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Re: [Phys-L] synchrophasor aka phasor measurement unit

Still in the vein of the 'smart grid', I have heard that the 'generator folks' have pushed thru a 'rule' that allows the frequency to drift away from 60 Hz WITHOUT requiring that any lag be made up for a return to a 60 Hz average over a 'long term'.

I'm having trouble finding the documentation for this idiocy - so it might just be a figment of my overactive imagination.

(I can see no reasonable advantage to allowing such long term drift other than laziness)

On Dec 19, 2012, at 5:20 AM, John Denker wrote:

Hi --

In the context of electrical power distribution, you may have heard the
term "smart grid" bandied about. There is some interesting real-world
physics in this.

One device that is required in order to create a smart grid is called
a _synchrophasor_ aka phasor measurement unit. It measures the amplitude
_and phase_ of the voltage and/or current. The phase is measured relative
to a well-defined standard, typically GPS. A new measurement is performed
many times per second. The results are transmitted to one or more analysis

Based on the analysis, the system can take various actions such as
throttling generators, throwing switches in the distribution network,
et cetera. This allows the system to respond to changing conditions,
e.g. changes in supply (such as a cloud moving over a solar power farm),
changes in demand, and changes in the distribution system (such as
overheating or whatever).

This has tremendous public-policy ramifications, because we can't switch
to renewable energy until we build a grid with lots more capacity and
lots more smarts.

This dovetails nicely with the existing curriculum, because understanding
a synchrophasor requires only one or two concepts beyond what "should"
already be covered. It's good for motivation, because it provides a
strong connection to the real world. A synchrophasor is conceptually
-- a phase-sensitive detector
-- with its timebase locked to GPS
-- plus a communication link.

Note that when physicists want a phase-sensitive detector they typically
grab a "lock-in amplifier". Electrical engineers have a conceptually
similar device that they call a "wave analyzer".

I am aware that lock-in amplifiers are not typically covered in the
introductory physics course, but IMHO they /should/ be. The lesson on
phasors is a lot more meaningful if students see that there is a way
to actually measure such things in practice.

The other point that is somewhat off the beaten track is this: The
power grid utterly depends on the physics of phase. That's how the
grid does load-sharing and achieves some semblance of voltage stability.
If you think of the grid in terms of Ohm's law, without regard to phase,
you'll never understand how it works.

The discussion and diagram here may help:

Rather than three abstract disks, I've thought of using three horses
in three separate horse mills, sharing the load, but I haven't gotten
around to making the required figure.

The point of a horse mill is that there is a notion of phase as the
horse goes around in a circle. A team of horses pulling a wagon in a
straight line illustrates load sharing but not phase.
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