By Phil Kreveld
Emergency backstop seems an ill-chosen term for constraining the power output from rooftop solar photovoltaic systems but what ‘emergency’ could be contemplated?
The Clean Energy Council appears to be okay with it—a shutting down of power export from solar inverters in case of an ‘emergency’, which is whatever the Australian Energy Market Operator (AEMO) proclaims it to be—as in Alice in Wonderland.
The punters are ‘assured’ that will happen rarely. But which is it—reducing power export or switch off? In essence, if AEMO has difficulty in maintaining voltage stability in the transmission grid, it will order distribution networks to remotely control (reduce) power export from rooftop solar PV, or demand a switch-off.
Related article: How much rooftop solar capacity can Australia handle?
AEMO’s big headaches
Unloaded transmission lines exhibit voltage rises beyond permissible values, occurring when solar PV not only takes care of native demand, but attempts to export the power excess. The easy way out is to make sure there is always minimum power flow in the transmission lines.
The alternative is investment in remotely-controllable reactive load banks and synchronous condensers. With rooftop solar PV capacity equalling large-scale-wind, solar, gas, hydro and coal-fired generation, ‘emergency backstop’ is likely to be a ‘business as usual’ feature in distribution grids.
Let’s capitulate the essential things required by AEMO—and distribution networks:
- No net power outflow (into the transmission grids).
- Minimum inflow into distribution networks to allow AEMO to operate the grid stably.
- Medium and low voltage levels in distribution to vary within permissible limits.
- Transformer ratings not to be exceeded.
- Voltage regulation equipment (on load tap changers, off line tap changing) to function properly without requiring re-engineering and CAPEX).
Rooftop solar needs ‘parenting’
As has been mentioned on many occasions, the growth in rooftop solar PV capacity is getting in the way of the ‘big end of town’ generation and transmission.
“Proponents say if households switched from gas to electricity for cooking and heating, and adopted rooftop solar, electric vehicles and home batteries, it would save them money and reduce the number of new power plants and transmission lines required to transition Australia’s grid to clean sources of energy.” — Bowen pans [Sen.] Bragg’s stalled report (AFR 18/02/2025).
Former Clean Energy Council chief executive Matthew Warren, at last year’s Australian Clean Energy Summit, in panel conversation with Australian Energy Regulator chair Clare Savage and independent energy economist David Leitch, opined that ‘rooftop solar needed parenting’.
Whoever Warren had in mind, it would appear that AEMO acts in ‘loco parentis’. Without going into detailed analysis, AEMO’s task of providing voltage and frequency stability is hugely challenging—and it has not been worked out if ‘everything were to be renewable’. To make the task less herculean, only one-way power (in)flow into distribution networks will be tolerated—and it has to be above some minimum level. One can expect that the minimum level will be redefined upwards as more transmission and large-scale generation enter the electricity market.
Switching off solar is political dynamite—enter dynamic operation
To keep rooftop solar operating as much as possible (there is a political price to pay if there is too much interference with its operation), dynamic operating envelopes (DOE) are being developed. However, as fancy as the term sounds, it is no failsafe against ‘emergency backstop’. South Australia, Victoria and New South Wales use CSPA protocol (Common Smart Inverter Profile-Australia) based on IEC 2030.5 communication protocol to control inverters.
The control strategy is based on AS 4755, which lists as control states, in addition to switch-off, watt-voltage, and reactive power (volt-ampR-voltage) modes in order to maintain voltage control at the points of connection of inverters. This is not DOE and furthermore, the bulk of inverters, as yet, do not comply with the comms protocol requirement nor is it universally applied across Australia!
Question marks on network documentation—yet another challenge
For inverter control to work effectively, distribution networks would have to have complete documentation including not only the complete ‘system wiring diagram’ but also the electrical impedances between all the connection points. Not one distribution network has that sufficiently documented. Therefore, DOE is based on a number of protocols which hopefully do away with having the complete network picture.
Dynamic operating schemes, reaching down to the low voltage ‘edge of network’ harbouring the inverter multitude, are either an interesting academic research project, for example based on machine learning or they are designed for simplicity—and conservative principles. Emergency backstop fits in with the latter concept—i.e., when things go pear-shaped, switch off inverters. Note: recent changes to AS/NZS 4777.1 allow for change-over switches so that an inverter can supply a domestic load if switched off from the distribution network.
Related article: Experts say AEMO’s ‘solar switch-off’ should be a last resort
Are we kicking the can down the road?
The background to emergency backstop and elaborate schemes such as DOE based on machine learning is a lack of appetite to grasp the nettle: the increasing energy independence of distribution networks! Yes, it does provide ammunition for complicated control schemes, beautiful in concept but hindered by non-uniformity of inverters. It avoids an inconvenient truth—the increasingly non-commercial basis of new transmission and large-scale generation development as energy independence grows and grows aided by household and commercial battery storage.
South Australian Power Networks’ scheme is an example based on Melbourne University research in machine learning. In principle individual householder and business smart meters providing voltage, voltage angle and current, act as perceptrons, and distribution transformers as the output layer. Backward propagation might reveal unique minima, but as networks grow the learning exercise keeps on having to be repeated. Smart meter data sets would provide the basis for correlating with power flow and terminal voltage data of transformers (‘learning’).
That can then lead to algorithm development whereby, based on transformer voltage and power measurements, inverters can have their output power output controlled. At present PV panels are permitted to have a rating of 30% more than the inverter rating. Given the emergency backstop initiative, expect that excess to be reduced over time. Also, given the slow roll-out of smart meters, emergency backstop is likely to be exercised more and more frequently as growth in large-scale generation, encouraged by the Commonwealth Government’s Capacity Investment Scheme proceeds apace.
