Ancillary services are the services (other than energy) required by system operators to ensure reliable operation of the electric grid. They are used to keep the system operating within acceptable frequency and voltage levels and to restore the system when contingencies occur. Examples of contingencies include unexpected power plant or transmission line outages, sudden changes in demand, sudden changes in supply from renewable resources, and unexpected flows to or from neighboring control areas.
The specific ancillary services utilized vary by market but may include the following examples:
- Regulating reserve – sources of supply that can ramp up or down within a few seconds
- Flexible ramp – sources of supply that have the ability to ramp up or down within five minutes
- Spinning reserve – sources of supply that are synchronized to the frequency of the grid and can ramp to full capacity within 10 minutes
- Non-spinning reserve – sources of supply that may not be synchronized to the frequency of the grid, but can be available within 10 minutes
- Replacement reserve – sources of supply that may not be synchronized to the frequency of the grid, but can be available within 30 minutes
- Voltage support – sources of supply that can provide reactive power to the grid as needed to support voltage
- Black start – sources of supply that can start without power from the grid for the purpose of restoring the system after an outage
Note that in various regions, slightly different titles or definitions for these services may be used.
Once units have been scheduled to provide any of these ancillary services in the day ahead, the resources must be available for dispatch by system operators throughout the period for which they are scheduled.
In vertically integrated wholesale markets, ancillary services are provided by utility resources under regulated cost-based tariffs. In competitive markets, the bulk system operator (the Independent System Operator or Transmission System Operator) utilizes ancillary services markets to obtain needed resources at market-based prices.
Ancillary services are traditionally provided by units such as hydropower, gas combustion turbines, gas combined-cycle turbines, and gas or coal steam turbine units that are providing energy from a portion of their capacity but have additional unused capacity. In some areas, flexible loads also provide frequency regulation and the various reserves services.
As the amount of variable renewable resources grows in regions without robust hydropower, the amount of traditional resources available to provide ancillary services will shrink. At the same time, the need for flexible ramp will increase. This will provide a significant challenge for grid operators to ensure they have access to sufficient ancillary services. New resources including enhanced flexible load programs, batteries, use of smart inverters for voltage support and frequency regulation, vehicle-to-grid, and other forms of storage will likely be required.