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Re: power-grid physics



Aha! That's where phase shifting devices come into play!

Remember the effects that inductors and capacitors have on phase shifting of
the line voltage with respect to the current. Distribution systems are
inherently inductive (lagging power factor) due to line inductance (which
usually is larger than line to ground capacitance) and loads sum up to an
inductive nature. There are a lot of motors out there!

Power companies use capacitor banks (and spinning capacitors) to improve their
power factors (minimizing the ohmic losses in the lines due to the
circulating reactive load currents) but they can also use those to shift
their voltages closer to or away from the current. When paralleling, it's the
voltage difference and phase across the soon-to-be-closed gap that's
important --- not the current between generators and loads. Ideally, when a
breaker gets shut, you want zero current to flow through the contacts.
Otherwise you get a heck of a big arc (too big to be called a spark and too
small to be called lightning). So one just phase shifts these voltages to
make the closure. By the way, there's not a lot of this going on in the grid;
the breakers are almost always shut. And that's a good thing because at
kilovolt (and now megavolt) line potentials, a small phase error can be a
heck of a lot of volts!

Adjusting the "line conditioning elements and controls" is how loads are
shifted one way or another. Of course, once the breaker is shut, the contacts
on eiither side of it are indeed in phase.

Jim

On Wednesday 2003 August 20 22:37, you wrote:
On 08/20/2003 10:31 PM, James Frysinger wrote:
it is damned important to match phases when paralleling generators!

OK, I'll bite.

What do you do if you're running a *grid*
and you've already got a C-shaped connection
pattern (three sides of a square) and you want
to close the switch to an additional tie-line
to complete the square.

I see no reason to expect that the phases
automagically match across the switch before
closure. So,
-- Do they have some way of arranging a match?
How?
-- Do they just close the switch without a match?
What happens then?

--
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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