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Power system oscillatory behavior can be analyzed in terms of modes, expressed as exponentially modulated sinusoids, exhibited in signals measured on the system. These signals are driven by the behavior of a large, nonlinear, time-variant system. In this paper, two modal identification methods are examined comparatively: Prony analysis and a method based on the Hilbert transform. Considerable structural differences exist between the two methods. Prony analysis yields modes which are directly expressed as exponentially modulated sinusoids, whereas the Hilbert method provides a more general solution. Synthetic and measured signals are used in the comparison. Some general conclusions are drawn from the analysis of several signals, including two sets of measured field data.Index Terms-Dynamics, empirical mode decomposition (EMD), Hilbert transforms, Hilbert-Huang method, interarea oscillations, power system dynamic stability, power system measurements, power systems, prony analysis.

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A considerable body of literature examines assessment, from measurements, of whether it is the network or a customer installation which makes the greater contribution to harmonic distortion at a point of common coupling. However, the customer contribution to harmonic distortion at a point of common coupling depends heavily upon the definition chosen for that contribution. For example, expressing contributions as currents instead of voltages or vice versa may lead to large changes in results. Further, it can be shown that the harmonic voltage at the point of common coupling cannot be expressed independently of the network conditions, meaning that the customer contribution under existing definitions is a function not just of the customer parameters but of the network parameters as well. In the harmonic framework described by the technical report IEC/TR 61000-3-6, adopted as a standard in some jurisdictions, each customer installation is entitled to a harmonic allocation; that is, the right to inject a certain quantity of harmonic distortion into the network. IEC/TR 61000-3-6 suggests procedures for determining the allocated emission levels based on the harmonic voltage planning level at each bus. The problem of harmonic source detection can be recast as a search for customer installations exceeding allocated injection levels. With this approach, the challenge ceases to be comparison of the contributions made by the network and customer sides to observed harmonic distortion. Instead, it is shown that the problem becomes a process of reconciliation of the allocated quantity with field measurements.