S5.2.5.1 is the clause of Schedule 5.2 of the National Electricity Rules that sets how much reactive power a generating system must be able to supply and absorb at its connection point. The automatic access standard is a reactive power of at least 0.395 times maximum active power, in both directions, at any active power level, across a defined voltage range. The minimum access standard requires no reactive capability at all. Almost every inverter-based project negotiates somewhere between the two, and the negotiation is won or lost on how carefully the capability has been calculated.
The trap is that 0.395 Pmax is required at the connection point, not at the inverter terminals. Everything between the two either consumes or produces reactive power, and the difference is rarely small.
What the Clause Requires
The current drafting, after the AEMC's 2025 access standards rule (Package 1), describes the automatic standard as a voltage against reactive power envelope rather than a single number.
| Connection point voltage | Supply (capacitive) | Absorb (inductive) |
|---|---|---|
| Mid-point voltage ±5% | At least 0.395 x Pmax | At least 0.395 x Pmax |
| Above that band, up to the normal upper limit | Tapers linearly from 0.395 x Pmax to zero | At least 0.395 x Pmax |
| Below that band, down to the normal lower limit | At least 0.395 x Pmax | Tapers linearly from 0.395 x Pmax to zero |
The mid-point voltage is set by the network service provider (NSP) for the connection point, somewhere between 95% and 105% of nominal. That choice matters. A mid-point at 102% pushes the full-capability band up, which is harder for the absorbing side of an inverter plant at high voltage.
Three other parts of the clause catch projects out:
- The plant must deliver the capability at any active power level, including full output. Inverters run out of current headroom at full output and high ambient temperature, so this is usually the binding corner.
- Plant that is connected but not generating must not shift the connection point voltage (automatic), or not by more than 1% (minimum), at the highest system impedance the NSP nominates. For a solar farm at night or a battery on standby, that means compensating the charging current of long cable runs.
- The performance standard must record the active power capability, any temperature derating, the agreed reactive capability, and any extra capability above the standard. Extra capability is recorded because AEMO and the NSP may use it for network support.
Where a plant cannot meet the level a negotiated standard needs, the rule lists the options: pay the NSP to provide the shortfall, install extra equipment (which then counts as part of the plant), buy the service from another participant, or agree operational arrangements for the conditions where the shortfall occurs.
The Loss Chain From Inverter to Connection Point
A capability study works from the inverter terminals outward. Each element changes the reactive power that arrives at the connection point.
| Element | Effect on reactive power at the connection point | What drives it |
|---|---|---|
| Inverter or PCS | Source of capability, limited by apparent power and current | DC voltage, ambient temperature, firmware limits, terminal voltage |
| Inverter step-up transformers | Absorb reactive power in proportion to current squared | Leakage reactance, number of units in service |
| MV collector cables | Produce reactive power (charging) and absorb a little through series reactance | Cable length and cross-section, MV voltage |
| Main transformer | Absorbs reactive power, often the largest single loss at full output | Impedance, tap position |
| HV line or cable to the connection point | Produces or absorbs depending on loading and length | Line length, conductor, voltage |
| Auxiliary load and filters | Filters produce reactive power; auxiliaries absorb a small amount | Harmonic filter size, cooling load |
The pattern is consistent. At full output the series reactances of the transformers absorb a lot of reactive power, so the plant's capacitive capability at the connection point shrinks. At low output and at night the cable charging dominates, so the plant appears capacitive and its inductive capability is used up absorbing its own charging.
Voltage at the inverter terminals
The point that most simple calculations miss is that terminal voltage moves. Supplying reactive power raises the voltage inside the plant, and absorbing it lowers the voltage. Inverters have upper and lower terminal voltage limits, and many reduce their reactive capability as they approach them. A plant can have enough current headroom on paper and still be unable to deliver full capacitive output at high grid voltage, because the inverter terminals hit their upper limit first.
This is why the study is a load flow, not a spreadsheet. Each operating point is solved with the real transformer taps, the real cable data and the inverter's voltage-dependent capability curve.
Temperature and DC voltage
Inverter capability usually falls with ambient temperature above a threshold. Solar inverters can also lose reactive capability at low DC voltage, typically on hot afternoons. A battery's PCS behaves differently in charge and discharge. The capability study should state the ambient temperature it assumes, and the performance standard records any derating the NSP and AEMO accept.
How the Capability Is Studied
A defensible S5.2.5.1 study covers a grid of operating points rather than a single case.
