South Australia experienced a state-wide blackout in 2016 due to a severe storm that damaged critical electricity transmission infrastructure and left 850,000 customers without power. Most electricity supplies were restored within eight hours, but it was a major event and prompted a multi-agency response involving emergency services and the Australian Defence Force.
Etik Energy Co-founder and consulting engineer, Sorrell Grogan, has studied the event at length. He is highly experienced in recreating real blackout events in computer simulation and analysing how to restart the grid.
Commissioned by CSIRO and the Australian Energy Market Operator (AEMO), he is the lead author of a new report on using inverter-based resources to restore power after a blackout.
The report is one of a series under the Australian Research in Power Systems Transition (AR-PST) initiative, and these reports are updated annually to support Australia’s transition to a stable, secure and affordable power system.
Generators versus batteries
Historically, Australia has been heavily reliant on gas and coal generator units for system restart after a blackout, but those units are quickly reaching their end-of-life.
The grid has also changed significantly in the last decade alone, and today’s electricity network looks very different, with large commercial wind and solar farms making up a higher percentage of Australia’s generation mix every year.
Sorrell’s work looks at how power systems can be restarted using large-scale, grid-forming batteries storing power from wind and solar sources as the primary restart source. While he recognises restarting the grid is not something most renewable plants were intentionally designed for in the first place, he remains confident in their ability.
“We’re 100 per cent moving in a direction where large-scale batteries are going to feature prominently, if not be the primary black starter of the grid after major blackouts,” Sorrell said.
Initial lessons learnt from a mixed-grid blackout
During the South Australian blackout, severe weather damaged powerlines and subsequently nearly all wind turbines across the state shut down in quick succession. This was caused by a protection setting unknown to operators. Losing the turbines caused a massive energy imbalance, and with far too much load for the generation available the system collapsed. Within seconds, the whole state lost power.
“It’s not because it’s wrong for those protection devices to be there. They’re there for very good reasons,” Sorrell said.
“What the problem tends to be, and what was the case in South Australia, was that despite being compliant with existing standards, these particular settings were not present in the models that the manufacturers provided.”
This meant the equipment was not being correctly represented, either in technical standards or in the simulation models that power system operators need, especially in understanding extreme circumstances.
Sorrell said there has since been a concerted effort across the industry to implement new standards in modelling so that they accurately represent the equipment in the field and their performance.
“Australia is a world-leader for setting modelling and performance standards,” he said.
In his latest System Restoration and Black Start DOCX (15 MB) report, Sorrell used these next generation computer models and simulations to explore how large-scale batteries, wind and solar can actively participate in system restart.
Wind and solar could be key to restarting after a blackout
Traditionally, it has been thought that large coal or gas generators have more capability and that large amounts of wind and solar in Australia will make our networks less stable.
CSIRO Power Systems Researcher, Dr Thomas Brinsmead, said one of the more interesting outcomes from the latest report is that this is not necessarily the case when it comes to restarting after a blackout.
“The capability of batteries with grid-forming inverter technology is better at supporting system restart than traditional black-start generators in many respects,” Thomas said.
The report found that grid-forming battery technology was capable of energising far larger areas of the network than an equivalent synchronous generator, be it gas, coal or hydro.
A synchronous generator is a type of electrical machine used to convert mechanical energy into electrical energy. It’s called ‘synchronous’ because its rotor rotates at the same speed as the magnetic field in the stator – this means it’s perfectly in sync with the frequency of the electricity being produced.
Grid-forming batteries use smart inverters that mimic the behaviour of traditional generators – such as coal or gas turbines – but without the fuel-burning. The inverters work by converting direct current (DC) from renewable energy sources into controlled alternating current (AC) to supply power to the grid. These can also be used to help restart the grid after blackouts.
“What we consistently found, and I was genuinely surprised by these results, was that grid-forming batteries outperformed the synchronous generators in almost all areas,” Sorrell said.
A slow and steady battery wins the race
A big challenge with inverter-based technology is that it is current limited. This means that the amount of energy it injects into the system must be tightly controlled, otherwise the transistors within it will fail. Synchronous generators are not current limited to the same extent. They’re capable of injecting immense amounts of current into the system as and when required.
“We originally subscribed to the idea that the best practice for re-energising a transformer during restart was to maintain a strong system capable of supplying that massive inrush of current to get it going,” Sorrell said.
“But what we found was the current-limited nature of grid-forming inverters might actually be helping in these circumstances. Because those inverters, they’re ramping to their maximum current and then they’re staying at that level for longer, resulting in these transformers being gradually re-energised over a second or more.”
This inherently gradual re-energisation from the inverters allowed large transformers to be re-energised without tripping network protection mechanisms more reliably than traditional rotating machine restart sources such as coal, gas or hydro. The current-limited nature of the inverters, although generally seen as a drawback of the technology, may be beneficial in this situation.
However, a grid with large amounts of solar, especially on rooftops, is not all good news when it comes to system restarts.
More testing to come
It’s important to have a steady load during system restart, especially in residential areas that rely heavily on electricity.
However, researchers discovered that during the early stages of system restart, the use of large-scale grid-forming batteries as the primary source may cause rooftop solar to become unstable. This happened at lower penetration levels when compared to the use of traditional restart sources.
These studies concluded that although a battery is more flexible than a coal or gas generator, or even a hydro generator, to accommodate changing load, they don’t initially provide the same system strength to rooftop solar.
Thomas said that there is presently still a need for black-start generators to be available in the National Electricity Market (NEM) as the primary source for restarts.
“We don’t like blackouts to happen, but we want to be very confident that when they do, we are able to get things started again as soon as possible,” he said.
However, the work continues to build confidence that the same restoration function can and will eventually be performed by newer technologies.
Given the published retirement schedule of synchronous machine-based generation in the NEM, approximately 2GVA of new grid-forming technology will be required by 2028 to maintain network restoration capability equivalent to today.
This is considerably lower than the capacity of synchronous machine-based generation being retired. This could be viewed as already recognising the greater capability of grid-forming inverters to restore network elements without activating protection mechanisms.
During the next stage of the system restoration work, energy system experts will investigate how new renewable energy zones – which include solar and wind farms – throughout the country can play an active role in system restoration. They will engage a transmission provider to devise a realistic test plan template for grid-forming batteries to restart a system. A successful test of a battery restarting a portion of the network in Australia is not far away.
“The industry is learning so quickly,” Sorrell said.
“From the inception of distributed electricity to when renewables came on board, we had 100 years. The world had 100 years to get electricity right. Meanwhile, we’ve had just two decades to go from the idea of large-scale wind and solar to getting it fully functional in the grid.”
This article was first published online in Energy Magazine.