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Pumped storage power plants are typically controlled using a two-degree-of-freedom control structure in which the feedforward part is static. For set point changes of the generator output, this control structure leads to pressure oscillations due to the non-minimum phase nature of pumped storage power plants. In order to improve the control performance, a flatness-based feedforward is developed as an add-on for the already existing two-degree-of-freedom control. Thereby a differentially flat dynamic model of a real power plant, derived on first principles, is used for both the design of a dynamic feedforward part and the design of consistent smooth trajectories for set point changes of the generator output. In view of trajectory planning, the effect of using different trajectory function types is addressed in detail. The improved control performance using the new control concept is shown in simulations, also in comparison with the widespread conventional two-degree-of-freedom control with static feedforward part.