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There is a low-frequency interarea oscillation mode in middle and western 60-Hz areas of Japan. Its stability determines available transfer capacity of a tie-line. We propose a method of monitoring its stability by phasor measurements synchronized using the global positioning system. The phase angle between two places deviates randomly. However, its Fourier spectrum shows a clear peak corresponding to the interarea mode. We estimate its frequency and damping from the spectrum. The estimation results agree well with those obtained by curve fitting for large disturbances. It is also possible to estimate the eigenvector. We examine how the eigenvalue varies with the total generation and power flow in the areas.

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A combined cycle power plant, which combines a gas turbine and a steam turbine, can achieve high energy efficiency. Many combined cycle plants have been installed in the world. However, a large-scale blackout occurred in Malaysia in 1996. Combined cycle and gas turbine plants sequentially tripped out. The cause of this chain trip was thought to be a system frequency drop. Considering these backgrounds, it is important to study dynamic behavior of combined cycle plants. Several dynamic models of the combined cycle plant have been proposed. In our analysis, we use some of them and build a model for a single-shaft combined cycle plant. We execute numerical simulations to see how the combined cycle behaves when the system frequency drops. Index Terms-Combined cycle, dynamic models, gas turbine, plant trip-out, under frequency. Naoto Kakimoto (M'78) was born in 1952 in Japan. He received the Ph.D. degree in electrical engineering from Kyoto University, Kyoto, Japan, in 1982. Currently, he is an Associate Professor at Kyoto University. His current interests are power system stabilities, power electronics devices, and distributed power generation. Dr. Kakimoto is a member of IEE of Japan. Kazuhiro Baba was born in 1978 in Japan. He received the M.S. degree in electrical engineering from Kyoto University, Kyoto, Japan, in 2002. His current interest is stability of combined cycle power plant. Mr. Baba is a member of IEE of Japan.

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.