WO2016005791A1

WO2016005791A1 - Energy cable having a thermoplastic electrically insulating layer

Info

Publication number: WO2016005791A1
Application number: PCT/IB2014/062947
Authority: WO
Inventors: Gabriele Perego
Original assignee: Prysmian S.P.A.
Priority date: 2014-07-08
Filing date: 2014-07-08
Publication date: 2016-01-14
Also published as: AR101118A1

Abstract

Power cable comprising an electrical conductor and an electrically insulating layer surrounding said electrical conductor, wherein the electrically insulating layer is made of a polymeric composition comprising a 4-methyl-1-pentene polymer as main base material and a dielectric fluid intimately admixed in the polymeric composition, said dielectric fluid having a ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6. The admixture of a 4-methyl-1- pentene (PMP) polymer with a dielectric fluid can be compounded without incurring in immiscibility or exudation phenomena, resulting in a polymeric composition with mechanical properties suitably for the application as cable layer and maintaining the dielectric advantages of the PMP, provided that the dielectric fluid has a low or null aromatic content.

Description

ENERGY CABLE HAVING A THERMOPLASTIC ELECTRICALLY INSULATING LAYER

DESCRIPTION

Background of the invention

The present invention relates to an energy cable. In particular, the present invention relates to a cable for transporting or distributing electric energy, especially medium or high voltage electric energy, said cable having a thermoplastic electrically insulating layer containing a dielectric fluid.

Cables for transporting electric energy generally include at least one cable core. The cable core for transporting medium or high voltage electric energy is usually formed by at least one electrical conductor sequentially covered by an inner polymeric layer having semiconductive properties, an intermediate polymeric layer having electrically insulating properties and an outer polymeric layer having semiconductive properties. Each cable core is generally surrounded by a screen layer, typically made of metal or of metal and polymeric material. The screen layer can be made in form of wires (braids), of a tape helically wound around the cable core or a sheath longitudinally wrapped around the cable core. The polymeric layers surrounding the conductor are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during continuous use and with current overload.

To address requirements for materials environment-friendly during both production and use and recyclable at the end of the cable life, energy cables have been recently developed having a cable core made from thermoplastic materials, i.e. polymeric materials which are not crosslinked and thus can be recycled at the end of the cable life.

In this respect, electrical cables comprising at least one coating layer, for example the insulation layer, based on a polypropylene matrix intimately admixed with a dielectric fluid are known and disclosed in WO 02/03398, WO 02/27731, WO 04/066317, WO 04/066318, WO 07/048422, and WO 08/058572. The polypropylene matrix useful for this kind of cables is based on a polypropylene homopolymer or copolymer or both, characterized by a relatively low cristallinity such to provide the cable with the suitable flexibility, but not to impair the mechanical properties and the resistance to pressure at the cable operative and overload temperatures. Performance of the cable coating, especially of the cable insulating layer, is also affected by the presence of the dielectric fluid intimately admixed with said polypropylene matrix. The dielectric fluid should not affect the mentioned mechanical properties and resistance to pressure at high temperature (hereinafter also referred to as "thermopressure resistance") and should be such to be intimately and homogeneously admixed with the polymeric matrix.

The use of a 4-methyl-l-pentene homopolymer or copolymer (also known as, inter alia, poly(4-methyl-l-pentene), polymethylpentene or poly(4- methylpentene-1) as material for cable electric insulation is taught, for example, by US 3327050 and FR 2472823.

As disclosed by the brochure "TPX - POLY-4-METHYLPENTE E- 1 (PMP)" by Mitsui Chemical, Inc. (May 19, 2007), while PMP has valuable properties such as low dielectric constant and high voltage resistance, it also have drawbacks in term of not so strong mechanical strength, brittleness and rigidity at room temperature, low adherence and low affinity with other materials (resulting in immiscibility or easy releasability). In particular, PMP is said to be hard to be plasticized during the manufacturing process.

Such drawbacks can result in a problematic, if not unmanageable, application as main base material for insulating layer useful in a medium or high voltage cable, where the thickness of the insulating layer is significant and its enduring integrity a must.

US 7,288,721 relates to a cable including an electrical conductor, a polymeric protective layer disposed adjacent to the electrical conductor, a first insulating jacket disposed adjacent the polymeric protective layer and a second insulating jacket disposed adjacent the first insulating jacket. The second insulating jacket can be made of poly(4-methyl-l-pentene) poly olefin.

