EP2095376A1

EP2095376A1 - Field grading material

Info

Publication number: EP2095376A1
Application number: EP07852215A
Authority: EP
Inventors: Erik Jonsson; Tommaso Auletta; Henrik Hillborg
Original assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Current assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Priority date: 2006-12-20
Filing date: 2007-12-05
Publication date: 2009-09-02
Also published as: CN101563734A; SE0602789L; SE531409C2; WO2008076058A1; EP2095376A4

Abstract

A field grading material comprising a polymeric matrix and a filler comprising zinc oxide and carbon black, wherein the zinc oxide is in the form of pure zinc oxide particles having at least one dimension smaller than or equal to 100 nm. An electric field control device (7) for grading an electric field at an interruption of the insulation layer arranged around the conductor in a medium or high-voltage cable, wherein the device comprises the field grading material.

Description

Field grading material

TECHNICAL FIELD

The present invention relates to a field grading material comprising a polymeric matrix and filler particles, as well as to a device for grading an electric field. The invention also relates to use of a field grading material at a joint or termination of an electric power cable.

BACKGROUND ART

At the transition of an electric field from a first medium to a second medium, electric stresses harmful to the electric equipment can ensue due to a discontinuity in the electric field. A cable comprises an insulating layer arranged around the conductor and a grounded shield arranged around the insulating layer. The grounded shield in the cable could also be referred to as a grounded outer conducting layer. In, for example, a medium or high voltage cable, the electric field is uniform along the cable axis and there is a variation in the field only in the radial direction. The entire potential difference between the conductor and the shield occurs across the insulation. When the cable is terminated or spliced, the shield of the cable is removed for a distance along the cable. The removal of the shield causes a discontinuity in the electric field at the shielded end, resulting in high electric stresses at the place where the shield is removed. The electrical potential lines, also referred to as equipotential lines, of the cable will be concentrated where the shield is removed. These high electric stresses must be reduced in order not to impair the expected life of the cable system. If the high stresses are not reduced the high electric stresses may cause flashover or breakdown at the surface of the terminated cable . In this description and subsequent claims medium voltage refers to a voltage in the interval 1-36 kV and high voltage to a voltage greater than 36 kV.

The electric stresses in question can be reduced by grading the electric field at the transition of the electric field from the first medium to the second medium, e.g. from a shielded cable part to a cable part where the original shield has been removed. There are a number of known methods for providing a stress control, for example, by means of geometrical 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 dependant on the field it is exposed to. In connection with a field grading material both capacitive and resistive field grading are mentioned.

The capacitive field grading is used in alternating current applications. Capacitive field grading can also be used to achieve field grading when voltages are occurring in the form of impulses. In 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 alternating 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. In 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 positive 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 and, consequently, the lower the electrical resistance in the body if the body exhibits such a non-linear electrical resistance. In this way, the voltage drop along the field grading body will become more uniformly 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, is achieved by combining at least two materials to a composite. Due to the area of use, the field grading material should be mechanically flexible. It is therefore suitable to use, for example, a rubber as a matrix and to mix it with semiconducting particles that is non-conducting at low voltages but becomes conducting at higher voltages. 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.

There are several known field grading material for guiding the electrical field. For example, a polymeric matrix containing nanoparticles of zinc oxide (ZnO) or silicon carbide (SiC) is described in published patent document WO2004/038735, where the polymeric matrix at least essentially consists of rubber, thermoplastics or a thermoplastic elastomer.

European patent document EP 0 088 450 Bl describes a field grading material comprising a polymeric matrix comprising particles of ZnO or SiC, and carbon black particles.

Published patent application WO 2005/036563 describes a field grading material with a polymeric matrix comprising a nanoparticle filler heterogeneously distributed in the matrix. The nanoparticle filler may be ZnO particles. The surface of the nanoparticle may be modified by treatment with an anorganosilane or organotitanante compound.

There is a constant need to improve said field grading materials with respect to known field grading materials in several aspects, such as having improved or similar electrical properties, and also as being a cost effective alternative to known field grading materials.

