WATER TREE RESISTANT ELECTRIC CABLE

Abstract

An electric cable includes at least one polymer layer obtained from a polymer composition having at least one polypropylene-based thermoplastic polymer material and at least one oxygen-containing compound having a melting temperature of about 110 C. or higher.

Claims

1. Electric cable comprising: at least one elongated electrically conductive element; and at least one polymer layer surrounding said elongated electrically conductive element, wherein the polymer layer is obtained from a polymer composition has at least one polypropylene-based thermoplastic polymer material has a propylene homopolymer or copolymer P.sub.1, and at least one water tree reducing agent, said water tree reducing agent being an oxygen-containing compound having a melting temperature of 110 C. or more.

2. Electric cable according to claim 1, wherein the oxygen-containing compound has a melting temperature of 120 C. or more.

3. Electric cable according to claim 1, wherein the oxygen-containing compound has a molecular weight ranging from 200 to 5000000 g/mol.

4. Electric cable according to claim 1, wherein the oxygen-containing compound comprises one or more functions selected from the functions alcohol, ester, acid, acid anhydride, and one of their mixtures.

5. Electric cable according to claim 1, wherein the oxygen-containing compound is selected from aliphatic polyols, polyols comprising at least two primary alcohol functions, and homo- and copolymers of propylene functionalised by acid anhydride functions.

6. Electric cable according to claim 1, wherein the oxygen-containing compound is dipentaerythritol.

7. Electric cable according to claim 1, wherein the oxygen-containing compound is selected from homopolymers of propylene functionalised by maleic anhydride functions.

8. Electric cable according to claim 1, wherein the polymer composition comprises from 0.1 to 15% by weight of oxygen-containing compound, based on the total weight of the polymer composition.

9. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises at least 50% by weight of propylene polymer(s), based on the total weight of the polypropylene-based thermoplastic polymer material.

10. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises a random propylene copolymer or a heterophasic propylene copolymer, as propylene copolymer P.sub.1.

11. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises a random propylene copolymer and a heterophasic propylene copolymer, or two different heterophasic propylene copolymers.

12. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material further comprises an olefin homopolymer or copolymer P.sub.2.

13. Electric cable according to claim 1, wherein the polymer composition further comprises a dielectric liquid.

14. Electric cable according to claim 1, wherein the polymer layer is a non-crosslinked layer.

15. Electric cable according to claim 1, wherein the polymer layer has a reduction in the breakdown voltage after ageing in a wet environment of at most 30%.

16. Electric cable according to claim 1, wherein said electric cable comprises: at least one semi-conductive layer surrounding the elongated electrically conductive element, and at least one electrically insulating layer surrounding the elongated electrically conductive element, at least one of the semi-conductive and electrically insulating layers being a polymer layer as defined in any one of the preceding claims.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0189] FIG. 1 shows a schematic view of an electric cable according to a preferred embodiment in accordance with the invention.

[0190] For reasons of clarity, only the elements essential for understanding the invention have been represented schematically, and are not to scale.

DETAILED DESCRIPTION

[0191] The medium- or high-voltage electric cable 1 conforming to the first object of the invention, shown in FIG. 1, comprises a central elongated electrically conductive element 2, in particular of copper or aluminium. The electric cable 1 further comprises several layers arranged successively and coaxially around this central elongated electrically conductive element 2, namely: a first semi-conductive layer 3 known as the internal semi-conductive layer, an electrically insulating layer 4, a second semi-conductive layer 5 known as the external semi-conductive layer, a metal shield 6 for earthing and/or protection, and an outer protective sheath 7.

[0192] The electrically insulating layer 4 is an extruded non-crosslinked layer, obtained from the polymer composition as defined in the invention.

[0193] The semi-conductive layers 3 and 5 are thermoplastic (i.e. non-crosslinked) extruded layers.

[0194] The presence of the metal shield 6 and the outer protective sheath 7 is preferential, but not essential, as this cable structure per se is well known to the skilled person.

Examples

[0195] 1. Polymer Compositions

[0196] Compositions I1, I2 and I3 in accordance with the invention, i.e. comprising at least one polypropylene-based thermoplastic polymer material and at least one oxygen-containing compound having a melting temperature of about 110 C. or more as a water tree reducing agent, were compared to a comparative composition C1, the composition C1 corresponding to a composition comprising a polypropylene-based thermoplastic polymer material identical to that used for the compositions of the invention I1, I2 and I3, but not comprising an oxygen-containing compound as defined in the invention.

[0197] Table 1 below lists the above-mentioned polymer compositions in which the amounts of the compounds are expressed as percentages by weight, based on the total weight of the polymer composition.

