WO1990001776A1

WO1990001776A1 - Method for increasing the resistance to humidity of a high voltage electric cable, material for implementing such method, cable thus obtained

Info

Publication number: WO1990001776A1
Application number: PCT/FR1989/000393
Authority: WO
Inventors: Jean Filippini; Claudine Meyer
Original assignee: Centre National De La Recherche Scientifique (Cnrs)
Priority date: 1988-07-29
Filing date: 1989-07-26
Publication date: 1990-02-22
Also published as: FR2634940A1

Abstract

Method for increasing the resistance to humidity of a high voltage electric cable of the type comprising a conducting core (1) surrounded by an insulating layer of polymer material (3), for example polyethylene, wherein a protection substance capable of preventing the degradation of the insulating layer because of moisture is incorporated at a cable region (2, 4), situated outside the isolating layer, said substance being at least partially water-soluble so as to be entrained to the insulating layer by any water trace present in the cable at the opposite end of the insulating layer with respect to said region, said protection substance being, after activation by water, appropriate to be involved in electrochemical reactions at the water/polymer material interface or appropriate to trap the free radicals resulting from electrochemical reactions, thereby preventing the formation and propagation of branched chains of water in the polymer material of the insulating layer and thus preventing the degradation of the insulating layer due to moisture.

Description

Method for increasing the resistance to humidity of a high voltage electric cable, material for carrying out the process, cable thus obtained

The invention relates to a method for increasing the moisture resistance of a high voltage cable for the transport of electrical energy.

It also relates to a new material for the implementation of the method, and the electrical cable of improved resistance to humidity, thus obtained.

High voltage electric cables are now frequently insulated with polymeric materials, especially polyethylene and especially crosslinked polyethylene. These polymer materials can be not only polymers in the strict sense, but also copolymers, alloys of polymers, etc. The operating conditions to which these cables are subjected become more and more severe due to the increase in the transported currents and therefore the operating temperature, and also by the increase in the operating voltages, the latter possibly reaching or exceed 400 kilovolts, which results in an increase in the electric field to which the insulator is subjected.

An insulated electrical cable of this type usually comprises a conductive core formed by a set of wires constituting a strand, for example made of copper, and surrounded by a first layer called "semi-conductor" or "internal layer". The latter is surrounded by an insulating layer, for example of polyethylene, which is itself surrounded by a second "semiconductor" or "outer layer". The latter is in turn surrounded by at least one layer. Usually the layer external semiconductor is surrounded by a layer forming a conductive screen, this being in turn surrounded by a layer or sheath of mechanical protection. The first and second semiconductor layers conventionally consist of a matrix of a polymer, such as polyethylene, which is loaded with carbon black so as to each form an equipotential surface around the cable.

One of the problems posed by this type of cable is that of the slow degradation of the insulation due to the presence of traces of water which have managed to penetrate within the insulating layer of the cable.

These traces of water can have several origins. They can be found in the insulating material, for example polyethylene, at the time of manufacture of the cable or they can penetrate by diffusion to the insulating material, either from the outside of the cable, or from the inside of it. In the latter case, this penetration of water to the insulation may be due to the water remaining in the cable strand or to water entered during the connection of two cable sections.

Under the effect of the electric field prevailing in the insulation and according to mechanisms which are not yet fully understood at present, the insulating material degrades in certain places where water is present. This deterioration is manifested by the appearance of small cavities, typically of the order of a micrometer, which are filled with water and which gradually invade the insulation.

This phenomenon is called "water trees" or "electrochemical trees", because of the characteristic aspect. that these cavities present.

Without wishing to be linked to a particular theory, it seems that these trees can be considered as a phenomenon of mechanical damage to the polymer, similar to that of the formation of cracks in polymers subjected to mechanical stresses in regime of fatigue.

In some cases, these trees can lead to breakdown of the insulation after an indefinite time or duration of service.

This phenomenon constitutes a problem all the more serious as the speed of propagation of the trees increases with the applied voltage.

To try to remedy this drawback, various solutions have been proposed so far, which can be divided into three main categories:

A first category of solutions consists in preventing water from reaching the outside of the cable by means, for example, of a metal screen such as a lead sheath or aluminum foil. In some cases where the metal screen does not provide an absolute seal, a hygroscopic layer is also applied, preferably between the external semiconductor layer and the metal screen.

