WO2003046592A1

WO2003046592A1 - Method for testing an electrical cable, modified electrical cable and process for producing it

Info

Publication number: WO2003046592A1
Application number: PCT/IB2001/002234
Authority: WO
Inventors: Luca Balconi; Alberto Bareggi; Sergio Belli
Original assignee: Pirelli & C. S.P.A.
Priority date: 2001-11-27
Filing date: 2001-11-27
Publication date: 2003-06-05
Also published as: AU2002223949A1

Abstract

The present invention relates to a method for the, electrical testing of a protective polymeric layer of an electrical cable comprising at least one metal element in a position radially internal to said protective polymeric layer. This method comprises the steps of applying a semiconductive polymeric layer in a position radially external to said protective polymeric layer, and of electrically connecting said semiconductive polymeric layer to said at least one metal layer. The present invention also relates to an electrical cable comprising a semiconductive polymeric layer in a position radially external to said protective polymeric layer, and also to a process for producing said cable.

Description

METHOD FOR TESTING AN ELECTRICAL CABLE, MODIFIED ELECTRICAL CABLE AND PROCESS FOR PRODUCING IT

The present invention relates to a method for the electrical testing of the outer polymeric sheath of an electrical cable, which is provided to protect and complete said cable.

In particular, the present invention relates to a method for the electrical testing of the outer polymeric sheath of an electrical cable for medium, high or extra high voltage power transmission or distribution.

In greater detail, the present invention relates to a method for checking the structural integrity of the outer polymeric sheath of an electrical cable, said method being able to be applied after the process for manufacturing said cable, directly at the production plant and/or after the operation for laying said cable.

The present invention also relates to an electrical cable whose structure is modified so as to enable the testing method according to the invention to be performed.

More particularly, the present invention relates to a modified electrical cable in which a semiconductive polymeric layer is arranged in a position radially external to the outer protective polymeric sheath which coats said cable.

The present invention also relates to a process for producing an electrical cable which is modified to enable the electrical testing method mentioned above to be performed.

In the present description, the expression "low voltage" means a voltage of less than about 1 kV, the expression "medium voltage" means a voltage of between about 1 kV and about 30 kV, the expression "high voltage" means a voltage of between about 30 kV and about 220 kV, while the expression "extra high voltage" means a voltage of greater than about 220 kV.

In the present description, the expression "cable core" refers to a semifinished structure comprising: a conductive element, an inner semiconductive layer arranged in a position radially external to said conductive element, a layer of electrical insulation arranged in a position radially external to said inner semiconductive layer, and an outer semiconductive layer arranged in a position radially external to said layer of electrical insulation.

For the purposes of the present description, the expression "unipolar cable" means a cable provided with a single core as defined above, while the expression "multipolar cable" means a cable provided with at least one pair of said cores. In greater detail, when the multipolar cable has a number of cores equal to two, said cable is technically defined as being a "bipolar cable", if there are three cores, said cable is known as a "tripolar cable", and so on.

In the case where a unipolar cable for medium voltage, high voltage or extra high voltage power transmission or distribution is considered, said cable generally has a metal screen in a position radially external to said core.

Usually, said metal screen is made of aluminium, steel, lead or copper.

In general, said metal screen consists of a continuous tubular component or a metal sheet folded on itself and welded or sealed to form said tubular component. In this way, said screen has a two-fold function: firstly, it ensures leaktightness of the cable to any water penetration in the radial direction, and secondly, it provides a function of electrical type by creating inside the cable, by means of the direct contact between the metal screen and the outer semiconductive layer of said core, a uniform electrical field of radial type, and at the same time nullifying the electrical field outside the cable. A further function is also that of supporting short-circuit currents.

In a configuration of unipolar type, said cable also has a protective polymeric sheath arranged in a position radially external to the abovementioned metal screen.

During the phases of transporting or laying a cable, accidental impacts can occur on said cable which may cause damages, even of considerable entity, to its structure (for example deformations of the insulating layer or detachment between the layers constituting the cable) , said damages possibly resulting in variations in the electrical gradient of the insulating layer with a consequent reduction in the insulating capacity of said layer.

For the purpose of protecting electrical cables for power transmission or distribution against damages caused by accidental impacts, said cables are generally provided with a metal armouring so as to give said cables suitable mechanical strength.

Said armouring, conventionally arranged in a position radially internal to said protective polymeric sheath, may be in the form of metal strips or wires (generally made of steel) or in the form of a metal sheath

(generally made of lead or aluminium) . An example of said cable structure is described in US patent N. 5,153,381.

As described in document WO 98/52197 in the name of the Applicant, said electrical cables for power transmission or distribution may comprise, in replacement for said metal armouring, a coating made of expanded polymeric material of suitable thickness capable of giving the cables high resistance to impacts. Preferably, said expanded polymeric coating is located in a position immediately below to said outer protective polymeric sheath.

In the case of a multipolar cable for medium voltage power transmission or distribution, the cores of said cable are generally combined together, for example by means of a helical winding of predetermined pitch.

Said winding results in the formation of a plurality of interstitial zones, i.e. the spaces between said cores, which are filled with a filling material which serves to give said multipolar cable a peripheral profile, in right cross section, of circular type so that a correct application of the subsequent coatings of the multipolar cable can be carried out, in a position radially external to said combined cores.

Said filling material may be of conventional type, for example a polymeric material applied by extrusion, or may be an expanded polymeric material similar to that used to make the expanded layer resistant to accidental impacts as mentioned above.

The multipolar cable with combined cores thereby obtained is then completed by applying at least one further coating, the nature of which, and also the number of coatings which may be used, depending on the type of cable to be obtained. For example, in accordance with one construction method of conventional type performed by means of techniques known in the art, in a position radially external to the abovementioned semifinished cable thereby obtained, it is possible to apply, successively, a metal armouring or a layer made of expanded polymeric material, and also an outer protective polymeric sheath as described above with reference to a unipolar cable. In certain cases, the application of the metal armouring is preceded by applying an inner polymeric sheath suitable for giving the combined component mechanical protection against said metal armouring.

In the case of a multipolar cable for high voltage or extra high voltage power transmission or distribution, the individual phases, each comprising a single core, are generally arranged substantially parallel to each other without any winding of said phases being carried out.