TABLE-US-00001 TABLE 1 Polymer compositions C1 (*) I1 I2 I3 heterophasic propylene copolymer A 53.8 52.3 48.8 51.3 heterophasic propylene copolymer B 15 15 15 15 linear low-density polyethylene 25 25 25 25 oxygen-containing compound 1 0 1.5 0 0 oxygen-containing compound 2 0 0 5 2.5 dielectric mineral oil 5.4 5.4 5.4 5.4 liquid benzophenone 0.3 0.3 0.3 0.3 antioxidant 0.5 0.5 0.5 0.5 (*) Comparative composition not part of the invention

[0198] The origin of the compounds in Table 1 is as follows: [0199] heterophasic propylene copolymer A marketed by LyondellBasell Industries with the product name Moplen EP2967; [0200] heterophasic propylene copolymer B marketed by LyondellBasell Industries with the product name Adflex Q 200F; [0201] linear low-density polyethylene marketed by ExxonMobil Chemicals with the product name LLDPE 1002 YB; [0202] oxygen-containing compound 1 marketed by Perstorp with the product name Voxtar D50, with a melting temperature of 222 C.; [0203] oxygen-containing compound 2 marketed by Honeywell under the product number A-C 907 P, with a saponification index of 87, and a melting temperature of 145 C. for composition I2; and marketed by Clariant under the product number Licocene 6452, with an acid number of 41, and a melting temperature of 134 C. for composition I3; [0204] dielectric liquid comprising about 5.4% by weight of a naphthenic mineral oil marketed by Nynas with the product name Nyflex 223 for compositions C1, I1, I2; and a naphthenic mineral oil marketed by Nynas with the product name Nyflex BNS28 for composition I3; and about 0.3% by weight of benzophenone marketed by Sigma-Aldrich under the product number B9300; and [0205] antioxidant marketed by Ciba with the product name Irganox B 225 which includes an equimolar mixture of Irgafos 168 and Irganox 1010.

[0206] 2. Preparation of Non-Crosslinked Layers and Cables

[0207] The compositions listed in Table 1 are implemented as follows.

[0208] The following components: mineral oil, antioxidant and benzophenone of compositions C1, I1, I2 and I3 referenced in Table 1, for each layer to be considered, are metered and mixed under stirring at about 75 C., in order to form a liquid mixture comprising the dielectric liquid.

[0209] The liquid mixture is then mixed with the following components: heterophasic propylene copolymer A, heterophasic propylene copolymer B, low-density polyethylene, and optionally oxygen-containing compound 1 or 2 of compositions C1, I1, I2 and I3 referenced in Table 1, for each polymer layer to be considered, in a container. Then the resulting mixture is homogenised using a Berstorff twin-screw extruder at a temperature of about 145 to 180 C., then melted at about 200 C. (screw speed: 80 rpm).

[0210] The homogenised and melted mixture is then put in the form of granules.

[0211] Cables are manufactured with a laboratory extruder and subjected to electrical characterizations. Each of the cables comprises: [0212] an electrically conductive element with a cross section of 1.4 to 1.5 mm, [0213] a first semi-conductive layer surrounding said electrically conductive element with a thickness of 0.7 mm, [0214] an electrically insulating polymer layer obtained from the polymer composition of the invention I1 or I2 or I3, or a comparative polymer composition C1, said electrically insulating polymer layer surrounding said first semi-conductive layer, and [0215] a second semi-conductive layer surrounding said electrically insulating layer.

[0216] The cables have a total external diameter of about 6.2 mm and a total length of about 200 m. They are stripped of the second semi-conductive layer to a thickness of 150 m.

[0217] The electrically insulating layer is 1.5 mm thick.

[0218] The semi-conductive layers are thermoplastic layers obtained from a composition comprising at least one polypropylene-based thermoplastic polymer material, and at least one conductive filler in an amount sufficient to render the layers semi-conductive.

[0219] This results in the electric cables Ci1, Ci2, Ci3 and Cc1, respectively, each comprising four extruded layers. The electrically conductive element with a cross section of 1.4 to 1.5 mm is then removed and replaced by an electrically conductive element with a cross section of 0.75 to 0.8 mm in order to be able to put water between said electrically conductive element and the internal semi-conductive layer.

[0220] Breakdown tests before and after ageing in a wet environment were then carried out.

[0221] The method used is that described in the document Model Cable Test for Evaluating the Ageing Behaviour under Water Influence of Compounds for Medium Voltage Cables, H. G. Land and Hans Schdlich, pages 177 to 182, published during the Conference Proceedings of Jicable 91, 24-28 Jun. 1991, in Versailles, France.

[0222] This method consists first in performing breakdown tests with an alternating voltage with a frequency of 50 Hz on unaged samples (conditioned at 90 C. for 16 hours in a non-wet environment) of electric cables Ci1, Ci2, Ci3 and Cc1 to determine the initial value of the breakdown voltage, and then to perform these breakdown tests on aged samples of electric cables Ci1, Ci2, Ci3 and Cc1, powered alternately, in a water tank heated to 70 C. for 1000 hours (according to the conditions referenced Ageing 2 in said document) and in the presence of water heated to 85 C. between the conductor and the internal semi-conductive layer to determine their breakdown voltage after 1000 hours.

[0223] The breakdown electric field (in kV/mm) of the electric cable corresponds to the voltage required to form an electric arc within the cable. It is typically returned to the electric field through the thickness of the electrically insulating layer, between the first semi-conductive layer (or internal semi-conductive layer) and the second semi-conductive layer (or external semi-conductive layer).

[0224] The results of the breakdown voltages are summarised in Table 2 below. 1.

TABLE-US-00002 TABLE 2 Initial value Value after 1000 h Cable (kV/mm) (kV/mm) Cc1 (*) 130 41 Ci1 58 49 Ci2 95 81 Ci3 133 77 (*) Comparative composition not part of the invention

[0225] All these results show that the incorporation of an oxygen-containing compound as defined in the invention into a polymer layer, in particular an electrically insulating polypropylene polymer-based layer, results in very good ageing resistance and improved resistance to water trees.