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An analysis method for low frequency oscillations in large power systems is presented in this paper. The method combines the eigenvalue analysis method and the Prony analysis method of PSS/E. The frequencies, mode shapes and participation factors of the system oscillation modes are calculated using the eigenvalue analysis method with the classical generator models. And the damping is calculated using the Prony analysis method with detail generator models and exciter models. The rigorous eigenvalue analysis results of a two-area, four-machine test system and New England 39-bus test system show that the basic characteristics of the system oscillation modes, such as the mode frequencies, mode shapes and participation factors, can be obtained by using the classical generator models. Thus, the method proposed by this paper is suitable for analyzing low frequency oscillations in real large-scale power systems. As an application of the proposed method, the low frequency oscillation characteristics of East China power grid is analyzed, and the frequencies, damping, mode shapes and participation factors of the inter-area oscillation modes in the grid are obtained.