These solutions are expensive, especially when using a lead sheath, and do not protect against penetration of water by the interior of the cable, that is to say by the conductive core.

A second category of solutions consists in preventing water to penetrate and circulate inside the conductive core of the cable, using a filling material to fill the spaces between the wires constituting the strand of the cable. These solutions are difficult and costly to implement and do not protect against penetration of water from the outside of the cable.

Finally, in a third family of solutions, it is sought to improve the resistance of the insulating layer to degradation by water, without opposing the penetration of water from inside and / or outside of the cable. .

It has thus been proposed to modify the polymer of the insulating layer by grafting or crosslinking, thus obtaining a modified polymer with a structure different from that of the starting polymer.

It has also been proposed to incorporate into the polymer of the insulating layer different types of additives which come to mix with the polymer, without modifying its structure. Among these additives, there may be mentioned in particular antioxidants, as taught by documents FR-A-2,378,338 and DE-A-3,537,438.

The latter solutions have the drawback of requiring the development and manufacture of specific materials which are much more expensive than the corresponding starting materials, that is to say unmodified or without additives.

In addition, modifying the polymer of the insulating layer can be done to the detriment of other properties of the polymer. Thus, in certain cases, the modification of the polymer results in an increase in the angle of loss of the insulator and, consequently, an increase in the dissipation of energy of this insulator. The incorporation of additives in the polymer of the insulating layer can lead to the formation of crystals which are then a source of electrical breakdowns. These crystals may be due to crystallization or recrystallization of the additive or of reaction products of this additive.

Furthermore, it has been proposed to prevent water from reaching the insulating layer of a cable by the use of special substances, as taught by documents FR-A-2,389,204, GB-A-2,032 678 and DE-A-3 507 449.

Note also the use of polyethylene glycol to eliminate the trees of water in an electric cable, as taught by the document EP-A-0 507 604. This substance whose role is apparently to increase the electrical conductivity at within the cable is of limited effectiveness because it tends to diffuse within the cable away from the insulating layer, so that it can no longer play the function which is assigned to it.

It is one of the aims of the invention to remedy the drawbacks of known solutions.

It is, in particular, an object of the invention to provide a method for increasing, by means of a protective substance, the resistance to humidity of a high voltage electric cable comprising an insulating layer in polymer material, for example polyethylene, which does not require modifications to the insulating layer.

The process of the invention is essentially characterized in that the said protective substance is incorporated into an area of the cable situated outside the insulating layer,

in that said protective substance is activatable by water and is at least partly water-soluble so that it can be entrained up to the insulating layer by any trace of water present in the cable opposite the insulating layer with respect to said zone.

and in that said protective substance is, after activation by water, suitable for intervening in electrochemical reactions at the water / polymer material interface or suitable for trapping free radicals resulting from these electrochemical reactions,

which makes it possible to prevent the creation and propagation of water trees in the polymer material of the insulating layer and thus to prevent degradation of the insulating layer by moisture.

Consequently, if there is no penetration of water into the cable and if no trace of water is present in the aforementioned zone of the cable where the protective substance is incorporated, said substance is not activated. and does not perform any function.

On the other hand, if there is water penetration in the cable and if traces of water are then present in the aforementioned zone of the cable, then the protective substance is activated, that is to say made useful.

As this protective substance is at least partly water-soluble, it is then carried along by the traces of water present in the cable, and transported by water to the insulating layer, that is to say precisely at the where it should act.

Without wishing to be bound by a theory, it seems that the action of the protective substance is located between water / polymer face of the insulating layer, that is to say on the internal surface and / or the external surface of the insulating layer. The protective substance, thus activated by water, is capable of intervening in electrochemical reactions at the water / polymer material interface or capable of trapping free radicals resulting from these chemical reactions.

In other words, the mode of action of the protective substance is located at the water / polymer material interface, either to intervene directly in electro-chemical reactions, or to intervene on the consequences of these reactions, it is that is to say, to trap the free radicals resulting from such reactions.

Thanks to this protective substance, which is only activated if traces of water are present in the cable, it is possible to prevent the creation and propagation of water arbo¬ resences in the polymer material of the insulating layer and thus preventing the degradation of this layer by moisture.

The invention provides for incorporating the protective substance in a region located either on the inside of the insulating layer or on the outside of this insulating layer, so that it can be entrained by any trace of water which may be present or which would filter either from the inside or the outside of the cable.

