EP1280167A1 - Semiconductive screen for power cable - Google Patents

Abstract

A semi-conductor screen for power cables consists of conductive material dispersed in a polymer matrix and comprises a layer of high longitudinal volume conductivity and a layer in contact with the insulating layer within the cable which restricts the injection of charges into the insulating layer from the screen. A semiconductor screen (3, 5) for a power cable comprises two layers (31, 32; 51, 52), each consisting of conducting material dispersed in a polymer matrix. The first of these layers (31, 51) has a longitudinal volume conductivity of more than 0.1 S/m at 20-90 degrees C. The second layer (32, 52) is designed to be in contact with an electrically insulating layer (4) in the power cable and is such that only a small quantity of space charges can be injected into (4) from the second layer (32, 52), i.e. the quantity of charge which can be injected into (4) from the screen (3, 5) is less than that which can be injected into (4) from the second layer alone, the second layer (32, 52) forming a barrier to the injection of charges into layer (4). An Independent claim is also included for a power cable with a screen as described above.

Description

The present invention relates to a semiconductor screen for power cable. It relates more particularly to such a screen intended for use in a medium, high and very high voltage direct current energy cable.

• une âme conductrice, comprenant par exemple des fils de cuivre
• un écran semi-conducteur dit intérieur en contact avec l'âme du câble
• une couche d'isolation électrique
• un écran semi-conducteur dit extérieur en contact avec la couche d'isolation électrique
• de manière optionnelle, un écran métallique de protection
• une gaine extérieure de protection.

A power cable of this type comprises in known manner the following elements, arranged coaxially from the inside to the outside of the cable:

• a conductive core, for example comprising copper wires
• a so-called interior semiconductor screen in contact with the cable core
• a layer of electrical insulation
• a so-called external semiconductor screen in contact with the electrical insulation layer
• optionally, a metal protective screen
• an outer protective sheath.

The electrical insulation layer is generally made of polyethylene, high or low density, crosslinked or non-crosslinked. Semiconductor screens, for their part, generally consist of a polar matrix, that is to say comprising polar groups such as hydrophilic groups, such as for example a copolymer of ethylene and acrylate d 'alkyl, charged by means of an electrically conductive charge, for example carbon black. The choice of a polar matrix is guided by the need to make the charge compatible with the matrix in order to ensure better interaction between these two constituents.

During the operation of such a direct current cable, especially at high and very high voltage, a very high electric field appears between the conductive core and the inner semiconductor screen, as well as between the aluminum screen. and the outer semiconductor screen. This electric field causes the diffusion (we speak in this case of injection) of electric charges from the semiconductor screen in the insulation layer electric. These electrical charges are then trapped in the electrical insulation layer.

However, the breakdown and aging phenomena of energy cables, in particular high voltage and very high voltage cables, are due to the strengthening of the electric field in localized areas of these cables. Under continuous tension, this reinforcement of field is induced by a particular distribution of space charges, function of the nature and the density of these.

Thus, the injection of space charges into the insulation and the amount of space charges thus injected depend on both the nature of the insulation and that of the semiconductors (the electrode), and more precisely the nature of the matrix of semiconductor screens, the nature and the rate of the conductive charge in this matrix, as well as the interaction between these constituents.

To limit the phenomenon of space charge injection, document EP-0 644 558 proposes to replace the polar matrix of semiconductor screens with an apolar matrix. In this case, the accumulation of space charges in the electrical insulation layer close to the interface of the latter with semiconductor screens is effectively reduced, but problems of compatibility between the charge and the matrix.

In addition, this solution limits the conductivity of semiconductor screens, necessary for electrical continuity with the cable core and allowing the insulation to resist lightning strikes.

The object of the present invention is therefore to develop a semiconductor screen making it possible to limit the injection of space charges into the adjacent electrical insulation layer during the application of an electric field, while ensuring the usual electrical functions.

