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 E0 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 dis- charges from the electrode surface and/or an electric breakdown through the medium to another electrically con- ducting 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 vol- tages, 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 electri- cally 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 R0 of curvature is about U/Eo. The radius of curvature, in its turn, is dimensioning for the dimensions of the screening elec- trode, 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 Figure 1, where the screen- ing electrode 1 is assumed to be a sphere with a radius r0 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 elec- trode is E = U(r0/r2) and it increases proportionally to the radius r0 of the electrode. The capacitance C of the electrode is C = where er is the relative dielectric permittivity of the medium and Q is the dielectric constant. Also the capaci- tance thus increases proportionally to the radius r0 of the electrode.

A typical embodiment of screening electrodes according to the state of the art is shown in Figure 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 Figures 3a, 3b and 3c from EP patent specification EP 0 075 884 B1, where Figure 3a shows a screening electrode 1 composed of 12 disc-shaped electrode segments la which are fixed to an icosahedron- shaped frame 5 composed of rods 6 according to Figure 3b, 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 la with the depression 4 is shown in cross section in Figure 3c. A composite elec- trode 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 configu- rations, for example line bends, the field strength in- creases since the curvature of the surface is larger for a sphere than for a cylinder with the same radius. Figure 7 shows a 90" line bend for GIS according to the state of the art. The cylinder-shaped inner conductor 10 has the radius rc, the tubular outer conductor/screen 11 has the radius Rc. In the actual line bend, the screen assumes the shape of a sphere 12 with the radius Rs and the inner conductor, the electrode, becomes a sphere 1 with the radius rg. For the line bend to be free of discharges, the following must apply, namely that, rS > re and Rs > Rc; 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 elec- tric non-conducting covering, for example to make diffi- cult 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 dis- charges in a gaseous insulating medium with reduced exter- nal dimensions and reduced capacitance.

An electrode for field control according to the invention comprises an inner electrode with an electrically conduc- ting surface which is surrounded by a thick layer of an

electrically insulating material with a low relative die--- lectric permittivity, preferably a polymeric foam con- taining 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 prefe- rably 0.25. For non-spherical electrodes, the diameter 2r of the inner electrode is considered to be the mean dia- meter of the electrode. A sufficiently good approximation of the mean diameter is here the mean value of the diame- ter in two directions perpendicular to each other. Inner electrode here also means an electrode element in a com- posite electrode. To achieve the effect of the invention, only those parts of the inner electrode, where a con- siderable increase of the electric field strength occurs, need to be covered with the non-conducting layer. Prefe- rably, at least that one-third of the surface of the inner electrode is covered which, without covering with the layer, would exhibit the highest field strength.

The mode of operation of the invention can be most readily illustrated by means of a model according to Figure 4, in which the inner electrode is assumed to be sphere 1' with the radius rl and at an electric potential U towards a dis- tant ground plane 2, where the inner electrode is surroun- ded by a concentric layer 8 of electrically insulating material with a thickness of r2 - rl where r2 is the outer radius of the insulating layer and 9 its outer edge. For the sake of simplicity, the insulating material is assumed to have such a high proportion of internal cavities, as gas bubbles or hollow microspheres, that the relative dielectric constant of the material is cr ~ 1, as is the dielectric constant of the gaseous medium. Since the diameters of the inner cavities are chosen so small that the dielectric strength of the gas contained in the cavity according to Paschen's law is considerably greater than the dielectric strength of a macroscopic gap within the same gas, this insulating material has a considerably

