Large-Scale Unit Commitment for Cascaded Hydropower Plants with Hydraulic Coupling and Head-Sensitive Forbidden Zones: Case of the Xiluodu and Xiangjiaba Hydropower System

Abstract

This study presents a methodology for optimizing the unit commitment (UC) of the Xiluodu and Xiangjiaba cascaded hydropower plants that rank as the second- and third-largest hydropower plants in China. This UC problem exhibits high complexity due to the integrated consideration of hydraulic coupling between plants, head-sensitive forbidden zones, and other hydropower system nonlinearities. Hydraulic coupling is considered through a group of nonlinear tailrace level curves that are represented using binary 0–1 variables. The interval segmentation is used to determine the binary value by quickly identifying the forebay level range of the immediate downstream plant. The irregular forbidden zones are divided into several quadrangles to approximately describe the upper and lower boundaries using linear functions of the net head. The described forbidden zone boundaries are further integrated with power generation limitations to determine discontinuous safe operating zones. With the aid of polynomial approximations of other nonlinearities, the presented problem is finally formulated as a mixed-integer nonlinear programming (MINLP) model and addressed by an optimization method based on the branch-and-bound technique. The practical behavior of the proposed methodology is demonstrated by real-world data. Moreover, the MINLP model is compared with a commonly used mixed-integer linear programming model and shows improvements in solution accuracy and computational efficiency.