The present invention provides for this purpose a semiconductor screen for power cable comprising two layers, each of said layers comprising a polymer matrix in which a conductive filler is dispersed, a first of said layers having a longitudinal electrical volume conductivity greater than 0.1 S / m between 20 and 90 ° C,
characterized in that the second of said layers is intended to be placed in contact with an electrically insulating layer of said power cable and is such that the amount of space charges capable of being injected from said second layer into said insulation layer is weak, so that the amount of space charges that can be injected from said semiconductor screen in said insulation layer is less than the amount of space charges that can be injected from said second layer alone in said electrical insulation layer, said second layer forming a barrier limiting the injection of space charges into said electrical insulation layer.

Thanks to the invention, the overall electrical properties of the semiconductor screen are preserved, that is to say an electrical conductivity sufficient to play its role of screen by homogenizing the distribution of the electric field inside the cable, while reducing the amount of space charges that can be injected into the electrical insulation layer thanks to the presence of a semiconductor layer of low injection of space charges directly in contact with the layer d electrical insulation.

The invention is based on the satisfaction of two constraints considered contradictory until now, namely the limitation of the injection of space charges into the electrical insulation layer and the good electrical conductivity of the semiconductor screen. .

In addition, it is important to note that the invention makes it possible to choose the material of the first layer intended to come into contact with the conductive core or a metal screen of an energy cable, without constraint as to the quantity of loads of space it is likely to inject into the cable insulation. This therefore opens the way to materials with interesting electrical properties but not used until now due to the too large injection of space charges which they entailed in the insulation.

Very advantageously, the amount of space charges capable of being injected from the semiconductor screen into the insulation layer is less than 200 nC between 25 and 70 ° C.

According to the invention, the amount of space charges capable of being injected from the second layer alone into the electrical insulation layer is less than 250 nC between 25 and 70 ° C.

Note that when talking about the amount of space charges injected, this is measured by the pressure wave method, which will be described in detail below.

Preferably, the longitudinal volume electrical conductivity of the first layer will be chosen to be greater than 5 S / m between 20 and 90 ° C. This allows in particular to withstand the shock stresses to which the cable may be subjected during its operation.

Preferably also, the longitudinal volume electrical conductivity of said second layer will be chosen between 10 ^-4 and 10 ^-1 S / m between 20 and 90 ° C. This has the advantage of reducing the constraints compared to the choice of the type and / or the rate of the conductive load.

Advantageously, the first layer may comprise a matrix chosen from copolymers of ethylene and of alkyl acrylate or mixtures of these copolymers with polyolefins, and a conductive filler dispersed in this matrix.

In fact, one can choose for this first layer any material known for conventional semiconductor screens, in particular based on a polar matrix.

Advantageously also, according to a first alternative embodiment, the second layer may comprise a polymer matrix chosen from polyethylene, polypropylene, polystyrene and their copolymers, alloys of polymers chosen from polyethylene, polypropylene, polystyrene and their copolymers, and mixtures of compounds chosen from polyethylene, polypropylene, polystyrene, their copolymer and the preceding alloys, and a conductive filler dispersed in this matrix.

According to a second variant, the second layer may comprise a polymer matrix chosen from polyolefinic thermoplastic elastomers and their mixtures.

More specifically, the polymer matrix can consist of a mixture comprising polyethylene and a hydrogenated block copolymer of styrene, chosen from copolymers of styrene and butadiene and of styrene and isoprene.

The filler can be chosen from carbon blacks, such as, for example, acetylene black.

The present invention also relates to an energy cable comprising at least one semiconductor screen as defined above.

• une âme conductrice
• un écran semi-conducteur intérieur
• une couche d'isolation électrique
• un écran semi-conducteur extérieur
• une gaine de protection extérieure

la première couche de l'écran semi-conducteur intérieur étant en contact avec l'âme conductrice, et
les deuxièmes couches des écrans semi-conducteurs intérieur et extérieur étant en contact avec la couche d'isolation électrique.More specifically, an energy cable according to the invention comprises, arranged coaxially and from the inside to the outside:

• a conductive soul
• an interior semiconductor screen
• a layer of electrical insulation
• an external semiconductor screen
• an outer protective sheath

the first layer of the inner semiconductor screen being in contact with the conductive core, and
the second layers of the interior and exterior semiconductor screens being in contact with the electrical insulation layer.

