Explained — Technology · 02.3
Export System & Landfall
ExportTo send goods or energy from one region to another · LandfallThe point where a subsea cable reaches the shore · The link between the farm and the national grid.
The export system is the long cable circuit connecting the offshore substation to the onshore grid connection point. On a short route it is mostly a scaled-up transmission problem. On a long or high-capacity route it becomes a separate project in its own right, with its own converter stations, its own route consenting, and its own cable procurement. The technology choice, HVAC against HVDC, drives the rest of the farm's electrical architecture.
In the UK, the export system is almost always built and owned by the developer and then transferred to a separate regulated asset owner, the Offshore Transmission Owner (OFTO), shortly after commissioning. This OFTO regime means the export system has a commercial structure that the rest of the farm does not, and a handover process that takes months after the cables first energise.
The HVAC vs HVDC Decision
The single most consequential decision in export system design. Every other aspect, cable size, substation design, converter requirement, reactive compensation, construction schedule, follows from it.
HVAC Export
Three-core AC cable at 220 to 275 kV (UK) or 132 to 155 kV (some European markets). No converter station needed offshore, the transformer at the OSS is sufficient.
Economic range: up to roughly 80 to 100 km, up to around 1 GW per circuit. Beyond that, reactive power losses and cable thermal ratings make HVAC uncompetitive.
Cost: lower capital, lower conversion losses, mature technology, many suppliers.
HVDC Export
Two single-core DC cables at ±320 to ±525 kV, with a VSC converter station at both ends (offshore and onshore). Higher voltage, no reactive power issue, lower resistive loss.
Economic range: from around 80 km out to several hundred kilometres. Preferred at very high capacities (2 GW+) even at shorter distances.
Cost: much higher capital, but lower line losses, viable at distances that HVAC cannot reach at all.
Below about 1 GW capacity and 80 km route length, HVAC wins. Above 2 GW, HVDC wins. The middle band depends on project-specific costs, available grid connection voltage, and reactive compensation requirements. UK Round 3 and 4 projects far from shore are almost all HVDC. North Sea "meshed" and hybrid interconnector-plus-wind projects are pushing HVDC as the default.
Reactive Power and HVAC Compensation
A long HVAC subsea cable acts as a distributed capacitor. It generates reactive power even when no active power is flowing, which consumes conductor current capacity and limits how much real power the cable can actually carry. At around 80 km, the cable is generating so much reactive power that its active power capacity approaches zero, which is the physics reason HVAC has a practical range limit.
Compensation mitigates this. Shunt reactors, large inductive loads at one or both ends of the cable, absorb the cable's reactive output. On longer HVAC routes, reactors are installed at both the offshore and onshore substations, sized to offset the cable's reactive generation at expected loading. Where compensation alone is insufficient, static synchronous compensators (STATCOMs) provide dynamic adjustment, but at significant capital cost.
HVDC Converter Technology
Every commercial offshore wind HVDC link uses voltage-source converter (VSC) technology, specifically modular multilevel converters (MMC). Unlike the older line-commutated converters used on some onshore HVDC interconnectors, VSC can operate independently of grid strength, provide its own voltage reference, and start up without an external AC source. This is essential offshore, where there is no stiff grid to commutate against.
The VSC valve hall on the offshore converter platform houses thousands of power electronic modules in series, switching at high frequency to synthesise the AC waveform on the farm side and the DC voltage on the export side. It is physically large, thermally demanding, and has a very narrow supplier base, Siemens Energy, Hitachi Energy, and GE Vernova being the dominant players. Lead times have extended well past four years.
Cable Corridor and Burial
The export cable corridor is the linear strip of seabed and foreshore along which the cable is routed. The corridor is consented separately from the farm itself, often by a different process, and it is typically the hardest part of the project to consent because it crosses multiple jurisdictions, fisheries grounds, shipping lanes, cable and pipeline crossings, and protected environmental sites.
Burial depth is typically 1 to 3 metres below seabed, increased at vulnerable sections (near shipping channels, fishing grounds, cable crossings). Where the cable cannot be buried, rock placement or concrete mattressing provides surface protection. Cable crossings of existing pipelines or other cables require purpose-built concrete mattresses and protective sleeves, designed in coordination with the owners of the crossed assets.
Landfall
Landfall is the engineering challenge of bringing a subsea cable from open water, through the surf zone, across the beach, and into a buried onshore section. It is almost always consented as a standalone piece of civil engineering, because beaches are sensitive, coastal erosion risk varies site by site, and local planning processes apply fully once the cable crosses mean high water.
Modern landfalls almost always use horizontal directional drilling (HDD). A drill rig on land bores a long curved hole from a shoreline launch pit out beneath the foreshore and seabed, emerging offshore at a point where the cable can be safely pulled through. The cable is then winched through the HDD duct from sea to land, avoiding any surface disturbance of the beach or dune system.
Transition Joints
At the end of the HDD duct, usually in a jointing pit a few hundred metres inland, the subsea export cable is joined to a different cable type, the onshore cable, which has different mechanical armouring (or none at all) and is designed for installation in buried trenches or ducts. The jointing pit is a critical piece of engineering, each joint is effectively a custom fabrication done on site, cured, tested, and buried. Joint reliability has been a focus of industry standards work because early project failures at transition joints caused significant outages.
Onshore Cable Route
From the jointing pit to the onshore substation, the cable is buried in a trench or cable duct. Onshore routes range from a few hundred metres to tens of kilometres, depending on where the nearest viable grid connection point is. Long onshore routes are often the most disruptive part of the project from a local community perspective, crossing agricultural land, minor roads, watercourses, and occasionally protected sites. Route consenting, compulsory purchase, and community engagement on long onshore sections can take years and are frequently the pacing item for the whole project schedule.