Dispatch

Electric dispatch is the act of a system operator instructing a supply resource to adjust its output to a specified level. Dispatch order among multiple available supply resources is commonly determined using a method called security constrained economic dispatch. This means that the supply resources with the lowest variable operating cost will be dispatched first subject to any system operating constraints such as transmission availability, power plant ramp speeds, the need for reserves, and any needs associated with local system operations. System operators use optimization and system flow models to determine optimal dispatch given system conditions. In competitive wholesale markets, unit price offers are used for economic optimization, while in vertically integrated markets variable operating costs are used. In some cases, non-economic factors may override economic dispatch.

Examples include a desire to utilize specific types of generation such as nuclear or renewables or certain power plant owners who choose to run regardless of market prices. System operators typically send dispatch orders based on day-ahead schedules at the start of the hour, and then dispatch certain units up or down across the hour to maintain frequency by balancing supply and demand (see frequency regulation).