Joint for electric cables with thermoplastic insulation and method for manufacturing the same

Abstract

A method for manufacturing an electric cable joint is described. The method includes: a step of providing two electric cables, each cable containing an electric conductor and a thermoplastic insulation system surrounding the electric conductor and containing an inner and an outer thermoplastic semiconducting layers and a thermoplastic insulating layer; a step of joining the terminal portions of the electric conductors of the first and second electric cables to form an electric conductor joint; a step of surrounding the electric conductor joint with a joint inner layer of a thermoplastic semiconducting material having a dynamic storage modulus E.sub.1; a step of surrounding the joint inner layer with a joint insulating layer of a thermoplastic insulating material having a dynamic storage modulus E.sub.2 smaller than E.sub.1; and a step of surrounding the joint insulating layer with a joint outer layer of a thermoplastic semiconducting material having a dynamic storage modulus E.sub.3.

Claims

1. A method for manufacturing an electric cable joint, the method comprising: providing a first electric cable and a second electric cable, each cable comprising an electric conductor and a thermoplastic insulation system surrounding the electric conductor; joining respective terminal portions of the electric conductors of the first electric cable and of the second electric cable placed axially adjacent to the first electric cable to form an electric conductor joint; surrounding the electric conductor joint with a joint inner layer of a first non-crosslinked thermoplastic semiconducting material having a first dynamic storage modulus (E.sub.1); surrounding the joint inner layer with a joint insulating layer of a non-crosslinked thermoplastic insulating material having a second dynamic storage modulus (E.sub.2); and surrounding the joint insulating layer with a joint outer layer of a second non-crosslinked thermoplastic semiconducting material having a third dynamic storage modulus (E.sub.3), wherein the first dynamic storage modulus (E.sub.1) of the first non-crosslinked thermoplastic semiconducting material of the joint inner layer is greater than the second dynamic storage modulus (E.sub.2) of the non-crosslinked thermoplastic insulating material of the joint insulating layer, the dynamic storage moduli (E.sub.1, E.sub.2) being measured at a substantially same measurement temperature of at least 130 C.

2. The method according to claim 1, wherein the first dynamic storage modulus (E.sub.1) is more than 20% greater than the second dynamic storage modulus (E.sub.2).

3. The method according to claim 2, wherein the first dynamic storage modulus (E.sub.1) is more than 50% greater than the second dynamic storage modulus (E.sub.2).

4. The method according to claim 3, wherein the first dynamic storage modulus (E.sub.1) is more than 100% greater than the second dynamic storage modulus (E.sub.2).

5. The method according to claim 1, wherein the second dynamic storage modulus (E.sub.2) is at least 10% of the third dynamic storage modulus (E.sub.3) of the second non-crosslinked thermoplastic semiconducting material of the joint outer layer, the dynamic storage moduli (E.sub.2, E.sub.3) being measured at a substantially same measurement temperature of at least 130 C.

6. The method according to claim 5, wherein the second dynamic storage modulus (E.sub.2) is equal to or greater than the dynamic storage modulus (E.sub.3).

7. The method according to claim 1, wherein in the thermoplastic insulation system of each cable, the non-crosslinked thermoplastic insulating layer is in direct contact with the inner non-crosslinked thermoplastic semiconductor layer, and the outer non-crosslinked thermoplastic semiconductor layer is in direct contact with the non-crosslinked thermoplastic insulating layer; and the joint insulating layer is in direct contact with the joint inner layer, and the joint outer layer is in direct contact with the joint insulating layer.

8. A joint for joining a first electric cable and a second electric cable, each cable comprising an electric conductor and a thermoplastic insulation system surrounding the electric conductor; the joint comprising: a joint inner layer of a first non-crosslinked thermoplastic semiconducting material having a first dynamic storage modulus (E.sub.1) and surrounding the electric conductors of the first and second electric cables; a joint insulating layer of a non-crosslinked thermoplastic insulating material having a second dynamic storage modulus (E.sub.2) and surrounding the joint inner layer; and a joint outer layer of a second non-crosslinked thermoplastic semiconducting material having a third dynamic storage modulus (E.sub.3) and surrounding the joint insulating layer, wherein the first dynamic storage modulus (E.sub.1) of the first non-crosslinked thermoplastic semiconducting material of the joint inner layer is greater than the second dynamic storage modulus (E.sub.2) of the non-crosslinked thermoplastic insulating material of the joint insulating layer, the dynamic storage moduli (E.sub.1, E.sub.2) being measured at a substantially same measurement temperature of at least 130 C.

