Effective modeling and resolution of generation-dependent ramp rates for unit commitment

The ramp rate of a combined cycle (CC) unit varies significantly with its generation level. In current unit commitment (UC) formulations, ramp rates of CC units are usually modeled as constants. Reserve capabilities may thus be significantly overestimated, causing serious consequences when reserves are needed. When generation-dependent ramp rates are considered, the performance of existing approaches decreases drastically and is not stable. This paper is on effective modeling and resolution of generation-dependent ramp rates in look-ahead UC problems. New ramp and reserve capability functions are established, and the UC formulation is further improved based on existing tight single-unit formulations. Testing results by using branch-and-cut (B&C) show that the solution quality and computational efficiency are improved and the performance is stable. These modifications can be directly incorporated in existing solvers. To exploit the tightened single-unit formation, the problem is also solved by the integration of our recent decomposition and coordination approach with B&C. Testing results show that time-consuming branching operations are much reduced as compared with those of B&C and the coordination is effective, demonstrating great potentials for complicated mixed-integer linear programming problems.