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How Offshore Wind Farms Send Electricity to Shore (HVAC vs HVDC)

Updated 8 October 20267 min readWind & Hydropower

Getting power from an offshore wind farm to the grid is a cable problem, not a turbine problem. A submarine cable is a long capacitor as well as a conductor: energise it with alternating current and it draws a charging current whether or not anything is connected at the far end. That current grows with length, it competes with the power you wanted to send, and at some distance it is the reason the industry stops using alternating current altogether.

Key takeaways

  • A submarine AC cable draws charging current along its whole length, so part of its rating is consumed before any wind power is carried.
  • That effect scales with length, which is why AC export becomes impractical beyond a certain distance rather than merely inefficient.
  • HVDC has no charging current in steady operation, so a DC cable's useful capacity does not fall away with distance.
  • The price of HVDC is a converter station at each end — large, complex equipment that AC schemes simply do not need.
  • The cable, not the electronics, is the usual point of failure, and a repair offshore is measured in weeks of weather windows.
On this page
  1. Everything happens inside the cable
  2. The current the cable takes before you send anything
  3. What HVDC changes
  4. Which one, and why
  5. The part that actually breaks

Everything happens inside the cable

An overhead line on land is mostly a conductor with air around it. A submarine cable is a conductor wrapped in insulation, wrapped in a metallic sheath that is bonded to the sea. That geometry — two conductive surfaces separated by a thin dielectric — is the definition of a capacitor, distributed along every metre of the route.

Overview of an offshore wind farm's electrical path. Turbines are connected in strings by medium-voltage array cables running along the seabed. The strings gather at an offshore substation platform where the voltage is stepped up. A single high-voltage export cable runs to a landfall point on the coast, continues underground to an onshore substation, and connects to the transmission grid.Overview of an offshore wind farm's electrical path. Turbines are connected in strings by medium-voltage array cables running along the seabed. The strings gather at an offshore substation platform where the voltage is stepped up. A single high-voltage export cable runs to a landfall point on the coast, continues underground to an onshore substation, and connects to the transmission grid.
Strings of turbines, one platform that makes them look like a single generator, and one export cable that everything depends on.

Inside the farm the picture is conventional enough. Turbines are wired into strings by medium-voltage array cables — typically a few tens of kilovolts — because running every turbine to shore separately would be absurd. The strings gather at an offshore substation, a platform whose transformers raise the voltage for the long run and whose switchgear lets a faulted string be isolated without stopping the farm.

Everything interesting happens after that platform, in the single export cable that carries the whole farm's output. And the first thing that cable does is draw current for itself.

The current the cable takes before you send anything

Energise an AC cable and the voltage across its insulation alternates, which means the distributed capacitance is charged and discharged every cycle. That charging current flows whether or not a single turbine is turning.

Cross-section and length view of a submarine AC cable. The cross-section shows conductor, insulation and an earthed metallic sheath, labelled as forming a capacitor along the whole length. The length view shows charging current entering the cable and flowing into the distributed capacitance all along it, so that the current measured at the sending end is larger than the current delivering power at the far end. A vector diagram shows the useful power current and the charging current at right angles, with the cable's total rating as the resulting limit.Cross-section and length view of a submarine AC cable. The cross-section shows conductor, insulation and an earthed metallic sheath, labelled as forming a capacitor along the whole length. The length view shows charging current entering the cable and flowing into the distributed capacitance all along it, so that the current measured at the sending end is larger than the current delivering power at the far end. A vector diagram shows the useful power current and the charging current at right angles, with the cable's total rating as the resulting limit.
The conductor and the earthed sheath form a capacitor along the whole route. Charging it consumes part of the cable's current rating before any wind power is carried.

The consequences follow directly. Charging current is proportional to length, so a longer cable draws more of it. It is also proportional to voltage and frequency, which removes the obvious escape route: raising the voltage to reduce the working current — the standard move for long distances on land — increases the charging current at the same time.

