High voltage direct current cable having an impregnated stratified insulation

Abstract

A high voltage direct current cable includes at least one electrical conductor, at least one semiconducting layer, at least one stratified insulation made from windings of at least one paper-polypropylene laminate, the stratified insulation being impregnated with at least one electrically insulating fluid having a kinematic viscosity of at least 1,000 cSt at 60EC, wherein the laminate includes at least one paper layer having an air impermeability of at least 100,000 Gurley sec.sup.1. Such a high air impermeability of the paper layer(s) remarkably reduces the swelling of the polypropylene layer(s) during impregnation with a high viscosity insulating fluid, thus preventing delamination, up to the end of the impregnation process for the whole stratified insulation.

Claims

1. A high voltage direct current cable comprising: at least one electrical conductor; at least one semiconducting layer; and at least one stratified insulation made from windings of at least one paper-polypropylene laminate, said stratified insulation being impregnated with at least one electrically insulating fluid, wherein the at least one electrically insulating fluid has a kinematic viscosity of at least 1,000 cSt at 60 C., and wherein the laminate comprises at least one paper layer having an air impermeability of at least 100,000 Gurley sec.sup.1.

2. The cable according to claim 1, wherein the paper-polypropylene laminate has a swelling not higher than 1%, after immersion of the laminate in an insulating fluid, having a kinematic viscosity of at least 1,000 cSt at 60 C., at a temperature of 120 C. for a time of 240 hours.

3. The cable according to claim 2, wherein the paper-polypropylene laminate has a swelling not higher than 0.2%, after immersion of the laminate in an insulating fluid, having a kinematic viscosity of at least 1,000 cSt at 60 C., at a temperature of 120 C. for a time of 240 hours.

4. The cable according to claim 1, comprising an inner semiconducting layer disposed between the conductor and the stratified insulation, and an outer semiconducting layer disposed between the insulating layer and an external metal shield.

5. The cable according to claim 1, wherein the at least one paper-polypropylene laminate comprises a central layer of polypropylene sandwiched between two paper layers.

6. The cable according to claim 1, wherein the at least one paper layer has an air impermeability of 100,000 to 150,000 Gurley sec.sup.1.

7. The cable according to claim 1, wherein the at least one paper layer has a density of at least 0.9 g/cm.sup.3.

8. The cable according to claim 7, wherein the at least one paper layer has a density not higher than 1.4 g/cm.sup.3.

9. The cable according to claim 7, wherein the at least one paper layer has a density of 0.9 to 1.2 g/cm.sup.3.

10. The cable according to claim 1, wherein the polypropylene is selected from: (a) thermoplastic propylene homopolymers; and (b) thermoplastic copolymers of propylene with at least one comonomer selected from: ethylene and alpha-olefins having from 4 to 10 carbon atoms.

11. The cable according to claim 10, wherein, in the thermoplastic copolymer, the comonomer is ethylene.

12. The cable according to claim 11, wherein the total amount of ethylene in the thermoplastic copolymer is 0.5 to 10 wt %.

13. The cable according to claim 12, wherein the total amount of ethylene in the thermoplastic copolymer is 0.5 to 5 wt %.

14. The cable according to claim 12, wherein the propylene homopolymer or thermoplastic copolymer has a Melt Flow Index of 7 to 50 g/10, measured at 230 C./2.16 kg according to ASTM D1238-04C.

15. The cable according to claim 11, wherein the polypropylene homopolymer or thermoplastic copolymer has a melting enthalpy of at least 100 J/g, measured by Differential Scanning Calorimetry according to Standard ASTM D3417-83.

16. The cable according to claim 15, wherein the melting enthalpy of the polypropylene homopolymer or thermoplastic copolymer is equal to or lower than 135 J/g.

17. The cable according to claim 15, wherein the melting enthalpy of the polypropylene homopolymer or thermoplastic copolymer is 105 J/g to 110 J/g.

18. The cable according to claim 10, wherein the propylene homopolymer or thermoplastic copolymer has a Melt Flow Index of at least 5 g/10, measured at 230 C./2.16 kg according to ASTM D1238-04C.

19. The cable according to claim 1, wherein the polypropylene has a value of swelling, measured as percentage weight increase, when immersed in a T2015 insulating fluid at 90 for 168 hours, not higher than 10%.

20. The cable according to claim 1, wherein the at least one electrically insulating fluid has a viscosity of at least 1,000 cSt at 60 C. according to ASRM D 445-09 (2000).

21. The cable according to claim 20, wherein the at least one electrically insulating fluid has a viscosity of 1100 to 1200 cSt at 60 C. according to ASTM D 445-09 (2000).

