The article considers the problem of defining the instantaneous values of power system state parameters for the purpose of investigating dynamic processes in non-linear systems. It is shown that existing methods of parameters calculation which provide the resulting values of sampling rate equal to industrial frequency does not allow to estimate the parameter ‘s value at an arbitrary time with high accuracy. The method of defining parameters’ instantaneous values based on the modified Hilbert transform is proposed. The method is validated involving simulation results and compared to the other known methods. The research results provide the possibility to investigate the phenomena taking place “inside” the industrial frequency period and to assess more precisely the dynamic characteristics of define power systems.