Non-synchronous renewable energy affects grid stability, but storage-as-transmission (SAT) assets offer grid companies a trump card. Whether it’s “virtual transmission” in Australia, Germany’s “Grid Booster” program or the giga-scale project emerging in the UK, energy storage will find a way.
In February 2023, construction began on a 200 MW 300 MW/600 MWh Battery Energy Storage System (BESS) in Blackhillock, Scotland. The developers want it to be the world’s first transmission-connected BESS to provide network stability. Instead of upgrading or replacing wires, Blackhillock offers full active and reactive power services.
As the UK aims to become carbon neutral by 2025, as part of its 2050 net-zero economy target, the National Grid ESO (NGESO) is buying grid stability services from private assets such as Blackhillock. While Phase 1 of NGESO’s Stability Pathfinder project included only synchronous condensers, Phase 2 accepted a grid of inverter resources for tender.
Four companies were awarded ten contracts in a tender last April to fix inadequate short-circuit levels (SCL) – the amount of current that flows during faults – across Scotland. Winners also provide “green” inertia to balance supply and demand after events such as power plant trips that change the frequency of the system. Five synchronous condensers and five battery bays were awarded contracts worth £323 million ($395 million).
“We believe this is the first time in the world that grid-forming inverters have been used in multiple locations across the region to provide a system-wide solution to short-circuit level and inertia,” said National Grid Networks CEO Julian Leslie. “These carbon-free, stability-enhancing devices enable more use of green electricity, are cheaper for consumers and enable the market to produce as much wind power as possible.”
Three battery contracts, including Blackhillock, were won by UK-based Zenobe. The company is also developing the 300 MW/600 MWh Kilmarnock South plant to be completed in 2025 and the 400 MW/800 MWh Eccles plant the following year. A gigawatt of batteries should produce 4.4 GVA (gigavolt ampere-seconds) of inertia – 5-10% of Britain’s needs.
“The reason we were so competitive is because we were able to stack the revenue we would get from the day-ahead and intraday markets, frequency response services, reactive power, balancing mechanism and other services combined with stability services,” says Semih Oztreves, head of network infrastructure at Zenobe. “When you put these services together, you can offer stability services for almost a tenth of what it would cost to offer them exclusively. Stability services are an additional income stack without much additional investment (investments) and operations (operating expenses). It’s about making the most of BESS.”
In Europe, EU law prohibits transmission storage projects from selling electricity on the energy or balancing markets, as batteries are owned by electricity transmission system operators and are considered “fully integrated network components”. Network booster batteries spend most of the time in standby mode to ease network congestion. Regulators are debating whether SAT assets should provide additional services.
“Revenue collection for SAT projects depends on their use case,” says Lars Stephan, senior policy and market development manager for Europe, Middle East and Africa at Germany-US-owned heavyweight Fluence. “In Germany, we see that Netzbooster (grid booster) SAT projects can generate significant economic benefits by increasing the use of existing power lines and implementing them much faster than building new traditional power lines. They do not need additional income, but are implemented solely on the basis of the SAT case.”
The Spanish grid company Red Electrica is bidding for the SAT project from the mainland to increase the use of interconnection lines between the Balearic Islands. “This use case also does not require additional revenue and could be of interest to the UK as well,” Stephan adds.
Fluence delivers SAT projects in Germany, Lithuania and Australia. Although BESS costs make it impossible to compete with traditional inertial, SCL and reactive power generators individually, such assets are versatile. For example, in Zenobe, stability contracts only make up 10% of the battery site’s turnover.
National Grid’s Stability Pathfinder 2 bid offered up to €6,500 ($6,985) per MWh of short-circuit current per year, with successful bids averaging around €4,000/MVA. It is difficult to assess which of the non-frequency ancillary services provided by the SAT assets has the greatest potential.
“The value of different services changes over time,” says Fluence’s Stephan. “Currently, the synthetic inertia and short-circuit portion acquired with Stability Pathfinder 2 creates a strong revenue potential in addition to the already existing battery-based energy storage reactive power contracts. It’s important to be able to stack them dynamically and to have a system that enables this.”
However, stacking services are not simple. “To provide all these services together requires a very complex control system, which many in the industry are unable or unwilling to design,” says Zenoben Oztreves. “We’ve shown through feasibility studies that we can do it, and now we’re effectively building these resources.”
Blackhillock is due to be completed in mid-2024, and Gabriele Buccini, development manager of the storage business at battery project partner Wärtsilä, says: “Although the system offers a new kind of service, the design and software testing required for the project runs parallel to the acquisition of the equipment. So from this perspective, it does not add new complexity.” Regarding the provision of short and energy-intensive services, additional testing is carried out.
Inertia, typically a byproduct of thermal power plants, requires a five-millisecond response to prevent a brownout or blackout. Grid-forming inverter batteries must change the response profile every five milliseconds when making offers to other markets; providing active and absorbing reactive power; and provides failover. It took Zenobe and inverter partner SMA almost a year to demonstrate such a capability to National Grid.
“We started talking with National Grid ESO over two years ago about the Stability Pathfinder program,” says SMA Business Development Manager Aaron Gerdemann. “It soon became clear that this is the first tender of its kind in the world where stability services are procured from inverter-based resources.” Gerdemann says SMA has since secured a number of UK recording-as-transmission projects, all related to Pathfinder 2. “Our solution allows project owners to stack services to improve their business and offer much more flexible and competitive services than traditional solutions,” he adds. .
The evidence included hundreds of pages of feasibility studies showing that the battery could provide grid stability under all conditions from a given connection point in accordance with grid code requirements. Bidders were asked to state how much shorting and inertia they could offer. “Based on that, we were able to optimize our bidding strategy based on the technical characteristics of the equipment we installed and find out what is stackable and what is not stackable,” says Zenoben Oztreves. “The availability requirements for SAT assets are high. However, NGESO set an availability requirement of over 90%, which allowed us to once again offer these services very competitively. In some other countries, SAT resources face extreme 99.95% availability requirements, which increases the cost of these solutions tremendously. National Grid was able to be more flexible because it has procured services from ten generators.
The utility knows its way around. In November 2022, Susana Neves e Brooks, NGESO’s general manager of customer relations, said: “The current approach takes a more conservative view of the possibility of the assumed behavior of storage, rather than being able to meet the needs of the energy market now.” He added that the company is “focused on leading the way in finding the best approach to enable the consolidation of storage projects.”