From: Paul
Date: Tue, 07 May 2002 09:36:43 +0100
Subject: Re: [TSSP] Secondary voltage stress factor
Terry wrote:
> I can see terminal voltage to about 100MHz and currents to 20MHz.
I had to go to 20 Mhz in the model in order to represent the
sharp step of the transient as applied to the top. But a lower
bandwidth than this would be fine for detecting it.
The experiment needs to look for
a) A burst of HF occuring in the base current at around a
quarter cycle of f1 after the top discharges. Maybe another
more diffuse burst a half-cycle after that.
b) An estimate of the peak amplitude of the burst, compared with
the resonant base current amplitude. Eg in
http://www.abelian.demon.co.uk/tssp/pn040502/tfsm1-h1d2.wave.gif
the pulse arrives at the base between 8.5uS and 9.0uS, and peaks
at around 4 times the normal base current peak.
c) The top voltage following the waveform predicted in the above
graph, or similar. The modeled discharge was made to occur
exactly at the Vtop peak. In the real coil I guess it would
occur sometime before, so I might have to model that case.
It would also be nice to observe in the real coil how long the arc
discharge clamps the topload to earth for. This information could
then be fed back into the model to refine the resonator's boundary
conditions. If the model is thus programmed to enforce the observed
discontinuities in the topvolts, we should as a consequence see
the predicted base current pulse appear in the real coil, with
roughly the same shape, timing, and amplitude.
Bart wrote:
> In my experience, breakout has occured first. As the power was
> increased, racing arcs started.
So perhaps I should be animating a model which contains a fair bit
of RC load hung off the topload. Maybe that drives the gradient up
higher than otherwise?
Bert wrote:
> I don't think the transients will be as nearly great as the
> model indicates for arrested leaders
Yes, I agree. I don't think there's any point in considering
in-air leaders as a contributor to secondary voltage stress.
> In addition to adding capacitive loading, the streamer channel
> is also quite resistive.
I can model the coil's response to a variable RC toploading, but
only if I know in advance how R & C vary as a function of topload
volts or charge. That bit of physics is the missing link at the
moment, so there's not much we can do with it for now.
The reason I try to model the arc discharge is first because it
may provide a mechanism through transients to stress the secondary,
and secondly it is something we can compute with only a limited
amount of guesswork.
Terry, I'll come back with some suggestions for tests after I've done
a bit more modeling on these transients. I don't think the momentary
topload discharge is a good facsimile so I need to code for a more
realistic discharge.
Malcolm wrote:
> I've seen racing arcs both with and without breakout depending
> on the particular setup.
OK, if we have examples of racing arcs occuring without breakout, then
we cannot subscribe to 'discharge-induced transients' as a cause.
John wrote:
> My TT-42 coil is coupled at only about k = 0.11,
> but it's very close to creating racing sparks
Can you let me know the details of this system. If I can model it we
can see what sort of voltage gradients are being reached along
the secondary.
Can it be that the combined effects of my stress factor due to
secondary volts distribution, and Boris's factor due to surface
dielectric field concentrations, is enough to lower the breakdown
voltage well below the average of 26kV/cm, eg lowered by a factor
of four or more? A closer look at John's low-k TT-42 would be
quite interesting.
> This coil was wound with a PVC insulated wire,
> which seemed especially plagued by racing sparks
Boris's factor there? Does space winding encourage racing
arcs by reducing the shielding effect that each wire receives
from its neighbours, thus allowing a very high local field
strength around the conductor? Are space wound coils generally
worse for racing arcs than closewound? I'm off now to work
out the max gradient between two parallel conductors...
--
Paul Nicholson,
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.