Transmission & distribution | Improving grid stability In the UK, Denmark and Brazil
Characteristically, the UK is adopting a market driven approach to the provision of inertia to the grid to deal with a growing share of inverter based resources and the loss of rotating masses with the closure of conventional power plants. In November, NESO (GB National Energy System Operator) announced the award of five contracts totalling an anticipated £25.4 million for the provision of 5 GVA.s of inertia to the GB grid between October 2025 and September 2026, under the new “Mid-Term (Y-1) Stability Market.” The successful bidders included three SGT5- 4000F based open cycle gas turbine power plants developed by Drax and currently under construction (Hirwaun, Millbrook, and Progress). These are equipped with clutches between gas turbine and generator to facilitate the provision of inertia. Two modified 13E2 gas turbines, formerly part of Triton’s Deeside combined cycle power plant, and a pumped storage hydro unit (at the Drax Cruachan station) were also successful. Modified gas turbine based power plants figure prominently because the Mid-Term (Y-1) Stability Market focuses on inertia-providing projects that can be operational within a year. It is expected these first contracts to be awarded under the Mid-Term (Y-1) Stability Market will deliver an anticipated consumer saving of £47.3 million. The award of the contracts will help contribute to the stability of the GB power system, says NESO, by “providing cost-effective, zero-carbon solutions which can be utilised to increase system inertia during periods of shortfall.”
The design of the new stability market arises from a project initiated in 2021 to determine the best way of establishing an “enduring stability market” for the UK, building on the previous programme of “Stability Pathfinders”, a series of tenders.
The market design project concluded with the intention of developing the provision of stability services across several time scales. The new stability markets are therefore broken down as follows:
Long-Term (Y-4), providing a four year delivery window;
Mid-Term (Y-1), for delivery across a single year; and
Short-Term (D-1) for delivery on a rolling day ahead basis.
Last year, NESO launched the first tender round for the Mid-Term (Y-1) Market. The Mid-Term (Y-1) Market was launched as a priority to access inertia capability from existing grid assets on a high-availability basis, as already noted. Through annual contracts, this market provides revenue certainty for market participants whilst reducing risks for NESO as periods of low inertia become more frequent and unpredictable.
Quinbrook/Welsh Power’s synchronous condenser at SSEN Transmission’s Rothienorman substation (photo: Quinbrook)
As outlined in NESO’s Frequency Risk and Control Report, it must operate the power system above the minimum target inertia threshold – currently 120 GVA.s – at all times. In November 2024, having concluded the opening round of the Mid-Term (Y-1) Stability Market with the award of the 5 GVA.s mentioned above, NESO launched the expression of interest phase for a second tender round. On 17 January 2025 NESO released the invitation to tender for this second round to the relevant market participants, with the aim of procuring 15 GVA.s of inertia for delivery between October 2026 and September 2027.
Stability pathfinder progress The twelve projects awarded in 2020 under phase 1 of the Stability Pathfinder tender process, total cost 328 GBP, are now in operation delivering about 12.5 GVA.s to the grid. The projects are: Cruachan (Drax), 533.3 MVA.s; Killingholme (Uniper), 2 x 1430 MVA.s; Grain (Uniper), 2 x 1729 MVA.s; Rassau (Welsh Power), 750 MVA.s; Deeside (Triton), 2 x 1553 MVA.s; Lister Drive (Statkraft), 2 x 450 MVA.s; and Keith (Statkraft), 2 x 450 MVA.s.
Phase 2 of Stability Pathfinder programme saw the award in April 2022 of ten contracts to four companies (Statkraft, TINZ Programme 1 ProjectCo 3, Zenobe, and WP Grid Services) for the provision of inertia (totalling 6.75 GVA.s) and short circuit level (11.55 GVA) in Scotland. See Modern Power Systems, June 2022, p 37. The Phase 2 awards were to “address potential grid stability problems arising from the imminent closure of nuclear power stations in Scotland and northern England, and the rising number of onshore and offshore wind farms in Scotland.” Five of the successful bids in Phase 2 involved synchronous condensers with flywheels, while
28 | March 2025|
www.modernpowersystems.com
the other five employ battery storage and “comprise what is thought to be a world-first use of new grid forming converters at multiple locations across a region to improve inertia and short circuit levels.”
The Stability Pathfinder Phase 2 winning tenders were: Coylton (Stakraft), grid forming battery storage; Neilston (Statkraft), grid forming battery storage; Beatrice (TINZ Programme 1 ProjectCo 3), synchronous condenser; Blackhillock (Zenobe Energy), grid forming battery storage; Kilmarnock South (Zenobe Energy), grid forming battery storage; Eccles (Zenobe Energy), grid forming battery storage; Gretna (WP Grid Services/Quinbrook), synchronous condenser; Rothienorman (WP Grid Services/Quinbrook), synchronous condenser; Thurso South (WP Grid Services/Quinbrook), synchronous condenser; and Neilston (WP Grid Services/Quinbrook), synchronous condenser.
The Stability Pathfinder Phase 3 tender process concluded in November 2022 with the award to six companies (Statkraft, Transmission Investment Holdings, Conrad, WP, Green Frog Ventures, and RWE) of synchronous condenser contracts totalling 1.3 billion GBP for supply of inertia (totalling 17.08 GVA.s) and short circuit level (some 8.72 GVA) in England and Wales. See Modern Power Systems, March 2023, p 28. The Rothienorman synchronous condenser installation has recently entered service, the first Phase 2 project to do so, with the remaining Phase 2 and 3 projects due to go live over the rest of 2025 and into 2026.
Quinbrook, which has seven synchronous condenser projects in the UK, started construction at Rothienorman in February 2023, partnered with Welsh Power in the development of the project from concept to commissioning, and key equipment supplied by Siemens Energy.
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