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Understanding Parametric Graphs

Category: PQ Theory

Understand how parametric graphs are useful for waveform analysis.


For monitoring of aggregate loads (e.g. at service entrance, substation, etc.), the parametric plots allow for an easier determination of load impedance.


Impedance can be estimated on a regular waveform capture graph by picking a point on the voltage and current sine waves (at the same time), and dividing, but a more accurate estimate is possible by using the parametric graph, and using the slope of a linear portion.


The vector diagram can also be used for impedance calculations, by dividing the voltage and current phasors, but this assumes a constant impedance throughout the waveform, and ignores harmonics.


Parametric graphs are a less common method of viewing waveforms, but can reveal details not obvious standard time-series displays.


They are especially useful when monitoring specific complex loads, and can be compared to reference signature plots to reveal changes in the equipment, or problems.

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