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HVAC is usually the practical choice for a short offshore export link, while HVDC becomes more attractive as subsea distance, transmitted power and grid-control requirements increase. HVAC connects directly to an alternating-current grid but long submarine AC cables draw charging current and generate reactive-power requirements. HVDC avoids cable charging current and provides tightly controllable real-power transfer, but it adds expensive converter stations, platforms and conversion losses at the ends.
There is no universal distance at which one technology always wins. A defensible choice compares the complete system—cables, converters, compensation, installation, land sections, losses, grid connection and ownership—under the wind farm’s actual route and capacity.
HVAC and HVDC in one view
| Decision factor | HVAC export | HVDC export |
|---|---|---|
| Connection to an AC grid | Direct AC connection, normally without conversion at both ends | AC must be converted to DC offshore or onshore and back to AC at the receiving end |
| Subsea-distance behavior | Cable capacitance creates charging current and reactive power; the impact grows with length | No charging current in the DC transmission cable, making long submarine routes more manageable |
| Main capital items | Export cables, switchgear and, on longer links, reactive-power compensation and possibly additional circuits | Export cable plus converter stations, valves, transformers, control systems and converter platforms or buildings |
| Power-flow control | Follows conventional AC-network operation and depends on the connected grid | Precisely controls active-power transfer and can connect asynchronous systems |
| Losses | Conductor losses plus losses associated with reactive current and compensation equipment | Conductor losses plus converter losses; the full chain must be compared on the same loading assumptions |
| Typical screening outcome | Often favored for shorter export routes and modest capacity | Often favored as route length and capacity increase, provided converter costs are justified |
These are engineering tendencies, not guarantees. A short route can still need HVDC for a specific grid or network-control reason, and a long route can remain HVAC if its capacity, voltage, ownership and connection conditions make that option cheaper.
How an HVAC offshore export system works
Three-phase AC cable from the wind farm
Most wind turbines generate variable-frequency power that is collected and converted by the wind farm’s electrical system before export. An HVAC export system sends three-phase alternating current through one or more high-voltage submarine cables to a landfall. At the onshore substation, transformers and switchgear match the transmission voltage to the receiving AC network.
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Because the receiving network is already AC, the interface is familiar to utilities. Protection, breakers, transformers and synchronization follow established AC practice, and no dedicated AC-to-DC converter station is required solely for the export link.
Why cable capacitance becomes a limitation
An insulated submarine AC cable behaves partly like a capacitor. Even when little useful power is delivered, it draws a charging current that circulates between the cable and the network. As the route gets longer, a larger share of the cable’s current-carrying capability is consumed by this charging current rather than by wind-farm power.
The resulting reactive-power flow must be managed with shunt reactors, compensation equipment, voltage-control devices or additional cable capacity. Those measures add cost, space and operating constraints. They also make the usable transfer capability depend on voltage, cable design, loading and the strength of the connected grid—not just the cable’s nameplate rating.
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- The export route is relatively short and has few land sections.
- The wind farm’s capacity fits within one or a small number of standard AC circuits.
- The onshore connection point can absorb the reactive-power behavior without extensive compensation.
- A project values the simpler AC interface and wants to avoid converter-station capital cost.
- Existing AC equipment, maintenance capability and ownership arrangements favor conventional infrastructure.
How an HVDC offshore export system works
Conversion at the sending and receiving ends
HVDC converts the wind farm’s AC output to direct current, transmits it through a submarine cable, and converts it back to AC for the receiving grid. The conversion equipment includes power-electronic valves, transformers, smoothing and filtering equipment, controls, protection and cooling systems. Depending on the design, converters may be installed on an offshore platform, integrated with an offshore energy hub, or located onshore with an AC collection arrangement offshore.
The submarine DC cable does not experience the continuous AC charging current that limits long AC cables. Its current is primarily associated with the real power being transferred, so long-distance transmission can use the cable’s thermal capability more effectively.
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Control and asynchronous connections
HVDC controls active-power flow directly. That can help a system operator schedule transfers, limit overloads and coordinate two networks that do not share the same operating frequency. The U.S. Department of Energy explains this role with the statement: “Transferring power between these regions without disrupting the frequency of either system is only possible with direct current (DC) links.” The statement concerns asynchronous interconnection generally; it is not a promise that every offshore project requires HVDC.
Converter controls, DC protection and interoperability are significant engineering tasks. A multi-terminal or meshed DC network requires coordinated protection and compatible equipment from different suppliers. Those requirements can increase design effort even when the cable itself is well suited to the route.
At what distance does HVDC become cheaper?
Distance is a screening variable, not a universal crossover point. The European Commission staff working document (2023) places a typical economic-preference range for HVDC over HVAC at about 50–100 km for offshore or underground cable links. The U.S. Department of Energy article (circa 2023), citing an ABB estimate, gives an approximately 37-mile break-even distance for submarine lines. The figures use different assumptions, so they should not be treated as conflicting universal rules or as a quotation for a particular wind farm.
