High-voltage electric cable

Abstract

A high-voltage alternate current electric cable [is provided. In the electric cable,] having at least one metallic electric conductor is surrounded by at least one extruded insulating layer. The insulating layer includes from 1 wt % to 30 wt % of a void-containing filler. The filler is made of particles having an average diameter up to 50 pm dispersed in an insulating polymeric material.

Claims

1. A high-voltage alternate current electric cable comprising at least one metallic electric conductor surrounded by at least one extruded insulating layer, wherein said insulating layer comprises from 1 wt % to 30 wt % of a void-containing filler made of particles having an average diameter of from 1 μm to 50 μm dispersed in an insulating polymeric material.

2. The high-voltage alternate current electric cable according to claim 1, wherein said polymeric material is selected from polyolefins comprising homopolymers of a single olefin and copolymers of two or more different olefins.

3. The high-voltage alternate current electric cable according to claim 1, wherein said insulating layer comprises from 5 wt % to 25 wt % of said void-containing filler.

4. The high-voltage alternate current electric cable according to claim 3, wherein said insulating layer comprises from 10 wt % to 20 wt % of said void-containing filler.

5. The high-voltage alternate current electric cable according to claim 1, wherein said particles have an average diameter up to 40 μm.

6. The high-voltage alternate current electric cable according to claim 1, wherein said particles have an average diameter of at least 10 μm.

7. The high-voltage alternate current electric cable according to claim 1, wherein said particles have an average diameter of from 15 μm to 20 μm.

8. The high-voltage alternate current electric cable according to claim 1, wherein said particles are selected from hollow particles, porous particles and particles expandable at an insulation manufacturing temperature and capable of developing a spongy structure after expansion.

9. The high-voltage alternate current electric cable according to claim 1, wherein said particles comprise inorganic or organic materials.

10. The high-voltage alternate current electric cable according to claim 9, wherein said inorganic material is glass or silica aerogel.

11. The high-voltage alternate current electric cable according to claim 9, wherein said organic material is a polymer selected from polypropylene, polymethyl methacrylate, poly-4-methylpentene and a fluorinated polymer.

12. The high-voltage alternate current electric cable according to claim 8, wherein said hollow particles have a wall thickness up to 0.50 μm.

13. The high-voltage alternate current electric cable according to claim 12, wherein said hollow particles have a wall thickness of from 0.20 μm to 0.50 μm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The present invention will be better understood by reading the following detailed description, given by way of example and not of limitation, to be read with the accompanying drawings, wherein:

(2) FIG. 1 shows a perspective view of a HVAC electric cable according to a first embodiment of the present invention;

(3) FIG. 2 shows a cross-section view of a HVAC electric cable according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 shows a first embodiment of an HVAC cable according to the present invention.

(5) According to this embodiment, cable 1 comprises a metallic conductor 11, an inner semiconductive layer 12, an insulating layer 13 and an outer semiconductive layer 14, which constitute the cable core. The cable core is surrounded by a metal shield 15, and an outer sheath 16.

(6) The metallic conductor 11 can be made of copper, aluminium or composites thereof. The metallic conductor 11 can be in form of a rod or can be made of wires stranded together by conventional technique.

(7) The cross sectional area of the metallic conductor 11 is determined in relationship with the power to be transported at the selected voltage. For example, the cross sectional areas for the HVAC cables according to the present invention range from 30 mm.sup.2 to 3,000 mm.sup.2.

(8) The insulating layer 13 according to the present invention is preferably made of a polyolefin polymeric material, for example a polyethylene homopolymer or copolymer or a polypropylene copolymer. In case of polyethylene, the polymeric material is advantageously cross-linked. In the case of a polypropylene copolymer, the insulating layer is preferably based on a polypropylene matrix intimately admixed with a dielectric fluid, as disclosed in WO 02/03398, WO 02/27731, WO 04/066318, WO 07/048422 and WO 08/058572.

(9) The glass hollow spheres suitable for the present invention are marketed, for example, by 3M Company, St. Paul, Minn.

(10) The inner semiconductive layer 12 and the outer semiconductive layer 14 are also made of suitable polymeric materials. According to a preferred embodiment, the polymeric material for the inner semiconductive layer 12 and/or the outer semiconductive layer 14 is similar to that of the insulating layer 13 added with an electro-conductive filler such as carbon black, for example electro-conductive furnace black or acetylene black, so as to confer semiconductive properties to the polymer material, in particular a volumetric resistivity value, at room temperature, of less than 500 Ωm, preferably less than 20 Ωm. Typically, the amount of carbon black can range between 1 wt % and 50 wt %, preferably between 25 wt % and 40 wt %, relative to the weight of the polymer.

(11) The polymeric material for the inner semiconductive layer 12 and/or the outer semiconductive layer 14 can be the same of the insulating layer 13 or chemically compatible thereto.

(12) The use of the same polymeric material for both the insulating layer 13 and at least one of the semiconducting layers 12 and/or 14 is particularly advantageous, since ensures excellent adhesion between adjacent layers and, as a consequence, a good electrical behaviour. This advantage is more evident when the insulating layer 13 and the semiconducting layers 12 and/or 14 are made of the same polymeric material because the electrical filed, as well as the risk of partial discharge, is high at the interface between said two layers.

