Operating reserve

Operating reserve describes the supply resource capacity that is available to the system operator within a short interval of time to maintain the balance of supply and demand on the bulk electric system. Definitions of operating reserve vary from region to region, but operating reserve always includes some form of spinning and non-spinning reserve. Some regions also include regulating reserve. And some also have other defined reserve services such as flexible ramp capability, supplemental reserve, or replacement reserve. The services that provide operating reserve are a part of the broader category of ancillary services, although not all ancillary services provide operating reserve. 

Operating reserve is used by the system operator when supply is short relative to demand due to load forecasting error, when renewable output is different from forecast, and/or during power plant or transmission outages

An example of the use of reserves to address unexpected loss of a 250 MW power plant. As the power plant drops offline at 0 minutes, regulating reserve responds within a few seconds to keep frequency above lower limits. Operators dispatch spinning and non-spinning reserve and as these units ramp up, the regulating reserve ramps down so it is available should it be needed later in the hour. Similarly, operators dispatch replacement reserve and as these units ramp up, the spinning reserve is ramped down so it too is available in case another event occurs during the hour.