EP0644558A1 - Câble insulative structure - Google Patents

Abstract

The subject of the present invention is an insulative structure for cables, comprising at least one first semiconducting layer that is contiguous and coaxial with the core of the cable and is surrounded by a second electrically insulating layer which is itself covered by a third semiconducting layer, characterised in that the said semiconducting layers are exclusively composed of a matrix, comprising non-polar polymers whose components have a molecular weight (molar mass) greater than 1000, and of a conductive filler.

Description

The present invention relates to an isolation structure for medium, high and very high voltage cables carrying direct or alternating current.

These cables generally consist of a conductive core surrounded by an isolation structure which is coaxial with it. This structure comprises at least a first semiconductor layer placed in contact with the core of the cable, itself surrounded by a second electrically insulating layer, in turn covered by a third semiconductor layer. Other outer layers are used to protect the cable.

The insulating layer is usually based on high density or low density polyethylene, crosslinked polyethylene, or alternatively ethylenepropylene-diene terpolymer with methylene main chain (EPDM).

The semiconductor layers are generally composed of a polar matrix, most often a copolymer of ethylene and alkyl acrylate, which is loaded with carbon black. The amount of filler varies according to the nature of the carbon black used. For an acetylene black or an oven black, the proportion of filler is generally between 28% and 40%.

The dielectric strength of such a cable is very much linked to the quality of the interface between the semiconductor layer and the insulating layer. The slightest roughness at this interface can cause a strengthening of the electric field and lead to breakdown and perforation of the insulating layer.

To obtain as smooth an interface as possible during extrusion, the matrix of semiconductor layers of high-voltage cables currently on the market is generally based on a polymer with a high melt index or "melt index" of around of 17 (A high "melt index" is the sign of the presence of low molar masses, it is measured according to ASTM standards reference D1238 or NFT 51-016), and having a very wide distribution in molar masses. However, it has been observed in the insulating layer, near the semiconductor layers, the appearance of space charges, the accumulation of which leads to a deterioration in the dielectric strength of the insulator, which can go as far as breakdown.

Some semiconductor manufacturers use nonpolar matrices based on an ethylene copolymer (EPR: thermoplastic ethylene-propylene elastomer, or EPDM: ethylene-propylene-diene terpolymer with methylene main chain), to which they add oils or plasticizers to facilitate obtaining a good surface condition of the semiconductor layer. However, these oils or plasticizers diffuse in the insulating layer and create at the interface between the semiconductor layer and the insulating layer, where the electric field is the highest, a region of lower dielectric strength.

The object of the present invention is to provide an insulation structure for medium, high and very high voltage cables carrying direct or alternating current, having dielectric characteristics more stable over time than those known up to now.

The object of the present invention is an insulation structure for a cable comprising at least a first semiconductor layer contiguous and coaxial with the core of the cable, surrounded by a second electrically insulating layer, itself covered by a third semiconductor layer. The semiconductor layers are composed exclusively of a matrix comprising apolar polymers whose components have a molar mass greater than 1000 and of a conductive filler.

Preferably, the components of the matrix have a molecular weight greater than 5000.

If the semiconductor layers contain low molecular weight compounds or additives, such as oils or plasticizers, these compounds migrate into the insulating layer. This phenomenon results in the formation of space charges which will cause an electrical field reinforcement and may subsequently lead to breakdowns. This field reinforcement is linked to the quantity of charges formed but also to their mobility: a quantity of uniformly distributed charges not giving as strong a field reinforcement as the same quantity of localized charges. This migration can occur during installation or during cable operation.

The use of a semiconductor layer of composition according to the invention comprising only compounds of high molar mass, prevents the migration of species in the insulating layer and thereby the accumulation of space charges nearby interfaces.

According to a first alternative embodiment, the matrix is chosen from polyethylene, polypropylene, polystyrene, and their copolymers, the polymer alloys chosen from polyethylene, polypropylene, polystyrene, and their copolymers, and mixtures of the selected compounds. among polyethylene, polypropylene, polystyrene, their copolymers, and the alloys mentioned above.

According to a second alternative embodiment, the matrix is chosen from polyolefinic thermoplastic elastomers and their mixtures.

On the choice of the constituent polymers, the matrix will depend on the quality of its interface with the insulating layer and the mechanical properties of the semiconductor layer obtained, without requiring the use of additives.

The present invention has the advantage of stabilizing the dielectric characteristics of the insulation structure by suppressing the migration of low molecular weight compounds. As a result, the quality of the interface between the different layers becomes a less critical parameter.

The filler is a carbon black containing as few impurities as possible. We can use a black oven or a black "KETJEN", but we will preferably choose an acetylene black which is much purer.

In some cases the matrix also contains a crosslinking agent. After the material has been shaped by extrusion, it can be crosslinked in order to improve its thermomechanical properties. These properties are particularly critical for cables carrying alternating current.

