From: Paul
Date: Sun, 24 Jun 2001 12:23:09 +0100
Subject: Re: [TSSP] Direct Voltage Measurement of Topload Voltage for TSSP
Hi Terry,
There's a problem with it - the E field gradient close to the
0.125" inner conductor considerably exceeds the oil breakdown
threshold:
Within the coax, E(r) = Q/(2 * pi * epsilon * r * L)
= C * V/(2 * pi * epsilon * r * L) volts/metre
where a <= r <= b, and L is the length 0.122m
so setting r to the inner conductor radius a,
r = a = 0.125 * 0.0254 = 3.175e-3
E(a) = 5e-12 * 500e3/(2 * pi * 2.2 * 8.854e-12 * 3.175e-3 * 0.122)
= 52.7 MV/m = 1339 kV/inch
Working the other way, the minimum radius for the inner of a 5pF coax
cap at 500kV terminal voltage would be
200e3 * 39.37 = E(a_min)
= 5e-12 * 500e3/(2 * pi * 2.2 * 8.854e-12 * a_min * len)
so a_min * len =
5e-12 * 500e3/(2 * pi * 2.2 * 8.854e-12 * 200e3 * 39.37) = 2.6e-3
For a length 0.122m that gives a min radius of 2.1 cm, giving a 20cm
radius to the outer. There might be a similar problem with the top rim
of the outer electrode, which might need to be rolled over to form a
nicely radiused lip.
The coaxial capacitor is appealing because of its inherent insensitivity
to anything but the voltage on its input terminal. Perhaps a co-linear
cascade of coaxial segments forming a stack of series connected caps?
The dimensions would be a lot more favourable, but it would be quite
hard to make I think.
Cheers,
--
Paul Nicholson,
Manchester, UK.
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.