EP0539905A1 - Electrical cable - Google Patents

Abstract

The present invention relates to a power cable comprising, arranged coaxially from the inside outwards: - a conductor core (2), - a jacket (4) made of an insulating material, - a metal screen (6), - an external protective sheath (7), characterised in that the insulating material consists of a continuous organic phase in which organic aggregates are dispersed. <IMAGE>

Description

The present invention relates to an electric cable intended to be used more particularly under high voltages (typically greater than 60 kV) in direct current.

• éventuellement d'un premier écran semi-conducteur,
• d'une enveloppe isolante,
• éventuellement d'un second écran semi-conducteur,
• d'un écran métallique,
• d'une gaine extérieure de protection en un matériau synthétique.

High voltage direct current power cables are used more and more today because they have a much better efficiency than that of high voltage AC cables. These cables generally consist of a conductive core surrounded:

• possibly a first semiconductor screen,
• an insulating jacket,
• possibly a second semiconductor screen,
• a metal screen,
• an outer protective sheath made of synthetic material.

With regard to the insulating envelope, several materials can be envisaged for its production.

First, one could think of using a material used for high-voltage alternating cables, that is to say for example chemically cross-linked polyethylene (noted PRC below), which has very good thermal and mechanical properties. and electric. The chemical crosslinking of polyethylene is obtained by adding organic peroxides to the latter which dissociate at high temperature to form free radicals which crosslink the linear chains of polyethylene. The decomposition or dissociation of these organic peroxides also leads to the formation of by-products. These by-products have been shown to have a harmful effect on direct current. Indeed, under the action of a continuous operating voltage, these by-products are at the origin of the formation of large charges which migrate near the interfaces between the semiconductor screens and the insulating envelope (or else between the insulating jacket and the conductive core on the one hand and between the insulating envelope and the metallic screen on the other hand, when the cable does not include semiconductor screens) where they are the cause of local reinforcements of the electric field. The intensity of the electric field can thus reach in the vicinity of the interfaces two to three times the intensity of the nominal electric field, so that the breakdown voltage of the insulating envelope can be quickly reached, in particular when a pulse of high amplitude (due to lightning, for example) is superimposed on the continuous operating voltage. In the long term, a perforation of this insulating envelope is observed, and consequently a deterioration of the cable. The use of PRC as cable insulation for high direct voltage is therefore not desirable.

One could then think of using crosslinked polyethylene by irradiation. The thickness of the insulating jacket necessary for high voltage and direct current applications (of the order of 2 cm) makes crosslinking by irradiation difficult and in practice of poor quality.

Another type of material has recently been proposed for the insulation of cables for high direct voltage. These are PRC-based materials containing mineral particles, the performance of which is described, for example, in an article entitled "Research and development of DC XLPE cables" published in JI CABLE 87. These materials would make it possible to avoid the harmful phenomenon of the accumulation of space charges at the interfaces. However, to achieve this result, it is necessary, as specified in the article mentioned above, that the purity of the mineral particles introduced into the PRC is carefully controlled in order to avoid the simultaneous introduction of various impurities in the PRC. Indeed, the presence of a very small amount of impurities is sufficient to cause the accumulation of space charges, because impurities can dissociate under the action of the electric field to form space charges. However, it is in practice difficult and tedious to introduce highly purified mineral particles into the PRC. The use of PRCs containing mineral particles is therefore hardly conceivable.

The object of the present invention is therefore to produce an electric cable in which the material constituting the insulating envelope makes it possible to reduce the phenomenon of accumulation of space charges in the presence of a high continuous voltage.

• une âme conductrice,
• une enveloppe en un matériau isolant,
• un écran métallique,
• une gaine extérieure de protection,

caractérisé en ce que ledit matériau isolant est constitué d'une phase organique continue dans laquelle sont dispersés des agrégats organiques.The present invention proposes for this purpose an electric cable comprising, arranged coaxially from the inside to the outside:

• a conductive soul,
• an envelope of insulating material,
• a metal screen,
• an outer protective sheath,

characterized in that said insulating material consists of a continuous organic phase in which organic aggregates are dispersed.

Thanks to the use of such an insulator, the accumulation of space charges at the interfaces between the insulating envelope and the conductive core on the one hand, and between the insulating envelope and the metallic screen on the other part, in the presence of a high DC voltage, is reduced compared to the cables of the prior art.

