🚀 go-pugleaf

In article <17838@sun.uucp> cmcmanis@sun.uucp (Chuck McManis) writes:
> "No, they [the diesels] are for cold starting the plant. When bringing a
> new generator on line power from the grid is allowed to spin it up to
> speed using it as a motor. As it gets closer to the speed and phase of
> the power line it is stabilized. Then the steam vents are opened and it
> switches over from using power to producing power at the same phase as
> the voltage that started it."

	I can't quite parse the above, so maybe I'm just missing something.
First you say the diesels are used to cold-start the plant, and then you
say that grid power is used to spin up the generators.  Can you clarify?

	I believe most commercial plants require power from the grid to
start up.  The large synchronous generators have smaller induction motors
on the same shaft for spin-up purposes.  (All synchronous motors need
induction motors to start them because until you get up to speed, the
synchronous motor can't produce any mechanical work.)  The assumption is
that the grid never goes down, just selected parts of it (people who have
built network time deamons will be familiar with this idea).  This is not
always a good assumption.  I believe there was a recent natural disaster in
Hawaii which caused the entire Hawaian grid to go down, causing some
consternation about how to start it all up again.  Of course, The Blackout
was a perfect example of this too.

	Earlier, somebody (I don't remember who) was wondering about
problems with starting up hydro plants which run at low RPM's.  I don't see
any particular problem with synching a low-RPM generator, at least not any
more than with a high-RPM generator.  For those not in the know, steam
driven plants (be they coal, oil, or nuclear fired) generally run at high
shaft speed; typically 4-pole machines running at 1800 RPM.  Hydro plants
usually run higher-pole machines at lower shaft speed.  The problem has to
do with the speed of sound and Reynolds numbers in the varous driving
fluids (high-pressure steam vs. water).  If you tried to drive a water
turbine at higher speeds, you would get turbulance, cavatation, and all
sorts of other horrible things.  Electrical engineers know better than to
worry about the details of stuff like that and leave it to the ME's to
grovel over. :-)

	If I remember, the generators at Robert Moses are 120 pole running
at 60 RPM (1 rotation per second).  The operative equation is f = 2F/N
where f is the electrical frequency in Hertz (60 in North America, 50 in
Europe), F is the mechanical shaft speed in revolutions per second, and N
is the number of poles, always an even integer.  Some texts will write it
as f = F/N, where N is the number of pole-pairs.  You rarely see 2-pole
(3600 RPM) machines because they are harder to wind and don't run very

	A good question to ask is "with all these random (albeit
synchronized) generators all over the place, what's to keep the grid
frequency from drifting?"  The answer is that somewhere on the grid,
somebody is monitoring the line frequency, and comparing it to a standard
(WWV, probably).  Each day, they add up how many cycles the grid lost or
gained compared to the standard and then try to make that up the next day
by running everybody a little fast or a little slow.  I understand that
over the course of a week, the power grid is accurate to a fraction of a
cycle per day.  Considering that you have about 5*10^6 cycles per day,
that's pretty amazing.
-- 
Roy Smith, {allegra,cmcl2,philabs}!phri!roy
System Administrator, Public Health Research Institute
455 First Avenue, New York, NY 10016

"you can't spell deoxyribonucleic without unix!"