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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsShort answer: HVAC is usually the simpler and more economical choice for shorter offshore export routes because it connects directly to an AC grid. As a submarine route becomes longer, cable-charging current and reactive-power requirements make HVAC harder to operate and may require extra compensation or circuits. HVDC removes cable charging current, gives precise control of real-power flow and can link asynchronous grids, but it adds expensive converter stations, platforms and conversion losses. A project-specific techno-economic and grid study—not a universal mileage rule—decides the better technology.
What HVAC and HVDC mean in an offshore wind project
An offshore wind farm generates electricity as alternating current (AC). The export system carries that power through one or more subsea cables to a landfall and then into the transmission network. The central choice is whether the long export link remains AC (HVAC) or is converted to direct current (HVDC) for the submarine section.
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HVAC: alternating current through the export cable
With HVAC, transformers raise the wind farm’s voltage and three-phase AC export cables carry power to shore. A landfall substation can then transform the voltage again and connect it to the receiving AC network. The interface is familiar to grid operators and normally does not require a power-electronic converter at both ends.
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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →The complication is that a long AC cable behaves partly like a capacitor. It draws charging current even when it is delivering little useful power. That current consumes part of the cable’s current-carrying capacity and produces reactive power that must be managed with shunt reactors, compensation equipment, additional cable circuits or operating restrictions. The effect grows with cable length and voltage.
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HVDC: converting AC for the submarine link
HVDC converts the wind farm’s AC output to DC at an offshore or onshore converter station, transmits it through a DC cable and converts it back to AC at the receiving grid. The DC cable does not have the same continuous charging-current problem as a long AC cable, so a greater share of its thermal rating can be used for real-power transfer over long routes.
Converters also provide controllable real-power flow. They can regulate how much power enters the receiving grid and, depending on the converter design and controls, provide reactive-power support and other grid services. A DC link can connect asynchronous AC systems because the two grids do not need to share the same frequency. The U.S. Department of Energy explains that transferring power between asynchronous regions without disrupting either frequency is possible with DC links.
Why subsea distance changes the HVAC calculation
Charging current reduces usable HVAC capacity
Every AC cable has capacitance between its conductors and between the conductors and metallic screens. On a short route, the resulting charging current is a manageable part of the design. On a long submarine route, it can approach a substantial portion of the cable’s current rating. The cable then carries current that does not deliver wind-farm megawatts to shore, increasing electrical losses and limiting the useful transfer capability.
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Reactive power also affects voltage control and the stability of the connected network. Compensation equipment can offset some of the cable’s reactive behavior, but it adds cost, footprint, maintenance and control requirements. If compensation cannot solve the constraint economically, the developer may need parallel HVAC circuits or a lower export rating.
HVDC avoids cable charging current, but not every difficulty
A DC export cable has conductor resistance and thermal limits, but it does not continuously exchange reactive power with the cable capacitance in the way an AC cable does. That is why HVDC often becomes attractive as export distance and project capacity increase. The trade-off moves from long-route AC compensation toward converter stations, converter platforms, DC switchgear, control systems, protection and specialized installation.
HVDC protection and interoperability are demanding engineering tasks, especially when several DC terminals or future offshore grids are planned. Converter controls must coordinate with the wind farm and the receiving network, and a fault on a DC system behaves differently from a conventional AC fault. These requirements do not make HVDC unsuitable; they mean that its benefits must be evaluated as a complete system.
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Cost structure and the break-even distance
HVAC generally has lower fixed equipment cost because it can avoid two HVDC converter stations. HVDC has a higher entry cost but can use the cable and corridor more effectively over a long submarine route. The distance at which the total costs cross is therefore the point where the added converter cost is offset by savings in cable circuits, compensation, losses, installation or other system elements.
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| Published screening figure | What it says | How to use it |
|---|---|---|
| About 50–100 km | The European Commission staff working document (2023) identifies this as a typical range in which HVDC can become economically preferable for offshore or underground cable links. | Use as an initial screening band only. The document stresses that assumptions and project conditions drive the choice. |
| Approximately 37 miles (about 60 km) | A U.S. Department of Energy article, citing an ABB estimate (circa 2023), gives this submarine-line break-even distance. | Treat it as a sourced general estimate, not a guaranteed crossover for a particular wind farm. |
| No universal cutoff | A 2009 comparative economic study found that ownership of the transmission cable changes the calculated break-even length. | State who owns, finances and operates the export asset before comparing technologies. |
These figures differ because they use different cable ratings, converter costs, installation assumptions, losses, financing and ownership arrangements. Capacity matters as much as distance: a high-capacity wind farm may justify converters sooner than a smaller project on the same route. Land cables, onshore routing, difficult seabed conditions, platform requirements and the point of grid connection can move the result in either direction.
