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Re: AC-rating for MMC caps // EMMC vs the GTL-WIMAs
Tesla List wrote:
>
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
<SNIP>
>
> Since the fields are contained in the poly dielectric. Corona should not
> be a problem. I was more concerned that having a cap say 10kV above ground
> may cause external corona to the air that may degrade the covering or
> eventually cause an arc from the cap to some external object. This is why
> I was concerned about putting it in oil. However, now, I think that this
> will not be a major problem The ozone and degradation of the epoxy coating
> will not be a factor in our limit time uses. A few data sheets mention
> some mysterious AC effects at high voltage but nothing beyond armwaving
> guesses.
>
> I quote from WIMAs data sheets "Furthermore the r.m.s. voltage derived from
> the peak voltage shall not be greater than the normal AC voltage rating of
> the capacitor to avoid the ionization inception level: V r.m.s. <<V a.c.
> rated."
>
> It's anybody's guess as to what that was supposed to mean...
>
> >
> >The answer from Wima on my question (how much the lifetime
> >will decrease if a cap is stressed beyond the AC-rating) still
> >hasn't arrived :'-((
> >
> >Terry wrote:
> >> Let me know if more info is needed. Since we use zillions
> >> of WIMA caps, we have "leverage" with them when we need
> >> questions answered...
> >Perhaps you could ask them if there is any truth in the
> >'corona-fear' (constant AC-rating at 'low' frequencies)?
> >
> >
>
> We use WIMAs at the DC/1.4 level all the time. As long as they do not get
> hot they last forever over (15 years of using them). WIMA would probably
> just base their answers off our data anyway. I bet somebody wrote some
> paper warning of doom once, or something like that, which caused this
> concern. No one I have talked to sees any problem with using them at the
> DC peak levels as long as the ESR heating does not raise the body
> temperature above 5 degrees C. I doubt WIMA could explain "ionization
> inception level". I'll try to check around for a definition, guidelines,
> equations, and data... However, I don't expect a darn thing....
>
<SNIP>
Terry and all,
"Ionization or Corona Inception Level" is an accepted term in capacitor
design, and it refers to the level at which ionization can begin to
occur inside a bubble of entrapped air or within an air-filled void
within the solid dielectric system. If one had "perfect" dielectrics and
could always exclude any entrapped air, derating for this phenomenon
would not be necessary.
Capacitor vendors will typically estimate the Inception Level by using
Paschen's Law, the anticipated maximum void size (in the direction of
the E-field), the dielectric constant(s) of the dielectric system, and
the thickness of the dielectric between plates. Basically, this is the
threshold at which ionization within an entrapped void COULD begin
during AC or pulsed stress. Most capacitor vendors multiply this by 2 to
estimate the applied stress level at which progressive damage WOULD
occur due to partial discharges within the dielectric itself. It's
basically a design parameter for the capacitor, and it permits the
manufacturer to estimate how much margin exists in the presence of
manufacturing process or material defects.
The actual stress at which ionization will occur (IF it occurs) for a
given capacitor is extremely variable, and is very sensitive to
manufacturing process control and incoming material quality control.
Ultimately, vendors do "torture" testing on large groups of capacitors
out of different manufacturing lots to determine the actual performance
of their products, analyze the statistics, and then do lifetime
projections. Damage due to partial discharges tends to be gradual, and
the smaller the defect size, the longer it takes to do damage. And, if
the vendor maintains tight materials and processs control, the batch of
caps we use my not even exhibit this failure mode!
Sooo..... as long as we do not exceed the breakdown voltage of the
dielectric system(s), and do not induce damage from excessively
dielectric heating (does not appear to be a problem for PP), we may see
a bit of long-term degradation and reduced life if we exceed the
manufacturers' AC ratings/ionization inception level. However, the
shortened life may not actually cause any problems for the short
run-times and low duty cycles seen by Tesla Coil application.
-- Bert --