The insulating material can consist, for example, of a thermoplastic rubber comprising an elastomeric phase and a thermoplastic phase.

According to a first possibility, the thermoplastic rubber can be of olefinic type. In this case, the thermoplastic phase can be chosen from polyethylene and polypropylene, and the elastomeric phase consisting of an ethylene-propylene rubber.

According to a second possibility, the thermoplastic rubber can be of styrenic type. In this case, the elastomeric phase, optionally hydrogenated, can be chosen from polybutadiene and polyisoprene, and the thermoplastic phase consisting of polystyrene.

Finally, a first semiconductor screen can be interposed between the conductive core and the envelope of an insulating material, and a second semiconductor screen can be interposed between the envelope of an insulating material and the metal screen.

The insulating jacket can be extruded.

The cable according to the invention can be used under high continuous voltages.

Other characteristics and advantages of the present invention will appear in the following description of a cable according to the invention, given by way of illustration and in no way limitative.

The single figure shows in exploded perspective a cable for DC voltage, and in particular for DC high voltage, according to the invention.

• une âme conductrice 2 en cuivre ou en aluminium,
• un premier écran semi-conducteur 3,
• une enveloppe isolante 4 constituée, selon l'invention, d'un caoutchouc thermoplastique,
• un second écran semi-conducteur 5,
• un écran métallique de protection 6,
• une gaine extérieure de protection 7 en un matériau synthétique.

In this figure, a cable 1 for direct high voltage comprises:

• a conductive core 2 of copper or aluminum,
• a first semiconductor screen 3,
• an insulating envelope 4 made up, according to the invention, of a thermoplastic rubber,
• a second semiconductor screen 5,
• a protective metal screen 6,
• an outer protective sheath 7 made of a synthetic material.

Thermoplastic rubbers (CT) consist of two mutually incompatible phases: a so-called thermoplastic phase (phase T), and a so-called elastomeric phase (phase E). The following is a non-limiting example of two families of possible CTs for the application of the invention: olefinic CTs and styrenic CTs.

In olefinic CTs, the T phase can be prepared from polypropylene or high or low density polyethylene, and the E phase is generally constituted by an ethylene-propylene rubber. The proportion of polyethylene in the CT is in this case, preferably but not limited to, between 10 and 25%. In order to obtain the CT having the desired structure, a dynamic crosslinking of phase E is carried out in the presence of phase T, that is to say that phase E is crosslinked by strongly kneading the assembly, which allows the fractionation of phase E and its dispersion in the form of aggregates in phase T.

In styrenic CTs, that is to say in block copolymers based on styrene, or block copolymers, phase T consists for example of a non-crystalline polystyrene, and phase E of non-crosslinked polybutadiene or polyisoprene . During the synthesis of the CT, the polystyrene is grafted onto the polybutadiene for example, at the end of the latter chain and by grouping into "domains" of small dimensions (diameter of the order of 30 nm), while the matrix rubber (or phase E) remains continuous. The material is thus made up of a succession of rigid segments in a continuous rubber phase.

CTs therefore generally have an organic phase dispersed in a continuous organic phase. This dispersion of aggregates creates numerous interfaces within the insulating envelope. As a result, any space charges no longer accumulate only at the interfaces between semiconductor screens and insulating envelope, but are also distributed at the numerous internal interfaces of the insulating envelope. Consequently, there are no longer any significant accumulations of space charges at the interfaces between semiconductor screens and insulating envelope, and the accumulations dispersed in the insulating envelope do not generate, under the effect of a continuous operating voltage, only weak reinforcements of the local electric field.

The insulation of cables for high continuous voltage by means of CT makes it possible to solve all the problems posed by the various possible prior art materials.

As has just been described, CTs give better results than PRCs with regard to the accumulation of space charges. In addition, they are much simpler to implement. Indeed, with the PRC, chemical crosslinking takes place during the manufacture of the cable and immediately after the extrusion of the insulating envelope. It is carried out under pressure and at a very high temperature (of the order of 200 ° C); cooling is also carried out under pressure. The manufacturing process is therefore very cumbersome. On the other hand, the CTs are synthesized before manufacture, and their implementation is carried out by heating and extrusion around the cable as for any other thermoplastic material. They do not lose their thermoplastic character when heated for extrusion.

