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RE: [TCML] Pigs killed in the line of [Tesla coil] duty (was: pole pig protection)



Hey, Gary:

Heating is certainly possible.  It would have had to be a very localized event given the low duty cycle of the pig together with its very large mass.  Another possibility, of course, would have been some preexisting pathology (the pig was, after all, old and used).  However, that would seem coincidental considering the low rate of pig failures in general and the unusual operating conditions in this scenario.

When I first started collecting pole pigs years ago, I also tried the 2x voltage trick, although at the time I merely evaluated the result qualitatively by looking at arc self-start distance.  It seemed to me at the time that I was achieving about 2x the rated HV out, however I did not look at the impact on open-circuit current or study the HV under load.  Those would be interesting things to do.

One other related datapoint is that, AFAIK, Justin and Aron (of hvguy.com) severely abused their 7200V 5kVA pole pig in similar fashion, dropping 240V across the 120V windings and running the poor beast up to 25kVA!  It survived this abuse, however.

The BIL rating of pigs is interesting, and I have often wondered what actual properties of the pig's construction contributed to that rating.  It appears to be somewhat independent of the actual insulation class of the transformer, or at least the two ratings do not seem to scale in proportion.  This isn't too surprising, given the high-frequency nature of the BIL test.  Perhaps inter-winding capacitance, for example, plays a large role that would be insignificant at 60Hz?  Some transformers actually have their insulation class printed on the nameplate.  I have a number of such units.  One 15.6kV potential transformer has a 34.5kV insulation class rating and 200kV BIL rating.  This is a high-BIL unit.  But if insulation class is the more indicative of the transformer's ability to stand up to 60Hz, and if the insulation class and BIL scale roughly in proportion (not saying they do, just a "what if"), then it would be clear to see that this particular
 transformer would be insufficiently insulated for 2x the HV if it were given a more standard 75-110kV BIL rating, which is probably what most peoples' pole pigs have.

That's all a bunch of speculative nonsense, so nobody call *me* on *that* either ;-)))

Cheers,
Aaron, N7OE

--- On Wed, 9/10/08, Lau, Gary <Gary.Lau@xxxxxx> wrote:

> Hi Aaron,
> 
> Thank you for chiming in here - it's very good that
> this data be shared!
> 
> In case #1 where the LV side was overvolted by a factor of
> 2, it's surprising to me that this would cause an
> insulation breakdown on the HV side, given the BIL rating of
> the HV side.  I would have guessed that it would have
> suffered from core saturation first.  I wonder if it did
> saturate and heated the core and surrounding insulation, and
> this caused the breakdown?  Do you know if the pig became
> detectably warm in use?
> 
> And before anyone calls me on it, I should admit that I was
> to a degree talking through my hat when I stated that the HV
> side of a pig is designed to survive a lightening strike. 
> This is probably not true - only that it is designed to
> survive HV transients _induced_ (?) by lightening or other
> means.
> 
> Regards, Gary Lau
> MA, USA
> 
> >
> > As long as we're keeping track of these incidents
> for posterity, I'll add a couple
> > more.
> >
> > Case 1)
> >
> > Friend and fellow Seattle coiler Matt Stiger toasted a
> 14.7kV 25kVA unit a couple of
> > years ago.  Granted, this was done whilst feeding 240V
> to the 120V windings
> > (which had been placed in parallel within the can, as
> would be done for 120V-only
> > service, or else 208Y three-phase service) in an
> effort to increase the output
> > voltage, which appeared to work fine.  Pig had
> operated without incident under
> > these conditions for some time, but it seemed to me a
> matter of when--not if--the
> > pig would be ruined.
> >
> > Sure enough, the pig did give up the ghost one day
> while driving Matt's monster
> > coil.  Coil suddenly stopped performing.  Further
> testing isolated the pig as the
> > cause of the problem.  Small arcs could be drawn off
> the pig, but running it open-
> > circuit caused it to arc internally.  The windings
> were later dissected and it was
> > discovered that multiple layers of paper on the HV
> side had burned through near
> > the end of the windings.  It was hard to tell exactly
> where the first fault had occurred
> > and what the sequence of destruction might have been.
> >
> > <snip>
> >
> > Cheers,
> > Aaron, N7OE
> >

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