greater dielectric strength than the gas surrounding the' electrode. Typical mean diameters are smaller than 300 micrometres. For a more detailed analysis of dielectric strengths in polymeric foams, reference is made to an article "Breakdown of Polyurethane Hard-foam Insulation under Short-time HV Stress" by D. Koenig, B. Bayer and H.J. Heller in IEEE Transactions on Electrical Insulation, Vol. 24, No. 2, April 1989, pages 239-248. In this case with Q = 1, the same relations apply to the field strength at the distance r from the centre of the electrode and the capacitance as to an electrically conducting sphere with the radius rl at potential U towards a distant ground plane as shown under the discussion of the background art: E = U(rO/r) C=47tQQr0 It can now be shown that, for an insulating medium with a dielectric strength which is kEo with E0 as the dielectric strength of the gaseous medium, an electrode according to the invention may be designed where the field strength at the surface of the inner electrode falls below the dielec- tric strength k-Eo of the insulating material, and the field strength at the outer surface of the insulating layer falls below the dielectric strength E0 of the gaseous medium if the conditions rl > r0/k and r2 > rO/lk are fulfilled, where r0 is the radius of an electrode according to the prior art (electrically conducting sphere) where the dielectric strength E0 of the gaseous medium is attained at the surface of the electrode. For example, the radius of the inner electrode is reduced, and hence also the capacitance, by a factor of four and the outer radius of the insulating layer is reduced by a factor of two if k = 4.

If the relative dielectric constant of the insulating material is Q > 1, the possible outer radius r2 becomes

somewhat larger. When maintaining the ratio r2/rl = 2, aS- in the example above, the following applies instead r2 = rO/ (l+er ) - For Q = 1.3, a typical value of a foamed plastic, thus r2 = 0.57 r0, which is still a considerable reduction of the size of the electrode. For the insulating layer to have a noticeable effect according to the invention, its relative dielectric constant Q should be smaller than 3, preferably smaller than 2 and still more preferably smaller than 1.5 An electrode according to the invention is preferably used in electric equipment for voltages exceeding 1 kV.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an explanatory sketch of a screening elec- trode according to the prior art as an electrically con- ducting sphere.

Figure 2 shows a toroidal screening electrode according to the prior art.

Figure 3a shows a screening electrode according to the prior art, composed of disc-shaped electrode segments.

Figure 3b shows an icosahedron-shaped frame to which the electrode segments according to Figure 3a are attached.

Figure 3c shows a disc-shaped electrode segment of the screening electrode according to Figure 3a.

Figure 4 shows an explanatory sketch of a screening electrode according to the invention with a concentric non-conducting layer on an electrically conducting sphere.

Figure 5 shows a toroidal screening electrode with a non-- conducting layer according to the invention.

Figure 6 shows an electrode segment with a non-conducting layer for a composite screening electrode according to the invention.

Figure 7a shows a GIS line bend according to the prior art.

Figure 7b shows a GIS line bend comprising a non-conduc- ting layer according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one embodiment of the invention, the toroidal screening electrode shown in Figure 2 is improved by providing, as shown in Figure 5, an inner toroidal electrode 1' with a non-conducting layer 8 on a major part of its surface.

That part of the electrode which is directed towards the symmetry axis (dash-dotted) is screened from high field strengths by the very toroid shape and hence need not be covered. The outside diameter of the toroidal screening electrode according to the invention becomes considerably smaller than in the corresponding toroidal screening elec- trode according to the prior art.

In another embodiment of the invention, the screening electrode composed of electrode segments, shown in Figures 3a-3c, is improved in that, as shown in Figure 6, at least one of its electrode segments la', on the major part of the surface of the inner electrode 1', comprises a non- conducting layer 8. Also in this case, the outside dia- meter of the composite screening electrode according to the invention is considerably smaller than in the corre- sponding composite screening electrode according to the prior art. Also the depression 4 may be filled with a non- conducting material (not shown).

It may be advantageous to allow the non-conducting layer- to constitute the mechanically supporting part of the screening electrode, where the inner electrode is made in the form of an electrically conducting covering on the inner side of the layer. The covering may, for example, be applied by painting, deposition of evaporated metal, or chemical precipitation of an electrically conducting sub- stance.

In an additional embodiment of the invention, the GIS line bend shown in Figure 7a is improved, as shown in Figure 7b, by placing a non-conducting material 8 between the spherical inner conductor 1' at the line bend and the outer conductor/the screen 12' at the line bend. The radius Rs may then be equal to the radius Rc of the straight sections, such that the space requirement for the line bend is reduced. Also the inner radius rs may be reduced. The inner conductor in the line bend may also have the same shape as an ordinary pipe bend (not shown).