The Z'Mutt pumping station, part of the Grande Dixence hydroelectric scheme, is one of the demonstrators of the XFLEX HYDRO project. A 5 MW reversible pump-turbine prototype equipped with a full-size frequency converter (FSFC) is used to investigate dynamic variable speed operation in pumping and generating mode. Since the FSFC converter is always connected to the electrical grid, the full rotational speed range of the motor-generator can theoretically be exploited. Furthermore, this technology enables fast operating point transitions and therefore increased grid regulation capacities. The advantages of the FSFC technology in generating mode are compared to a conventional fixed speed start-up with a variable speed startup. The operating point trajectories are extracted from 1-D hydraulic transient simulations. Detailed hydrodynamic and structural aspects of the pump-turbine during the two start-up scenarios are further studied. Simplified unsteady 3-D CFD simulations and transient structural FEM analyses of the pump-turbine prototype are carried out to assess the harshness of the flow and to anticipate runner fatigue. The present work aims to point out potential mitigation of partial runner damages during start-up in generating mode using FSFC technology.