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National Grid ESO stresses importance of planning for lower inertia ahead of a net zero system

Inertia is important due to its impact on Rate of Frequency Change, which in turn impacts on frequency response services. Image: National Grid ESO

Inertia is important due to its impact on Rate of Frequency Change, which in turn impacts on frequency response services. Image: National Grid ESO

National Grid ESO needs to plan and operate the system with lower levels of inertia as the UK moves to net zero, with pathfinder projects exploring the procurement of inertia.

“We’ll be going to a future where we’ll be spending more time on lower levels of inertia for most of the time in the year,” ESO operability manager Shurooque Baloch, said during the latest Current± Briefings webinar.

She suggested that in the short-term the country will be spending less time in a year on lower levels of inertia compared to the longer term, with the ESO expecting that in order for it to operate a zero carbon system by 2025, minimum system inertia inertia could be as low as 96GVA.s, while today system inertia is always above 140GVA.s.

". . . so we need to plan and operate the system with those new levels of inertia going forward," Baloch said.

Inertia is a measure of how much energy is available in the rotating mass of all machines that are directly coupled to the system to instantaneously balance any surplus or deficit in power.

Inertia then affects the rate of frequency change (RoFC) of the system - if inertia is low, RoFC is faster, and vice versa. This is important, Baloch said, as if the frequency changes very fast, the ESO's frequency response services may not be able to provide response in time, meaning there's the potential for frequency to fall outside of statutory limits and certain generators to trip due to their protection settings.

Inertia response is a “very, very fast response”, Baloch said, with other services in the system then kicking in once the inertia response has happened.

Of these other services, Dynamic Containment is the fastest, with this usually responding within half a second.

“The purpose of the whole package is to ensure the system frequency stays within the limits and doesn’t fall outside of the limits and doesn’t fall or increase too fast,” she said.

While inertia is one aspect of maintaining system stability, other elements include short circuit levels and dynamic voltage, with these all working together to ensure National Grid ESO is operating a stable system.

Short circuit levels and inertia form a key part of the ESO’s Stability Pathfinder, the first phase of which was the first time the ESO had procured inertia in the system as a commercial service.

Phase two of the project looked to address short circuit level decline in Scotland, although the ESO also procured inertia alongside this, with tender results coming out earlier this month.

This was the first time the ESO had procured new technologies such as grid forming inverters, which Baloch explained are different to traditional inverter based technologies and are able to provide an inertia response.

Another of National Grid ESO’s activities in this space is a Grid Code working group, which looked at defining technical specifications of grid forming inverters. This has now formed a part of the Grid Code.

Lastly, the third stage of the Stability Pathfinder – which is currently running – focuses on short circuit level requirement in England and Wales alongside inertia procurement.

As it stands, the ESO has been focusing on transmission level inertia, with Baloch stating that it doesn’t currently see distributed solutions providing the same level of inertia to help with transmission system issues.

However, she said that going forward, if distribution levels start to see issues with regards t new technologies coming on and reduced short circuit levels of inertia, then there is potential for grid forming technologies to be able to provide support to distribution networks.

When it comes to battery storage providing inertia, Baloch explained it must be a grid forming battery.

In National Grid ESO’s stability pathfinders, it does encourage participants to consider the stacking of services, with the stacking of inertia and Dynamic Containment considered.

However, the provider of the technology must make sure they can provide a guaranteed level of inertia at any given time alongside providing other services.

“So yes, it’s technically possible it’s just understanding how those services are allocated for that asset in terms of reserved MWs to be able to do different elements,” she said.

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