AU683076B2 - A power cable with improved dielectric strength - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Societe Anonyme dite ALCATEL CABLE Actual Inventor(s): Hakim Janah Jose Bezille Jean Becker Jean-Claude Assier Bernard Aladenize Alain Le Mehaute Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: A POWER CABLE WITH IMPROVED DIELECTRIC STRENGTH Our Ref: 383281 POF Code: 1501/188345 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): llle -31 A POWER CABLE WITH IMPROVED DIELECTRIC STRENGTH The present invention relates to high tension power cables for DC or AC. It relates more particularly to such a power cable having improved dielectric strength.

This power cable includes polymer insulation which is preferably extruded. In general, the insulation is covered by an outer semiconductive screen and covers an inner semiconductive screen, itself covering the conductive core of the cable. The polymer insulation has high intrinic dielectric strength. Its practical dielectric strength as obtained on the cable is less than its intrinsic strength. This diffe:rence is due essentially to the presence of impurities or of cavities, which are included or formed prior and/or during S* 15 application of the insulation on the cable, thus giving rise to local concentrations of the electric field in the insulation which can give rise to possible electrical faults through the insulation of the cable.

Document JP-A-2 18811 describes a power cable with polymer insulation containing 0.2% to 1.5% by weight of carbon black. The insulation modified in that way can be applied directly to the conductive core of the cable.

The small quantity of carbon black that it contains reduces the risks of electrical faults that might occur due to peripheral irregularities of the core snd to internal impurities or cavities of the insulation, by improving the uniformity of electric field distribution and thus the reliability of the cable. It makes the insulation slightly electrically conductive, i.e. its conductivity is low but nut zero.

This conductivity is constant and directly related to the intrinsic electrical conductivity of the carbon black contained in the insulation, which is typically S/cm to 100 S/cm. It encourages leakage currents through the insulation and increases its dielectric losses. It reduces the intrinsic dielectric strength of the insulation as modified in this way, and thereby

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reduces its practical dielectric strength on the cable, with this being independent of whether or not irregularities or internal cavities are present.

An object of the present invention is to provide a power cable in which the polymer insulator is of high dielectric strength, and which adapts to any impurities or cavities, but only locally.

The present invention provides a DC power cable having improved dielectric strength, the cable 4eaan electrical core and a first polymer dielectric layer for insulating said core, wherein said first dielectric V layer is constituted by an insulating polymer matrfx *o containing at least one conductive polymer incorporated .in said polymer matrix at a concentration by weight such 15 that the resulting electrical conductivity of said first "dielectric layer is less than 10- 1 4 S/cm.

The present invention also provides an AC power cable having improved dielectric strength, the cable i a~~i an electrical core and a first polymer dielectric layer for insulating said core, wherein said first dielectric layer is constituted by an insulating polymer matrix containing at least one conductive polymer incorporated in said polymer matrix at a concentration by weight such that the resulting electrical conductivity of said first dielectric layer is less than 10 10 S/cm.

The characteristic and advantages of the jpresent o C~ cris er oCV .e e invention appear from the following description k9***nf with reference to the accompanying drawing. In the drawing: Figure 1 is a section view of a power cable of the invention; and Figure 2 is a section view of a variant embodiment of the Figure 1 cable.

As an initial point it will be observed that the electrical conductivity values given herein are all ambient temperature values.

The cable shown in Figure 1 comprises a conductive core 1 fore-d by twisted conductive strands, but it could equally well be formed by a single conductor. It is surrounded by an inner semiconductive screen 2, itself surrounded by a dielectric layer of insulation 3, in turn surrounded by an outer semiconductive screen 4. A protective sheath 5 surrounds the outer semiconductive screen 4 and serves to protect the cable. In particular it may be made of lead or of a lead alloy. It may be insulative, in which case it is preferably associated with a metal grounding screen that underlies it directly.

According to the invention, in this cable the dielectric layer of insulation 3 is constituted by an insulative polymer matrix in which at least one 15 conductive polymer is incorporated, at a concentration by weight such that the resulting electrical conductivity of the dielectric layer 3 is less than 10- 14 S/cm for DC, rad *less than 10-10 S/cm for AC.

