Non-spinning reserve comprises units that are not required to be synchronized to the frequency of the system but can be available within 10 minutes of notification. Non-spinning reserve can also include demand response that is available within the 10-minute window. While non-spinning reserve has the same 10-minute requirement as spinning reserve, these units take longer to begin contributing partial generating capacity since they must first be synchronized to the system. Many regions have a requirement that resources providing non-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.
Non-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 so that they are available later in the hour. Most commonly a system operator will maintain a mix of spinning and non-spinning reserve. Spinning reserve begins providing power to the grid more quickly, but the advantage to non-spinning reserve is that the units do not need to burn fuel to stay synchronized to the frequency of the system while in standby mode.
Non-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 which allows the system operator to obtain necessary non-spinning reserve resources via a price auction. Non-spinning reserve may also be called quick start reserve, supplemental reserve, 10-minute non-spinning, 10-minute non-synchronized reserve.