DE69402494T3 - Power cables with improved dielectric strength - Google Patents

Description

The present invention relates to high-voltage power cables for direct or alternating current. It relates in particular to a power cable with improved dielectric breakdown strength.

This power cable contains an insulating material made of polymeric materials, preferably introduced by extrusion. In general, the insulating material covers an inner semi-conductive shield, which in turn surrounds the conductive cable core, and is surrounded by an outer semi-conductive shield. This polymer insulation has a high intrinsic dielectric strength. The practical dielectric strength of the cable is lower than its intrinsic dielectric strength. This difference is essentially due to impurities or voids introduced before and/or during the manufacture of the insulating layers of the cable, leading to local concentrations of the electric field in the insulating material and causing possible electrical defects in the insulating material of the cable.

The publication JP-A-2 -18811 describes a power cable with polymer insulation containing 0.2 to 1.5 mass percent carbon black. The insulating material modified in this way can be applied directly to the conductive core of the cable. The small amount of carbon black it contains reduces the risks of electrical defects due to irregularities in the surface of the core and due to impurities or internal voids in the insulating material by improving the homogeneity of the distribution of the electric field and therefore the reliability of the cable. The carbon black gives the insulating material a slight electrical conductivity, which is low but not zero.

This conductivity is constant and directly related to the intrinsic electrical conductivity of the insulating material, typically between 10 and 100 S/cm. This promotes leakage currents in the insulating material and increases its dielectric losses. The conductivity reduces the intrinsic dielectric strength of the insulating material thus modified and thus also the practical dielectric strength of the cable, regardless of surface irregularities or internal cavities.

The aim of the present invention is to provide a power cable whose polymer insulation has a high dielectric strength that only locally adapts to possible impurities or cavities.

The invention relates to a direct current power cable with improved dielectric strength, comprising an electrical core and a first dielectric layer of insulation made of polymer material around the core, characterized in that the first dielectric layer consists of an insulating polymer matrix containing at least one conductive polymer incorporated in the polymer matrix in such a mass fraction that the resulting electrical conductivity of the first layer is less than 10-14 S/cm, and which makes the resulting electrical conductivity of the first layer locally self-adaptive in the presence of defects, increasing as a function of the electric field due to a defect at a given point.

The invention also relates to an alternating current power cable with improved dielectric strength, comprising an electrical core and a first dielectric layer for insulating the core made of polymer material around the core, characterized in that the first dielectric layer consists of an insulating polymer matrix containing at least one conductive polymer incorporated in the polymer matrix in such a mass fraction that the resulting electrical conductivity of the first layer is less than 10-10 S/cm, and which makes the resulting electrical conductivity of the first layer locally self-adaptive in the presence of defects, increasing as a function of the electric field due to a defect at a given point.

The invention will now be explained in more detail with reference to the drawings.

Fig. 1 shows a cross-section of a power cable according to the invention.

Fig. 2 shows a cross-section of a variant of the cable from Fig. 1.

First of all, it should be noted that all electrical conductivity values are related to ambient temperature.

The cable shown in Fig. 1 contains a conductive core 1 consisting of a stranded conductor, but the core can also be formed by a single conductor. The core is surrounded by an inner semiconductive screen 2, which in turn is embedded in a dielectric insulation layer 3 and further in an outer semiconductive screen 4. A protective sheath 5 surrounds the outer semiconductive screen 4 and protects the cable. This protective sheath is made in particular of lead or a lead alloy. It can also be insulating and then preferably combined with a metallic ground screen which lies immediately underneath.

In this cable, the dielectric insulation layer 3 consists, according to the invention, of an insulating polymer matrix in which at least one conductive polymer is contained with such a mass fraction that the resulting electrical conductivity of the dielectric layer 3 is below 10⁻¹⁴ S/cm for direct current or 10⁻¹⁴ S/cm for alternating current.

The electrical conductivity or dielectric constant of layer 3 in the cable according to the invention increases significantly locally in the presence of a defect at any point, depending on the type of conductive polymer contained in a small amount, and is variable from one point to the next, depending on the defects at these points.

