Unit commitment with transmission security and voltage constraints

In this paper, optimal power flow (with transmission security and voltage constraints) is incorporated in the unit commitment formulation. Using Benders decomposition, the formulation is decomposed into a master problem and a subproblem. The master problem solves unit commitment with prevailing constraints, except transmission security and voltage constraints, by augmented Lagrangian relaxation. The subproblem minimizes violations of transmission security and voltage constraints for a commitment schedule given in the master problem. Since transmission constraints are decoupled from voltage constraints in the subproblem, the subproblem could further be decomposed into two smaller subproblems. The resulting transmission subproblem minimizes transmission flow violations for the worst contingency case by adjusting unit generation and phase shifter controls. The corresponding reactive subproblem minimizes voltage constraints by reactive power generation and tap changing controls. In the case of transmission flow or voltage infeasibility, Benders cuts are produced for unit commitment rescheduling. The iterative process between the master problem and the subproblems provides a minimum production cost solution for generation scheduling while satisfying the constraints. A comprehensive unit commitment software package is developed using Visual C++. A 36-unit system in the IEEE-118 bus network is used to demonstrate the efficiency of the method.