The method of the invention applies in particular to cables of the type further comprising an internal semiconductor layer located between the conductive core and the insulating layer and an outer semiconductive layer surrounding the insulating layer.

In a preferred embodiment of the invention, said protective substance is then incorporated into the material of the internal semiconductor layer and / or of the external semiconductor layer.

This material is, in a manner known per se, usually consisting of a polymer, such as polyethylene, loaded with carbon black.

According to the invention, the protective substance is then mixed with the polymer and with carbon black during the manufacture of the material intended to form the semiconductor layer.

A new material is thus obtained, which also constitutes another aspect of the invention.

To ensure the mixing of the protective substance with the polymer and with carbon black, this substance is advantageously presented in the form of a particle size powder, preferably less than 1 micrometer, for example between 0.1 and 0.2 micrometer.

This powder is present in the mixture in a relatively low proportion, for example less than 5% by weight.

The manufacture of this new material is carried out by means of a suitable mixer, as is well known in the polymer manufacturing technique.

In another embodiment of the invention, the protective substance is applied to an area of the cable located towards the inside and / or outside of the cable with respect to the insulating layer.

In the case where the cable further comprises a layer internal semiconductor located between the conductive core and the insulating layer and an external semiconductive layer surrounding this insulating layer, the protective substance is then applied to an area of the cable located inwardly relative to the layer internal semiconductor and outside this layer and / or on an area of the cable situated towards the outside with respect to the external semiconductor layer and outside this layer.

Preferably, the protective substance is applied to the conductive core, since this is the region of the cable where the electric field is the most intense, and this before manufacture of the cable. This embodiment is therefore particularly suitable for preventing the degradation of the insulating layer by penetration of water through the interior of the cable, that is to say by the conductive core.

In this case, the application of the protective substance can be done over the entire length of the conductive core or else only at the ends of the conductive core.

In this embodiment of the invention, the protective substance is advantageously constituted by a powder having the particle size indicated above.

The application of the protective substance can be in the form of an aqueous solution or suspension, in particular by spraying or immersion, followed by drying.

Thus, in the case where the protective substance is applied over the entire length of the conductive core of the cable, this application can be done by immersion in a bath or else by spraying using one or more suitable nozzles.

Alternatively, the protective substance can be applied in the form of a powder, which then avoids the need for a drying operation. The powdering operation can be carried out using any suitable device, such as powder sprayers.

In another variant, the protective substance is applied by means of a textile tape impregnated with said substance, this tape thus forming another layer of the cable. The impregnation of the tape can be carried out in the form of an aqueous solution or suspension or in the form of a powder. This tape can be applied directly around the conductive core and / or directly around another part of the cable, for example around the external semiconductor layer.

The protective substance to be used in the context of the invention can be chosen from different types of substances capable of preventing degradation of the insulating layer by humidity.

These are essentially chemical substances capable of intervening in electrochemical reactions or capable of trapping free radicals resulting from these electrochemical reactions, so as to prevent the creation and propagation of water trees in the polymer material of the insulating layer.

These chemicals must have at least one of the following properties:

- property of delaying the appearance of trees,

property of reducing the speed of propagation of trees,

- property of giving a less dense internal structure trees.

In all cases, these chemical substances have the property of delaying the breakdown of the insulating material due to these trees.

The protective substance can be a "free radiophobic trap", that is to say a substance preventing the formation of such radicals or reacting on these radicals to make them inactive.

These "free radicals" can come, for example, from the decomposition of the residues of hydroperoxides present in the polymeric material or possibly appeared in this material following electrochemical reactions.

These radicals are responsible for the chain reaction of degradation of the polymeric material of the insulating layer, for example polyethylene. In particular, they make possible an action of oxygen.

The free radical scavengers that can be used in the context of the invention must satisfy a certain number of constraints. A priori, all free radical scavengers that are usually incorporated into polyethylene or other polymers used for the manufacture of insulating layers of electric cables are suitable. These substances must be stable at the temperatures to which the cable is subjected during its manufacture, and soluble at least partly in water at the maximum operating temperature of the cable, that is to say of the order of 80 ° C when this antioxidant is located inside the cable, that is to say on the internal side of the insulating layer.

Among the free radical scavengers that can be used, may cite those belonging to the class of amines.

Preferably, n-phenyl-n * -cyclohexyl-p-phenylene diamine (PCPD) is used, although its solubility in water is relatively low.