Losses: compare the whole system, not just the cable
It is inaccurate to say that “DC always has lower losses.” HVDC removes the AC cable’s charging-current burden, but every AC-to-DC and DC-to-AC conversion stage consumes energy. The European Commission’s 2023 staff working document includes this illustrative sample:
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| Configuration | Conductor losses | Converter-station losses | Sample total |
|---|---|---|---|
| HVAC | 2% | No converter-station entry | 2% |
| HVDC with line-commutated converters (LCC) | 1% | 1.4% | 2.4% |
| HVDC with voltage-source converters (VSC) | 1% | 2.0% | 3% |
Those are the document’s sample comparison figures, not guaranteed performance for current offshore projects. They demonstrate why conductor efficiency alone cannot establish system efficiency. A credible comparison models cable and converter losses at the wind farm’s expected loading profile, includes transformer and compensation losses, and considers how often the link operates below its rated power.
The Department of Energy article states that HVDC losses can be 30–50% lower than comparable HVAC lines over long distances. That is a broad estimate attributed to other analysis, not a project-specific result. It should be used to frame a study, not to promise a percentage saving.
Grid connection, controllability and network role
| Decision axis | HVAC | HVDC |
|---|---|---|
| Connection to an AC grid | Direct electrical connection through transformers and switchgear. | Requires conversion between AC and DC at the system endpoints. |
| Long subsea routes | Charging current and reactive power become increasingly significant; compensation or additional circuits may be needed. | No cable charging current in the DC transmission cable; long-distance transfer is often more practical. |
| Power-flow control | Follows conventional AC-network behavior and dispatch controls. | Converters can set and regulate active-power transfer precisely and can support selected grid-control functions. |
| Asynchronous grids | Cannot directly couple systems that do not share synchronized frequency. | Can exchange power between asynchronous systems through the converters. |
| Fixed equipment | Cable, transformers, switchgear and possibly reactors or other compensation. | Cable plus converter stations, platforms or buildings, DC equipment, controls and protection. |
| Engineering maturity | Established AC protection and operating practices. | Proven for point-to-point links, but DC protection, multi-terminal operation and interoperability require specialized design. |
HVAC and HVDC equipment in the export chain
Typical HVAC arrangement
- Wind-turbine generators feed the offshore collection network.
- An offshore substation raises the collection voltage to the export-cable voltage.
- Three-phase HVAC subsea cables carry power to landfall.
- Onshore transformers and switchgear connect the export circuit to the AC transmission system.
- Reactors or other compensation equipment manage cable-generated reactive power when required.
At shorter distances, this arrangement can be compact and straightforward. At greater distances, engineers must check voltage profile, charging current, thermal rating, resonance, compensation switching and the number of parallel circuits needed to deliver the required megawatts.
Typical HVDC arrangement
- The offshore collection network gathers the turbines’ AC output.
- An offshore or onshore converter changes AC to DC.
- A high-voltage DC cable transmits power to the receiving end.
- A second converter changes DC back to AC.
- Transformers and switchgear connect the recovered AC to the transmission grid.
Voltage-source converter (VSC) technology is common in modern offshore applications because it can control active and reactive power independently and can connect to relatively weak grids. Line-commutated converter (LCC) systems have different operating characteristics and have historically been used for very high-power links. The appropriate converter type depends on the grid strength, black-start or voltage-support requirements, fault behavior, losses and cost—not on distance alone.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A project-by-project method for choosing the technology
Use distance as the first screen, then work through the complete decision sequence below.
- Define the export requirement. Record the wind farm’s rated and expected output, collection voltage, export voltage, commissioning date, availability target and whether the link must be expandable.
- Fix the physical route. Measure subsea length, landfall length, onshore cable sections, seabed conditions, crossings, burial depth, route bends and the distance to the grid connection point. A nominal “distance to shore” can understate the actual electrical route.
- Build matched HVAC and HVDC configurations. Use realistic cable ratings, number of circuits, conductor sizes, insulation systems, installation methods and spare philosophy. Do not compare one HVAC circuit with an overbuilt HVDC design or vice versa.
- Model HVAC charging and compensation. Calculate charging current, reactive-power flow, voltage rise, resonance risk, compensation locations and how the system behaves at minimum and maximum generation.
- Size the HVDC converters and auxiliaries. Include converter stations, offshore platforms or buildings, transformers, smoothing and DC equipment where applicable, cooling, controls, protection, communications and harmonic filtering.
