NIST offers power grid calibration service

Published 23 August 2007

Terms like “phasor and “sinor” are not taken from a Star Trek episode: NIST offers a phasor-based service to help US. grid operators better calibrate surges

Worried about blackout or brownouts in your area, or how the local utility will cope with a sudden disruption caused by a natural disaster or act of terrorism? There is a new service to help operators do just that. You may think that the new calibration service for phasor measurement units (PMUs) offered by the National Institute of Standards and Technology (NIST) would appeal to Star Trek fans, but it is actually the operators of America’s electrical power grid — and all of us who value uninterrupted current — who benefit. The new NIST service provides calibrations for the instruments that measure the magnitude and phase of voltage and current signals in a power system — a combined mathematical entity called a phasor — and reports the data in terms of Coordinated Universal Time (UTC, also known as “the official world atomic time”). Use of absolute time allows measurements called phase angles taken at one location on a power grid to be comparable to others across different systems. Phase angles and their derivations allow grid managers to know the operating condition of their portion of the system and determine if action is needed to prevent a power blackout.

The new NIST calibration service has already yielded two additional benefits. First, a major PMU manufacturer reports that using the calibrations during the manufacture of its instruments has improved their accuracy by a factor of five. Secondly, some PMUs that have been calibrated using the NIST service have revealed incompatibilities in the message format they send out, leading to corrections that have improved interoperability between PMUs across power grids.

For the mathematically inclined:

In circuit analysis a phasor is a vector with constant length (A) and constant phase angle (θ). It is typically represented as a complex exponential, A•ejθ. Phasors are used to simplify computations involving sine waves, where they can often reduce a differential equation problem to an algebraic one (in school, we were always grateful for this). In that application, A and θ represent the amplitude and phase of the wave. Now, phasors and sinors are important concepts for many different problems, and readers familiar with complex numbers and plotting complex numbers would appreciate this. They basically replot a sinusodial onto the complex plane with the y-axis being imaginary and the x-axis being real. They do this by relating A cos (\omega t + \phi) = Re(Ae^{\phi}e^{\omega t}) from Euler’s formula. The phasor is a vector in this domain with origin at 0,0. The phasor is stripped of the time component. The sinor is a rotating phasor (that is, time component added).

Talking about the time component: To understand how phasor and sinor represent the time dependant function sin (\omega t + \phi), drop down a shadow from the sinor vector onto the real plane: This is the magnitude of the signal in the real plane at any time t. You see that as the sinor rotates, the shadow on the x-axis shrinks and grows, just as you do with the time-dependant sinusodial. The component in the imaginary axis is like a “conserved” portion of the signal. Full to appreciate phasors, one needs to try and solve some problems. When you do, it will be immedialtey apparnet that the elimination of the time dependant term helps simplify calculations considerably.