As traditional inertia sources retire, operators seek new ways to maintain frequency
The UK’s National Grid Electricity Systems Operator (ESO) launched its Dynamic Containment frequency response service on September 30, 2020. The service is the first of a suite of digital services being introduced by the ESO to speed up the grid’s resilience to disturbances caused by faults such as the loss of a generator.
The rise of renewables is displacing thermal generation and the inherent inertia of its heavy spinning turbines, synced with the grid’s frequency, contribute to power system stability. As a result, additional means of containing rapid changes in frequency during system imbalances are needed. Dynamic containment in real time is the answer, made possible by artificial intelligence technology.
Deviations in system frequency can take just seconds to travel the full length of the network. Imbalances need to be managed extremely rapidly—dynamic containment can achieve this in under one second. Digitalisation supports how the ESO procures dynamic containment, explains Colm Murphy, electricity market change delivery manager at the ESO.
“In the past, we secured frequency response at quite long procurement horizons—one or six months ahead. It was a fairly manual process, with bidders filling in prices and availability on a spreadsheet, sending it by email to someone who would assess the tenders and work out who won,” he says. In contrast, dynamic containment is procured through day-ahead auctions seven days a week every 24 hour period from 23:00, giving operators of demand-side response assets such as batteries much more certainty in how their assets will be used and what prices they should propose, based on conditions closer to real time.
An algorithm rapidly optimises the markets based on what the ESO needs and the price it is willing to pay, while providers submit the price they are willing to sell dynamic containment for, which speeds up the process of procuring dynamic containment, Murphy says. “We’ve taken the process and turbocharged it using digital tools to create auction platforms and algorithms that result in a really user-friendly, easy way to submit bids,” he adds.
The service’s agreements with generators require them to be available and respond when needed. Any technology can take part, but the speed and flexibility with which batteries can store and release power makes them well-suited to the task and the ESO expects them to make up the majority of providers in the early stages. The first round of tenders in September 2020 saw two battery energy storage units accepted to provide 90 megawatts (MW) of fast response services over 24 hours—with six units and 165 MW available to compete in the next day ahead tender.
This use of digitalisation has enabled a speed of response to imbalances in frequency that is critical to the low carbon transition, Murphy says. As ever larger renewable energy assets such as offshore wind farms and interconnectors are connected to the system, the risk of fast changes in frequency from system losses rises, as does the range of those risks.
“The original frequency response services were designed for older technologies. Operationally, they’re just not as efficient as they could be, and they’re not fit for purpose commercially,” he explains. The dynamic containment service also means a cheaper solution for consumers compared with paying renewable energy operators to reduce generation.
“To operate at zero carbon, we want to enable a huge penetration of renewables and all the variability that comes with changing generation output according to the weather. We need to accommodate less big synchronous spinning plants on the system, as well as providing a dampening effect when there is a loss of frequency, which needs a faster response,” he says. Initially, 500 MW of low frequency response is being bought from providers, which will grow to one gigawatt (GW) this year, and also include high frequency response.
The ESO is planning to introduce two other frequency response services—Dynamic Moderation and Dynamic Regulation. The first will manage sudden imbalances in intermittent generation such as during gusting winds, while the second will manage small deviations when frequency is close to the required 50 hertz (Hz). One of the first participants in the ESO’s new service is software platform operator Arenko. Andy Hadland, the company’s chief product officer, agrees the previous way of controlling frequency via mechanical valves is too slow for modern network requirements.
“We’ve seen the digitalisation of power technology through inverters, which bring a whole new world of possibility. Batteries are only limited by communication delays and the protocols inside the inverter, the IT and software, which is how we’re able to get response times down to tenths of a second or quicker. This means that we can react when there’s the first hint of a problem, before it fully manifests. If you solve the problem before it gets worse, there’s less to clean up afterwards and it’s a much more stable system,” he says.
So far, the service has been called into action one to two times a month, according to the ESO. In January 2021, Arenko’s battery storage was called into action when an interconnector with France tripped. Frequency dropped 0.25 Hz, indicating that around 300 MW fell off the system, though in reality was more like a drop of 900 MW, with the rest being smoothed out by Arenko automatically discharging its battery as part of its dynamic containment contract with the ESO.
The event also provided a good example of the interaction between the balancing mechanism and dynamic containment, following a move by the ESO in January to allow stacking between the two for the first time. Arenko’s batteries had charged just hours earlier, having received a signal from the ESO to charge at a negative price through the balancing mechanism.
“One of the absolutely fantastic things about digitalisation of assets is that we don’t just have to be in one market—we’ve been able to offer flexibility not only in frequency markets, but also in the wholesale market, and in the balancing mechanism at the same time,” Hadland explains.
“This means we can provide free services from the same asset, which is better and cheaper for customers,” he adds. •
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Catherine Early
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Shane Rounce