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Re: [TCML] first light disaster



Hi Marko,

Your MMC configuration is the problem if the data you gave is correct. Each cap is a 0.5uF at 1500V rating. And you said an 8 x 3 array (meaning 8 caps per string and 3 strings total)?

Each string is then 0.063uF and when three strings are placed in parallel, the total cap bank capacitance is 0.188uF and it's rated only at 12kVp. Not only is the cap bank standoff voltage a whopping low 12kV, but the cap value is off by one magnitude. Also, the initial description of your cap failures sound like voltage punch through which for me further supports an MMC configuration blunder.

If the string was one string of 24 caps, then yes 36kV at .0201uF. But as soon as you divided up the string into an 8x3 array, you changed the whole setup for 0.188uF and lowered the string voltage to 12kV.

Take care,
Bart




mark olson wrote:
At 09:07 PM 12/27/07 -0800, you wrote:


The cap failures may or may not have been due to the tap disconnect.

Did you notice if the caps failed before the tap issue? I expect you simply noticed the tap, reconnected and then tried with no results and later realized the caps had failed.

I noticed the failure completely after the fact


To be safe, describe your connections for us. If there is an obvious blunder, someone will recognize it.

From one HV bushing on the NST: To one side of the SRSG and to the primary inner. From the other HV bushing on the NST: To the other side of the SRSG and to one side of the cap bank.
From the other side of the cap bank:    To the outer primary.

 Also, describe the caps your using and MMC in general would be helpful.

The cap bank is comprised of 24 each .5 MFD @1500 V connected in series for a value of .0208 MFD @ 36 KV. The caps are oil filled "snubbers". The terminals are 1/4-20 studs. They are bussed with copper strap, arranged
in an 8 x 3 array with a 20 Meg 15 KV "tiger stripe" across each triplet.

 Also your SRSG electrode spacing would be of interest here.

The SRSG is a propeller type with two pairs of stationary electrodes and one flying electrode @ 1800 RPM.
The gaps were approximately..015" for a total of about .030"

One thing of note here, I had the gap motor phase shifted "all the way" on one occasion and hit the main kill switch. The resulting kick back actually snapped the flying electrodes which are made of 1/8"
welding tungstens.


Take care,
Bart"



What about the SAFETY GAP directly across the transformer??????????????//

Ed

I do not understand the significance of the gap across the NST, Therefore I did not incorporate this feature.

Thank you very much,

Marko
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