Process for producing an energy cable having a thermoplastic electrically insulating layer

Abstract

A process for producing an energy cable including at least one electrically conductive core and at least one thermoplastic electrically insulating layer, includes the steps of: impregnating a thermoplastic material in subdivided solid form, having a melting enthalpy equal to or lower than 70 J/g, with a dielectric fluid to obtain an impregnated thermoplastic material; feeding the impregnated thermoplastic material in subdivided solid form to a single-screw extruder; and extruding the impregnated thermoplastic material onto the at least one electrically conductive core, so as to form the at least one thermoplastic electrically insulating layer, whereby the impregnated thermoplastic material is not subjected to any mechanical homogenization step in a molten state. Energy cables having a large amount of the dielectric fluid in the electrically insulting layer, e.g. higher than 10 wt %, are obtained without showing any morphological defects in the layer itself and any drawbacks in the extrusion process, even when the rotation speed of the extruder screw, and therefore, the cable production speed, are high (e.g. higher than 20 m/min for medium voltage cable).

Claims

1. A process for producing an energy cable comprising at least one electrically conductive core and at least one thermoplastic electrically insulating layer, which comprises the steps of: impregnating a thermoplastic material in subdivided solid form, having a melting enthalpy equal to or lower than 70 J/g, with a dielectric fluid to obtain an impregnated thermoplastic material; feeding said impregnated thermoplastic material in subdivided solid form to a single-screw extruder; and extruding the impregnated thermoplastic material by means of the single-screw extruder onto said at least one electrically conductive core, so as to form said at least one thermoplastic electrically insulating layer, whereby said impregnated thermoplastic material is not subjected to any mechanical homogenization step in a molten state.

2. The process according to claim 1, wherein the thermoplastic material is impregnated in the form of granules or pellets, having an average dimension of from 2 to 7 mm.

3. The process according to claim 2, wherein the thermoplastic material is impregnated in the form of granules or pellets, having an average dimension of from 3 to 6 mm.

4. The process according to claim 1, wherein the thermoplastic material is impregnated with an amount of the dielectric fluid of from 8% to 40% by weight, with respect to the weight of the thermoplastic material.

5. The process according to claim 4, wherein the thermoplastic material is impregnated with an amount of the dielectric fluid of from 0% to 30% by weight, with respect to the weight of the thermoplastic material.

6. The process according to claim 4, wherein the thermoplastic material is impregnated with an amount of the dielectric fluid of 15% to 25% by weight, with respect to the weight of the thermoplastic material.

7. The process according to claim 1, wherein the impregnation step is carried out on the thermoplastic material pre-heated at a temperature of from 30 C. to 110 C.

8. The process according to claim 7, wherein the impregnation step is carried out on the thermoplastic material pre-heated at a temperature of from 50 C. to 90 C.

9. The process according to claim 1, comprising temporarily storing the impregnated thermoplastic material between the impregnating and the feeding steps.

10. The process according to claim 1, wherein a medium voltage energy cable is produced with a production speed of at least 20 m/min.

11. The process according to claim 10, wherein a medium voltage energy cable is produced with a production speed of at least 30 m/min.

12. The process according to claim 1, wherein the thermoplastic material has a melting enthalpy from 30 to 60 J/g.

13. The process according to claim 1, wherein the thermoplastic material is selected from: at least one copolymer (i) of propylene with at least one olefin comonomer selected from ethylene and an -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 70 J/g; a blend of at least one copolymer (i) with at least one copolymer (ii) of ethylene with at least one -olefin, said copolymer (ii) having a melting enthalpy of from 0 J/g to 120 J/g; and 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.

14. The process according to claim 1, wherein the dielectric fluid has a melting point or a pour point of from 130 C. to +80 C.

15. The process according to claim 1, wherein the dielectric fluid has a viscosity, at 40 C., of from 1 cSt to 100 cSt (measured according to ASTM standard D445-03).

16. The process according to claim 15, wherein the dielectric fluid has a viscosity, at 40 C., of from 5 cSt to 100 cSt (measured according to ASTM standard D445-03).

17. The process according to claim 1, wherein the dielectric fluid is selected from: aromatic oils, either monocyclic, polycyclic (condensed or not) or heterocyclic, wherein aromatic or heteroaromatic moieties are substituted by at least one alkyl group C.sub.1-C.sub.20, and mixtures thereof, and wherein, when two or more cyclic moieties are present, such moieties may be linked by an alkenyl group C.sub.1-C.sub.5.

18. The process according to claim 1, wherein the dielectric fluid is selected from: mineral oils, naphthenic oils, aromatic oils, paraffinic oils, polyaromatic oils, said mineral oils optionally containing at least one heteroatom selected from oxygen, nitrogen or sulfur; and liquid paraffins.

19. The process according to claim 1, wherein one or more additives selected from: antioxidants, processing aids, voltage stabilizers, and nucleating agents, are added to the thermoplastic material during the impregnation step.

20. The process according to claim 19, wherein one or more additives in solid form are dispersed into the dielectric fluid before impregnation.

Description

BRIEF DESCRIPTION OF THE DRAWING

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

(2) FIG. 1 is a perspective view of an energy cable, particularly suitable for medium or high voltage, which can be produced according to the present invention;

(3) FIG. 2 is a schematic representation of a plant to carry out the process according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) In FIG. 1, the cable (1) comprises a conductor (2), an inner layer with semiconductive properties (3), an intermediate layer with insulating properties (4), an outer layer with semiconductive properties (5), a metal screen layer (6) and a sheath (7). The combination of conductor (2) and inner layer with semiconductive properties (3) corresponds to the electrically conductive core as described above.

