Power swings in hydroelectric power plants as well as oscillations and vibrations, are common problems in operation of some power units. Researches of such phenomena on bulb type units with horizontal Kaplan turbines are rare. Because of their complex design, such units can have a variety of problems with oscillations and vibrations, and due to small moment of inertia have poor stability characteristics. Power oscillations have been present on the HPP Dubrava since the operation beginning. In the case of large disturbances appearance, power oscillations become problematic for unit operation. Numerous researches were carried out, but explanations of the phenomenon are different. One approach starts from mechanical torque disturbance, and second from electromechanical resonance as the cause of power oscillations. Analyzing a large number of cases, power oscillations phenomena are identified, and comparison of measured and simulated results have shown that power oscillations could be interpreted by simulation on mathematical model. It may be concluded that occasional power oscillations appear due to mechanical torque disturbances caused by hydraulic phenomena in water conduit system. Furthermore, effects of additional control signal in excitation system are analyzed, but with presence of forced oscillations possibilities of power system stabilizer action are limited. Considering that the possibility of oscillations appearance can’t be eliminated, decision algorithm for treatment of large power oscillations based on disturbance parameters is defined.