Once completed, an electrical cable conventionally undergoes checking according to conventional testing methods intended to evaluate the structural quality of the cable, that is to say the possible presence of defects present in one or more constitutive components of the cable, said defects possibly arising during the process for producing said cable and/or possibly being caused by the process for laying the cable.

Among the testing methods to which a cable is conventionally subjected there is also the electrical testing of the protective polymeric sheath arranged in a position radially external to the core of said cable.

Said protective sheath usually represents the radially outermost coating layer of the cable and has the function of being a protective barrier to the external environment (for example, as a barrier to the penetration of water or moisture into the cable) , and also a mechanically strong component, for example intended to give said cable a desired mechanical strength with respect to impacts or abrasion. Said sheath can also have a function of electrical insulation, although only moderate, of the metal screen inside which flow induced currents caused by the presence of an electromagnetic field generated by the conductor during the passage of an electrical current through it.

The abovementioned electrical testing, also known as an electrical withstand test, of said protective polymeric sheath is intended to check the structural integrity of said protective sheath.

More particularly, said testing consists in checking for the possible presence of defects in the sheath, such as, for example, fissures, formed during the process for producing the cable, or damages, for example, cuts, produced following the operations for laying the cable.

In general, the testing of the protective sheath of a cable is performed directly at the production plant after the process for producing the cable.

Sometimes, said testing is also repeated once the cable has been installed, so as to check for any evidence of damage produced in the sheath due to the laying operations of the cable.

Repeating the testing once the cable has been installed is particularly desirable, especially in the case of underground installations in which the electrical cable is placed directly in the ground without the aid of conduits to contain it. Specifically, the installation of a cable by means of using containing duct is a solution that is capable of ensuring better protection of the structural integrity of said cable during the laying phase and, therefore, said testing is generally not repeated in this case.

On the contrary, said testing method is generally repeated in the case of cable installations for high voltage or extra high voltage power transmission or distribution, said cables being traditionally arranged directly in the laying trench.

Any defect in and/or damage to the protective polymeric sheath of the cable constitute a discontinuity in the polymeric material of the protective layer, which gives rise to problems that reduce, even, drastically, the cable's capacity for power transmission and distribution, and also the cable's life.

For example, the presence of an incision in the outer polymeric sheath of the cable represents a preferential route for the entry of water or moisture to the interior (that is to say towards the core) of the cable.

The entry of water into a cable is particularly undesirable since, in the absence of suitable solutions provided to stop the leak, once the water has entered, it is able to run freely inside the cable. This particularly causes damages in terms of the integrity of the cable, since corrosion problems (affecting, for example, the armouring, if present, or the metal screen) may arise inside the cable, as well as problems of premature ageing with degradation of the electrical properties of the insulating layer. Said phenomenon of premature ageing is better known with the term "water treeing" and is manifested by the formation of micro- fractures of branched shape ("trees") due to the combined action of the electrical field generated by the passage of current in the conductor, and of the moisture that has penetrated into said insulating layer .

In addition, once it flows onto joints, terminals or any apparatus connected to an end of the cable, the water not only interrupts the functioning of the cable, but also damages said apparatuses causing damages that frequently are irreversible and very expensive.

One testing method known in the art and commonly used to evaluate the structural integrity of the outer protective polymeric sheath of an electrical cable involves installing an electrically conductive layer placed in a position radially external to said sheath.

In greater detail, said testing method (described in IEC Standard - Publication 229 - Second Edition - 1982 - page 7 - paragraph 3.1) consists in applying, by means of a voltage generator, a preset DC voltage between said electrically conductive layer and the metal layer immediately below said sheath, said metal layer being represented by the metal screen or the metal armouring depending on the type of electrical cable under consideration.

On the basis of said testing method, if the protective polymeric sheath has any major damage or defect, such as, for example, an incision substantially equal in depth to the entire thickness of the sheath, said electrically conductive layer and said metal layer come into electrical contact via the abovementioned incision.

It should be pointed out that if said incision is not equal in depth to the thickness of the protective sheath and if it stops close to said metal layer, the passage of electrical current inside said incision produces an electric discharge to complete the abovementioned incision down to the metal layer, thus achieving the electrical contact mentioned above.

Said electrical contact between the electrically conductive layer and the metal layer thus results in the establishment of a short-circuit which is identified by a person skilled in the art in the form of the detecting of an overcurrent which the generator mentioned above is unable to cope with.

The IEC standard mentioned above envisages that the electrically conductive layer, applied to the protective sheath to be subjected to electrical testing, consists of a graphite coating obtained by applying said graphite in liquid form or in solid form (for example in the form of powder) .

In accordance with said testing method, the abovementioned graphite coating serves as a first electrode, while the second electrode is represented by the metal component arranged in a radially internal position relative to the sheath to be tested.

In this way, it is possible to check the electrical continuity of the sheath, and thus its integrity, since, in the absence of defects and/or damages, the sheath is capable of withstanding the voltage applied between said electrodes.

In other words, in the absence of defects in and/or damages to the sheath, whenever it is necessary to measure the voltage at the end of the cable that is opposite to the end at which the DC voltage is applied between the abovementioned first and second electrodes, the voltage measured will be substantially unchanged relative to the applied voltage, since the electrical current will be able to pass undisturbed in the graphite coating and in the metal component immediately below the sheath from one end of the cable to the other, apart from a small reduction in voltage due to the resistance of the sheath.

If, however, said sheath has a defect and/or damage such as to create an electrically conductive path in the thickness of the sheath between the electrodes mentioned above, the electrical circuit described above is in a short-circuit condition and an overcurrent is produced which, above a given threshold, causes disconnection of the voltage generator which is no longer capable of supplying the required voltage to the circuit .

The establishment of said overcurrent condition thus enables a person skilled in the art to confirm the presence of damage to and/or a defect in the protective sheath of the cable.

In the case of a multipolar cable for medium voltage power transmission or distribution not provided with armouring, said testing method involves applying a DC voltage between the electrically conductive layer and the metal screen present in one of the phases of said cable. In this case, specifically, the metal screens present in each of the phases are generally in a condition of mutual contact in at least one point and, therefore, the electrical connection with the voltage generator can be made with any one of the metal screens present in the cable under consideration.

