Explained — Technology · 02.4
Onshore Substations & Grid Connection
GridThe interconnected high-voltage electricity network of a country or region · ConnectionThe point where a generator is physically and contractually joined to the grid · The last link between offshore wind and the consumer.
The onshore substation is where the offshore wind farm stops being a generator and becomes part of the national transmission system. Electrically, it is a reception point and, where the export is HVDC, a converter station. Commercially, it is where the developer meets the Transmission System Operator (TSO) and the grid code becomes enforceable. Geographically, it is often in a muddy field several kilometres from the coast, sized for a hundred years of life, consented under full onshore planning rules.
For most UK projects, this is also where the offshore scope ends and the regulated Offshore Transmission Owner (OFTO) asset begins. The export cables, offshore substation, and onshore reception substation together form the "transmission asset" that is transferred to an OFTO after commissioning.
What the Onshore Substation Does
The role depends on whether the export system is HVAC or HVDC, but in all cases the onshore substation:
- Terminates the export cable and manages the interface with the transmission grid.
- Steps voltage up or down to match the grid connection voltage (typically 275 or 400 kV in the UK).
- Provides switchgear, protection, and isolation for operational control and fault management.
- Houses reactive compensation equipment, harmonic filters, and where required, STATCOMs.
- Meters the electricity handed over to the grid, for settlement purposes.
- Hosts the onshore end of the control and communications system to the offshore farm.
For HVAC Export: The Reception Substation
An HVAC onshore substation is a fairly conventional transmission substation. It takes the 220 or 275 kV export cable, passes it through switchgear and isolators, and steps it up to the grid connection voltage (typically 400 kV for GB transmission or 275 kV for the Scottish transmission system). Primary equipment includes the main transformers, air-insulated switchgear (AIS) in an outdoor yard, shunt reactors for reactive compensation, harmonic filters, and the control and protection building. Footprint is typically 5 to 15 hectares, depending on capacity and the connection voltage.
For HVDC Export: The Onshore Converter Station
An HVDC link needs a converter at each end. Where the offshore platform hosts one, the onshore substation hosts its mirror image. The onshore converter station is physically enormous, a valve hall the size of a warehouse, 25 to 35 metres tall, housing the VSC modular multilevel converter, surrounded by DC switchyard, AC switchyard, filter banks, reactors, cooling plant, and control buildings. Footprint is typically 10 to 30 hectares, larger than most equivalent HVAC reception substations.
Onshore converter stations are visually prominent and tend to attract local planning objections on that basis. Projects have taken years of consenting, and in several cases, including Norfolk Boreas and sections of the East Anglia cluster, the onshore infrastructure has been the pacing item for project delivery. There is ongoing UK policy work on "coordinated" or "integrated" offshore networks that would rationalise the number of onshore converter stations required, but implementation is proceeding slowly.
Grid Code Compliance
Every generator connected to the GB transmission system must comply with the Grid Code, maintained by the Electricity System Operator (NESO, formerly National Grid ESO). The Grid Code and its supporting codes (including the Engineering Recommendations G99 and G98 for distribution-connected generators, and NC RfG for transmission-connected) specify the technical behaviour required at the grid connection point. Key requirements include:
- Fault Ride Through (FRT): the farm must remain connected during short, severe voltage dips caused by network faults, rather than tripping offline.
- Frequency response: the farm must increase or reduce active power output in response to frequency deviations.
- Reactive power capability: a specified range of reactive power exchange must be available at the connection point, regardless of active power output.
- Voltage control: the farm must control voltage at the connection point within a specified range.
- Harmonic distortion limits: the injected current must not exceed specified harmonic levels.
- Power quality and flicker: within defined limits.
Meeting the Grid Code is a system-level task. The turbines, the park controller, the reactive compensation equipment, and the substation protection all have to work together, and compliance testing is a formal process with the TSO taking several months from first energisation to full commercial operation.
The Connection Agreement
Before any of this can happen, the developer needs a grid connection agreement, a contract with the TSO specifying the connection point, the connection voltage, the connection capacity, and the date the connection will be physically available. In Great Britain, connection offers are issued by NESO, and the connection queue is currently one of the most contested issues in the sector.
As of 2025, the connection queue contained several hundred gigawatts of applications, many from projects with no realistic prospect of delivery in the offered timeframe. NESO's "Connections Reform" process is reshaping the queue to prioritise projects with genuine delivery readiness, but offshore wind projects can still face connection dates 10 years or more in the future, with all the commercial implications that entails for investment decisions. This is a regulatory issue, not a technology issue, but it shapes every part of offshore wind project planning.
A useful heuristic: for a UK offshore wind project in 2025, the onshore substation and grid connection are more likely to delay commercial operation than any offshore engineering issue. Turbine delivery, foundation fabrication, vessel availability, and cable manufacturing all matter, but the binding constraint is usually a row of pylons waiting for reinforcement somewhere inland, and a connection date that has slipped twice.
Black Start and Grid Services
A "black start" is the restoration of the grid after a total system collapse, starting from generators capable of energising the grid without any external electricity supply. Historically this has been the role of large thermal plants (hydro, gas, a few specific CCGTs). Offshore wind is now being evaluated as a black start resource, with UK pathfinder projects exploring whether a wind farm plus its HVDC export can restart part of the grid without relying on thermal units. Early trial work at Dogger Bank and elsewhere suggests it is technically viable, but commercial and regulatory frameworks are still being developed.
Beyond black start, offshore wind farms are increasingly expected to provide ancillary services, frequency response, reactive power, synthetic inertia, and system voltage support, that historically came from thermal plant. Grid codes are evolving to mandate these capabilities, and the commercial markets for ancillary services are a growing revenue stream, though still secondary to the main wholesale energy and CfD revenue.