Solution to Short-term Unit Commitment Problem

Abstract

The Lagrangian relaxation approach to solve the unit commitment problem for a large system comprising both thermal and hydro generating units is presented. Commitment states of thermal units are obtained by solving thermal subproblems of Lagrangian dual problem. To get the output levels of hydro units, the hydrothermal scheduling is performed with a thermal unit commitment schedule obtained by solving thermal subproblems. Extensive constraints are considered. Nonlinear functions are used for thermal generation cost, water discharge rate and sulfur oxide emission. A general transmission loss formula is utilized for incorporating transmission loss. The variable metric method is used for updating the Lagrangian multipliers during maximization of the dual function. The Lagrangian multipliers are adjusted by the linear interpolation method during searching for a feasible suboptimal solution near the dual optimal point. A refinement algorithm is used to fine tune the schedule. A unit commitment expert system is employed for checking the feasibility of the solution and for handling constraints, which are difficult or impractical to be implemented in commitment algorithm. Results of the implementation on a utility are shown.