[TCML] CD942 Capacitor failure

["Phil Tuck" <phil@xxxxxxxxxxx>]

Hello,

I have suffered several CD942 cap failures lately and I am wondering on the
cause. Each time it has just been the one isolated cap. I would not have
thought that my voltage was excessive for these caps, but after consulting
with another coiler, I am now wondering about a primary current issue.

I was running a SRSG on a pig rated at 11.55Kv but running at 12Kv at times.
Secondary current is around 400 to 450ma depending on my ballast setting.

I was using nine strings of 19 in series giving 71nF @ 38Kv. Damage to the
caps was an exterior blackened hole where the blowout had occurred. I
unwound the last cap' to find where the damage had occurred, this turned out
to be about half way down into the cap in its middle layers. The damage at
this point being a circular hole about 0.25-inch diameter.

Safety gap was set at 9mm or 0.354 inches at the time with the coil running
very smoothly for 3 minutes or more while producing some nice streamers. The
dv/dt time should not have been exceeded either as the res frequency is 92
KHz. 

The primary was tapped at about 8 turns I think, but he primary coil itself
only ever feels slightly warm while the wiring temperature seems to remain
unchanged.

Admittedly, none of the caps are now new, as I have used them on previous
set-ups before, but I would not say they have ever been abused, as the MMC
voltage ratings they have been used in have always been adequate.

As none of the damage was from proximity issues, it can only really be down
to voltage or current I suppose. Thoughts anyone?

 

Phil

 

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