The balancing act of stabilising the grid

The need for inertia in electricity system operations

How does a large piece of spinning machinery which sits on the electricity grid but doesn’t provide any power contribute to decarbonization? Steve Scrimshaw, Vice President, Siemens Energy UK&I takes a look at how this technology will help the transition to cleaner energy sources.


The demand for energy is increasing worldwide, and with that comes the need to contain the effects of climate change and reduce CO2 emissions. This means many countries are looking at how to phase out CO2 intensive forms of power generation, such as coal, and expanding renewable energies and interconnections with other countries.


In the UK, the Government committed that all coal-fired power generation would be switched off by 2025, and for the past decade has installed around 20GW of renewable power into the system. As a result, just over a third (37.1%) of the UK’s electricity comes from renewable sources and the Government has ambitious plans to install a further 40GW of offshore wind over the next decade.


Renewable power is connected to the grid electronically rather than directly as a large centralised power station would be. As a result of the shift away from coal there is fewer large spinning turbine on the grid, and this has led to a reduction in the amount of inertia in the system.  


Why is a lack of inertia a problem?


Inertia is the resistance of a physical object to any change in velocity – essentially the kinetic energy that keeps something moving. Think about how you keep moving forward when you stop pedalling your bike.


In energy terms, inertia is energy stored in a generator or motor which keeps it rotating. It helps slow the rate at which the grid frequency changes, as rapid changes can create instability in the system. In Great Britain, the grid needs to be kept at around 50Hz, as blackouts will occur if it dips below that and consumers are disconnected.

Traditionally, inertia has been provided as a by-product of large-scale power plant operation. However, as more of these close, and the system becomes more decentralised using wind, solar and interconnectors, a new approach is needed to add more inertia into the system.

Generate inertia without generating power


Britain’s electricity system operator, National Grid ESO launched a program named the Stability Pathfinder which is unique in the world. This competition looked for companies who could provide inertia without generating power.


While that sounds complicated, the answer lies in innovative technology called a rotating grid stabilizer. This uses synchronous condenser technology which has been around for years and used across the world.


It is a large piece of spinning machinery usually made up of a generator and a flywheel, which when connected to the grid, provides this all-important inertia by spinning continuously, but doesn’t generate any power. By spinning in sync with the grid frequency, it contributes to the stability of the system, dampening any fluctuations in the grid frequency, just as car shock absorbers dampen a bump in the road. 


As the name suggests, the synchronous condenser spins in-sync with the grid frequency. In the UK, this is 3,000 rotations per minute which matches the 50Hz frequency of the grid.


In addition, synchronous condensers support voltage stability by providing reactive power and enhance grid strength at connection points by providing short circuit power. Through using this type of technology, it means more renewable power can be integrated into the UK’s electricity system, ultimately leading to further decarbonisation.

National Grid ESO estimates the Stability Pathfinder programme will save British consumers up to £128 million over the six-year period of the first contracts, reducing the costs currently associated with managing system balancing and stability. Additionally, it will also provide a significant step towards the ESO’s ambition of being able to operate the GB electricity system carbon free by 2025.


As we move to a net zero world, this type of technology and innovative thinking will become all the more important.


Find out more about our synchronous condensers

A proven solution to stabilize the grid