In the case of a multipolar cable for high voltage or extra high voltage power transmission or distribution in which, as mentioned above, the individual phases are arranged substantially parallel to each other, said testing method calls for the testing to be carried out on each individual phase.

In accordance with the IEC standard mentioned above, the electrically conductive coating consisting of the layer of graphite can be replaced with the electrical conductivity of water by means of immersing in water the cable wound on a reel. In this way, by keeping the ends of the cable to be tested out of the immersion tank, the water is used as the first electrode to which the voltage is applied in a manner similar to that described above, while the second electrode is again represented by the metal component which is below the sheath to be tested.

Document DE-3,931,340 describes a further method for the electrical testing of the outer protective sheath of an electrical cable, said method involving the use of a gas introduced under pressure into the cable at one end, while the opposite end is appropriately sealed off. Said gas is made to travel inside cavities which are obtained in the space between the insulating layer and the protective sheath to be tested of this cable. Any defects and/or damages present in said sheath are detected by the use of a suitable detecting device equipped with a sensor for detecting said gas.

Document US-4,370,610 describes a further method for the electrical testing of the outer protective sheath of an electrical cable, said method involving applying DC pulses to said sheath at a given point in the cable and searching along the length of the cable, by using a magnetometer, for the magnetic field generated by the breakdown current towards earth circulating in the sheath, said current being produced by applying said pulses. The Applicant has found that the electrical testing methods of the prior art have several drawbacks.

In particular, the method for testing the protective sheath of an electrical cable comprising the step of applying a graphite coating to said sheath has several drawbacks mainly connected with the deposition of said graphite coating.

In fact, in general, the graphite coating is obtained by by means of two possible procedures according to which the graphite is deposited either in solid form

(graphite powder) or in liquid form (for example dispersed in water) .

In detail, when graphite powder is used, the deposition process involves the step of moving the cable to be coated inside a chamber containing graphite powder.

Thus, after passing the cable into the bath of graphite powder, said powder becomes deposited on the outer sheath of the cable by means also of the action of rotatable brushes arranged downstream of said chamber, said brushes having the function of producing the best possible adhesion of the graphite powder to said sheath and, simultaneously, of removing the graphite in excess .

According to one variant, in order to obtain better adhesion of the graphite to the outer protective sheath, said sheath is electrostatically charged, for example by passing the cable through a solenoid in which a given electrical current passes.

When graphite in liquid form is used, the deposition process involves the step of moving the electrical cable in a chamber provided with nozzles or drips capable of bathing the outer surface of the protective sheath of said cable.

In accordance with said procedure, a system of rotatable brushes is also provided downstream of said nozzles/drips and, as mentioned above, said brushes have the function of spreading said liquid so as to wet the entire outer surface of the cable and to remove the excess of liquid.

Said deposition method also involves a step of drying said liquid before the step of storing the cable thus finished on a reel.

However, the Applicant has found that depositing a coating of graphite, either in the form of powder or liquid, involves a number of problems.

In fact, firstly, the methods mentioned above do not ensure the deposition of a graphite coating of uniform thickness, this being confirmed by the case where said coating has an excessively high thickness in certain portions of cable and, simultaneously, is entirely absent or is present in insufficient amounts in other portions of said cable.

In other words, said deposition methods are incapable of evaluating, and even less of correcting, the amount of graphite actually deposited on the protective sheath of the cable.

Said aspect is a particular disadvantage since the presence of discontinuities in the graphite coating does not allow a correct and reliable execution of the electrical testing of said sheath.

A further problem is represented by the adhesion of the graphite to the sheath which is below either in the case where it is deposited in powder form or in liquid form.

Above all, in the case where graphite powder is used, the problem of adhesion is particularly acute since said adhesion is mainly ensured by the degree of rugosity of the sheath and by the action of compacting and spreading said graphite on said sheath performed by the abovementioned rotatable brushes .

Thus, especially during the operations for storing the cable on a reel and also during the operations for laying the cable in a trench, due to accidental impacts and/or abrasion, said graphite coating may be partially or totally removed with the inevitable ^consequences mentioned above.

In the case where the conductive coating is obtained by depositing graphite in liquid form, the adhesion to the protective sheath, which is below the conductive coating, is greater than in the case where said graphite is deposited in powder form. However, there is a greater risk of said liquid not being distributed uniformly over the entire outer surface of the cable or of being completely removed therefrom as a result of the action of the rotatable brushes or as a result of the sliding of the liquid on the outer surface of the cable.

Moreover, in said latter case, the deposition process has the further drawback of requiring a further drying step so as to fix the graphite coating on said sheath, said step thus making the deposition process more complex and expensive.

As previously mentioned, in order to ensure better adhesion between the graphite coating and the protective sheath, the deposition methods mentioned above can involve an initial step in which the outer surface of said sheath is electrostatically charged.

However, said step has a negative impact both on the complexity of the plant for producing the cable, and on the costs of said plant.

The methods for applying the graphite coating mentioned above moreover have non-negligible problems in terms of contamination and safety of the working environment.

Said aspect is particularly critical when said coating is obtained from graphite in powder form since, as said powder is of extremely fine particle size (for example of the order of few microns) , it is readily dispersed into the atmosphere and can be inhaled by the personnel working on the deposition line.

The Applicant has also found that the further methods for the electrical testing of the outer protective sheath of an electrical cable that are known in the art, such as, for example, those described above with reference to documents DE-3,931,340 and US-4, 370, 610, are difficult to be carried out because they are complex and expensive to carry out.

Thus, the Applicant has found that it is necessary to develop a method for the electrical testing of the outer protective sheath of an electrical cable that is simple and economical to be performed, and also highly reliable compared with the techniques conventionally used in the prior art.

In particular, with reference to the abovementioned IEC standard, said standard conventionally being used in the sector for the simplicity and speed of the testing method codified therein, the Applicant set itself the objective of overcoming the drawbacks related to the use of a graphite layer as a conductive coating placed in a position radially external to the protective sheath of the cable, and also to the methods for depositing said graphite layer.

In particular, the Applicant has found a simple, fast and safe method for the electrical testing of the protective sheath of an electrical cable, said method comprising the step of depositing by extrusion a conductive coating in a position radially external to said sheath during the process of manufacturing the cable.

