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Posted by R.H. Allen on December 27, 2005, 3:20 pm
 
Johnny wrote:

It's probably fair to say that they're primarily from resistance. There
are other factors too -- losses during transformation up and down in
voltage, for example. And because we're talking AC, there's something
called the skin effect that ensures the effective resistance (or, more
correctly, the impedance) will be higher at low voltages than at high
voltages, which provides another reason to use high voltages for
long-distance transmission.

Posted by Jeff Thies on December 27, 2005, 4:13 pm
 
R.H. Allen wrote:

I think you mean frequency. Not that 60Hz is that high a frequency and
should not have much skin effect, which is more a concern at RF frequencies.

   It appears that corona losses become significant over 1,000,000 V,
which is probably why you don't see voltages that high.


     Cheers,
Jeff


  than at high


Posted by R.H. Allen on December 28, 2005, 7:43 pm
 Jeff Thies wrote:

No, I meant voltage, though I admit I was pretty obtuse about why.


Here's what I was getting at: The skin effect limits the useful diameter
of the transmission wire. Granted, at 60 Hz it's still quite thick (the
skin depth is something like 8.5 mm), but when you're transmitting
megawatts or gigawatts it imposes a significant limitation. In a DC
cable you can simply increase the wire thickness to combat heating and
the associated increase in resistance, but in an AC cable that only
works until the cable thickness is equal to the skin thickness. After
that, increasing the wire thickness has no effect on resistance. It's
not a big deal when you're talking about low-power applications, but in
high-power transmission you rapidly reach the point that you can no
longer control losses by increasing wire diameter and must increase
transmission voltage instead. Indeed, heavy transmission line cables are
often made of steel with a low-resistance outer layer such as aluminum,
since the skin effect ensures the steel at the center won't be carrying
any current, and bus bars for AC power applications are often made from
hollow tubes.



Posted by Dan Bloomquist on December 28, 2005, 8:24 pm
 

R.H. Allen wrote:

Another solution is multi cable runs. We have high tension running up
highway 87, AZ, and the cables are in groups of three with about two
diameter spacing.

Best, Dan.

--
"We need an energy policy that encourages consumption"
George W. Bush.

"Conservation may be a sign of personal virtue, but it is not a
sufficient basis for a sound, comprehensive energy policy."
Vice President Dick Cheney


Posted by Derek Broughton on December 28, 2005, 9:39 pm
 Dan Bloomquist wrote:


Is that a solution or a requirement?  I thought the "high tension" had to be
in groups of three for multi-phase.
--
derek

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