Solar farms, wind farms and batteries in Australia’s National Electricity Market (NEM) meet the grid at a connection point. Four kinds of people answer for each one: the owner’s and operator’s engineers, the EPC (engineering, procurement and construction) contractor or connection consultant taking the plant to registration, the manager who carries its schedule, and the network service provider (NSP) engineer who assesses it.
When the voltage at that connection point starts to oscillate, their questions arrive in order. Is it real? What do we do? What will we be able to show afterwards?
The National Electricity Rules (NER) make detecting it and responding as agreed an obligation in clause S5.2.5.10, “Detection and response to unstable operation”, in force since 21 August 2025. None of the three questions is settled by installing a monitor. Each turns on what no purchase supplies: the settings, the order of actions, and a record that lines up in time with the NSP’s and the market operator’s.
Each is taken in turn below, with what VECTO, CT LAB’s synchronised measurement platform, does about it.
Why installing a monitor does not settle it
It would be simpler if S5.2.5.10 were a purchase: buy a monitor, tick the item, move on. Its history shows why it is not.
Before August 2025, S5.2.5.10 was a trip-only clause, “Protection to trip plant for unstable operation”.
From 13 September 2019, the Australian Energy Market Operator (AEMO) constrained five solar farms in the West Murray zone of north-west Victoria and south-west New South Wales to 50% output for about seven months. The curtailments, pv magazine Australia reported, “are estimated to have cost the combined solar farms $1 million a day in lost revenue”. Further oscillations followed in the same zone between 2020 and 2022.
AEMO opened its consultation on a guideline with an S5.2.5.10 Initial Summary Guide of 27 July 2022; no final guideline followed.
In 2025 the Australian Energy Market Commission (AEMC), which makes the rules, rewrote the clause. It said that uncertainty over the access standards had delayed multiple connection projects for asynchronous plant, and recorded: “Some events required manual disconnection of the plant contributing to the instability because there was no automatic system to eliminate the instability or disconnect the plant.” Its final determination of 22 May 2025 put detection and an agreed response at the centre.
A rewrite sets duties; it does not settle how to meet them. In July 2026, a report by Amplitude Consultants on protection in networks rich in inverter-based resources listed what was unclear: quantitative thresholds for instability, consistent verification methods, sufficient test evidence, and pass/fail criteria.
Not one item on that list is hardware. The obligation is a function the plant demonstrates, built into its production units or provided by added equipment. A monitor can support that function. It cannot settle the settings, the response or the record.
Detecting it
The first question on any project is what to look for, and on which settings. The clause answers the first and leaves the second to agreement.
Under the automatic access standard, the plant needs facilities to detect instability in voltage, reactive power and active power at the connection point, for its asynchronous production units; the minimum access standard asks for voltage, reactive power and, where relevant, active power. The clause names no frequency band, sampling rate or instrument class. The settings are agreed with the NSP and AEMO and recorded in the plant’s performance standards.
Agreement is where the arguments start. Responding to the AEMC’s draft rule in 2025, Transgrid wrote that “Instability can have broad definitions and this has led to numerous debates between stakeholders.”
Some sites have behaved in ways the studies did not predict, which is a reason to measure at the connection point. At the Broken Hill battery, Ekistica’s June 2025 summary for the Australian Renewable Energy Agency (ARENA) records reactive power oscillations during commissioning that simulations did not replicate.
VECTO detects at the instrument, on CT LAB’s edge-computing architecture: it monitors voltage, active power and reactive power continuously, detects oscillatory behaviour across configurable frequency bands, and measures oscillation magnitude and persistence. Detection, alarm and response thresholds are configurable, with hold-off, hysteresis and reset logic; a trigger needs the configured magnitude held for the configured time. The plant’s own engineers set all of it, locally or remotely through VECTO Grid OS™, to what the NSP and AEMO agree. The bands are on the Sub-Synchronous Oscillation Phasor Monitoring page.
Responding as agreed
Here the worry is the wrong trip: a plant disconnected, or disconnecting itself, for an oscillation it did not start.
The clause answers with an order. Under the automatic standard, on detection, the plant must promptly execute a hierarchy of actions, on trigger conditions, thresholds and timeframes agreed and recorded in the performance standards. The hierarchy must put eliminating the instability ahead of disconnecting the plant: runback or curtailment, for example, before any trip. Under the minimum standard the duty is a process to manage the instability promptly, agreed and recorded the same way.
The agreed response need not end in a trip. AGL’s December 2025 report to ARENA on its Liddell battery records that the plant’s instability monitoring system can raise alarms and initiate disconnection, but that it was agreed with Transgrid and AEMO, under a negotiated standard, that the plant would not be disconnected in response to such events. In Victoria, VicGrid expects the alarm enabled and the trip isolated until AEMO and VicGrid advise.
VECTO gives each agreed step a route to the plant: relay outputs for configured plant response, including runback, curtailment or trip; SCADA (supervisory control and data acquisition) and plant-control-system integration, over Modbus IP by default, with IEC 61850 and DNP3 as licensed options; and independent threshold levels for alarms and relay control.
Showing what happened
After an event, AEMO and the NSP ask what the plant did. The plant’s answer is only as good as its record.
West Murray shows why. Investigating the oscillations there, AEMO wrote of the data it received from registered generators: “Due to the absence of timestamped, synchronised high-speed measurement (HSM) data, no conclusions could be drawn regarding the root cause of, or individual contributions to, the oscillations”. A plant’s own account of what it did rests on its own record, aligned in time.
Even with that record, whose oscillation it was is not the plant’s question to settle: the clause leaves the contribution determination to AEMO. The plant supplies its side of the account.
