EP1280167B1 - 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 for use in a medium power cable, high and very high voltage direct current.

• 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.

An energy 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, comprising, for example, copper wires
• an internal semiconductor screen in contact with the core of the cable
• a layer of electrical insulation
• an external semiconductor screen in contact with the electrical insulation layer
• optionally, a protective metal screen
• an outer protective sheath.

The electrical insulation layer is generally made of polyethylene, high or low density, crosslinked or uncrosslinked. Semiconductors screens, in turn, generally consist of a polar matrix, that is to say comprising polar groups such as hydrophilic groups, such as a copolymer of ethylene and acrylate. alkyl, charged by means of an electrically conductive filler, 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.

When operating such a DC cable, especially at high and very high voltage, a very high electric field appears between the conductive core and the inner semiconductor screen, and between the aluminum screen and the external semiconductor screen. This electric field causes the diffusion (we speak in this case of injection) of electrical charges from the semiconductor screen in the insulation layer electric. These electric charges are then trapped in the electrical insulation layer.

However, the phenomena of breakdown and aging of power cables, especially high voltage and very high voltage cables, are due to the strengthening of the electric field in localized areas of these cables. Under DC voltage, this field enhancement is induced by a particular distribution of space charges, depending on the nature and density of these.

Thus, the injection of space charges into the insulation and the quantity of space charges thus injected are a function of both the nature of the insulation and that of the semiconductors (the electrode), and more precisely, the nature of the matrix of the semiconductor screens, the nature and the level of the conductive filler in this matrix, as well as the interaction between these constituents.

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

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

The purpose of the present invention is therefore to develop a semiconductor screen for limiting the injection of space charges in the adjacent electrical insulation layer when applying an electric field, while ensuring the electric functions of use.

The present invention proposes for this purpose a semiconductor screen for an energy 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 conductivity greater than 0.1 S / m at 20 to 90 ° C,
characterized in that the second of said layers is placed in contact with an electrical insulation layer of said energy cable and is such that the amount of space charge that can be injected from said second layer into said layer of insulation is low, so that the amount of space charge that can be injected from said semiconductor screen into said insulation layer is less than the amount of space charge that can be injected from said second layer only 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 retained, that is to say a sufficient electrical conductivity to play its role of screen by homogenizing the distribution of the electric field inside the while reducing the amount of space charge that can be injected into the electrical insulation layer by virtue of the presence of a semiconductor layer of low space charge injection directly in contact with the electrical insulation.

The invention is based on the satisfaction of two constraints considered as contradictory until now, namely the limitation of the injection of space charges in 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 a power cable, without any restriction as to the quantity of loads of space that it is likely to inject into the insulation of the cable. This opens the way for materials with interesting electrical properties but not used until now due to the injection too large space loads that they entailed in, insulation.

Very advantageously, the amount of space charge that can be injected from the semiconductor screen into the insulation layer is less than 200 nC at 25 to 70 ° C.

According to the invention, the amount of space charge that can be 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 charge injected, it is measured by the pressure wave method, which will be described in detail below.

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

Also preferably, the longitudinal 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 with respect to the choice of the type and / or the rate of the conductive charge.

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

In fact, it is possible to choose for this first layer any material known for conventional semiconductor screens, in particular based on polar matrix.

Advantageously still, according to a first variant embodiment, the second layer may comprise a polymer matrix chosen from polyethylene, polypropylene, polystyrene and their copolymers, polymeric alloys selected from polyethylene, polypropylene, polystyrene and copolymers thereof, and mixtures of compounds selected from polyethylene, polypropylene, polystyrene, their copolymers and the foregoing alloys, and a conductive filler dispersed therein.

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

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

The filler may be chosen from carbon blacks, 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 towards the outside:

• a conductive soul
• an internal 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 inner and outer semiconductor screens being in contact with the electrical insulation layer.

A cable according to the invention may furthermore comprise, between the external semiconductor screen and the outer protective sheath, a protective metal screen.

The invention is particularly applicable to DC power cables.

Other features and advantages of the present invention will appear in the following description of an embodiment of the invention, given for illustrative and not limiting.

