PT. Vale Indonesia has three hydropower plants (called PLTA). They have three dams cascaded in one river. The water sources from three lakes at around company operational area. The hydropower plants are main supporting units for Nickel production. Thus the hydropower plants must be have high performance and long live time. Here the preventive maintenance is needed. This paper discusses about study result of dam operating strategy of the hydropower plants in order to optimal water management during preventive maintenance period, by considering of power demand at the smelting furnace and the auxiliary power. Linear programming is used as an optimization method. It is proved that the electric energy of 15616.8 MW hours be obtained through water flow rate of 17625.6 m3/seconds, for 2015's operation.