An analytical approach allowing modelling transient phenomena in pipes, valves, surge tank and Francis turbines based on impedance method is developed. These models are introduced in a software called “SIMSEN”, which simulates the behaviour of complex applications in electrical machines and power networks. This program is based on a modular structure, which enables the numerical simulation in transient modes of systems exhibiting arbitrary topologies. The numerical simulation for transient phenomena in hydropower plants with “SIMSEN” has the benefit of an algorithm that generates and solves automatically the main set of differential equations. This algorithm solves simultaneously the electrical, hydraulic and regulation equations assuring the interaction between the three parts of the systems. Three cases are investigated demonstrating the capability of these models to reproduce the “wave propagation”, the ”mass oscillations” and the transient behaviour of Francis turbine.