Energy cable having stabilized dielectric resistance

Abstract

A cable includes at least one electrical conductor and at least one electrically insulating layer surrounding the electrical conductor, wherein the at least one electrically insulating layer includes: (a) a thermoplastic polymer material selected from: at least one copolymer (i) of propylene with at least one olefin comonomer selected from ethylene and an a-olefin other than propylene, the copolymer having a melting point greater than or equal to 130 C. and a melting enthalpy of 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, the copolymer (ii) having a melting enthalpy of 0 J/g to 70 J/g; 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; (b) at least one dielectric fluid intimately admixed with the thermoplastic polymer material; and (c) at least one water tree retardant selected from ethoxylated fatty acids and amide derivatives thereof.

Claims

1. A cable comprising at least one electrical conductor and at least one electrically insulating layer surrounding said electrical conductor, wherein the at least one electrically insulating layer comprises: (a) 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 130 C. and a melting enthalpy of 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 0 J/g to 70 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; (b) at least one dielectric fluid intimately admixed with the thermoplastic polymer material; and (c) at least one water tree retardant selected from ethoxylated fatty acids and amide derivatives thereof.

2. The cable according to claim 1, wherein the copolymer (i) is a propylene/ethylene copolymer.

3. The cable according to claim 1, wherein, in copolymer (i) or copolymer (ii) or both, when heterophasic, an elastomeric phase is present in an amount equal to or greater than 45 wt % with respect to the total weight of the copolymer.

4. The cable according to claim 1, wherein copolymer (i) has a melting enthalpy of 25 J/g to 80 J/g.

5. The cable according to claim 1, wherein copolymer (ii) has a melting enthalpy of 10 J/g to 30 J/g.

6. The cable according to claim 1, wherein weight ratio between the at least one dielectric fluid (b) and the thermoplastic polymer material (a) is 1:99 to 25:75.

7. The cable according to claim 1, wherein the water tree retardant (c) is an ethoxylated fatty acid of formula Ia: ##STR00005## wherein R1 is a fatty acid residue C.sub.6-C.sub.31, and n is an integer of 4 to 25.

8. The cable according to claim 7, wherein R1 is a fatty acid residue C.sub.8-C.sub.25.

9. The cable according to claim 7, wherein the ethoxylated fatty acid is selected from: stearic acid ethoxylate, lauric acid ethoxylate, oleic acid ethoxylate, myristic acid ethoxylate, coconut fatty acid ethoxylate, palmitic acid ethoxylate, linoleic acid ethoxylate, linolenic acid ethoxylate, tallow fatty acid ethoxylate, sebacic acid ethoxylate, azelaic acid ethoxylate, and mixtures thereof.

10. The cable according to claim 1, wherein the water tree retardant (c) is an amide derivative of ethoxylated fatty acid of formula Ib: ##STR00006## wherein R1 is a fatty acid residue C.sub.6-C.sub.31, and n is an integer of 4 to 25.

11. The cable according to claim 10, wherein R1 is a fatty acid residue C.sub.8-C.sub.25.

12. The cable according to claim 10, wherein the amide derivative is selected from: stearoyl ethanolamide ethoxylate, lauryl amide ethoxylate, oleyl amide ethoxylate, myristyl amide ethoxylate, coconut monoethanolamide ethoxylate, elaidyl amide ethoxylate, sebacoyl amide ethoxylate, azelaoyl amide ethoxylate, and mixtures thereof.

13. The cable according to claim 1, wherein said at least one water tree retardant (c) is present in an amount of 0.05 to 2% by weight with respect to the total weight of the insulating layer.

14. The cable according to claim 13, wherein said at least one water tree retardant (c) is present in an amount of 0.1 to 1% by weight with respect to the total weight of the insulating layer.

15. The cable according to claim 1, having at least one semiconductive layer comprising at least one water tree retardant (c).

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, according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) 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).

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

(5) The insulating layer (4) may be produced by extrusion, around the conductor (2), of a composition according to the present invention.

(6) The semiconductive layers (3) and (5) are also made by extruding polymeric materials usually based on polyolefins, preferably a composition according to the present invention, made to be 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 aluminum 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) The cable according to the present invention may be manufactured in accordance with known methods, for example by extrusion of the various layers around the central conductor. The extrusion of two or more layers is advantageously carried out in a single pass, for example by the tandem method in which individual extruders are arranged in series, or by co-extrusion with a multiple extrusion head. The screen layer is then applied around the so produced cable core. Finally, the sheath according to the present invention is applied, usually by a further extrusion step.

(11) The cable of the present invention is preferably used for alternating current (AC) power transmission.

(12) FIG. 1 shows only one embodiment of a cable according to the 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.

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

Examples 1-4

(14) The following compositions were prepared with the amounts reported in Table 1 (expressed as % by weight with respect to the total weight of the composition).

(15) In all of the examples, the polypropylene material was fed directly into the extruder hopper. Subsequently, the dielectric fluid, previously mixed with the antioxidants and the water tree retardant (if any), was injected at high pressure into the extruder. An extruder having a diameter of 80 mm and a L/D ratio of 25 was used. The injection was made during the extrusion at about 20 D from the beginning of the extruder screw by means of three injection points on the same cross-section at 120 from each other. The dielectric fluid was injected at a temperature of 70 C. and a pressure of 250 bar.