The dielectric layer 3 can therefore be considered as locally self-adapting according to the various defects contained. It allows the distribution of the electric field to be made homogeneous over the entire length of the cable. and reduce the risk of breakdown due to these defects.

The conducting polymer can, for example, be an undoped, dedoped or autodoped polymer.

It should be recalled that an undoped conducting polymer is a polymer whose synthesis is carried out without the introduction of a dopant, such as polyaniline obtained by polycondensation of aniline and quinone, or polyacetylene whose polymerization is initiated by means of a Ziegler-Natta catalyst.

An autodoped conducting polymer is a polymer that is obtained by incorporating a dopant during synthesis, for example polyaniline, into which a sulfur group is incorporated in the cycle.

A dedoped conducting polymer is a polymer that is doped during its synthesis, such as polyaniline, which is treated with hydrochloric acid and subsequently dedoped by removing the acid by a suitable agent.

In order to obtain the intrinsic conductivities according to the invention, when using an autodoped polymer, its mass fraction in the dielectric layer 3 is at most about 2 mass percent, both for a direct current and for an alternating current cable. If an undoped or dedoped polymer is used, its mass fraction in the layer 3 is preferably at most about 5 mass percent, both for direct current and for alternating current.

At least one of the semiconductive screens 2 and 4 preferably corresponds to the type described in EP-A-0 507 676 and contains an insulating polymer matrix and at least one conductive polymer selected from among undoped and doped and subsequently dedoped polymers and incorporated into the polymer matrix in a mass fraction of 5 to 70%, preferably 20 to 30%. designed to achieve a conductivity of the semiconducting screens of no more than 1 S/cm.

According to an equally advantageous variant, at least one of these semiconducting screens consists of an insulating polymer matrix and at least one autodoped conductive polymer of the type described in EP-A-0 512 926, which is incorporated into the polymer matrix with a mass fraction of more than 5% and preferably between 10 and 40%.

The concentrations and conductivities specified for the semiconducting shields apply to both direct and alternating current.

The polymer matrix of the dielectric layer 3, like that of the semiconductive screens 2 and 4, contains at least one thermoplastic polymer chosen from acrylic, styrene, vinyl and cellulose 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 material is selected from polypropylene (PP), polyethylene (PE), the copolymer of ethylene and vinyl acetate (EVA), the ethylene-propylene-diene monomer (EPDM), the fluorinated polyvinylidene (PVDF), the ethylene-butyl acrylate (EBA), alone or in mixtures.

According to a variant, the polymer matrix contains at least one thermosetting polymer selected from polyesters, epoxy resins and phenolic resins.

The undoped or first doped and then dedoped polymer materials of the dielectric layer 3 like those of the semiconducting screens 2 and 4 are selected from the group containing polyaniline, polythiophene, polypyrrole, polyacetylene, polyparaphenylene, polyalkylthiophene, their derivatives and mixtures.

These undoped and dedoped polymers do not contain any ionic groups. Their intrinsic electrical conductivity measured at direct current is very low and is around 10⁻¹⁰ to 10⁻⁹9; S/cm. The conductivity of the dielectric layer 3, which contains at most 5% of the undoped or dedoped polymer, is of the order of 10⁻¹⁴ S/cm or even less for a direct current application and for low electric fields, and less than 10⁻¹⁴ S/cm for an alternating current application and for low electric fields, i.e. in the absence of defects or in the presence of only negligible defects which do not affect the high dielectric strength of this layer. It can be locally around 10⁻¹⁴ S/cm in the presence of high electric fields and defects, whereby the dielectric strength is only impaired locally and in a manner adapted to the defects, while a distribution of the high fields at these points results and the resulting dangers of breakdown are avoided.

The self-doped polymers of the dielectric layer 3, like those of the semiconducting screens 2 and 4, are selected from the self-doped polyanilines with benzene rings or with benzene and quinone rings bearing radicals consisting, on the one hand, of a hydrocarbon radical with 2 to 8 carbon atoms and interrupted by at least one heteroatom, and, on the other hand, of a strong acid function or one of its salts, the heteroatom being itself selected from oxygen and sulfur, and the strong acid function being selected from the radicals of sulfuric, phosphonic and phosphoric acids and their salts.

