Spinning reserve is provided by resources that are not putting energy onto the grid but are synchronized to the frequency of the system and thus can begin providing energy upon receiving a dispatch call. Capacity included in spinning reserve must be fully available to the system operator within 10 minutes of notification. Many regions have a requirement that resources providing spinning reserve must be capable of running for a minimum of two hours after dispatch. In some cases, spinning reserve can also be provided by flexible load that can be curtailed within a 10-minute time window.
Spinning reserve is used by bulk electric system operators for protection against contingency events such as the unexpected loss of a generator or a transmission line. These resources can be brought online quickly to replace the lost resource and to allow other resources such as automatic generation control units to ramp back down to be available later in the hour.
Typical sources of spinning reserve include hydropower; gas combustion; turbines; gas combined-cycle turbines; and gas, coal, or oil steam turbine units that are providing energy from a portion of their capacity but have additional unused capacity that can be ramped in the 10-minute window. Spinning reserve can also be provided by battery storage and flexible loads.
Spinning reserve is a defined ancillary service. In vertically integrated wholesale markets, it is provided by utility resources under regulated cost-based tariffs. In competitive markets, the bulk system operator (the Independent System Operator or Transmission System Operator) facilitates a market that allows the system operator to obtain necessary spinning reserve resources via a price auction. Spinning reserve may also be called contingency reserve, responsive reserve, synchronous reserve, and 10-minute spinning.