S5.2.5.13 is the clause of Schedule 5.2 that sets how a generating system controls voltage and reactive power at its connection point: which control modes it must have, how accurately it must regulate, and how quickly it must respond to a setpoint change or a change in grid voltage. For inverter-based plant the response is delivered by the power plant controller (PPC) and the inverters together, and the clause is effectively a tuning specification.
It is also one of the most commonly negotiated clauses, usually because a response that is fast at a strong connection point becomes slow or oscillatory at a weak one.
Control Modes
Under the automatic access standard a plant must be able to regulate voltage, as its primary control mode unless the NSP requires otherwise, and reactive power or power factor as a secondary mode. The minimum access standard requires regulation of voltage, or of reactive power or power factor with the agreement of AEMO and the NSP.
In practice almost every transmission-connected inverter plant runs in voltage droop control: the PPC adjusts reactive power in proportion to the difference between the measured and target voltage at the connection point, with a droop setting that shares the voltage regulation task with other plant and the network. A droop of a few percent on the plant's reactive capability base is common. Too little droop and the plant chases every small voltage change; too much and it contributes little.
The clause also requires that the plant does not cause instability, including hunting of tap-changing transformers, and that it does not degrade the damping of critical oscillation modes. For a voltage-controlled plant near an on-load tap changer, the interaction between the PPC's reactive response and the tap changer's deadband and time delay deserves a specific check.
Rise and Settling Times
The automatic access standard for asynchronous plant is set in Table S5.2.1 of the Rules.
| Quantity | Automatic access standard | Test condition |
|---|---|---|
| Reactive power rise time | Less than 3 seconds | Voltage disturbance of 2% to 5%, across typical to highest system impedance |
| Settling time, no limiter | Less than 5 seconds | 5% setpoint change or voltage disturbance, for voltage, reactive power and active power |
| Settling time, with limiter | Less than 7.5 seconds | As above, where the step reaches or starts at a limit |
| Minimum access standard settling time | Less than 7.5 seconds, or longer if agreed with the NSP | 5% voltage disturbance, in the relevant control mode |
Three details shape how the test is run:
- System impedance range. The NSP nominates a typical and a highest system impedance under paragraph (m). The highest impedance corresponds to the minimum fault level at the nearest system strength node combined with the worst single network outage. The plant must meet the times across that range.
- Limiters. A limiting device condition applies when half the specified step would just reach a limit, or when the plant is already at a limit. The longer settling time then applies.
- Rate limits. If the plant applies setpoint rate limits in normal operation, they must be recorded in the performance standard, and can be disabled for testing.
Earlier versions of the Rules set different values for some of these times, and older GPS documents will show them. Check which version of the access standards applies to your project before you tune to a number. Our article on Package 1 explains the transition.
The Study Matrix
A complete S5.2.5.13 study tests every mode the plant will use, at the strongest and weakest grid it must work in.
S5.2.5.13 Step Test Matrix
- Voltage control: positive and negative voltage setpoint steps (typically 5%)
- Grid voltage disturbance: positive and negative steps in the source voltage (typically 5%)
- Reactive power and power factor modes: positive and negative reference steps (typically 50% of the agreed range)
- Limiter tests: a small step away from a limit, settle, then a step into it
- Operating points at maximum and minimum active power (and full charge for a BESS), with reactive power at zero and at the capability limits
- Grid strength at maximum SCR and at the NSP's highest system impedance
- Repeat of key steps in wide-area network cases
The SMIB results establish the characteristic response. The wide-area cases confirm there is no interaction with nearby plant, tap changers or reactive devices.
Measuring the response
Reviewers re-derive rise and settling times from the plots, so the report should make it easy:
- annotate each plot with the step time, the 10% and 90% points used for rise time, and the settling band
- state the error band used for settling time
- state the measured quantity and its base
- zoom in where the response is small
- include steady-state error against the regulation accuracy (for example within 0.5% of the voltage setpoint)
A recurring argument is whether quantities a mode does not control should be judged for settling. For example, in reactive power mode, active power may wander slightly after a reactive step at zero output. Present it, explain it, and propose wording that reflects what the mode is meant to do.
Why Plants End Up Negotiating
The usual drivers of a negotiated S5.2.5.13 standard are:
- Weak grids. At high system impedance a fast reactive response causes a large voltage change, so the controller must be slower to stay stable. The rise time at minimum SCR is the commonest shortfall.
- Steps into limits. A small step into a reactive limit at minimum SCR can take several seconds to settle because the controller's anti-windup and limit logic dominate.
- Communication and sampling delays. The PPC measures at the connection point, computes, and sends setpoints to dozens of inverters over a network. Delays of a few hundred milliseconds add directly to the rise time and reduce the stability margin.
- No power oscillation damping capability. The automatic standard requires one with sufficient flexibility. Many inverter platforms offer it; some do not.
Reviewers push back hard on long times proposed without a narrative. A credible proposal identifies the specific cases that drive the number, lists the tuning steps taken, and shows that performance across the normal operating range is much better than the worst case. Re-tuning at a realistic minimum SCR, rather than at an artificially low one, often recovers most of the gap.
Partial Plant and Droop Scaling
When some inverters are out of service, the plant's reactive capability falls. If the PPC's droop is defined on the full-plant base, the effective gain per remaining inverter rises, which can destabilise the response or push units into limits. If it is defined on the in-service base, the plant's contribution to voltage regulation falls. Either way, the performance standard and the model documentation need to say how the droop base scales with units in service. This has delayed commissioning on more than one project.
Get Voltage Control Tuned Once
Voltage control tuning is iterative, and each iteration means re-running the study matrix. The fastest route is to agree the system impedance range with the NSP first, tune the PPC against the weakest case, and then confirm the strong-grid response.
ElectraGlobe tunes plant controllers, runs S5.2.5.13 studies in PSCAD and PSS/E, and supports the setpoint and step testing at commissioning. See our steady-state and dynamic studies service, and our article on S5.2.5.1 reactive power capability for the capability the controller works within.
Frequently Asked Questions
What are the rise and settling time requirements in S5.2.5.13?
For asynchronous plant under the current automatic access standard in Table S5.2.1, reactive power rise time must be less than 3 seconds for a 2% to 5% voltage disturbance, and settling time less than 5 seconds for a 5% setpoint change or voltage disturbance, or 7.5 seconds where a limiter operates. The minimum access standard settling time is 7.5 seconds or a longer period agreed with the NSP.
What control mode do solar farms use?
Most transmission-connected solar farms, wind farms and batteries operate in voltage droop control at the connection point as the primary mode, with reactive power or power factor control available as a secondary mode.
What is the highest system impedance?
It is the system impedance at the connection point that the NSP nominates as the weakest condition the plant must be tuned for. It normally corresponds to the minimum fault level at the nearest system strength node with the single network outage that most reduces the connection point fault level.
Why is voltage control slower on a weak grid?
At a weak connection point, a given change in reactive power causes a larger change in voltage, so the effective loop gain is higher. The controller has to be tuned more conservatively to stay stable, which lengthens the rise time.