Discover the silent force that ‘pumps’ electricity round the grid
Electricity that turns on light bulbs and charges phones is called ‘active power’. However, getting active power to go around the country efficiently, economically and safely requires something called ‘reactive power’.
The pump powering electricity
Reactive power is generated in the same way as active power, by large power stations, but is fed into the system in a slightly different manner.
It does not travel far. So a network of regional reactive power distributors services each local area.
Power stations aren’t the only source of reactive power. Electronic devices such as laptops and TVs also produce and feed small amounts of reactive power back into the grid. This can increase the amount of reactive power on the grid, so power stations must absorb the excess.
That’s because, although reactive power is essential, it is more important to have the right amount on the grid. Too much and power lines can become overloaded, creating volatility on the network. Too little and efficiency decreases.
Managing reactive power ensures active power is delivered to the places it needs to be. But it also means controlling voltage across the grid.
How voltage control keeps volatility at bay
Across Great Britain, all electricity on the national grid must run at the same voltage (either 400kV or 275kV). A deviation as small as 5% either way can lead to equipment being damaged or large-scale blackouts.
National Grid ESO, the system operator, monitors and manages the nationwide voltage level to ensure it remains within the safe limit, and doing this relies on managing reactive power.
Ian Foy, Drax Head of Ancillary Services, explains: “When cables are ‘lightly loaded’ such as overnight when electricity demand is lower, they start emitting reactive power, causing the voltage to rise.”
To counter this, generators such as Drax Power Station, under instruction from the national control room, can change their transformers from exporting to absorbing reactive power in just two minutes.
This relies on 24-hour coordination across the grid, but as our power system continues to evolve, so do our reactive power requirements.
The changing needs for reactive power
“Large industrial power loads bring voltage down and create a demand for more reactive power,” explains Foy. “Now, with more consumer product usage, the demand for active power is falling and the voltage is rising.”
So Drax Power Station and other electricity generators now spend more time absorbing reactive power than exporting it to keep voltage levels down.
As Great Britain’s energy system decarbonises, the load on powerlines also becomes lighter as more and more decentralised power sources such as wind and solar are used to meet local demand, rather than large power plants supplying wider areas.
This increases the voltage and creates a greater need for generators to absorb reactive power from the system — highlighting that balancing reactive power remains an essential service.
This short story is adapted from a series on the lesser-known electricity markets within the areas of balancing services, system support services and ancillary services. Read more about black start, system inertia, frequency response, reactive power and reserve power. View a summary at The great balancing act: what it takes to keep the power grid stable and find out what lies ahead by reading Balancing for the renewable future.
