The final thesis describes hydroelectric power plant like a big drive which consists of more parts, here were described cycles through which energy passes, conversion from the potential and kinetic energy into mechanical and then into electrical energy. The final thesis briefly describes the basic parts of hydroelectric power, advantages and disadvantages, how to and why to regulate frequency, voltage, active and reactive power. There are basic calculations that are essential for understanding hydropower plant. At the end, the final thesis describes modern ways of converting energy of water in tidal plants and wave power stations that are still under development and not so widespread