SE1200131A1 - Fältstyrande material - Google Patents

Abstract

Fältstyrande gummimaterial används generellt i kabeltillbehör. De vanliga fältstyrande materialen som används i tillbehör innehåller SiC och CB som fyllmedel, vilket ger att man antingen har ett svagt fältstyrande material med en högelektrisk hållfasthet eller så har man ett kraftigt fältstyrande material med en låg elektrisk hållfasthet.Genomatt ha kombinationen ledande eller halvledande flingor och SiC som fyllmedel så kan man få ett material som är kraftigt fältstyrande och fortfarande ha en hög elektrisk hållfasthet.Figur 5.

Description

15 20 25 30 are a number of known methods for providing a stress control, for example, by means of geometrical or refractive field control in the form of a stress cone arranged at the position where the shield ends, or to have a material that has the ability to distribute the field by itself. The material is usually referred to as a field grading material, but is also called a stress grading material. The electrical properties of a field grading material are dependent on the field it is exposed to. ln connection with a field grading material both capacitive and resistive field grading are mentioned.

The capacitive or refractive field grading is used in altemating current applications.

Capacitive field grading can also be used to achieve field grading when voltages are occurring in the form of impulses. ln case of a cable ending of the kind indicated above, a body of a material having a dielectric constant higher than that of the insulation and with as low losses as possible is introduced around the unshielded part of the cable in the area closest to the shielded part of the cable and in electrical contact with the shield, whereby the spreading of the equipotential lines will be achieved.

Capacitive field grading properties are also desired in a material adapted for grading the electric field in high- voltage direct current (HVDC) applications so as to achieve an effective field grading in case of suddenly occurring voltage surges.

Resistive field grading can be used in altemating current as well as direct current applications. Resistive field grading can also be used in order to achieve field grading when voltages are occurring in the form of impulses. ln case of a cable ending of the kind indicated above, a body having a suitable resistance is introduced around the unshielded part of the cable in the area closest to the shielded part and in electric contact with the shield. When a voltage is applied across the cable a current flows through the body towards the shield of the cable which shield is at earth potential. A resistive voltage drop then occurs in the body, which results in a more uniform distribution of the potential. This potential distribution will be more linear if the body consists of a material exhibiting a non-linear electrical resistance that decreases with an increasing electric field. The non-linearity starts in a specific region of the electric field.

The closer the edge of the shield, the higher the electric field in the field grading body 10 15 20 25 30 and, consequently, the lower the electrical resistance in the body if the body exhibits such a non-linear electrical resistance. ln this way, the voltage drop along the field grading body will become more unifomtly distributed in a body that exhibits such a nonlinear electrical resistance than in a body that does not.

A field grading material with a non-linear electrical resistance, i.e. a material that does not follow Ohms law, can be achieved by combining at least two materials to a composite. Due to the area of use, the field grading material should be mechanically flexible. lt is therefore suitable to use, for example, a rubber as a matrix and to mix it with (semiconducting) particles.The particles forms a network within the matrix and the result is a composite being insulating at low electrical fields and having increased conductivity at higher electrical fields.

Currently, there is a problem of reaching high enough conductivity with conventional field grading materials and still maintain a sufficiently high breakdown strength, particularly at switching impulse (SI) and lightning impulse (LI). A possible solution to this problem could be to raise the amount of fillers or using a highly conducting SiC or CB type. However, this would result in lower dielectric breakdown strength of the field grading material.

SUMMARY lt is an object of the present invention to provide an improved alternative to the above techniques and prior art. More specifically, it is an object of the present invention to produce an electrical field grading material with high electrical breakdown strength.

To achieve these and other objects, a method and an electrical field grading material in accordance with the independent claims are provided.

The invention is based on the insight that the current electrical field grading materials need to have higher conductivity and higher breakdown strength without increasing the filler contents of CB or changing the type of SiC used in the material. 10 15 20 25 30 According to a first aspect of the invention, an electrical field grading material with improved breakdown strength is provided as defined in claim 1.

According to a second aspect of the invention, a method for producing an electrical field grading material with improved breakdown strength is provided as defined in claim 5. ln accordance with an embodiment of the invention, an electrical field grading material includes a matrix and at least two filler particles including silicon carbide (SiC) or ZnO and at least one electrically conducting or semiconducting flake. ln accordance with another embodiment of the invention, a method for producing an electrical field grading material with improved breakdown strength includes mixing at least two filler particles, wherein the filler particles include silicon carbide (SiC) or zinc oxide (ZnO) and electrically conducting or semiconducting flakes.

By using a new kind of filler particle, a conducting or semiconducting flake filler instead of CB, it is possible to reach high enough conductivity without lowering the dielectric breakdown strength of the material. The conducting or semiconducting flakes have such shape and size that they might form a percolating network by themselves.

One advantage with the concept of the present invention is that cable accessories, such as joints or tenninations, which will survive both lightning and switching impulse tests as well as other dielectric tests, can be produced.

The person skilled in the art realises that that the present invention is not in any way restricted to the embodiments described above. On the contrary, several modifications and variations are possible within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail by the following non-limited detailed description of embodiments with reference to the attached drawings, wherein: 10 15 20 25 30 Figure 1 is a schematic representation of an electric insulation arrangement according to the prior art, Figure 2 shows a diagram on the lightning impulse (LI) breakdown data for four different field grading materials according to the invention, Figure 3 shows a diagram on the switching impulse (SI) breakdown data for four different field grading materials according to the invention, Figure 4 shows a diagram of the electrical DC resistivity as a function of applied electrical field for four different field grading materials at 30°C according to the invention, Figure 5 shows a diagram of the electrical DC resistivity as a function of applied electrical field for four different field grading materials at 70°C according to the invention.

