Reactive power shortage is one of the current issues that endanger the stability of the power grid systems. Some massive blackouts have been experienced in the past years due to the unbalance of reactive power. One of the solutions to provide the necessary reactive power to the grid is to operate generators as motors and supply or absorb reactive power when needed. Hydraulic turbines are the most suitable machines to perform this task since they can be easily and quickly dewatered to work as motors (Synchronous Condenser (SC) operation). Nowadays, hydraulic turbines are operating as SC for several hours per day, especially in grids like in US or Canada. The change from generating mode to SC and vice versa can be very fast, depending on the grid requirements. However, these fast changes may affect the mechanical behavior of the machine, reducing its useful life-time or increasing vibrations. In this paper, the effect in the mechanical system of SC operation in a Francis turbine prototype is evaluated. Strains in the runner and vibrations in the stationary parts are analyzed in detail in the fast changes between generating to SC and vice versa.