Re: coax cable , blumlein, etc.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Syd,

On 7 Nov 2005, at 22:42, Tesla list wrote:

> 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

The problem I ran into had to do with the high di/dt generated by the
gap firing sending a pulse racing back towards the transformer
terminals over about 8 feet of wire. I didn't scope it and don't know
how many reflections were involved but observing discharges in the
safety gap across the transformer just before it died showed that the
energies were small but the voltages were enormous.

Malcolm

>
>
>
>
> 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???
> >>
> >>
> >>
> >
> >
> >
>
>
>
>