A cable according to the invention may further comprise, between the outer semiconductor screen and the outer protective sheath, a metallic protective screen.

The invention applies very particularly to direct current power cables.

Other characteristics and advantages of the present invention will appear in the following description of an embodiment of the invention, given by way of illustration and in no way limitative.

• la figure 1 est une vue en perspective éclatée d'un câble d'énergie incorporant deux écrans semi-conducteurs selon l'invention
• la figure 2 est une vue en coupe transversale du câble de la figure 1
• la figure 3 est une vue en coupe d'un échantillon servant à effectuer l'essai dit de l'onde de pression.

In the following figures:

• Figure 1 is an exploded perspective view of an energy cable incorporating two semiconductor screens according to the invention
• Figure 2 is a cross-sectional view of the cable of Figure 1
• Figure 3 is a sectional view of a sample for performing the so-called pressure wave test.

• une âme conductrice 1 formée d'un toron de conducteurs de cuivre 2
• un écran semi-conducteur intérieur 3 en contact avec l'âme conductrice 1
• une couche d'isolation électrique 4 en un matériau diélectrique tel que le polyéthylène haute ou basse densité, le polyéthylène réticulé ou le terpolymère d'éthylène-propylène-diène à chaîne principale méthylène (EPDM)
• un écran semi-conducteur extérieur 5 en contact avec la couche d'isolation électrique 4
• un écran métallique de protection 6, optionnel, constitué d'un ruban d'aluminium
• une gaine extérieure de protection 7 en un matériau tel que le polychlorure de vinyle, le polyéthylène ou un mélange de polymère et de charges ignifugeantes.

FIG. 1 shows a cable 10 comprising, arranged coaxially from the inside to the outside:

• a conductive core 1 formed of a strand of copper conductors 2
• an interior semiconductor screen 3 in contact with the conductive core 1
• an electrical insulation layer 4 made of a dielectric material such as high or low density polyethylene, crosslinked polyethylene or ethylene-propylene-diene terpolymer with methylene main chain (EPDM)
• an external semiconductor screen 5 in contact with the electrical insulation layer 4
• an optional protective metal screen 6, consisting of an aluminum tape
• an outer protective sheath 7 made of a material such as polyvinyl chloride, polyethylene or a mixture of polymer and flame retardant fillers.

According to the invention, the interior semiconductor screen 3 is a composite which comprises (see FIG. 2) a layer 31 of high longitudinal volumetric electrical conductivity, typically greater than 0.1 S / m between 20 and 90 ° C, and preferably greater than 5 S / m at these temperatures, in contact with the conductive core 1, and a layer 32 capable of injecting a small amount of space charges into the insulation layer 4 after polarization, so that the amount of space charges injected from the screen 3 in the electrical insulation layer 4 is typically less than 200 nC between 25 and 70 ° C, the layer 32 being in contact with the electrical insulation layer 4.

Still according to the invention, the outer semiconductor screen 5 is a composite which comprises a layer 51 of high volume electrical conductivity, typically greater than 0.1 S / m between 20 and 90 ° C, and preferably greater than 5 S / m at these temperatures, the layer 51 being in contact with the metal screen 6, and a layer 52 capable of injecting a small amount of space charges into the insulation layer 4 after polarization, so that the amount of space charges injected from the screen 5 into the electrical insulation layer 4 is typically less than 200 nC between 25 and 70 ° C, the layer 52 being in contact with the electrical insulation layer 4.

As mentioned above, the semiconductor screens 3 and 5 according to the invention allow both to obtain a satisfactory electrical conductivity in the vicinity of the conductive elements of the cable 10 in order to perform the function of homogenizing the distribution of the field. electric, and to limit the injection of space charges into the electrical insulation layer 4 since the layers 32 and 52 of the semiconductor screens 3 and 5 in contact with the latter inject a small amount of space charges after polarization.