Depending on the type of conductive polymer it contains in small quantity, the layer 3 in the cable of the invention exhibits electrical conductivity and dielectric constant that both increase in substantial manner on a local basis in the presence of a defect at any point, and varies from one point to another as a function of the defects at the points.

The dielectric layer 3 is consequently said to be self-adaptive locally depending on the various defects that it presents. It thus serves to homogenze the distribution of the electric field therethrough over the entire length of the cable, reducing risks of breakdown due to the defects.

By way of example, the conductive polymer may be a polymer that is non-doped, de-doped, or self-doped.

A non-doped conductive polymer is a polymer whose synthesis does not require the addition of a doping agent, e.g. polyaniline obtained by the polycondensation reaction of aniline and quinone, or polyacetylene in

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which polymerization has been initiated by means of a Zeigler-Natta type catalyst.

A self-doped conductive polymer is a polymer obtained by grafting a doping agent during synthesis, e.g. polyaniline with a sulfone group grafted on the ring.

A de-doped conductive polymer is a polymer that is doped during synthesis, such as polyaniline treated with hydrochloric acid, and then de-doped by eliminating said acid by appropriate means.

To obtain the intrinsic conductivities of the invention when using a self-doped polymer, its weight concentration in the dielectric layer 3 is not more than V about 2% by weight, both for DC use and for AC use. When 15 a non-doped or a de-doped polymer is used, its weight concentration in the layer 3 is preferably not more than about 5% by weight, both for DC use and for AC use.

One or each of the inner and outer semiconductive screens 2 and 4 is advantageously of the type described in document EP-A-0 507 676, which is constituted by an 0 "insulating polymer matrix and at least one conductive polymer selected from non-doped polymers and polymers that have been doped and then de-doped, and incorporated in the polymer matrix at a concentration of 5% to 70% by weight, and preferably 20% to 30%, so as to obtain semiconductive screens having conductivity of not more than 1 S/cm.

In a variant that is equally advantageous, one or each of the semiconductive screens is constituted by an insulating polymer matrix and at least one self-doped conductive polymer, in particular of the type described in document EP-A-0 512 926, which is incorporated in the polymer matrix at a concentration of more than 5% by weight, and preferably lying in the range 10% to 40% by weight The concentrations and the conductivities given for the semiconductive screens apply both to DC and to AC.

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The polymer matrix of the dielectric layer 3 comprises, like that of the semiconductive screens 2 and 4, at least one thermoplastic polymer selected from acrylic, styrene, vinyl, and cellulose resins, polyolefins, fluoropolymers, polyethers, polyimides, polycarbonates, polyurethanes, silicones, copolymers thereof, and mixtures of homopolymers and mixtures of homopolymers with copolymers.

In particular, the thermoplastic polymer is selected from polypropylene polyethylene ethylenevinyl acetate copolymer (EVA), ethylene-propylene-dienemonomer (EPDM), polyvinylidene fluoride (PVDF), ethylene- **butyl acrylate (EBA), singly or in a mixture.

In a variant, the polymer matrix comprises at least one thermosetting polymer selected from polyesters, epoxy resins, and phenol resins.

The non-doped or doped and then de-doped polymer or polymers of the dielectric layer 3, and also that or those which may be used in the semiconductive screens 2 and 4 are selected from the group comprising: polyaniline, polythiophene, polypyrole, polyacetylene, polyparaphenylene, polyalkylthiophenes, derivatives thereof, and mixtures thereof.

These non-doped and de-doped polymers do not contain ionic groups. Their intrinsic electrical conductivities, measured using DC are very low and about 10 10 S/cm to 9 S/cm. The conductivity of the dielectric layer 3 containing no more than 5% non-doped or de-doped polymer is about 10 14 S/cm or even less for DC use with low electric fields, and is less than 10- 10 S/cm for AC use with low electric fields, i.e. in the absence of any defects or in the presence of defects that are negligible, which would otherwise degrade the high dielectric strength of this layer. It may locally be about 10 9 S/cm in high electric fields and in the presence of defects, but that degrades dielectric strength only locally and in a manner that matches the -r I I 6 defects, allowing high fields to be distributed at such points and avoiding the risk of breakdown resulting therefrom.

