Distinguishing Between Natural and Forced Oscillations Using a Cross-Spectrum Index

Two different types of oscillatory responses often occur in power systems: natural and forced. Natural oscillations are characterized by the electromechanical eigenvalues (or modes) and are typically initiated by a system discontinuity (e.g., a fault). Forced oscillations are caused by some external input driving the system into a sustained oscillation. A fundamental goal is to distinguish between a very lightly-damped (or undamped) natural oscillation and a forced oscillation based upon system-wide measurements. In this paper, a cross-spectrum index function is developed for distinguishing between the two types of oscillations based upon time-synchronized PMU measurements. A simulation and a real-life event are used to demonstrate key points.