The crossover exists because HVAC usually starts with lower fixed cost, while HVDC starts with the additional cost of two converter stations. As an AC route lengthens, however, reactive-power equipment, larger or additional cables and reduced usable transfer capability raise the HVAC cost. At some project-specific length, those incremental AC costs can exceed the converter premium.
Variables that move the break-even point
| Variable | Effect on the comparison |
|---|---|
| Export capacity | Higher power can make converter-station cost easier to spread and can increase the number or size of HVAC circuits and compensation equipment. |
| Cable voltage and rating | Higher-voltage or higher-current designs change cable count, losses, accessories and installation cost. |
| Route geometry | Longer subsea sections, land crossings, bends, difficult seabed and separate landfall work alter both technologies’ installation costs. |
| Converter technology and price | Converter cost, efficiency, footprint and control capability determine how much distance is needed to repay the fixed investment. |
| Grid connection | Reactive-power limits, fault levels, available land and the need to connect asynchronous networks can favor one technology independently of distance. |
| Ownership and tariff structure | Who owns and pays for the export cable changes the economic break-even calculation. A 2009 comparative study by Eeckhout and co-authors specifically identified cable ownership as a factor that changes the calculated break-even length. |
| Loss valuation | Energy prices, expected loading and the value assigned to losses can materially change lifecycle economics. |
Losses: compare the complete electrical chain
It is incorrect to say simply that “DC always has lower losses.” HVDC removes the cable’s AC charging-current burden, but its converter stations consume power. HVAC has no HVDC converter loss, yet reactive current and compensation affect its conductor and equipment losses.
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The European Commission staff working document (2023) gives an illustrative, assumption-dependent sample comparison:
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| Configuration | Conductor losses | Converter-station losses | Total in the sample |
|---|---|---|---|
| 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% |
These are the document’s sample figures, not guaranteed performance for a current offshore wind project. They illustrate why a study must state voltage, cable design, loading, converter type, compensation and operating conditions before comparing percentages.
The Department of Energy article gives a broad estimate that HVDC losses can be 30–50% lower than comparable HVAC lines over long distances. That is a general estimate attributed to other analysis, not a project-specific result. A wind farm developer should calculate annual energy lost at the expected dispatch profile and price that energy over the asset life.
What to include in a loss model
- Conductor resistance at operating temperature and the expected current profile.
- HVAC charging and reactive-power effects, including losses in reactors, transformers and compensation equipment.
- HVDC converter efficiency at minimum, typical and maximum loading.
- Auxiliary consumption for cooling, controls and platform equipment.
- Availability and outage assumptions, because a lower-loss link that is unavailable more often may deliver less energy.
- Losses in any onshore land cable, overhead line or intermediate substation.
Capital cost and lifecycle cost structure
HVAC cost elements
- Three-core or single-core export cables, joints, terminations and testing.
- Submarine installation, burial, protection and route preparation.
- Onshore transformers, switchgear, protection and metering.
- Shunt reactors or other reactive-power compensation where cable charging is material.
- Potentially additional parallel circuits if the desired real-power transfer cannot be carried within the reactive-power and thermal limits of one circuit.
HVDC cost elements
- One or more DC submarine cables, joints, terminations and installation work.
- AC/DC converter stations at the relevant endpoints, including valves, transformers, filters, cooling, controls and protection.
- Offshore converter platforms or substantial onshore converter sites, depending on architecture.
- DC fault protection, telecommunications and interoperability engineering.
- Replacement strategy for specialized converter equipment and access for maintenance.
HVDC can reduce the number of high-capacity export circuits and avoid long-route AC compensation, but those savings must repay the converter stations. A financial model should separate one-time capital expenditure from recurring losses, maintenance, availability, replacement and decommissioning costs.
Grid integration and operational trade-offs
HVAC connection
HVAC behaves like a conventional AC transmission connection. It can be straightforward where the point of interconnection has adequate short-circuit strength, voltage-control range and reactive-power capability. The grid operator still needs studies for faults, harmonics, cable energization, insulation coordination and dynamic voltage behavior.
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HVDC connection
HVDC gives the operator a controllable active-power set point and electrical separation between the two AC systems. That is valuable when connecting asynchronous areas or when the receiving grid needs controlled ramping. The trade-off is dependence on converter controls and specialized protection. Converter interaction with weak AC networks, harmonic performance, control coordination and DC fault clearing must be demonstrated during planning and commissioning.
Network expansion and future links
A single radial export link is simpler than a future multi-terminal network. If several wind farms may later share an offshore hub, planners should decide early whether equipment, voltage levels, protection philosophy and ownership rules can support expansion. Retrofitting a mixed AC/DC architecture can be more expensive than reserving space and interface requirements in the initial design.
