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Within the XFLEX HYDRO project, the possibility of increasing the flexibility of hydro power plants to support the Electric Power System (EPS) is investigated. The flexibility of the pumped-storage power plant Frades 2, as the target, should increase by extending the operating range for each unit and by using the hydraulic short circuit (HSC) operation mode. Transient investigations of the additional plant conditions are performed to ensure the safety of the plant. With a 1D model of the entire hydro power plant including both pump-turbines, valves, surge tanks, and the water-bearing components, extensive calculations are carried out to verify the safety of the existing plant for extended operation conditions. In particular, the study focuses on the synchronous and asynchronous emergency shutdowns of the plant in the new operating conditions as well as other operation-related power plant transients regarding the HSC mode. With the results presented in this paper, the flexibility of the plant Frades 2 can be increased. Delayed emergency shutdowns are identified as particularly critical during the study and should always be given additional consideration in transient investigations. [ABSTRACT FROM AUTHOR]