The two currents are out of phase with one another, so they do not simply add, but they do share the same conductor and the same thermal rating. Past a certain length, the cable is fully occupied carrying its own charging current and has nothing left for the farm.

Chart of useful transmitted power against cable length for alternating current and for direct current. The AC curve starts high and falls away as length increases, dropping steeply towards zero once charging current dominates the cable's rating. Adding reactive compensation shifts the AC curve outward to greater lengths without changing its shape. The DC line stays almost flat across the whole range, declining only gently with resistive losses.Chart of useful transmitted power against cable length for alternating current and for direct current. The AC curve starts high and falls away as length increases, dropping steeply towards zero once charging current dominates the cable's rating. Adding reactive compensation shifts the AC curve outward to greater lengths without changing its shape. The DC line stays almost flat across the whole range, declining only gently with resistive losses.
Illustrative shapes. AC capacity falls away with distance; compensation pushes the wall further out but does not remove it. DC declines only through resistance.

Reactive compensation is the mitigation: shunt reactors at one or both ends, and sometimes on a platform partway along the route, that absorb the charging current locally so it does not have to travel the full length. It works, and it is why most AC export schemes reach further than the raw physics would suggest. It also means more offshore structure to build, inspect and maintain.

What HVDC changes

Direct current does not alternate, so in steady operation the cable's capacitance is charged once and then simply sits there. No repeated charging, no charging current, no length-dependent penalty.

The two export architectures compared side by side. The HVAC scheme runs from the offshore substation transformer directly into an AC export cable, with shunt reactors for compensation at each end, and into the onshore substation. The HVDC scheme adds a converter station offshore that rectifies AC to DC, a DC cable pair, and a second converter station onshore that inverts it back to AC before the grid connection. Labels note that the AC scheme has fewer components offshore while the DC scheme has no charging current in the cable and can control its power flow directly.The two export architectures compared side by side. The HVAC scheme runs from the offshore substation transformer directly into an AC export cable, with shunt reactors for compensation at each end, and into the onshore substation. The HVDC scheme adds a converter station offshore that rectifies AC to DC, a DC cable pair, and a second converter station onshore that inverts it back to AC before the grid connection. Labels note that the AC scheme has fewer components offshore while the DC scheme has no charging current in the cable and can control its power flow directly.
HVDC removes the cable problem and adds two converter stations to do it. One of those stations has to live offshore, on a platform, for its whole service life.

Modern schemes use voltage source converters, which can set the power they transfer independently of what the grid is doing and can help support grid voltage rather than merely leaning on it — the same split between following a grid and forming one that separates grid-tied and off-grid inverters at household scale. That controllability matters beyond the cable: a wind farm connected through a converter is decoupled from the grid's frequency, which changes how it behaves during disturbances: it does not naturally offer the stored rotational energy that a synchronous machine, such as a pumped hydro storage unit, contributes the instant frequency moves.

The price is the converters themselves. Two stations, one of which is an offshore platform carrying power electronics that must run for decades with limited access. They introduce conversion losses at both ends — the same kind of loss a rooftop solar inverter pays turning DC into AC, at vastly higher power — and those losses are paid on every kilowatt-hour regardless of distance.

Which one, and why

So the decision is a race between a fixed penalty and a growing one. HVDC pays its converter penalty immediately; HVAC pays a penalty that grows with every kilometre of route. Where they cross is commonly cited at roughly 50-100 km (30-60 mi), depending on cable design, voltage and project specifics — a range to reason with, not a threshold to design to.

What pushes a scheme towards each option
FactorPoints towards HVACPoints towards HVDC
Distance to shoreShort routes, where charging current stays modestLong routes, where AC capacity has fallen away
Farm capacitySmaller farms a single AC circuit can carryLarge farms needing full capacity at distance
Equipment offshoreTransformers and switchgear onlyAccepts a converter platform and its maintenance
LossesNo conversion losses to payLower cable losses, paid for with converter losses
Grid supportGrid must tolerate a direct AC connectionConverter can control power flow and support voltage
Connecting two gridsNot possible if they are not synchronisedStraightforward — DC does not care about frequency

Engineering relationships, not a rule. Break-even is commonly cited at roughly 50-100 km (30-60 mi), depending on cable design, voltage and project specifics — a range rather than a hard threshold, and one that also moves with the farm's capacity and what the receiving grid requires.

