PURPOSE: To output a period of electric power fluctuation and a damping time constant, by always calculate-processing by on-line a data of a system disturbance of some extent, which occurs frequently in an electrical power system.

CONSTITUTION: With respect to electric power of a transmission line of an electrical power system, a voltage phase difference between two separated buses, and a difference of frequencies of two buses, an electric power fluctuation mode of the worst damping is extracted by an electric power fluctuation component discriminating filter 2 from an input signal 1. An output X0(t) is subjected to time differentiation by a differentiator 4, X1(t)=dX0/dt 5 is obtained, and also is subjected to time differentiation by a differentiator 6, and X2(t)=dx1/dt 7 is obtained. x0(t) 3, x1(t) 5 and x2(t) 7 are sampled by a sampler 8, and numerical signals x0(k), x1(k) and x2(k) 9 are generated. Subsequently, a fluctuation period, an attenuation time constant and magnitude of fluctuation 11 of the electric power fluctuation mode of the worst damping of the input signal 1 are estimated by a linear least square method parameter estimating device 10.