Switching off rooftop solar will become an important feature of a renewables grid

Australia’s biggest grid problems have traditionally been framed as one of matching supply and demand, with the focus on ensuring enough supply to meet the demand at any one point in time. But that network challenge is about to be turned on its head: the biggest problem in the future will be ensuring there is enough demand, because most consumers have their own supply in the form of rooftop solar.

Last weekend in South Australia gave a perfect illustration of this transition. On Friday evening, it was a test of supply: network maintenance on the main transmission line to Victoria inhibited cross-border trade, a fire in a sub-station took out half the state’s gas capacity, rarely used diesel generators sent the price to the market cap, and the Australian Energy Market Operator declared an LOR2, a lack of reserve level that signalled that if another major generator went offline, then power cuts would likely ensue.

In the end, there was no further incident, apart from sustained high prices. But on Sunday, the situation was reversed: The gas generators were back on line, but relatively low power consumption and sunny conditions – and the still inhibited cross-border trade – meant there was too much supply, and not enough “grid demand”, because most demand was being provided by rooftop solar systems.

The market prices were negative, but that is not what was bothering AEMO. It needs a certain number of controllable assets to keep the grid stable. So at around 3pm local time, the rooftop solar systems of more than 10,000 Adelaide households, along with some larger ground-mounted systems, were quietly turned off.

By around 5.15pm they were all back on line, but it is likely that few of the solar households affected even noticed the intervention, and certainly not their place in Australia’s grid history – for this was the first time that AEMO had intervened in such a way. But it certainly won’t be the last.

AEMO, the South Australia government and energy regulators have been working up to this moment for some time. It has long become obvious that rooftop solar – still growing at around 3,000MW a year across the country – has become Australia’s largest power station, but it’s a resource that had been beyond the bounds of network and operator control.

Sunday was the first time that this newly installed harness had been put to good use. Instead of summoning back-up generators or shedding load to ensure grid stability, the operators and the networks had created load – this time by turning off around 50MW of rooftop solar, out of the 1,000MW that was generating at the time in South Australia. It meant that those households affected would now draw their power from the grid.

This was achieved through two newly installed protocols that will soon become common in the rest of Australia. The first was the South Australia government’s “Smarter Homes” initiative that now requires all newly installed rooftop solar systems to have inverters with “ride through capability” and to sign up to an “agent” that will cut off the inverter, and the rooftop solar supply, when instructed.

On Sunday, around 10MW was removed that way, with the local distribution network South Australia Power Networks acting as the agent. Another 40MW of rooftop solar was cut off through SAPN’s new voltage management system at seven different sub stations, again deployed for the first time. A further 17MW of larger, ground mounted solar systems were also cut off for around one hour, although this particular mechanism has been deployed before.

Why was all this necessary? On Sunday, it was because AEMO forecast that “scheduled demand” – the demand that the grid sees – was likely to fall below 400MW. This was below AEMO’s level of comfort in the situation it faced when the main transmission link was undergoing repairs. In the end it fell to 358MW, but the instruction to the local network operator ensured that the 400MW level was quickly maintained.

By later this year, those minimum demand levels are forecast to fall as low as zero in South Australia, which has the highest penetration of rooftop solar in the country, as well as the highest level of “variable” renewables, which amount to more than 60 per cent in the last 12 months, a level unmatched in the world by a grid its size.

By 2024, the “minimum demand” in all Australian mainland states is expected to move from the traditional night time period to the middle of the day, as the amount of rooftop solar systems surges from around 13.8GW now to more than 20GW. And AEMO and others want to have the tools needed to ensure there is enough demand to keep the grid stable, either by switching off rooftop solar when needed, or creating a “solar sponge”, demand such as pool pumps and hot water systems, and eventually battery storage, more pumped hydro and even hydrogen.

It is expected that by the end of 2021, new inverter standards that will require ride-through capabilities and the appointment of ”agents” will come into force across the main grid. But South Australia couldn’t wait for the regulatory process to take effect, so with the help of AEMO and the local networks it fast-tracked the new rules, which came into effect last September and already give it a deployable resource of up to 140MW of rooftop solar. It says Sunday’s events justify that early move.

An SAPN spokesman said the loss of rooftop solar would likely not have been noticed by the households affected. It did not mean that they lost power. And he pointed out that the use of the Smarter Homes mechanism, and the voltage management system, meant that solar systems would be able to export more over the course of the year than they would otherwise. SAPN only expects the mechanisms to be deployed on a few days each year.

AEMO says the ability to curtail a range of large and residential solar customers, including those through the South Australian government’s ‘Smarter Homes’ initiatives, is vital to support the transition into a modern renewable power system. Exactly what level of grid demand will trigger its deployment in the future will depend on a range of factors such as the generation mix, weather forecasts, and any infrastructure outages.

“AEMO has been working with governments and market bodies to support the development of a range of measures to continue to facilitate greater amounts of renewable energy in the network and maximise consumer benefits,” a spokesperson said.

“Minimum demand events will become increasingly common throughout Australia as we continue to integrate rooftop PV installations.”

South Australia energy minister Dan van Holst Pellekaan said the expert advice was that the state needed to implement these new smart solar rules.

“We took that advice, and whilst initially there were some people who had concerns, Sunday shows the real world situation where it was needed,” he said in an emailed statement.

“We implemented AEMO’s recommendations as soon as possible, and in hindsight it’s clear that any delays could have been very harmful for consumers. AEMO, ElectraNet and SAPN should be congratulated for how effectively they have worked together and prepared.”

The premier also reaffirmed his desire to get final approvals for the proposed new transmission link to NSW, which will offer increased grid security and open the avenue to more renewable energy projects to be developed in the state.

“This weekend also showed why EnergyConnect is needed to increase our resilience with a second interconnector,” he said.