Frequency Control Ancillary Services — markets where generators and batteries earn revenue by providing fast-response reserves that keep grid frequency at 50 Hz.
Frequency Control Ancillary Services (FCAS) are a set of markets AEMO operates alongside the energy market to keep the National Electricity Market's frequency within its normal operating band around 50 Hz. Generators and batteries can offer to hold capacity in reserve — to raise output (or reduce consumption) if frequency falls, or lower output (or increase consumption) if it rises — and are paid for that reserved capacity whether or not it is ever called on, plus a further payment if it is actually dispatched.
There are ten FCAS markets in total: six contingency services (6-second, 60-second and 5-minute, each in a raise and a lower direction) that respond to sudden plant trips or large frequency deviations, and two regulation services (raise and lower) that respond continuously to AEMO's automatic generation control (AGC) signal, correcting the small, constant drift in frequency during normal operation.
Batteries are well suited to FCAS because they can respond within milliseconds and can switch between charging and discharging far faster than thermal generation, but a battery committing power to FCAS in a given interval has that power unavailable for energy arbitrage in the same interval — the two compete for the same MW of headroom. NEMPulse's optimal-dispatch benchmark co-optimises energy and all ten FCAS markets together on one shared battery for exactly this reason.
See also: Actual vs optimal performance
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