TSSP: List Archives

From: Paul
Date: Fri, 25 Oct 2002 13:25:26 +0100
Subject: Re: [TSSP] Top Voltage

Hi All,

Things do seem to be looking up.  I'm pleased and relieved to
see that there's a huge amount of professional research out
there on the subject of breakout in long gaps.

A few comments on what I've seen so far, (thanks Marco for
Larsson, et al, that was bang on target)

There appears to be off-the-shelf physical quantitative models
available for each stage of the breakout process:  corona onset,
diffuse corona (streamers), streamer-leader transition, leader
development.

These models extend to discharges inhibited by finite source
charge and resistance.

They seem to involve straightforward integrals over field
quantities, with things like coefficients for ionisation,
recombination, etc.

The characteristic time for leader formation seems to be
quite long - several tens to a hundred or more uS, which gives
plenty of scope for delayed breakout in say, OLTC.  I begin to
see now why breakrate, frequency, and coupling will have a big
impact on the breakout.

I'm coming to agree with Boris in that although 30kV/cm may
predict the corona onset, the initial effects are pretty
negligible, and a whole lot more physics happens (and a much
higher voltage is required) before any distinctly visible
leaders begin to propagate.

We should, I think, take some time to study the available
literature and set up some models of our own, before we propose
too many more experiments.  There's an awful lot of stuff out
there and some effort will be required to filter out the relevant
bits without drowning in information and getting hopelessly lost.

Hopefully, we can achieve our aims without having to do any
original research of our own.  We can continue our tradition of
working from well established physical principles - no need to
launch off into the dark.

To summarise our goals as a guide to what info is relevant:

Our good understanding of the secondary needs to be extended
to account for the interaction with breakout loading. We would
like to be able to predict the breakout performance of a TC,
given the geometry and firing voltage.[*]

Predicting the performance, in this sense, means being able to
estimate the typical number and length of inhibited leaders when
discharging into air, for a given firing voltage.

To do so, we will likely have to take account of the dynamically
varying load applied to the secondary, working in the time domain
to derive a numerical solution to the non-linear system.  Only then
will we cope with arbitrary topload waveforms (wide range of Ctop,
Fres, and k).

Thus we will need to assemble quantitative models for each step
in the formation of inhibited leaders.  We should be able to
predict, not only the corona onset, but the voltage at which
diffuse corona forms, and the voltage at which visible leaders
begin to start, as well as work out the max leader length.

One possible difficulty is that all the stuff I've seen so far
deals with single polarity impulses.  Our HF alternating
waveforms, modulated by a beat envelope, then modulated again
by a break rate, will add some complications.  We'll just have
to see how things come out.

I'll start a new web page this weekend to summarise existing
applicable research on breakout phenomena.  I'll use it to keep
track of which references we need to obtain or have got, so that
Marco will know exactly what to go for.   There's such a huge
amount of material out there that we have to be pretty specific.

[*] Given this, we would then be able to close the loop
on secondary design and answer questions such as which
combination of BPS, Fres, k, h/d and topload size makes
the longest leaders from a given input power.
--
Paul Nicholson,
--


Maintainer Paul Nicholson, paul@abelian.demon.co.uk.