Grid inertia is the energy stored in the heavy rotating parts of machines that are directly linked to the electricity network, such as the turbines and generators in gas, nuclear and hydro power stations. Because they spin in step with the grid, they resist sudden changes in speed, and that slows down how quickly the system frequency moves when something goes wrong.
Why it matters
Great Britain’s grid runs at 50 hertz. If a large power station suddenly trips off, demand briefly exceeds supply and the frequency starts to fall. With plenty of inertia, it falls slowly, giving fast-acting reserves time to respond. With little inertia, it falls faster, and some protection systems may disconnect more equipment, which makes the problem worse.
These generators are called synchronous because they turn at a speed locked to the grid frequency. Wind turbines, solar panels and batteries connect through power electronics instead, so on their own they add little or no inertia.
How the system operator keeps enough of it
In the past, inertia came free as a by-product of running fossil-fuel plants. As those plants run less, the National Energy System Operator (NESO) has to obtain stability in other ways:
- Running synchronous plants through the Balancing Mechanism, which costs money and can mean turning down wind.
- Stability pathfinders, tenders under which NESO contracted synchronous condensers (large spinning machines that provide inertia without burning fuel) and, in later rounds, grid-forming batteries.
- A stability market, which NESO is now using to buy inertia for set periods ahead.
The 9 August 2019 power cut
On 9 August 2019, a lightning strike was followed by the near-simultaneous loss of two large generators and smaller local generation. Ofgem found this combined loss triggered automatic disconnection of demand that cut power to more than a million customers. The companies involved paid a total of £10.5 million, and the event added urgency to the work on keeping the system stable as it changes.
Read more: Smart grids and microgrids: how a flexible electricity system works.