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Re: [TCML] My last question on caps. I swear. (maybe)

Hi DC,
I agree that the standard 2.5 derating is a good idea for long life. But, it does get expensive for coilers trying to make ends meet. If one can afford it, great, go for it. But if not, you can get away with a little less derating. How much depends on the coiler, the coil, gap distance, etc. I'm not sure what kind of financial freedom Nicholas has, but if he can afford to go with more caps per string, then he will certainly need 2 strings to accommodate the LTR size and sure, that would great.
I personally do fine with 16 to 18 caps per string with both 12 and 15 kV setups. I suspect a 9kV NST can probably do ok with say 13 caps as along as the static gap (and preferably a static gap) is set to the NST output voltage. Of course, cap life may degrade a little faster, but even that depends on actual use and there's not a much data yet on that (with coilers in general). It's probably nearing that for some however since these MMC's were conceived several years ago. I suspect in the next year or so that it is possible we may start seeing some end of life failures on MMC's. That will be interesting when they start to occur. That's something we just haven't seen yet, but time will tell the tale. 

Take care,
Bart

DC Cox wrote:

Use xmfr Erms rating x 2.5 and you will not have problems.  At 24 kV AC x
2.5  =  60 kV DC rating

With your 9 kV nst, then, 9,000 x 2.5  =  22,500/2 kV per cap  =  11.25 MMC
caps   so, use 12 in series in each string.  24 kV DC rating.

In our larger caps, for pole xmfrs, I use .06 uF and 0.1 uF (60 and 100 nF
respectively) conservatively rated at 80 kV DC.  Peak currents with the .06
uF cap are hitting 2,850 Amperes and with the 0.1 (100 nF) cap it's hitting
4,600 Amps peak.

Dr. Resonance




On Tue, Jul 8, 2008 at 2:03 PM, Nicholas J. Goble <ngoble@xxxxxxx> wrote:


With a 9/30 NST  I found my LTR capacitance to be .0124uF. (right?).  I
placed an order for 13 (1 more just in case I need an extra)
942C20P15K-F caps.  They're rated at 2000VDC and .15uF.  If I string
them together in series, I'll get an MMC at 24000 Volts and .0125uF.
Two things worry me.

1).  The website says that it's rated for 2000VDC.  I'm not using
direct current.  There's a link to a data sheet for the caps and it
says that the voltage range is 600-2000 VDC or 300-500 VAC.  If I
series 12 of these together, will I get an MMC rated at 24000 VDC and
only 6000VAC?  Does this mean that I've got to string 48 caps to get an
MMC that will work at 24000VAC?  Or will 12 2000VDC caps work just fine?

2).  The website shows a picture of the caps.  On the side of the caps,
it says it's rated .15MFD.  Why would it say that?  I thought
942C20P15K-F caps were rated at .15uF.  This isn't a huge concern of
mine, but it's making me worry about my purchase.

Thanks for putting up with me.

Nicholas Goble
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