From: Paul
Date: Mon, 21 Oct 2002 14:33:20 +0100
Subject: Re: [TSSP] Top Voltage
Summary
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Assumptions, Definitions, Hypotheses
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1) Assume that corona onset begins when the surface field
reaches 30kV/cm (low frequency or DC), as per the North report.
Lets call this the corona onset field, COF.
1a) The COF may fall, say to 26kV/cm above 30kHz.
1b) The COF may rise, say by a factor of 2 or 3 for
short fast signals, perhaps due to inertia at molecular
level.
1c) We expect the COF to be proportional to air density,
specifically particle number density. Hence from this a
fairly straightforward variation with temp and pressure.
1d) For a given electrode geometry, a given COF leads to a
corona onset voltage, COV, based on quasistatic E-field
calcs in the usual way (eg north, tssp).
1e) Below the COV, the field is loss-free and described
by the quasistatic approximation.
1f) Only local physics is involved in the corona onset,
no long range effects, so COV should be in good agreement
with calcs.
1g) Variations to COF due to frequency, temperature, and
pressure, can be treated as adjustments to the 'standard'
value of 30kV/cm.
2)
As the voltage rises above the COV, corona forms into
recognisable leaders. Let's call this the breakout
voltage, BV.
2a) For short gaps, we expect BV to be close, perhaps
indistinguishable, from the COV.
2b) In some cases the leaders will initiate a full discharge
arc. We'll not bother to draw distinction between leaders
in air and arc discharges, whichever happens first counts
as the BV. Thus BV can stand for breakout or breakdown volts.
2c) For long gaps, and TC terminals, we expect BV to be
distinctly higher than the COV.
2d) The interval between BV and COV is likely to depend on
frequency and air particle number density.
2e) Long range effects become significant when considering
BV, as opposed to only local for COF.
2f) COV-BV interval would likely be reduced by memory
(channels of low air density) of previous shots.
2g) As voltage increases beyond COV towards BV, we expect a
fairly smooth and continuous increase in corona, therefore
corona-related effects should smoothly increase: Q drop,
terminal current, DC offset, etc, all should increase steadily.
Some comments
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We'd like to be able to calculate the COV and BV for
typical TC geometries.
Lets stick to signals with slow risetimes, say > 1uS,
to avoid effects (1b), so that would be fres < 250kHz,
to begin with. Big, low frequency coils, line frequency,
or DC.
Lets stick to slow BPS to avoid effects (2f), to begin with.
Things to do
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Let's concentrate on recognising and measuring COV and BV.
We must learn to be good at predicting and measuring COV, and
get good agreement, before we can go any further.
Firm up the geometry around OLTC topload to better define
the E field.
Look for signs of corona onset occuring earlier than the
observed breakout:-
a) Onset of decline of Q factor;
b) Appearance of DC base current component;
c) Low level light output;
d) Increase in apparent terminal capacitance;
e) Slight drop of Fres;
f) Increase of HF noise output from the system.
Measure breakout thresholds for various arrangements of
terminals:-
a) Various TCs;
b) Various gaps of a few cm;
c) Must be well defined geometries, prefer one terminal
grounded, rather than balanced.
Some specific expts:
--------------------
For OLTC...
Plot Q as function of Vfire, for small sphere and large
sphere (at same height). Look to see if Q starts to
decline later with large sphere.
Test also with no rod/sphere. Q should not decline at
all until much later. That would prove Q drop is due
to corona onset.
Plot BV (and COV if detectable) as function of adjustable
ceiling height. Ceiling in 2 or 3 positions only. Just
small sphere and large sphere.
Capture rod/sphere terminal current in the voltage regime
below the obvious BV. Analyse to look for signs of corona
onset.
If corona onset is discernable by Q or rod/sphere current,
see if DC base current appears in region between COV and BV.
If so, then space charge is an issue in determining COV-BV
interval. If DC base current delays until BV reached, then
we can ignore space charge and treat the initial corona as
bulk neutral.
For other coils...
Try to observe effects of corona onset prior to obvious
leader formation.
Try to measure peak base current at COV and BV for well
defined TC geometries.
Also,
Find published data on gap breakdowns.
Find published estimates of equilibrium between charged
particle density and prevailing E-field.
Attempt to model virtual enlargement of terminal due to
(hopefully neutral) initial corona. Estimate radial
current in the extra 'virtual' region and see if breakout
coincides with a certain total or max current or current
density.
--
Paul Nicholson,
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.