• 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:

• the figure 1 is an exploded perspective view of an energy cable incorporating two semiconductor screens according to the invention
• the figure 2 is a cross-sectional view of the cable from the figure 1
• the figure 3 is a sectional view of a sample used to perform 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.

The figure 1 shows a cable 10 comprising, arranged coaxially from the inside towards the outside:

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

According to the invention, the inner semiconductor screen 3 is a composite which comprises (see figure 2 ) a layer 31 of high longitudinal 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 charge into the insulation layer 4 after polarization, so that the amount of space charge 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 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 charge into the insulation layer 4 after polarization, so that the the amount of space charge injected from 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 make it possible both to obtain a satisfactory electrical conductivity in the vicinity of the conductive elements of the cable 10 in order to ensure the homogenization function of the distribution of the field electrical, and to limit the injection of space charges in 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 charge after polarization.

In order to demonstrate 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, whose basic structure is shown in figure 3 . The thicknesses of the SC1 and SC2 layers 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 the sample A, the two layers SC1 and SC2 are composite semiconductor screens according to the invention, each consisting of a layer made of a Composition 1 material given below, capable of inducing a large quantity of charges. and a layer of a Composition 2 material given below, inducing a small amount of space charge in the electrically insulating layer 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 molecular weight of 126,500 g / mol: 100 parts
• Hydrogenated SBS (block copolymer, hydrogenated styrene and butadiene): 20 parts
• Black 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 molecular weight of 126,500 g / mol: 100 parts
• Hydrogenated SBS: 20 parts
• Acetylene black DENKA: 39 parts
• Antioxidant: 0.25 part

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

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

It is important to emphasize that compositions 1 and 2 above have been chosen in order to make comparisons of amounts of space charge injected, regardless of their electrical conductivity.

The test implemented, called the pressure wave test, consists in sending the beam of a YAG laser onto the tested sample, each semiconductor screen of which constitutes a (+) and (-) electrode. This beam absorbed at the surface of the (-) electrode decomposes this surface by pyrolysis, and the gases emitted cause a pressure wave which passes through the sample, causing a displacement of space charge 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 the 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 during 4 hours under a continuous electrical voltage of +40 kV at ambient temperature (25 ° C.). <b><u> Table 1 </ u></b> 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 shows that at room temperature, a semiconductor screen according to the invention injects 19 times less space charge than the layer causing the highest injection alone (sample C), but also 15 times less the one with the lowest space-only charge injection (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 space charge injection, 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 DC voltage of +40 kV at 70 ° C. <b><u> Table 2 </ u></b> 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 elevated temperature.

Other measurements were made, under the same conditions as those described above, on samples D, E, F and G, whose basic structure is also that of figure 3 .

In the sample D, the two layers SC1 and SC2 are composite semiconductor screens according to the invention, each consisting of a layer made of a Composition 3 material given below, capable of inducing a large quantity of charges. space, and a layer of a Composition 4 material given below, inducing a small amount of 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 molecular weight of 212 000 g / mol: 100 parts
• Hydrogenated SBS: 20 parts
• Black oven ENSACO 250G: 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 molecular weight of 212 000 g / mol: 100 parts
• Hydrogenated SBS: 20 parts (
  
  To be explained by the inventors)
• Acetylene black DENKA: 39 parts
• Antioxidant: 0.25 part

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

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

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

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

Here again, it is important to emphasize that compositions 3 to 5 above have been chosen in order to make comparisons of quantities of injected space charges, independently of their electrical conductivity.

Table 3 below gives the results obtained outside the applied field, after polarization of the samples during 4 hours under a continuous electrical voltage of +40 kV at ambient temperature (25 ° C.). <b><u> Table 3 </ u></b> 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 BOY WUT 1.68 0.85 235

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

Table 4 below gives the results obtained after polarization of the samples for 4 hours under a DC voltage of +40 kV at 70 ° C. <b><u> Table 4 </ u></b> 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 BOY WUT 1.15 0.98 205

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

We will not go into the details of the method of manufacturing a cable according to the invention. It is simply stated that a screen according to the invention can be obtained by coextrusion 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 just described.

