RE: Cap life testing - Re: MMC cap bank - ion inception testing

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Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Gary,

At 10:22 PM 7/6/2003 -0400, you wrote:
>Hi Terry:
>
>A couple of times, you suggested that a streamer strike to an MMC cap 
>might be the cause of  trouble.  I'm having trouble imagining a scenario 
>where an MMC might be (re)charged to a higher potential than Vgap.  Am I 
>missing something?  I just hate to propagate imaginary failure modes.
>
>Regards, Gary Lau
>MA, USA

I was thinking more that a streamer might hit the body of a cap and tear 
through the insulation and into the cap plates.  At that point in time, the 
cap should be pretty well discharged.  However, the streamer has enough 
voltage to easily pass through the cap covering and such.  Mostly physical 
damage to the cap is what I was worried about.  If a streamer could tear 
say 1/8th inch into the body of the cap, that would probably kill it.  But 
the outer plates "should" guide the streamer away.

One may wonder if say 30pF at 200000V (top load energy) has the energy to 
recharge the cap.  I.e., if a full discharge from the secondary into a cap 
could blow it.  I guess we have to assume the streamer loss would be low 
and the energy would go straight into the cap.

E = 1/2 x C x V^2 = 1/2 x 30e-12 x 300000^2 = 1.35J

1/2 x 150nF x V^2 = 1.35J   V == 4243V

So, it may be possible that a "perfect" streamer hit could pop a single MMC 
cap, if everything were just right, just from over voltage.  Of course, the 
cap could easily self heal from that odd event.

I guess there is also a chance you could get a primary to secondary power 
arc going with the MMC cap in the middle.  That would do more damage.  but 
just a new single cap or two and it would be all fixed in any case.

A streamer hit might cause much more havoc if it attached to a bleeder 
resistor.  But I would think the streamer would be guided to one of the end 
caps rather than stay on the fragile film.

I can't remember anyone reporting MMC damage from a streamer hit.  I know 
they certainly have been hit, but no damage was seen. I don't know if any 
commercial caps have been hit from the side into the cap.  I can imagine 
that may do serious harm.

Cheers,

         Terry