Specifically, the Applicant has found that said method makes it possible to provide, in a position radially external to the protective sheath of an electrical cable, for an electrically conductive coating which is uniformly distributed over said sheath and firmly adhered thereto.

In other words, the Applicant has found that producing the abovementioned conductive coating by means of a deposition step by extrusion makes it possible to considerably increase the reliability of the testing method since it makes possible to produce a conductive layer of predetermined and substantially constant thickness over the protective sheath of the cable during the process for producing said cable.

Thus, in accordance with the present invention, the Applicant has found that, in comparison with the testing methods of the prior art, many of the drawbacks among those mentioned above can be successfully overcome by producing an electrically conductive coating layer by means of depositing by extrusion a suitably charged polymeric material so as to ensure the desired electrical conductivity of said layer. Thus, in a first aspect, the present invention relates to a method for testing a protective polymeric layer of an electrical cable comprising at least one metal component in a position radially internal to said protective polymeric layer, said method comprising the steps of:

• applying a semiconductive polymeric layer in a position radially external to said protective polymeric layer, and

• electrically connecting said semiconductive polymeric layer to said at least one metal layer.

According to one embodiment, said step of electrically connecting comprises the step of connecting the semiconductive polymeric layer to at least one metal screen of said cable.

According to another embodiment, said step of electrically connecting comprises the step of connecting the semiconductive polymeric layer to a metal armouring of said cable.

According to one embodiment, said step of applying comprises the step of depositing said semiconductive polymeric layer by extrusion.

According to another embodiment, said application step comprises the step of depositing said semiconductive polymeric layer and said protective polymeric layer by co-extrusion.

In a second aspect thereof, the present invention relates to an electrical cable comprising:

• at least one core comprising at least one conductive element and at least one layer of electrical insulation in a position radially external to said at least one conductive element;

• at least one metal element placed in a position radially external to said at least one core, and

• a protective polymeric layer in a position radially external to said at least one metal element, characterized in that said cable further comprises a semiconductive polymeric layer in a position radially external to said protective polymeric layer.

Preferably, said semiconductive polymeric layer is placed in contact with said protective polymeric layer.

According to one embodiment, said semiconductive polymeric layer is compact.

According to another embodiment, said semiconductive polymeric layer is expanded.

In a further aspect thereof, the present invention relates to a process for producing an electrical cable comprising at least one core and a protective polymeric layer in a position radially external to said at least one core, said process comprising the step of depositing by extrusion a layer of semiconductive polymeric material in a position radially external to said protective polymeric layer.

In one embodiment thereof, said process comprises the step of depositing said layer of semiconductive polymeric material and said protective polymeric layer by co-extrusion.

The present description, given hereinbelow, makes reference to the attached drawings, which are provided purely for the purpose of illustration and with no intended limitation, in which:

- Figure 1 is a view in right cross section of one particular embodiment of a unipolar electrical cable for power transmission or distribution according to the invention;

- Figure 2 is a view in right cross section of a variant of the embodiment of Figure 1;

- Figure 3 is a view in right cross section of one particular embodiment of a tripolar electrical cable according to the invention;

- Figure 4 is a view in right cross section of a variant of the embodiment of Figure 3, and

- Figure 5 is a view in right cross section of a further variant of the embodiment of Figure 3.

The present invention may advantageously be applied not only to electrical cables for power transmission or distribution, but also to data transmission cables or cables of mixed power/ elecommunications type. Thus, in this sense, for the purposes of the present description, the term "conductive element" means a conductor of metallic type, of circular or sectorial configuration, or of mixed electrical/optical type.

For the purpose of simplifying the description, in the attached figures, similar or identical components have been given the same reference numerals.

Figure 1 illustrates in cross section a unipolar electrical cable 10 for power transmission or distribution. In detail, said cable 10 comprises: a central conductor 11, an inner semiconductive coating 12 radially external to said central conductor 11, a layer of electrical insulation 13 in a position radially external to said inner semiconductive coating 12 , and an outer semiconductive coating 14. Said semifinished structure constitutes the core 15 of the cable 10.

Said layer of electrical insulation 13 may consist of a crosslinked or non-crosslinked polymeric composition with electrical insulation properties, which is known in the art and chosen, for example, from: polyolefins

(homopolymers or copolymers of various olefins) , olefin/ethylenically unsaturated ester copolymers, polyesters, polyethers, polyether/polyester copolymers, and blends thereof. Examples of such polymers are: polyethylene (PE) , in particular linear low-density polyethylene (LLDPE) ; polypropylene (PP) ; propylene/ ethylene thermoplastic copolymers; ethylene-propylene rubbers (EPR) or ethylene-propylene-diene rubbers (EPDM) ; natural rubbers; butyl rubbers; ethylene/vinyl acetate (EVA) copolymers; ethylene/methyl acrylate (EMA) copolymers; ethylene/ethyl acrylate (EEA) copolymers; ethylene/butyl acrylate (EBA) copolymers; ethylene/α-olefin copolymers, and the like.

According to one embodiment, which is not shown, the unipolar cable 10 may be provided, in a position radially external to the outer semiconductive layer 14, with a water-swellable tape capable of acting as a barrier to the penetration of water into the core 15 of the cable .

With reference to Figure 1, in a position radially external to said core 15, the cable 10 also comprises a metal screen 16 and an outer protective polymeric sheath 17. In accordance with the present invention, in a position radially external to said protective polymeric sheath 17, the cable 10 has a semiconductive polymeric layer 18 which, as mentioned above, acts as an electrode during the electrical testing of said outer protective polymeric sheath 17 according to the abovementioned IEC standard.

Said semiconductive polymeric layer 18 may be obtained from a crosslinked or non-crosslinked polymeric composition chosen, for example, from the polymers mentioned above, made semiconductive by introducing a conductive material .

Preferably, said conductive material is an electrically conductive carbon black such as, for example, acetylene black, furnace black or the like.

In general, a carbon black with a surface area of greater than 20 m²/g and preferably between 40 and 500 m²/g is used.

However, it is also possible to use a highly conductive carbon black, with a surface area of at least 900 m²/g, such as, for example, the furnace black known commercially by the brand name Ketjenblack® EC (Akzo Chemie NV) .

The amount of carbon black to be added to the polymeric matrix may vary as a function of the type of polymer and of carbon black used.

