EP0644558B2 - Câble insulative structure - Google Patents

Description

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

These cables are generally made up of a conductive core surrounded by an isolation structure which is coaxial to it. This structure includes minus a first semiconductor layer placed at the contact of the cable core, 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 high density or low density polyethylene, polyethylene crosslinked, or even terpolymer of ethylenepropylene-diene to the main methylene chain (EPDM).

Semiconductor layers are generally composed of a polar matrix, most often a copolymer of ethylene and alkyl acrylate, which is charged with carbon black. The amount of charge varies depending on the nature of the carbon black used. For a black acetylene or a black oven, the proportion of filler is generally between 28% and 40%.

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

To obtain an interface during extrusion as smooth as possible, the matrix of semiconductor layers high voltage cables currently marketed is generally based on a polymer high melt index or melt index of around 17 (A high "melt index" is the sign of the presence of low molar masses, it is measured according to standards ASTM reference D1238 or NFT 51-016), and having a very wide distribution in molar masses. But we found in the insulating layer, near semiconductor layers, the appearance of charges of space whose accumulation leads to deterioration dielectric strength of the insulation up to breakdown.

Some semiconductor manufacturers use apolar matrices based on a copolymer of ethylene (EPR: thermoplastic ethylene-propylene elastomer, or EPDM: ethylene-propylene-diene terpolymer to main chain methylene), to which they add oils or plasticizers to facilitate obtaining a good surface condition of the layer semiconductor. 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 lower dielectric strength.

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

The object of the present invention is a structure insulation for cable comprising at least one first contiguous and coaxial semiconductor layer at the core of the cable, surrounded by a second layer electrically insulating, itself covered by a third semiconductor layer. Semiconductor layers consist exclusively of a matrix containing only apolar polymers of molar mass greater than 1000, and a conductive charge.

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

If the semiconductor layers contain low molecular weight compounds or additives, like oils or plasticizers, these compounds migrate into the insulating layer. This phenomenon has as a consequence the formation of space charges which will cause electric field strengthening and can later lead to breakdowns. This field reinforcement is linked to the amount of loads trained but also their mobility: a quantity of uniformly distributed charges giving no field reinforcement as important as the same amount of localized charges. This migration may occur during implementation or at during cable operation.

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

According to a first alternative embodiment, the polymers are chosen from polyethylene, polypropylene, polystyrene, and their copolymers, alloys polymers chosen from polyethylene, polypropylene, polystyrene, and their copolymers, and mixtures of the compounds chosen from polyethylene, polypropylene, polystyrene, their copolymers, and the alloys previously mentioned.

According to a second variant embodiment, the polymers are chosen from thermoplastic elastomers polyolefins and their mixtures.

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

The present invention has the advantage of stabilizing the dielectric characteristics of the structure isolation by suppressing compound migration of low molar mass. As a result, the quality of the interface between the different layers becomes a parameter less critical.

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

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

A photodiode 12 makes it possible to synchronize a detector 13 with laser 10. The circuit is electrically powered by a high voltage power supply 14 provided resistance 15. The data recorded are transferred to be processed by a computer 16 and represented as a function of time on a graphic recorder 17. The laser 10 sends a wave to the target 11 causing space charges to appear and modification of the distribution of the electric field which then is 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-conductnce 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%, at 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 wave test pressure using the installation shown on Figure 1. We see the appearance in proportions significant negative charges at cathode 2. The reinforcement of the field is then greater than 20% and the charges remain trapped in the material several hours after the power cut.

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%, at 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 wave test pressure using the installation shown on Figure 1. We see the appearance in proportions significant negative charges near cathode 2 and which remain trapped in the matrix of the insulating layer after cutting the voltage. The reinforcement of the field is greater 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 10 ³ and 10 ⁷ and centered on 1.1.10 ⁶ , 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 10 ³ and 10 ⁷ and centered on 1.2.10 ⁵ ,
• 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 wave test pressure using the installation shown on Figure 1. The reinforcement of the electric field is less at 10%, and after switching off the voltage it does not remain no 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 × 10 ⁶ , 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 of 40 and whose molar mass is between 10 ³ and 10 ⁷ and centered on 1 , 2.10 ⁵ ,
• 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 wave test pressure using the installation shown on Figure 1. The reinforcement of the electric field is less at 10%, and after switching off the voltage it does not remain no 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 semiconductor layers, paraffinic oil at a rate of 5% by weight relative to the matrix.

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

Claims (5)

1. Cable insulation structure including at least a semiconductor first layer contiguous and coaxial with the cable core surrounded by an electrically insulating second layer covered with a semiconductor third layer, characterised in that said semiconductor layers comprise exclusively a matrix including only apolar polymers whose components have a molecular weight greater than 1 000 and a conductive charge.
2. Structure according to claim 1 wherein the components of said matrix have a molecular weight greater than 5 000.
3. Structure according to claim 1 or claim 2 wherein said matrix is selected from polyethylene, polypropylene, polystyrene and their copolymers, alloys of polymers chosen from polyethylene, polypropylene, polystyrene and their copolymers, and mixtures of the aforementioned substances.
4. Structure according to claim 1 or claim 2 wherein said matrix is chosen from polyolefin thermoplastic elastomers and mixtures thereof.
5. Structure according to any one of the preceding claims wherein said charge is acetylene black.

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