S5.2.5.1 Study Matrix
- Active power from zero (and full charge for a BESS) to Pmax in steps
- Connection point voltage across the full required range, including the mid-point ±5% band and the tapered regions
- Maximum supply and maximum absorption at each point
- Main transformer tap ranges, with the on-load tap changer allowed to act where the control philosophy uses it
- Ambient temperature at the design maximum, and at the derating breakpoints if any
- Partial plant cases: units or feeders out of service
- Night-time or standby case with all inverters connected but not producing
The results are presented as a PQ chart (active power against reactive power at the connection point) and a VQ chart (connection point voltage against reactive power). AEMO and many NSPs provide a template for these charts, with the required envelope drawn in, and reviewers expect the plant's capability to be plotted against it. If the plant falls short in a corner, the chart should show exactly where, and the proposed negotiated standard should describe the shortfall precisely, for example as a function of active power.
Partial plant
The clause allows the capability to scale with the number of units in service, provided each unit still performs at its terminals as it would with the whole plant running. In practice reviewers check two things: that the per-unit performance really is the same, and that the plant's own fixed elements, such as cable charging and filters, do not leave the connection point voltage outside limits when most of the plant is offline. If they do, the NSP or AEMO may require operational arrangements in the performance standard.
Closing a Shortfall
When the inverters alone cannot meet the target, there are four engineering options. Each has a cost and a control consequence.
| Option | Typical use | Watch out for |
|---|---|---|
| Oversize the inverter fleet (more kVA per MW) | Small capacitive shortfall at full output | Cost scales with the whole plant; check terminal voltage limits still allow it |
| Switched capacitor banks | Steady capacitive shortfall | Switching steps cause voltage changes; harmonic resonance with the network |
| Shunt reactors | Absorbing shortfall at night from cable charging | Reactors in service when not needed reduce daytime capacitive capability |
| STATCOM or SVC | Fast, continuous capability, often for weak grids | Becomes part of the plant: modelled, tested and under the same performance standards |
Any reactive plant used to meet S5.2.5.1 is deemed part of the generating system. It has to be in the PSS/E and PSCAD models, it is covered by the voltage control standard in S5.2.5.13, and it is tested at commissioning like the inverters. A capacitor bank added late in the design to fix a capability gap can open a new set of harmonic and control questions, so it is far cheaper to settle the capability early.
Documenting It in the GPS
The S5.2.5.1 section of a GPS proposal should leave the reviewer nothing to reconstruct:
- the active power capability and maximum temperature for operation
- the agreed reactive supply and absorption at the connection point, as a function of active power and voltage where needed
- any temperature derating, and how it is calculated
- the night-time or standby reactive compensation arrangement, in MVAr, and how it is controlled
- any additional capability above the agreed standard
- for a negotiated standard, the reason the automatic standard cannot reasonably be met and evidence that the proposed level still meets the system standards
Our guide to automatic, minimum and negotiated access standards explains how that justification is tested, and S5.2.5.13 voltage and reactive power control covers how the capability is controlled once it exists.
Get Your Reactive Capability Right First Time
Reactive capability decides inverter sizing, compensation plant and the voltage control design, so it is worth settling before procurement rather than after the first AEMO review.
ElectraGlobe builds connection point PQ and VQ capability studies, sizes compensation, and prepares the S5.2.5.1 sections of GPS proposals for solar, wind and BESS projects. See our AEMO GPS applications service, or contact us to review your plant's capability.
Frequently Asked Questions
What does 0.395 Pmax mean?
It is the automatic access standard for reactive power under S5.2.5.1. A plant must be able to supply and absorb reactive power of at least 0.395 times its maximum active power at the connection point, at any active power level, across a voltage band set around the NSP's mid-point voltage. The figure corresponds to a power factor of about 0.93 at full output.
Is reactive power measured at the inverter or the connection point?
At the connection point. Transformers, cables, lines and filters between the inverters and the connection point all add or consume reactive power, so the inverters normally need more capability than 0.395 Pmax to deliver it at the connection point.
Why does a solar farm need reactive compensation at night?
MV and HV cables produce reactive power whenever they are energised. With the inverters not producing, that charging current can raise the connection point voltage. S5.2.5.1 requires a connected but idle plant not to change the connection point voltage (or not by more than 1% under the minimum standard), so the plant needs inverters that provide reactive power at night, or a shunt reactor.
Can a plant negotiate below 0.395 Pmax?
Yes. The minimum access standard requires no capability, and a negotiated standard can sit anywhere above that, provided the NSP and AEMO are satisfied that the system standards are still met. If they are not, the applicant must make up the shortfall through extra equipment, a payment to the NSP, a commercial arrangement or agreed operational arrangements.