9. The joint according to claim 8, wherein the second dynamic storage modulus (E.sub.2) is at least 10% of the third dynamic storage modulus (E.sub.3) of the second non-crosslinked thermoplastic semiconducting material of the joint outer layer, the dynamic storage moduli (E.sub.2, E.sub.3) being measured at a substantially same measurement temperature of at least 130 C.

10. The joint according to claim 9, wherein the second dynamic storage modulus (E.sub.2) is equal to or greater than the dynamic storage modulus (E.sub.3).

11. The joint according to claim 8, which is a diameter joint.

12. The joint according to claim 8, wherein in the thermoplastic insulation system of each cable, the non-crosslinked thermoplastic insulating layer is in direct contact with the inner non-crosslinked thermoplastic semiconductor layer, and the outer non-crosslinked thermoplastic semiconductor layer is in direct contact with the non-crosslinked thermoplastic insulating layer; and the joint insulating layer is in direct contact with the joint inner layer, and the joint outer layer is in direct contact with the joint insulating layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a side view of a medium/high voltage electric cable, shown during an initial step of the method for manufacturing a joint according to the present invention; and

(3) FIG. 2 is a cross-section view of the medium/high voltage electric cable of FIG. 1, at the completion of the method for manufacturing a joint according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) In FIG. 1 a first electric cable 100 and a second electric cable 200 are schematically represented. The first electric cable 100 and the second electric cable 200 are placed axially adjacent one to another, so as to be subsequently joined together.

(5) Each cable 100, 200 comprises an electric conductor 10, 20 and an insulation system 12, 22 surrounding the respective electric conductor 10, 20. The insulation system 12, 22 comprises an inner thermoplastic semiconducting layer 14, 24 that encircles and is in direct contact with the respective electric conductor 10, 20 of the electric cable 100, 200. The insulation system 12, 22 of each electric cable 100, 200 further comprises a thermoplastic insulating layer 16, 26 that encircles and is in direct contact with the inner thermoplastic semiconducting layer 14, 24, and an outer thermoplastic semiconducting layer 18, 28 that encircles and is in direct contact with the thermoplastic insulating layer 16, 26. The insulation system 12, 22 of each electric cable 100, 200 is then sequentially surrounded by a metal screen 30, 40 and by one or more outer jackets 32, 42 made, for example, of polyethylene.

(6) In FIG. 2 a joint 300 for joining together the first electric cable 100 and the second electric cable 200 is schematically represented in cross-section. The joint 300 comprises a joint inner layer 50 made of a first thermoplastic semiconducting material, a joint insulating layer 60 made of a thermoplastic insulating material, and a joint outer layer 70 made of a second thermoplastic semiconducting material. The joint inner layer 50, the joint insulating layer 60 and the joint outer layer 70 are respectively configured for rebuilding the inner thermoplastic semiconducting layer 14, 24, the thermoplastic insulating layer 16, 26 and the outer thermoplastic semiconducting layer 18, 28 of the cable insulation system 12, 22.

(7) The method for manufacturing the joint 300 comprises the step of joining respective terminal portions of the electric conductors 10, 20 of the first 100 and the second 200 electric cables, so as to form an electric conductor joint 80. The electric conductor joint 80 can be obtained, for example, through a compression clamp if the electric conductors 10, 20 are made of copper, or through metal inert gas (MIG) welding if the electric conductors 10, 20 are made of aluminium.

(8) The method for manufacturing the joint 300 further comprises the step of surrounding the electric conductor joint 80 with the joint inner layer 50 made of a first thermoplastic semiconducting material having a first dynamic storage modulus E.sub.1.

(9) The dynamic storage modulus E rates the stored energy of a specific polymeric material when a sinusoidal force (stress ) is applied to said material and the resulting displacement (strain ) is measured.

(10) The method for manufacturing the joint 300 further comprises the step of surrounding the joint inner layer 50 with the joint insulating layer 60 made of thermoplastic insulating material having a corresponding second dynamic storage modulus E.sub.2. Thereafter, the joint insulating layer 60 is surrounded with the joint outer layer 70.

(11) The joint outer layer 70 can be made of a second thermoplastic semiconducting material having a corresponding third dynamic storage modulus E.sub.3.

(12) The dynamic storage modulus E.sub.1, E.sub.2, E.sub.3 of each corresponding joint layer 50, 60, 70 is measured at a temperature of at least 130 C., said measurement temperature being substantially the same for all of the dynamic storage moduli of the electric cable joint.

