The problem of control of electric generator whose model is described by a third-order differential algebraic equation with a priori unknown parameters under uncontrollable external disturbance defined by an unknown time-dependent power is solved. When the problem is solved, it is assumed that the relative speed of the generator's rotor can be measured. A control algorithm ensuring small amplitude of the rotor vibrations in the quasi-steady-state mode due to changes in the voltage at the excitation coil of the rotor is proposed. This algorithm also ensures the stable operation under emergency conditions caused by changes in the reactive resistance of the power transmission line. Numerical examples and the results of computer simulation are presented to illustrate the performance of the proposed control algorithm.