(13) As represented in FIG. 1, the metal shield 15 is made in the form of metallic wires, for example copper wires, helically wound around the outer semiconductive layer 14.

(14) According to an alternative embodiment, the metal shield is made of a continuous metal tube or sheet, preferably aluminium or copper, surrounding the outer semiconductive layer. In this embodiment, the metal shield is welded or sealed using an adhesive material so as to make it watertight.

(15) The outer sheath 16 preferably is made of polymeric material, such as polyvinyl chloride (PVC) or polyethylene (PE).

(16) The cable according to the present invention can be manufactured by processes known to the person skilled in the art.

(17) In particular, the insulating layer can be produced by (a) mixing an insulating polymeric material with an amount from 1 wt % to 30 wt % of a void-containing filler made of particles having an average diameter up to 50 μm, (b) extruding the resulting polymeric composition onto the metallic conductor, covered with an inner semiconducting layer, and, optionally, (c) cross-linking said polymeric composition.

(18) In the step (a), the void-containing filler is added to the polymeric material when the latter is in form of a molten mass contained in a continuous mixer of Ko-Kneader type (Buss) or of co- or counter-rotating double-screw type; or in a single screw extruder. The resulting mechanical mixture of void-containing filler and insulating polymeric material is extruded (step (b)) onto the metal conductor, covered with an inner semiconducting layer, through an extrusion head comprising at least one extruder. Advantageously, said extrusion head comprises three extruders (triple-head extrusion) for the co-extrusion of insulation layer and semiconductive layers. The co-extrusion gives place to an improved adhesion among the co-extruded layers resulting in a better cable performance.

(19) The cross-linking step (c), if any, can be carried out immediately after the extrusion step, by passing the metallic conductor covered with said layer of polymeric composition through a vulcanising tube.

(20) Other conventional additives that can be added to the polymeric composition of the insulation layer, such as antioxidants, heat stabilizers, processing aids, lubricants, voltage stabilizer additives, antiscorching agents, and mixtures thereof.

(21) The HVAC cable according to the present invention can also be completed by the addition of other protective coverings (armouring), not represented in the figures.

(22) FIG. 2 shows another embodiment of the HVAC cable according to the present invention. According to this embodiment, cable 2 comprises three cable cores, each comprising a metallic conductor 21, an inner semiconductive layer 22, an insulating layer 23, and an outer semiconductive layer 24. Each cable core is surrounded by a metal shield 25. The three cable cores are stranded and embedded into a filler (or bedding) 30 which, in turn, is surrounded by an outer sheath 26.

(23) In the embodiment represented in FIG. 2, the HVAC cable 2 also comprises three ground conductors 29, each comprising a metallic conductor 27 and an insulating layer 28.

(24) The metallic conductors 21 and 27, the inner semiconductive layer 22, the insulating layers 23 and 28, the outer semiconductive layer 24, the metal shield 25 and the outer sheath 26 can be made of materials already disclosed in connection with cable 1 of FIG. 1 for analogous cable layers.

(25) The present invention will be explained in more detail below by way of examples, which are not intended to be limiting of the present invention.

EXAMPLE 1

(26) The following compositions were prepared with the amounts reported in the following table 1.

(27) As for sample B, low density polyethylene (LDPE) was compounded with hollow glass microspheres in the amounts according to the following table 1 in an open mixer at the temperature of 170° C. The polymeric material was melted in the mixer and the glass microspheres were subsequently added step by step within 20 minutes, by continuous mixing. The material was then collected and grinded.

(28) TABLE-US-00001 TABLE 1 Sample A(*) B LDPE (g) 700.00 700.00 XLD 6000 (g) — 125.00 (*)comparative

(29) XLD 6000: hollow glass microspheres having an average diameter of 18 μm, a wall thickness of 0.35 μm and filled with a mixture of CO.sub.2+N.sub.2 (manufactured by 3M Company, St. Paul, Minn.).

(30) The density of the samples was measured according to CEI EN 60811-1-3 (2001). The results are disclosed in the following table 2.

(31) TABLE-US-00002 TABLE 2 Sample A(*) B Density (g/cm.sup.3) 0.914 0.762 (*)comparative

(32) The analysis showed a density decrease for sample B according to the present invention when compared to the reference sample A.

EXAMPLE 2

(33) The dielectric constant of the samples of Example 1 was measured by a bridge impedance tester LCR HP4284A (Hewlet-Packard) in term of relative permittivity (∈.sub.r) at different frequencies. The results are set forth in table 3.

(34) TABLE-US-00003 TABLE 3 Sample A(*) B ∈.sub.r at 50 Hz 2.41 2.34 ∈.sub.r at 50 kHz 2.41 2.33 ∈.sub.r at 100 kHz 2.41 2.31 ∈.sub.r at 300 kHz 2.43 2.31 ∈.sub.r at 400 kHz 2.45 2.33 ∈.sub.r at 600 kHz 2.38 2.25 (*)comparative

(35) The dielectric constant of sample B according to the invention is significantly lower than that of comparative sample A at all of the frequencies tested.

EXAMPLE 3

(36) Sample B of Example 1 was tested in a cell according to CIGRE II method for revealing the possible insurgence of partial discharge phenomena.

(37) Despite the presence of the hollow glass microspheres equivalent to micro-voids in the insulating layer, no partial discharge was detected in one hour up to 15 kV/mm.