In order to show the effectiveness of the invention, space charge measurements were carried out by the pressure wave method, as known, on three different samples A, B and C, the basic structure is shown in figure 3. The thicknesses of the layers SC1 and SC2 of the sample A are double those of the layers SC1 and SC2 of the samples B and C.

Each of these samples comprises an electrically insulating layer I of thickness 0.8 mm placed between two semiconductor layers SC1 and SC2 of identical compositions.

In sample A, the two layers SC1 and SC2 are composite semiconductor screens according to the invention each consisting of a layer of a material of Composition 1 given below, capable of inducing a large amount of charges d space and a layer of a material of Composition 2 given below, inducing a small amount of space charges in the electrically insulating layer I with which it is in contact.

Composition 1

• Low density polyethylene (0.919 g / cm ³ ) having a Melt Flow Index of 2 and a weight average molar mass of 126,500 g / mol: 100 parts
• Hydrogenated SBS (block, hydrogenated copolymer of styrene and butadiene): 20 parts
• Black in oven ENSACO 250 G: 39 parts
• Antioxidant: 0.25 part

Composition 2

• Low density polyethylene (0.919 g / cm ³ ) having a Melt Flow Index of 2 and a weight average molar mass of 126,500 g / mol: 100 parts
• SBS hydrogenated: 20 parts
• DENKA acetylene black: 39 parts
• Antioxidant: 0.25 part

In sample B, the two layers SC1 and SC2 consist only of a material of Composition 2 above.

In sample C, the two layers SC1 and SC2 consist solely of a material of Composition 1 above.

It is important to insist on the fact that the compositions 1 and 2 above were chosen in order to be able to carry out comparisons of quantities of space charges injected, and this independently of their electrical conductivity.

The test implemented, called the pressure wave test, consists in sending the beam of a YAG laser onto the test sample, each semiconductor screen of which constitutes an electrode (+) and (-). This beam absorbed on the surface of the electrode (-) decomposes this surface by pyrolysis, and the gases emitted cause a pressure wave which crosses the sample, causing a displacement of space charges and the appearance of image charges at the electrodes. , giving rise to the measured signal. The processing of this signal gives an indication of the distribution of the electric field and of the volume charge density in the sample.

The values highlighted during this test on the samples tested are the maximum density D ₊ of positive charges in the insulating layer I, the maximum density D _- of negative charges in the insulating layer I and the total quantity Q _T of charges in the insulating layer I (in fact its image).

Table 1 below gives the results obtained outside the applied field, after polarization of the samples for 4 hours under a continuous electrical voltage of +40 kV at room temperature (25 ° C). Table 1 Sample D ₊ (C / m ³ ) D _- (C / m ³ ) Q _T (nC) AT 0,019 0.03 5 B 0.9 0.05 76 VS 1.8 0.5 98

This table highlights the fact that, at room temperature, a semiconductor screen according to the invention injects 19 times less space charges than the layer causing the highest injection alone (sample C), but also 15 times less than the one with the lowest injection of space charges alone (sample B). This result is therefore quite surprising.

It shows that one can choose for the first layer of a semiconductor screen according to the invention, a material with high, medium or low injection of space charges, as long as its electrical conductivity characteristics are satisfactory.

Table 2 below gives the results obtained after polarization of the samples for 4 hours under a continuous electrical voltage of +40 kV at 70 ° C. Table 2 Sample D ₊ (C / m ³ ) D _- (C / m ³ ) Q _T (nC) AT 0.15 0.19 48 B 1.1 0.6 126 VS 2.5 1.8 196

Table 2 shows that the results obtained at room temperature are also valid at high temperature.

Other measurements were carried out, under the same conditions as those described above, on samples D, E, F and G, the basic structure of which is also that of FIG. 3.

In sample D, the two layers SC1 and SC2 are composite semiconductor screens according to the invention each consisting of a layer of a material of Composition 3 given below, capable of inducing a large amount of charges d space, and a layer of a material of Composition 4 given below, inducing a small amount space charges in the electrically insulating layer I with which it is in contact.

