Replacement reserve

Replacement reserve comprises units that are available with a longer lead time than other reserves, commonly 30 minutes from notification. Replacement reserve is used to substitute for other resources that have been used in response to a system contingency such as the unexpected loss of a generator or a transmission line. The role of the replacement units is to allow other units to return to their original role as spinning or non-spinning reserve to be available in the event of an additional contingency. 

Typical sources of replacement 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. 

In some regions, replacement 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 replacement reserve resources via a price auction. Replacement reserve may also be called supplemental reserve, 30-minute non-synchronized reserve, 30-minute operating reserve, 30-minute synchronized reserve.