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.
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 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 (Statkraft), 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.
Huge growth in ancillary service needs in Denmark
Meanwhile, in Denmark, there is a growing need for ancillary services to balance the grid. Towards 2040, Energinet expects the need for ancillary services to increase by approximately 150%. A new report highlights this, prompting a search for flexible megawatts to balance the future power system.
Reliance on renewable energy sources such as wind turbines and solar farms, combined with fewer conventional thermal power plants, makes it increasingly difficult to maintain the balance between electricity consumption and production. Energinet therefore anticipates that the need for ancillary services to balance the power grid will grow significantly in the coming years.
“We will need substantially more megawatts for ancillary services in the future power system. By 2030, we’ll be heavily reliant on fluctuating renewable energy sources. This will challenge security of supply,” says Thomas Dalgas Fechtenburg, Senior Manager at Energinet.
Energinet has just published a report, Outlook for ancillary services 2024-2040, which projects Energinet’s ancillary services needs towards 2040. Energinet says these projections provide its best estimates for ancillary grid services requirements in a still greener energy system where solar and wind create larger fluctuations in electricity production, and where electricity consumption is expected to increase as heat pumps, electric cars, etc, replace fossil fuels.
Thomas Dalgas Fechtenburg says he and his Energinet colleagues “have a solution to restore and maintain the balance in the power system, ensuring a stable and secure supply on a calm, frosty January weekday as well as on a windy, sunny July afternoon.
“We need everyone with flexible electricity production or consumption to participate. Energinet can guide and support you through market entry. But it’s essential that everyone contributes to the green transition and makes their flexible megawatts available. Every megawatt counts, and companies providing ancillary services to Energinet will gain on both the financial and green bottom line,” he notes, emphasising that ancillary service providers can profit from helping to keep the power grid balanced.
Also, he stresses that it is not just “classic” market players like power plants that can provide ancillary services. “Electric vehicles, supermarket refrigeration, greenhouses, and various industries can also contribute. The possibilities are vast, and Energinet hopes that many will see the potential to get involved.”
Fortunately, more and more companies recognise the economic benefits of flexibility in their electricity production and consumption. However, the ancillary service markets are far from saturated, notes Energinet.
Synchronous condensers for Brazil
Brazil is also grappling with the problem of accommodating a growing share of fluctuating renewables on its grid and Andritz reports that it is helping to enhance Brazilian grid stability by supplying synchronous condensers.
New regulatory requirements and a cleaner, more diversified energy mix pose new challenges for grid operators tasked with maintaining a stable power supply, the company says, noting that synchronous condensers have positioned themselves as a reliable, proven, cost-effective and smart solution for these issues, and are bolstering grid resilience, “which is negatively impacted by wind and solar power plants worldwide.”
Specifically, Andritz has signed two contracts to supply new synchronous condenser systems to Brazil, with both projects including the supply of a 330 MVAr machine. “The installation of these systems is an important step towards balancing and stabilising the Brazilian power system,” says Andritz, “improving its efficiency, and ensuring a more reliable power supply for the future.” One of the synchronous condensers will support the transmission lines between Bahia and Minas Gerais, and the other will support the Ceará Crateús region.
Sérgio Gomes, Sales Director of Andritz Hydropower in Brazil, said the existence of the Andritz manufacturing hub in Araraquara, São Paulo, played a central role in the winning of these contracts thanks to its ability to produce “high-quality synchronous condensers that meet the demanding requirements of modern power grids.”
The Andritz scope of supply, as well as the two 330 MVAr synchronous condensers, includes mechanical and electrical auxiliaries, automation, protection, step-up transformer, field assembly services, and commissioning.