However, said amount needs to be such that it gives the polymeric material sufficient semiconductive properties, in particular so as to obtain a volume resistivity for the polymeric material, at ambient temperature, of less than 500 Ω-m, preferably less than 20 Ω-m. Typically, the amount of carbon black may range between 5 and 80%, preferably between 10 and 70% by weight relative to the weight of the polymer.

For an electrical cable for medium voltage, high voltage or extra high voltage power transmission or distribution, the thickness of said semiconductive polymeric layer 18 according to the present invention is preferably between 0.05 mm and 3 mm and more preferably between 0.2 mm and 0.8 mm.

In accordance with the present invention, said semiconductive polymeric layer 18 may be compact or expanded.

For the purposes of the present description, the expression "compact polymeric layer" means a layer of non-expanded polymeric material, that is to say a material with a zero degree of expansion.

For the purposes of the present description, the expression "expanded polymeric layer" means a layer of polymeric material in which is provided a predetermined percentage of "free" space, that is to say of space not occupied by the polymeric material, but instead by gas or air.

In general, said percentage of free space in an expanded polymer is expressed by means of the "expansion degree" (G) , defined as follows:

G = (do/de - 1)*100

in which d₀ indicates the density of the non-expanded polymer and d_e indicates the apparent density measured for the expanded polymer. The expanded semiconductive polymeric layer according to the present invention is obtained from an expandable polymer optionally subjected to crosslinking, after expansion, as indicated in greater detail in the present description hereinbelow.

Said expandable polymer may be chosen from the group comprising: polyolefins, various olefin copolymers, olefin/unsaturated ester copolymers, polyesters, polycarbonates, polysulphones, phenolic resins, urea resins, and blends thereof. Examples of suitable polymers are: polyethylene (PE) , in particular low density polyethylene (LDPE) , medium density polyethylene (MDPE) , high density polyethylene (HDPE) and linear low-density polyethylene ( LDPE) ; polypropylene (PP) ; ethylene/propylene elastomeric copolymers (EPM) or ethylene/propylene/diene terpolymers (EPDM) ; natural rubber; butyl rubber; ethylene/vinyl ester copolymers, for example ethylene/vinyl acetate (EVA) copolymers; ethylene/ acrylate copolymers, in particular ethylene/methyl acrylate (EMA) , ethylene/ethyl acrylate (EEA) , ethylene/butyl acrylate (EBA) copolymers; ethylene/α- olefin thermoplastic copolymers; polystyrenes; acrylo- nitrile-butadiene-styrene (ABS) resins; halogenated polymers, in particular polyvinyl chloride (PVC) ; polyurethane (PUR) ; polyamides; aromatic polyesters, for instance polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) ; and copolymers or mechanical blends thereof.

Preferably, the polymeric material is a polyolefinic polymer or copolymer based on ethylene and/or propylene, and is chosen in particular from:

(a) copolymers of ethylene with an ethylenically unsaturated ester, for example vinyl acetate or butyl acetate, in which the amount of unsaturated ester is generally between 5% and 80% by weight, preferably between 10% and 50% by weight;

(b) elastomeric copolymers of ethylene with at least one C₃-Cι₂ α-olefin, and optionally a diene, preferably ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, preferably having the following composition: 35 mol%-90 mol% of ethylene, 10 mol%-65 mol% of α- olefin, 0 mol%-10 mol% of diene (for example 1,4- hexadiene or 5-ethylidene-2-norbornene) ;

(c) copolymers of ethylene with at least one C₄-Cι₂ α- olefin, preferably 1-hexene, 1-octene and the like, and optionally a diene, generally having a density of between 0.86 g/cm³ and 0.90 g/cm³ and the following composition: 75 mol%-97 mol% of ethylene, 3 mol%-25 mol% of α-olefin, 0 mol%- 5 mol% of a diene;

(d) polypropylene modified with ethylene/C₃-Cι₂ α- olefin copolymers, in which the weight ratio between the polypropylene and the ethylene/C₃-Cι₂ α-olefin copolymer is between 90/10 and 30/70, preferably between 50/50 and 30/70.

For example, the commercial products Elvax® (Du Pont) , Levaprene® (Bayer) , Lotryl® (Elf-Atochem) are included in class (a) , the products Dutral® (Enichem) and Nordel® (Dow-Du Pont) are included in class (b) , and the products Engage® (Dow-Du Pont) and Exact® (Exxon) are included in class (c) , while polypropylene modified with ethylene/α-olefin copolymers is commercially available under the brand names Moplen® or Hifax® (Montell), or Fina-Pro® (Fina) , and the like.

Thermoplastic elastomers that are particularly preferred in class (d) include those comprising a continuous matrix of a thermoplastic polymer, for example polypropylene, and small particles (generally with a diameter of about 1-10 μm) of a vulcanized elastomeric polymer, for example crosslinked EPR or EPDM, dispersed in the thermoplastic matrix. The elastomeric polymer may be incorporated in the thermoplastic matrix in non-vulcanized form and then dynamically crosslinked during the process by adding a suitable amount of a crosslinking agent. Alternatively, the elastomeric polymer may be vulcanized separately and then dispersed in the thermoplastic matrix in the form of small particles. Thermoplastic elastomers of this type are described, for example, in documents US- 4,104,210 and EP-324,430. Among the polymeric materials that are particularly preferred is a high melt strength polypropylene, as described, for example, in patent US-4, 916, 198, commercially available under the brand name Profax®

(Basell S.p.A.). Said document illustrates a process for producing said polypropylene by means of a step of irradiating a linear polypropylene, performed by using high-energy ionizing radiation for a period that is sufficient to form a large amount of long chain branches, after which phase is also envisaged a suitable treatment of the irradiated material so as to deactivate substantially all the free radicals present in the irradiated material .

Even more preferably, among the polymeric materials that are particularly preferred is a polymeric composition comprising the abovementioned polypropylene with a high degree of branching, in an amount generally of between 30% and 70% by weight, blended with a thermoplastic elastomer of the type belonging to class (d) mentioned above, in an amount generally of between 30% and 70% by weight, said percentages being expressed relative to the total weight of the polymeric composition. In accordance with the present invention, the expansion degree can range from 5% to 200%, preferably from 30% to 70%.