It is also possible to use other amines, for example n, n'-diphenyl-p-phenylene diamine (DPPD), available for example from the company Bayer, chemically modified to increase their solubility in water.

One can also use free radical scavengers belonging to the class of phenols.

The protective substance can also be constituted by a substance capable of intervening in redox phenomena and having at least one of the properties indicated above.

In particular, it may be a substance comprising trivalent ions such as, for example, Fe ^ ^"1" or l- ⁺ ions. By way of example, it is possible to use, as a protective substance, FeCl3 or AICI3.

It is also possible to use substances comprising bivalent ions, for example Cu ⁺ ions.

Of course, these substances involved in oxidation-reduction phenomena must have the same properties of temperature stability and solubility as those indicated above for the free radical scavengers.

The invention relates, in another aspect, to the high voltage cables obtained by implementing the method.

In the following description, made only as example, reference is made to the appended drawing, in which:

the single figure is a schematic sectional representation of a high voltage cable to which the invention applies.

The cable shown in section in the drawing comprises a conductive core 1 formed of a set of wires constituting a strand of an electrically conductive material, preferably copper. The conductive core 1 is surrounded by a first semiconductor layer 2, also called "internal layer", formed of a mixture of a polymer, such as polyethylene and carbon black, to form a first equipotential surface around of the conductive core. The semiconductor layer 2 is surrounded by an insulating layer 3 of a polymer material, for example polyethylene. This layer 3 forming an insulating sheath is itself surrounded by a second semiconductor layer 4 or "outer layer". Formed from the same material as the inner layer 2. This layer 4 forms a second equipotential surface around the conductive core .

The semiconductor layer 4 is surrounded by a metal screen 5, for example made of aluminum, the latter itself being surrounded by an insulating protective sheath 6.

The metal cable, the structure of which has just been described above, constitutes a cable in itself known, capable of transporting current under particularly high electrical voltages, which can reach for example 400-kilovolts.

For example, the conductive core 1 can have a total diameter of 30 mm, the semiconductor layers 2 and 4 a thickness of 2 mm, and the insulating layer 3 a thickness of 30 mm.

In the case where the protective substance is incorporated into the semiconductor layer 2 and / or the semiconductor layer 4, this incorporation preferably takes place in the two layers, the material constituting them being the same .

For this, during the manufacture of the material, the protective substance is incorporated into the mixture of the polymer material, for example polyethylene, and carbon black.

Preferably, the protective substance is incorporated into the mixture in the form of a powder with a particle size of less than 1 micrometer, for example between 0.1 and 0.2 micrometer, this powder being present in a proportion of less than 5% by weight relative to the total mixture.

The preparation of this material is carried out according to the same processes as those applied in the polymer manufacturing technique.

In a preferred embodiment of the invention, the protective substance incorporated into the material of the semiconductor layers 2 and 4 is n-phenyl n'-cyclohexyl-p-phenylene diamine. This substance is available for example from the company BAYER and is in the form of grains or flakes which should be ground to obtain the desired particle size.

Of course, other protective substances may be incorporated into the material of the semiconductor layers 2 and 4, whether they be free radical scavengers, for example amines or phenols, or else intervening substances. nant in redox phenomena, such as trivalent ions or bivalent ions.

In the case where an electrical cable is manufactured, in which the internal semiconductor layer 2 and the external semiconductor layer 4 incorporate a protective substance according to the invention, the other elements of the cable also remain unchanged. In particular, it is not necessary to modify the insulating layer 3 which remains made from polyethylene or another suitable polymer material, unmodified.

It is not necessary to take rigorous precautions to prevent the introduction of water inside and / or outside the cable. In particular, it is not necessary to provide an external waterproof sheath of metallic material such as a lead sheath.

In another embodiment of the invention, the protective substance is applied to the conductive core 1 before manufacturing the cable. For this, it is possible, for example, to pass the conductive core 1 through an aqueous solution or suspension containing said protective substance, or else spray this solution in suspension on the conductive core.

The application of this solution or suspension can be done over the entire length of the conductive core or else only at the ends thereof, in the case where it is sufficient to guard against the effects of possible penetration of water when connecting sections of electric cables.

In the case where the protective substance is applied in the form of an aqueous solution or suspension, it is necessary to carry out a drying operation after the operation of applying the solution or suspen¬ sion.