22. The cable according to claim 20, wherein the at least one electrically insulating fluid comprises a naphthenic or paraffinic oil or a synthetic hydrocarbon oil or a mixture thereof, and optionally, at least one viscosity increasing additive in an amount so as to obtain the desired viscosity.

23. The cable according to claim 1, wherein the paper-polypropylene laminate has an overall thickness of 50 to 300:m.

24. The cable according to claim 23, wherein the paper-polypropylene laminate has an overall thickness of 70 to 200:m.

25. The cable according to claim 1, wherein the polypropylene layer has a thickness of 35% to 75% of the laminate overall thickness.

26. The cable according to claim 25, wherein the polypropylene layer has a thickness of 50 to 65% of the laminate overall thickness.

Description

(1) The present invention is further illustrated with reference to the attached figures wherein:

(2) FIG. 1 shows a cross-section view of the cable according to FIG. 1;

(3) FIG. 2 shows a cross-section view of a laminate according to the present invention;

(4) FIG. 3 shows the diagrams of thickness variation over time during impregnation of different laminates with a high viscosity insulating fluid.

(5) With reference to the aforesaid figures, the cable (1) according to the present invention comprises, sequentially from the centre to the exterior, a conductor (2), an inner semiconducting layer (3), a stratified insulation (4), an outer semiconducting layer (5), and a metal sheath (6).

(6) The conductor (2) is generally formed by a plurality of single conductors, preferably made from copper or aluminum, for example in the form of wires stranded together by conventional methods, or, preferably (as illustrated in FIG. 1), the conductor (2) is of the copper shaped or Milliken type.

(7) Around the conductor (2) a layer (3) is placed having semiconducting properties, consisting, for example, of windings of cellulose paper tapes filled with conductive carbon black. Analogous construction can be made for the second semiconductive layer (5) placed around the stratified insulation (4).

(8) The stratified insulation (4) is generally formed by successive windings of the paper-propylene laminate (12) as illustrated above.

(9) The metal sheath (6), usually made from lead or lead alloys, encloses the cable core formed by the aforementioned elements, and any space within the sheath (6) is filled by the insulating fluid so as to thoroughly impregnate the cable layers, particularly the stratified insulation (4).

(10) Around the metal sheath (6), an armoured structure is usually disposed, in order to provide a mechanical protection to the cable. This armoured structure may comprise, for example, a sheath (7) made from a plastic material, on which a metal reinforcement (8), formed e.g. by steel tapes, is placed. Outwardly, at least one armour (10), made e.g. of carbon steel, combined with at least one bedding layer (9), made e.g. of tapes or yarns, may be applied, the bedding layer (9) being able to prevent the armour (10) from damaging the internal layers. As the outermost layer, a serving sheath (11) is usually present, made of polymeric material, provided for protection and uniformity of the cable surface.

(11) FIG. 2 shows a cross-section view of a preferred embodiment of the laminate (12) according to the present invention, wherein a central layer (13) made from polypropylene is sandwiched between two paper layers (14).

(12) The laminate may be manufactured according to known techniques, preferably by extrusion coating wherein the two paper layers (14), usually at room temperature, are put into contact with a film of polypropylene in the melted state, usually at a temperature of from 200 C. to 320 C., namely at a temperature much higher than the melting temperature of the polymer. Afterwards the contacting layers are calendered at low temperatures, usually by means of chilled rolls.

(13) The following working examples are given to better illustrate the invention, but without limiting it.

EXAMPLE 1

(14) Two layers of kraft paper (pure conifer cellulose) having a thickness of 0.025 mm, a density of 0.93 g/ml and an air impermeability of 100,000 Gurley sec.sup.1 were coupled with a layer of Pro-Fax PF611 (Basell), a propylene homopolymer (PP) having a density of 0.902 g/ml (ASTM D 792) and a MFI @ 230 C./2.16 kg of 30.0 g/10 (ASTM D 1258). The resulting paper/PP/paper laminate had a thickness of 0.100 mm, a PP percentage content of 60% by weight and a weight of 100 g/m.sup.2. The peeling strength between PP and paper in the dry laminate was measured according to Standard ASTM D 1876-08 and resulted to be 13 g/15 mm.

(15) The so obtained laminate was dried in an oven under vacuum for 8 hours at 135 C. and then impregnated at 125 C. with an insulating fluid having a viscosity at 100 C. of 1200 cSt (commercial product T2015 by H&R ChemPharm (UK) Ltd.). During the impregnation process, the thickness variation (swelling) was measured at regular intervals: the results are reported in the diagram of FIG. 4. After 240 hours the overall swelling was 0.14%. The peeling strength between PP and paper in the impregnated laminate was measured to be 25 g/15 mm.

