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Coherency between generators is tracked, by examining the instantaneous phase differences among interarea oscillations and swing curves in disturbed multiarea power systems. Huang's empirical mode decomposition is applied to extract dominant oscillatory modes from interarea oscillations/swing curves. Hilbert transform on these modes yields their instantaneous phase. The coherency is thus revealed by the temporal evolution of the phase difference among interarea oscillations/generator swing curves. The technique is illustrated using simulations on a test system, as well as actual wide-area measurements.