Since many years the EPFL Laboratory of Electrical Machines develops SIMSEN, a numerical software package for the simulation of electrical power systems and adjustable speed drives, in transient and steady-state conditions, having an arbitrary topology. SIMSEN is based on a collection of modules, each corresponding to one system component (machines, converters, transformers, control devices, etc.). For more information, one refers to: http//simsen.epfl.ch. This contribution presents the extension of SIMSEN to the hydraulic components of a hydroelectric power plant including pump-turbine, valve, penstock, surge tank, gallery, reservoir etc. The basic idea is to define for each hydraulic component an equivalent electric component which can be introduced in the existing electric version of SIMSEN. Doing this, it becomes possible to use the modularity of the electric version to define the complete topology of the hydroelectric power plant and the connected electrical network. This extension makes possible a numerical simulation taking precisely into account the interactions between the hydraulic and the electric parts of the system during transients. It is therefore useful for transient and stability analyses as well as for the design optimization.