Re: Oil Filled Capacitors

Miroslav Pokorni
 

Yes, Morris, you are right, I did not consider that an area where there is a
transition between air and high dielectric constant material makes for some
very ugly field distribution. However, that is at the fringes of the cap,
not between 'plates' of it.

Your reply made me go and look up dielectric constants, what I should have
done in the first place. Paper shows values between 3.3 and 3.7, while oils
that are used for caps, chlorinated phenyls and naphthalenes (trade names
HB-40 Oil, Pyranol 1476, Arochlor 1260, Halowax Oil 1000), are between 2.6
and 5.0. There is not much disparity there, ratio is less than 1:2, so there
would not be much of electric field change going from solid paper particles
to liquid oil, at least not between plates in an oil field cap. On the
fringes of a cap, oil is held between dielectric layers by capillary action
so there is a continuity of dielectric there. Besides, applicable to both,
inter-plate and on fringes, oil being liquid and remaining so, is kind of
self-healing, through diffusion.

Looking at the dielectric constant of most frequently used plastic foil
materials, (polyethylene, polypropylene and polystyrene), there is nothing
dramatic, they are bunched at 2.4. The only other common capacitor
dielectric material, polycarbonate, stands at 3.2. These are not quite the
values that would make field discontinuity which might lead to local corona.

So, I guess, that corona stuff at 200 Vac is an artifact of Wima's marketing
department. I was aware that Wima was trying to tell world that there is no
such a thing as Wima when it comes to line voltage caps, but Taiwanese seem
to have held pretty good.

Regards

Miroslav Pokorni

----- Original Message -----

From: "Morris Odell" <morriso@...>
To: <TekScopes@...>
Sent: Thursday, May 29, 2003 11:15 PM
Subject: RE: [TekScopes] Re: Oil Filled Capacitors


Miroslav wrote:

Corona discharge would be pretty hard to induce at 200 Vac. You would
have
to hit frequency around cap's resonance, but then Q factor would be quite
miserable, so there would not be much of voltage amplification.
I can see that if you are getting corona, that can ruin a day for a high
voltage power supply, like yourself. Corona would cause impedance of a cap
to drop to next to zero and burn everythin in the path, it is just that
you
need pretty high voltage for that.

I am a high voltage power supply and I can shed some light on oil
filled capacitors.

The challenge in capacitor design is to make an A.C. cpacitor that
is
free from corona.

<snip>

I'm pretty sure the power supply was talking about corona initially on a
pretty small scale. It's well known that the voltage stress across
insulating voids is higher than that across a corresponding volume of
insulator due to the distribution of the electric field between areas of
different dielectric constant. The problem would be worse for AC because
of
the changing field - perhaps due to dielectric loss effects too. Once a
micro corona develops it would lead to local destruction and eventually
failure of the entire unit. One of the reasons for impregnating the
dielectric with oil is to fill the voids and prevent this from happening.
I
suppose the oil would really be the dielectric and the cellulose fibres of
the paper merely a carrier/spacer.

Plastic film dielectrics don't have the porous structure of paper with its
inherent voids and so doesn't need to be impregnated. I suppose you can
think of plastic films as "solid oil" anyway.

Morris

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