> From: Mike Keitz <
KILLspammkeitzKILLspam
JUNO.COM>
> To:
RemoveMEPICLISTTakeThisOuT
MITVMA.MIT.EDU
> Subject: Re: [OT] Cable retained energy, was Super Caps
> Date: Tuesday, March 10, 1998 10:18 PM
>
> On Tue, 10 Mar 1998 21:38:31 -0500 Sean Breheny <
spamBeGoneshb7spamBeGone
CORNELL.EDU>
> writes:
> > Jorge has mentioned that this phenomenon is
> >observable when dealing with power transmission line. Power
> >transmission
> >line is not coaxial, is it?
>
> The stuff that is buried underground and used from about 2 KV up to maybe
> 50 KV (e.g. for distribution to transformers in neighborhoods) is. It
> has a copper or aluminum conductor at the core, thick polyethylene
> insulation, then a shield made out of metal braid and conductive plastic,
> and finally a black plastic outer covering. The shield is likely not
> intended to normally carry current. It is connected to ground at both
> ends for safety reasons. Maybe some single-phase circuits use it as the
> return conductor. In any case, the coaxial construction makes it
> difficult for high voltage to appear on the outside of the cable if there
> is a leakage, cut or hole in the insulation. Such a circumstance will
> cause the cable to arc and the fuse or circuit breaker to open. This
> line is of course routinely "hipot" tested before installation by
> connecting a high voltage between the center and the shield and measuring
> any leakage. A big spool of it charged to many KV would indeed be
> dangerous. The polyethylene insulation would prevent the charge from
> leaking off for a long time as well as maybe causing a phantom "recharge"
> from dielectric absorption.
>
> Of course such cable is intended for use at 60 Hz and is not designed for
> any particular impedance.
>
> >I have always been told that it is single
> >conductor woven cable,
>
> The wire strung overhead between poles is laid up of strands of aluminum
> on the outside, and steel on the inside. The steel is to make it
> stronger. Even at 60 Hz, there is enough of a "skin effect" to reduce
> the effectiveness of the conductors in the center, so the aluminum is
> outside. Except in special cases, it is bare. Please do not climb
> power poles and touch the wires. You will get hurt. Really high power
> lines have several cables in parallel. They are held apart in a triangle
> or square arrangment to make the apparent diameter of the conductor as
> large as possible. This improves the impedance and current distribution
> along the line. When dealing with hundreds of km of cable, AC effects
> such as phase shifts are very real at 60 Hz.
>
> sometimes with a hollow core with a coolent run
> >through it.
>
> In power "substations" the conductors are sometimes placed inside much
> larger outer pipes. The pipes are filled with sulfur hexafluoride gas.
> SF6 is a much better insulator than air, as well as being certain to be
> clean and dry, so the conductors can be in closer proximity than if they
> were strung out in the open. This is rather expensive so it is done only
> for extremely high voltages or where the real estate to lay out the
> station with larger seperations would be even more expensive.
>
> I don't think the conductors in transmission systems are hollow or
> actively cooled in any way. Hollow cooled wire is used to wind
> generators and big motors. Hydrogen (H2) gas was used as the coolant in
> one example I saw. It's both very "thin" and light (i.e. easy to move at
> high velocity through small hollow passages) and absorbs heat well. It
> doesn't seem that H2 gas and big electrical machines should be used near
> each other.
>
> This is what I remember from several years ago; current practices may be
> different.
>
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