This paper presents an approach for solving the generation scheduling problem of a complex system consisted of conventional and renewable energy sources (RES). Wind power plants are integrated into the system in order to minimize the total thermal unit fuel costs. The gained results for wind farm power production are used as input in the system to determine the optimal amounts of generated power for the thermal generating units and hydro generating units over the study period. The optimization problem consists of minimizing the total production costs, respecting power balance equations for each time interval and all operational system constraints. The proposed approach is applied on a specific system consisted of thermal power plants (TPPs), storage hydro power plants (HPPs), pumped-storage hydro power plant (PSHPP) and wind power plant (WPP). The benefits of energy production from WPP, in terms of reducing the production costs of conventional thermal power plants are also investigated. In the proposed paper two cases are analyzed. In the two analyzed cases power unit's generation, thermal unit's fuel costs and stream flows of hydro units are calculated over the study period. (C) 2012 Elsevier Ltd. All rights reserved.