How inertia keeps the lights on 

Resisting change isn’t always considered a good thing in life. But in the world of physics, it’s vital.   
 
That’s because without it, none of us would have the power we need to lead our lives or run our businesses.  
 
Our phones and laptops would die. We’d be stuck eating cold food. And well, we would all be sat in the dark. 
 
Britain’s electricity system operates at a constant frequency of 50 Hz, meaning the electric current cycles in the cables 50 times a second. If this changed much from this level it could put our electricity system at risk.   
 
System inertia is something which helps to prevent this, alongside the actions we take at NESO to keep the system secure. 
 
You may remember from GCSE physics, inertia is the resistance of an object to any change in its state of motion. 
 
But Britain’s demand for electricity is changing all the time. And so is the supply available. It’s our job at NESO to ensure that both are in sync, every second of every day. Ensuring there is enough inertia to resist the impact of these changes is essential.  
 
This isn’t just to ensure that we all have the electricity we need, but also to ensure the safe running of the national transmission system. That’s the ‘motorway’ of cables carrying high voltage power from where it’s generated to towns and cities around the country.  
 
So, when supply reduces during the evening as the sun stops shining on solar panels, but demand starts increasing as we all put our cookers on for dinner, frequency tries to fall. Think of the imbalance in terms of a car slowing down as it starts to go uphill.  
 
We anticipate this, so our control room increases the supply of energy to keep it steady – a bit like pressing down on the accelerator to keep the car’s speed constant.   
 
Conversely, when we switch off the lights at bedtime and demand falls, but supply increases from wind farms during gusty overnight weather - the frequency tries to rise. Just like when a car tries to roll downhill, the driver takes their foot off the accelerator. 

NESO uses frequency response services to manage the normal ups and downs of frequency as the balance shifts. These are flexible sources of supply, such as fast standby generators, pumped hydro power stations, and batteries - able to respond to any disturbance in frequency levels.  
 
But if there is a sudden problem on the network, like a lightning strike causing damage or a power station trip, there can be a rapid fall in frequency. It’s like a sudden bump in the road. Inertia essentially acts as a natural shock absorber - instantaneously stabilizing the grid by resisting changes in frequency during sudden power imbalances. In this case, inertia is supported by our frequency services and buys our control room time to restore balance.  
 
Historically, the heavy spinning turbines in gas and nuclear power stations have kept turning at a constant speed, helping generate and maintain inertia on the electricity grid.  
 So, when there is a sudden unexpected fault, their steady spins have helped keep frequency stable at 50 Hz. And similarly a big heavy truck is able to cope with a sudden bump in the road better than a lighter car.
 
But as Britain has decarbonised and shifted to renewables, NESO’s faced the challenge of how to maintain the levels of inertia we need.  
 
That’s because wind turbines and solar panels don’t naturally have inertia, as they’re connected to the grid through power electronics, or what’s known as inverters - instead of direct mechanical links to a spinning generator.   
 
It’s a challenge faced by electricity grids across the world, as we transition to clean power to tackle climate change.  
 
As you would expect, NESO has been planning ahead for this for many years, ensuring we maintain one of the most secure energy systems in the world.  
 
Thanks to a huge increase in batteries connected to the grid, which can store clean renewable energy, we can respond almost instantaneously to dips or peaks in frequency - providing a much larger volume of frequency response services and meaning we don’t actually need as much inertia as we did.  
 
But in addition, to compensate for the lack of inertia generated by renewables, we now have the world’s first market to supply zero-carbon inertia to Britain’s grid, helping us reduce our reliance on gas for inertia.  
 
For example, fifteen synchronous compensators – essentially giant electric spinning wheels - are providing alternative sources of inertia, with more set to come online next year.   
In Britain, we’ve also installed a handful of what are called grid-forming inverters. These control the output of renewables such as wind and batteries in a way which effectively mimics larger power stations as the grid's primary voltage source.  
 
These grid-forming inverters are able to respond more directly to a drastic change in frequency. They provide what we call synthetic inertia.  
 
These additions to Britain’s electricity network help our transition to a zero-carbon power grid, without affecting our energy security.  
 
So, while the sources of electricity we all use to power our lives are changing with the times - becoming cleaner and more diverse – one thing will always remain constant: inertia.

And as electrical engineers, that’s just how we like it.