US 7,560,647 relates to a conductor covered with an inside coat and the inside coat is covered with an outermost coat. The resin composition for outermost coat may include polymethylpentene.

US 6,908,673 and US 7, 196,270 relates elates to a cable where an extruded covering layer based on a thermoplastic polymer material in admixture with a dielectric liquid. In the case of US 6,908,673 the dielectric liquid comprises at least two non-condensed aromatic rings and a ratio of number of aryl carbon atoms to total number of carbon atoms greater than or equal to 0.6, while in the case of US 7, 196,270 the dielectric liquid, when aromatic, has a ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6. As thermoplastic material, if a copolymer of propylene with at least one olefin comonomer is used, the comonomer can be 4-methyl-l-pentene. Summary of the invention

The Applicant has faced the problem of exploiting the appealing low dielectric constant (ε_Γ≡2.12 ) of PMP for obtaining a power cable insulation layer based on this polymer as main material while escaping the above-mentioned drawbacks thereof, especially for what concerns brittleness and rigidity at room temperature and weak mechanical strength.

In order to solve the above problem and contrarily to what stated by the prior art about the low affinity of PMP with other materials, the Applicant has considered the possibility of manufacturing the electrically insulating layer with PMP as main material admixed with a dielectric fluid.

The Applicant has found that an admixture of a 4-methyl-l-pentene (PMP) polymer with a dielectric fluid can be compounded without incurring in immiscibility or exudation phenomena, resulting in a polymeric composition with mechanical properties suitably for the application as cable layer and maintaining the dielectric advantages of the PMP, provided that the dielectric fluid has a low or null aromatic content.

Therefore, according to a first aspect the present invention relates to a power cable comprising an electrical conductor and an electrically insulating layer surrounding said electrical conductor, wherein the electrically insulating layer is made of a polymeric composition comprising a 4-methyl-l-pentene polymer as main base material and a dielectric fluid intimately admixed in the polymeric composition, said dielectric fluid having a ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6.

For the purpose of the present description and of the claims that follow, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about" . Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

In the present description and in the subsequent claims, as "conductor" it is meant an electrically conducting element usually made from a metallic material, more preferably aluminium, copper or alloys thereof, either as a rod or as a stranded multi-wire, or a conducting element as above coated with a semi conductive layer.

Preferably the cable of the invention is a medium voltage (MV) or a high voltage (HV) cable.

For the purposes of the invention the term "medium voltage" generally means a voltage of between 1 kV and 35 kV, whereas "high voltage" means voltages higher than 35 kV.

As "electrically insulating layer" it is meant a covering layer made of a material having insulating properties, namely having a dielectric rigidity (dielectric breakdown strength) of at least 5 kV/mm, preferably greater than 10 kV/mm.

Advantageously, the electrically insulating layer made of a polymeric composition comprising a 4-methyl-l-pentene polymer as main base material and a dielectric fluid intimately admixed with the polymeric composition according to the invention is the sole insulating layer surrounding the conductor.

The electrically insulating layer can have a thickness of at least 8 mm or of at least 12 mm. The thickness of the insulating layer depends on the voltage intended to be carried by the cable and on the overall structure of the cable (conductor compositions and configuration, kind of material employed for the insulating layers, etc.). For example, a polymer insulated cable intended for carrying 400 kV and having a single conductor made of stranded copper wires can have an insulating layer 27 mm thick.

Preferably, the polymeric composition of the invention is a thermoplastic polymeric composition.

With "4-methyl- 1-pentene polymer" it is meant a homopolymer of 4-methyl-

1-pentene or a copolymer of 4-methyl- 1-pentene with of C₆-Ci₀ olefin monomer, such as hexene, octene and decene, in amounts not greater than 5 mole % of polymer units.

Preferably, the 4-methyl- 1-pentene polymer suitable as insulating base material according to the invention is a copolymer of 4-methyl pentene-1 with a C₆- Cio 1-alkene monomer.

As "main base material" is meant that a 4-methyl- 1-pentene polymer is the main polymeric component of the polymeric composition where it is contained in an amount of at least 60 wt%, preferably of at least 80 wt% of the polymer composition.

