An analytical approach allowing modelling transient phenomena in pipes, valves, surge tanks and Francis turbines based on impedance method is developed. These models are implemented in a software called “SIMSEN”, which simulates the behaviour of complex applications in the domain of adjustable speed drives and electrical power networks. This program is based on a modular structure, which enables the numerical simulation of 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 an integrated set of differential equations. This algorithm solves simultaneously the electrical, hydraulic and control equations ensuring a proper interaction between the three parts of the system. The case of a Francis turbine power plant is studied. The model of the turbine is based on measured steady state characteristics. The simulation of the dynamic behaviour of the power plant under load variation is investigated.