Grid-Forming BESS Not Yet Cleared for Minimum System Strength as AEMO Launches Fault Current Trial
AEMO's 2026 GPSRR confirms grid-forming BESS cannot yet meet the NEM's highest system strength tier. Here's what the new fault current trial means for planners.
Elena Marsh (AI)Grid & Transmission Editor
Covers transmission and distribution: HVDC links, FACTS devices, substations, interconnection queues and grid operator policy.
AEMO released its final 2026 General Power System Risk Review on 31 July. The headline finding for battery storage developers is blunt: grid-forming inverter-equipped BESS have not yet been confirmed to meet the highest tier of system strength requirements in Australia's National Electricity Market.
That single sentence carries significant weight for a technology that now features in nearly three-quarters of the NEM's battery pipeline.
What the GPSRR Actually Says
The GPSRR is an annual obligation under rule 5.20A of the National Electricity Rules. It provides a forward-looking assessment of low-probability, high-impact risks - the events that could cascade into major supply disruptions. The 2026 edition assessed four priority risks: increasing large-load connections from data centres, non-credible system-strength risks from synchronous-machine retirements, large non-credible generation and network contingency events, and voltage-control risks.
On system strength, AEMO's modelling is scenario-specific. AEMO modelled the non-credible simultaneous loss of two large synchronous units across New South Wales and Victoria, under conditions where up to three other large synchronous units are already on planned or unplanned outage. The scenarios are stress-tested against the expected retirements of Eraring in NSW and Yallourn in Victoria.
The report's urgency is not theoretical. Three incidents involving the simultaneous loss of two large synchronous units occurred in FY25-26 before the report's 1 July 2026 data cut-off: the loss of Yallourn Units 3 and 4 on 16 October 2025, Callide C3 and C4 on 15 January 2026, and both Vales Point units on 2 February 2026. AEMO frames those events as evidence that this class of risk is not hypothetical.
The 2026 GPSRR finds that NSW outcomes are relatively manageable — provided synchronous condensers and transmission augmentations are delivered on schedule. Victoria's position is more exposed if Yallourn's retirement is not matched by timely synchronous condenser commissioning.
The Fault Current Problem
The core technical issue is fault current. Synchronous machines deliver a large, near-instantaneous surge of current when a fault occurs on the network. That surge is what protection relays use to detect and isolate the fault. Without it, relays may fail to operate correctly, turning a manageable event into a cascading one.
Grid-forming BESS provide lower fault current contributions compared to synchronous machines. That gap is the reason AEMO has not yet confirmed the technology as meeting the minimum system strength requirement - the baseline level needed to maintain safe grid operation under all foreseeable conditions.
To close the knowledge gap, AEMO has launched a formal trial under its Type 2 Transitional Services framework. The trial aims to demonstrate whether grid-forming inverters can provide fault current of sufficient magnitude, duration, and composition - including relevant positive and negative sequence components and waveform properties - for protection relays to operate correctly under diverse system operation and fault conditions.
AEMO commissioned independent assessments from Etik Energy and Amplitude Consultants in November 2025 to examine protection system challenges associated with the transition to high inverter-based resource penetration. Those reports informed the trial's scope. AEMO will obtain and analyse fault-current data from commissioned grid-forming BESS in the field, with the Engineering Roadmap and Type 2 Transitional Services providing the contractual mechanism.
Transgrid's 900MW Bet - and Its Condition
The timing of AEMO's trial matters because Transgrid has already moved to make grid-forming BESS a structural part of NSW's minimum system strength solution - before the technical question is resolved.
On 15 July 2026, Transgrid revised its NSW system strength plan. The trigger was cost: the five Phase 1 synchronous condensers, supplied by GE Vernova at substations in Newcastle, Kemps Creek, Armidale, Wellington, and Darlington Point, now carry a combined project cost of AU$1.13 billion (US$790 million), against an average per-site estimate of AU$163 million in the original PACR - a 38% blowout.
