What are ancillary services?

How do we make sure the power system operates in a safe, efficient and stable way — and get electricity to where it’s needed on the grid

Ancillary services are the name we give a range of measures that make sure our power system provides enough electricity where it is needed, but also keeps it stable.

We always need to maintain a balance between the amount of electricity that’s being generated, and the amount that’s being used.

That takes very careful management. Ancillary services are one of the ways the power system is managed to keep it safe, efficient, and stable.

Ancillary services help manage:

Frequency: To stay balanced, the UK’s power system always has to run at a constant frequency. This means that turbines and generators automatically change the rate at which they spin to raise or lower the amount of power that is being produced to match demand and makes sure the whole system is stable.

Voltage: The UK transmission system does not all operate at the same voltage. Parts of it operate at 400, 275 or 132 kilovolts. To keep the system safe for domestic electricity, we need to ensure that voltage is always within 5% of those figures. To help achieve this, power stations can produce or re-absorb energy that is not needed in the form of reactive power.

Inertia: To help smooth out unexpected changes in frequency across the network, the electricity system uses the weight of spinning turbines to create stability.

Reserve: One of the most important roles for ancillary services is making sure there are no unpleasant surprises in the electricity system. One of the ways they do this is by holding power in reserve in case the unexpected happens.

Who is in charge of ancillary services?

Ancillary services in the UK are currently managed by National Grid Electricity System Operator (ESO).This is a separate company of National Grid Electricity Transmission.

The ESO works with partners to provide the ancillary services that are required. These are often generators like Drax Power Station. Their large turbines can help control voltage and frequency as well as providing inertia. They can also supply reserve power when it is needed.

Do more renewables mean we need more ancillary services?

In the past, we depended on a few very large power stations for our electricity. Now we can call on many more different sources. Some of them depend on the weather. Others can only generate in daylight. At the same time, the ways we use electricity are changing. So there is more need for ancillary services to keep the grid stable.

Ancillary services: the facts

• Sometimes, batteries can be cheaper sources of reserve power than more traditional ancillary services such as power stations. For example, the Hornsdale Power Reserve, which runs on a Tesla battery in South Australia, cut the cost of frequency ancillary services by 90% after just four months of use.
• While the UK was locked down because of COVID-19 in 2020, electricity consumption on weekdays fell by 13%. So the ESO had to call on ancillary services to balance the system and keep the lights on.
• Ancillary services in the UK move 300 terawatt hours (TWh) of electricity to where it is needed every year. That’s as much energy as it would take to boil four trillion kettles at the same time.