SUMMARY OF THE INVENTION

An object of the invention is to provide a field grading material of the type indicated in the preamble of claim 1 having improved or similar properties with respect to field grading materials already known. According to a first aspect of the invention this object is obtained by a field grading material according to claim 1. Advantageous embodiments of the invention will be clear from the description below and in the dependent claims.

According to one embodiment of the invention the field grading material comprises a polymeric matrix and a filler comprising zinc oxide and carbon black. The zinc oxide is essentially in the form of pure zinc oxide particles having at least one dimension smaller than, or equal to, 100 nm. The combination of pure zinc oxide particles smaller than, or equal to, 100 nm and carbon black in a polymeric matrix gives a field grading material with excellent field grading properties. Excellent field grading properties refers to a material having the desired non-linear properties, as described under "background art", for the field grading material. Also by using pure zinc oxide particles the field grading material is cost effective to produce, because pretreatment of the zinc oxide particle by surface treatment or by addition of doping substances is not necessary.

In this description and the subsequent claims the term "pure zinc oxide" refers to zinc oxide where the properties of the pure zinc oxide has not been modified by, for example, surface treatment or by addition of, for example, doping substances. However, the term "pure zinc oxide" does not exclude zinc oxide that contains a small amount of other substances than zinc oxide, for example, that is naturally present in the zinc oxide or origin from a contamination during the manufacturing of the zinc oxide .

In this description and the subsequent claims the expression "particles having at least one dimension smaller than or equal to 100 nm" refers to nano-sized particles having a width, length and/or height smaller than or equal to 100 nm. The nano-sized particles may of course have several or all dimensions smaller than or equal to 100 nm. The nano-sized particles may have any shape as long as they in at least one of their dimensions are in the interval of 0.1-100 nm.

According to one embodiment of the invention the pure zinc oxide particles have at least one dimension in the interval of 20-70 nm.

According to one embodiment of the invention the pure zinc oxide particles constitute less than 40% by volume of the field grading material.

According to one embodiment of the invention the pure zinc oxide particles constitute less than 30% by volume of the field grading material.

According to one embodiment of the invention the carbon black has an average particle size in the interval 1-100 nm, preferably 10-60 nm, and most preferably 15-50 nm. The carbon black constitutes between 0.2-10 % by volume, preferably 0.5-4 % by volume, and most preferably 1-3% by volume of the field grading material. According to an embodiment of the invention the carbon black is self-assembling and is adapted to form a percolated network structure of carbon black and ZnO in the polymeric matrix. At a certain level of the electrical field applied to the field grading material conducting paths of carbon black and zinc oxide is formed in at least part of the network in the polymeric matrix. The electrical resistance of the material changes from a linear to a non-linear behavior. The amount of carbon black is chosen in dependence on the desired resistivity of the material. The self-assembling carbon back makes it possible to use very small amounts of carbon black to achieve the percolated network. The carbon black is, for example, the commercially available

Ketjenblack^®.

According to one embodiment of the invention the carbon black has an average size between 40-500 nm. The carbon black constitutes between 5-40 % by volume, preferably 10-30 % by volume, and most preferably 10-15 % by volume of the field grading material. When the carbon black is between 40-500 nm it is present in the form of ball-shaped graphite. The ball- shaped graphite is present in form of filaments in the polymer matrix. At a certain level of the electrical field in the field grading material the carbon filaments and pure zinc oxide particles turns into conducting paths and give the material its non-linear properties which are desired for grading the electric field.

According to one embodiment of the invention the polymeric matrix comprises EPDM (Ethylene Propylene Diene Monomer) rubber. EPDM has a good dielectric strength and has a low price compared to other polymeric matrix that could be used.

According to one embodiment of the invention the polymeric matrix comprises silicone. When the polymeric matrix is silicone the field grading material can be injection molded to an electric field control device. Thereby the time for production of an electric field control device can be shortened. Also it is easier to manufacture a device with a complex shape due to the low viscosity of EPDM.

According to one embodiment of the invention the polymeric matrix comprises EVA (Ethylene Vinyl Acetate) .

