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Re: AC-rating for MMC caps // EMMC vs the GTL-WIMAs



Hi Terry, Coilers,


Terry> 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... 

I guess with 'V a. c.' they mean only sinusodial waveforms. So 
I think it means that you should not exceed the rms value 
regardless of the waveform. With other words, you could go beyond
1.4 times the rated VAC with your peak voltage if your waveform 
is not sinusodial. This applies for example for a 'bipolar 
rectangular' signal (+U,0,-U,0,+U,0,-U...) with t_off=3*t_on.
But their formula is not a very reliable thing because it would
allow a far to high voltage for t_off=100*t_on :-(

--------------------

Terry> 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....

Bert> "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. 

Well, today I got a phone call from WIMA and we discussed the things.
The guy said that there ARE partial discharges occuring above a 
certain voltage level. This phenomenon takes place ONLY at AC.
Above a certain VERY low frequency (about 1Hz, he guessed), it is
INDEPENDENT of frequency. So we have the same problem at 50/60Hz 
as well as in the 100kHz range.

This Corona Inception Level depends on the internal geometry of 
the cap. With one floating electrode (= series connection of two 
plates like the FKP1-type from WIMA), it is 700VAC. With an internal 
series connection of 4 plates, it only raises up to 1200VAC.

I told him that we want the caps to last for 2000 hours and asked 
him how hard we could drive them above the rated 700VAC. At first 
he said factor '1.5 above', later he only suggested factor 1.25
or with other word 900VAC. BUT: he couldn't tell me how much
of the caps will die in this time (50% or 1% or whatever...). Of 
course I nailed him down to tell me that important information in
the next days. I think the number of dead caps will be quite low
as they count failures per 10^9 cap hours. So I think we can go
beyond these 125% AC stress.

Due to the low duty cycle we typically have (<5%), we don't have to
derate the caps HV wise. Only in high breakrate systems we have to 
check the temperature (as Terry said).

The max. amperage (pulse risetime) is not as critical as the 
Corona Inception Level is, he told me. 

----------------------------

Terry> 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)....
How high is the factor Udc/Uac for the caps you mentioned? There
are types where it is pretty low (1000Vdc/600Vac) and where this 
should be no problem as the peak voltage (850Vpeak) is nearly the 
rated dc voltage (1000Vdc). The caps the GSTCs ('German Speaking 
Tesla Coilers) ordered are 6kVdc/700Vac and therefore there is a BIG
difference!


-----------------------------


STK> BTW, did I understand right, that your 'EMMC' is just a new
     type of cap used in a configuration where it is stressed
     by using 'real peak AC = rated DC'? 

Terry> Don't get to excited about my silly acronyms :-))  

No, don't worry, I'm used to read Reinhards mails both on the
pupman as on the GTL - looks like you two have gone through the 
same school ;-))) (Hi Reinhard, congratulation to your sparklength!)
On our GTL-website, we have a dictionary 'coiler slang <-> German'
which becomes a lookup table for abreviations more and more :)
[:) is the shortcut for :-) <G>]

Terry> I was just jokingly trying to name caps that were used beyond 
       their manufacturer's rating (with a good degree of 
       understanding).  Manufacturer's rate their caps so anyone
       will be able to use the caps within that "rating" without 
       a problem.  The parts can often be pushed much harder.  

Terry> My "EEMMC" cap was meant to take the rating to very near 
       destructive levels.  It is right on the edge where my best 
       guess said it "probably" would survive

What about using the WIMAs in an XEEMMC-arrangement with just
two 6kVDC caps (33nF) in series on an 8kV-neon? This would meet 
your 'peak voltage < DC rating'-requirement (peak 11.3kV for 
2x 6kVDCr) and will result in an 16.5nF cap for only 1.70$US ;-)
I think I have to perform this test just for fun. The 700VAC
rating compared to their 6kVDC rating is very strange and if
the caps withstand the test for any reasonable time, those
caps seem to be the ultimate TC-caps. I hope they'll do....
(we should get them in KW26)

Terry> And you now see why the manufacturers have so much darn 
       trouble rating capacitors :-)))  It just isn't easy....  
       Take your best guess as they do....
Yes, that seems to be true. The guy had my data for three weeks 
and still couldn't say how high the risk due to the partial 
discharges will be. Hope he gives me some more concrete nuumbers
in the next days when he looked over their statistics.


Stefan
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