FR2710183A1 - Energy cable with improved dielectric strength. - Google Patents

Abstract

The dielectric strength enhanced power cable has a conductive core and an insulating dielectric layer. It is characterized in that said dielectric layer (3) consists of a polymeric matrix containing a conductive polymer, chosen from undoped, dedoped or autodoped polymers and incorporated with a rate at most equal to 5% by mass in said matrix. Application: medium or high voltage power cable and direct or alternating current.

Description

Energy cable with improved dielectric strength.

  The present invention relates to energy cables in medium or high voltage and direct or alternating current.

  It relates more particularly to such an energy cable5 with improved dielectric strength.

  This energy cable comprises a polymeric insulation preferably extruded. In general, the insulator covers an internal semiconductor screen, itself covering the conductive core of the cable, and is covered with an external semi-conductive screen. This polymeric insulation has a high intrinsic dielectric strength. Its practical dielectric strength, obtained on the cable, is lower than its intrinsic rigidity. This difference is mainly due to the presence of impurities or cavities, which were introduced or formed before and / or during the implementation of the insulation on the cable, give rise to local concentrations of electric field in the insulation and cause possible electrical faults through the cable insulation. JP-A-2-18811 discloses a polymeric insulator energy cable containing from 0.2 to 1.5% by weight of carbon black. The insulation thus modified can be implemented directly on the conductive core of the cable. The small amount of carbon black it contains reduces the risk of electrical faults that may be due to peripheral irregularities of the core and impurities or

  internal cavities of the insulation, improving the homogeneity of distribution of the electric field and thus the reliability of the cable. It gives the insulation a slight electrical conductivity, as such low but not zero.

  This conductivity is constant and directly related to the intrinsic electrical conductivity of the carbon black, typically 10 to 100 S / cm, contained in the insulation. It promotes leakage currents in the insulation and increases its dielectric losses. It reduces the intrinsic dielectric strength of the insulation thus modified and thereby its practical dielectric strength on the cable, regardless of the presence or absence of irregularities or internal cavities. The present invention aims to provide a power cable whose polymeric insulation is of high dielectric strength, which adapts only locally to possible impurities or cavities. It relates to a power cable with improved dielectric strength, comprising an electrical core and a first polymeric dielectric insulation layer of said core, characterized in that said first layer

  dielectric is constituted by an insulating polymeric matrix containing at least one conductive polymer, incorporated with a mass ratio of at most 5% in said polymeric matrix and selected from undoped polymers, dedoped polymers and autodoped polymers.

  The cable according to the invention advantageously also has at least one of the following additional features: the cable further comprises an internal semiconductor screen, between the core and the first polymeric insulation layer, and a screen external semiconductor, between said first polymeric insulation layer and an outer protective sheath, each consisting of an insulating polymeric matrix containing from 5 to 70% by weight of at least one conductive polymer selected from undoped polymers, dedoped polymers and autodoped polymers; it furthermore comprises a second internal additional dielectric layer, underlying said first dielectric layer, and a third additional external dielectric layer covering said first dielectric layer, each consisting of an insulating polymeric matrix containing a rate of between 5 and 20% by weight of conductive polymer, selected from the undoped and autodoped undoped polymers and preferably from the only undoped and dedoped polymers; at least one of said second and third dielectric layers comprises a plurality of elementary layers having different levels of conductive polymer between them, these levels being decreasing in the elementary layers of said second dielectric layer from that of the innermost elementary layers, and being croissants

  in the successive elementary layers of said third dielectric layer, from the innermost elementary layer of this third dielectric layer. The features and advantages of the present invention will emerge from the description given below in

  reference to the attached drawings. In these drawings: - Figure 1 is a sectional view of an energy cable according to the invention.

  - Figure 2 is a sectional view of an alternative embodiment of the cable of Figure 1.

  The cable shown in FIG. 1 comprises a conductive core 1, formed by a conductive strand but equally well formed by a single conductor, which is surrounded by an internal semiconductor screen 2, itself surrounded by a dielectric layer. 3, in turn surrounded by an external semiconductor screen 4. A protective sheath 5 surrounds the outer semiconductor screen 4 and provides protection of the cable. It is in particular lead or lead alloy. It can be insulating and then preferably associated with a metal screen of mass directly underlying. In this cable, the insulating dielectric layer 3 is constituted by an insulating polymeric matrix and at least one conductive polymer, which is chosen from undoped polymers, doped polymers and then dedoped polymers and autodoped polymers and is incorporated in the matrix. , with a rate of less than or equal to 5%, by mass. This dielectric layer 3 has a quasi-zero electrical conductivity, in the absence of impurities, cavities or other, so-called defects overall, in one of its areas along the cable. Its conductivity or its constant

  According to the type of conductive polymer which it contains in a limited quantity, the dielectric increases substantially locally, in the presence of a defect at any point, being variable from one point to another depending on the defects in these conditions. points.

