Re: coax cable , blumlein, etc.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: syd <tesla@xxxxxxxxxxxxxxx>

I've never removed the shield conductor on any of my hv cable and 
I've never experienced this overvolting problem. I've run various 
lengths of AC and DC shielded lines up to 38 kilovolts and never had 
a pig, diode stack, or cable fail. If this effect depends on the 
capacitance generated between the center conductor and the grounded 
shield, then removing the outer shield will only change things if the 
cable is suspended in the air away from any grounded object, or the 
ground itself. This type of cable is used ubiquitously for 
underground power transmission so the 'other plate' of this 
capacitance is present regardless.

The only time I've seen someone have problems with crawling sparks 
back at the pole xformer bushings was when their coil wasn't 
sufficiently rf grounded so that the whole coil was lifting in 
voltage and grouning out to the pig which was better grounded because 
of its placement.

I know the power utilities need to consider these effects when 
they're running miles and miles of cable at hundreds of thousands of 
volts, but I just can't see the issue arising in a 10, 20, or 50 foot 
run of cable, especially when this is basically the use for which 
this cable was designed, conveying high voltage electricity.

syd klinge





Tesla list wrote:

> Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
>
>
>
>
> A transmission line is essentially a capacitor, ie, plate, 
> dielectric, and another plate.  Removing the shield of x-ray cable 
> leaves the PE/PP and no second plate, so the normal xmission line 
> effect is now missing.  Common example of xmission line would be a 
> coaxial cable with its inner conductor and outer braided shield.  RF 
> currents travel along a capacitvely equal balanced line.
>
> I used coax cable and noted the entire HV bushing on the 14.4 kV pig 
> crawling with capacitive sparks --- that's why I mentioned beware 
> using coax at all with pole pigs.  I don't have the pri cap size or 
> lengths of cable because this was back in 1967.  I also didn't have 
> the equipment back then to do precise measurements.
>
> This was posted only to be a warning to anyone thinking of using 
> coax with a pole pig --- don't!  or else you may damage the pig.
>
> Dr. Resonance
>
>
>
> > Is the effect one of two I mention below?? or is it a third 
> > mechanism?? If it is a transmission line effect, can you explain 
> > why removing the shield fixes it and why proper termination doesn't???
> > If a resonant charging effect, can you descibe the L that the coax 
> > capacitance is resonanting with, the resonant frequency, and how 
> > this works???   If a third mechanism, I would like to challange you 
> > for the benefit of all the readers here to give a quick synopsis of 
> > this effect :o)).
> >
> > Knowledge is everything and sharing it is golden.
> > Gerry R.
> >
> >
> >
> > Also is it
> >
> > > Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
> > >
> > > These effects are well documented in Sargent & Dolan's book -- 
> > > Power Electronics.  Also covered are both strip line and circular 
> > > HV generators. Very novel designs.  It's out of print but I 
> > > believe that Bert Pool (Stoneridge Engineering) has some copies for sale.
> > >
> > > > Anyway, going to the intent of the original post,  I'm trying to 
> > > > understand the mechanism of voltage growth when using a shielded 
> > > > coax (not because I want to use shielded coaxes, but because I 
> > > > want to understand). Is the phenominum transmission line effects 
> > > > being underdamped while energy is feeding the line,  is it a 
> > > > resonant charging effect (series LC) due to the capacitance of 
> > > > the coax, or is it some other mechanism???  Also in the TC 
> > > > context, is the blumlein effect the same as the transmission line 
> > > > effect I described above or something else???