Abstract: In-plant economical operation is an important measure to improve the benefit of a hydropower plant. In cases where a large number of generator sets of various types are in operation, traditional methods or intelligent algorithms come up against problems, such as too much time-consuming and uncertainty in solutions. Through a case study of the large number of complex generator sets at the Xi-Luo-du hydropower plant, a practical method has been developed to achieve in-plant economical operation of generator sets, including operation schemes of how to add new generator sets and shut down some of the existing sets. Results show that our method can give solutions very close to the optimal at less computational cost. The method is fast enough to meet on-line operation needs and it would be useful to the design and operation of similar hydropower plants.