It should be recalled that, in the case where the semiconductive polymeric layer is expanded, the amount of carbon black present in the polymeric matrix also varies as a function of the chosen expansion degree and of the expanding agent used.

Figure 2 illustrates an electrical cable 20 similar to the electrical cable 10 of Figure 1, except for the presence of a metal armouring 21 placed between the metal screen 16 and the outer protective sheath 17.

Said metal armouring 21 can consist of metal wires, for example steel wires, of a continuous metallic tubular element made of aluminium, lead or copper, or of a metal strip in the form of a tube which is welded or sealed with an adhesive material so as to ensure adequate leaktightness. In general, said metal armouring is obtained by means of an armouring apparatus with wires or tapes of known type.

In accordance with one embodiment, which is not shown, said cable 20 can include a tubular element coaxial to said metal armouring 21 and in a position radially external thereto, said tubular component being intended to act as a barrier to the penetration of water in the radial direction.

Said tubular component can consist, for example, of a thin sheet of aluminium, lead, copper or steel welded or bonded longitudinally with the edges overlapping (by using suitable machinery, for example of laser type) or a plurality of tapes so as to form said tubular element, or alternatively may be extruded (for example by using extrusion presses) to form a sheath, or may be obtained by using one or more strips of aluminium or steel .

Said tubular component is generally bonded to the outer polymeric sheath 17, in a position radially internal thereto.

Figure 3 shows, in right cross section, a tripolar electrical cable 30 for medium voltage power transmission or distribution.

Said cable 30 comprises three conductors, each covered, in a radial direction from the inside outwards for each individual phase, with an inner semiconductive coating, a layer of electrical insulation and an outer semiconductive coating, respectively, to define three cores 15 (one core for each phase) .

Each core 15 also has a metal screen 16 in a radially external position.

The three cores 15 thus coated are twisted together and the star-shaped areas formed between said cores are filled with a filling material of conventional type, generally EPR-based elastomeric blends filled with recovered material such as, for example, calcium carbonate, to define a filling layer 31 of substantially circular cross section.

In accordance with the embodiment illustrated in Figure 3, the cable 30 comprises, in a position radially external to said filling layer 31, an outer protective sheath 17 coated with a semiconductive polymeric layer 18 according to the present invention.

Figure 4 illustrates, in right cross section, a further embodiment of a tripolar electrical cable 40 which, relative to the tripolar cable 30 of Figure 3, further comprises a metal armouring 21, in a position radially internal to said outer protective sheath 17, and an inner polymeric sheath 41, in a position radially internal to said metal armouring 21, said inner polymeric sheath 41 being capable of imparting to the twisted cores 15 mechanical protection from the metal armouring itself.

Figure 5 illustrates, in right cross section, a further embodiment of a tripolar electrical cable 50 which, relative to the tripolar cable 30 of Figure 3, includes a layer 51 of expanded polymeric material for protecting the cable against accidental impacts as described in document WO 98/52197 in the name of the Applicant.

The figures mentioned above show only a few of the possible embodiments of cables in which the present invention may advantageously be used.

Specifically, it is clear that the embodiments mentioned above may be suitably modified, without thereby limiting the application of the present invention.

As regards the process for producing a cable according to the present invention, the main steps which characterize the abovementioned process when it is desired to produce a unipolar cable of the type featured in Figure 1 are given hereinbelow.

When it is desired to produce a multipolar cable, for example of tripolar type, the process described for a unipolar cable may be suitably modified on the basis of the indications given and of the technical knowledge of a person skilled in the art. The inner semiconductive layer 12 and outer semiconductive layer 14, produced according to known techniques, in particular by extrusion, choosing a polymeric material and a carbon black from those mentioned above, are applied over a conductive component 11, unwound from a suitable reel.

Similarly, the insulating layer 13, placed in an intermediate position between said semiconductive layers 12 and 14, is also preferably obtained by extruding a polyolefin chosen from those mentioned above, in particular polyethylene, polypropylene, ethylene/propylene copolymers, and the like.

At the end of the extrusion step, the material is preferably crosslinked according to known techniques, for example by using peroxides or via silanes.

Alternatively, the core 15 of the cable may be produced by means of a process of co-extrusion of the abovementioned layers according to known techniques and, once completed, is stored on a first collecting reel .

In a different line of the production plant, the core is unwound from said first reel and a metal screen 16 is applied thereto according to known methods. For example, a tape screening machine is used, which places thin strips of copper (for example about 0.1-0.2 mm thick) helically, via suitable rotating heads, preferably producing an overlap of the edges of said strips equal to about 33% of their surface. Alternatively, when the metal screen consists of a plurality of copper wires (for example 1 mm in diameter) , said wires are unwound from reels positioned on suitable rotating cages and helically applied onto said core. In general, in said cases, a counter-spiral (represented, for example, by a strip of copper 0.1- 0.2 mm thick) also needs to be applied, and serves to keep the copper wires mentioned above in position during the subsequent processing steps.

Once completed, the semifinished product thus obtained, that is to say the core and metal screen, is generally stored on a second collecting reel.

The process then continues by deposition by extrusion of the outer protective polymeric sheath 17 directly over the semifinished product mentioned above, followed by deposition by extrusion of the semiconductive polymeric layer 18 according to the present invention in a position radially external to said outer protective polymeric sheath 17.

As an alternative to the production process mentioned above including several extrusion steps in series (for example by means of the "tandem" technique) , said semiconductive polymeric layer 18 and said outer protective polymeric sheath 17 may advantageously be obtained by co-extrusion by using a single extrusion head.

According to a further embodiment of the production process, the extrusion or co-extrusion of the outer protective polymeric sheath 17 and of the semiconductive polymeric layer 18 is carried out on the same production line intended for producing the core/metal screen semifinished product.

In the case where said semiconductive polymeric layer 18 is expanded, the expansion of the polymer is carried out during the extrusion step performed directly on the outer protective polymeric sheath 17.

Said expansion may take place either chemically, by means of adding a suitable expanding agent that is capable of generating a gas under given pressure and temperature conditions, or physically, by means of injecting a gas at high pressure directly into the extruder cylinder.

Examples of suitable expanding agents are: azodicarba ide, para-toluenesulphonyl hydrazide, mixtures of organic acids (for example citric acid) with carbonates and/or bicarbonates (for example sodium bicarbonate) , and the like.

