Ancillary Services

Ancillary services are the support functions required to maintain reliable, secure operation of the bulk power system beyond the basic delivery of energy from generators to loads. They ensure that frequency, voltage, and system stability remain within acceptable bounds at all times.

Key Aspects of Ancillary Services:

• Frequency Services: These include primary frequency response (automatic governor action within seconds), secondary frequency control via AGC (restoring frequency over minutes), and tertiary reserves (manual replacement reserves over 15–30 minutes). Together they form a layered defense against generation-load imbalances.
• Voltage Support: Reactive power provision by generators, synchronous condensers, STATCOMs, SVCs, and capacitor banks keeps voltages within ±5% of nominal at every bus. Unlike frequency, voltage must be managed locally because reactive power does not travel well over long distances.
• Operating Reserves: Spinning reserve (synchronized, fast-responding capacity) and non-spinning reserve (offline but startable within 10–30 minutes) provide backup against unexpected outages. Reserve requirements are typically sized to cover the largest single contingency (N-1 criterion).
• Black Start: Designated plants capable of self-starting without external power supply ensure the grid can be restored after a complete blackout. Hydroelectric stations, gas turbines, and increasingly battery systems provide this critical capability.
• Market Procurement: In liberalized systems, ancillary services are procured through competitive markets or bilateral contracts, creating revenue streams separate from energy sales. The growing share of variable renewables has increased both the volume and the value of these services.