(16) TABLE-US-00001 TABLE 1 EXAMPLE 1* 2 3 4 Adflex Q200F 89.7 89.2 89.2 89.45 Nyflex 800 10 10 10 10 Neopal CO 5 0.5 Sinerex AS 0.5 0.25 antioxidant 0.3 0.3 0.3 0.3 *comparative Adflex Q200F: propylene heterophase copolymer having melting point 165 C., melting enthalpy 30 J/g, and flexural modulus 150 MPa (Lyondell Basell); Nyflex 800: naphthenic oil, CAS No. 64742-53-6 (Nynas AB) Neopal CO 5: coconut oil fatty acids, ethoxylated monoethanolamide, CAS 68425-44-5; ethylene oxide EO residue: 5 (Industria Chimica Panzeri); Sinerex AS: ethoxylated stearic acid, CAS 9004-99-3, ethylene oxide EO residue: 5-20 (Industria Chimica Olimpia Tensioattivi) antioxidant: 4,6-bis (octylthiomethyl)-o-cresol.

(17) The dielectric breakdown strength (DS) of the polymer compositions obtained was evaluated on test-pieces of insulating material having the geometry proposed by the EFI (Norwegian Electric Power Research Institute) in the publication The EFI Test method for Accelerated Growth of Water Trees (IEEE International Symposium on Electrical Insulation, Toronto, Canada, Jun. 3-6 1990). In this method, the cable is simulated with glass-shaped test-pieces of insulating material having their base coated on both sides with a semiconductive material coating. The glass-shaped test pieces were formed by moulding discs of insulating material at 160-170 C. from a plate 10 mm thick obtained by compressing each blend of Examples at about 190 C.

(18) The inner and outer surfaces of the base, which had a thickness of about 0.40-0.45 mm, were coated with a semiconductive coating. The DS measurement was made by applying to these specimens, immersed in saline at 40 C., an alternating current at 50 Hz starting with a voltage of 0 kV and increasing in steps of 2 kV/sec until perforation of the test-piece occurred. The voltage stress was a measure of a 12 kV/mm electric field gradient. Each measurement was repeated on 15 test-pieces. The values given in Table 2 are the arithmetic mean of the individual measured values. Four series of experiments were carried out. The above screening of the specimens is necessary to exclude those which are defective because of the moulding process.

(19) Dielectric strength (DS) values of the tested samples are reported in Table 2.

(20) TABLE-US-00002 TABLE 2 DS kV/mm DS kV/mm DS kV/mm DS kV/mm EXAMPLE 0 day 12 days 30 days 60 days 1* 95 76 58 2 96 99 103 98 3 97 89 106 84 4 92 104 101 113 *comparative

(21) Even after 60 days at 40 C. in saline, the samples containing the water tree retardants according to the invention maintained a substantially unchanged dielectric strength, whereas the comparative sample (not containing any water tree retardant) showed a significant decrease in dielectric strength just after 12 days under the same experimental conditions.

Examples 5-7

(22) Following the same preparation procedure of Examples 1-4, the following comparative compositions were prepared with the amounts reported in Table 3 (expressed as % by weight with respect to the total weight of the composition).

(23) TABLE-US-00003 TABLE 3 EXAMPLE 5 6 7 Adflex Q200F 94.0 84.0 84.0 Jarylec Exp3 5.7 5.7 5.7 Lotryl 17 BA 04 10 Lotryl 30 BA 02 10 antioxidant 0.3 0.3 0.3 Adflex Q200F: propylene heterophase copolymer having melting point 165 C., melting enthalpy 30 J/g, and flexural modulus 150 MPa (Lyondell Basell); Jarylec Exp3: dibenzyltoluene (DBT) (Elf Atochem); Lotryl 17 BG 04: ethylene butyl acrylate copolymer (Arkema); Lotryl 30 BA02: ethylene butyl acrylate copolymer (Arkema); antioxidant: primary (phenolic) antioxidant.

(24) The dielectric breakdown strength (DS) of sample cables (5 m long) having an insulating layer based on the composition of Examples 1-5 was evaluated in alternating current condition. The DS measurements were made by applying to these sample cables. In water at 80 C., an alternating current at 50 Hz starting with a voltage of 50 kV and increasing in steps of 10 kV every 10 minutes until perforation of the test-piece occurred. The results are given in Table 4.

(25) TABLE-US-00004 TABLE 4 DS kV/mm DS kV/mm DS kV/mm EXAMPLE 0 day 30 days 90 days 5 78 37 25 6 75 36 39 7 67 40 34

(26) Sample cables wherein the insulating polypropylene base material of the invention was added with water tree retardant copolymer containing as a comonomer a monomer containing an ester group, according to the prior art showed a dielectric behavior substantially similar to that of a sample cable not added with such a water tree retardant.

(27) The addition of ester containing polymers known as water tree retardant did not provide significant results in insulating layer based on the polypropylene/dielectric fluid mixture according to the invention.

(28) Other compounds known as water-tree retardants, such as PEG 20,000 (disclosed, for example, by U.S. Pat. No. 4,305,849) and ethylene-oxide/propylene-oxide block copolymer (disclosed, for example, by EP 0 814 485) gave place to manufacturing problems when compounded with the polypropylene/dielectric fluid insulating material of the invention. In particular, the stability of the above mentioned water tree retardants was challenged and, above all, the extrudates containing these additives resulted to be thermally spoiled and unsatisfactory from an industrial point of view.