  This dielectric layer 3 is consequently self-adaptive locally according to the various defects that it presents. It thus makes it possible to homogenize the distribution of the electric field across it over the entire length of the cable, reducing the risks of breakdown due to

to these defects.

  One or each of the internal and external semiconductor shields 4 is advantageously of the type described in document EP-A-0507676, which consists of an insulating polymeric matrix and at least one polymer

  conductive, the latter being selected from doped polymers and doped polymers and then dedoped and being incorporated in the polymer matrix with a level of 5 to 70% by weight.

  As an equally advantageous variant, one or each of these semiconductor screens is constituted by a matrix

  polymeric insulator and at least one autodoped conductive polymer, particularly of the type described in EP-A-0512926, which is incorporated with a mass ratio greater than 5% in the polymer matrix.

  The polymeric matrix of the dielectric layer 3 comprises, like that of the semiconductor screens 2 and 4, at least one thermoplastic polymer chosen from acrylic, styrenic, vinyl and cellulosic resins, polyolefins, fluorinated polymers, polyethers, polyimides, polycarbonates, polyurethanes, silicones, their copolymers, and mixtures between homopolymers and between homopolymers and copolymers. In particular, this thermoplastic polymer is chosen from polypropylene (PP), polyethylene (PE), copolymer of ethylene and vinyl acetate (EVA),

  ethylene-propylene-diene-monomer (EPDM), fluorinated polyvinylidene (PVDF), ethylene-butylacrylate (EBA), alone or as a mixture.

  In a variant, the polymeric matrix comprises at least one thermosetting polymer chosen from polyesters, epoxy resins and phenolic resins. The undoped or doped polymer (s) and then dedoped from the semiconductor layer 3, such as that or those possible from the semiconductor screens 2 and 4, are chosen from the group comprising polyaniline, polythiophene, polypyrrole and polyacetylene. , polyparaphenylene, polyalkylthiophenes, their derivatives and mixtures thereof. These undoped and dedoped polymers do not contain ionic groups. Their intrinsic electrical conductivity is very low and of the order of 10-7 to 10-8 S / cm. The conductivity of the dielectric layer 3, containing at most 5% of the undoped or dedoped polymer, is of the order and even less than 10-16 S / cm at the low electric fields, that is to say at 1 absence of defects or in the presence of negligible defects, which does not degrade the high dielectric strength of this layer. It can locally be of the order of 10-9 S / cm at high electric fields in the presence of defects, which degrades only locally and in a manner adapted to the dielectric strength defects but allows the distribution of the high fields at these points. avoiding the risk of breakdown. The self-doped polymer or polymers of the semiconductor layer 3, such as that or those possible of the semiconductor screens 2 and 4, are chosen from autodoped polyanilines having benzene or benzenic and quinonic rings, which carry grafts constituted for each by a hydrocarbon radical containing from 2 to 8 carbon atoms and interrupted by at least one hetero atom, and for the others by a strong acid function or a salt thereof, said hetero atom being itself chosen from O and S and the strong acid function among the residues of sulfonic acid, phosphonic acid and phosphoric acid or of their salts. The intrinsic electrical conductivity of these autodoped polymers is on the order of 10-3 to 10-2 S / cm. It is also adjustable at will between 10-5 to 1 S / cm, by variation of the molecular ratio of the two types of grafts. The electrical conductivity of the dielectric layer, consisting of the above polymer matrix to which is added at most 5% by weight of this autodoped polymer, is itself adjustable and on average of the order of 10-5 to 10- 6 S / cm at low electric fields. This dielectric strength drops with the increase of the electric field. On the other hand, the dielectric constant of such a dielectric layer increases strongly with the high electric fields and thus makes it possible to withstand a significant local concentration of space charges without problem and to distribute these charges. As a variant given with respect to this FIG. 1 and not shown, the aforementioned dielectric layer 3 directly surrounds the core of the cable and is directly covered by the protective sheath 5, the two internal and external semiconductor shields being removed. In the variant embodiment according to FIG. 2, the references identical to those of FIG.

  parts identical to those of this figure 1.

