Spinning Reserve

Spinning reserve is generation capacity that is already online, synchronized to the grid, and operating below its maximum output so that it can ramp up rapidly (typically within 10 minutes) to compensate for sudden generation losses or unexpected load increases. It is one of the most important ancillary services for maintaining system security.

Key Aspects of Spinning Reserve:

• Already Synchronized: Unlike non-spinning (or supplemental) reserve, spinning reserve units are already connected and injecting power into the grid. This eliminates startup delays and allows response within seconds to minutes, depending on the technology.
• Sizing Criteria: The required amount of spinning reserve is typically set to cover the largest single contingency in the system (N-1 criterion), ensuring that if the biggest generator trips, enough headroom exists on other machines to replace the lost output without load shedding.
• Response Hierarchy: Following a disturbance, spinning reserve acts as part of the primary and secondary frequency response. Governor action on spinning units provides the initial arrest of frequency decline within seconds, while AGC then adjusts outputs over minutes to restore frequency to nominal.
• Opportunity Cost: Providing spinning reserve requires generators to operate below full capacity, forgoing potential energy sales. This opportunity cost is reflected in reserve market prices and is a key factor in the economic optimization of unit dispatch.
• Evolving Providers: Traditionally supplied by partially loaded thermal and hydro plants, spinning reserve is increasingly provided by battery energy storage systems and demand response, which can offer faster ramp rates and lower emissions than conventional generators.