(16) By using the above laminate, a cable specimen was produced with a copper conductor of 2000 mm.sup.2 cross-section and a stratified insulation of 18.1 mm thickness. After impregnation of the stratified insulation with the same insulating fluid T2015, some tests (bending test based on three repeated cycles and electrical tests, as High Voltage Direct Current with loading cycles up to 1080 kV and impulse test up to 1650 kV) were carried out to check the cable functioning: no shortcoming were encountered.

EXAMPLE 2

(17) Two layers of kraft paper (conifer pure cellulose) having a thickness of 0.025 mm, a density of 0.93 g/ml and an air impermeability of 100,000 Gurley sec.sup.1 were coupled with a layer of HD601CF (Borealis), a propylene homopolymer (PP) having a density of 0.90 g/ml (ISO 1183) and a MFI @ 230 C./2.16 kg of 8 g/10 (ISO 1133). The resulting paper/PP/paper laminate had a thickness of 0.100 mm, a PP percentage content of 60% by weight and a weight of 100 g/m.sup.2. The peeling strength between PP and paper in the dry laminate was measured according to Standard ASTM D 1876-08 resulted to be 100 g/15 mm.

(18) The so obtained laminate was dried in an oven under vacuum for 8 hours at 135 C. and then impregnated at 125 C. with an insulating fluid having a viscosity at 100 C. of 1200 cSt (commercial product T2015 by H&R ChemPharm (UK) Ltd.).

(19) During the impregnation process, the thickness variation (swelling) was measured at regular intervals: the results are reported in the diagram of FIG. 4. After 240 hours the overall swelling was 0.84%. The peeling strength between PP and paper in the impregnated laminate was measured to be 25 g/15 mm.

EXAMPLE 3 (COMPARATIVE)

(20) Two layers of kraft paper (mixed conifer/broad leaved tree pure cellulose) having a thickness of 0.025 mm, a density of 1.01 g/ml and an air impermeability of 40,000 Gurley sec.sup.1 were coupled with a layer of Pro-Fax PF611 (Basell), a propylene homopolymer (PP) having a density of 0.902 g/ml (ASTM D 792) and a MFI @ 230 C./2.16 kg of 30.0 g/10 (ASTM D 1258). The resulting paper/PP/paper laminate had a thickness of 0.100 mm, a PP percentage content of 60% by weight and a weight of 100 g/m.sup.2. The peeling strength between PP and paper in the dry laminate was measured according to Standard ASTM D 1876-08 and resulted to be 50 g/15 mm.

(21) The so obtained laminate was dried in an oven under vacuum for 8 hours at 135 C. and then impregnated at 125 C. with an insulating fluid having a viscosity at 100 C. of 1200 cSt (commercial product T2015 by H&R ChemPharm (UK) Ltd.). During the impregnation process, the thickness variation (swelling) was measured at regular intervals: the results are reported in the diagram of FIG. 4. After 240 hours the overall swelling was 1.95%. The peeling strength between PP and paper in the impregnated laminate was measured to be 30 g/15 mm.

(22) By using the above laminate, a cable specimen was produced having a copper conductor of 2000 mm.sup.2 cross-section and a stratified insulation of 18.1 mm thickness. After impregnation of the stratified insulation with the same insulating fluid T2015, it was found that an excessive swelling of the external windings of the laminate hindered penetration of the insulating fluid through the inner laminate layers, thus causing an unacceptable lack of homogeneity in the insulation impregnation.

EXAMPLE 4 (COMPARATIVE)

(23) Two layers of kraft paper (conifer pure cellulose) having a thickness of 0.025 mm, a density of 0.75 g/ml and an air impermeability of 1,000 Gurley sec.sup.1 were coupled with a layer of Pro-Fax PF611 (Basell), a propylene homopolymer (PP) having a density of 0.902 g/ml (ASTM D 792) and a MFI @ 230 C./2.16 kg of 30.0 g/10 (ASTM D 1258). The resulting paper/PP/paper laminate had a thickness of 0.100 mm, a PP percentage content of 60% by weight and a weight of 88 g/m.sup.2. The peeling strength between PP and paper in the dry laminate was measured according to Standard ASTM D 1876-08 and resulted to be 50 g/15 mm.

(24) The so obtained laminate was dried in an oven under vacuum for 8 hours at 135 C. and then impregnated at 125 C. with an insulating fluid having a viscosity at 100 C. of 1200 cSt (commercial product T2015 by H&R ChemPharm (UK) Ltd.). During the impregnation process, the thickness variation (swelling) was measured at regular intervals: the results are reported in the diagrams of FIG. 4. After 240 hours the overall swelling was 3.5%. The peeling strength between PP and paper in the impregnated laminate was measured to be 30 g/15 mm.