The self-doped polymer(s) of the dielectric layer 3, like that or those which may be used in the semiconductive screens 2 and 4, are selected from selfdoped polyanilines having benzene rings or benzene and quinone rings, carrying grafted chains some constituted by a hydrocarbon radical having two to eight carbon atoms interrupted by at least one hetero-atom, and others constituted by a strong acid function or a salt thereof, said hetero-atom itself being selected from O and S and the strong acid function being selected from the residues of sulfonic, phosphonic, and phosphoric acids or salts 15 thereof.

The intrinsic electrical conductivity of these selfdoped polymers as measured using DC is on average about 10-3 S/cm to 10" 2 S/cm. It can also be adjusted at will over the range 10 5 S/cm to 1 S/cm by varying the molecular ratio of the two types of grafted side chain.

The electrical conductivity of the dielectric layer 3 as.

constituted by the above polymer matrix having no more than 2% by weight of said self-doped polymer added thereto is itself adjustable and not greater than about 10- 14 S/cm for DC use with low electric fields, and not greater than about 10 10 S/cm for AC use with low electric fields. This dielectric strength decreases with increasing electric field. In contrast, the electrical conductivity and the dielectric constant of such a dielectric layer increase itrongly with electric field thus making it possible to withstand wi*ji-ut problem a high local concentration of space charge and to distribute such charge.

In a variant of Figure 1 that is not shown in the drawing, the above dielectric layer 3 directly surrounds the cable core and is directly covered by the protective L, sheath 5, with the inner and outer spmiconductive screens both being omitted.

In the variant embodiment shown in Figure 2, references identical to those in Figure 1 designate parts that are identical to those of Figure 1.

The cable shown in Figure 2 includes an inner 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 Each of these two dielectric layers 7 and 8 is "constituted by at least one of the above-specified polymers of the insulating polymer matrix and at least one conductive polymer incorporated in said matrix at a 15 concentration of 5% to 20% by weight. The conductive polymer is at least one of the three above-mentioned types of conductive polymer, but it is preferably selected only from non-doped or de-doped polymers. It is added to the polymer matrix of the dielectric layer at a concentration that is not greater than 20% by weight and that is greater than 5% by weight.

The resulting electrical conductivity of the layers 7 and 8 is 10 14 S/cm to 1 S/cm for DC use and 10- 1 0 S/cm to 1 S/cm for AC use.

The electrical conductivity of these inner and outer dielectric layers 7 and 8 having non-doped or de-doped polymer can reach a few Siemens per centimeter when they are subjected to high electric fields.

In a variant of the Figure 2 embodiment, the cable includes two semiconductive screens as in Figure 1, the inner screen being covered by the inner dielectric layer 7 and the outer screen covering the outer dielectric layer 8.

In another variant, represented by dashed lines in Figure 2, one or both of the inner and outer dielectric layers 7 and 8 is subdivided into a plurality of sublayers such as 7A and 7B or 8A and 8B, each having a concentration of conductive polymer that remains in the range 5% to 20% by weight, but that differs from one sublayer to the next. The concentration of conductive polymer in the sublayers of the inner layer 7 decreases in succession going from the innermost sublayer 7A that is contact with the core. In contrast, the concentration increases through the outer layer 8 going from the innermost sublayer 8A in contact with the dielectric layer 3 to the outermost sublayer 8B that is in contact with the protective sheath The inner and outer dielectric layers 7 and 8, or Stheir sublayers, if present, act as inner and outer semiconductive screens when they are subjected to high electric fields due to their own internal defects, and 15 also to peripheral irregularities on the conductive core "or to defects of the protective sheath. They act as dielectric layers at low electric fields.

For DC use, the electrical conductivity of the sublayer 7A lies in the range 10- 9 S/cm to 1 S/cm, that of the sublayer 7B in the range 10- 14 S/cm to 10- 9 S/cm, that of the sublayer 8A in the range 10- 14 S/cm to 10- 9 S/cm, o" and that of the sublayer 8B in the range 10-9 S/cm to 1 S/cm.

SFor AC use, the electrical conductivity of the sublayer 7A lies in the range 10- 5 S/cm to 1 S/cm, that of the sublayer 7B in the range 10 10 S/cm to 10- 5 S/cm, that of the sublayer 8A in the range 10- 10 S/cm to 10- 5 S/cm, and that of the sublayer 8B in the range 10- 5 S/cm to 1 S/cm.

