Re: Skin Effect in Solid/Stranded/Litzendraht Wire -Guy Macon

On Oct 11, 10:01 pm, ChairmanOfTheBored <RUBo...@xxxxxxxxxxxxxxx>
wrote:
On Thu, 11 Oct 2007 18:02:22 -0700, Tom Bruhns <k7...@xxxxxxx> wrote:
On Oct 11, 1:29 pm, Winfield Hill <h...@xxxxxxxxxxx> wrote:
Tom Bruhns wrote:

Yes, at 60Hz, skin effect is significant even in long distance power
transmission lines, whose conductors may be a couple inches in
diameter. In copper at 20C, skin depth is only about a third of an
inch. If the conductor is a bundle of seven strands (each of which
may comprise a multitude of smaller strands, of course), it seems
reasonable to make the center strand steel, for strength, since it's
not conducting much current. Is that done? Or is the corrosion from
dissimilar metals reason to not do it?

I have a one-foot-long cross-section of an ac 115kV HV
transmission cable, from when the 234MW power plant
across the street was installed. It's 4.2" thick with
a 2" dia copper interior. Although the very center of
the conductor carries no current, it's still made of
copper, like the rest. BTW, this is an underground HV
cable. They installed three sets of four cables for
the entire plant output, IIRC, one set being a spare.
The cable has a 1/8" outer plastic or rubber insulation
followed by a 1/6" thick inner lead sheath. I was told
the lead sheath was grounded, to act as a short-inducing
element in the event of a cable breach. The cable was
manufactured by Pirelli, the tire people. It's marked
115kV / 138kV.

Hi Win,

Ah, back when copper was cheap... ;-)

Of course, that's an entirely different application than the one I was
thinking of, where the wire is strung between towers and generally is
not insulated -- and where strength matters. Tim W. says that yes,
cores are commonly steel. I tried a web search just now but didn't
turn up anything useful. Maybe I'll drop a note to our local PUD or
the like and see what they say. I suppose with the current price of
copper, Pirelli would consider making the core from some relatively
inexpensive and much lighter material. I wonder if the construction
of long distance transmission lines was covered in that old power
engineering book you got from me about three years ago. Might be
interesting to compare "then" and "now."

Cheers,
Tom

"Commonly" Try ALWAYS! Long spans between tie points ALWAYS require a
steel carrier strand.

Even long cable TV coax runs need a steel carrier strand to keep them
from damaging the coax at the tie points. Not talking about hard line
here, as that gets bundled to a STEEL carrier strand as well!

Thanks for the input. However, my further research into this,
including conversations with engineers who actually design and install
power transmission systems, makes it clear that the real answer isn't
anywhere near that simple. It's been fun to look into this more
deeply and learn a little more about the infrastructure that delivers
electrical power to us. The answers are out there for anyone
interested enough to look for them.

Cheers,
Tom

.

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