S5.2.5.11 is the clause of Schedule 5.2 that sets a generating system's capability to change its active power in response to power system frequency. The automatic access standard requires a plant to be capable of operating in frequency response mode, automatically reducing output when frequency rises and increasing it when frequency falls, proportionally and fast enough to offer frequency control ancillary services (FCAS). The plant must be able to set its deadband anywhere from 0 to ±1.0 Hz and its droop anywhere from 2% to 10%.
The clause sets what the plant can do. How it must actually run day to day is set separately, by AEMO's Primary Frequency Response Requirements (PFRR) under clause 4.4.2(c1) of the Rules. Projects that tune to one and forget the other get caught at commissioning.
Droop and Deadband in Plain Terms
Droop is the frequency change, as a percentage of 50 Hz, that would move the plant's output by 100% of its active power capability. A 5% droop means a 2.5 Hz change would swing the plant across its full range. A 2% droop means a 1 Hz change would.
Deadband is the band around 50 Hz inside which the plant does not respond. Outside the deadband, the response follows the droop line until the plant reaches a limit: its maximum output, its minimum operating level, or the energy actually available.
| Setting | S5.2.5.11 capability range | Typical operational setting under the PFRR |
|---|---|---|
| Deadband | 0 to ±1.0 Hz, independently for rise and fall | Narrow, around ±0.015 Hz |
| Droop | 2% to 10%, or other agreed settings | No more than 5%, unless an exemption is agreed |
| Response delay | No delay beyond that needed for stable operation or inherent in the controls | As per the capability |
The Rules define droop at the connection point, measured outside the deadband and within the plant's available power transfer, considering the units in operation. If the NSP agrees droop can reasonably be controlled at the unit terminals, the performance standard must record how it is calculated.
What the Minimum Standard Allows
The minimum access standard is much looser. It requires that output does not move the wrong way by more than 2% per Hz, and that the plant can operate in frequency response mode with a change in active power that is either proportional or as agreed. Very few new utility-scale plants propose the minimum, because the PFRR obligations apply to scheduled and semi-scheduled plant regardless.
Solar, Wind and Batteries Respond Differently
Solar and wind can only increase output for under-frequency if they are operating below the power available from the sun or wind. Normally they run at maximum available power, so their under-frequency response is limited to what headroom exists. Over-frequency response, reducing output, is always available. The Rules recognise this: the response is subject to energy source availability.
Batteries can respond in both directions across their full range, and the integrated resource system drafting requires a smooth transition between charging and discharging. A battery charging at full rate during high frequency should reduce charging smoothly through zero and into discharge as frequency falls, without a step or a pause at the crossover. That transition is worth testing specifically in simulation and at commissioning.
How the Clause Is Studied
The standard approach injects frequency disturbances at the network source in a single-machine infinite-bus model, with the grid impedance included, and measures the active power response at the connection point.
S5.2.5.11 Study Cases
- Frequency steps and ramps above and below 50 Hz, large enough to reach the droop limit and small enough to sit inside the deadband
- Ramps at realistic rates of change of frequency, as well as steps
- Operating points at full output, part output and, for a BESS, full charge and idle
- Grid strength at maximum and minimum SCR
- For a BESS, a ramp through the charge to discharge transition
- Settings at the proposed operational deadband and droop, and at the extremes of the capability range
Present the droop calculation explicitly: frequency deviation beyond the deadband, the resulting change in active power, and the droop that implies. Reviewers re-derive it, and measured droop that differs from the setting is a common finding, both in simulation and on site. The difference usually comes from the base used (installed capacity versus available capacity versus connection point export), losses between the inverters and the connection point, or a PPC that calculates droop on one base and reports it on another.
The response must be adequately damped. Plot enough of the run to show the response settling, not just the initial movement.
Partial load rejection
The neighbouring clause S5.2.5.7 requires the plant to stay connected through a partial load rejection. For inverter plant it is usually studied by ramping frequency high enough that the droop response cuts output by the required fraction, and confirming the plant stays connected and stable. The frequency needed is simply the deadband plus the droop multiplied by the fraction and by 50 Hz.
Over-Frequency Generation Reduction
Separately, clause S5.2.5.8 requires plant above a size threshold to have an automatic droop response that reduces active power when frequency rises towards the upper extreme limits. This is a protection-style backstop rather than a market service, but it uses the same control function. Make sure the settings for S5.2.5.8 and S5.2.5.11 do not conflict, and that the over-frequency trip settings sit outside both.
Active Power Control (S5.2.5.14)
Frequency response modifies output around a target. S5.2.5.14 sets how the target itself is controlled. For semi-scheduled plant, the automatic access standard requires the plant to:
- reduce or increase output at a constant rate to the level in an electronically issued instruction, within five minutes
- limit output to that level
- not change output within five minutes by more than the raise and lower amounts specified
- ramp linearly from one dispatch level to the next
- receive and respond to AGC signals updated every four seconds
Scheduled plant must follow dispatch instructions, ramp linearly and respond to AGC. The study is a series of active power reference steps and ramps at maximum and minimum SCR. At a weak connection point, a fast change in active power can depress the voltage enough to push the plant towards instability, so do not skip the minimum SCR case.
Commissioning Traps
Frequency Control Checks Before Commissioning
- Operational droop and deadband comply with the PFRR, not just the S5.2.5.11 capability range
- Droop base scales correctly when units are out of service
- Measured droop at the connection point matches the setting within tolerance
- BESS response is smooth through the charge to discharge transition
- FCAS registration amounts are consistent with the performance standard
- S5.2.5.8 over-frequency reduction and S5.2.5.11 response settings do not conflict
- Model settings match the commissioned settings for R2 validation
An exemption from the PFRR droop limit has to be in place before commissioning if the plant cannot comply. Leaving it to the hold point tests is a reliable way to lose weeks.
Get Frequency Control Settled Early
Frequency control is usually one of the easier clauses to meet at the automatic level, but it is easy to get the operational settings, the base and the FCAS intent out of step.
ElectraGlobe prepares the frequency control and active power sections of GPS proposals, runs the supporting studies and supports the injection tests at commissioning. See our AEMO GPS applications service, or contact us.
Frequently Asked Questions
What droop and deadband settings does S5.2.5.11 require?
The plant must be capable of a deadband settable from 0 to ±1.0 Hz and a droop settable from 2% to 10%, or other settings agreed with the NSP and AEMO. The settings actually used in operation are governed by AEMO's Primary Frequency Response Requirements.
Can a solar farm provide under-frequency response?
Only if it is running below its available power. A solar farm operating at maximum power point has no headroom to increase output, so its under-frequency response is limited by energy source availability. It can always reduce output for over-frequency.
How does a battery respond to frequency under S5.2.5.11?
A battery responds in both directions, increasing discharge or reducing charge as frequency falls, and the reverse as it rises. The Rules require a smooth change between charging and discharging when the response crosses zero.
What does S5.2.5.14 require for semi-scheduled plant?
The ability to follow electronically issued dispatch instructions within five minutes at a constant rate, to limit output to the instructed level, to ramp linearly between dispatch levels, and to respond to AGC signals updated every four seconds.