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Re: coax cable , blumlein, etc.

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Syd,

Although I do respect Dr. R's opinions regarding anything
"Tesla", since he's obviously been doing it professionally longer
than many of us fellow list members have even been living, from
my personal experience, I'd have to totally agree with you on
this matter. I, too, have been using x-ray cables (which are
basically BIG coax cables) for the last several years as
transmisison lines from my pig to the base of the coil sys-
tem and I have also never experienced any of the over-
voltage related failures or even any signs of overvoltage.
And just in case there IS a serious overvoltage, like be-
cause of the RF kickback from the primary circuit of the
coil, I have an 18 kV rated distribution arrester wired
directly in parallel with the HV output bushings of my pig.
IMHO, distribution arresters are the BEST overvoltage
protection for the HV side of a pole pig since that's exactly
what they were designed for in the first place! Maybe if
Dr. R (or anyone else who's experienced Blumlein first
hand) could expound more on the details and conditions
that were present when this was noted so that we could
all take precaution to make sure that we don't replicate
these particular conditions when we do decide to use
grounded shielded HV cables for our transmission lines.?
As a side note, come to think of it, I do think that I have
noted some Blumlein with HV cables, but only when using
them with the >100 KV voltage from the x-ray transformer
itself. I did note that the sparks got longer,much louder and
brighter (capacitance) when allowed to go through the 20 to
60 feet (the longer the cable, the more pronounced this effect)
of cable, as opposed to cutting the leads very short to take the
HV directly from the HV ports of the x-ray transformer.
So maybe the relatively "low" voltage of a pig isn't going
to cause noticable Blumlein effects with run lengths of
less than several hundred feet or something like that?
I'm open to further facts, figures, comments, ect  ;^))

David Rieben


----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, November 07, 2005 11:42 PM
Subject: Re: coax cable , blumlein, etc.


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???