Composition 3

• Low density polyethylene (0.920 g / cm ³ ) having a Melt Flow Index of 2 and a weight average molar mass of 212,000 g / mol: 100 parts
• SBS hydrogenated: 20 parts
• ENSACO 250G baked black: 39 parts
• Antioxidant: 0.25 part

Composition 4

• Low density polyethylene (0.920 g / cm ³ ) having a Melt Flow Index of 2 and a weight average molar mass of 212,000 g / mol: 100 parts
• Hydrogenated SBS: 20 parts (
  
  To be explained by the inventors)
• DENKA acetylene black: 39 parts
• Antioxidant: 0.25 part

In sample E, the two layers SC1 and SC2 consist only of a material of Composition 4 above.

In sample F, the two layers SC1 and SC2 consist only of a material of Composition 3 above

In sample G, the two layers SC1 and SC2 consist solely of a semiconductor material with high injection of space charges and high commercial electrical conductivity, based on a mixture of polyethylene and copolymer of ethylene and vinyl acetate.

Note that for all samples D to G, the thicknesses of the layers SC1 and SC2 are identical.

Here again, it is important to insist on the fact that the compositions 3 to 5 above have been chosen in order to be able to carry out comparisons of quantities of space charges injected, and this independently of their electrical conductivity.

Table 3 below gives the results obtained outside the applied field, after polarization of the samples for 4 hours under a continuous electrical voltage of +40 kV at room temperature (25 ° C). Table 3 Sample D ₊ (C / m ³ ) D _- (C / m ³ ) Q _T (nC) D 0.09 0.04 5 E 1.05 0.21 43 F 1.95 0.69 80 G 1.68 0.85 235

The results shown in Table 3 lead to the same qualitative conclusions as those of Table 1.

Table 4 below gives the results obtained after polarization of the samples for 4 hours under a continuous electrical voltage of +40 kV at 70 ° C. Table 4 Sample D ₊ (C / m ³ ) D _- (C / m ³ ) Q _T (nC) D 0.52 0.3 53 E 1.25 0.65 102 F 2.82 1.91 422 G 1.15 0.98 205

The results shown in Table 4 lead to the same qualitative conclusions as those of Table 2.

We will not go into the details of the process for manufacturing a cable according to the invention here. It is simply indicated that a screen according to the invention can be obtained by co-extrusion of its two constituent layers in a suitable device, well known to those skilled in the art.

Of course, the invention is not limited to the embodiment which has just been described.

Similarly, the energy cable structure described is only for example, and an energy cable according to the invention may comprise only a semiconductor screen according to the invention, for example the interior semiconductor screen only or the exterior semiconductor screen only. In addition, the cable according to the invention may include other types of metallic protective screens, for example a laminated or welded aluminum screen.

In addition, the protective structure which comprises the metal screen and the outer sheath may also include other protective elements such as in particular a swelling protective strip in the presence of water. Such a protective strip can be interposed between the outer semiconductor screen and the metal protective screen. It provides itself or is associated with conductive means ensuring electrical continuity between the outer semiconductor screen and the metal screen.

Furthermore, the materials indicated for the various elements of the cables according to the invention are for information only, and can of course be replaced by equivalent materials which are within the reach of those skilled in the art.

• la teneur massique en styrène dans la matrice polymère peut être de 0,1 à 20 %, préférentiellement de 1 à 10 %,
• la charge conductrice peut être du noir de carbone, de préférence du type « acétylène » qui est plus propre que les noirs de carbone du type « furnace » (ou noirs au four),
• la teneur massique en noir de carbone (par rapport à la matrice) peut être de 15 à 40% préférentiellement de 20 à 30 %,
• l'anti-oxydant utilisé est l'Irganox 1010 ; la teneur massique en antioxydant est de 0,1 à 0,2 %, préférentiellement de 0,15 %.

