In this paper, a decomposition method is proposed which relates the unit ramping process to the cost of fatigue effect in the generation scheduling of thermal systems. The objective of this optimization problem is to minimize the system operation cost, which includes the fuel cost for generating the required electrical energy and starting up decommitted units, as well as the rotor depreciation during rampi processes, such as starting up, shutting down, loading, and J o a d i n g . According to

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... he unit fatigue index curves provided by generator manufacturers, k e d unit ramping-rate limits, which have been used by previous studies, do not reflect the physical changes of generator rotors during the ramping processes due to the fatigue effect. Even though the use of properly pre-selected ramprate limits can guarantee a conservative generation schedule, it fails to provide the flexibility of choosing appropriate ramping rates for minimizing the system operation cost. Since ramping rates do not change linearly with ramping costs, we have to know the ramping duration beforehand in order to select the proper ramp ing. This is the most difficult aspect of incorporating ramping costs into the generation scheduling procedure since the ramp ing costs vary with the generation schedule. By introducing ramping costs, the unit on/off states can be determined more economically by the proposed method. The Lagran 'an relaxation method is proposed for unit commitment anfeconomic dispatch, in which the original problem is decomposed into several subproblems corresponding to the optimization process of individual units. The network model is employed to represent the dynamic process of searching for the optimal commitment and generation schedules of a unit over the entire study time span. The experimental results for a practical system demonstrate the effectiveness of the proposed approach in optimizing the power system generation schedule.