  The cable represented in this FIG. 2 comprises an internal dielectric layer 7, between the conductive core 1 and the dielectric layer 3, and an outer dielectric layer 8, between the dielectric layer 3 and the protective sheath 5. Each of these two dielectric layers 7 and 8 is constituted by at least one of the polymers of the abovementioned polymeric insulating matrix and at least one conductive polymer incorporated in this matrix, with a content of 5 to 20% by weight. Its conductive polymer is at least one of the three conductive polymer types mentioned above, but is preferably chosen from the only undoped or dedoped polymers. It is added to the polymer matrix of the dielectric layer at a rate of less than or equal to 20% by mass and greater than 5%. The electrical conductivity of these doped or undoped polymer internal and external dielectric layers can reach a few. cm, when subjected to high electric fields. As a variant given with respect to this FIG. 2, the cable comprises two semiconductor screens as in FIG. 1, the inner one being covered by the inner dielectric layer 7 and the outer one covering the outer dielectric layer 8. In a variant also, illustrated in FIG. dotted in this same FIG. 2, one or each of the inner and outer dielectric layers 7 and 7 is divided into several elementary layers, such as 7A and 7B and 8A and 8B, having a mass ratio of conductive polymer which remains between 5 and 20. % but is different from one elemental layer to another. This level of conductive polymer of the elementary layers of the inner layer 7 decreases successively from the innermost elementary layer 7A in contact with the core. It is instead increasing in the outer layer 8, from the innermost elementary layer 8A in contact with the dielectric layer 3 to the outermost elementary layer 8B in contact with the protective sheath 5. The inner dielectric layers 7 and external 8 or their possible elementary layers act as internal and external semiconductor screens when they are subjected to high electric fields, which are due to their internal defects and in addition to the peripheral irregularities of the conductive core or to defects in the

  protective shealth. They act as a dielectric layer at low electric fields.

  Among the main advantages presented by the energy cables according to the invention and which lead to greater reliability of these power cables, mention is made in particular of: the overall increase in the dielectric strength of the cable, due to a part of the intrinsic dielectric strength left high of the dielectric layer (s) of this cable and secondly to the resulting practical dielectric strength, which remains equal to this intrinsic value in all areas free from defects and is modified only locally in the presence of a defect and depending on what

  defect, - an increase in the maximum permissible electric field locally, which can pass from a typical value of the order-

  of 10 KV / mm in certain types of power cables according to the known art at a value of 20 to 30 KV / mm in these same types of cables according to the invention, - a possible increase in the energy transmitted by the cable, by increasing the high voltage service cables,

  - Reduction of the thickness of the insulating dielectric layer, with the performance of the preserved cables.

Claims (3)

  1 / energy cable with improved dielectric strength, comprising an electrical core and a first polymeric dielectric insulation layer of said core, characterized in that said first dielectric layer is constituted by an insulating polymeric matrix   containing at least one conductive polymer, incorporated with a mass ratio of at most 5% in said polymeric matrix and selected from undoped polymers, dedoped polymers and autodoped polymers.   2 / A cable according to claim 1, further comprising an internal semiconductor screen, between the core and the first dielectric layer, and an external semiconductor screen, between said first dielectric layer and an outer protective sheath, characterized in that each of said screens is constituted by a polymeric matrix   insulating material containing from 5 to 70% by weight of at least one conductive polymer chosen from undoped polymers, dedoped polymers and self-doped polymers. 3 / Cable according to one of claims 1 and 2, characterized in that it also comprises a second layer   additional internal dielectric (7), underlying said first dielectric layer (3), and an additional third external dielectric layer (8), covering said first dielectric layer (3), each consisting of an insulating polymeric matrix containing a rate inclusive of between 5 and 20% by weight of conductive polymer chosen from undoped polymers, dedoped and autodoped.  4 / cable according to claim 3, characterized in that at least one of the second and third dielectric layers comprises a plurality of elementary layers having between them different levels of conductive polymer, these levels being decreasing in the elementary layers of said second dielectric layer since that of its innermost elementary layers, and being increasing in the successive elementary layers of said third layer   dielectric from its innermost elementary layer.