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Re: just wonderin... expanded, And more thoughts
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.
Pole pigs tend to behave more like low impedance voltage sources
(particularly when used in utility power distribution without series
ballasting). Power companies often parallel them to obtain higher output
current, but this is normally done using identically-rated transformers.
Transformers connected in parallel are normally selected to have the
same "Percent Impedance" ratings so that the output current is equally
shared between the pair.
-- Bert --
Tesla List wrote:
>
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
> Hello Ed, Bert,
>
> Comments down below.
>
> >Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
> >> Original Poster: "Seekins" <seeked-at-sover-dot-net>
> >>
> >> To expand on Christopher's Questions,
> >> I was wondering if anyone could maybe comment on NST matching.
> >> I've seen what seems to be conflicting info. regarding how close the
> >> xformers should be.
> >> Some seem to be saying that even a few volts difference on the
> output(s) of
> >> a pair of xformers paralleled together can cause problems.
> >> Others seem to imply that they just throw together a couple or more
> with the
> >> same faceplate ratings and let 'um rip.
> >> Anyone have any comments that might illuminate this area?
> >> If it is an issue, how much difference in output voltage is
> "tolerable"?
> >> (recommendations on the poor man's approach to measuring the
> difference
>
> Bertīs comments:
> >As long as the transformers have the same output voltage faceplate
> >rating you should have no problems mixing and matching. They do not
> need
> >to have the same output current ratings. You should verify that each
> >transformer works independently (by drawing arcs from each bushing to
> >the transformer's case). Don't connect a transformer that's got a
> "weak"
> >side, since it'll reduce the output of all the others when you connect
> >them in parallel. There's no need to actually measure the transformer's
> >output voltage.
> >
>
> Edīs question is indeed a good one and not (I think) that easy to
> answer. While most coilers probably do as Bert suggests and
> get away with it, Iīm not sure if this is THE correct way.
>
> Iīll try and explain what I mean: The problem we coilers face, when
> paralleling xfomers, is the spark gap. Huh?!?. Yes, because the
> spark gap is either an open circuit or an almost(!) short. So, it
> is a non constant load and that is where the problem arises.
>
> This means during non conduction, we soley have to match the open
> voltage. If (in a 2 xformer setup) the first transformer only supplies
> 14kV and the other puts out 15kV (real measured output, I mean),
> there is a potential difference of 1kV between the two. This, in turn,
> means transformer one acts a load for the second (15kV) xformer.
> So, effectively, the true output voltage of this paralleled setup will
> be between 14 and 15kV (depending on which transformer is
> "stronger").
>
> Now, when the gap conducts, we have a somewhat different case:
> The spark does not present a full short. Now the question is which
> transformer break down first and then how much. If the 14kV (e.g.)
> unit is much stiffer, then the 15kV unit is now the "load", etc. NSTs
> are current limited and the current usually isnīt very high (which
> doesnīt mean it isnīt deadly), so paralleling NSTs might not be a
> true problem in the real world (esp. since coil(er) run times are
> pretty low and NSTs are designed for a 24hr/day/year operating
> period). However, paralleling a PT or a pole pig, will probably
> lead to the problems mentioned above, unless they are of the
> exact same type.
>
> FWIW
>
> Coiler greets from germany,
> Reinhard