According to a second aspect of the invention the object is achieved with an electric field control device for grading an electric field at an interruption of the insulation layer arranged around the conductor in a medium or high voltage cable, wherein the device comprises a field grading material according to any of claims 1-12.

An interruption in the insulation of an electric cable occurs for example at a joint or at a termination of a cable.

According to one embodiment of the invention the field grading material in the electric field control device is arranged circumferential around the interruption of the insulation and in contact with the conductor of the cable and with a grounded outer conducting layer of the cable. This provides a uniform distribution of the electric field in a joint or termination for a cable for DC or AC.

According to a third aspect of the invention the object is achieved with the use of a field grading material according to claim 15 or 16.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in greater detail by description of embodiments with reference to the accompanying drawings, wherein

Figure 1 is a diagram showing the electrical resistivity as a function of applied electrical field for a field grading material according to an embodiment of the invention,

Figure 2 is a schematic cross-section of the field grading material according to an embodiment of the invention, and

Figure 3 is a schematic longitudinal cross-section of a joint of an electric power cable, provided with a body comprising a field grading material according to the invention. DESCRIPTION OF PREFERRED EMBODIMENTS

The field grading material according to the present invention comprises a polymeric matrix and a filler comprising zinc oxide and carbon black. The zinc oxide is in the form of pure zinc oxide particles having at least one dimension smaller than or equal to 100 nm.

The electrical resistivity (Ohmm) as function of applied electrical field (kV/mm) for a field-grading material comprising pure ZnO nanoparticles and carbon black is illustrated in figure 1. Curve A represents a field grading material comprising an EPDM matrix with 23 % by volume of pure zinc oxide nanoparticles and 1 % by volume of carbon black. The average diameter of the pure zinc oxide particles is 53 nm. The carbon black is Ketjenblack^® EC 300, with an average particle size of 20-30 nm. Ketjenblack^® EC products are electroconductive carbon black of very high purity. Due to their unique morphology in comparison to conventional carbon blacks, substantially lower amounts of Ketjenblack^® EC-300J, and even lesser amounts of Ketjenblack^® EC-600JD, are required for making plastics and elastomers conductive. This results in improved processing and mechanical properties of the finished electroconductive articles. The field grading material was prepared by drying the ZnO particles in vacuum at 190⁰C over night before use. Carbon black and the ZnO nanoparticles were added to the EPDM matrix. The matrix comprises EPDM as well as a stabilizer (stearic acid) and a peroxide curing agent (VAROX) . The compound was carefully mixed using a Brabender internal mixer at a temperature less than 80⁰C. Thereafter the samples were compression molded at 180⁰C during 30 min. The samples were subsequently degassed in a vacuum-oven during 24 h and 80⁰C to evaporate by-products from the peroxide vulcanization. The final dimensions of the samples were approximately 1 mm in thickness and 210 mm in diameter.

Curve A shows that the above described material has a non- linearity which starts between l,0E9 and 1,0ElO ohmm and a conductivity at high electrical fields which makes the material suitable for use as a field grading material. The level of the resistivity of curve A shows that the material is especially suitable to be used as a field grading material for High Voltage Direct Current (HVDC) applications. Figure 1 also shows, as comparison, the electrical resistivity as function of applied electrical field for a material comprising EPDM rubber and 25 vol % pure ZnO particles with an average diameter of 53 nm, curve B. In this case, where no carbon black is added, an insulating material without field grading properties is obtained within the electric field of interest, 0.1-8 kV/τnτn.

Figure 2 shows an example of the distribution of pure zinc oxide and the carbon black in the EPDM matrix of the electric field grading material according to curve A in figure 1. The carbon black and pure zinc oxide are distributed in the form of aggregates, or a network, and when the electrical field is sufficiently high for the resistivity to become non-linear the ZnO particles conduct current and the aggregates, or network, forms conducting paths in the polymer matrix.

