Inertia – the resistance of an object to a change in its motion – in the power grid helps limit frequency variations in the case of sudden load or generation changes. Traditionally, this inertia (or stored energy) has been provided by the rotating mass of synchronous generators and has been essential to stabilizing power grids.
With the onset of high penetration of asynchronous renewable generation from inverter-based/grid-forming technologies like solar which has no synchronous rotating mass, energy providers like AEMO and others are looking to other solutions to provide the frequency response of ‘grid following’ fossil-fueled units.
Enter ‘synthetic inertia’
Synthetic inertia is the ability of a generator to sense and respond to system frequency changes. It’s produced by asynchronous generators as a way to bear the burden of their own frequency dynamics.
A typical response to grid instability due to inverter-based generation has been to impose constraints on wind and solar farms or install synchronous condensers which are essentially unpowered motors linked to the grid to provide voltage stability via rotating mass. It’s old technology being used in a new way but it’s an expensive add-on proposition. Now, however, emerging technologies are demonstrating they can provide many of the services previously thought only possible through spinning machines, ironically due to the absence of rotating mass.
Renewable generation + storage = synthetic inertia
When the Callide C power station in Queensland exploded in May 2021 causing cascading, state-wide issues, the region’s electricity grid was under pressure. The subsequent frequency issues were able to be immediately addressed by South Australia’s big battery, Tesla’s Hornsdale Power Reserve. The battery was able to slow down the rate of frequency changes due to the explosion, acting as a “virtual machine” and providing a crucial grid service normally provided by spinning machines.
Another example is ElectraNet’s ESCRI-SA large-scale battery at Dalrymple substation. This battery allows the transmission grid operator to provide grid stability and prevent outages in case of disruptions to the power grid.
Storage solutions like these and others are being used in Australia and beyond for the express purpose of providing reserve capacity and ancillary grid services to have generation available for immediate dispatch if frequency drops. The upside is that these utility-scale batteries don’t need to be synchronized and generating and can provide immediate inertial response in milliseconds of a trip. Further to this, they also provide revenue-generating services as opposed to installing costly synchronous condensers.
There are 16 large-scale batteries under construction in Australia as of December 2020 indicating the important role battery storage is expected to play in the future energy landscape. The role of battery storage is further highlighted by the large pipeline of battery storage projects going through feasibility and the connection application process. These include stand-alone battery energy storage systems as well as augmenting existing renewable generation projects with a battery storage component.
 AEMO is currently developing an Advanced Inverter White Paper to increase understanding of the application of grid-forming inverters, including the provision of synthetic inertia. This is currently planned for release by July 2021.
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