That last row is easy to overlook. Because a DC link imposes no frequency relationship between its ends, it can join grids that are not synchronised with each other — which is why the same technology appears in interconnectors that have nothing to do with wind.

The part that actually breaks

The converters get the attention; the cable causes the outages. It lies on or under the seabed for its whole life, and the hazards are mechanical rather than electrical.

A seabed section showing an export cable buried in a trench, with the hazards it faces. A ship's anchor dragging across the route. A trawl board scraping the seabed. Scour, where current washes sediment away and leaves a span of cable unsupported and free to vibrate. Abrasion where the cable crosses rock. A separate inset shows the repair sequence: locate the fault, recover the cable ends to a vessel, joint them, and rebury the repaired section.A seabed section showing an export cable buried in a trench, with the hazards it faces. A ship's anchor dragging across the route. A trawl board scraping the seabed. Scour, where current washes sediment away and leaves a span of cable unsupported and free to vibrate. Abrasion where the cable crosses rock. A separate inset shows the repair sequence: locate the fault, recover the cable ends to a vessel, joint them, and rebury the repaired section.
Anchors, fishing gear, scour and abrasion — all mechanical. Burial is the defence, and a repair depends on a vessel and a weather window.

Burial is the main protection, with depth chosen against the seabed conditions and the traffic overhead. Scour is the subtler hazard: currents wash sediment away from beneath a cable, leaving an unsupported span that can vibrate and fatigue. Route surveys, burial depth verification and periodic inspection exist to catch these before they become faults.

When a fault does occur there is rarely an alternative route, so the farm stops exporting until a cable vessel can locate the break, lift both ends, joint them and rebury the section — work that waits on weather in exactly the way that turbine shutdowns in storms do. It is the same theme that runs through all marine energy engineering, and the reason wave and tidal devices are judged as much on survivability and access as on output.

That is the honest summary of offshore transmission: the physics of the cable decides the architecture, and the sea decides what it is like to own. The rotor questions — how many blades, what the Betz limit permits — are settled long before any of this begins.

Frequently asked questions

Why not just use a higher voltage for AC and be done with it?

Raising voltage reduces the current needed for a given power, which helps with resistive losses — but charging current rises with voltage as well, so it makes the capacitive problem worse rather than better. That is the trap: the usual remedy for distance on land works against you at sea.

What is reactive compensation?

Equipment — typically shunt reactors — that absorbs the cable's charging current locally instead of letting it travel the length of the cable. It is installed at one or both ends, and sometimes at a platform partway along. It extends the workable length of an AC link, at the price of more offshore structure to build and maintain.

Does HVDC lose less energy?

The cable itself does, because there is no charging current and no skin effect to contend with. The converter stations at each end introduce losses of their own that an AC scheme never pays. So HVDC is not automatically more efficient — it wins on long links, where the cable advantage has had enough distance to outgrow the fixed converter penalty.

What is inside an offshore substation?

Transformers to raise the array voltage to export voltage, switchgear to isolate faults and individual strings, protection and control systems, and for a DC scheme the converter equipment as well. It is a platform whose job is to make the whole farm look like a single connection point to the export cable.

What happens when an export cable fails?

The farm stops exporting, entirely, until it is repaired — there is usually no second route. Locating the fault, mobilising a cable vessel, recovering the cable from the seabed, jointing it and reburying it depends on weather windows, so outages are measured in weeks rather than hours. This is why burial depth and route surveys receive so much attention.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

Editorial Team

Research, drafting and review

Articles are drafted from primary engineering and physics references with AI-assisted tools, then reviewed and fact-checked line by line by a human editor before publication. We publish explanations, not recommendations: no products, no pricing, no country-specific rules, and no invented author personas.

Last reviewed 8 October 2026. How we research and review

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