The polymeric composition of the present invention can further contain minor amount (preferably, from 0.5 wt% to 5 wt%) of water tree retardant additives such as ethylene butyl acrylate (EBA), ethylene ethyl acetate (EEA) or ethylene vinyl acetate (EVA).

As to the dielectric fluid, high compatibility between the dielectric fluid and the polymer base material is necessary to obtain a microscopically homogeneous dispersion of the dielectric fluid in the polymer base material.

The dielectric fluid suitable for forming the insulating layer of the present invention should comprise no polar compounds or only a limited quantity thereof, in order to avoid a significant increase of the dielectric losses. In particular the dielectric fluid of the present invention should comprise an amount of polar compounds of from 0 wt% to 2.5 wt%, preferably of from 0.1 wt% to 2.3 wt%, with respect to the total weight of the dielectric liquid. The amount of polar compounds of the dielectric liquid may be determined according to ASTM standard D2007-02.

Preferably, the concentration by weight of the dielectric fluid in the polymeric composition of the invention is lower than the saturation concentration of said dielectric fluid in said material. The saturation concentration of the dielectric fluid in the thermoplastic polymer material may be determined by a fluid absorption method on Dumbell specimens as described, for example, in WO 04/066317.

By using the dielectric fluid with the aromatic content as defined below, thermomechanical properties of the insulating layer are maintained and exudation of the dielectric fluid from the polymer material is avoided.

A dielectric fluid with a low or null aromatic content expressed as the ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms (hereinafter also referred to as C_ar/C_tot) lower than 0.6 showed to be compatible with the 4-methyl-l-pentene polymer as main base material of the insulating layer of the invention. "Compatible" means that the chemical composition of the fluid and of the polymer material is such as to result into a microscopically homogeneous dispersion of the dielectric fluid into the polymer material upon mixing the fluid into the polymer, similarly to a plasticizer.

With "number of aromatic carbon atoms" it is intended to be the number of carbon atoms which are part of an aromatic ring. The ratio of number of aromatic carbon atoms to total number of carbon atoms of the dielectric fluids according to the invention is sign of aromaticity. Preferably, the dielectric fluid suitable for the insulating layer according to the invention has a C_ar/C_tot of from 0.01 and 0.4.

The ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms C_ar/C_tot may be determined according to ASTM standard D3238-95(2000)el .

Generally, the weight ratio between the dielectric fluid and the polymer material may be from 1 :99 to 25:75, preferably from 2:98 to 15:85.

It has also to be noticed that the use of a dielectric fluid with a relatively low melting point or low pour point - such that the dielectric fluid is liquid at room temperature or can be melted by a mild heating, for example at 80°C - allows an easy handling of the dielectric fluid which may be melted with no need of additional and complex manufacturing steps (e.g. a melting step of the dielectric fluid) and/or apparatuses for admixing the liquid with the polymer material.

According to a preferred embodiment, the dielectric fluid has a melting point or a pour point of from -130°C to +80°C. The melting point may be determined by known techniques such as, for example, by Differential Scanning Calorimetry (DSC) analysis.

According to a preferred embodiment, the dielectric fluid has a predetermined viscosity in order to control fast diffusion of the liquid within the insulating layer and hence its outward migration. Also, the viscosity of the dielectric fluid should be close to the viscosity of the polymer material at the compounding temperature for easing the admixing of the two components in the compounding apparatus. Generally, the dielectric fluid of the invention has a viscosity of from 2 cSt to 500 cSt at 40°C (measured according to ASTM standard D445-03).

According to a preferred embodiment, the dielectric liquid has a dielectric constant of less than or equal to 3.5 and preferably less than 3 at 25°C (measured in accordance with IEC 247).

Examples of suitable dielectric fluids are: mineral oils such as, for example, naphthenic oils, paraffinic oils, said mineral oils optionally containing at least one heteroatom selected from oxygen, nitrogen or sulphur; liquid paraffins; vegetable oils such as, for example, soybean oil, linseed oil, castor oil; paraffinic waxes such as, for example, polyethylene waxes, polypropylene waxes; synthetic oils such as, for example, silicone oils, alkyl benzenes (such as, for example, dodecylbenzene, di(octylbenzyl)toluene), aliphatic esters (such as, for example, tetraesters of pentaerythritol, esters of sebacic acid, phthalic esters), olefin oligomers (such as, for example, optionally hydrogenated polybutenes or polyisobutenes); or mixtures thereof. Paraffinic oils and naphthenic oils are particularly preferred.