(13) According to the invention, the thermoplastic materials of the joint 300 are provided such that the first dynamic storage modulus E.sub.1 of the joint inner layer 50 is greater than the second dynamic storage modulus E.sub.2 of the joint insulating layer 60.

(14) Advantageously, the joint outer layer 70 is made of a second thermoplastic semiconducting material having a corresponding third dynamic storage modulus E.sub.3. The second dynamic storage modulus E.sub.2 of the joint insulating layer 60 is at least 10% of the third dynamic storage modulus E.sub.3 of the joint outer layer 70, the dynamic storage moduli being measured at a substantially same measurement temperature of at least 130 C.

(15) In a preferred configuration, each layer of the joint 300 has a dynamic storage modulus E that is greater than the dynamic storage modulus E of the radially external layer when measured at a measurement temperature of at least 130 C., said measurement temperature being the same for all of the dynamic storage moduli of the electric cable joint.

(16) However, the joint insulating layer 60 can be made of a thermoplastic material having a second dynamic storage modulus E.sub.2 lower than that of the thermoplastic material of the radially external layer, i.e. the joint outer layer 70, because the joint insulating layer 60 has a thickness significantly greater than that of the joint outer layer 70 (typically from 15 to 30 times greater). This superior thickness allows the joint insulating layer 60 keeping its shape and homogeneity when the thinner joint outer layer 70 is applied thereupon and heated, providing that the second dynamic storage modulus E.sub.2 has a value not lower than 10% of the dynamic storage modulus E.sub.3 of the radially external joint outer layer 70, the dynamic storage moduli being measured at the substantially same measurement temperature of at least 130 C.

(17) Each joint layer 50, 60, 70 can be made in the form of a tape to be helically wound around the electric conductor 10, 20. Each tape is made of a thermoplastic insulating or semiconducting material chemically compatible with and having substantially the same electrical properties of the corresponding thermoplastic insulating or semiconducting material of the corresponding inner 14, 24, intermediate 16, 26 and outer 18, 28 cable layer, so as to restore the cable continuity over the electric conductors 10, 20. Each tape is preferably obtained by extrusion.

(18) Each joint layer 50 in form of tape is wound around the electric conductor joint 80 according to a conventional procedure known in the field of cable joint manufacture. Subsequently, each joint layer 50, 60, 70 is submitted to a heating step to a temperature suitable for melting the thermoplastic material thereof and for converting the material from the shape of a tape winding to a shape of a homogeneous cylinder.

(19) The joint inner layer 50 is submitted to two heating steps further the one for its deposition and homogenization, i.e. the step for melting and homogenization of the joint insulating layer 60 and the step for melting and homogenization of the joint outer layer 70. Analogously, the joint insulating layer 60 is submitted to one heating step further the one for its deposition and homogenization, i.e. the step for melting and homogenization of the joint outer layer 70. The joint outer layer 70 is thus submitted to a single heating step for its own melting and homogenization.

(20) As sketched in FIG. 2, the insulating layer 60 has a thickness significantly greater than the thickness of the joint semiconducting layer 50 and 70. Such a greater thickness results in a longer heat treatment to have the tape/s for obtaining the insulating layer 60 melted and, accordingly, the stability of the already applied joint and underlying semiconducting layer 50 is particularly challenged.

(21) The material used to apply the joint inner layer 50 has the highest thermomechanical resistance in terms of a relatively high dynamic storage modulus E.sub.1, while the material of the joint insulating layer 60 and the material forming the joint outer layer 70 has relatively low dynamic storage moduli E.sub.2, E.sub.3, the dynamic storage moduli being measured at the same measurement temperature.

(22) Each heating step of the thermoplastic joint layers 50, 60, 70 can be performed by enclosing the layer still in form of wound tape in a metal cylindrical case made, for example, of a suitable number of metal slit collars (e.g., copper split collars), which is then connected to a thermocouple. The shape of the cylindrical caseand/or of the slit collarseases the obtainment of a layer with a suitable shape.

(23) Between the tape to be melted and the cylindrical case various coatings can be provided, to protect the tape from overheating due to direct contact with the metal cylindrical case, to ease the removal of the case or of other coatings at the end of the heating step and/or to mechanically cushion the joint layer 50, 60, 70 during the heating treatment. These coatings can be similar to those known in the art for the manufacturing of cross-linked polymer joints.

(24) The process of the invention is preferably carried out at atmospheric pressure. It has been found that the application of a pressure greater than the atmospheric one not only brings no benefit to the stabilization and homogenization of the joint layers, but also could cause deformation of the joint layers.

