This PhD thesis is framed within the XFLEX HYDRO project, funded by the European Union's Horizon 2020 research and innovation program under grant agreement No 857832. The ultimate objective of the XFLEX HYDRO project is to increase hydropower potential in terms of plant efficiency, availability and provision of flexibility services to the Power System. Within this project, the research activities focus on the improvement and integration of flexible technological solutions to optimize operation and maintenance of Hydro Power Plant (HPP) while providing ancillary services to the power system. The achievements of these activities will be implemented and demonstrated at seven demonstration sites located in Europe. The demonstration on-site of the implemented flexible technologies will also allow for the validation of a methodology for increasing the HPP flexibility and to draw a road-map for future HPP projects. The proposed PhD work will significantly contribute to the achievement of these objectives through the investigation and modelling of the hydraulic machines behaviour operating under fast dynamics and extended operating range by performing innovative experimental campaigns and forecasting techniques. In this framework, the three main research questions which are addressed in this PhD thesis are listed here below, and the proposed solutions will be demonstrated in the corresponding full-size HPP: • Pump-storage Power Plant : How to improve the transient sequences such as starts and stop for a unit equipped with a Full Size Frequency Converter (FSFC). Z'Mutt demonstrator is a pumping station feeding the lac des Dix in Wallis, Switzerland. One of the 5 units of the station is under refurbishment and will then be operated as a variable speed unit with a FSFC • Run-of-the-River power plant : How to reduce wear and tear in Kaplan turbines by hybridization with a battery energy storage system or by implementing variable speed technology. Vögelgrun run-of river power plant will be hybridized with a BESS. The hybridization will allow for a reduction of components wear and tear as well as improving ancillary services. This solution will be compared with the upgrade of the same unit with variable speed technology. • Reservoir Storage Power Plant How to enhanced fixed speed technology through operation range extension. The main goal is to achieve an almost continuous power from near zero to rated power and compare the these results with the variable speed technology. Alto Lindoso demonstrator site will provide demonstration of the proposed solutions in its two high-head Francis units operated with fixed speed generators. To answer to these research questions, four experimental campaigns will be held at EPFLPTMH facilities. The reduced scale physical models of the corresponding hydraulic turbine geometries will be instrumented by means of both on board pressure sensors and strain 4 gauges. The test-rig is modified to allow high dynamic transient sequences under different real grid behaviours and to integrate a battery energy storage system. The measurements collected during these experimental campaigns will be used to feed meta-models such as efficiency and damage hillchart which will characterize and predict the behaviour of the hydraulic machines under enhanced flexible operation. An advanced control system will then leverage these meta-model for an optimal HPP management.