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DC Power Transmission

J

JosephKK

Jan 1, 1970
0
Tom Bruhns [email protected] posted to sci.electronics.design:
...

If you assume a two-wire line made from highly conductive round
wires two inches in diameter and spaced 200 inches apart, well away
from any ground and essentially air-insulated, the differential mode
impedance
will be pretty nearly 636 ohms resistive. To get to 1000 ohms,
they'd need to be spaced 4160 inches apart, and kept much further
than that
from ground. It's tough to make a really high impedance RF
transmission line.

If you're operating a line at a frequency where the series
resistance of the conductors is an appreciable fraction of the
series inductive reactance, or the shunt conductance is an
appreciable fraction of the parallel capacitive reactance (in other
words, at low frequencies),
the line characteristic impedance may be quite reactive. The calc I
just did for this hypothetical line at 60Hz and assuming room
temperature copper conductivity suggests about 636-j10 ohms;
assuming I didn't mess it up, you can probably forget about the
reactive part for most calcs.

Cheers,
Tom

From the few handfuls of times that i have made the calculation, it
seems to be about 370 ohms. That can't be a coincidence can it?
 
T

Tom Bruhns

Jan 1, 1970
0
Tom Bruhns [email protected] posted to sci.electronics.design:






From the few handfuls of times that i have made the calculation, it
seems to be about 370 ohms. That can't be a coincidence can it?

Sure it can. The serious high-power ones around here are certainly
higher than 370 ohms, which would have a spacing/wire-diameter ratio
of only 11:1 -- that's for a two-wire line in free-space but should be
quite close for differential mode of two wires of a three-wire line,
well above ground. What I see around here is a spacing/diameter ratio
more like 100:1.

If I'm not mistaken, another advantage of a high-voltage DC line is
much less corona loss than an AC line. -- Yes, since I was unsure
about this, I did a little search and uncovered a paper from ABB at
http://search.abb.com/library/ABBLi...LanguageCode=en&DocumentPartID=&Action=Launch
that has a lot of info comparing high-end (multi-gigawatt) AC and DC
power transmission lines. Looks like typically corona loss from DC
lines and AC lines is similar in clear weather, but much less for the
DC lines in rain or ice conditions. See page 5 of the reference.
Anyway, looks like a pretty good paper that will answer much of what
has come up in this thread, a lot more authoritatively and
quantitatively than the postings here. ;-)

Cheers,
Tom
 
J

JosephKK

Jan 1, 1970
0
Tom Bruhns [email protected] posted to sci.electronics.design:
Sure it can. The serious high-power ones around here are certainly
higher than 370 ohms, which would have a spacing/wire-diameter ratio
of only 11:1 -- that's for a two-wire line in free-space but should
be quite close for differential mode of two wires of a three-wire
line,
well above ground. What I see around here is a spacing/diameter
ratio more like 100:1.

If I'm not mistaken, another advantage of a high-voltage DC line is
much less corona loss than an AC line. -- Yes, since I was unsure
about this, I did a little search and uncovered a paper from ABB at
http://search.abb.com/library/ABBLi...LanguageCode=en&DocumentPartID=&Action=Launch
that has a lot of info comparing high-end (multi-gigawatt) AC and DC
power transmission lines. Looks like typically corona loss from DC
lines and AC lines is similar in clear weather, but much less for
the
DC lines in rain or ice conditions. See page 5 of the reference.
Anyway, looks like a pretty good paper that will answer much of what
has come up in this thread, a lot more authoritatively and
quantitatively than the postings here. ;-)

Cheers,
Tom

Nice link.
 
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