(25) The outer layer 70 could also be prepared in a different way with respect to the tape.

(26) The materials used for manufacturing each joint layer 50, 60 and preferably 70 are substantially of the same nature than the corresponding ones of the cable insulation system 12, 22, but the composition of these material is preferably varied in terms of crystallinity content and filler content to reach the desiderata properties.

(27) Preferably, the thermoplastic materials for the insulating system of the electric cables to be joined and, accordingly, of the diameter joint of the present invention and, accordingly, of the electric cables to be joined, can be based on a polypropylene matrix intimately admixed with a dielectric fluid disclosed, for example, in WO 02/03398, WO 02/27731, WO 04/066317, WO 04/066318, WO 07/048422, WO 2011/092533 and WO 08/058572.

(28) The polypropylene matrix useful for thermoplastic cables and relevant joints can be a thermoplastic polymer material 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 140 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 -olefin, said copolymer (ii) having a melting enthalpy of from 0 J/g to 70 J/g; a blend of at least one propylene homopolymer with at least one copolymer (i) or copolymer (ii).

(29) Suitable compatibility between the dielectric fluid and the polymer base material is advantageous to obtain a microscopically homogeneous dispersion of the dielectric fluid in the polymer base material. The dielectric fluid suitable for forming the thermoplastic layers 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.

(30) The thermoplastic polymer material is characterized by a relatively low crystallinity such to provide the cable with the suitable flexibility, but not to impair the mechanical properties and thermopressure resistance at the cable operative and overload temperatures. Performance of the cable insulating system 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 thermopressure resistance and should be such to be intimately and homogeneously mixed with the polymeric matrix.

(31) Once the construction of the joint outer layer 70 has been completed, a screening of each joint layer 50, 60, 70 through an X-ray inspection can be performed, so as to check the presence of any defects or undesired inclusions in one or more of said joint layers 50, 60, 70. One or more X-ray inspections can be carried out on single or couples of joint layers during the electric cable joint construction.

(32) The joint outer layer 70 is finally covered by subsequently rebuilt layers of the metal screen 30, 40 and of the one or more outer jackets 32, 42. The metal screen rebuilding can be performed, for example, with a brazing process, while the outer jackets are usually rebuilt by using polymer (e.g. polyethylene) shrinkable tubes or adhesive tapes.

(33) An example is provided of a diameter joint according to the present invention. The joint comprises the following layers: an inner semiconducting layer (IS) made of a 30/70 mixture of a polypropylene random copolymer (melting enthalpy: 65 J/g; density: 0.900 g/cm.sup.3; melting temperature: 144 C.) and an ethylene-propylene heterophase copolymer (melting enthalpy: 30 J/g; density: 0.880 g/cm.sup.3; melting temperature: 160.5 C.) containing 6 wt % of dibenzyltoluene as dielectric fluid and 65 wt % of carbon black; an insulating layer (I) made of an ethylene-propylene heterophase copolymer (melting enthalpy: 30 J/g; density: 0.880 g/cm.sup.3; melting temperature: 160 C.) containing 5 wt % of dibenzyltoluene as dielectric fluid; an outer semiconducting layer (OS) made of an ethylene-propylene heterophase copolymer (melting enthalpy: 12 J/g; density: 0.870 g/cm.sup.3; melting temperature: 154 C.) containing 6 wt % of dibenzyltoluene as dielectric fluid and 40 wt % of carbon black.

(34) The thermoplastic materials of the three joint layers have the characteristics set forth in the following Table 1.

(35) TABLE-US-00001 TABLE 1 E @ T.sub.room E @ 130 C. E @ 150 C. E @ T > 170 C. Layer (MPa) (MPa) (MPa) (MPa) IS 417.7 39.8 10.7 4.01 I 85.8 5.2 1.5 <1 OS 80 3.6 1.6 0.6

(36) The evaluation of the dynamic storage moduli of the materials has been made by dynamic mechanical thermal analysis (DMTA) with oscillation strain 0.1%, static force 0.01 N and force track 125%. The test samples had a rectangular shape (length 57 mm; width 4.06 mm; thickness 0.50.7 mm). All samples have been equilibrated at 20 C. for 5 minutes.

(37) The measurement of the dynamic storage moduli of the three joint layers was performed at various measurement temperatures. The difference among modulus values makes the three thermoplastic materials suitable for being part of the same electric joint cable. In particular, the dynamic storage modulus of the inner semiconducting layer (IS) is far greater than that of the insulating layer (I). Such a difference between the dynamic storage moduli provides a suitable safety margin in the electric cable joint manufacturing method.