(5) The conductor (2) generally consists of metal wires, preferably of copper or aluminium or alloys thereof, stranded together by conventional methods, or of a solid aluminium or copper rod.

(6) The insulating layer (4) is produced according to the present invention. The semiconductive layers (3) and (5) are also made by extruding polymeric materials usually based on polyolefins, preferably a thermoplastic material as described above, which is made semiconductive by adding at least one conductive filler, usually carbon black.

(7) 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 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 thereof.

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

(9) The cable can be also provided with a protective structure (not shown in FIG. 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.

(10) In FIG. 2, a schematic representation of a plant to carry out the process according to the present invention is provided. The plant comprises a mixer (8) wherein the thermoplastic material and the dielectric fluid are fed, which may come from, respectively, a pellet container (9) and a tank (10). Before being fed into the mixer (8), the thermoplastic material is preferably heated in a heater (17), for example at a temperature of 50-100 C. Alternatively, the thermoplastic material can be heated into the mixer (8) before the addition of the dielectric fluid and, optionally, of additives, e.g. antioxidant.

(11) In the mixer (8) the step of impregnation occurs, and the impregnated thermoplastic material is then fed to the extruder (13) usually by means of a hopper (12). Advantageously, a storage unit (11) can be provided between the mixer (8) and the hopper (12) in order to temporarily store the impregnated thermoplastic material so as to guarantee a continuous feeding of the extrusion apparatus and a maturation of the impregnated material.

(12) The extruder (13) comprises a barrel (14) and a screw (15) wherein the impregnated thermoplastic material (11) is melted and kneaded. The extruder (13) is driven by an engine to cause rotation of the screw and is provided by suitable heating units, in order to heat and melt the polymer material (not represented in FIG. 2), according to well known techniques.

(13) The deposition and shaping of the thermoplastic material is usually carried out by means of an extrusion head (16) placed at the end of the extruder (13).

(14) The extrusion head is preferably a triple extrusion head, which allows to co-extrude onto the conductor, in a single pass, the inner semiconductive layer, the intermediate electrically insulating layer, and the outer semiconductive layer. Alternatively, a tandem method can be performed, wherein individual extruders are arranged in series. Further apparatuses are included in the production plant to provide the cable with the metal screen layer and the sheath.

(15) In the schematic representation of FIG. 2, the thermoplastic insulating material is extruded on a cable core (18), comprising an electrical conductor surrounded by an inner semiconductive layer, by the extrusion head (16). Subsequently, the outer semiconductive layer is formed onto the external surface of the thermoplastic insulating layer by means of another extruder (not represented in FIG. 2).

(16) FIGS. 1 and 2 show only one embodiment of the present 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.

(17) The following examples are provided to further illustrate the invention.

EXAMPLES

(18) Two prototype cables were prepared with a laboratory extrusion line, with the same composition of materials but by different manufacturing procedures for the insulating composition. The composition comprised, as polymeric base, a first polypropylene copolymer having a melting enthalpy of 30 J/g and a second polypropylene copolymer having a melting enthalpy of 65 J/g, the first and second polypropylene copolymer being in a weight ratio of 75/25. As dielectric fluid, a naphthenic oil having a viscosity of 25 cSt (at 40 C.) was used. The composition further comprised an antioxidant in an amount of 0.3 wt % which was added to the thermoplastic material together with the dielectric fluid.

(19) Both prototypes had a 70 mm.sup.2 aluminum conductor and were extruded with a catenary line. The semiconductive composition, the same in both cables, was the same thermoplastic polypropylene composition as indicated above, added with conductive carbon black.

(20) The insulation of the first cable was prepared and extruded as follows. Polypropylene pellets were charged in a turbomixer, mixed and heated up to 90 C. Upon reaching said temperature, a dielectric fluid in an amount of 15 wt % was added to the polypropylene pellets and the mixing was continued at 90 C. After 25 minutes of mixing, the dielectric fluid was absorbed by the polypropylene pellets, which resulted to be dry. The polypropylene/dielectric fluid material was discharged and fed into a single screw extruder and the extrusion was carried out with the following extrusion temperature profile:

(21) zone 1: 160 C.; zone 2: 180 C.; zone 3: 200 C.; zone 4: 200 C.; zone 5: 200 C.; zone 6: 210 C. The screw rotation speed was 7 rpm.

(22) The insulation of the second cable was extruded by directly injecting 15 wt % of dielectric fluid in the barrel of an identical single screw extruder as used above. Polypropylene feeding was carried out with pellets as obtained from the raw material supplier, without any preliminary treatments. The extrusion temperature profile was the same as indicated above. The screw speed was 10 rpm.

(23) The insulation extrusion speed was initially fixed at 2 m/min for both cables and apparently no significant phenomena were observed. Then, the extrusion speed was increased to 3 m/min: in the second cable quite evident morphological defects appeared, due to the incomplete mixing and absorption of the dielectric fluid. These defects significantly affected the insulation quality and cannot be tolerated. Moreover, an abnormal bulging of the insulation layer was observed in some points of the extruded cable, with breakage of the outer semiconductive layer, due to local accumulation of the dielectric fluid. Conversely, the insulation layer of the first cable, obtained by the process according to the present invention, showed no defects at 3 m/min speed.