Why do plylon transmission towers carry six wires?
Tom
Why do pylons, most that catch my attention anyway, carry six wires,
three, vertically stacked on each side.
THen sometimes there's one right at the top from the apex of each
tower.
My wild guess was that the six cables represent a send and return path
for the 3 phases.
Can anybody correct me?
Michael Stemper
>A question that I have been curious of for years.
>
>Why do pylons, most that catch my attention anyway, carry six wires,
>three, vertically stacked on each side.
Because they're carrying two transmission lines. In US terminology,
this would be called a "double circuit" structure.
>THen sometimes there's one right at the top from the apex of each
>tower.
That would be a shield wire, designed to reduce the likelihood of a
lightning strike on one of the circuits or lines.
>My wild guess was that the six cables represent a send and return path
>for the 3 phases.
Nope. Each of the three phases acts as a "return path" for the other
two. Or, turning it around, for any phase, both of the other two act
as its "return path".
>Can anybody correct me?
Gladly.
--
Michael F. Stemper
#include <Standard_Disclaimer>
Nostalgia just ain't what it used to be.
phil-new...@ipal.net
| Because they're carrying two transmission lines. In US terminology,
| this would be called a "double circuit" structure.
[...]
| Nope. Each of the three phases acts as a "return path" for the other
| two. Or, turning it around, for any phase, both of the other two act
| as its "return path".
But why two circuits?
FYI, I have seen the same tower design with only one side populated.
There is one location I saw where the transmission line with 6 wires goes to
a point where it splits 2 ways. One set of 3 from one side went one way and
the other set of 3 from the other side went the other way. There was also a
set of 3 that come from one of the forks and went down the other. So it is
not obvious which way the power actually comes from. Maybe it could be
rerouted, anyway. Maybe there is redundancy here.
I also see those larger towers around which stand from a single corner of a
triangle, with the opposing side up high and horizontal. That horizontal
bar has some extension to the side, and the side supporting it have some
extension above (with thing "ground" wires I'm guessing). The far ends of
the horizontal extension carry big heavy wires, along with a point in the
middle doing so, so the wires are (approximately, if not exactly) in line
horizontally. I've been told in reference to one of these, that they are
carrying 765000 volts, which I presume to be delta. So if it is currently
carrying 1000 amps, that would be about 1325 MW if my arithmetic is right.
Those things are crackling quite a bit, so I wonder how much loss that is.
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phil-new...@ipal.net
| My wild guess was that the six cables represent a send and return path
| for the 3 phases.
You don't actually need to return power that way. But, maybe they could
be phased that way (6 different phase angles) in some circumstances.
Try these on for size:
http://phil.ipal.org/usenet/miue/2008-07-25/powerlines.jpg
I'm quite sure these _are_ phased evenly around a circle.
Paul Hovnanian P.E.
>
> On Fri, 25 Jul 2008 22:19:36 +0200 (CEST) Michael Stemper <mste...@walkabout.empros.com> wrote:
>
> | Because they're carrying two transmission lines. In US terminology,
> | this would be called a "double circuit" structure.
>
> [...]
>
> | Nope. Each of the three phases acts as a "return path" for the other
> | two. Or, turning it around, for any phase, both of the other two act
> | as its "return path".
>
> But why two circuits?
>
> FYI, I have seen the same tower design with only one side populated.
The other side is for future expansion.
--
Paul Hovnanian mailto:Pa...@Hovnanian.com
------------------------------------------------------------------
I bet the human brain is a kludge. -- Marvin Minsky
phil-new...@ipal.net
| phil-new...@ipal.net wrote:
|>
|> On Fri, 25 Jul 2008 22:19:36 +0200 (CEST) Michael Stemper <mste...@walkabout.empros.com> wrote:
|>
|> | Because they're carrying two transmission lines. In US terminology,
|> | this would be called a "double circuit" structure.
|>
|> [...]
|>
|> | Nope. Each of the three phases acts as a "return path" for the other
|> | two. Or, turning it around, for any phase, both of the other two act
|> | as its "return path".
|>
|> But why two circuits?
|>
|> FYI, I have seen the same tower design with only one side populated.
|
| The other side is for future expansion.
For the tower with an empty side, sure, there is room for expansion. But
there are quite few of those. The vast majority of towers with 6 points to
hold wires have all 6 in place (I not going to climb up there to see if all
6 are actually energized).
IWSTM, if all 6 are used, then they could run them at 6 distinct phase angles
at 60 degree intervals and get a 79.7kV difference between near wires instead
of the 138kV of just 3 wires. IOW, 2 separate 3 phase systems interleaved
with each other, in approximately circular geometry by spacing the mid-height
wires on each side a bit further out from each other. I don't know if that
would impact the reactance issues, or raise the cost of transformers to supply
it (e.g. as a double-wye instead of delta).
You've seen this: http://phil.ipal.org/usenet/miue/2008-07-29/powerlines.jpg
What I'm thinking of is the configuration on the left with 6 wires, but on a
larger scale and instead of a structure around the conductors, a more common
tower "up the middle" with arms out to hold the conductors.
Paul Hovnanian P.E.
Either configuration (your photo or your description of a single tower
with horizontal insulators) can carry either 2 3-phase circuits or a
single 6 phase circuit.
The major issues with 6 phase lines are with the small improvement in
efficiency gained for the expense of interconnecting it with a largely 3
phase world.
I also don't want to be on the crew that has to rig up and pull the
conductors on that crazy line shown in the photo above. :-/
--
Paul Hovnanian mailto:Pa...@Hovnanian.com
------------------------------------------------------------------
Incorrigible punster -- Do not incorrige.
phil-new...@ipal.net
Producing 6 phase is easy. You just have 3 transformer winding sets (either
3 separate cores or an E-core) with whatever primary, and a secondary that
is an Edison style center tapped (or a pair of windings wired as such).
Converting 6 phase back to 3 phase is easy. You just have 3 windings that
are connected at each of the most opposite phases (phase A is between 0
degrees and 180 degrees, phase B is between 120 degrees and 300 degrees,
and phase C is between 240 degrees and 60 degrees). You now have 3 phase
in the secondaries.
Converting to/from 12 phase is a little more complex, but doable. I think
it's probably not worth the effort. I think 6 phase will be good enough.
The 6 phase conversion requires no more transformer cores that would be used
with ordinary 3 phase. You just wire the windings differently and have twice
as many bushings. So it's a bit more expensive, but not a great deal more.
So that 345kV system would now be a 398kV system.
| I also don't want to be on the crew that has to rig up and pull the
| conductors on that crazy line shown in the photo above. :-/
Neither do I. Just stick with the 6 armed pylons. Forget the 12 phase stuff.
What about those EHV ones that have support structures going up on both
sides of the middle phase?