The Generator Maintenance and Operation Scheduling problem is presented as a large scale mixed integer nonlinear programming case. Several relaxations of the conditions of variables and constraints are discussed. The optimal solution of the models based on these relaxations is viewed as the lower bound of the optimal solution in the original problem. A combined implicit enumeration and branch-and-bound algorithm is used. Typical dimensions of the problems for which computational experience is reported are 25 generator in the system, 19 of these are to be maintained and a planning horizon of 52 weeks; the corresponding dimensions of the model are about 2300 constraints, 700 binary variables and 1300 bounded nonlinear separable variables.

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