• la figure 1 montre le schéma général de l'installation d'essai de l'onde de pression,
• la figure 2 représente une vue de dessus de l'échantillon de la structure d'isolement pour l'essai de l'onde de pression,
• la figure 3 une coupe schématique de l'échantillon de la figure 2.

Other characteristics and advantages of the present invention will appear on reading the following examples, given of course by way of non-limiting illustration, and with reference to the appended drawing in which:

• Figure 1 shows the general diagram of the pressure wave test facility,
• FIG. 2 represents a top view of the sample of the insulation structure for testing the pressure wave,
• Figure 3 a schematic section of the sample of Figure 2.

The pressure wave test is carried out using the installation shown in FIG. 1. This test makes it possible to evaluate the strengthening of the electric field in an isolation structure.

• une première couche 2 semi-conductrice d'épaisseur B 0,5mm,
• une deuxième couche 3 électriquement isolante d'épaisseur C 0,8mm,
• une troisième couche 4 semi-conductrice identique à la couche 2.

A sample 1 of the insulation structure for testing the pressure wave is shown seen from above in FIG. 2 and in section in FIG. 3. On a circular surface of diameter A 20mm, there are superimposed:

• a first semiconductor layer 2 of thickness B 0.5 mm,
• a second electrically insulating layer 3 of thickness C 0.8 mm,
• a third semiconductor layer 4 identical to layer 2.

The installation shown in Figure 1, consists of a laser 10 "YAg" whose beam is sent to a target 11 corresponding to the sample 1 of which each semiconductor constitutes an electrode (+) and (-). This beam absorbed at the surface of electrode 2 (-) decomposes this surface by pyrolysis, and the gases emitted cause a pressure wave which crosses the sample. This wave modulates the image charges on the electrodes and gives access to the volume charge density in the sample.

A photodiode 12 makes it possible to synchronize a detector 13 with the laser 10. The circuit is electrically supplied by a high voltage supply 14 provided with a resistor 15. The recorded data are transferred to be processed by a computer 16 and represented as a function of time over a graphic recorder 17. The laser 10 sends a wave to the target 11 causing the appearance of space charges and the modification of the distribution of the electric field which is then measured by the detector 13.

EXAMPLE 1

• une première couche semi-conductrice composée d'une matrice polaire à base d'un copolymère d'éthylène et d'acrylate d'alkyl dont le "melt index" a pour valeur 8 et dont la teneur en ester est de 20%, à laquelle est ajoutée une charge de noir d'acétylène dans une proportion de 66 parts en poids par rapport à 100 parts de la matrice,
• une deuxième couche électriquement isolante composée d'un élastomère thermoplastique oléfinique,
• une troisième couche semi-conductrice identique à la première couche.

A sample of the isolation structure according to the prior art is produced, analogous to the sample shown in FIG. 2, comprising:

• a first semiconductor layer composed of a polar matrix based on a copolymer of ethylene and alkyl acrylate whose "melt index" has the value 8 and whose ester content is 20%, which is added a charge of acetylene black in a proportion of 66 parts by weight relative to 100 parts of the matrix,
• a second electrically insulating layer composed of an olefinic thermoplastic elastomer,
• a third semiconductor layer identical to the first layer.

This sample is then subjected to the pressure wave test using the installation shown in FIG. 1. We note the appearance in significant proportions of negative charges at the cathode 2. The reinforcement of the field is then more than 20% and the charges remain trapped in the material for several hours after the voltage is cut off.

EXAMPLE 2

• une première couche semi-conductrice composée d'une matrice polaire à base d'un copolymère d'éthylène et d'acrylate d'alkyl dont le "melt index" a pour valeur 8 et dont la teneur en ester est de 20%, à laquelle est ajoutée une charge de noir d'acétylène dans une proportion de 66 parts en poids par rapport à 100 parts de la matrice,
• une deuxième couche électriquement isolante composée d'un polyéthylène réticulé chimiquement(PRC),
• une troisième couche semi-conductrice identique à la première couche.

A sample of the isolation structure according to the prior art is produced, analogous to the sample shown in FIG. 2, comprising:

• a first semiconductor layer composed of a polar matrix based on a copolymer of ethylene and alkyl acrylate whose "melt index" has the value 8 and whose ester content is 20%, which is added a charge of acetylene black in a proportion of 66 parts by weight relative to 100 parts of the matrix,
• a second electrically insulating layer composed of a chemically crosslinked polyethylene (PRC),
• a third semiconductor layer identical to the first layer.

This sample is then subjected to the pressure wave test using the installation shown in FIG. 1. We note the appearance in significant proportions of negative charges near the cathode 2 and which remain trapped. in the matrix of the insulating layer after cutting the voltage. The reinforcement of the field is more than 20%.

