The principle and testing scheme of a novelon-line method of identifying the lower frequency oscillationin power system is presented in this paper. For the firsttime, the application of this on-line method to test the lowerfrequency oscillations in power system is described. Usingthe method the lower frequency oscillation in operating system can be studied at any time. According to the result ofusing this method in Oct. 1991 in Fujian power system, itwas confirmed that the SKX hydroelectric power station wa...