Method for extracting crosslinking by-products from a crosslinked electrically insulating system of a power cable and related power cable

Abstract

An energy cable comprises at least one cable core comprising an electric conductor, a crosslinked electrically insulating layer, and particles of a zeolite system comprising at least a first zeolite and a second zeolite placed in the cable core. A method for extracting crosslinking by-products from a cross-linked electrically insulating layer of an energy cable core comprises manufacturing the energy cable core comprising particles of the above-said zeolite system, heating the energy cable core up to a temperature causing migration of the crosslinking by-products from the crosslinked electrically insulating layer, and placing a metal screen around the energy cable core.

Claims

1. A power cable comprising at least one cable core comprising an electric conductor surrounded by a crosslinked insulating system made of at least one polyolefin crosslinked by reaction with at least one peroxide crosslinker and comprising: an inner semiconducting layer surrounding the electric conductor; an electrically insulating layer surrounding the inner semiconducting layer; an outer semiconducting layer surrounding the electrically insulating layer; wherein a zeolite system comprising particles of a first zeolite and particles of a second zeolite is placed in the cable core, the first zeolite having a SiO.sub.2/Al.sub.2O.sub.3 ratio higher than 5 and equal to or lower than 20, and a maximum diameter of a sphere than can diffuse along at least one of the cell axes directions higher than 5 ; and the second zeolite having a SiO.sub.2/Al.sub.2O.sub.3 ratio equal of 5 at most, and a maximum diameter of a sphere than can diffuse along at least one of the cell axes directions of from 3 to 5 .

2. The power cable according to claim 1, wherein the conductor comprises a plurality of stranded electrically conducting wires defining a bundle of wires and the particles of the zeolite system are placed between the outer perimeter of the bundle of wires and the inner semiconducting layer.

3. The power cable according to claim 1, wherein the electric conductor is formed by a plurality of stranded electrically conducting wires defining a bundle of wires and the particles of the zeolite system are placed within voids among said wires.

4. The power cable according to claim 1, wherein the particles of the zeolite system are placed in contact with the inner surface of the inner semiconducting layer.

5. The power cable according to claim 1, wherein the particles of the zeolite system are in the inner semiconducting layer.

6. The power cable according to claim 3, wherein the particles of the zeolite system are dispersed in/on a substrate.

7. The power cable according to claim 1, wherein the total amount of particles of the zeolite system is of 0.008 g/cm.sup.3 at most.

8. The power cable according to claim 1, wherein the total amount of particles of the zeolite system is of at least 0.003 g/cm.sup.3.

9. The power cable according to claim 1, wherein the first zeolite has a charge compensating cation content, expressed as oxide, of at most 0.3 wt % based on the weight of the first zeolite.

10. The power cable according to claim 1, wherein the second zeolite has a charge compensating cation content, expressed as oxide, of at least 10 wt % based on the weight of the second zeolite.

11. The power cable according to claim 1, wherein the second zeolite is present in an amount of from 1 wt % to 50 wt % based on the weight of the zeolite system.

12. A method for extracting crosslinking by-products from a crosslinked electrically insulating system of a power cable core, said method comprising the following sequential stages: (a) manufacturing a power cable core comprising: an electric conductor, an inner semiconducting layer surrounding the electric conductor; an electrically insulating system surrounding the electric conductor and made of at least one polyolefin crosslinked by reaction with at least one peroxide crosslinker thereby containing cross-linking by-products; and; a zeolite system comprising particles of a first zeolite and particles of a second zeolite placed in the cable core, the first zeolite having a SiO.sub.2/Al.sub.2O.sub.3 ratio higher than 5 and equal to or lower than 20, and a maximum diameter of a sphere than can diffuse along at least one of the cell axes directions higher than 5 ; and the second zeolite having a SiO.sub.2/Al.sub.2O.sub.3 ratio of 5 at most, and a maximum diameter of a sphere than can diffuse along at least one of the cell axes directions of from 3 to 5 ; (b) heating the power cable core up to a temperature causing migration of the crosslinking by-products and water molecules from the crosslinked electrically insulating system to the zeolite system, thereby the crosslinking by-products are absorbed by the particles of the first zeolite and the water molecules are absorbed by the particles of the second zeolite; (c) placing a metal screen around the power cable core.

13. Method according to claim 12, wherein the heating step is carried out at a temperature of from 70 C. to 80 C., for a time from 7 to 15 days.