Examples of gases which may be injected at high pressure into the extruder cylinder are: nitrogen, carbon dioxide, air, low-boiling hydrocarbons, for example propane or butane, halohydrocarbons, for example methylene chloride, trichlorofluoromethane, 1- chloro-1, l--difluoroethane, and the like, or mixtures thereof .

Preferably, the aperture of the extruder head has a diameter that is slightly less than the final diameter of the cable having the expanded coating which it is desired to be obtained, such that the expansion of the polymer outside the extruder results in the desired diameter being reached.

The expanded polymeric material may be crosslinked or non-crosslinked. The crosslinking is performed, after the step of extrusion and expansion, according to known techniques, in particular by heating in the presence of a free-radical initiator, for example an organic peroxide such as dicumyl peroxide. Alternatively, a crosslinking can be performed via silanes, involving the use of a polymer belonging to the group mentioned above, in particular a polyolefin, to which are covalently bonded silane units comprising at least one hydrolysable group, for example trialkoxysilane groups, in particular trimethoxysilane groups. The coupling of the silane units may be performed by free-radical reaction with silane compounds, for example methyltriethoxysilane, dimethyldiethoxysilane, vinyldimethoxysilane, and the like. The crosslinking is performed in the presence of water and of a crosslinking catalyst, for example an organic titanate or a metal carboxylate. Dibutyltin dilaurate (DBTL) is particularly preferred.

In the case where the cable is provided with metal armouring (as illustrated, for example, in Figure 2), the production process described above also involves the implementation of a further step which involves the use of an armouring machine with wires or tapes, operating according to the principle of the tape screening machines mentioned above .

A number of illustrative examples are given hereinbelow to describe the invention in further detail.

EXAMPLE 1

A medium voltage cable was produced according to a structural scheme similar to that given in Figure 1.

The core of said cable consisted, respectively, of: a) a copper conductor (with a cross section of 35 mm²) ; b) an inner semiconductive layer (based on EPR and having a thickness of 0.5 mm); c) an insulating layer (based on EPR and having a thickness of 5.5 mm); d) an outer semiconductive layer (based on EVA and having a thickness of 0.5 mm).

In a radially external position, the core thus obtained was provided, by means of bonding, with a metal screen consisting of an aluminium foil 0.15 mm thick. An outer protective sheath made of MDPE with a thickness of 1.8 mm was then deposited on the cable thus obtained by extrusion.

The outside diameter of the cable thus obtained was 24.7 mm.

A semiconductive polymeric layer according to the invention was deposited on said outer protective sheath, by extrusion.

Said semiconductive polymeric layer comprised 100 phr of Levapren® 450 (based on EVA, produced by Bayern) and 50 phr of Vulcan® XC72 (carbon black produced by Cabot Corporation, having a surface area of 254 m²/g) , where the term "phr" means parts by weight per 100 parts by weight of rubber.

The thickness of said semiconductive polymeric layer was 1 mm.

An 80 mm single-screw Bandera extruder, in configuration 20 D, was used to deposit said semiconductive polymeric layer.

Tables 1 and 2 show the temperature profile and operating parameters of the extruder used to obtain the semiconductive polymeric layer.

The cable was subsequently cooled in water and wound on a reel.

Table 1

Table 2

The extruder pressure values given in Table 2 were measured upstream of the extruder filtration zone.

A piece of cable about 100 m long was obtained and a sample of cable 20 m long was taken therefrom.

Testing (IEC Standard 229)

In accordance with the testing method codified in IEC Standard 229 described above, a DC voltage of 15 kV was applied between the metal screen and the semiconductive polymeric layer of said cable sample.

The outer polymeric sheath present on said cable sample was found to be capable of withstanding the applied voltage, reflecting the integrity of said sheath, that is to say the absence of defects and/or damages of an extent such as to compromise the functioning of the cable. In order for said measurement to be indicative only of the soundness or not of said outer protective sheath, it is necessary for the electrical resistance of the semiconductive polymeric layer to be much less than the electrical resistance of the outer protective sheath.

If such were not the case, due to the greater electrical resistance of the semiconductive polymeric layer relative to the electrical resistance of the outer protective sheath, there would be a reduction in said voltage along the cable, which reduction could not then be attributable to the presence of defects in the outer protective sheath.

The outer protective polymeric sheath and the semiconductive polymeric layer of the cable sample mentioned above were subjected to a measurement of electrical resistance by using an ohm-meter. The electrical resistance values measured (3 MΩ for the semiconductive polymeric layer and 5 TΩ for the outer protective polymeric sheath of said sample) revealed that, for the cable under examination, the electrical resistance of the semiconductive polymeric layer was actually much less than the electrical resistance of the outer protective sheath.

Next, the outer protective sheath of said sample was cut by using a cutting edge so as to simulate possible damage to the sheath.

Said incision was made transversely relative to the sample and concerned the entire thickness of the sheath. In other words, said incision, of extremely narrow width, was carried out so as to give rise to an electrically conductive path, within the thickness of said sheath, between the semiconductive polymeric layer and the metal screen. The same voltage as mentioned above was then applied to one end of said sample. However, due to the short- circuit produced by the incision mentioned above, even at an applied voltage value equal to 5 kV, an overcurrent was detected which was such as to cause the disconnection of the voltage generator, reflecting the presence of the defect in the sheath.

EXAMPLE 2

A cable similar to that of Example 1 was produced, the only difference being that the semiconductive polymeric layer was expanded rather than compact.

The expansion of said semiconductive polymeric layer was obtained chemically, by adding to the above composition 5% by weight (relative to the total) of the expanding agent Hydrocerol® BIH 40 (carboxylic acid/sodium bicarbonate) produced by Boehringer Ingelheim.

The material constituting the semiconductive polymeric llaayyeerr hhaadd aa ffiinnaall ddeennssiittyy of 0.85 kg/dm³ and an expansion degree equal to 30%

The expanded semiconductive polymeric layer had a thickness of 1.3 mm.

The final outside diameter of the cable was 27.3 mm.

The cable was obtained by using the same operating conditions as those described in Example 1.

In a similar manner to that described in Example 1, a sample of cable 20 m long was subjected to a voltage test (as codified by IEC Standard 229) in the absence and presence of an incision in the protective sheath. Said test gave results similar to those obtained in Example 1 reflecting the reliability of said testing method even when the semiconductive polymer coating is expanded rather than compact.

