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Posted by Dan Bloomquist on December 28, 2005, 10:30 pm
 


Derek Broughton wrote:

It is a solution. It is a six phase line so has 18 total conductors.

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 Iain McClatchie on December 29, 2005, 1:58 am
 
It turns out the loss from moving electricity over large distances is
a fairly simple economic calculation.  Maximum practical distances are
about what you'd expect: a thousand miles or two.  The distance
"limit" is proportional to the transmission voltage.

The biggest steel-supported aluminum wire that I could find was from
Southwire:

http://appprod.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet30
   3048 lbs/1000 feet
   1.762 in diameter
   51300 pounds rated breaking strength
   $41/100 pounds = $345/1000 feet
   19 strands steel, 64 strands aluminum
   8.7 mohm/1000 feet

There is an average current target and a peak limit:

1) Cost of material.  Minimum cost is achieved when the present value
   of the electricity lost is equal to cost of wire.

   For a given cross-section, both costs are proportional to the length
   of the wire, so that the optimal cross-section depends directly on
   RMS current, and so on the present value of the watts lost at that
   RMS current.  For example, the above thick cable optimally carries
   347 RMS amps of electricity at 4 cents/kW-hr and a 5% discount rate.

   It should be noted that RMS is a kind of average, and the timespan
   for this average is the operating life of the cable.

2) Self-heating.  This wire is rated up to 200 degrees C, which it will
   achieve in average ambient conditions carrying 3403 amps.  Note that
   these amps are an RMS average over a relatively short time period,
   perhaps a minute, so this is more of a peak limit.

A 350 kV transmission line operating at the RMS current target would
lose about a kilowatt per 1000 feet, but transmit 120 megawatts (per
cable).  You could move those megawatts 1100 miles and see 5% loss.
Since cables usually come in groups of 6, 12, or 18, a link using
these cables could move as much as 2.1 gigawatts, *average*, and have
a peak rating of 21 gigawatts.

A 500 kV transmission line would transmit 173 megawatts per cable and
go almost 1570 miles with a 5% loss.

Note that fatter conductors to reduce losses do not make economic
sense -- the additional cost of the wire is larger than the cost of
the electricity saved.  If electricity costs are projected to go up in
the future, the conductors might get somewhat larger, but not a lot.
Conductor sizing is mostly driven by capacity planning.

Note also that for long distances, higher voltages or HVDC (which is
effectively a higher voltage for more of the time) can make a lot of
sense.


Posted by Jeff Thies on December 29, 2005, 4:11 pm
 <snip>

How do you convert that back (HVDC)? I would think the losses or
technical issues would be substantial.

  Cheers,
Jeff


Posted by Dan Bloomquist on December 29, 2005, 4:54 pm
 

Jeff Thies wrote:
 >someone wrote:

google: hvdc transmission


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 Jeff Thies on December 29, 2005, 5:23 pm
 Dan Bloomquist wrote:


I'm not really seeing this (efficiency) in the search results. I know
that years ago these conversions were done mechanically, but I would
imagine that this is now done with large banks of series/parallel
connected thyristers and some substantial sized caps.

   It does seem that HVDC has substantial benifits in asynchronous
grids. Also in cold starting downed systems, fault tolerance, and
underground cables (no capacitance recharging).  Seems to me that the
major benefits lie other than efficiency.

   Of course, it's not really important that I know this. It's not
likely I'll be running an HVDC line!

   Cheers,
Jeff


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