Re: [TCML] high frequency wave propagation along secondary coils

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Bert,

On Fri, Sep 21, 2012 at 11:02 PM, Bert Hickman <bert.hickman@xxxxxxxxxx> wrote:
> Hi Steve,
>
> Sudden external voltage changes have causes insulation failures between
> adjacent turns near the termination ends of power and distribution
> transformer windings. This has been a rather common problem for transformers
> that are hit with switching transients, especially from vacuum contactors.
> However, when this occurs, it tends to show up on the end of the winding
> nearest to the voltage transient, not at the remote end.

Yes, im familiar with that problem too ;-).  I too would expect the
most stress to be at the top end of the coil.

Somewhat related: I had to install a plastic disc over the top few
turns of these coils because we were observing racing sparks from end
to end on some coils when operating more than 2 coils in close
proximity (and all in phase in terms of RF output voltage).  I blamed
this issue on the poor shielding offered by the spheres, and the
plastic disk eliminated this issue by stopping the spark from
propagating down the coil (or so i think).

>
> A more likely candidate may be ground bounce as charged topload capacitance
> rapidly dumps into local ground. If there is significant path inductance
> between the secondary base/strike rail and the local ground (where the
> grounded ends of the HV E-fields terminate), AND if the ground arc connects
> to the strike rail, it's possible that the coil base voltage spikes to the
> point of causing a secondary-primary flash-over and subsequent turn-to-turn
> fusing.

Yep, ran into that problem as well during initial testing of these
systems.  We have a *substantial* counterpoise system here, so i do
not believe ground bounce is of the magnitude required to cause a
failure here.

When i ran into this ground bounce issue, i did some rough analysis
that showed it was relatively easy to bounce the ground up to 50kV if
there was no local ground capacitance via counterpoise or direct
connection to earth.  The 50kV spikes are on the time-scale of just
100-200nS, which is probably why they arent even more problematic for
coilers.

>
> You might try adding a small grounded counterpoise that's also connected to
> the strike rail and other nearby metallic objects so that most of the
> capacitive discharge current flows directly between the topload and its
> matching capacitor "plate" rather than having to take a more roundabout
> (i.e., more inductive and resistive) path back to the local ground.

Agreed, and already done in practice.

>
> This has not been a common problem, so the Pupman Archive will likely not be
> of much help. Since it only affects one coil, you may have accidentally
> damaged the wire or the winding or that particular spool of wire may have
> had an insulation defect.

I have other suspected reasons for this failure now.  During testing,
our rig got rained on by a leaky roof.  This caused one immediate
failure due to the water running down the coil.  This failure was
obvious, but resulted in attempts to rid the rest of the systems of
water.  My idea to bake out the water from the remaining coils
involved applying about 660W of power to the windings directly (350VDC
applied across secondary winding of 185 ohms DCR).  We made the
surface temperature of the coil (with a .063 thick lexan shield
wrapped around them) get up to ~140F.  Upon disection of the coil that
had the bottom winding fail, we found the wire was separated from the
PVC form.  It was likely the mechanical stress of heating the coil
(which was likely a lot hotter than 140F) that caused the winding to
be compromised.  I think i learned my lesson here.

So for now im dropping my concerns about HF waves and blaming this on
simple material failure.  I would still be interested in seeing
simulations for what happens during ground discharges on the TSSP site
if Paul could make that happen.  At least then i can adjust my mental
simulator ;-).

Steve

>
> Good luck,
>
> Bert
> --
> Bert Hickman
> Stoneridge Engineering
> http://www.capturedlightning.com
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