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Re: power-grid physics



I believe multiple-generator matching within a power plant, and matching
generators to the grid, are of utmost importance. Otherwise, when the
generator-to-generator connection is made, or when the generator-to-grid
connection is made, tremendous strains are placed on the generator. I
believe this can literally tear the generator apart.

I think I have a basic idea of how it is done, and I describe this
below.
Some of my info is from having taken many tours of dams and other
hydro plants. Much to my wife's chagrin, I cannot pass a major dam
without stopping for the tour, if a tour exists.

We control the frequency of the generator by the throttle-governor
system. We control the voltage of the generator by the DC excitation
current that magnetizes the field magnet.

We bring the generator very close to the same frequency as the grid or
generator we want to connect to. Then we match voltage by varying the
DC excitation current on the generator. Then we monitor the voltage
difference between the generator output and the system we want to
connect to. If the frequency, phase, and voltage perfectly match, there
will be no potential difference between the two. That's when we would
like to close the switch. Most likely the frequency and/or phase will
not match exactly, so the throttle-governor is fine tuned. If the
frequency is very close, but not exact, the potential difference will
slowly oscillate from zero (in phase) to double the voltage (180 degrees
out if phase). When we get the frequencies almost matched this
oscillation will be very slow. When the oscillation is slow enough
(more like minutes than seconds) we close the switch at the zero
crossing of the voltage difference.

Contrary to what another respondent stated, the newly added generator
will not slow down. When connected in this manner it neither supplies
energy to the grid nor takes energy from the grid. If there's no
potential difference between the generator and the grid (or other
generator) then no current flows. That's the condition under which we
want to make the connection. If a perfect connection is made, neither
system "realizes" a connection has been made. After the connection has
been made we can slowly raise the excitation current to raise the
voltage of the new on-line generator. This is when the new generator
begins to supply power to the grid. This is done slowly so that the
governor-throttle can maintain the frequency and phase lock.

There is an instrument used by power companies to more easily allow this
matching. It is called a synchroscope. If you enter this word into a
Google search you will get about 1000 hits. Some of them explain how
the synchroscope works.

This one has a Java applet to show a synchroscope in action...
http://www.eece.maine.edu/Power/Java/synchro/synchro.html

This one explains how it works and how it is used...
http://www.tpub.com/fireman/121.htm


Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Chair of Sciences
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu