Ancillary services
Services beyond bulk energy that system operators procure to keep the grid stable and secure, such as frequency reserves, voltage control and black start.
Ancillary services are the products a transmission system operator (TSO) buys to maintain frequency, voltage and system restoration capability. They are separate from energy sold on wholesale markets and are typically procured through auctions, bilateral contracts or mandatory grid-code obligations.
Main categories:
- Frequency control: in ENTSO-E terminology, FCR (full activation within 30 s), aFRR (automatic, within 5 minutes) and mFRR (manual, within 12.5 to 15 minutes). In North America, NERC and ISOs use regulation, spinning and non-spinning reserves.
- Voltage and reactive power control.
- Black start capability.
- Inertia and stability services, newly procured in Great Britain and Ireland.
- Congestion management and redispatch.
Ancillary service costs are usually a small share of total system costs, typically 2 to 5% of wholesale market value, but they are rising with higher VRE shares. Batteries have transformed these markets: in Great Britain, Germany and Texas, BESS captured most frequency response volume within a few years, pushing prices down sharply; FCR prices in Germany fell from above 20 EUR/MW/h in 2022 to under 10 in 2024. Wind, solar and demand-side resources can also provide many services through fast-acting inverters.
Related terms
All terms →- Frequency response
The automatic change in generation or load that arrests and corrects a frequency deviation after a sudden imbalance on the grid.
- Grid frequency
The rate at which alternating current changes direction on a power system, 50 Hz or 60 Hz, which reflects the instantaneous balance of supply and demand.
- Black start
The process of restoring a power system after a total or partial blackout using units that can start without power from the grid.
- Reactive power
The component of AC power that oscillates between source and load without doing net work, needed to sustain magnetic and electric fields and control voltage.
- Inertia (grid)
The kinetic energy stored in synchronously rotating generators and motors that resists sudden changes in grid frequency.
- Virtual power plant (VPP)
A software platform that aggregates many distributed resources, such as batteries, EVs, heat pumps and rooftop solar, and operates them as a single dispatchable unit.