Set Your Phasors: Employing phasors for correct measurements

摘要

Though It Wasn't The Result Of Watching any "Star Trek" reruns, the concept of phasors (or phasers to Trekkies) has been raised in a couple of recent discussions. The first was relative to using a phasor diagram to verify the proper connection of the voltage and current sensors to a power quality-monitoring instrument to get the correct value for all power parameters. The second instance came about in the review of several recent articles in the trade journals on phase-measuring units (PMUs), instruments that don't just monitor the voltage and frequency on electrical distribution and transmission systems. They have the ability to monitor the phase angle of the voltage (and current) phasors with respect to time. The phasor is a mathematical representation of a periodic signal from the time domain into a single line with a magnitude and direction on a polar chart, rather than the X and Y coordinate-based graphs used to plot the sine waves themselves in the time domain. In three-phase power systems, each phase is represented by a single vector or phasor. In a normal three-phase system, one would expect that the three voltage phasors would be of equal magnitude and located 120 degrees apart from each other. By convention, we usually plot the Phase A voltage of a wye circuit or Phase A-B in a delta circuit at zero degrees (at the three o'clock position). Whether the system has a clockwise or counterclockwise rotation will determine where the Phase B and Phase C (or Phase B-C and Phase C-A phasors) will be located on the polar plot.