A Hybrid Modeling Approach For Approximating The Hydro Unit Commitment Constraints In A Large-Scale Day-Ahead Dispatch Model In Brazil

This paper presents a hybrid modeling approach to approximate hydro unit commitment (HUC) constraints, especially the discontinuous operating ranges, in the Brazilian large-scale day-ahead dispatch model (DADM). The current DADM uses a plant-based hydro production function (HPF), which is computationally manageable but cannot accurately capture unit-level constraints, such as forbidden operating zones and different unit types. Although more precise, full unit-based models are too computationally intensive for the Brazilian context, which includes 156 hydro plants and around 745 hydro units. The proposed hybrid method combines plant-level and grouped unit-level HPFs, enabling an accurate representation of operating zones and different units while managing combinatorial complexity. The hybrid approach is tested and compared with the actual HPF model using real data from the Brazilian system under various hydrological and load conditions. The results show that the hybrid model ensures unit-level feasibility, reduces the need for post-processing adjustments, and improves the predictability of the day-ahead dispatch process.