The main advantages presented by power cables of the invention and which lead to greater reliability for such power cables, include the following in particular: an overall increase in the dielectric strength of the cable due firstly to the intrinsic dielectric strength of the dielectric layer(s) of the cable, which strength remains high, and secondly to the resulting practical dielectric strength which remains equal to the intrinsic value in all zones that are free from defects, and is modified only locally in the presence of a defect and as a function of the defect; an increase iL the maximum electrical field that is acceptable locally, which may rise from a typical value of about 10 kV/mm in certain types of prior art power cable to a value of 20 kV/mm to 30 kV/mm in cables of the same types and implementing the invention; the possibility of increasing the power transmitted by the cable by increasing its operating high tension; and S" for given cable performance, the thickness of the insulating dielectric layer can be reduced.

Naturally, the invention is not limited to the S* 15 embodiments described above.

In particular, when the cable of the invention includes semiconductive screens as such, they may be made out of the materials described above or else out of the materials conventionally used in the semiconductive screens of prior art cables.

•In addition, cables of the invention can be made by.

using the manufacturing techniques that are conventional for cables of this type.

Finally, in the embodiment shown in Figure 2, it is possible to provide more than two sublayers in each of the layers 7 and 8.

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Claims (1)

10- 14 S/cm. 2/ An AC power cable having improved dielectric strength, the cable eman electrical core and a first polymer dielectric layer for insulating said core, 15 wherein said first dielectric layer is constituted by an i* .5 insulating polymer matrix containing at least one conductive polymer incorporated in said polymer matrix at a concentration by weight such that the resulting electrical conductivity of said first layer is less than 10-10 S/cm. S 3/ A cable according to claim 1 or 2, wherein said conductive polymer is a polymer that is non-doped or de- doped and that its concentration in said polymer matrix is not more than about 5% by weight. 4/ A cable according to claim 1 or 2, wherein said conductive polymer is a self-doped polymer whose concentration in said polymer matrix is not more than about 2% by weight. A cable according to any one of claims 1 to 4, further including an inner semiconductive screen between the core and the first dielectric layer, and an outer semi- conductive screen between said first dielectric layer and an outer protective sheath, wherein each of said screens is constituted by an insulating polymer matrix containing L ~L~PI a conductive polymer at a concentration by weight such that the electrical conductivity of said screens is not greater than 1 S/cm. 6/ A cable according to claim 5, wherein said conductive polymer is a polymer that is non-doped or de-doped at a concentration in said polymer matrix of said screens lying in the range 5% to 70% by weight. 7/ A c5ble according to claim 5, wherein said conductive polymet is a self-doped polymer whose concentration in Ssaid polymer matrix of said screens is greater than 5% by weight. a 8/ A cable according to any one of claims 1 to 7, further including an additional second inner dielectric layer underlying said first dielectric layer, and an additional outer third dielectric layer overlying said first dielectr,c layer, each constituted by an insulating 20 polymer matrix containing a conductive polymer selected from non-doped polymers, de-doped polymers, and self- doped polymers, the electrical conductivity of said second and third dielectric layers lying in the range 10- 14 S/cm to 1 S/c;a for DC use and in the range 10-10 S/cm to 1 S/cm for AC use. 9/ A cable according to claim 8, wherein at least one of the second and third dielectric layers comprises a plurality of sublayers having mutually different concentrations of conductive polymer, said concentrations decreasing through the successive sublayers of said second dielectric layer going from the innermost sublayer, and increasing through the successive sublayers of said third dielectric layer going from the innermost sublayer of said third dielectric layer. I A DC power cable substantially as herein described with reference to and as shown in the accompanying drawing. 11/ An AC power cable substantially as herein described with reference to and as shown in the accompanying drawing. DTTED: 13th September, 1994 PHILLIPS ORMONDE FITZPATRICK Attorneys for: SOCIETE ANONYME DITE ALCATEL CABLE o 0 0* 00 I ABSTRACT A power cable having improved dielectric strength including an electrical core and a first polymer dielectric layer insulating the core, the first dielectric layer being constituted by an insulting polymer matrix containing at least one conductive polymer incorporated in said polymer matrix at a concentration by weight such that the resulting electrical conductivity of 10 the first layer is less than 10 14 S/cm for DC and less than 10- 10 S/cm for AC. 0 e 0 0909 I -1