14. Method according to claim 12, wherein the heating step causes at least one fraction of the crosslinking by-products to be irreversibly absorbed into the particles of the first zeolite and at least one fraction of the water molecules to be irreversibly absorbed into the particles of the second zeolite.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 is a transversal cross section of a first embodiment of a power cable, particularly suitable for medium or high voltage, according to the present disclosure;

(3) FIG. 2 is a transversal cross section of a second embodiment of a power cable, particularly suitable for medium or high voltage, according to the present disclosure.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

(4) In FIG. 1, a transversal section of a cable (1) according to the present disclosure is schematically represented. Cable (1) comprises an electric conductor (2), an inner semiconducting layer (3), an electrically insulating layer (4), an outer semiconducting layer (5), a metal screen (6) and a sheath (7). Electric conductor (2), inner semiconducting layer (3), electrically insulating layer (4) and the outer semiconducting layer (5) constitute the core of cable (1). Cable (1) is particularly intended for the transport of medium or high voltage current.

(5) The conductor (2) consists of metal wires (2a), for example of copper or aluminium or both, stranded together by conventional methods. The electrically insulating layer (4) and the semiconducting layers (3) and (5) are made by extruding and cross-linking polymeric materials according to known techniques. Around the outer semiconducting layer (5), a metal screen layer (6) is positioned, made of electrically conducting wires or strips, for example helically wound around the cable core, or of an electrically conducting tape longitudinally wrapped and overlapped (and, optionally, glued) onto the underlying layer. The electrically conducting material of said wires, strips or tape is usually copper or aluminium or both. The screen layer (6) may be covered by a sheath (7), generally made from a polyolefin, usually polyethylene, in particular high density polyethylene.

(6) In accordance with an embodiment of the present description, a tape (8) with particles of the zeolite system according to the present disclosure dispersed upon, is wound between the conductor (2) and the inner semiconducting layer (3). In FIG. 2, a transversal section of another cable (1) according to the present description is schematically represented. This cable (1) comprises the same elements as described in FIG. 1, with the addition of further particles of a zeolite system according to the present disclosure dispersed in a filling material (2b), for example a buffering filling material, placed within voids among the wires (2a) of the electric conductor (2) or between the outer perimeter of the electric conductor (2) and the tape (8). This filling material can also have the function of avoiding the propagation of water or humidity possibly penetrated within the cable conductor (2), especially when the cable (1) is to be installed in very humid environments or under water.

(7) Also, the cable (1) of FIG. 2 has a tape (8), similar to the tape (8), wound between the outer semiconducting layer (5) and the metal screen (6), the tape (8) bearing particles of the zeolite system of the present disclosure.

(8) FIGS. 1 and 2 show only two embodiments of the present disclosure. Suitable modifications can be made to these embodiments according to specific technical needs and application requirements without departing from the scope of this disclosure. For example, a cable according to the present disclosure can comprise particles of the zeolite system herein taught in one, two or all of the following positions: (i) between the electric conductor and the inner semiconducting layer, (ii) among the electric conductor wires, and (iii) between the outer semiconducting layer and the metal screen.

(9) The following examples are provided to further illustrate the subject matter of the present description.

Example 1

(10) Some tests were carried out to evaluate the ability of tapes bearing a zeolite system comprising particles of a first zeolite (suitable for absorbing crosslinking by-products deriving from crosslinking reaction of polyethylene with cumyl peroxide, particularly cumyl alcohol), and particles of a second zeolite (suitable for trapping water molecules).

(11) The tape carried particles of a zeolite system comprising: zeolite CBV 600 (Y-type zeolite having: charge compensating cation=H.sup.+; specific surface area=660 m.sup.2/g; SiO.sub.2/Al.sub.2O.sub.3 ratio=5.2; Na.sub.2O %=0.2; dimensionality=3; maximum diffusing sphere diameter=7.35 ) to absorb the crosslinking by-products zeolite A3 (A-type zeolite [(Na.sup.+.sub.12(H.sub.2O).sub.27].sub.8[Al.sub.12Si.sub.12O.sub.48].sub.8 having: charge compensating cation=Na.sup.+; specific surface area=800 m.sup.2/g; SiO.sub.2/Al.sub.2O.sub.3 ratio=1; Na.sub.2O %=13 wt %; dimensionality=X; maximum diffusing sphere diameter=4.2 ) to absorb water.

(12) The weight ratio between the CBV 600 first zeolite and the A3 second zeolite was about 90:10.

(13) In a first cable (SAMPLE A) according to the present disclosure, the tape was placed between the conductor and the inner semiconducting layer. The conductor had a cross-section of 2,500 mm.sup.2, the inner semiconducting layer had an inner diameter of about 64 mm and the outer semiconducting layer had an outer diameter of about 107 mm. The conductor was made of a multiplicity of copper wires, the tape being placed around the bundle of wires and in contact with its outer perimeter. The voids among the wires were filled with a buffer material made of 92 AC JV (a mixture based on EPDM and EVA, marketed by Sigea S.p.A.). The insulation layer, which was made of XLPE like the semiconducting layers, had a thickness of 20 mm.

