SA Power Networks will be able to switch off new rooftop solar systems beginning on September 28 as South Australia wrestles with soaring PV production. Around 270,000 electricity customers in the state have rooftop solar, adding up to some 1.5 gigawatts of capacity.

The electricity distribution supplier will disconnect systems as a last resort if other measures fail to stabilize the grid during periods of peak solar generation.

Also beginning on Sept. 28, new solar systems in the state will be required to have inverters with voltage ride-through capabilities, in order to avoid the need for disconnection.

The requirements are the latest in a growing range of measures aimed at maintaining grid stability as South Australia moves toward a target of net 100 percent renewables by 2030.

Last month, SA Power Networks announced it was upgrading the voltage management systems across approximately 140 major substations, serving 790,000 of South Australia’s 900,000 electricity customers.

And in July, the distribution system operator introduced a "solar sponge" tariff to encourage electricity use during the midday residential PV production peak.

Under the tariff, SA Power Networks cuts network charges to a quarter of its normal rates, which account for around 30 percent of the final amount charged to customers by energy retailers in the state. Customers must have a smart meter installed in order to take advantage of the offer.

South Australia already has one of the highest levels of rooftop solar penetration in the world and is adding new capacity at a rate of more than 200 MW a year, according to the Australian Energy Market Operator (AEMO).

Solar customers in the dark

The preponderance of rooftop PV on the grid has raised concerns of a blackout risk since distributed solar inverters disconnect from the grid in response to voltage drops.

A May 2020 study by AEMO warned: “A severe but credible fault near the Adelaide metropolitan area could cause disconnection of up to half the distributed PV in the South Australian region.”

By the end of this year, such an event could lead to the uncontrolled loss of up to 400 megawatts from the grid, the report said.

This scenario seems perilously close to what happened in 2016 when a storm knocked out 445 MW of South Australian wind capacity and caused demand to switch to the main interconnector between the state and the rest of Australia’s National Electricity Market.

That surge overloaded the Heywood interconnector, which shut down and islanded the South Australian grid, leaving almost the entirety of the state in the dark.

“When South Australia is operating as an island, it is now almost impossible to maintain the frequency operating standard for certain credible fault events if they cause distributed PV disconnections,” said AEMO in May. “This means that AEMO may no longer have the means to operate a South Australian island securely at times of high distributed PV generation, and as such, mitigation actions are required urgently.”

A further problem for AEMO is that if South Australia’s grid is islanded, then there has to be enough demand on it to keep synchronous generating units online, since these provide vital system strength, inertia, frequency control and voltage management.

Solar sinks demand

But with so many households consuming rooftop solar, the operational demand on the grid can drop way below the minimum 550 MW that AEMO considers is needed.

“Demand on our network can be as low as 200 MW,” said Paul Roberts, manager of corporate affairs at SA Power Networks, in an email to GTM. 

“AEMO has highlighted [that] this situation in the middle of the day is particularly problematic for managing energy security on a weekend spring day if the South Australia energy grid is separated from the [National Electricity Market] due to the loss of the interconnector with Victoria,” he said.

“It is also problematic for AEMO, and us, in that we currently have no real-time visibility of what solar is doing in the low-voltage network, nor any ability to manage it.”

Despite the challenges, Roberts said SA Power Networks is “excited to support” the state government’s renewables target.

“We see our distribution network as being central to the achievement of this transition, and we have a goal to double the amount of solar we can accommodate on our distribution network by 2025,” he said.

However, it remains to be seen how much more solar can be squeezed onto the grid using the measures at hand.

“South Australia, next to probably Hawaii, has the highest penetration of solar PV anywhere in the world,” observed Darren Miller, CEO of the Australian Renewable Energy Agency, in an interview in July.

“The distribution system is now having to accommodate excess exports from solar during the middle of the day, which is bringing into reality the problem that solar is great, but if everyone has it, and you’re not consuming energy, then you’ve got a mismatch there that needs to be solved.”