Water hammer is an undesirable event, caused by sudden flow changes in confined pipe systems. When it occurs, its consequences can be very costly and even sometimes deadly. In general, it may be encountered in the penstocks of hydropower plants, water transmission lines, water networks, etc. Therefore, the operation guidelines of the hydropower plants should be defined correctly and if necessary, preventive measures should be considered. In the present study, water hammer problems in pumped-storage hydropower plants supported by wind energy (water-wind hybrid systems) are investigated. Time dependent flow conditions in the penstocks are studied by the use of the computer software, HAMMER. The software solves nonlinear differential equations by using method of characteristics. Firstly, hydraulic transients for various operational cases are investigated using some scenarios. In this study, only the scenario 5 which gives the most critical results is investigated.