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Paralleling NSTs (was Re: just wonderin... expanded, And more thoughts)
Hello Bert, Alex,Chris, all,
>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
>Reinhard and all,
>Because of the comparatively high source impedance of NST's, voltage
>imbalances don't present excessive strain on slightly mismatched NST's.
>identical voltage-rated NST's are typically within a couple of hundred
>volts of each other under no-load, and under short-circuit will share
>current according to their faceplate current ratings. However, I
>wouldn't recommend paralleling NST's with significantly different
output
>voltages since the designed insulation breakdown voltage capability of
>the lower voltage NST may be exceeded. For example, if 7500 and 15000
>volt NST's were paralleled, the output voltage of the pair will now be
>significantly less than 15,000 volts. While this will not damage the
>higher voltage transformer, it may tend to overvolt the 7500 volt unit.
>The combined system will run at less than optimal efficiency since
there
>will now be a significant circulating current flowing between the two
>NST's. This will also cause the combined output voltage of the pair to
>lie somewhere between the lower and higher faceplate voltages of the
>NST's.
Agreed. If we now take this to an extreme (like in the case of
paralleling a 15kV with a 7.5kV unit) it will most likely be that the
total setup won´t deliever longer TC sparks than one of the single
units. With such a great difference in voltage, the lower (current)
rated unit will probably be "using up" all it´s current just trying to
equalize the voltages. So, effectively the second xformer won´t
be "present". Paralleling xformers with different voltages is in
MHO not the way to go. Using different voltages requires the
actual measurement of no load voltage and short circuit current
(the latter should be done anyway) of the total setup to find the
proper cap. I couldn´t tell you what might happen, if you happend
to use a cap that mains resonates with one xformer but not the
other one. I could imagine this might make the situation worse
(following Murphy´s Law). My advice is to keep the xformers
"voltage-matched" as much as practically possible. This kind of
rules out paralleling a 12kV and a 15kV (etc) xformer. However,
paralleling different (e.g.) 15kV rated xformers, even if they
have a few hundred volts difference, should not be a problem.
Coiler greets from germany,
Reinhard