Interconnectors
Nine HVDC cables, 9 GW of capacity, 11 percent of 2024 electricity supply.
Great Britain has nine operational electricity interconnectors to France, Netherlands, Belgium, Norway, Denmark and Ireland. Together they can import up to 9 GW, about 17 percent of winter peak demand. Post-Brexit, the UK trades through a new market coupling arrangement that is less efficient than pre-2021 EU membership. The cables work. The trading rules are still being written.
10Route 10 of 12 · Energy vectorsAfter this route you will be able to
- Name the nine operational GB interconnectors, their capacities, and their counterparties.
- Explain how HVDC technology works at a basic level and why interconnectors use it.
- Describe the cap-and-floor regulatory regime and what it incentivises.
- Identify the post-Brexit market coupling arrangements and what they cost in allocation efficiency.
- Make a reasoned call on further interconnector capacity.
15 September 2021IFA-1 Sellindge fire · 1 GW import capacity lost overnight
Fire at the Sellindge converter station removed half the UK-France import capacity for a winter.
Shortly after 07:00 BST on 15 September 2021, a fire at National Grid's Sellindge converter station in Kent damaged a key component of the 2 GW IFA-1 link to France. The link dropped to 1 GW of capacity. IFA-1 had been operational since 1986; the station had the scale of a small industrial complex.
The timing was unfortunate. GB wholesale day-ahead prices were already rising in the run-up to the 2021-22 energy crisis; losing 1 GW of French imports at the start of winter removed a valuable tool. Full IFA-1 capacity was not restored until
The Sellindge incident illustrated a truth that the 2021 Storm Arwen and the 2022 crisis both reinforced: interconnector capacity is a physical asset with physical failure modes. The benefits (cheap imports, export revenue, resilience options) depend on the cables and their converter stations working.
Interconnectors deliver more than a tenth of GB electricity. How much of this capacity is real firm reliance, and how much is optional when counterparty prices rise?
The answer starts with the nine cables. Each has a capacity, a partner, and a market coupling mechanism.
Section 01 · The nine cables
9 GW combined capacity, six counterparties.
All nine are HVDC (High Voltage Direct Current) undersea cables. HVDC is used rather than AC because DC can transmit bulk power over long subsea distances with lower losses and without the phase-synchronisation problems AC transmission has.
Data: NESO interconnector register; operator commissioning records. Viking Link is the longest subsea HVDC interconnector in the world at 765 km.
Section 02 · How trading works
Day-ahead auction, within-day trading, explicit after Brexit.
Capacity on each interconnector is allocated through auctions. Before Brexit, this happened through EU market coupling. Since 2021, GB uses a coarser explicit allocation that costs efficiency.
Before Brexit, day-ahead market coupling meant that cross-border capacity and energy prices were cleared in one integrated auction across all the participating exchanges. A generator in France offered into a single pool that included GB buyers. Flows matched price signals automatically.
Since 1 January 2021, GB has been outside EU market coupling. Capacity is now auctioned explicitly (traders buy cross-border capacity separately from energy), which reduces allocation efficiency by an estimated 5-10 percent. The Trade and Cooperation Agreement commits both sides to re-establishing "multi-region loose volume coupling", but implementation has moved slowly.
Interconnector flows in 2024 were a net 31 TWh of imports. The direction changes by hour: GB exports during windy afternoons (cheap GB wholesale price); GB imports during peak demand hours and when wind is low. Norway's NSL also brings hydro-stored flexibility that GB domestic storage cannot match.
Interconnector operators are subject to a cap-and-floor regime that guarantees a minimum return if capacity revenues fall below an agreed floor, and caps returns above a ceiling. The regime applies for a regulated period (typically 25 years) to support the investment case for new cables.
Ofgem Cap and Floor Regime for Electricity Interconnectors
Section 03 · Capacity expansion
Build to 18 GW by 2030 or hold at 9 GW?
Six further interconnectors are in development or regulatory approval. Each adds cost and each adds option value. The decision below is about collective scale.
Check your understanding
Three questions on what you have just read.
Key takeaways
- Nine operational HVDC interconnectors. 9 GW combined capacity. Six counterparties.
- Net imports covered 11 percent of 2024 GB electricity (~31 TWh).
- Post-Brexit capacity is auctioned explicitly rather than through EU coupling; ~5-10 percent less efficient.
- Viking Link (Denmark, 765 km, 2023) is the longest subsea HVDC interconnector in the world.
- NESO targets ~18 GW capacity by 2030. Each new cable requires Ofgem cap-and-floor determination.
References
- NESO: Interconnector operation
Operational register, capacity and flow data.
Primary operational source.
- Ofgem: Cap and floor regime
Regulatory framework for interconnector revenue.
Primary regulatory reference.
- UK-EU Trade and Cooperation Agreement
Title 8 (energy) covers interconnector market arrangements.
Post-Brexit trading framework.
- Elexon BMRS
Half-hourly interconnector flow data.
Operational transparency source.
The next route opens voltage management. The technical layer beneath the cascade, where specific constraints determine who can connect and when.