FR2710447A1 - Isolation structure for cable. - Google Patents

Abstract

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

Description

The present invention relates to a cable insulation structure.

  insulation structure for medium, high and very high voltage cables carrying direct or alternating current. 5 These cables generally consist of a conductive core surrounded by an insulation 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, polyethylene

  crosslinked, or alternatively ethylene-propylene-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 charged 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 in

  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 the 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 5 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 cables comprising at least one 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 nonpolar polymers whose components have a mass

  molar 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, 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 electric field reinforcement and may subsequently lead to breakdowns. This field reinforcement is linked to the quantity of charges formed but also to their

  mobility: an amount of uniformly distributed charges not giving as much field strength as the same amount 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 mass compounds

  high molar, prevents migration of species into the insulating layer and thereby the accumulation of space charges near the 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 compounds chosen. among polyethylene, polypropylene, polystyrene, their copolymers, and the alloys previously cited.25 According to a second variant 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. Consequently, 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. Other characteristics and advantages of the present invention will appear on reading the following examples, of course given by way of illustration and not limitation, and with reference to the appended drawing in which: - Figure 1 shows the general diagram of the installation d pressure wave test, FIG. 2 represents a top view of the sample of the insulation structure for the pressure wave test,

  - Figure 3 a schematic section of the sample of Figure 2.

  The pressure wave test is carried out using the installation shown in Figure 1. This test allows

  to assess the strengthening of the electric field in an isolation structure.

  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 25 of diameter A 20 mm, we find 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 represented in FIG. 1, is composed of a laser 10 "YAg" whose beam is sent to a target 11 corresponding to the sample 1 of which each semiconductor 35 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.5 A photodiode 12 makes it possible to synchronize a detector 13 with the laser 10. The circuit is electrically supplied by a high voltage power supply 14 provided of a resistor 15. The recorded data are transferred to be processed by a computer 16 and 10 represented as a function of time on 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

  A sample of the insulation 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%, to which is added a charge of acetylene black in a proportion of 66 parts by weight relative to at 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. The appearance is observed in proportions

  significant negative charges at the cathode 2. The reinforcement of the field is then greater than 20% and the charges remain trapped in the material for several hours after the voltage has been cut off.

EXAMPLE 2

  A sample of the isolation structure according to the prior art is produced, analogous to the sample shown in FIG. 2, comprising: 5 - a first semiconductor layer composed of a polar matrix based on a copolymer d ethylene and alkyl acrylate whose "melt index" has the value 8 and whose ester content is 20%, to which is added a charge of acetylene black in a proportion of 10 66 parts by weight per 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 on the

  Figure 1. There is the appearance in significant proportions of negative charges near the cathode 2 and which remain trapped in the matrix of the insulating layer20 after cutting the voltage. The reinforcement of the field is more than 20%.

EXAMPLE 3

  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 black d acetylene 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% d '' 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 FIG. 1. The reinforcement of the electric field is less

  at 10%, and after switching off the voltage, no charges remain trapped in the insulating material.

EXAMPLE 4

  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 black d acetylene 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 1.106, and on the other hand 80% of an ethylene copolymer and of propylene containing approximately 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 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 FIG. 1. The reinforcement of the electric field is less

  at 10%, and after switching off the voltage, no charges remain trapped in the insulating material.

  EXAMPLE 5 Comparison A sample analogous 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 on the

  figure 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

  many variants accessible to those skilled in the art without departing from the spirit of the invention. In particular, without going beyond the ambit of the invention, any means can be replaced by equivalent means.

Claims (5)

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