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Re: power-grid physics



Geez, you guys just lost me with the numbers you are using and the way you are
using them. In my past experiences I have supervised the operation and
maintenance of a submarine nuclear power plant and have operated a few others
personally. So, I'm going to start there and see if I can shed any light on
this in a way that makes sense.

The scale of electrical generation that I worked at, without giving away any
classified information, may be considered to be in the order of magnitude of,
say, 10 MW. Propulsion and other loads were in addition to that. The systems
I operated included DC on a battery "float" and 3-phase AC (those two were
linked via motor-generators). This is all open source material so don't
bother wasting time trying to report me to Naval Reactors. For the sake of
comaprison to my ~10 MW total generation, the power flow reversal on the main
trunks in this blackout was on the order of 5000 MW, according to Time
magazine. That's just the size of the *change* in power transmission.

AC electrical generators are designed to have a "speed droop". As the load on
a generator increases, its speed slows slightly and system frequency
decreases slightly. A little thought should convince you that generators with
no droop or with positive droop are an inherently unstable system. A sudden
load increase (as happens with a short circuit!) could cause the generators
to spin up to dangerously high speeds. Control of such generators can be done
but only with much more complicated control circuits than I am used to.
Perhaps commercial plants have such designs, but I doubt it very much.

If an AC system is being fed by #1 generator and one wishes to bring #2
generator online, then #2 generator must be running slightly faster than #1
generator, so that when the #2 generator breaker is shut, some real
("active") load is shifted to it. The amount of load sharing between those
two generators can controlled by varying the "zero load speed" setting of
each generator. If #2 generator were brought online while running slower than
#1 generator it would become a motor and thus a further load on #1 generator.
Turbine generators are not designed to act as motors; the watchstanders in
the area would be spending some time picking turbine blades out of their
teeth.

Generators also have "voltage droop"; increased loading causes voltage to fall
slightly. Normally one operates them with equal or at least similar "zero
load voltage" settings. If they do not, then when they are carrying equal
real ("active") loads, the two generators would be carrying unequal reactive
loads. As I recall, the reactive loads go to the generator with the higher
"zero load voltage" setting.

Loading of parallel DC power sources is controlled soley by voltage settings,
of course, and DC power sources have voltage droop characteristics for the
reasons given above (for speed droop on AC sources).

The "grid" that I am familiar with on submarines is a small example of what
commercial power systems use, but it is much, much simpler. Essentially,
every "bus" ("sector" on a commercial grid) has two or more feed points.
Those feed points share the load on that bus (or sector). If one feed point
opens up (say, a breaker tripping on overload), then the other feed point(s)
try to pick up that extra load. This is what "protects the poweer to" the bus
in the case of a failed generator. But that in turn can cause the breaker(s)
on the remaining source(s) to trip on overload.

Keep in mind that the size of the grid I operated was roughly the length (or
thereabouts) of a submarine. Commerical grids are BIG! That leads to other
and very significant problems, which are related to this blackout, but I had
better save that for another email.

Please (!) let me know if this is boring you. I'll gladly shut up.

Jim

On Wednesday 2003 August 20 19:32, you wrote:
At 02:37 PM 8/20/2003 -0500, you wrote:
Maybe we need a tutorial for those of us somewhat AC-impaired. Let me
propose this simplified scenario and see if anyone on the list knows how
this works in reality.

Power is off. Two generating plants, each with a max output of 1000
Watts--generators at 100 Volts. One transmission line at 1000 volts. A
load (still switched on) of 1600 Watts--say two factories each wanting 800
Watts at 100 volts.

OK--how do you turn the power back on? How does the phase get
synchronized? What, if any, role do the transformers play in this
scenario?

??Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu

Treating this as a hypothetical puzzle - which I suppose it really is
if I mention there is no relation to reality:
I spin up generator 1 to 60 Hz, at 70 volts, at which point the
load is drawing 70^2 / 100/16 watts. That's 784 watts.
Then as soon as possible, I spin up the second, and when it
self-synchronizes, it then lifts network volts slowly so as not
to exceed 1000 watts at either source.



Brian Whatcott Altus OK Eureka!

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