The demonstration of flexibility technologies to enhance ancillary services of Hydropower Plants (HPPs) is showcased by the XFLEX HYDRO H2020 European Project. The potential of Full Size Frequency Converters (FSFC) is numerically and experimentally assessed by a case study on the Z’Mutt HPP. The investigated unit is a 5 MW variable speed reversible Francis pump-turbine fed by a FSFC that provides full speed control from -100% to +100%, allowing for fast start-up and mitigation of damage caused to different components. In this context, several turbine start-up sequences with controlled guide vane opening and rotational speed are developed, implemented, and tested on prototype scale. The definition of the different sequences, their impact on penstock fatigue and further FSFC capabilities like fast active power control in pump mode and fast power reversion are presented in part 1 of this contribution. Part 2 deals with fatigue damage investigations of the runner with respect to variable speed operation in generating mode using coupled numerical and experimental techniques. Strain signals from the runner blades are extrapolated to the critical areas to assess the relative fatigue damage induced by each turbine start-up. The scaled normal stress approach is applied, which is a critical plane method, adapted for multiaxial stress states. The results reveal that runner damage is mitigated by more than two orders performing a variable speed start-up compared to a classical one with fixed speed technology. Simultaneously, the delay for active power provision is significantly shortened. Thus, the study approves that fast and frequent turbine start-ups are realizable without affecting the runner’s service life thanks to the FSFC.