Short-term scheduling of electric power systems under minimum load conditions

Thermal unit operating constraints, hydro spill conditions, must-take resources and inflexible demand can all contribute to an electric power system´s minimum load, or “backdown”, problem´s severity. We present two approaches to addressing minimum load constraints in the unit commitment problem. The first, traditional approach is to perform a unit commitment without considering the minimum load constraints, and then make the solution feasible by applying a predetermined “noneconomic” backdown order to curtail must-take resources. The second approach considers minimum load constraints as system constraints similar to operating capacity constraints, and characterizes resources to be backed as having both a decremental cost and an incremental value to the system, where these are not necessarily equal and so impact the decision to make economy sales or pump with pumped-storage resources