Instability of low frequency oscillation close mode shapes in Power System

In a n-machine power system, there are n-1 low frequency oscillation modes and corresponding n-1 low frequency oscillation mode shapes. Frequencies of these modes are distributed in a small range of 0.2-3.0 Hz, thus close modes may occur. This paper first analyzes sensitivity of low frequency oscillation close mode shapes to small change of system parameters, points out the instability of close mode shapes in power system. Then, eigenvector turning angles criterion is presented to identify close modes. At last New England 10-machine system is employed to validate the conclusion.