EXAMPLE 3

• une première couche semi-conductrice composée d'une matrice apolaire à laquelle est ajoutée une charge de noir d'acétylène dans une proportion de 66 parts en poids par rapport à 100 parts de la matrice; la matrice contient d'une part 20% de polyéthylène (PE) dont le "melt index" a pour valeur 2 et dont la masse molaire est comprise entre 10³ et 10⁷ et centrée sur 1,1.10⁶, et d'autre part 80% d'un copolymère d'éthylène et de propylène contenant environ 50% en poids d'éthylène dont la viscosité "MOONEY" (mesurée selon la norme NFT 43005) est de l'ordre de 40 et dont la masse molaire est comprise entre 10³ et 10⁷ et centrée sur 1,2.10⁵,
• une deuxième couche électriquement isolante composée d'un polyéthylène réticulé chimiquement (PRC),
• une troisième couche semi-conductrice identique à la première couche.

A sample of the isolation structure according to the invention is produced, analogous to the sample shown in FIG. 2, comprising:

• a first semiconductor layer composed of an apolar matrix to which is added a charge of acetylene black in a proportion of 66 parts by weight relative to 100 parts of the matrix; the matrix contains on the one hand 20% of polyethylene (PE) whose "melt index" has the value 2 and whose molar mass is between 10³ and 10⁷ and centered on 1.1.10⁶, and on the other hand 80% of a copolymer of ethylene and propylene containing approximately 50% by weight of ethylene whose viscosity "MOONEY" (measured according to standard NFT 43005) is of the order of 40 and whose molar mass is between 10³ and 10⁷ and centered on 1.2.10⁵,
• a second electrically insulating layer composed of a chemically crosslinked polyethylene (PRC),
• a third semiconductor layer identical to the first layer.

This sample is then subjected to the pressure wave test using the installation shown in Figure 1. The reinforcement of the electric field is less than 10%, and after cutting the voltage there does not remain charges trapped in the insulating material.

EXAMPLE 4

• une première couche semi-conductrice composée d'une matrice apolaire à laquelle est ajoutée une charge de noir d'acétylène dans une proportion de 66 parts en poids par rapport à 100 parts de la matrice; la matrice contient d'une part 20% de polyéthylène (PE) dont le "melt index" a pour valeur 2 et dont la masse molaire est centrée sur 1,1.10⁶, et d'autre part 80% d'un copolymère d'éthylène et de propylène contenant environ 50% en poids d'éthylène dont la viscosité "MOONEY" (selon la norme NFT 43005) est de l'ordre de 40 et dont la masse molaire est comprise entre 10³ et 10⁷ et centrée sur 1,2.10⁵,
• une deuxième couche électriquement isolante composée d'un élastomère thermoplastique oléfinique,
• une troisième couche semi-conductrice identique à la première couche.

A sample of the isolation structure according to the invention is produced, analogous to the sample shown in FIG. 2, comprising:

• a first semiconductor layer composed of an apolar matrix to which is added a charge of acetylene black in a proportion of 66 parts by weight relative to 100 parts of the matrix; the matrix contains on the one hand 20% of polyethylene (PE) whose "melt index" has the value 2 and whose molar mass is centered on 1.1.10⁶, and on the other hand 80% of a copolymer of ethylene and propylene containing about 50% by weight of ethylene whose viscosity "MOONEY" (according to standard NFT 43005) is of the order 40 and whose molar mass is between 10³ and 10⁷ and centered on 1.2.10⁵,
• a second electrically insulating layer composed of an olefinic thermoplastic elastomer,
• a third semiconductor layer identical to the first layer.

This sample is then subjected to the pressure wave test using the installation shown in Figure 1. The reinforcement of the electric field is less than 10%, and after cutting the voltage there does not remain charges trapped in the insulating material.

EXAMPLE 5 Comparison

A sample similar to that described in Example 4 is prepared but by adding to the matrix of the semiconductor layers, a paraffinic oil in an amount of 5% by weight relative to the matrix.

This sample is then subjected to the pressure wave test using the installation shown in FIG. 1. The reinforcement of the field in this case is 140%.

Of course the present invention is not limited to the embodiments described, but it is susceptible to numerous variants accessible to those skilled in the art without departing from the spirit of the invention. In particular, without going beyond the scope of the invention, any means can be replaced by equivalent means.

Claims (6)

1. / Insulation structure for cable comprising at least a first semiconductor layer contiguous and coaxial with the core of the cable, surrounded by a second electrically insulating layer, itself covered by a third semiconductor layer, characterized by the fact that said semiconductor layers are composed exclusively of a matrix comprising nonpolar polymers whose components have a molar mass greater than 1000 and of a conductive filler. 2. / Structure according to claim 1, wherein the components of said matrix have a molar mass greater than 5000. 3. / Structure according to one of claims 1 and 2, wherein said matrix is chosen from polyethylene, polypropylene, polystyrene, and their copolymers, the polymer alloys chosen from polyethylene, polypropylene, polystyrene, and their copolymers, and mixtures of the above-mentioned compounds. 4. / Structure according to one of claims 1 and 2, wherein said matrix is chosen from polyolefinic thermoplastic elastomers and their mixtures. 5. / Structure according to one of the preceding claims, wherein said filler is acetylene black. 6. / Structure according to one of the preceding claims, wherein said matrix further contains a crosslinking agent.

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