Re: Polarized HV/ what's + and what's -

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:

>Original poster: "David Rieben" <drieben-at-midsouth.rr-dot-com>
>Hi all,
>I recently scored a pretty nice x-ray transformer. I think it's
>rated around 150 kV -at- ~ 600 mA. I've already repaired a
>carbon tracked phenolic rectifier mount twice. The new
>phenolic mounting block also carbon tracked after the
>first repair but so far the second repair has held up (keep-
>ing my fingers crossed). Anyway, I'm wanting to keep
>the rectifiers intact in this transformer since I'm wanting
>this one as a very high DC voltage source. Since I have
>it all back together after the last repair I really don't want
>to dig into the oily mess again just to get a close look at
>the diodes to determine the polarity of the HV outputs. I
>have heard that one polarity tends to form corona easier
>and actually arc further at a given voltage than the other.
>I've noticed this phenomena from the output leads of this
>transformer as I slowly ramp up the input voltage to it with
>a big variac. Of course I'm also seriesing the variac's out-
>put to it with a ballast assembly since x-ray trannies are
>NOT current limited. Anyway, I was thinking that the ne-
>gative terminal would be the one that would most likely
>tend to more corona and arc further at a given voltage
>but I simply can't remeber for sure. I think Antonio de
>Quieros was talking about this once. Can anyone tell
>me which polarity will form more corona and arc further
>at a given DC voltage?
>Thanks,
>David Rieben
>
>.

Hi David,

For terminals with identical radius of curvature in air and highly 
nonuniform fields, the negative HV terminal will begin to exhibit corona 
breakdown, forming relaxation oscillator-like Trichel pulses (a localized 
space charge effect). However, once corona breakdown is achieved at the 
positive HV terminal (at slightly higher voltages), the positive terminal 
will then tend to initiate hotter positive corona and, as the voltage is 
further increased, this evolves to bursts of corona streamers, and then 
longer streamers. BTW, the ability of the positive HV terminal to more 
easily generate streamers is true for HV terminals in virtually any all 
gases and dielectric liquids, including mineral oil. Finally, at very long 
distances and very high voltages, the advantage of the positive terminal in 
streamer propagation disappears.

Insulating HV DC under oil for extended periods can be a bit challenging at 
close spacings since any impurities (especially cellulose or clothing 
fibers) will tend to align themselves along the E-field in regions of 
highest stress, and the oil itself tends to become "thicker" in the region 
with the stronger E-field . Over time, chains of debris can be formed which 
will try to bridge the regions with the highest electrical stress. Since 
most common fibers are typically hygroscopic, these chains can then lead to 
partial or complete dielectric breakdown. Circulating and filtering the oil 
may help. However, in the case of your rectifier mountings, using a 
different dielectric (PP, PE, PVC, or G10) will likely be a better solution.

Best regards,

-- Bert --
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