In addition, the formation of organic aggregates being an intrinsic characteristic of CTs, the risks of presence of external impurities are low compared to the case of the introduction of mineral particles in PRC. In addition, the implementation of CT is simpler than that of a PRC with mineral particles.

Tests carried out in the laboratory show that, under the same experimental conditions, the PRC and the CT have a completely different behavior. Thus, the local electric field reinforcements due to the accumulation of space charges are much weaker for the CTs: after one hour of continuous polarization at 20 ° C, the field reinforcement in the vicinity of the interfaces is of the order 110% compared to the value of the field applied for PRC, while it is less than 20% for CT.

est égal à 0,7 ; pour les CT, le rapport $\frac{\text{Vp}}{\text{Vo}}$

est égal à 1.High amplitude impulse withstand tests were also carried out. The resistance of the materials tested to pulses of high amplitude is determined either by direct application of a pulse of increasing voltage until the breakdown of the insulator, or by application of this pulse of increasing voltage after a prepolarization of one hour under a DC voltage equal to one third of the expected breakdown voltage. We call Vo the breakdown voltage without prepolarization, and Vp the breakdown voltage with prepolarization. The relationship between these two values gives an idea of the resistance to high amplitude pulses superimposed on a continuous operating voltage of the materials tested: for PRCs, the ratio $\frac{\text{Vp}}{\text{Vo}}$

is 0.7; for TCs, the report $\frac{\text{Vp}}{\text{Vo}}$

is equal to 1.

CTs are currently available on the market and are used as insulators in cables for low AC voltage. CTs have the particularity, because of their molecular constitution, of behaving both as plastic materials at the temperatures at which they are used for the manufacture of cables, and as rubbery materials at current temperatures of use . They are therefore used in the field of alternative low voltages for their ease of implementation and for their advantageous mechanical and thermal properties.

It is also well known that the resistance to high amplitude pulses of a material increases with its rate of crystallinity. The article entitled "The effect of morphology on the impulse breakdown in XLPE cable insulation" published in IEEE Vol. EI17 n ° 5 of October 1982, on page 386, shows in this respect a curve giving the resistance to pulses of high amplitude as a function of the crystallinity rate. However, the CTs are not very crystalline, and therefore have a resistance to impulses of high mediocre amplitude. This is why they have not been considered so far. as cable insulation materials for high continuous voltage.

Contrary to what was commonly accepted, it has therefore been discovered that thermoplastic rubbers, although having a resistance to impulses of high amplitude less good than that of PRCs, show themselves much better than the latter when they are subjected to pulses. high amplitude superimposed on a continuous operating voltage, and can therefore be used as insulators for cables for high continuous voltage.

Obviously, the invention is not limited to the embodiment which has just been described: the numerical values provided are only for information purposes, and any means may be replaced by equivalent means without departing from the part of the invention.

Claims (12)

Electric cable comprising, arranged coaxially from the inside to the outside: - a conductive core (2), - an envelope made of an insulating material (4), - a metal screen (6), - an outer protective sheath (7), characterized in that said insulating material consists of a continuous organic phase in which organic aggregates are dispersed. Cable according to claim 1 characterized in that said insulating material consists of a thermoplastic rubber comprising an elastomeric phase and a thermoplastic phase. Cable according to claim 2 characterized in that said thermoplastic rubber is of olefinic type. Cable according to claim 3 characterized in that said thermoplastic phase is chosen from polyethylene and polypropylene. Cable according to one of claims 3 and 4 characterized in that said elastomeric phase consists of an ethylene-propylene rubber. Cable according to claim 2 characterized in that said thermoplastic rubber is of styrenic type. Cable according to claim 6 characterized in that said elastomeric phase is hydrogenated. Cable according to either of Claims 6 or 7, characterized in that the said elastomeric phase is chosen from polybutadiene and polyisoprene. Cable according to one of Claims 6 to 8, characterized in that the said thermoplastic phase consists of styrene. Cable according to one of claims 1 to 9 characterized in that a first semiconductor screen (3) is interposed between said conductive core (2) and said envelope made of an insulating material (4), and in that a second semiconductor screen (5) is interposed between said envelope made of an insulating material (4) and said metallic screen (6). Cable according to one of Claims 1 to 10, characterized in that the said envelope is extruded. Cable according to one of Claims 1 to 11, characterized in that it is used in direct high voltage.

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