The intrinsic electrical conductivity of these autodoped polymers measured at direct current is on average about 10⁻³ to 10⁻² S/cm. It can also be adjusted on request between 10⁻⁵ and 1 S/cm by changing the molecular ratio of the two types of residues. The electrical conductivity of the dielectric layer 3, which consists of the above polymer matrix to which at most 2% by mass of the autodoped polymer is added, is itself adjustable and is about 10⁻¹⁴ S/cm or less for use in weak electric fields and direct current, and about 10⁻¹⁴ S/cm or less for use in alternating current and weak electric fields.

This dielectric strength decreases with the increase of the electric field. In contrast, the electrical conductivity and the dielectric constant of such a dielectric layer increase strongly with the electric field and thus allow a significant local concentration of space charges to be tolerated without problems and these charges to be distributed.

In a variant with respect to Fig. 1, which is not shown, the mentioned dielectric layer 3 directly surrounds the core of the cable and is directly covered by the protective sheath 5, so that the inner and outer semiconducting shields are then omitted.

In the variant shown in Fig. 2, the same reference numerals as in Fig. 1 were used for the same parts.

The cable shown in Fig. 2 contains 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 5.

Each of these two dielectric layers 7 and 8 consists of at least one of the polymers of the mentioned insulating polymer matrix and at least one conductive polymer incorporated in this matrix in a mass fraction of between 5% and 20%. The conductive polymer is constituted by at least one of the three conductive polymer types mentioned above, but preferably it is chosen only from among the non-doped or de-doped polymers.

It is added to the polymer matrix of the dielectric layer with a mass fraction of no more than 20% and no less than 5%.

The resulting electrical conductivity of layers 7 and 8 is between 10⁻¹⁴ and 1 S/cm for use with direct current and between 10⁻¹⁴ and 1 S/cm for use with alternating current.

The electrical conductivity of these inner and outer dielectric layers 7 and 8 with doped or undoped polymer can reach several S/cm when exposed to high electric fields.

In a variant of the embodiment according to Fig. 2, the cable contains two semiconductive shields as in Fig. 1, wherein the inner shield is covered by the inner dielectric layer 7, while the outer shield surrounds the outer dielectric layer 8.

Also in accordance with a variant, indicated in dashed lines in this Figure 2, one or each of the inner and outer dielectric layers 7 and 8 is divided into several elementary layers such as 7A and 7B or 8A and 8B, in which the mass fraction of the conductive polymer remains between 5% and 20%, but varies from one elementary layer to another. This mass fraction of the conductive polymer in the elementary layers of the inner layer 7 decreases from the innermost elementary layer 7A in contact with the core. On the other hand, in the outer layer 8, it increases from the innermost layer 8A in contact with the dielectric layer 3 to the outermost elementary layer 8B in contact with the protective sheath 5.

The dielectric inner and outer layers 7 and 8 or their possible elementary layers act as inner and outer semiconducting screens when exposed to high electric fields acting on internal defects and also on surface irregularities of the conductive core or defects in the protective cover. They have the function of a dielectric layer in the weak electric fields.

For use with direct current, the electrical conductivity of layer 7A is between 10⁻⁹⁴ and 1 S/cm, that of layer 7B is between 10⁻¹⁴ and 10⁻⁹⁴ S/cm, that of layer 8A is between 10⁻¹⁴ and 10⁻⁹⁴ S/cm and that of layer 8B is between 10⁻⁹⁴ and 1 S/cm.

For use with alternating current, the electrical conductivity of layer 7A is between 10⁻⁵ and 1 S/cm, that of layer 7B is between 10⁻¹⁰ and 10⁻⁵ S/cm, that of layer 8A is between 10⁻¹⁰ and 10⁻⁵ S/cm and that of layer 8B is between 10⁻⁵ and 1 S/cm.

