Electrode for field control

Details Electrode for field control Electrode for field control

1999-07-09

2002-08-13

Copenheaver, Blaine (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C428S313500, C521S056000, C174S034000, C336S08400R, C336S08400R

Reexamination Certificate

active

06432524

ABSTRACT:

This application claims priority from PCT/SE97/01953 filed on Nov. 21, 1997, which claims priority from Swedish application Ser. No. 9604282-5 filed on Nov. 22, 1996.
TECHNICAL FIELD
The present invention relates to an electrode for control of an electric field in a gaseous electrically insulating medium.
BACKGROUND ART
Electrically conducting objects at a high electric potential relative to the surroundings give rise to high electric field strengths in their vicinity. This applies particularly to objects with a small radius of curvature and parts projecting from the object into the surrounding insulating medium.
If the electric field strength exceeds the dielectric strength E
0
of the medium, in dependence on the degree of inhomogeneity of the electric field and the change of the voltage with time, this leads to electric partial discharges from the electrode surface and/or an electric breakdown through the medium to another electrically conducting object or a ground plane.
Electric breakdowns must inevitably be prevented since they entail an electric short-circuit of the equipment, where the electrodes are included.
Also partial discharges are undesirable, since, among other things, they give rise to radio interference, energy losses and chemical degradation of material in that the partial discharges generate chemically aggressive and poisonous substances such as ozone and nitric oxides in air and a number of fluorine and sulphur compounds in the technical insulating gas sulphur hexafluoride. Partial discharges, which are harmless per se, may make the measurement of partial discharges of other parts of the equipment impossible.
Commonly used methods of reducing the electric field strength in the vicinity of electrically conducting objects comprise selecting, in the entire design and dimensioning of devices intended for high electric voltages, mutual distances between electrically conducting objects and radii of curvature on the surfaces thereof, such that the dielectric strength of the medium is not attained at any place in the device.
Since such dimensioning would often lead to very large dimensions of the equipment, areas with especially high local field strengths, such as the high-voltage side of bushings or conductor bends in gas-insulated switchgear, GIS, are provided with screening electrodes which have sufficient radii of curvature to keep the electric field strength below the dielectric strength of the medium. The electric field strength E in the vicinity of an electrically conducting object is of the order of magnitude of E≈U/R, where U is a typical electric potential difference for the equipment and R is the radius of curvature. This implies that the smallest allowable radius R
0
of curvature is about U/E
0
. The radius of curvature, in its turn, is dimensioning for the dimensions of the screening electrode, which often becomes a significant part of the dimensions of the whole device.
Large screening electrodes increase the capacitance of the screening part. For many fields of use, the increasing capacitance implies an unnecessary load on the voltage/current source. The electrostatic energy in the device which is released during a short-circuit, which, for example, is caused by an electric breakdown, increases with increased capacitance.
A large screening electrode reduces the maximum electric field strength in its vicinity. However, it causes a higher field strength at larger distances than a smaller electrode, which is often disturbing, since maximum field strength outside a device are often specified and must not be exceeded.
These conditions are illustrated in the simplest way by means of a model according to
FIG. 1
, where the screening electrode
1
is assumed to be a sphere with a radius r
0
and at an electric potential U towards a distant ground plane
2
. With these assumptions, the electric field strength at the distance r from the centre of the electrode is
E=U
(
r
0
/r
2
),
and it increases proportionally to the radius r
0
of the electrode. The capacitance C of the electrode is
C=4&pgr;∈
r
∈
0
r
0
where ∈
r
is the relative dielectric permittivity of the medium and ∈
0
is the dielectric constant. Also the capacitance thus increases proportionally to the radius r
0
of the electrode.
A typical embodiment of screening electrodes according to the state of the art is shown in
FIG. 2
, where a high-voltage apparatus
3
, for example a capacitor, terminates in a toroidal screening electrode
1
. For high voltages, composite screening electrodes are often used, a typical embodiment being shown in
FIGS. 3
a
,
3
b
and
3
c
from EP patent specification EP 0 075 884 B1, where
FIG. 3
a
shows a screening electrode
1
composed of
12
disc-shaped electrode segments
1
a
which are fixed to an icosahedron-shaped frame
5
composed of rods
6
according to
FIG. 3
b
, with the fixing elements placed in a depression
4
in the electrode element to fix the electrodes to the icosahedron corners
7
. An electrode segment
1
a
with the depression
4
is shown in cross section in
FIG. 3
c
. A composite electrode according to EP 0 075 884 is lighter and simpler to manufacture than a corresponding electrode which is made up of a coherent electrode, but its dimensions have not be diminished.
When changing from two-dimensional electrode devices as, for example, GIS lines, where the inner conductor is here interpreted as an electrode, to three-dimensional configurations, for example line bends, the field strength increases since the curvature of the surface is larger for a sphere than for a cylinder with the same radius.
FIG. 7
shows a 90° line bend for GIS according to the state of the art. The cylinder-shaped inner conductor
10
has the radius r
c
, the tubular outer conductor/screen
11
has the radius R
c
. In the actual line bend, the screen assumes the shape of a sphere
12
with the radius R
s
and the inner conductor, the electrode, becomes a sphere
1
with the radius r
s
. For the line bend to be free of discharges, the following must apply, namely that, r
s
>r
c
and R
s
>R
c
; usually, the radii at the line bend are about 50% larger than in the straight lines.
It is also known to provide the electrodes with an electric non-conducting covering, for example to make difficult the emission of photoelectrons from the electrode surface, or as corrosion protection. However, these coverings are thin compared with the dimensions of the electrode, and their influence on the electric field around the electrode is therefore negligible. In this way, the dimensions of the electrode cannot be reduced.
SUMMARY OF THE INVENTION
An electrode for field control according to the invention solves the task of preventing breakdown or partial discharges in a gaseous insulating medium with reduced external dimensions and reduced capacitance.
An electrode for field control according to the invention comprises an inner electrode with an electrically conducing surface which is surrounded by a thick layer of an electrically insulating material with a low relative dielectric permittivity, preferably a polymeric foam containing gas bubbles or a matrix in which hollow gas-filled microspheres are embedded. By a thick layer is here meant a layer in which the ratio of the thickness d of the layer to the diameter 2r of the inner electrode is greater than 0.05, preferably greater than 0.15 and still more preferably 0.25. For non-spherical electrodes, the diameter 2r of the inner electrode is considered to be the mean diameter of the electrode. A sufficiently good approximation of the mean diameter is here the mean value of the diameter in two directions perpendicular to each other. Inner electrode here also means an electrode element in a composite electrode. To achieve the effect of the invention, only those parts of the inner electrode, where a considerable increase of the electric field strength occurs, need to be covered with the non-conducting layer. Preferably, at least that one-third of the surface of the inner electrode is covered which

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