ESO gets green light to reduce minimum system inertia

Ofgem has given the green light to the Electricity System Operator (ESO) to lower its minimum requirement for system inertia, despite concerns from some industry parties.

National Grid ESO estimated that the reduction will cut annual balancing costs by £65 million without increasing the risk of substantial frequency deviations beyond an acceptable level.

System inertia describes the resistance of the electricity system to sudden changes in frequency, for example, following an unexpected plant outage.

Inertia has historically been provided by synchronous generators with large turbines spinning in harmony with the frequency of the power grid. The replacement of synchronous coal and gas generators with asynchronous wind and solar farms is leading to a reduction system inertia.

The ESO is currently required to maintain a system inertia of 140 gigavolt-amp seconds (GVA.s). The requirement was set at this level to keep the Rate of Change of Frequency (RoCoF) below 0.125Hz/s and thereby avoid tripping distributed generators with RoCoF-based loss-of-mains protections.

As part of its plans to make the power grid ready for zero-carbon operation by 2025, the ESO called for the minimum inertia requirement to be lowered to 120GVA.s in March.

The ESO made the recommendation in the third iteration of its Frequency Risk and Control Report, which it has been mandated produce on annual basis since late 2020. The first report was issued in April 2021.

The requirement was introduced in response to major blackout in August 2019 in which more than a million customers were disconnected.

The initial cause of the event was a lightning strike on the transmission network, which led to the near-simultaneous loss of two large generators. The resulting drop in frequency also triggered the loss-of-mains protections for 500MW of distributed generation – 350MW with RoCoF relays and 150MW with vector shift protections.

The ESO and electricity distribution networks launched the Accelerated Loss of Mains Change programme in September 2019 to provide funding to distributed generators to replace vector shift protections and reduce the sensitivity of RoCoF relays to prevent so-called “nuisance tripping”.

In its March report, the ESO said the completion of this programme, which closed in September 2022, means it is now able to operate the electricity system with a lower system inertia as it was already planning to do.

The body said the operation of the electricity system with lower system inertia has also been enabled by the launch and subsequent growth of its Dynamic Containment service. The fast-acting post-fault frequency response service was launched in 2021 as one of three new frequency response services – the others being Dynamic Moderation and Dynamic Regulation.

The ESO said it has so far procured 1,148MW of low frequency response and 983MW of high frequency response through the Dynamic Containment market.

Furthermore, the ESO said it has procured 41GVA.s of additional inertia through its Stability Pathfinder programme – a trial of a new stability service incorporating system inertia as well as reactive power and short-circuit level contribution. It said it is already able to access the majority of the 12.5GVA.s of inertia procured in phase one of the programme, while the inertia procured in the second and third phases will start to become available from 2024 and 2025 respectively.

The report explored the impacts of five different minimum levels of inertia – the current requirement of 140GVA.s and incremental 10GVA.s reductions down to 100GVA.s.

The ESO found that the smaller reductions to 130GVA.s and 120GVA.s would cut the annual costs of maintaining a stable frequency by £44 million and £65 million respectively. It found further reductions would deliver diminishing marginal savings, which would only rise to £70 million by going all the way down to 100GVA.s.

The body identified 120GVA.s as the optimal minimum inertia requirement. It said the completion of the Accelerated Loss of Mains Change programme means it will be able to operate the system with this level of inertia but with a lower risk of significant frequency deviations than it did in 2022.

The ESO proposed that the minimum inertia requirement should be initially reduced 130GVA.s. It said there should then be a monitoring period before the requirement is lowered to 120GVA.s if no issues emerge.

In line with the recommendation of the Security and Quality of Supply Standard panel, Ofgem has now decided to approve the ESO’s proposals. The regulator this is despite all of the respondents to the ESO’s consultation on the proposals raising concerns over the impact of reduced system inertia on grid stability.

Responding to their complaints that the ESO provided insufficient information on its assessment of these impacts, Ofgem said the ESO was unable to share its detailed view as it “relies on commercially sensitive data which has the potential to distort or adversely influence markets or tenders.”

The regulator also noted concerns raised by one respondent that the estimated cost savings from reducing the inertia requirement have been influenced by the current high gas prices and may be lower in future.

Given that a low-inertia system is necessary to meet net-zero targets, Ofgem said it considers the ESO’s proposals to gradually lower system inertia requirements, with monitoring and assessment between reductions, “a sensible, cautious approach.” It said they also represent “an appropriate balance between the cost and residual likelihood of frequency deviations.”

Ofgem said the decision, which was issued on 9 June, will take effect immediately.