- Compare total delivered-energy losses. Simulate realistic wind output rather than only full-load operation. Include cable, transformer, converter, reactor and auxiliary consumption and account for curtailment caused by electrical limits.
- Test grid and regulatory requirements. Check fault-ride-through, voltage and frequency support, harmonic limits, energization, restoration, protection coordination and the system operator’s connection requirements. For HVDC, test converter controls and DC-fault scenarios; for HVAC, test reactive-power and resonance behavior.
- Price the life cycle. Include procurement, manufacturing slots, installation vessels, landfall works, platforms, spares, insurance, maintenance, outage consequences, decommissioning and financing. Identify whether the wind developer, transmission owner or a third party owns the cable.
- Run sensitivity cases. Vary route length, capacity factor, cable and converter prices, installation weather windows, electricity value, losses, delay and future interconnection options. Report a range rather than a single break-even number.
- Obtain system-operator review. A technically cheaper export design can fail if it cannot satisfy the receiving network’s stability, protection or control requirements. Use the operator’s formal planning process before selecting the final architecture.
What real projects illustrate
South Fork Wind: a supplier-reported HVAC example
Nexans describes South Fork Wind as a 132 MW offshore wind project with approximately 110 km of three-phase 138 kV HVAC subsea export cable. This is a useful example of a relatively moderate-capacity project using an HVAC export configuration. The figures are the supplier’s project description, with data refreshed 16 March 2026; they are not a universal rule that a 110 km route must use HVAC.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteRevolution Wind: mixed cable functions and technologies
Nexans reports Revolution Wind at 704 MW with about 150 km of subsea cable, including 300 kV three-core HVAC cable and two 320 kV HVDC XLPE export circuits. The project description shows why a label such as “the project uses HVAC” or “the project uses HVDC” can be misleading: a large development may contain different cable systems for different functions or circuits. Nexans marks the project completed and calls it the first use of HVDC technology for an offshore wind project in the United States; those are supplier claims.
LionLink: a developing cross-border HVDC concept
National Grid’s LionLink FAQ describes a proposed link intended to carry power from a Dutch offshore wind farm to landfall in the United Kingdom over HVDC, where converter equipment changes it back to AC for distribution. Because the project is under development, route, configuration and schedule details may change. Its concept demonstrates how HVDC can serve both offshore-wind export and interconnection between national networks.
Common mistakes when comparing HVAC and HVDC
- Using a hard mileage rule. The 50–100 km European screening range and approximately 37-mile DOE/ABB estimate are starting points with different assumptions, not procurement thresholds.
- Counting only cable losses. HVAC compensation and HVDC converter losses belong in the same system boundary.
- Ignoring cable ownership. The economic break-even changes when a transmission owner, generator or shared network pays for the export asset.
- Comparing unlike capacities. A single high-voltage HVAC circuit and a multi-circuit HVDC scheme must be sized to deliver the same firm power and availability.
- Forgetting land segments. Onshore underground sections, transition joints and landfall constraints can materially change a route’s cost and reactive-power behavior.
- Assuming controllability is free. HVDC’s power-flow control requires converters, controls, communications and protection engineering.
- Treating supplier examples as universal evidence. Published project descriptions show that a design is possible; they do not establish the least-cost solution for another wind farm.
- Leaving grid requirements until the end. Voltage support, fault ride-through, harmonics and restoration obligations can eliminate an apparently attractive configuration.
Which technology is likely to fit?
| Project condition | Initial inclination | Checks that can change the answer |
|---|---|---|
| Short subsea export route, moderate capacity and nearby strong AC grid | HVAC often has the simpler and lower-fixed-cost design. | Charging current, number of circuits, landfall constraints and required reactive support. |
| Long subsea route or high transfer capacity | HVDC becomes increasingly attractive because it avoids cable charging current. | Converter price and availability, platform space, losses, installation and maintenance. |
| Connection between asynchronous networks | HVDC is usually the enabling architecture. | Interconnector rules, converter controls, protection and ownership. |
| Future multi-terminal offshore network | HVDC may provide a controllable backbone. | DC-grid standards, breaker technology, interoperability and staged expansion. |
| Highly constrained onshore route or difficult landfall | Either technology may win depending on where equipment can be located. | Platform and building footprint, cable sections, permitting and construction logistics. |
Bottom line
Choose HVAC when a shorter export route, moderate capacity and a conventional AC connection make its charging-current and reactive-power requirements manageable. Investigate HVDC seriously when subsea distance, transfer capacity, asynchronous interconnection or precise power-flow control increases the value of converters and the savings from a more efficient long-distance cable. The defensible decision is the result of a matched, project-specific study covering route, capacity, compensation, converters, losses, ownership, grid performance and life-cycle cost.
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