The invention presents a plurality of advantages over the prior art.

In particular, the present invention makes it possible to eliminate the drawbacks associated with the deposition of a conductive layer based on graphite, which is either in solid form (in powder form) or in liquid form.

Firstly, the fact that it is possible to arrange an extruded semiconductive polymeric layer in a position radially external to the protective polymeric sheath of a cable allows said cable to be given a continuous and uniform layer along its length.

The application by extrusion of a semiconductive polymeric layer according to the invention makes it possible, in fact, to ensure a uniform deposition both in terms of thickness (and thus of amount of material deposited) and in terms of total covering of the surface of the sheath of said cable.

In addition, the application by extrusion of said semiconductive polymeric layer makes it possible to obtain a coating that adheres advantageously to the layer immediately below said semiconductive layer, thus avoiding problems of detachment of the conductive material from the cable due to operations of transportation, laying or winding on a reel. A further advantage of the present invention consists in that the provision of a semiconductive layer by means of deposition by extrusion makes it possible to establish a production process of continuous type, said extrusion step being subsequent to or at the same time as the extrusion step of the outer protective sheath.

In addition, said production process is more simplified from the point of view of plant engineering, and also faster and less expensive than the prior art processes including the graphitization step since it is not necessary to provide apparatuses specifically intended for depositing said graphite layer.

In addition, the fact that the graphite can be eliminated allows the working environment to be considerably improved in terms of both cleanliness and safety for the technical staff working on the production process.

Claims

1. Method for testing a protective polymeric layer

(17) of an electrical cable (10, 20, 30, 40, 50), said electrical cable comprising at least one metal component (16, 21) in a position radially internal to said protective polymeric layer (17), said method comprising the steps of:

• applying a semiconductive polymeric layer (18) in a position radially external to said protective polymeric layer (17) , and

• electrically connecting said semiconductive polymeric layer (18) to said at least one metal layer (16, 21) .

2. Method according to Claim 1, in which said step of electrically connectining comprises the step of connecting said semiconductive polymeric layer

(18) to at least one metal screen (16) of said cable (10, 20, 30, 40, 50) .

3. Method according to Claim 1, in which said step of electrically connectining comprises the step of connecting said semiconductive polymeric layer (18) to a metal armouring (21) of said cable (10, 20, 30, 40, 50).

4. Method according to Claim 1, in which said step of applying comprises the step of depositing said semiconductive polymeric layer (18) by extrusion.

5. Method according to Claim 1, in which said step of applying comprises the step of depositing said semiconductive polymeric layer (18) and said protective polymeric layer (17) by co-extrusion.

6. Electrical cable (10, 20, 30, 40, 50) comprising: • at least one core (15) comprising at least one conductive element (11) and at least one layer of electrical insulation (13) in a position radially external to said at least one conductive element (11) ;

• at least one metal element (16, 21) placed in a position radially external to said at least one core (15) , and

• a protective polymeric layer (17) in a position radially external to said at least one metal element (16, 21),

characterized in that said cable (10, 20, 30, 40, 50) further comprises a semiconductive polymeric layer (18) in a position radially external to said protective polymeric layer (17).

7. Electrical cable (10, 20, 30, 40, 50) according to Claim 6, in which said semiconductive polymeric layer (18) is in contact with said protective polymeric layer (17) .

8. Electrical cable (10, 20, 30, 40, 50) according to Claim 6, in which said semiconductive polymeric layer (18) is compact.

9. Electrical cable (10, 20, 30, 40, 50) according to Claim 6, in which said semiconductive polymeric layer (18) is expanded.

10. Electrical cable (10, 20, 30, 40, 50) according to Claim 9, in which the expansion degree of said semiconductive polymeric layer (18) is between 5% and 200%.

11. Electrical cable (10, 20, 30, 40, 50) according to Claim 10, in which said expansion degree is between 30% and 70%.

12. Electrical cable (10, 20, 30, 40, 50) according to Claim 9, in which the expandable polymeric material of said expanded semiconductive polymeric layer (18) is chosen from polyolefinic polymers and copolymers based on ethylene and/or propylene.

13. Electrical cable (10, 20, 30, 40, 50) according to Claim 12, in which said expandable polymeric material is chosen from:

a) copolymers of ethylene with an ethylenically unsaturated ester, in which the amount of unsaturated ester is between 5% and 80% by weight;

b) elastomeric copolymers of ethylene with at least one C₃-Cι₂ α-olefin, and optionally a diene, having the following composition: 35 mol%-90 mol% of ethylene, 10 mol%-65 mol% of α- olefin, 0 mol%-10 mol% of diene;

c) copolymers of ethylene with at least one C-Cι₂ α-olefin, and optionally a diene, having a density of between 0.86 and 0.90 g/cm³;

d) polypropylene modified with ethylene/C₃-Cι₂ α- olefin copolymers, in which the weight ratio between the polypropylene and the ethylene/C₃-Cι₂ α-olefin copolymer is between 90/10 and 30/70.

14. Electrical cable (10, 20, 30, 40, 50) according to Claim 6, in which the polymeric material of said semiconductive polymeric layer (18) comprises a conductive material .

15. Electrical cable (10, 20, 30, 40, 50) according to Claim 14, in which said conductive material is an electrically conductive carbon black.

16. Electrical cable (10, 20, 30, 40, 50) according to Claim 6, in which the thickness of said semiconductive polymeric layer (18) is between 0.05 mm and 3 mm.

17. Electrical cable (10, 20, 30, 40, 50) according to Claim 16, in which said thickness is between 0.2 mm and 0.8 mm.

18. Process for producing an electrical cable (10, 20, 30, 40, 50) comprising at least one core (15) and a protective polymeric layer (17) in a position radially external to said at least one core (15) , said process comprising the step of depositing by extrusion a layer of semiconductive polymeric material (18) in a position radially external to said protective polymeric layer (17) .

19. Process according to Claim 18, in which said layer of semiconductive polymeric material (18) is deposited by co-extrusion with said protective polymeric layer (17) .

20. Process according to Claim 18, in which the semiconductive polymeric material of said layer of semiconductive polymeric material (18) is expandable.