What the Transmission Network Is

The transmission network operates at two main voltage levels in Great Britain. 400 kV is the top tier, used for the longest-distance bulk power transfer and for the connections to the largest generators and demand centres. 275 kV is the secondary tier, typical of older sections of the network and of Scottish transmission south of the Highlands. Below 275 kV, the network steps down into distribution, which is a separate system operated by Distribution Network Operators (DNOs) and is out of scope of this page.

Physically, the network is a mixture of overhead lines on lattice steel pylons (the majority), underground cables (expensive, used for specific routes), and subsea cables (used for the interconnectors to mainland Europe and Ireland, and increasingly for domestic Scotland-to-England links). The topology is meshed, not radial, meaning there are generally multiple paths between any two points, so the loss of a single line does not black out any part of the system.

NESO

On 1 October 2024, the Electricity System Operator function was formally separated from National Grid plc and reconstituted as the National Energy System Operator (NESO), a public corporation owned by the UK government through DESNZ. The separation had been planned for years to remove a long-standing conflict of interest, National Grid being both the system operator and a major owner of the transmission infrastructure it dispatched power across.

NESO now has a single remit: operate the GB electricity system (and, from a future date, the gas system too) in the public interest. It runs the Balancing Mechanism, dispatches generation in real time, manages reserve and response services, and publishes the network design and investment planning that shapes the system for decades to come. It does not own wires. It instructs the Transmission Owners, who do.

The name change matters for accuracy, and it is a recurring source of confusion in sector writing. If you see a reference to "National Grid ESO" in anything published after October 2024, it is out of date.

The Three Transmission Owners

The physical transmission network is owned and operated by three regulated monopolies, each with a defined geographic area:

• National Grid Electricity Transmission (NGET), a subsidiary of National Grid plc, owns the network in England and Wales. The largest of the three by asset base and circuit length.
• SP Transmission (SPT), part of SP Energy Networks (itself part of Iberdrola), owns the network in southern Scotland.
• Scottish Hydro Electric Transmission (SHE-T), part of SSE plc, owns the network in northern Scotland and the north of Scotland's islands.

Each TO plans, builds, maintains, and repairs its portion of the network under a regulatory price control set by Ofgem (currently RIIO-T3 for the 2026-2031 period). Major projects proceed under NESO's Network Options Assessment (NOA) process, which identifies the reinforcement needed to meet future demand and generation patterns. The TOs then bid to build those reinforcements, or are directed to.

The OFTO Regime

Offshore wind transmission assets, the export cables, offshore substation, and the onshore substation up to the point of connection with the wider grid, are treated separately under the Offshore Transmission Owner (OFTO) regime. An OFTO is a regulated asset owner that takes over the transmission assets after a wind farm is built and commissioned, operating them for the remainder of their design life under a long-term revenue licence from Ofgem.

The regime exists to separate generation economics from transmission economics and to ensure that offshore wind transmission assets are operated to neutral standards even when the generator changes hands. In practice, the OFTO tender process runs for months to years after first energisation, during which the developer continues to operate the assets under a transitional arrangement. OFTO ownership is dominated by a handful of specialist infrastructure funds, pension funds, and utility subsidiaries, Diamond Transmission Partners, Equitix, Balfour Beatty Infrastructure, Macquarie-backed vehicles, and similar.

System Boundaries

NESO models the transmission network as a set of zones separated by notional "boundaries", lines across the network where the transfer capability between the two sides is engineering-limited. The boundary capability is the maximum power that can safely flow from one side to the other without overloading lines, causing unacceptable voltage excursions, or breaching stability margins. When actual flows approach the boundary limit, NESO has to intervene by redispatching generation, curtailment, in practice, to keep the flow within limits.

The critical boundary for UK offshore wind is B6, the north-to-south line across the Scotland-England border. Most of the UK's wind generation is now north of B6, most of the demand is south of it, and B6 is the wire that connects them. When B6 is full, the wind has to stop. B4, an internal Scottish boundary between the far north and central Scotland, layers a second constraint on top for the largest offshore wind farms off the east and north coasts of Scotland.

The Connection Queue and TMO4+ Reform

Getting a new generator onto the transmission network requires a connection offer from NESO specifying when, where, and at what capacity the connection will be made. Historically, offers were issued on a first-come, first-served basis. That process collapsed under the weight of applications through the early 2020s. By early 2025, the connection queue held more than seven hundred gigawatts of applications, approximately four times what Britain actually needed to meet its Clean Power 2030 target, with connection dates for new projects routinely falling into the 2030s and 2040s.

The fix is a package of reforms collectively known as TMO4+ Connections Reform, approved by Ofgem in April 2025 and implemented on 10 June 2025. TMO4+ replaces first-come-first-served with "first ready, first needed, first connect". Every project currently in the queue has had to resubmit under the new Gate 2 criteria demonstrating genuine readiness (land rights, planning, financing milestones) and strategic alignment with the 2030 or 2035 pipeline. Projects that do not make the cut are being removed, freeing network capacity for those that can actually deliver.

NESO confirmed the new pipeline of Gate 2 projects on 8 December 2025. It is, at minimum, the biggest reform of the GB connections process in its history. Early implementation is not without friction, NESO has already had to revise the rollout timeline in February 2026 to accommodate engineering review of the higher-than-forecast Gate 2 application volumes in some locations, but the direction of travel is fixed.

The Great Rebuild

The transmission network is now in the middle of the largest investment cycle in its history. The Eastern Green Links (EGL) programme is a set of four to five subsea HVDC cables from Scotland to England, each rated at around 2 GW, designed to increase B6 transfer capability by 8 GW or more. EGL2, approved by Ofgem in 2025 at a capital cost of £4.3 billion, is currently the single largest electricity transmission investment ever approved in Britain. EGL1, 3, and 4 are in various stages of consent and procurement.

Parallel to the EGLs, the existing onshore transmission system is being upgraded. Major corridors are being reconductored or rebuilt to 400 kV where they were previously 275 kV. Phase-shifting transformers are being installed at key nodes to provide dynamic control of boundary flows. The Western Link, an existing HVDC link between Scotland and north Wales, has already increased B6 capability materially and is complemented by the new programme.

Together, the Clean Power 2030 transmission investment programme adds up to roughly ten billion pounds a year between 2025 and 2030. The majority of the subsea reinforcement will not deliver until 2029 at the earliest, and the onshore reconductoring is limited by its own scheduling, the upgrades to a major power corridor themselves require outages, which themselves reduce current transfer capability. The current peak curtailment is therefore partly a self-inflicted consequence of building the capacity needed to fix it.

Why This Is the Bottleneck

Transmission is the bottleneck because it is slow, expensive, politically contested, and was under-invested for a decade while generation was rapidly added. New overhead lines take ten to fifteen years from first concept to energisation in the UK, dominated by consenting, compulsory purchase, and local opposition. Underground cabling is four to six times more expensive per kilometre. Subsea HVDC is in the same ballpark as onshore cabling on a pure steel-and-copper basis, with the added complexity of marine installation and vessel scheduling.

None of these problems are fundamentally technical. The engineering is mature and well understood. The problems are institutional, a system designed for slow, incremental network growth is being asked to accommodate explosive growth in renewable generation, and the mismatch between the two timescales is what produces the curtailment bill and the connection queue. TMO4+ Reform, the RIIO-T3 price control, the EGL programme, and the wider reinforcement work together are a deliberate attempt to change that. Whether they do it fast enough to keep curtailment costs from tripling before 2030 is the question the sector will answer between now and then.