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Re: power-grid physics



Comments below...

On Thursday 2003 August 21 11:59, you wrote:
....
.... It is my understanding that when the synchroscope is used, an
exact frequency match is not achieved. It's the slight frequency
difference that allows the phase to vary and therefore provides perio=
dic
opportunities for closing the switch. Once the phase is varying
sufficient slowly (ie frequencies matched close enough), the generato=
r
is brought on-line during one of the times that the synchroscope
indicates an in-phase condition. This could be done with the generat=
or
running either a bit slow or a bit fast. But having the generator
running a bit fast makes better sense than having it running a bit sl=
ow.
As Jim said, having the generator come on line as a motor would be mo=
re
harmful than having it come on line as a generator with a small load.

Mike, you've got it nailed! Yep, the synchroscope needle goes completely
around the meter since it is driven by a syncro receiver. At "12 o'clock",
the two inputs are in phase. Having watched about 4 moles of synchronizations
over my career, I would put the average sweep rate for that needle at about 1
revolution per 4 s or 5 s. So, we're talking about a frequency difference of
0.20 Hz to 0.25 Hz --- ballpark. I'll leave out the gory details of how we
got there and what made the desparate move necessary, but I saw a very
skilled electrical panel operator make a "no jump", dead-on synchronization
with the needle moving at about 2 to 3 revolutions per second. (His hand
started to move the switch at about "a quarter till" and the breaker contacts
closed right on time!) I suppose I should have written him up for violating
procedurally defined "best practices", but what the heck --- I'm sitting here
on dry land today and that's nice.

I'm still a bit confused about power sharing. It is clear to me that=
a
generator hooked to a grid could provide power to the grid either by
have a voltage slightly higher than the grid, or by having a phase th=
at
slightly leads the grid. I always assumed adjustment of the voltage =
was
how the loading was adjusted. I understand that a temporary speed
increase of the generator can make it lead the grid and thus increase
its power contribution to the grid. But why would one do that as
opposed to keeping the phases matched and raising the voltage?

In a DC circuit that is what is done and that is intuitively obvious. In an
AC circuit, it's a whole lot more complicated and I cannot think of a simple,
emailable, written explanation that would explain that. I suppose I could dig
back into my 30+ year old notes on how phasors work and how to use them to
describe non-steady state systems, but that would take a lot of time and I'm
not sure I would succeed in comprehending the mathematical reasoning anymore.

....
How does a power plant make its initial connection to the grid? Do w=
e
have to have some sort of synchroscope connected to the grid and
power-plant output at the high-voltage end (after the transformers an=
d
at the location where the connection to the grid takes place)?

Mike, I don't know the answer to that, but it seems like there's got to be a
synchroscope and an operator that you trust a whole lot or there's got to be
a computer controlled system (that you trust a whole lot!) to do it for you.
I'll ask my dad the next time we talk. He designed some of that stuff that's
in the grid (including the first megavolt transmission line, as I recall).

....
Can anyone shed any light on what happens beyond the power plant?=

There are some thoughts that I would like to share on that issue because it
relates to what we frequently hear: It's difficult to re-energize a cold
(dead) transmission grid sector. Heck, it's exciting enough just to
re-energize a dead bus with its loads! I had better address that later,
though, and get a few necessary things done here first.

Jim

--

James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

http://www.cofc.edu/~frysingj
frysingerj@cofc.edu
j.frysinger@ieee.org

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