ELECTRIC CABLE HAVING A PROTECTING LAYER

Abstract

An electric cable comprising: a cable core having a radius and comprising an electric conductor and an insulating system; a first metal layer in radially outer position with respect to the cable core; and a semiconductive protecting layer between the cable core and the first metal layer. The insulating system comprises an insulating layer based on a propylene copolymer and the protecting layer is made of non-expanded material and has a thickness of from 0.5% to 3% the radius of the cable core. The cable is apt to safely operate at thermal cycles of from 20 C. to at least 90 C. (for example up to 130 C. or more), with substantially no deformation of the metal layer and of the overall cable structure.

Claims

1. Electric cable comprising a cable core having a radius and comprising an electric conductor and an insulating system; a first metal layer in radially outer position with respect to the cable core; and a semiconductive protecting layer between the cable core and the first metal layer, characterized in that the insulating system comprises an insulating layer based on a propylene copolymer and in that the protecting layer is made of non-expanded material and has a thickness of from 0.5% to 3% of the cable core radius.

2. Electric cable according to claim 1 wherein the insulating system comprises an inner semiconductive layer and an outer semiconductive layer provided, respectively, in a radially inner and outer position with respect to the insulating layer and in direct contact with the insulating layer, and the inner semiconductive layer and an outer semiconductive layer are made of a propylene copolymer.

3. Electric cable according to claim 2 wherein the protecting layer is provided between the outer semiconducting layer and the first metal layer and in direct contact with these layers.

4. Electric cable according to claim 1 wherein the protecting layer is water swellable.

5. Electric cable according to claim 4 wherein the protecting layer comprises water swellable powder constituted by particles having an average diameter of from 10 to 150 m.

6. Electric cable according to claim 1 wherein the protecting layer is made of a nonwoven fabric based on one or more non-expanded polymer.

7. Electric cable according to claim 1 wherein the protecting layer is in form of a tape helically wounded or longitudinally folded around the cable core.

8. Electric cable according to claim 1 wherein the thickness of the protecting layer is of from 1% to 2% of the cable core radius.

9. Electric cable according to claim 1 comprising a second metal layer in a radially outer position with respect to the first metal layer.

10. Electric cable according to claim 1 comprising a cushioning layer provided in radial external position with respect to the first metal layer, the cushioning layer is made of a non-expanded material.

11. Electric cable according to claim 1 comprising a second metal layer and a non expanded and semiconductive cushioning layer provided in radial external position with respect to the first metal layer in radial internal position with respect to the second metal layer.

12. Electric cable according to claim 1 wherein the propylene copolymer is an ethylene-propylene copolymer.

13. Electric cable according to claim 12 wherein the ethylene propylene copolymer is a heterophasic copolymer, a random copolymer or a mixture thereof.

14. Electric cable according to claim 13 wherein the heterophasic ethylene-propylene copolymer for the cable of the invention comprises an elastomeric phase in an amount of from 45 to 85 wt % with respect to the total weight of the copolymer.

15. Electric cable according to claim 1 wherein the propylene copolymer is admixed with a dielectric fluid in an amount of from 1 wt % to 10 wt %.

Description

[0049] Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawing, in which:

[0050] FIG. 1 shows a cross-section of an embodiment of an electrical cable according to the present invention;

[0051] FIG. 2 shows a cross-section of another embodiment of an electrical cable according to the present invention.

[0052] The cable (10) of FIG. 1 is a single core cable comprising a conductor (11) sequentially surrounded by an inner layer semiconducting layer (12), an insulating layer (13) and an outer semiconducting layer (14), these three layers constituting the insulating system. All of the layers of the insulating system are made of a propylene copolymer.

[0053] The outer semiconducting layer (14) is surrounded by and in contact with a semiconducting, water-swellable and non-expanded protecting layer (15) which, in turn, is surrounded by and in contact with a first metal layer (16). In the present embodiment, the first metal layer (16) is in form of a longitudinally welded aluminium foil and acts as electric screen and as radial water barrier.

[0054] An outer sheath (17), in polymeric material, e.g. HDPE, is the outermost layer.

[0055] The cable (20) of FIG. 2 is a single core cable comprising a conductor (21) sequentially surrounded by an inner semiconducting layer (22), an insulating layer (23) and an outer semiconducting layer (24), these three layers constituting the insulating system. All of the layers of the insulating system are made of a propylene copolymer.

[0056] The outer semiconducting layer (24) is surrounded by and in contact with a semiconducting, water-swellable and non-expanded protecting layer (25) which, in turn, is surrounded by and in contact with a first metal layer (26). In the present embodiment, the first metal layer (26) is in form of helically wound copper wires and acts as electric screen.