Project examples: why labels need context
| Project | Supplier-reported details | What the example demonstrates |
|---|---|---|
| South Fork Wind | Nexans describes a 132 MW offshore wind project with approximately 110 km of 138 kV three-phase HVAC subsea export cable. Project-page data were refreshed 16 March 2026. | An HVAC export can be selected for a substantial project and a route near the screening distances when its complete grid and cost conditions support it. |
| Revolution Wind | Nexans reports a 704 MW project with about 150 km of subsea cable, including 300 kV three-core HVAC cable and two 320 kV HVDC XLPE export circuits. The supplier marks the project completed and calls it the first use of HVDC technology for an offshore wind project in the United States. | A project description can contain multiple cable functions and technologies; saying only “the project is HVAC” or “the project is HVDC” would omit important architecture. |
These are supplier-reported descriptions, not independent cost or performance audits. They are useful illustrations of the range of real project architectures, not templates for selecting technology elsewhere.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose a technology for a new wind farm
- Define the electrical boundary. Record wind-farm capacity, collection-system voltage, export voltage options, landfall, onshore route, point of interconnection and the required energization and commissioning sequence.
- Fix realistic route quantities. Separate subsea, land-cable and overhead sections. Include burial depth, seabed conditions, crossings, joints, repair access and installation-vessel constraints.
- Create comparable HVAC designs. Model cable type, number of circuits, conductor size, reactive-power compensation, voltage limits, thermal rating and protection. Do not compare one optimized HVDC design with an underspecified AC option.
- Create comparable HVDC designs. Specify converter technology, voltage, power rating, offshore versus onshore converter location, platform footprint, cooling, filters, control modes, DC protection and spare strategy.
- Run grid studies. Check load flow, short circuit, harmonics, transient and dynamic behavior, reactive-power range, weak-grid performance and interactions with neighboring projects.
- Calculate lifecycle cost. Include cable and converter capital cost, installation, compensation, landfall, losses at realistic loading, maintenance, availability, replacement and decommissioning. State who owns each asset and who pays for losses and outages.
- Test sensitivities. Vary route length, capacity factor, energy price, cable price, converter price, construction delay, loss assumptions and connection date. A technology that wins under one forecast may not win under another.
- Confirm standards and interfaces. Verify requirements from the transmission owner, system operator, maritime authorities and environmental permits. Confirm that proposed equipment has compatible ratings, protection behavior and communications.
- Select using a documented decision record. Record assumptions, rejected alternatives, uncertainty ranges and the conditions that would trigger a redesign. Distance alone should never be the sole approval criterion.
Common mistakes and how to avoid them
Treating a distance rule as a quotation
Ranges such as 50–100 km or approximately 37 miles are screening guidance from different sources and assumptions. Use them to decide whether a full comparison is warranted, not to issue a procurement decision.
Comparing cable losses only
A lower conductor-loss percentage does not prove lower system losses. Add converter, transformer, reactor, filter, cooling and auxiliary losses, and compare annual energy delivered rather than a single nameplate condition.
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Ignoring reactive power in HVAC
Leaving cable charging out of an AC model overstates transfer capability and understates equipment cost. The model should show reactive current, compensation steps, voltage limits and the resulting real-power margin.
Hiding land sections and interface equipment
Land cables, transition joints, onshore substations and permitting can be material. Put the same route boundaries and equipment scope into both the HVAC and HVDC alternatives.
Assuming one project architecture fits another
Capacity, route, grid strength, ownership and future network plans differ. Supplier project pages show that a single wind farm may include both HVAC and HVDC circuits for different functions.
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The 2009 economic comparison’s finding on cable ownership is still a useful warning: the party paying capital cost, losses and maintenance can change the apparent break-even length. Align the financial model with the actual regulatory and commercial structure.
Developing hybrid and interconnector architectures
Offshore wind export does not have to be a single radial cable choice. A project may use HVAC for collection or a shorter segment and HVDC for a longer export section, or combine export with an interconnector between national grids. The boundaries between those functions affect converter location, metering, control responsibility and outage consequences.
National Grid’s LionLink FAQ describes a developing link intended to carry electricity from a Dutch offshore wind farm to landfall in the United Kingdom over HVDC, where converter equipment returns it to AC for distribution. The route and project details remain subject to change because the link is under development. The example shows how an offshore connection can serve both wind integration and cross-border transfer, but it does not establish a universal architecture for other projects.
Practical checklist for a feasibility study
- Is the stated route length measured along the actual cable corridor rather than as a straight-line map distance?
- Are export capacity, voltage, conductor size and number of circuits identical in the alternatives being compared?
- Does the HVAC case include charging current, reactive compensation, voltage limits and any additional circuits?
- Does the HVDC case include both converter stations, platforms or buildings, filters, cooling, controls, protection and auxiliaries?
- Are cable installation, burial, crossings, joints, landfall and repair assumptions common to both cases?
- Are converter and cable losses calculated over the expected hourly loading profile?
- Have grid strength, asynchronous operation, harmonics, fault behavior and control interactions been studied?
- Who owns each asset, pays for losses and carries outage risk?
- What future wind farms, hubs or interconnectors could change the preferred architecture?
- Which assumptions are uncertain enough to require sensitivity ranges or supplier quotations?
A project team that can answer these questions has moved beyond a distance rule and toward an auditable technology decision.
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