Pixmaker said:
A few days ago, I stopped beside the highway to talk with the foreman of a
line crew that was
stringing a new, 240-KVolt transmission line on 80-foot concrete towers.
I noticed that the three
transmission cables (not yet energized) were tied togetherand grounded
with what looked like
humongous clip leads. I thought this was a routine safety measure to avoid
injury if a new cable
might accidently fall onto the pre-existing, 130-KVolt lines.
The foreman explained that, with 3-5 miles of continuous cable swaying in
the wind (the supports
were about 600 feet, or more apart) there was significant danger from
voltages generated by the
cables' motion within the earth's magnetic field. In fact, he said, men
have been killed when these
new cables are not grounded.
I can understand that a few miles of high-line is a really serious antenna
but didn't think that a
one-turn (or half a turn) coil could generate any real energy. It seems to
me that the wind motion
would cause a polarity reversal with each reversal of cable direction thus
cancelling any serious
generation. Also, there's a good probability that different catenary spans
would move in
out-of-phase ways, with further cancelling effects.
Then there's the possibility of inductive coupling with existing
high-voltage cabling parrallel to
the new lines. Now that, I think, could pick up some significant energy!
And what about lightning
in the area?. I would guess there could be some serious voltage pulses
generated, especially here in
"lightning alley, USA." (South Florida.)
Funny how my brain works. . . (funny peculiar, not funny ha ha.) I wonder
if it is possible, within
reason, to build a pickup coil big enough to string in your back yard and
(if you lived beside one
of those easements containing a dozen or so transmission lines at about
250,000 Volts) pick up
enough energy to provide power for lighting, etc.
I live in a condo quite a long distance from any high lines so my
experimental urges must remain
frustrated. But I think it's a kinda fun idea! I'm many decades removed
from my courses in
electromagnetic induction so the local utility is safe from my poaching.
Has anyone any practical experience with this matter? Or was that old guy
running that crew just
having a little fun with me? What do you think?
Pixmaker in FLL
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I suggest that the foreman doesn't know what he is talking about, or, more
likely is having a bit of fun with you. First of all, the voltage that can
be induced from one end of the line to the other (not to ground) depends not
only on the length of conductor but also on the strength of the magnetic
field component which is perpendicular to the motion and the line -as well
as the velocity of movement- the latter two are negligable. The fact that
polarity reversed means nothing except that the miniscule signal which
might be generated would be AC. Your comment about different sections being
out of phase is on target.
Two factors do exist.
a) the pre-existing lines and inductive coupling to these lines can result
in lethal voltages induced. One ground point is fine, but when somebody
makes a second ground point, there is a problem. The procedure you mention
is common when work is done on a "dead" line in the vicinity of live ones.
b) ungrounded lines, just floating, even when not near any others, can pick
up static charges from charged clouds moving about. No problem unless you
are the path to ground.
Oh yes, one can get an inductive coupling to a transmission line,
preferrably a single phase rural line (polyphase lines tend to cancel their
fields) and with a large enough loop closed at a far enough distance from
the line, run a light bulb. However the output voltage isn't constant,
and,depending on line current, you may have a dim bulb (say less than 50V)
or have a problem with a bulb blowing up and the socket flashing over (at,
say 1000V+), Of course if you escape being killed, you can end up in court
with a charge of theft. Cheaper, safer and more reliable to simply connect
to the grid legally.