The main advantages of the power cables according to the invention, which lead to a high level of reliability of these cables, are as follows:

- the overall increase in the dielectric strength of the cable due, on the one hand, to the high remaining intrinsic dielectric strength of the dielectric layer or layers of this cable and, on the other hand, to the resulting practical dielectric strength which remains at the intrinsic value in all zones, except for defects, and is only modified locally in the presence of a defect or depending on this defect;

- an increase in the maximum permissible local electric field, which increases from a typical value of about 10 kV/mm in some types of power cables according to the prior art to a value of 20 to 30 kV/mm in cables of the same type but according to the invention;

- a possible increase in the energy transmitted through the cable by increasing the operating voltage of the cables,

- a reduction in the thickness of the dielectric insulation layer without impairing the properties of the cables.

Of course, the invention is not limited to the embodiments described above.

In particular, if the cable according to the invention contains semiconductive shields as such, these can be formed either from the materials described above or from known materials which are used for the semiconductive shields in the known cables.

Incidentally, the cables according to the invention can also be manufactured using the known manufacturing processes for such cables.

Finally, in the embodiment according to Fig. 2, it is possible to realize more than two elementary layers for each of the layers 7 and 8.

Claims (9)

1. DC power cable with improved dielectric strength, comprising an electrical core (1) and a first dielectric layer (3) of polymer material for insulation around the core, characterized in that the first dielectric layer consists of an insulating polymer matrix containing at least one conductive polymer incorporated in the polymer matrix in such a mass fraction that the resulting electrical conductivity of the first layer is less than 10-14 S/cm at ambient temperature, and which makes the resulting electrical conductivity of the first layer locally self-adaptive in the presence of defects, increasing as a function of the electric field due to a defect at a given point. 2. AC power cable with improved dielectric strength, comprising an electrical core (1) and a first dielectric layer (3) of polymer material for insulation around the core, characterized in that the first dielectric layer consists of an insulating polymer matrix containing at least one conductive polymer incorporated in the polymer matrix in such a mass fraction that the resulting electrical conductivity of the first layer is less than 10-10 S/cm at ambient temperature, and which makes the resulting electrical conductivity of the first layer locally self-adaptive in the presence of defects, increasing as a function of the electric field due to a defect at a given point. 3. Cable according to one of claims 1 or 2, characterized in that the conductive polymer is an undoped or dedoped polymer, the mass fraction of which in the polymer matrix is at most about 5%. 4. Cable according to one of claims 1 or 2, characterized in that the conductive polymer is an autodoped polymer whose mass fraction in the polymer matrix is at most 2%. 5. Cable according to one of claims 1 to 4, which also contains an inner semiconductive screen (2) between the core (1) and the first dielectric layer (3) and an outer semiconductive screen (4) between the first dielectric layer (3) and an outer protective sheath (5), characterized in that each of the screens is formed from a polymer matrix of insulating material with a conductive polymer, the mass fraction of which is chosen so that the electrical conductivity of the screens is at most 1 S/cm. 6. Cable according to claim 5, characterized in that the conductive polymer is an undoped or dedoped polymer whose mass fraction in the polymer matrix of the shields is between 5% and 70%. 7. Cable according to claim 5, characterized in that the conductive polymer is an autodoped polymer whose mass fraction in the polymer matrix of the shields is greater than 5%. 8. Cable according to one of claims 1 to 7, characterized in that it further comprises a second additional inner dielectric layer (7) under the first dielectric layer (3) and a third additional outer dielectric layer (8) over the first dielectric layer (3), each formed by an insulating polymer matrix and a conductive polymer selected from undoped, dedoped and autodoped polymers, the electrical conductivity of the second and third dielectric layers being between 10⁻¹⁴ and 1 S/cm for use with direct current and between 10⁻¹⁰ and 1 S/cm for use with alternating current. 9. Cable according to claim 8, characterized in that at least at least one of the second and third dielectric layers contains a plurality of elementary layers (7A, 7B; 8A, 8B) which differ in the proportion of conductive polymer, these proportions decreasing in the successive elementary layers of the second dielectric layer starting from the innermost elementary layer and increasing in the successive elementary layers of the third dielectric layer starting from the innermost elementary layer of this third dielectric layer.