Britain’s electricity system operator restricted export capacity to European markets during July’s heatwave, highlighting the emergence of summer as a more challenging period for national power supplies.
National Energy System Operator data analysed by the Financial Times showed that day-ahead export capacity was reduced to zero on one or more of the Nemo Link, Viking Link, North Sea Link and ElecLink interconnectors during several hours on 24, 29, 30 and 31 July.
The four cables connect Britain with Belgium, Denmark, Norway and France respectively. The restrictions covered morning and evening periods on 24 July and evening hours on the other three days.
The measure did not mean that Britain was close to blackouts. It limited the capacity available to traders in the day-ahead market, rather than necessarily interrupting electricity already flowing across every affected cable.
Its significance lies in NESO’s use of “margin extremes” as the stated reason for setting export capacity to zero. The designation applies when the system’s spare generating margin is sufficiently tight that NESO might otherwise need to issue a wider market warning seeking additional supply.
NESO had informed the industry that it would make greater use of day-ahead Net Transfer Capacity restrictions from the operational day beginning on 24 July. It said limits would be calculated using forecasts for demand, generation, network outages and expected interconnector flows, with capacity potentially restored later if conditions improved.
The mechanism is available on only four of Britain’s ten electricity interconnectors. NESO’s published policy says it can be used for transmission constraints, frequency-management risks and margin extremes, as a last resort to maintain system security.
NESO described the limits as “one part of NESO’s established approach” to maintaining security in an increasingly dynamic system. It said such restrictions were agreed between system operators and did not diminish the wider importance of interconnectors to efficient cross-border trading.
The July measures followed several earlier indications of unusual summer stress. NESO issued Britain’s first summer Electricity Margin Notice on 24 June after high temperatures increased demand while wind and gas-fired generation availability was lower than expected. The notice was withdrawn when more capacity became available and supplies remained secure.
Ofgem has nevertheless commissioned a formal review of NESO’s operational decisions during the week beginning 22 June, alongside an independent investigation into whistleblowing allegations concerning decision-making and record-keeping. The regulator stressed that there had been no concern about meeting demand or disruption to customers.
The wider issue is that Britain’s adequacy challenge is no longer confined to cold, low-wind winter evenings. Higher temperatures increase demand from air conditioning and refrigeration while reducing the efficiency of some thermal generation. Solar output can be strong during the day but falls quickly towards the evening peak.
Similar weather can also affect neighbouring markets simultaneously. Drought and low river levels have constrained hydroelectric and nuclear generation elsewhere in Europe, reducing the likelihood that surplus continental power will always be available when Britain needs it.
Interconnectors remain valuable because different countries have different generating mixes and demand profiles. However, their contribution to security of supply is partly dependent on the willingness and ability of neighbouring system operators to maintain exports during regional stress.
For Britain’s clean-power strategy, the July restrictions strengthen the case for treating hot, low-wind summer periods as a distinct planning scenario. Additional storage, flexible demand, dispatchable generation and transmission capacity will all be required to manage the evening transition from high solar output.
NESO’s action appears to have worked as intended. The more important warning is that a system increasingly built around cross-border electricity trading cannot assume that national supply pressures will occur in isolation.