(14) The amount of particles of the zeolite system that were placed between the conductor and the inner semiconducting layer was about 0.0054 g/cm.sup.3.

(15) For comparison purposes, a power cable having the same structure of Sample A described above, but without any the addition of zeolite particles, was also prepared and tested (SAMPLE C).

(16) The concentrations of cross-linking by-products were measured by column gas chromatography of samples of cross-linked insulating material as a whole slice (S) or cut at different positions of the insulating layer (close to outer semiconducting layer (VSE), central part (C), close to the inner semiconducting layer (VSI)).

(17) The samples were cut into small pieces and extracted by speed extractor at the following operating conditions: Solvent: Acetone Volume: 100 ml Temperature: 90 C. Pressure: 100 bars Extraction time: 5 hours Sample weight: 5 g

(18) To determine the content of by-products in the tapes with zeolites, a sample of each tape was extracted by means of a soxhlet extractor at the following operating conditions: Solvent: ethyl ether Volume: 100 ml Extraction time: 24 hours Sample weight: 5 g

(19) The analyses were carried out on the cables after degassing at 70 C. for a time up to 49 days, unless otherwise stated. The results are reported in Tables 1 to 2, where the by-products content at each position are listed and compared to the corresponding ones of the sample before degassing (fresh).

(20) TABLE-US-00001 TABLE 1 Sample A Acetophenone Cumyl alcohol wt % wt % TOTAL % Fresh S 0.30 0.62 0.94 VSE 0.21 0.32 0.59 C 0.45 0.81 1.28 VSI 0.38 0.84 1.23 Degassed 28 days S 0.13 0.29 0.48 VSE 0.11 0.25 0.39 C 0.20 0.44 0.71 VSI 0.22 0.30 0.67 Degassed 49 days S 0.10 0.19 0.38 VSE 0.07 0.15 0.26 C 0.13 0.25 0.47 VSI 0.17 0.17 0.54

(21) From the data reported in the Table 1, it is apparent that the zeolite system contained in Sample A according to the present disclosure is able to reduce the cross-linking by-products concentration in the insulating material and, in particular, the cumyl alcohol concentration in substantially shorter time compared to the known degassing procedure without incorporating any zeolite in the cable. Notably, the presence of the zeolite system allows to reduce the total amount of by-products below 0.5 wt % after 28 days of degassing (Sample A, Slice).

(22) TABLE-US-00002 TABLE 2 Sample C Acetophenone Cumyl wt % alcohol wt % TOTAL % Fresh S 0.30 0.61 0.93 VSE 0.21 0.33 0.59 C 0.42 0.77 1.21 VSI 0.30 0.72 1.02 Degassed 35 days S 0.15 0.39 0.57 VSE 0.11 0.27 0.40 C 0.22 0.54 0.79 VSI 0.21 0.40 0.63 Degassed 49 days S 0.12 0.32 0.45 VSE 0.07 0.21 0.29 C 0.16 0.42 0.59 VSI 0.19 0.39 0.60

(23) In the same cable as Sample A without any zeolite, a concentration below 0.5 wt % of by-products in the insulation is obtained not earlier than a five-to-seven-week degassing (Sample C, Slice).

Example 2

(24) To determine the moisture content of the insulating layer, the Samples A and C were analyzed by a Karl Fischer titrator at the following conditions: Oven temperature: 130 C. Environmental humidity<5% Sample weight: 200 mg Analysis repetition: 5
The results of the water content analysis at different insulation positions are reported in Table 3.

(25) TABLE-US-00003 TABLE 3 Water content in the insulating material Sample A H.sub.2O (ppm) S 51.7 VSE 49.0 C 53.2 VSI 48.4

(26) As it can be inferred from Table 3, both the zeolite system of Sample A according to the present disclosure was able to keep the moisture content into the insulation layer to a value significantly lower than 100 ppm.

(27) In similar experiments carried out on an 525 kV DC cable containing first zeolite particles (CBV 600) only, placed within the voids of the conducting wires as well as distributed on tapes between the conductor outer perimeter and the inner semiconducting layer, and on tapes surrounding outer semiconducting layers, the water content in the insulation layer center was found to be higher than 350 ppm after degassing for 42 days at 70 C. The highest concentration of water (nearly 400 ppm) was found close to the inner semiconducting layer, i.e. in the region of the cable containing the highest portion of first zeolite particles (on the tape between the inner semiconducting layer and the conductor and in the conductor body). Such a high amount of water observed in this experiment cannot be due to a water present in the freshly extruded insulating system and it is conjectured to be generated by the dimerization/oligomerization or decomposition reaction of the crosslinking by-products upon their absorption on the particles of the first zeolite.