The polymeric matrix of the field grading material according to the present invention suitably consists, or at least essentially consists, of rubber, thermoplastics or a thermoplastic elastomer. It is preferred that the matrix consists, or at least essentially consists, of polyolefin rubber or thermoplastic polyolefin elastomer/plastomer, preferably including EPDM rubber, silicone rubber or EVA (Ethylene Vinyl Acetate) , or of semi-crystalline thermoplastics, preferably polyethylene or cross-linked polyethylene (PEX) .

A field grading material according to the invention is particularly suitable for use in an electric field control device for grading an electric field in medium or high voltage applications. Figure 3 schematically shows a cable joint comprising such an electric field control device 7. The cable joint 1 comprises a conductor 2 and cable insulation 3. Further, the cable usually comprises an inner conducting layer (not shown) arranged between the conductor and the insulation, and an outer conducting layer 4 arranged outside and surrounding the insulation. At the place where two conductor ends are joined together the insulation 3 and conducting layer (s) 4 is cut off for a distance and the two conductor ends is connected with a metallic cable sleeve 5. The sleeve is surrounded by a deflector 6 of conducting rubber. An electric field control device 7 comprising a field grading material according to the invention is arranged over the end of the outer conductive layer 4 to achieve a uniformly distributed electric field in the cable insulation 3. Outside the field grading material there is provided a second insulation layer 8. A semi-conducting layer 9 with a grounded screen 10 that is in contact with the grounded outer conducting layer 4 of the cable is arranged around the field control layer 7 and the second insulation layer 8

Although not shown, the field grading material may be used in a cable termination or other electrical equipment where it is suitable to use a field grading material.

The invention is not in any way limited to the preferred embodiments described above. On the contrary, several possibilities to modifications thereof should be evident to a person skilled in the art, without deviating from the basic idea of the invention as defined in the appended claims.

Claims

1. A field grading material comprising a polymeric matrix and a filler, the filler comprising zinc oxide and carbon black, characterized in that the zinc oxide essentially is in the form of pure zinc oxide particles having at least one dimension smaller than or equal to 100 nra.

2. A field grading material according to claim 1, wherein the pure zinc oxide particles have at least one dimension in the interval of 20-70 nm.

3. A field grading material according to claim 1 or 2, wherein the pure zinc oxide particles constitute less than 40% by volume of the field grading material.

4. A field grading material according to claim 1 or 2 , wherein the pure zinc oxide particles constitute less than 30% by volume of the field grading material.

5. A field grading material according to any of the preceding claims, wherein the carbon black has an average particle size in the interval of 1-100 nm, preferably in the interval of 10- 60 nm, and most preferably in the interval of 15-50 nm.

6. A field grading material according to claim 5, wherein the carbon black constitutes between 0.2-10 % by volume, preferably 0.5-4 % by volume, and most preferably 1-3% by volume of the field grading material.

7. A field grading material according to claim 5 or S₁ wherein the carbon black is self-assembling and is adapted to form a network structure in the polymeric matrix.

8. A field grading material according to any of claims 1-4, wherein the carbon black has a particle size in the interval between 40-500 nm.

9. A field grading material according to claim S, wherein the carbon black constitutes between 5-40 % by volume, preferably 10-30 % by volume, and most preferably 10-15 % by volume of the field grading material .

10. A field grading material according to any of the preceding claims, wherein the polymeric matrix comprises silicone.

11. A field grading material according to any of claims 1-9, wherein the polymeric matrix comprises EPDM (Ethylene Propylene Diene Monomer) rubber.

12. A field grading material according to any of claims 1-9, wherein the polymeric matrix comprises EVA (Ethylene Vinyl Acetate) .

13. An electric field control device (7) for grading an electric field at an interruption of the insulation layer arranged around the conductor in a medium or high-voltage cable, wherein the device comprises a field grading material according to any of claims 1-12.

14. An electric field control device (7) according to claim 13, wherein the field grading material is arranged circumferential around the interruption of the insulation and in contact with the conductor of the cable and with the grounded screen of the cable .

15. Use of a field grading material according to any of claims 1-12 for grading an electric field in a cable termination.

16. Use of a field grading material according to any of claims

1-12 for grading an electric field in a cable joint.