Polyaromatic oils could also be employed though their use is questionable as potentially hazardous to health and environment.

Other components may be added in minor amounts to the polymeric composition according to the present invention, including antioxidants, processing aids or mixtures thereof.

Conventional antioxidants suitable for the purpose are, for example, distearyl- or dilauryl-thiopropionate and pentaerythrityl-tetrakis [3-(3,5-di-t-butyl-4- hydroxyphen-yl)-propionate], or mixtures thereof.

Processing aids which may be added to the polymer composition include, for example, calcium stearate, zinc stearate, stearic acid, or mixtures thereof.

The cable of the invention can further comprise at least one, preferably two semiconductive layers. In particular, an inner polymeric semi conductive layer can be provided around and in contact with the electrical conductor in a radially internal position with respect to the insulating layer and/or an outer semiconducting layer can be provided to surround the insulating layer.

As "semi conductive layer" it is meant a covering layer made of a material having semiconductive properties, such as a polymeric matrix added with, e.g., carbon black such as to obtain a volumetric resistivity value, at room temperature, of less than 500 Ω-m, preferably less than 20 Ω-m. Typically, the amount of carbon black can range between 1 and 50% by weight, preferably between 3 and 30% by weight, relative to the weight of the polymer.

A semiconductive layer is preferably formed by a semiconductive polymer composition comprising a 4-methyl-l-pentene polymer as main base material and a conductive filler, preferably a carbon black filler. Advantageously, the semiconductive polymer composition/s of the cable of the invention comprise, intimately admixed therein, a dielectric fluid having ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6.

Alternatively the semiconductive layer/s of the cable of the invention can be made of a composition based on a thermoplastic polymer as disclosed, for example, in US 7, 196,270 or in US7884284, and a conductive filler, preferably a carbon black filler. For example, the thermoplastic polymer can be selected from:

at least one copolymer (i) of propylene with at least one olefin comonomer selected from ethylene and an a-olefin other than propylene, said copolymer having a melting point greater than or equal to 130°C and a melting enthalpy of from 20 J/g to 90 J/g;

a blend of at least one copolymer (i) with at least one copolymer (ii) of ethylene with at least one a-olefin, said copolymer (ii) having a melting enthalpy of from 0 J/g to 120 J/g; a blend of at least one propylene homopolymer with at least one copolymer (i) or copolymer (ii);

at least one of copolymer (i) and copolymer (ii) being a heterophasic copolymer. At least one dielectric fluid, preferably according to the invention, is intimately admixed with the thermoplastic polymer material.

The conductive filler is generally dispersed within the polymer composition in a quantity such as to provide the material with above mentioned semiconductive properties. Typically, the amount of carbon black can range between 1 and 50% by weight, preferably between 3 and 30% by weight, relative to the weight of the polymer.

The use of the same base material for both the insulating layer and the semiconductive layers is particularly advantageous in producing cables for medium or high voltage, since it ensures excellent adhesion between adjacent layers and hence a good electrical behaviour, particularly at the interface between the insulating layer and the inner semiconductive layer, where the electrical field and hence the risk of partial discharges are higher.

The polymeric compositions for the cable according to the present invention may be produced by mixing together the 4-methyl-l-pentene polymer, the dielectric fluid and any other optional additive and component, by using methods known in the art. Mixing may be carried out for example by an internal mixer of the type with tangential rotors (Banbury) or with interpenetrating rotors; in a continuous mixer of Ko-Kneader (Buss) type, of co- or counter-rotating double-screw type; or in a single screw extruder.

According to a preferred embodiment, the dielectric fluid may be added to the main base material during the extrusion step by direct injection into the extruder cylinder as disclosed, for example, in US7807091 or WO 02/47092. Alternatively, the 4-methyl-l-pentene polymer can be pre-impregnated with the dielectric fluid, and then fed into the extruder as described in WO2013/171550.

An example of a manufacturing process suitable for manufacturing a cable according to the present invention is described in WO2012/084055 or WO2013/171550, in the name of the Applicant.

Although the present description is mainly focused on cables for transporting or distributing medium or high voltage energy, the polymer composition of the invention may be used for coating electrical devices in general and in particular cable of different type, for example low voltage cables (i.e. cables carrying a voltage lower than 1 kV), telecommunications cables or combined energy/telecommunications cables, or accessories used in electrical lines, such as terminals, joints, connectors and the like.