Similarly, the energy cable structure described is only exemplary, and an energy cable according to the invention may comprise only a semiconductor screen according to the invention, for example the inner semiconductor screen only or the outer semiconductor display only. In addition, the cable according to the invention may comprise other types of metal shields, 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 band in the presence of water. Such a protective strip may be interposed between the outer semiconductor screen and the metal shield. It ensures 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 indicative, and may of course be replaced by equivalent materials that are within the reach of ordinary skill 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, those skilled in the art can vary the compositions given above by way of example, as follows:

• the mass content of styrene in the polymer matrix may be from 0.1 to 20%, preferably from 1 to 10%,
• the conductive filler may be carbon black, preferably of the "acetylene" type which is cleaner than "furnace" type carbon blacks (or furnace 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)

1. A semiconductor shield (3, 5) for a power cable, comprising two layers (31, 32; 51, 52), each of said layers comprising a polymer matrix wherein a conductive filler is dispersed, a first one of said layers (31, 51) having a longitudinal volume electrical conductivity of more than 0.1 S/m between 20 and 90 °C,
   characterized in that the second one of said layers (32, 52) is contacted with an electric insulating layer (4) of said power cable and is such that the amount of space charges, which can be injected from said second layer (32, 52) into said electric insulating layer (4) is low, so that the amount of space charges, which can be injected from said semiconductor shield (3, 5) into said insulating layer (4) is less than the quantity of space charges, which can be injected from said second layer (32, 52) only into said electric insulating layer (4), with said second layer (32, 52) being a barrier limiting the injection of space charges into said electric insulating layer (4).
2. The shield according to claim 1, characterized in that the amount of space charges, which can be injected from said semiconductor shield (3, 5) into said insulating layer (4) is less than 200 nC between 25 and 70 °C.
3. The shield according to any of claims 1 or 2, characterized in that the amount of space charges, which can be injected from said second layer (32, 52) only into said electric insulating layer (4) is less than 250 nC between 25 and 70 °C.
4. The shield according to any of claims 1 to 3, characterized in that the volume electrical conductivity of the second layer (31, 51) is more than 5 S/m between 20 and 90 °C.
5. The shield according to any of claims 1 to 4, characterized in that the longitudinal volume electrical conductivity of said second layer (32, 52) is comprised between 10^-4 and 10^-1 S/m between 20 and 90 °C.
6. The shield according to any of claims 1 to 5, characterized in that said first layer (31, 51) includes a matrix chosen from ethylene and alkyl acrylate copolymers or blends of such copolymers with polyolefins, and a conductive filler dispersed in said matrix.
7. The shield according to any of claims 1 to 6, characterized in that said second layer (32, 52) includes a polymer matrix chosen from polyethylene, polypropylene, polystyrene, and copolymers thereof, polymer alloys chosen from polyethylene, polypropylene, polystyrene, and copolymers thereof, and the blends of compounds chosen from polyethylene, polypropylene, polystyrene, copolymers thereof, and the preceding alloys, and a conductive filler dispersed in said matrix.
8. The shield according to any of claims 1 to 6, characterized in that said second layer (32, 52) includes a polymer matrix chosen from polyolefin thermoplastic elastomers and blends thereof, and a conductive filler dispersed in said matrix.
9. The shield according to claim 7, characterized in that said polymer matrix of the second layer (32, 52) is composed of a blend including polyethylene and a hydrogenated styrene block copolymer, chosen from styrene butadiene and styrene isoprene copolymers.
10. The shield according to any of claims 1 to 9, characterized in that said filler is chosen from carbon blacks, such as acetylene black.
11. A power cable, characterized in that it comprises a shield (3, 5) according to any of claims 1 to 10.
12. The 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 internal semiconductor shield (3),

    - an electric insulating layer (4),

    - an external semiconductor shield (5),

    - an external protective sheath (7),

    said first layer (31) of said internal semiconductor shield (3) contacting said conductive core (1), and
    said second layers (32, 52) of said internal and external semiconductor shields (3, 5) contacting said electric insulating layer (4).
13. The cable according to claim 12, characterized in that it comprises, between said external semiconductor shield (5) and said external protective sheath (7), a protective metal shield (6).
14. The cable according to any of claims 11 to 13, characterized in that it is designed for direct current operation.

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