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Re: HV wire



Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

I'd worry about dielectric "treeing", which is the usual failure mode in HV
wire (some colleagues of mine at work are having all sorts of trouble
running 18-20 kV through a (small) wire potted in silicone, so I've been
hearing a lot about it)...

The basic theory is this:
A small void in between the insulation and the conductor creates a field
concentration because of low dielectric constant.
Corona forms in the void, dissipating some heat.
The heat breaks down the insulation, enlarging the void.
Charge deposits on the edges of the void, allowing a "creeping spark" to get
to the "end of the void", etc...

If you look at thin sections of the failed wire, you see many little
microscopic voids.  Over time, the number of voids increases, and
eventually, one gets to the outside.  Now you have a pin hole through the
insulation, a nice current path, and bang, the wire's is useless.

The problem is that in a reasonable length of wire, it's impossible to
guarantee there are no voids, etc., they don't show up ahead of time (and
all it takes is one), so it's not like inspection helps.

About the only strategy to find the problem seems to be using partial
discharge testing, which looks for the corona starting as the voltage comes
up, but the testing in itself tends to degrade the insulation, which, in a
one of a kind space probe is considered an unacceptable risk.

If you can hear corona hissing in your conduit, your insulation is doomed...
Of course, it might take 5-10 years for it to degrade to the point where
it's a problem, by which time you'll have likely replaced it. And, of
course, the "cost" of insulation failure in your situation is not that high
(inconvenience, etc.).
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, January 13, 2002 6:47 AM
Subject: Re: HV wire


> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Tesla729-at-cs-dot-com>
>
> In a message dated 1/12/02 8:49:43 AM Central Standard Time,
tesla-at-pupman-dot-com
> writes:
>
> Jim Lux wrote:
>
> >
> > ).  Vinyl tubing (e.g Tygon) is another
> > possibility, but I don't know how good an insulator vinyl is.
> >
> >
> >
>
>
>
> Jim,all,
>
> I have personally found the transparent vinyl 1/8" thick walled
> tubing to work quite well to hold off > 15 kVAC when used as
> an outer sheathing around a # 10 AWG standard insulated
> stranded wire. Then I insert the vinyl cover "hot" wire thru an
> equal length of RF grounded aluminum, flex Romex conduit
> (sort of like a big homemade coax cable). I know there has to
> be some corona losses in this setup though, because when
> I energize the "hot"wire inside with my overdriven 14.4 kV pole
> pig with up to about 17 kV out, without any load from being con-
> nected to my coil, I hear a considerable amount of corona
> "hiss" inside the Romex conduit. I know the field is pretty
> even though and I'm not overly concerned about power losses
> from the corona since I'm running a 10 kVA pole pig. In about
> a year, this setup hasn't failed me yet.
>
> Sparkin' in Memphis,
> David Rieben
>
>