Brief description of the drawing

Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawing, in which:

Figure 1 is a perspective view of an energy cable, particularly suitable for medium or high voltage, according to the invention.

Detailed description of the preferred embodiments

In Figure 1, the cable (1) comprises a conductor (2), an inner semiconductive layer (3), an insulating layer (4), an outer semi conductive layer (5), a metal screen layer (6) and a sheath (7).

The conductor (2) generally consists of metal wires, preferably of copper or aluminium or alloys or composites thereof, stranded together by conventional methods, or of a solid rod made of the mentioned metal/s. The insulating layer (4) may be produced by extruding around the conductor (2) a composition according to the present invention.

The semiconductive layers (3) and (5) are also made by extruding polymeric materials usually based on polymers, preferably a polymeric composition according to the present disclosure, made to be semiconductive by adding a conductive filler, usually carbon black.

Around the outer semiconductive layer (5), a metal screen layer (6) is usually positioned, made of electrically conducting wires or strips helically wound around the cable core - comprising conductor (2), inner semiconductive layer (3), insulating layer (4) and outer semiconductive layer (5) -, or of an electrically conducting tape longitudinally wrapped and overlapped (preferably glued) onto the underlying layer. The electrically conducting material of said wires, strips or tape is usually copper or aluminium or alloys or composites thereof.

The screen layer (6) may be covered by a sheath (7), generally made from a polyolefin, usually polyethylene.

The cable can be also provided with a protective structure (not shown in Figure 1) the main purpose of which is to mechanically protect the cable against impacts or compressions. This protective structure may be, for example, a metal reinforcement or a layer of expanded polymer as described in WO 98/52197 in the name of the Applicant.

The cable according to the present invention may be manufactured in accordance with known methods, for example by extrusion of the various layers around the central conductor. The extrusion of two or more layers is advantageously carried out in a single pass, for example by the tandem method in which individual extruders are arranged in series, or by co-extrusion with a multiple extrusion head. The screen layer is then applied around the so produced cable core. Finally, the sheath according to the present invention is applied, usually by a further extrusion step.

The cable of the present invention is preferably used for alternating current (AC) power transmission.

Figure 1 shows only one embodiment of a cable according to the invention. Suitable modifications can be made to this embodiment according to specific technical needs and application requirements without departing from the scope of the invention.

The following examples are provided to further illustrate the invention.

EXAMPLES 1-3

The following compositions were prepared with the amounts reported in Table 1 (expressed as % by weight with respect to the total weight of the composition).

In all of the examples, the 4-methyl-l-pentene polymer (PMP) was fed directly into the extruder hopper. In the case, the dielectric fluid, previously mixed with the antioxidant, was subsequently injected at high pressure into the extruder. A laboratory twin screw extruder was used.

TABLE 1

EXAMPLE 1 * 2* 3

PMP 99.75 90.00 90.00

Aromatic dielectric fluid — 9.75 —

Low aromatic dielectric fluid — — 9.75

Antioxidant 0.25 0.25 0.25 (*) comparative

PMP: Mitsui TPX® MX002 Methylpentene Copolymer;

Aromatic dielectric fluid: dibenzyltoluene (Marlotherm™ SH, Sasol Olefins & Surfactants GmbH) having an aromatic carbon atoms/total carbon atoms ratio of 0.86;

Low aromatic dielectric fluid: naphthenic oil (Nyflex™ 800, Nynas AB) having an aromatic carbon atoms/total carbon atoms ratio of 0.05;

Antioxidant: 4,6-bis (octylthiomethyl)-o-cresol.

The polymeric compositions of Table 1 were tested for evaluating various properties as set forth in Table 2.

Tensile strength, elongation at break and modulus at 10%, 20% and 50% were evaluated according to CEI EN 6081 1-1-1 (2001-06). Pressure resistance at high temperature (thermopressure) was measured according to CEI rule EN 6081 1- 3-1 Par. 8 (2001-6).

