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20 | 28TH FEBRUARY - 5TH MARCH 2020 | UTILITY WEEK Operations & Assets Analysis B y 2025, the electricity system operator (ESO) at National Grid aims to be able to run the power grid without any fos- sil fuel generation whatsoever. Since making the commitment in April last year, the ESO has been busy overhauling its ancillary and balancing services to make them ready for zero-carbon operation. Inertia One of the biggest changes being imple- mented by the ESO concerns inertia – the resistance of the electricity system to sudden changes in frequency. Inertia can only be provided by syn- chronous generators – usually coal and gas power stations – with heavy masses spinning in harmony with the frequency of the power grid. When there is an outage elsewhere on the system, the kinetic energy stored in the turbines is automatically absorbed into the power grid, limiting changes of frequency. Until recently, the ESO had never pro- cured inertia as a discrete service, however, at the beginning of November, it launched the first phase of its Stability Pathfinder – the trial of a new service incorporating inertia. The tender sought to procure inertia with- out buying the generation. "This is about buying the spinning element rather than the megawatts," says Julian Leslie, head of net- work capability at the ESO, which has previ- ously been forced to constrain renewables and interconnectors to make up for fossil fuel generation and maintain inertia. The body looked to buy up to 25 gigavolt- ampere seconds (GVAs) of inertia starting in April 2021. According to Leslie, the short lead time meant it was only really suitable for existing assets, most likely gas plants – per- haps modified to enable them to keep their turbines spinning whilst generating little or no electricity. The ESO received offers for 22.5GVAs and awarded contracts for 12.5GVAs to seven sites operated by five companies. They included Deeside, Killingholme and Grain combined- cycle gas turbines, as well as the Cruachan pumped hydro storage facility. The contracts are worth £328 million in total. Leslie says the ESO is the first system operator in the world to create a market for inertia separate from power. And he says phase two of the project, which will seek to procure the service for two to three years ahead, will bring "the really exciting stuff." He continues: "There's some really cool devices out there that provide a lot of inertia for not a lot of megawatts." These technolo- gies could include flywheels or synchronous compensators – essentially electric motors with free-spinning sha¡s. They could also include wind and solar farms with smart electronics that mimic the "look and feel" of synchronous generators. Frequency response The benefit of high inertia is that it allows the ESO more time to inject a "squirt of energy" onto the power grid in the form of frequency response. But nevertheless, these services will still need to be reformed to reflect the changing make-up of the energy system, which will be increasingly driven by fluctua- tions in renewable output due to weather. They will need to be faster. "With all of the old thermal plant the fast- est you can really get frequency response is in about ten seconds," says ESO director of operations Duncan Burt. "We're redefining all of those products from 10 and 30 seconds to effectively down to 1 second." Its fastest acting service will be called dynamic containment. As the name sug- gests, the service will be used to contain sud- den changes in frequency a¡er a fault. What is perhaps less obvious is what is meant by dynamic in this context. Frequency response services are presently split between those that are static, activated when the frequency falls outside of certain limits, and those that are dynamic, meaning they are operated within this band to main- tain a steady 50 Hertz. The dynamic containment service would be classified as static under this old definition. The new definition refers to the way in which the response ramps up and down. "It acts like static in that it only comes in once you go out- side of a band, but then it works continuously – dynamically – to regulate against where the frequency is," Burt explains. Dynamic containment, due to be launched later this year, will be part of an "integrated suite" of three new services. The second – dynamic moderation – will also be used to counteract sudden imbalances and activated when frequency is outside a central range but will be slower responding. The third – dynamic regulation – will be used continu- ously to correct small deviations in frequency. All three will be procured over shorter timeframes and in closer to real time than is currently the norm. The ESO is currently trialling weekly auc- tions for existing services but eventually wants to move to buying frequency response just a day ahead and perhaps even sooner. Reserves and the balancing mechanism The ESO's work on reserve services is in large part being led by EU regulations, in particu- lar new guidelines for electricity balancing introduced in 2017, which required the ESO to participate in the Trans European Replace- ment Reserves Exchange (TERRE). The preparations for TERRE have included the widening of access to the bal- ancing mechanism, in particular for inde- pendent aggregators, which are now able to operate aggregated units without becoming the supplier for the component assets by instead registering as a virtual lead party. The ESO was due to join TERRE in Janu- ary but was permitted to delay its entry in line with the French transmission system operator RTE. The ESO said in November that it is hold- ing back on further reforms to reserve ser- vices until the implications of both this work and its overhaul of frequency services have become clearer. The ESO said it also needs to consider the implications of the clean energy package passed by the EU last year, which calls for all balancing services, including reserves, to be procured through day-ahead pay-as-clear auctions, as opposed to long-term contracts. "Once we've worked through that we'll make some announcements," says Burt Creating a zero-carbon grid Tom Grimwood speaks to senior figures at the electricity system operator about the challenges they face – and the progress they have made so far – towards a zero-carbon electricity grid.