Thus in particular, a person skilled in the art can vary the compositions given above by way of example, in the following manner:

• the mass content of styrene in the polymer matrix can be from 0.1 to 20%, preferably from 1 to 10%,
• the conductive filler can be carbon black, preferably of the “acetylene” type which is cleaner than carbon blacks of the “furnace” type (or oven blacks),
• the mass content of carbon black (relative to the matrix) may be from 15 to 40%, preferably from 20 to 30%,
• the antioxidant used is Irganox 1010; the mass content of antioxidant is 0.1 to 0.2%, preferably 0.15%.

Finally, any means can be replaced by equivalent means without departing from the scope of the invention.

Claims (14)

Semiconductor screen (3, 5) for power cable comprising two layers (31, 32; 51, 52), each of said layers comprising a polymer matrix in which a conductive filler is dispersed, a first of said layers (31, 51 ) having a longitudinal volume electrical conductivity greater than 0.1 S / m between 20 and 90 ° C,
characterized in that the second of said layers (32, 52) is intended to be placed in contact with an electrically insulating layer (4) of said power cable and is such that the amount of space charges capable of be injected from said second layer (32, 52) into said electrical insulation layer (4) is low, so that the amount of space charges that can be injected from said semiconductor screen (3, 5) in said insulation layer (4) is less than the amount of space charges capable of being injected from said second layer (32, 52) alone in said electrical insulation layer (4), said second layer (32 , 52) forming a barrier limiting the injection of space charges into said layer of electrical insulation (4). Screen according to claim 1 characterized in that the amount of space charges capable of being injected from said semiconductor screen (3, 5) into said insulation layer (4) is less than 200 nC between 25 and 70 ° C. Screen according to one of claims 1 or 2 characterized in that the amount of space charges capable of being injected from said second layer (32, 52) alone into said electrical insulation layer (4) is less than 250 nC between 25 and 70 ° C. Screen according to one of Claims 1 to 3, characterized in that the volumetric electrical conductivity of the first layer (31, 51) is greater than 5 S / m between 20 and 90 ° C. Screen according to one of Claims 1 to 4, characterized in that the longitudinal volume electrical conductivity of the said second layer (32, 52) is between 10 ^-4 and 10 ^-1 S / m between 20 and 90 ° C. Screen according to one of claims 1 to 5 characterized in that said first layer (31, 51) comprises a matrix chosen from copolymers of ethylene and alkyl acrylate or mixtures of these copolymers with polyolefins, and a conductive filler dispersed in said matrix. Screen according to one of Claims 1 to 6, characterized in that the said second layer (32, 52) comprises a polymer matrix chosen from polyethylene, polypropylene, polystyrene and their copolymers, the polymer alloys chosen from polyethylene, the polypropylene, polystyrene and their copolymers, and mixtures of compounds chosen from polyethylene, polypropylene, polystyrene, their copolymer and the above alloys, and a conductive filler dispersed in said matrix. Screen according to one of Claims 1 to 6, characterized in that the said second layer (32, 52) comprises a polymer matrix chosen from polyolefinic thermoplastic elastomers and their mixtures, and a conductive filler dispersed in the said matrix. Screen according to Claim 7, characterized in that the said polymer matrix of the said second layer (32, 52) consists of a mixture comprising polyethylene and a hydrogenated block copolymer of styrene, chosen from the copolymers of styrene and butadiene and of styrene and isoprene. Screen according to one of Claims 1 to 9, characterized in that the said filler is chosen from carbon blacks, such as acetylene black. Power cable characterized in that it comprises a screen (3, 5) according to one of claims 1 to 10. Power cable according to Claim 11, characterized in that it comprises, arranged coaxially and from the inside to the outside: • a conductive core (1) • an interior semiconductor screen (3) • an electrical insulation layer (4) • an external semiconductor screen (5) • an outer protective sheath (7) said first layer (31) of said inner semiconductor screen (3) being in contact with said conductive core (1), and
said second layers (32, 52) of said interior and exterior semiconductor screens (3, 5) being in contact with said electrical insulation layer (4). Cable according to claim 12 characterized in that it comprises, between said external semiconductor screen (5) and said external protective sheath (7), a metal protective screen (6). Cable according to one of Claims 11 to 13, characterized in that it is intended to operate on direct current.

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