From: Paul
Date: Sun, 05 May 2002 22:58:49 +0100
Subject: Re: [TSSP] Secondary voltage stress factor
Boris wrote: > In this way dielectric objects (insulators here) act like > sort of lens and the effect of coverging E-field lines is most > expressed in surface of the dielectric while voltage > between big plate electrodes remained same as before. > I hope I made my point clearer now. Yes, I do see what you mean. With a perfect smooth coating this probably wouldn't be an issue, but as you say, local non-uniformities in the dielectric coating, eg a bubble, or something stuck to the surface, would give uniformity to the E-field in that region, thereby creating points where the local field exceeds the average. So we should think in terms of two sources of non-uniformity, a) that due to the voltage profile, giving rise to the stress factor under discussion; b an additional non-uniformity due to surface irregularity, giving rise to an additional factor increase in the peak field strength. Perhaps we should have a stress factor and a surface quality factor, the product of the two multiplying the average gradient to give a working value for the peak gradient. We really do need some hard data on this. If we new at what primary voltage particular coils developed racing arcs, then we could calculate the voltage gradient due to (a), and if this falls short of 26kV/cm, we could lump the remaining coefficient into a 'surface' factor. Of course, we'd need to look at quite a few coils to see if there was any consistency to such a surface quality factor. Q to all: Do racing arcs tend to occur before topload breakout, as the power is turned up, or do they appear at the same time? I think I may have asked this before, perhaps someone could remind me? The traditional cure seems to be to reduce coupling, so K must be a factor, but it doesn't look very promising from the data in pn040502. I'll redo some of those tomorrow, normalising the primary energy instead of the primary voltage. I think I'll also plot a set of those gradient animations, ie showing the |dV/dx| rather than V. And concerning strikes that hit the secondary, do these occur only when the top is breaking out? -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.