supagridFrequency Control Ancillary Services (FCAS) are services that help maintain the power system frequency at 50 Hz. When supply and demand are unbalanced, frequency drifts; FCAS providers respond by increasing or decreasing output to restore balance. The NEM operates separate markets for energy and FCAS, but NEMDE co-optimises them so that the same unit can provide both energy and FCAS in a single dispatch interval.
FCAS is split into regulation (continuous, automatic adjustment) and contingency (discrete response to large disturbances such as generator trips). Each service type has its own price and is procured regionally. FCAS prices can be a significant component of market outcomes and generator revenue, especially during scarcity or when specific regions lack local FCAS capability.
Why frequency control matters
Power system frequency must stay within narrow bounds (nominally 50 Hz in Australia). If demand exceeds supply, frequency falls; if supply exceeds demand, frequency rises. Prolonged deviation can damage equipment, trigger under-frequency load shedding, or cause cascading outages. FCAS ensures that the system can correct imbalances quickly, both for normal fluctuations and for major contingencies such as the loss of a large generator.
As the NEM transitions to more variable renewables and fewer synchronous generators, the availability of FCAS—especially inertia and fast frequency response—has become more important. New technologies such as batteries and grid-forming inverters are increasingly providing FCAS.
Regulation vs contingency FCAS
Regulation FCAS (raise and lower) provides continuous, automatic adjustment in response to small frequency deviations. Regulation units receive a setpoint from AEMO and automatically increase or decrease output as frequency changes. Regulation is procured in each region to meet normal variability in supply and demand.
Contingency FCAS (R6, R60, R5) provides discrete response to large disturbances. When a contingency occurs—such as the trip of a major generator—frequency can fall or rise sharply. Contingency FCAS is designed to arrest the frequency change and support recovery. R6 responds within 60 seconds; R60 and R5 respond within 5 minutes. Each region must procure sufficient contingency FCAS to cover the largest credible contingency (e.g. loss of largest unit) in that region.
FCAS services at a glance
The table below summarises the main FCAS service types in the NEM. Raise services increase output (or decrease demand) when frequency falls; lower services decrease output (or increase demand) when frequency rises.
| Service | Description |
|---|---|
| Regulation Raise | Automatically adjusts output up to correct frequency deviations above 50 Hz. |
| Regulation Lower | Automatically adjusts output down to correct frequency deviations below 50 Hz. |
| R6 Raise | Contingency reserve; responds within 60 seconds of a frequency drop. |
| R6 Lower | Contingency reserve; responds within 60 seconds of a frequency rise. |
| R60 Raise | Contingency reserve; responds within 5 minutes of a frequency drop. |
| R60 Lower | Contingency reserve; responds within 5 minutes of a frequency rise. |
| R5 Raise | Contingency reserve; responds within 5 minutes for delayed contingencies. |
| R5 Lower | Contingency reserve; responds within 5 minutes for delayed contingencies. |
Co-optimisation with energy
NEMDE solves for energy and FCAS simultaneously. A unit can provide energy and one or more FCAS services in the same interval, subject to its capability envelope. For example, a battery might discharge for energy while also holding capacity for regulation raise; a hydro unit might generate energy and provide R60 raise. The co-optimisation ensures that the total cost of meeting both energy demand and FCAS requirements is minimised.
When a unit provides FCAS, it may need to reserve capacity that could otherwise be used for energy. This can affect energy dispatch and prices. Conversely, high energy prices can draw capacity away from FCAS, potentially raising FCAS prices. Understanding this interaction is important when analysing price spikes in either market.
Regional FCAS markets
FCAS is procured and priced separately in each NEM region. A unit provides FCAS to the region where it is electrically located. If a region has insufficient local FCAS, it cannot rely on surplus from another region in real time—FCAS is not transferable like energy across interconnectors (with limited exceptions, e.g. Basslink for Tasmanian FCAS). Regions with few FCAS providers—such as South Australia—can experience high FCAS prices when local supply is scarce.
FCAS demand is determined by AEMO based on forecast load, largest contingent unit, and security requirements. FCAS prices clear at the marginal cost of the last unit needed to meet the requirement. When few units offer FCAS or many are already committed to energy, FCAS prices can spike to the admin cap.
When and why FCAS prices spike
FCAS prices can reach very high levels when:
- Scarcity: Few units in a region offer the required FCAS, or many are unavailable.
- High energy prices: Units prefer to sell energy rather than hold capacity for FCAS, reducing FCAS supply.
- Outages: Key FCAS providers are offline, increasing reliance on remaining units.
- Low renewable output: When solar/wind are low, synchronous plant may be running and providing FCAS; when they are high, fewer synchronous units run, potentially reducing FCAS availability.
FCAS revenue can be significant for fast-response assets such as batteries, hydro, and open-cycle gas. Analysing FCAS prices alongside energy prices gives a fuller picture of market outcomes.
Related
Dispatch & Pricing explains energy dispatch and pricing. Bidding describes how offers (including FCAS) are structured. Interconnectors notes Basslink's FCAS capability. For AEMO variables, see AEMO variables.