DETAILED DESCRIPTION Electrical field grading materials are electrically stress- controlling compositions. They are generally used in cable accessoriesand are key parts in high voltage/medium voltage (HV/MV) cable accessories. The electrical field grading material reduces the electrical stresses in electrically critical positions on the cable accessories.

Figure 1 illustrates a cable termination 1 provided with a body 2 of a field grading material according to the prior art. The cable 3 comprises a conductor 4 surrounded by an electric insulation 5. A shield 6 is arranged outside the insulation 5, said shield 6 being connected to ground. The end of the cable 3 is unshielded, i.e. at the end of the cable the insulation 5 is not covered by any shield. The body 2 of the field grading material is introduced around the unshielded part of the cable in the area closest to the shielded part of the cable and in electric contact with the shield 6. The body 2 of field grading material will secure a unifomi distribution of the potential at the cable terrnination, as illustrated by the equipotential lines 7. lt should also be mentioned that only the upper half of the Iongitudinal section of the cable termination is shown in Figure 1. 10 15 20 25 30 Four different field grading materials FGM 1 - FGM 4 have been used for tests. FGM 2 is less conducting than FGM 1. The conducting or semiconducting flakes in FGM 3 and FGM 4 comprise between about 1% and about 20% of the volume of the composition.

FGM 4 comprises less vol.% of conducting or semiconducting flakes than FGM 3.

Results from switching impulse (SI) and direct current (DC) step tests on experimental DC joints performed, with high voltage level, has shown that the existing field grading material FGM 2 is a too weak field grader to be able to be used in the joint for higher voltages. Also the FGM 1, generally used in field grading adapters for DC termination was tested with SI and DC step test. This FGM is a good field grader but has too low breakdown strength in SI. Screening activities for new field grading materials which has a high enough conductivity and high enough breakdown strength at SI has been performed. The results ended up in a new material combination containing two different fillers, silicon carbide (SiC) and conducting Mica flakes. ln Figure 2 and 3 Weibull plots are seen from the lightning impulse (LI) and switching impulse (SI) breakdown data for four different field grading materials. The two conventional materials FGM 1 and FGM 2 containing SiC and CB as fillers, but also two new materials containing SiC and conducting Mica flakes, FGM 3 and FGM 4. ln Figure 4 and Figure 5 the DC resistivity at 30°C (Figure 4) and 70°C (Figure 5) are plotted as a function of electric field. The resistivity for the FGM 1 is much lower than for the FGM 2, but the FGM 1 has a much lower breakdown strength compared to the FGM 2. This is because the FGM 1 contains a more conducting SiC and higher amcunts of CB.

When changing the CB to Mica flakes one can keep the high breakdown strength. The FGM 4 has the same breakdown strength as the FGM 2 material and the same DC resistivity level. The amount of filler in vol.% is the same in FGM 2 and FGM 4. The FGM 2 contains a certain amount of SiC and CB. The FGM 4 contains SiC and Mica flakes. ln the FGM 4 the Mica flakes is probably not a percolated system. For the FGM 3, with SiC and more vol.% Mica flakes than in FGM 4, it has the same low DC resistivity as the FGM 1 but the breakdown strength is much higher. 10 One conclusion from the tests is that by using a field grading material comprising at least two types of particles, wherein at least one of the at least two types of particles is a conducting or semiconducting flake and wherein the conducting or semiconducting flake has at least one dimension which is smaller than its other two dimensions, percolation can be achieved.

One example of conducting or semiconducting flakes can be Mica flakes.

The invention shall not be considered limited to the examples described, but can be modified and altered in many ways by the person skilled in the art, without departing from the scope of the invention.

Claims (8)

10 15 20 25 30 CLAIMS

1. An electrical field grading material with improved breakdown strength including a matrix and at least two types of filler particles, wherein the at least two types of filler particles include silicon carbide, SiC, or zinc oxide, ZnO, and at least one electrically conducting or semiconducting flake.

2. An electrical field grading material with improved breakdown strength according to claim 1, wherein the matrix includes a dielectric material.

3. An electrical field grading material with improved breakdown strength according to claim 1, wherein the at least one electrically conducting or semiconducting flake has at least one dimension which is smaller than its other two dimensions.

4. An electrical field grading material with improved breakdown strength according to claim 1, wherein the at least one electrically conducting or semiconducting flake includes Mica flakes.

5. A method for producing an electrical field grading material with improved breakdown strength, the method including mixing at least two types of filler particles in a matrix, wherein the at least two types of filler particles include silicon carbide, SiC, or zinc oxide, ZnO, and at least one electrically conducting or semiconducting flake.

6. A method for producing an electrical field grading material with improved breakdown strength according to claim 5, wherein the matrix includes a dielectric material.

7. A method for producing an electrical field grading material with improved breakdown strength according to claim 5, wherein the at least one electrically conducting or semiconducting flake has at least one dimension that is smaller than its other two dimensions.

8. A method for producing an electrical field grading material with improved breakdown strength according to claim 5, wherein the at least one electrically conducting or semiconducting flake includes Mica flakes.