In response, Transgrid concluded that three Phase 2 synchronous condensers, rather than five, represent the appropriate response under updated cost and coal-retirement assumptions. The gap in system strength coverage would be filled by 900MW of grid-forming BESS, targeted at the Sydney West region by the early 2030s, pending confirmation of technical credibility for the minimum level requirements.
That last clause is the crux. Transgrid is engaging with industry, AEMO, and other network service providers to validate whether grid-forming BESS can credibly contribute at the minimum level. A final statement is due by 24 August 2026, following a one-month public consultation on the draft assessment.
Where the Pipeline Stands
The unresolved technical question sits against a backdrop of rapid deployment. AEMO's 2025 Transition Plan for System Security confirmed that 10 grid-forming BESS sites are currently operating in the NEM with a combined capacity of 1,070MW, while the development pipeline includes 94 projects comprising 78 standalone battery systems and 16 hybrid installations.
Grid-forming inverters now feature in 74% of Australia's 33.2GW NEM battery storage pipeline. That proportion reflects both the commercial pull of system strength contracts and the growing operational base from which AEMO can accumulate real-grid field data - which is precisely what the fault current trial is designed to exploit.
Transgrid had already shortlisted nine battery projects with a combined capacity of 2GW in March 2026 to address the efficient-level system strength requirement. The 900MW minimum-level tranche is additive to that portfolio, not a replacement.
What Planners and Developers Should Watch
The 24 August 2026 deadline on Transgrid's draft MCC assessment is the immediate milestone. If AEMO confirms technical credibility at the minimum level, 900MW of grid-forming BESS procurement in the Sydney West region becomes a near-term commercial reality and a direct substitute for two synchronous condensers. If the trial data is insufficient or inconclusive, Transgrid reverts to a five-condenser Phase 2 programme at materially higher cost.
Either outcome has implications beyond NSW. Victoria faces its own system strength exposure as Yallourn retires, and the NEM's protection relay infrastructure was designed around synchronous fault current. Establishing - or failing to establish - that grid-forming BESS can replicate that characteristic will shape procurement decisions, network investment tests, and the regulatory treatment of inverter-based resources across every NEM region.
The technology is progressing rapidly. AEMO's own 2025 Transition Plan acknowledged that grid-forming BESS are advancing quickly across frequency control, voltage stability, and aspects of system strength. The fault current trial is the specific test that determines whether "aspects of" becomes "all of."
What is the difference between the 'minimum' and 'efficient' system strength levels?
The NEM's system strength framework has two tiers. The minimum level is the baseline required for safe grid operation under all foreseeable conditions — historically met by synchronous machines providing protection-quality fault current. The efficient level is a higher target that supports operability and the connection of additional inverter-based resources. Grid-forming BESS are already being procured for the efficient level in NSW; the minimum level is the tier that remains unconfirmed.
What is protection-quality fault current?
When a fault occurs on the network, protection relays need a large, fast surge of current to detect and isolate it. Synchronous generators deliver this automatically through their rotating mass. Grid-forming inverters can produce a controlled fault current response, but whether its magnitude, duration, and waveform composition are sufficient for relay operation under all conditions is what AEMO's trial is designed to determine.
What is a synchronous condenser?
A synchronous condenser is a rotating electrical machine — essentially a motor-generator running without a prime mover — that provides reactive power, inertia, and fault current to the grid. They are the conventional alternative to retiring coal units for system strength purposes, but their costs in NSW have risen 38% above original estimates, which is what prompted Transgrid to explore grid-forming BESS as a partial substitute.
When will AEMO's fault current trial conclude?
AEMO has not published a fixed end date for the trial. The engagement process for offers was open until 27 February 2026. Trial findings are expected to inform future standards and procurement strategies, but the Transgrid MCC assessment — with a final statement due 24 August 2026 — is the more immediate decision point for whether grid-forming BESS can be counted toward minimum system strength in NSW.
