From: Bert Hickman
Date: Fri, 18 Oct 2002 10:31:49 -0500
Subject: Re: [TSSP] Top Voltage
Paul, The onset of corona and discharge voltages are two different things. When the sphere diameter is significantly smaller than the gap distance, corona onset will occur at a significantly lower voltage than that required for sparkover. And corona will initially be observed at the negative terminal. However, for long, nonuniform gaps, long sparks will be preferentially launched when the sphere is positive than when it's negative in a sphere-plane gap. Makes things interesting... The model we should use is closer to a sphere-plane (if Terry adds an element of control by adding a grounded tinfoil ceiling). The AIEE tables are empirically derived and require compensation for surroundings, humidity, temperature, air pressure, etc... In a TC environment, the onset of corona may stimulate easier formation (due to heating from RF displacement currents) of streamers and leaders than the lower frequency or DC cases covered in the AIEE tables. In the sphere-sphere case with large diameter terminals and comparatively short gaps, only a spark discharge occurs with no steady or pulsed corona. Believe your assessment is probably correct... Best regards, -- Bert -- -- Bert Hickman Stoneridge Engineering "Electromagically" (TM) Shrunken Coins! http://www.teslamania.com Paul wrote: > Thanks Bert, I thought maybe I was making another mistake > there. Can't seem to engage brain today (:. > > With this correction to fig 7.4 to redefine V, the tssp > sphere-sphere breakdowns agree with North, very well at > long range, deteriorating for short gaps. > > When I compare these figures with > > http://hot-streamer.com/TeslaCoils/Misc/SGapVolt.jpg > > the north/tssp values for breakout voltage become quite > a bit less than the breakout given in this table. > > Eg 2.5cm diam electrodes, 4.05cm gap > > SGapVolt.jpg: 70 kV > North: 52.05 kV > tssp: 51.2 kV > > this is about the worst case discrepancy, and things improve > as the two spheres become closer. I take it that the AIEE tables > are derived from measured values? If so, does this indicate > a systematic departure from the values expected purely on the > basis of a fixed surface field (north & tssp). Are we seeing > a long range effect that holds off breakout despite the surface > field reaching 30kV/cm. > > I'm wondering if north/tssp predicts the onset of corona, > whereas AIEE tables show discharge voltages. For small > gaps the discharge follows immediately after corona starts, > but when the gap is wider, the initial corona 'softens' the > field gradient just above the electrodes, effectively increasing > the ROC a little, and allowing some further voltage rise before > discharge. Is that what we're seeing with this difference > between north and AIEE? > > Extrapolating to the case of the small sphere on OLTC, this > could easily account for considerably higher breakout voltage > measured. > > Is this the space charge effect that others have mentioned? If > so we seem to be acquiring some sort of numerical handle on it. > > Terry, > Does the AIEE have spark-gap voltages for sphere-plane discharges? > For our purposes these would be much better. Any other good > sources of data? > > The amount of space charge forming along a line of E-flux must be > some function of the field gradient along that line of flux. > Just that we don't know that function. If we could take a guess > at this, we could calculate the correction to the breakout voltage. > We could refine our guess by adjusting it first to match published > sphere-plane and sphere-sphere discharge voltages, and then against > measured TC breakouts. > > Is this on the right lines? Perhaps we can make an initial > guess of how much space charge is formed per unit volume for > a given background E-field uniform in that volume? Maybe this is > in a book somewhere. > > Then we just work out the location of peak surface field. Follow > a flux line out of that point, calculating the computed space charge > along the way, and subtracting the field that it induces at > the surface from the initial surface field. Well something along > those lines anyway. > > Of course, it would be nice to have some evidence that space charge > is holding off breakout. It would have to show up as an increase > in the apparent C of the terminal. And maybe it would also leave > behind a negative static charge on the terminal, as we've seen. > > So if any of this is realistic, we should be seeing signs of an > apparent reactance change and/or a DC residual, in the voltage > regime between the north/tssp predicted breakout and the observed > visible breakout. > > Does this approach consolidate everyone's thoughts on the subject? > -- > Paul Nicholson, > -- >
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.