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Ancillary services are reliability services required to support the secure and stable operation of the electricity system.
While the energy market determines which generators produce electricity, ancillary services provide additional capabilities needed to maintain system stability, including rapid response to system disturbances, frequency control, and voltage regulation.
The Independent Electricity System Operator (IESO) coordinates and procures ancillary services to ensure that the power system can respond effectively to unexpected changes in supply, demand, or system conditions.
Ancillary Services
Reliability services that support the operation of the electricity grid but do not directly supply energy to consumers.
These services help maintain system stability and ensure reliable electricity delivery.
⚡ Electricity systems must maintain stability at every moment.
Ancillary services provide the rapid-response capabilities needed to manage sudden changes in supply, demand, or system conditions.
Operating reserve provides backup generation capacity that can be quickly activated if electricity supply is unexpectedly disrupted.
Operating reserve resources must be capable of responding rapidly to system events.
In many electricity systems, operating reserve is divided into categories based on response time, such as:
These reserves ensure that additional electricity supply can be brought online quickly when needed.
Regulation service helps maintain the electricity system’s operating frequency.
Power systems operate at a specific frequency (typically 60 Hz in North America). Small imbalances between electricity supply and demand can cause frequency deviations.
Regulation resources respond automatically to these fluctuations by adjusting their output continuously.
These adjustments help maintain stable system frequency.
Electricity systems must also maintain stable voltage levels across the transmission network.
Voltage support services help maintain appropriate voltage conditions so that electricity equipment operates safely and efficiently.
Generators, transmission equipment, and specialized devices may contribute to voltage support.
Related topic:
Transmission System
The system operator ensures that sufficient ancillary services are available to maintain reliability.
These services may be obtained through:
The procurement process ensures that the system has sufficient reserves and stability services available at all times.
Ancillary services operate alongside the electricity energy market.
While generators may earn revenue by producing electricity, they may also receive compensation for providing ancillary services such as reserve capacity or regulation service.
Participation in ancillary service markets can provide additional revenue streams for generation resources, energy storage systems, and demand response providers.
Consider a situation where a large generator suddenly trips offline.
Ancillary services allow the system to respond quickly and prevent widespread disruptions.
➡ Next Topic:
Electricity Settlements
This topic explains how financial transactions are calculated and settled between participants in Ontario’s electricity market.
Last Updated: 2026-03-26