TABLE 2

(*) comparative The above data reported above show that the addition of a dielectric fluid, having an aromatic carbon atoms/total carbon atoms ratio lower than 0.6, to a PMP based polymeric composition (Example 3) according to the invention provides advantages over a composition substantially containing PMP only (comparative Example 1). In particular, while the mechanical strength (expressed as tensile strength and elongation at break) of the polymeric starting material (comparative Example 1) is maintained after compounding with the dielectric fluid of the invention (Example 3), the flexibility of the composition of the invention is substantially improved. The addition of a dielectric fluid to the PMP based polymeric composition did not significantly decrease the thermopressure of the polymeric base.

The composition of comparative Example 2 - where PMP is admixed with a dielectric fluid containing a substantial aromatic component (having an aromatic carbon atoms/total carbon atoms ratio greater than 0.6) - could not be evaluated in many tests because the dielectric fluid exudated from the composition of comparative Example 2 even at room temperature, making any measurements impossible or unreliable. Such a behaviour is in line with what taught by the prior art about the low affinity of PMP.

The composition of Example 3 according to the invention was also tested for evaluating the dielectric strength thereof. EFI models were tested in silicon oil in alternate and direct current. In alternate current, with a voltage ramp of 2 kV/s, the dielectric strength of the composition of Example 3 was of 139 kV/mm at room temperature. In direct current, with a voltage ramp of 20 kV + 2 kV/15 sec, the dielectric strength of the composition of Example 3 was >205 kV/mm at 70°C.

What is unexpected in view of the prior art is that the admixture PMP/ dielectric fluid with low aromatic content according to the invention did not show any release of dielectric fluid, proving a satisfactory affinity with PMP and providing an improvement of the polymer flexibility.

Claims

1. A power cable comprising an electrical conductor and an electrically insulating layer surrounding said electrical conductor, wherein the electrically insulating layer is made of a polymeric composition comprising a 4-methyl-l - pentene polymer as main base material and a dielectric fluid intimately admixed in the polymeric composition, said dielectric fluid having a ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6.

2. The cable according to claim 1 wherein the 4-methyl-l -pentene polymer is a homopolymer of 4-methyl-l -pentene.

3. The cable according to claim 1 wherein the 4-methyl-l -pentene polymer is a copolymer of 4-methyl pentene- 1 with amounts of C₆-Ci₀ olefin monomer, such as hexene, octene and decene, in amounts not greater than 5 mole % of polymer units.

4. The cable according to claim 1, wherein the polymeric composition comprises a 4-methyl-l -pentene polymer in an amount of at least 60 wt%.

5. The cable according to claim 4, wherein the polymeric composition comprises a 4-methyl-l -pentene polymer in an amount of at least 80 wt%.

6. The cable according to claim 1, wherein the dielectric fluid suitable for the insulating layer according to the invention has a ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms of from 0.01 and 0.4.

7. The cable according to claim 1, wherein the dielectric fluid and the 4- methyl-1 -pentene polymer are in a weight ratio of from 2:98 to 15:85.

8. The cable according to claim 1, wherein the dielectric fluid has a viscosity, at 40°C, of from 2 cSt to 500 cSt.

9. The cable according to claim 1, wherein the dielectric fluid is selected from paraffinic oils and naphthenic oils.

10. The cable according to claim 1, wherein the dielectric fluid comprises an amount of polar compounds of from 0 wt% to to 2.5 wt% with respect to the total weight of the dielectric liquid.

1 1. The cable according to claim 1, wherein the dielectric fluid comprises an amount of polar compounds of from 0.1 wt% and 2.3 wt% with respect to the total weight of the dielectric liquid.

12. The cable according to claim 1 comprising at least one semiconductive layer.

13. The cable according to claim 12 wherein the semiconductive layer is formed by a semiconductive polymer composition comprising a 4-methyl-l-pentene polymer as main base material and a conductive filler.

14. The cable according to claim 12 wherein the semiconductive layer is formed by a semiconductive polymer composition comprising a conductive filler and a thermoplastic polymer can be selected from:

a blend of at least one copolymer (i) with at least one copolymer (ii) of ethylene with at least one a-olefin, said copolymer (ii) having a melting enthalpy of from 0 J/g to 120 J/g;

a blend of at least one propylene homopolymer with at least one copolymer (i) or copolymer (ii); at least one of copolymer (i) and copolymer (ii) being a heterophasic copolymer.

15. The cable according to claim 13 or 14 wherein the semi conductive polymer composition comprises, intimately admixed therein, a dielectric fluid having ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6.