Value chain · CLM-0793
In plain terms
Britain's system operator plans to run the grid with less spinning inertia, and it sets that limit from the size of the biggest single generator loss it must survive, currently 1800 megawatts.
The claim as recorded
The GB system operator's Frequency Risk and Control Report 2025 proposes reducing the minimum inertia requirement from 120 to 102 GVA.s and states its full basis: the 102 GVA.s figure rests on a future largest loss of 1800 MW and on keeping the rate of change of frequency within 0.5 Hz/s, an 1800 MW loss requiring 90 GVA.s of inertia for that limit, plus approximately 12 GVA.s of inertia assumed to be lost with the 1800 MW unit itself; the requirement had already been reduced from 140 GVA.s to 130 GVA.s on 28 February 2024 and to 120 GVA.s on 19 June 2024; and the report gives an estimated annual cost of managing frequency risks of GBP 173 million including a GBP 96 million saving from the inertia reduction.
Scope and limits
This is a Great Britain policy figure, not a physical constant, and it is under annual review by the process that produced it. The 1800 MW "future largest loss" is a planning input tied to specific nuclear units, not a general limit. The cost figures are frequency-risk management costs for the whole system and may not be attributed to any one plant.
What it assumes
That the money figures are in the money of the report's own year, 2025, which the report does not state. The report is titled "The Report - For Approval", so the 102 GVA.s figure was a recommendation awaiting an Ofgem decision as at the report's date; the Clean Power 2030 operability annex describes it as "expected to be reduced to 102 GVAs in the future", consistent with that.
Where it comes from
Basis, verbatim, section 4.1: "The minimum inertia of 102 GVA.s is based on a future largest loss of 1800 MW and ensures the Rate of Change of Frequency (RoCoF) remains within 0.5 Hz/s. An 1800 MW loss would require 90 GVA.s of inertia to ensure RoCoF remains within 0.5 Hz/s. This assumes a loss of approximately 12 GVA.s of inertia from the 1800 MW unit. This results in the pre-fault minimum inertia requirement of 102 GVA.s." Recommendation, verbatim: "Reduce the minimum inertia requirement from 120 GVA.s to 102 GVA.s". Implementation history, verbatim: "Phase 1 of the implementation, which reduced the minimum inertia requirement from 140 GVA.s to 130 GVAs, was implemented on 28 February 2024. Phase 2, which reduced the requirement further from 130 GVA.s to 120 GVA.s, was implemented on 19 June 2024." Cost, verbatim: "The estimated annual cost of managing frequency risks based on FRCR 2025 recommendations is GBP 173 million. This includes a GBP 96 million cost saving from reducing the minimum inertia requirement resulting in cost of GBP 170 million for BMU only and an extra GBP 3.2 million cost for procuring additional DC." (The source prints the pound sign; transcribed here as GBP.) Future largest loss context, verbatim: "As our largest loss size increases, with sites such as Hinkley-C power station connecting to the network in the future, we will see a significant increase in DC requirements to cover a larger individual loss."
Basis: pre-fault system inertia in GVA.s; RoCoF limit of 0.5 Hz/s; largest single loss of 1800 MW; costs as printed, currency year not stated by the source
Related records
- Answers question
- QST-012 not published in this release
Cite this
energyaperture.com/c/clm-0793
CLM-0793, Energy Aperture, as of 2025.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-242
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