[0057] Cable (20) further comprises a second metal layer (29) and a semiconductive and non-expanded cushioning layer (28) in a radially internal position. Due to its semiconductive property, the cushioning layer (28) is suitable for establishing an electric contact between the first metal layer (26) and the second metal layer (29).

[0058] The second metal layer (29) is in form of a longitudinally welded aluminium foil and acts as electric screen and as radial water barrier.

[0059] An outer sheath (27), in polymeric material, e.g. HDPE, is the outermost layer.

EXAMPLE 1

[0060] Thermal expansion of high voltage cables was calculated at temperature increasing from 20 C. to 130 C.

[0061] Two samples of cable cores 300 mm long have been tested, the samples having the following features:

Sample 1 (comparative): [0062] 1200 mm.sup.2 Al conductor (diameter: 43 mm) [0063] 2.1 mm inner semiconducting layer [0064] 21.2 mm insulating layer [0065] 1.6 mm outer semiconducting layer

[0066] The insulating layer was made of crosslinked low density polyethylene (LDPE), while the semiconducting layers were made of crosslinked ethyl butyl acetate (EBA).

[0067] The outer diameter was of about 93 mm.

Sample 2 (according to the invention): [0068] 1000 mm.sup.2 Cu conductor (diameter: 40 mm) [0069] 1.4 mm inner semiconducting layer [0070] 16.8 mm insulating layer [0071] 1.1 mm outer semiconducting layer

[0072] The insulating system was made of a mixture 75:25 of, respectively, a heterophasic ethylene-propylene copolymer having about 70 wt % of elastomeric phase and a random ethylene propylene copolymer, the mixture being admixed with 6 wt % of dibenzyltoluene.

[0073] The outer diameter was of about 79 mm.

[0074] Preliminary heating cycles up to 130 C. and return to ambient temperature were carried out to allow shrink back and stabilize insulation length. The thermal expansion was calculated as percentage of the thickness of the insulating layer. The temperatures are measured on the conductor.

TABLE-US-00001 TABLE 1 Temprature Thermal expansion (%) Thermal expansion (%) ( C.) Sample 1* Sample 2 20 0 0 50 2 1.8 70 3.4 2.3 90 5.4 3.9 110 8.2 4.4 130 9.8 5.9

[0075] The cable core according to the invention has a thermal expansion percentage of its diameter much lower than a cable core having an insulating system based on crosslinked polyethylene.

[0076] It has to be noted that at temperatures greater than 90 C., the insulating layer based on crosslinked polyethylene suffered from a drop of hardness (Shore D hardness from 30 after 15 seconds at 90 C. to 0 after 15 seconds at 110 C.) and this prevents the use of this cable at temperature greater than 130 C. (on the conductor). Adversely, in a cable according to the invention, the insulating layer had a Shore D hardness decreasing from 40 after 15 seconds at 90 C. to about 26 after 15 seconds at 130 C.; accordingly, such a hardness drop value is compatible with an acceptable cable performance at 130 C. operating temperature and up to 150 C. or more on the conductor (with a thermal expansion which still remains within limits acceptable for cable integrity).

EXAMPLE 2

[0077] A cable according to the invention having [0078] insulating system made of a mixture 75:25 of, respectively, a heterophasic ethylene-propylene copolymer having about 70 wt % of elastomeric phase and a random ethylene propylene copolymer, the mixture being admixed with 6 wt % of dibenzyltoluene, [0079] cable core outer diameter of 78 mm, [0080] a semiconducting, water-swellable and non-expanded protecting layer made of a polyester nonwoven fabric tape having a thickness of 0.5 mm (1.3% of the cable core radius), and [0081] a first metal layer in form of a longitudinally welded aluminium foil having a thickness of 1 mm,
was tested under repeated thermal cycles according to IEC 62067-01 (2001-10) and successfully passed them (no deformation of the first metal layer was detected).

[0082] A comparative cable having the insulating layer made of cross-linked polyethylene, a cable core outer diameter of 78 mm and a first metal layer in form of a longitudinally welded aluminium foil having a thickness of 1 mm, did not pass the test according to IEC 62067-01 (2001-10) with a semiconducting, water-swellable and non-expanded protecting layer made of a polyester nonwoven fabric tape having a thickness of 0.78 mm (2% of the cable core radius).

[0083] In particular, the repeated thermal cycles caused a plastic deformation of the first metal layer such that, after a cooling step and the relevant cable layer contraction, discontinuities were formed at the interface between the first metal layer and the outer semiconducting layer. Such discontinuities gave place to lack of electrical contact between the metal layer and the underlying outer semiconducting layer, such that sparks have been observed, thereby compromising the electrical performance of the cable; in addition, the presence of such discontinuities compromised also the longitudinal water barrier performance of the protecting layer.