This paper explains the general principle of hydropower plant system. Specifications and data are listed for each element of the supply system for HE Senj. Losses are grouped into local and longitudinal, with the methods of of calcualtion explained. Different ways of fluid flow are listed (laminar and turbulent flow), with their differences explained. In addition to the numerical loss calculation, losses are shown via simulation in the AMF Fantom program. AMF Fanthom is a program used for dynamic fluid simulation, pressure drop calculation and flow display in liquid and gas piping and duct systems. The paper explains the most significant undesirable phenomenon of hydropower plants - hydraulic shock. The path of the wave that occurs during the closing of the valve in the valve chamber and the change of pressures inside the pipe between the supply pipe and the valve chamber is shown in detail. The hydraulic shock was simulated by the AMF Impulse program to create a dynamic simulation of pressure overvoltage in piping systems caused by water jet. The simulation shows the oscillation of water masses in the water chamber of HE Senj and a comparison of simulations with different valve closing times and changes that occur in the chamber itself is also made.