The heart of grid stability is frequency. The frequency regulation market is the segment of ancillary services dedicated to keeping the grid's frequency within strict limits (e.g., 60 Hz ± 0.05 Hz). When supply and demand are balanced, frequency is nominal. When they are not, frequency deviates. The frequency regulation market provides the resources that respond instantly to correct these deviations. As renewables displace synchronous generators, the frequency regulation market is growing, with new technologies like batteries and fast-ramping gas plants.

The broader ancillary services power market includes this segment. The frequency regulation market is typically divided into "primary," "secondary," and "tertiary" response. Primary response (or "fast frequency response") occurs within seconds. The frequency regulation market for primary response is the fastest, with batteries and specially tuned hydro turbines providing it. Primary response arrests the frequency deviation. The frequency regulation market for "droop" control (a proportional response) is standard for generators.

Secondary response (or "automatic generation control" or "regulating reserve") brings the frequency back to the nominal value after a primary event. The frequency regulation market for secondary response has a response time of seconds to minutes. Traditional generators (gas, hydro) and batteries provide secondary response. The frequency regulation market for "area control error" (ACE) is the signal used to dispatch secondary response. Tertiary response (or "replacement reserve") relieves the secondary response units and restores reserve levels.

The frequency regulation market is being transformed by battery storage. A battery can respond to a frequency deviation in milliseconds, far faster than a gas turbine. The frequency regulation market for "fast" regulation (e.g., PJM's RegD signal) pays a premium for this speed. The frequency regulation market for "combined" fast and slow regulation is also common. The frequency regulation market for "synthetic inertia" (using a battery to emulate the physical inertia of a spinning generator) is a new product.

The frequency regulation market is also influenced by "under-frequency load shedding" (UFLS). If frequency falls too low, utilities automatically shed load to prevent a blackout. The frequency regulation market for "UFLS" is not a market product but a last resort. The frequency regulation market for "over-frequency" events (e.g., when wind generation surges) can be handled by curtailment or pumped storage.

Looking ahead, the frequency regulation market will see the participation of "grid-forming" inverters. Unlike "grid-following" inverters (which rely on a stable grid reference), grid-forming inverters can create that reference, providing virtual inertia. The frequency regulation market for "grid-forming" batteries will be essential for 100% renewable grids. The frequency regulation market will also see "decentralized" approaches, where devices respond to local frequency measurement without a central dispatcher.

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