Partial Discharges.....A good reason for cap failure?

To: tesla-at-pupman-dot-com
Subject: Partial Discharges.....A good reason for cap failure?
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Mon, 01 Mar 1999 05:40:54 -0700
Approved: twf-at-verinet-dot-com

Hello All,

A while ago, we were discussing the phenomenon of partial discharges
within caps on the GTL. I have never seen this mentioned on the
Pupman List (okay, I have only been on this list for 3/4 year), so I
thought I would mention it:

It seems as if all HV RF caps using PE, PS, PP, etc. (read: plastic)
as dielectrica will experience the phenomenon of "partial discharge".

Now, what exactly is this?

Partial discharges are local corona discharges that occur to some
extent in just about any HV isolation material. They occur because
of non uniform material (thickness), minute dirt particles, entrapped
air and so called "Lunker areas" (very, very small hollow areas).

Corona and the free radicals (O3, NOx) it produces, will split up
the long polymer chains, that the plastic is made of. This
structural damage to the plastic is also called treeing, because
the shape of the damage sort of looks like a tree. The insulation
voltage (volts/mil) within these zones is sharply reduced. It takes
a while for this to happen, but it DOES happen. As soon as the
V/mil has been "eaten" low enough, the dielectrica will be punctured,
of course, leading to cap failure. Elevated temperatures will increase
the rate of destruction. This might explain, why even commercial
PS caps (very lossy at RF frequencies) will die very quickly in HV/
RF enviroments. I´m not quite sure how the self healing aspects of
some commercial caps will counteract the phenomenon of partial
discharges, but no homemade PE cap has any self healing
capability at all.

Any kind of disturbance (i.e. non uniformity) in the E-Field (sharp
metal edges for example) will also increase the production of
partial discharges (->corona formation). Important to know is that
this phenomenon does NOT lead to instant failure, but rather, it
accumulates over time and then leads to failure. I believe this to
be one of the major reasons why commercial pulse caps are built
with a very low volt per mil rating (lots of series caps). And now it
should make sense why coilers like Bert Hickman (72-96 mils)
or Ed Sonderman (90 mils) suggest such "gigantic" mil numbers
for homebrew caps.  Oil DOES NOT prevent the formation of
corona!! Of course a pulse cap always has a limited lifetime,
simply because it is a highly stressed component.

Interestingly enough, mica caps are totally insensitive to partial
discharges. However, mica caps are VERY sensitive to voltage
spikes exceeding their rated limit, so they really aren´t any
better in coiling usage :o).

Coiler thoughts.............FWIW.

Reinhard