charging current takes up an increasingly large share of that space. As the cable gets longer, the room left for actual, usable electricity steadily shrinks.

By the time an AC cable reaches 80 to 100 kilometres in length, the charging current consumes almost the entire thermal ampacity of the conductor. The cable is effectively full of its own reactive power, leaving no room for the wind farm's output.

"It is possible for a very long cable to carry 100% capacitive charging current making the cable unable to supply any useful load," the Voltage Disturbance analysis confirms. At this point, the infrastructure becomes entirely self-defeating.

The 80-Kilometre Physical Wall

At this critical 80-kilometre distance, developers hit a hard physical wall. There is simply no thermal headroom left to carry the actual electricity generated by the offshore wind turbines to the mainland grid, rendering standard AC transmission physically unviable.

A 2023 study by the National Offshore Wind Research and Development Consortium modeled a 1.2-gigawatt wind farm using 220-kilovolt AC cables over an 80-kilometre distance. The results highlighted the sheer scale of the reactive power problem.

The researchers found that the cables generated a staggering 1,074 megavars of reactive power. Managing this required massive reactor installations at both the onshore and offshore substations just to keep the grid connection stable and compliant.

Without this expensive reactive compensation, the AC transmission capacity declines rapidly beyond 80 kilometres, eventually reaching zero. The physics of the cable simply will not allow the power to flow any further without catastrophic overheating.

While mid-point compensation stations can extend this range slightly, they are highly impractical. Building an artificial island halfway along the route just to house massive shunt reactors adds immense capital costs and environmental complexities to the project.[2]

The Direct Current Solution

The only definitive way to bypass the dielectric charging limit is to abandon alternating current entirely and switch to High Voltage Direct Current (HVDC). This technology fundamentally changes how the electricity moves through the subsea environment.[3]

Direct current flows continuously in one direction, meaning the voltage does not alternate 50 times a second. Because of this stability, the cable's capacitor only needs to be charged once when the system is initially energized.

"HVDC doesn't suffer from capacitive charging losses," Das explains in his review. Because there is no continuous charging cycle, the entire cross-section of the conductor remains available for transmitting real, usable power to the shore.

This physical advantage allows HVDC cables to transmit gigawatts of electricity over hundreds of kilometres with minimal losses. For remote offshore wind installations located far beyond the 80-kilometre mark, this technology becomes absolutely mandatory.[3]

A survey by the European Commission's Joint Research Centre quantified this efficiency gap. They found that long HVDC submarine cables lose only 3.5 to 5 percent of their power over 1,000 kilometres, including the converter station losses.[3]

In stark contrast, comparable AC cables would lose roughly 6.7 to 10 percent over the exact same distance, assuming they could even be compensated enough to function. This makes DC the only viable choice for long-haul subsea transmission.[3]

The Economic Trade-Off

While HVDC neatly solves the physics problem, it introduces a severe economic hurdle for project developers. Wind turbines naturally generate AC power, and the onshore electrical grids operate entirely on AC power, creating a massive integration challenge.[3]

Using a DC cable requires building massive converter stations at both ends of the line. These facilities rectify the alternating current into direct current for the journey, and then invert it back to AC at the destination.[3]

These converter stations are colossal, highly complex feats of engineering. A typical multi-gigawatt HVDC converter station costs approximately $350 million and requires an offshore platform roughly the size of a football pitch to house the power electronics.[3]

For wind farms located close to shore, the staggering cost of these converters far outweighs the benefits of DC transmission. In these shallow-water scenarios, standard AC cables remain the cheaper and more practical option.[2][3]

However, the offshore wind industry is increasingly moving into deeper waters to capture stronger, more consistent wind resources. As these new projects push past the 80-kilometre mark, the physical limits of AC cables make the massive investment in HVDC converters unavoidable.[2][3]

The transition to direct current is driven entirely by the immutable laws of electromagnetism rather than developer preference. Until material science produces an insulation that can defy the capacitor effect, the 80-kilometre wall will continue to dictate the architecture of the global offshore grid.[1]