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Re: Partial Discharges.....A good reason for cap failure?
Tesla List wrote:
>
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
> 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:
<big big snip>
It wouldn't be too hard to set up a test setup with a NST and a variac
to actually look for partial discharge activity. This might give you a
way to evaluate a home built (or used) cap. Typically, HV components
show an increase in partial discharge activity as they reach the end of
their life, shortly before catastrophic failure.
All you need is an oscilloscope and a suitable HV coupling cap (a few pF
is all you need, so it would be cheap). Hook the NST across the cap and
hook the scope up with a series cap to one of the terminals of the cap.
As the voltage increases, you get bursts of HF noise (from the corona
and partial discharges). Since the voltage is varying sinusoidally, the
bursts will tend to be synchronised with the line frequency, so by
syncing the scope to line freq, you should see them.
One diagram I have shows a series C of 3300pF and a load impedance
(scope input) of 50 ohms. I would think that something like 33 pF and a
load of 5K would work just as well. The 33 pF cap could be as simple as
a few feet of RG-8 coax
--
Jim Lux Jet Propulsion Laboratory
ofc: 818/354-2075 114-B16 Mail Stop 161-213
lab: 818/354-2954 161-110 4800 Oak Grove Drive
fax: 818/393-6875 Pasadena CA 91109