Re: Oil Filled Capacitors


I am not really a high voltage power supply !!

I am a high voltage power supply engineer. I work for CPI designing power supplies for Medical X-ray generators. These are up to 100kW, 150kV and 1000mA.
Some of my work can be seen at
These incidently use resonant technology.

I learned about the corona effects from a company called ASC. This company was TRW. They make and have made many custom capacitors for HP, Tek, Varian, Fluke etc. They are generally white wrap and fill construction.

They explained the corona effects in capacitors to me. They have an application note at:

Wima built their capacitors with two sections in series to help with the self-healing properties. When one of the sections arcs the other still presents impedance.

ASC has built some snubber capacitors for me that have four sections in series to obtain an a.c. rating of 800 v rms. The dc. rating of these was 1600 v.

As someone pointed out the corona arises because the electric field is not uniform in the area of a void or mismatched dielectric constants.

These comments on capacitors also apply to high voltage transformers. This why vacumn impregnating is used.

I have done some work with solid dielectric materials, potting, matching the dielectrics and preventing voids is quite a challenge.


John Barnes

P.S. just got home from the Rochester Hamfest with a pile of 7K plugins to work on.

---------- Original Message ----------------------------------
From: "Miroslav Pokorni" <mpokorni2000@...>
Reply-To: "Miroslav Pokorni" <mpokorni2000@...>
Date: Fri, 30 May 2003 02:42:39 -0700

I guess, it does not help, but then everything has a resonant frequency. The
only way to cope with it, if you can not avoid operating at that frequency,
is to make cap mechanically lossy so that Q factor is down in mud.

Possibly, this acoustical phenomena, together with high current through cap,
made resonant power supplies pretty much disappear. The last one that I have
seen was a Hewlett Packard unit that used a cap branded with their name,
what I guess was a custom cap.

There seems to be no such a thing as a simple component, there is only the
simple demand.


Miroslav Pokorni

----- Original Message -----
From: "Craig Sawyers" <c.sawyers@...>
To: "TekScopes" <TekScopes@...>
Sent: Friday, May 30, 2003 12:27 AM
Subject: RE: [TekScopes] Re: Oil Filled Capacitors

Corona discharge would be pretty hard to induce at 200 Vac. You would
to hit frequency around cap's resonance
That reminds me of another failure mechanism - acoustic. Try the
experiment. Make a simple power amp for square waves with a couple of
transistors or FETS, driving +/- 30V (say). Connect capacitor from output
to low impedance load to get a reasonable current flowing. Now sweep an
audio generator.

Many capacitors howl like a banshee at particular frequencies where the
internal mechanics resonate. These are usually ones where the windings
inside are relatively "loose". Very tightly wound capacitors are very
quiet - but there are some real shocks out there when you try the
(like some respected 10kV polypropylene which were more like a loudspeaker
than a capacitor).

Needless to say, such mechanical vibration does nothing to help the
reliability of the capacitor.


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