For that side, VECTO records events and waveforms with time synchronisation less than 100 ns from absolute time, ± 10 ns typical, taken from dual-band GNSS (global navigation satellite systems) or IEEE 1588 Precision Time Protocol. An event triggers high-speed root mean square (RMS) and synchronised waveform capture, with pre- and post-event data. With Xross Trigger™, a local event on one VECTO 3 triggers event snapshots on geographically distant VECTO 3 meters. VECTO Grid OS™ provides centralised monitoring, event analysis and reporting, from a single analyser to a fleet.
One platform for the plant’s other measurement
The same platform carries the plant’s other measurement jobs, on one time base across instruments. The VECTO 3 is itself an IEC 61000-4-30 Edition 4.0 Class A power quality analyser, and licensed firmware options add frequency control ancillary services (FCAS) verification through Contingency FCAS Metering, which is AEMO FCAS verification compatible, and synchrophasor measurement through PMU+. Not every option runs on every device at once.
Measured in the NEM
A solar plant in the West Murray zone used VECTO, and AEMO used its recordings in its analysis of the further oscillations. CT LAB’s analysers were measuring at Wemen Terminal Station from October 2021, on a GPS-synchronised 50 kHz waveform stream, and by April 2022 were at three sites. CT LAB’s case study rests on one terminal station and says the pilot was too small in scope to investigate how the oscillation propagated. It is a record of measurement, not of a detection-and-response scheme.
The clause in brief
The engineer who has to act needs the clause and the plant’s own performance standards.
- Where it applies. The NEM’s five regions: Queensland, New South Wales (which includes the Australian Capital Territory), Victoria, Tasmania and South Australia. It does not reach New Zealand. It binds generating, integrated resource and synchronous condenser systems under schedule 5.2. The text is in the AEMC’s National Electricity Rules.
- Which standard binds. The one the connection agreement records: automatic, negotiated or minimum. A negotiated standard is taken to be the performance standard applicable to the plant (NER 5.3.4A(i)).
- The automatic standard, for asynchronous production units: facilities to detect instability in voltage, reactive power and active power at the connection point; facilities capable of disconnecting units, on settings agreed with the NSP and AEMO; and a prompt hierarchy of actions on agreed trigger conditions, thresholds and timeframes, recorded in the performance standards.
- At 100 MW of active power capability or more (or 100 MVA of synchronous condensers): access to a phasor measurement unit (PMU) that can send data to AEMO and the NSP, and the capability to receive AEMO’s information on the system’s contribution to instability, when available. Under the minimum standard, both apply only if required.
- The minimum standard, for a plant that can change the voltage at its connection point by more than 1% from the voltage with the plant not connected: detection in voltage, reactive power and, where relevant, active power, and, for asynchronous units, a process to manage instability promptly, agreed and recorded the same way.
- Paragraphs (c) to (f). Elimination of the instability before disconnection (c); regard to AEMO’s Power System Stability Guidelines (d); if required, detection information to the NSP’s and AEMO’s control centres (e), and a remote tripping signal from the NSP (f).
- An operating plant. Rule 4.15 requires it to meet its recorded performance standard, monitor its performance under a compliance program, keep records for seven years, and notify AEMO immediately of an actual or likely breach. An alteration reopens the impacted performance standards under NER 5.3.9.
- R1. The R1 assessment of pre-connection data runs between the connection agreement and registration. AEMO’s R1 Capability Assessment Guideline asks how the required instability detection and protection are implemented, with original equipment manufacturer data sheets on stability monitors or relays as one example of evidence.
- VicGrid. In Victoria, VicGrid’s Oscillation Detection System settings and functionality guidelines set general expectations, negotiable per project, including a rolling two-month record and commissioning tests; they are not NER obligations.
- The penalty. Rule 4.15(a) is a tier 1 civil penalty provision. For a body corporate, the maximum is the amount the Australian Energy Regulator (AER) publishes, or three times the benefit where a court can determine it, or 10% of annual turnover where it cannot. A breach of a plant’s S5.2.5.10 performance standard reaches 4.15(a) on a reading of the two clauses together.
For the NSP engineer assessing a scheme
The sources point to three things; what is enough on a given plant is the NSP’s decision, with AEMO.
- Settings in the terms that apply: as agreed and recorded under the clause, and in Victoria against VicGrid’s guideline.
- The alarm handled apart from the trip: VicGrid’s expectation; the negotiated standard at AGL’s Liddell battery.
- A commissioning test plan: end-to-end injection tests on site, and injected signals that demonstrate appropriate triggering of alarms (Connections & Power Systems Advisory and Energy Queensland, in AEMO’s 2022 consultation on the previous clause).
Questions
Must the plant trip, or can an alarm be the agreed response?
Under the automatic standard the plant must be capable of disconnecting units, but its enablement conditions are agreed with the NSP and AEMO, and the hierarchy puts eliminating the instability first. AGL’s Liddell battery runs alarm without disconnection under a negotiated standard. See Responding as agreed.
Who sets the thresholds, delays and bands?
They are agreed with the NSP and AEMO and recorded in the plant’s performance standards. On VECTO, the plant’s own engineers enter the agreed values, locally or remotely through VECTO Grid OS™. See Detecting it.
What binds a plant that is already registered?
Its recorded performance standard: rule 4.15 requires an operating plant to meet it, and an alteration reopens the impacted standards under NER 5.3.9. See The clause in brief.
What does R1 ask for, and can the scheme hold up registration?
AEMO’s guideline asks how the required instability detection and protection are implemented, and AEMO and the NSP may require confirmation of installation. One of its example findings is a plant that will not have implemented the means to detect instability by the time of registration. See The clause in brief.
A conversation about your project
If you are working through S5.2.5.10 on a plant in the NEM, CT LAB offers a project-specific assessment, and it starts with a conversation with Michael Guy.