Instead of using a solution or suspension, the protective substance can be applied directly in the form of a powder, and this by means of a powdering device known per se. In this case, it is not necessary to carry out a drying operation after the operation of applying the protective substance.

Of course, the protective substance can be applied to the conductive core of the cable and / or to other parts of the cable situated on the outside of the cable with respect to the external semiconductor layer.

In another variant, as indicated above, the protective substance permeates a textile tape which forms another layer of the cable. This tape can be applied directly around the conductive core and / or around another part of the cable, for example around the external semiconductor layer.

To impregnate the textile tape, the protective substance may be present in the form of an aqueous solution or suspension or else in the form of a powder.

It should be noted that the method of the invention applies only to protection against trees which arise from the internal or external surface of the insulating layer. It does not apply to trees which arise within the insulating layer and which are called "bow-tie trees" and which are mainly due to water which pre-exists in the mass of the polymer. The latter do not constitute a serious danger for the isolation, since they have a limited volume, the quantity of available water being limited, and that they do not generally lead to breakdown.

Claims

claims

1.- Method for increasing, by means of a protective substance, the resistance to humidity of a high-voltage electric cable comprising a conductive core surrounded by an insulating layer of polymeric material, for example polyethylene,

characterized in that the said protective substance is incorporated into an area of the cable situated outside the insulating layer,

in that said protective substance is activatable by water and is at least partly water-soluble so as to be able to be entrained up to the insulating layer by any trace of water present in the cable opposite the insulating layer with respect to to said area,

2.- The method of claim 1, wherein the cable further comprises an inner semiconductor layer located between the conductive core and the insulating layer and an outer semiconductor layer surrounding the insulating layer, characterized in that incorporates said protective substance into the material of the semiconductor layer internal and / or external semiconductor layer.

3.- Method according to claim 2, wherein the material of the inner and outer semiconductor layers is a polymer, such as polyethylene, loaded with carbon black, characterized in that the protective substance is mixed with the polymer and carbon black during the manufacturing of the material.

4.- Method according to claim 1, characterized in that the protective substance is applied to an area of the cable located towards the inside and / or outside of the cable with respect to the insulating layer.

5.- Method according to claim 4, wherein the cable further comprises an inner semiconductor layer located between the conductive core and the insulating layer and an outer semiconductor layer surrounding the insulating layer, characterized in that applies the protective substance to an area of the cable located inwardly relative to the internal semiconductor layer and outside this layer and / or to an area of the cable located outwardly relative to the semi-conductive layer conductive externally and outside this layer.

6.- Method according to one of claims 4 and 5, caracté¬ ized in that the protective substance is applied to the conductive core before manufacture of the cable.

7.- Method according to claim 6, characterized in that the protective substance is applied over the entire length of the conductive core.

8.- Method according to claim 6, characterized in that the protective substance is applied to the ends of the conductive core.

9.- Method according to one of claims 4 to 8, caracté¬ ized in that one applies the protective substance in the form of an aqueous solution or suspension by spraying or immersion, followed by drying.

10.- Method according to one of claims 4 to 8, caracté¬ ized in that one applies the protective substance in the form of a powder.

11.- Method according to one of claims 4 to 7, caracté¬ ized in that the protective substance is applied by means of a textile tape impregnated with said substance, this tape thus forming another layer of the cable .

12.- Method according to one of claims 1 to 11, caracté¬ ized in that the protective substance is a powder of particle size less than 1 micrometer, this powder being present in the mixture in a proportion less than 5% in weight.

13.- Method according to one of claims 1 to 12, charac¬ terized in that the protective substance is a scavenger of free radicals.

14.- Method according to claim 13, characterized in that the free radical scavenger is an amine.

15.- Method according to claim 14, characterized in that the free radical scavenger is n-phenyl n'-cyclohexyl-p-phenylene diamine.

16.- Method according to claim 13, characterized in that the free radical scavenger is a phenol.

17.- Method according to one of claims 1 to 12, charac¬ terized in that the protective substance is a substance suitable for intervening in electrochemical phenomena.

18.- Method according to claim 17, characterized in that the protective substance comprises trival ions such as Fe ⁺⁺⁺ or Al ⁺⁺⁺ .

19.- As a new material for the implementation of the method according to one of claims 2 and 3, a material consisting of a mixture of a polymeric material, for example polyethylene, carbon black and a protective substance as defined in claim 1.

20.- High voltage cable obtained by the implementation of the method according to one of claims 1 to 18.