There is a low-frequency interarea oscillation mode in middle and western 60-Hz areas of Japan. The mode is damped, i.e., stable for small disturbances, but diverges in an oscillatory manner for large disturbances. It restricts power transmission in the areas. This paper shows that the instability is caused by nonlinear interactions between a few modes. The areas are represented with West30-machine system model prepared by the IEE of Japan. Numerical simulations are executed to obtain a critical case in which generator swings last without diverging or damping. Natural oscillation modes contained in the swings, are then calculated. Two modes have strong interactions with the interarea mode, and act so as to deteriorate its damping. Last, we examine how their influence changes with load level of the system.

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