What is synthetic inertia?
An explainer on how big batteries can stabilise the energy grid
Investment in big batteries is only getting bigger. As reported by RenewEconomy, the first big battery in New South Wales reached full production in late 2022. It will soon be joined by at least seven big batteries in the state in coming years — and likely more to come.
As the journey towards 100 per cent renewables marches on, the topics of inertia and frequency control come up. Our power grid has always relied on old-school generators to help stabilise the grid, but now big batteries are stepping up to the plate.
This explainer will demystify inertia and frequency control concepts from an Australian perspective. It will also address concerns about a lack of inertia in the grid as renewables begin to dominate the National Energy Market (NEM).
What is frequency control and inertia?
Managing the power system requires a balance of generation and load. The grid operates within the band of 50 (+- 0.15) Hz, and deviation from this frequency can lead to instability and in extreme cases, grid failure and blackouts. To keep supply and demand in balance, the Australian Energy Market Operator AEMO employs frequency control and ancillary services (FCAS].
In a generator-dominated power system, FCAS has largely been the domain of synchronous generators: gas, coal and hydro. These spinning machines provide inertia, a force which refers to resistance to change. To provide inertial response, mechanical rotors start to accelerate or decelerate to respond to a change in electrical output (see Inside Energy for more info). This also enables time for other supporting controls (i.e. powering up other generators, load shedding) to balance the grid.
Synchronous condensors (syncons) can also provide inertial response (along with system strength — this property refers to the robustness and stability of the grid with respect to properties other than inertia.) Syncons are essentially spinning motors running at no load — an old technology that has long been used to deliver inertia and system strength. Numerous syncons have been installed around the country this year to deliver grid services as gas and coal power stations are retired.
However, it’s highly likely that more cost competitive grid-scale batteries will take their place in the medium to long term.
How can batteries stabilise the grid?
Increasingly, our power system will be dominated by inverter-connected technologies, which include solar PV, most wind turbines and batteries. These systems rely on inverters to interface with the electricity grid and they don’t provide any “real” inertia to the grid — but they can still keep the grid stable and reliable.
Large scale battery storage (LSBS) systems are highly adept at stabilising the grid through fast frequency response (FFR). When disturbances outside the normal frequency are detected, FFR pushes grid frequency back in its normal operating range by rapidly injecting or drawing power from the grid. FFR generally refers to a response within two seconds: much faster than synchronous generators can provide FCAS.
The difference between fast frequency response and inertia
Virtual inertia (often used interchangeably with synthetic inertia, simulated or digital inertia) is a fundamentally different mechanism of regulating frequency to FFR. As energy engineer Stephen Sproul explains, “Virtual inertia and FFR are both valuable to the grid but they are very different… FFR is an important service in its own right, responding quicker than traditional assets [synchronous generators]”.
The only grid-scale batteries in Australia (as of 2020) that can provide virtual inertia are the Dalrymple ESCRI-SA battery, the new Wallgrove Grid Battery project in NSW, and Hornsdale Power Reserve, the original ‘big battery’ currently being expanded to greater capacity and testing new services, including virtual inertia. All three batteries provide revenue through energy arbitrage and FCAS services.
Big batteries have already proven their ability to stabilise the grid, particularly during major incidents like the grid disruption in August 2018. Extensive modelling suggests that LSBS can provide inertial services to the grid.
Grid-following vs grid-forming inverters: what does virtual inertia look like in action?
ESCRI-SA is the first project in Australia that has exhibited virtual inertia in action. The battery mainly serves the South Australian grid, but can also disconnect from the NEM and operate as an island by virtue of being a grid forming inverter system.
Most inverters operating in Australia at wind and solar farms are grid following: they operate using the electrical grid as voltage reference and convert their power output to the grid frequency by controlling current waveform. Grid forming inverters, on the other hand, provide their own voltage source. This means the system’s internal frequency doesn’t change instantaneously when grid frequency changes.
To provide virtual inertia, grid forming inverters use the same mechanism as synchronous machines If the grid frequency increases or decreases from the inverter’s internal frequency, this causes an active power transfer. This is because of the laws of physics that set out that power transfer is proportional to the phase angle between two voltage sources (see Watt Clarity’s article for more details).
These plots illustrate ESCRI’s virtual response during a separation incident in November 2019.
Separation from the NEM caused an over frequency event (supply outstripped demand) which required a reduction in active power output within South Australian. The bottom figure shows ESCRI’s virtual inertia response starting at the red line: power output begins to reduce, even before the frequency increase is measured (as seen in the graph FFR measurement delay; top figure). For further details, go to Green Tech.
Virtual inertia advantages
The key difference between FFR and virtual inertial is that FFR will always have a delay (about 10–100 milliseconds) to measure and respond to the condition of the grid frequency. Virtual inertia is inherent and doesn’t rely on measurement. This ultra-fast response will become even more critical to grid stability as more synchronous generators retire to make way for more renewables and batteries.
Commercial testing is ongoing to demonstrate that big batteries can provide inertial services to the grid. If AEMO recognises that LSBS can provide virtual inertia, this would be a turning point. Inertia would no longer be the domain of synchronous generators and we’ll be one step closer to 100 per cent renewables.
