From: Paul
Date: Thu, 13 Jun 2002 15:01:22 +0100
Subject: Re: [TSSP] Re: Top V Probe Design
Bert wrote:
> On page 288 Sarjeant presents a chart...
> 550 kV/cm R-Temp
> 500 kV/cm 20 weight motor oil(!)
> 400 kV/cm Water white mineral oil (Macrol 2930)
> 310 kV/cm Mineral Transformer Oil
> significant derating should be applied - at
> least a factor of 4-5.
For coaxial cylinders, the radial field strength is
E = Q/(2*pi*epsilon*Rx)
= C*V/(2*pi*epsilon*Rx)
where Q, C is charge, capacitance per unit length,
Rx is some radius between inner and outer, and V is
the potential across the cylinder.
So V = 2*pi*epsilon*Rx*E/C
and with C given by
C = 2*pi*epsilon/ln(R1/R2)
where R1, R2, are the outer, inner radii, we get
V = Rx*E*ln(R1/R2)
for the volts across the cylinder at which the field at
radius Rx is E volts/metre.
Terry wrote:
> http://hot-streamer.com/temp/HVProbe-2.gif
For R2 = 0.5" = 0.0127m;
R1 = 3.0" = 0.0762m;
and setting E = 6000kV/m at Rx = R1, we have a max
working voltage of
V = 0.0127 * 6000 * ln(0.0762/0.0127)
= 136 kV
based on breakdown of the inner capacitance.
For the outer outer shield to the secondary coil, we have
R2 = 3.0" = 0.0762m;
R1 = 5.5" = 0.1397m;
V = 0.0762 * 6000 * ln(0.1397/0.0762)
= 277 kV.
So the system is likely to be limited to quite small
toploads due to breakdown of the inner capacitance.
A larger inner electrode would improve things a little, eg
R2 = 1.5" = 0.0381m;
R1 = 3.0" = 0.0762m;
V = 0.0381 * 6000 * ln(0.0762/0.0381)
= 158 kV
but then the total capacitance goes up quite a bit unless
the inner electrode is made a lot shorter.
I have a modification to suggest, which I'll sketch out
and post later today. Unfortunately it's quite a large
modification...
--
Paul Nicholson,
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.