The transient behavior of hydroelectric power plants is of high interest for ensuring stability of islanded electrical power networks. Therefore, it is suitable to determine and validate the set of parameters of a turbine speed governor using a realistic simulation model taking into account the hydraulic circuit, the rotating inertias, the electrical installations and the control systems. This paper presents the modeling of a 4x250 MW Francis turbine hydroelectric power plant taking into account the dynamics of all the aforementioned components. The transfer function of the turbine is determined through a time domain simulation using white noise excitation for the determination of an appropriate set of parameters. The performance of the regulator parameters are assessed using two different models: (i) a hydraulic model and (ii) a full hydroelectric model including the model of an islanded power network. The stabilization effect of Standard Power System Stabilizer (PSS) is evidenced.