ENERGY CABLE HAVING A CROSSLINKED ELECTRICALLY INSULATING SYSTEM, AND METHOD FOR EXTRACTING CROSSLINKING BY-PRODUCTS THEREFROM

Abstract

An energy cable comprising at least one cable core comprising an electric conductor, a crosslinked electrically insulating system comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer and zeolite particles placed between the electric conductor and the inner semiconducting layer of the insulating system. The zeolite particles are able to efficiently extract and irreversibly absorb the by-products deriving from the cross-linking reaction, so as to avoid space charge accumulation in the insulating material during cable lifespan. This allows to eliminate the high temperature, long lasting degassing process of the energy cable cores having a crosslinked insulating layer, or at least to reduce temperature and/or duration of the same, so as to increase productivity and reduce manufacturing costs.

Claims

1. An energy cable comprising at least one cable core comprising an electric conductor, a crosslinked electrically insulating system comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer, and zeolite particles placed between the electric conductor and the inner semiconducting layer.

2. Energy cable according to claim 1, wherein the zeolite particles are placed in contact with the inner semiconducting layer.

3. Energy cable according to claim 1, wherein the zeolite particles are further placed into or in contact with the outer semiconducting layer.

4. Energy cable according to claim 3, wherein the zeolite particles are placed in contact with the outer semiconducting layer.

5. Energy cable according to claim 1, wherein the zeolite particles are dispersed on a substrate, such substrate including any of yarn or tape.

6. Energy cable according to claim 1, wherein the zeolite particles are present in an amount less than 0.01 g/cm.sup.3.

7. Energy cable according to claim 6, wherein the zeolite particles are present in an amount at most of 0.008 g/cm.sup.3.

8. Energy cable according to claim 1, wherein the zeolite particles have a charge compensating cation selected from the group consisting of ammonium (NH.sub.4.sup.+) and hydron (H).

9. Energy cable according to claim 1, wherein the zeolite particles have a SiO.sub.2/Al.sub.2O.sub.3 molar ratio equal to or lower than 20.

10. Energy cable according to claim 1, wherein the zeolite particles have a SiO.sub.2/Al.sub.2O.sub.3 molar ratio equal to or lower than 15.

11. Energy cable according to claim 1, wherein the zeolite particles have a maximum diameter of a sphere than can diffuse along at least one of the cell axes directions equal to or greater than 3 .

12. Energy cable according to claim 1, wherein the zeolite particles have a sodium content, expressed as Na.sub.2O, equal to or lower than 0.3% by weight.

13. A method for extracting crosslinking by-products from a cross-linked electrically insulating system of an energy cable core, said method comprising the following sequential stages: manufacturing an energy cable core comprising an electric conductor, a crosslinked electrically insulating system containing cross-linking by-products and comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer, and zeolite particles placed between the electric conductor and the inner semiconducting layer; heating the energy cable core up to a temperature causing migration of the crosslinking by-products from the crosslinked electrically insulating system to the zeolite particles, thereby the zeolite particles absorb the crosslinking by-products; and then placing a metal screen around the energy cable core.

14. Method according to claim 13, 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.

15. Method according to claim 13, wherein the heating step causes at least one fraction of the crosslinking by-products to be irreversibly absorbed into the zeolite particles.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0061] Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawings, in which:

[0062] FIG. 1 is a transversal cross section of a first embodiment of an energy cable, particularly suitable for medium or high voltage, according to the present invention;

[0063] FIG. 2 is a transversal cross section of a second embodiment of an energy cable, particularly suitable for medium or high voltage, according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] In FIG. 1, a transversal section of a first preferred embodiment of a cable (1) according to the present invention is schematically represented, which comprises an electric conductor (2), a cross-linked electrically insulating system composed by an inner semiconducting layer (3), an electrically insulating layer (4) and 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 outer semiconducting layer (5) constitute the core of cable (1). Cable (1) is particularly intended for the transport of medium or high voltage current.

[0065] The conductor (2) consists of metal filaments (2a), preferably of copper or aluminium or both, stranded together by conventional methods. The electrically insulating layer (4), 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 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 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.

[0066] In accordance with an embodiment of the present invention, a tape (8) wherein the zeolite particles are dispersed is wound around the conductor (2) and the inner semiconducting layer (3) is extruded thereon.

[0067] The zeolite particles can be dispersed in a filling material, preferably a buffering filling material which is placed among the filaments (2a) of the electric conductor (2) in order to avoid propagation of water or humidity that can penetrate within the cable conductor (2), especially when the cable (1) is to be installed in very humid environments or under water.

[0068] The filling material is preferably a polymeric filling material which can be provided in the cable core by a continuous deposition process, especially by extrusion or by pultrusion. The filling material can comprise a polymeric material and, advantageously, a hygroscopic material, for example a compound based in an ethylene copolymer, for example an ethylene/vinyl acetate copolymer, filled with a water absorbing powder, for example sodium polyacrylate powder.

[0069] In FIG. 2, a transversal section of another embodiment of the cable (1) according to the present invention is schematically represented, which comprises the same elements as described in FIG. 1, with the addition of a tape (8), wound onto the outer semiconducting layer (5), wherein the zeolite particles are dispersed. In a further embodiment, the zeolite particles may be also dispersed in a filling material placed within voids (2b) among the metal filaments (2a) forming the electric conductor (2).

[0070] FIGS. 1 and 2 show only two embodiments of the present invention. Suitable modifications can be made to these embodiments according to specific technical needs and application requirements without departing from the scope of the invention.

[0071] The following examples are provided to further illustrate the invention.

EXAMPLES 1-3

[0072] Some tests were carried out to evaluate the ability of tapes with zeolite particles to absorb by-products deriving from crosslinking reaction of polyethylene with cumyl peroxide and in particular cumyl alcohol, one of the major of these by-products.

[0073] The tape carried zeolite particles. The zeolites particles were 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 ).

[0074] The tape was placed between the conductor and the inner semiconducting layer and, optionally, also around the outer semiconducting layer of cables having a conductor cross-section of 1800 mm.sup.2, where the inner semiconducting layer had an inner diameter of about 51 mm and the outer semiconducting layer had an outer diameter of about 97 mm.

[0075] The amount of zeolite particles placed between the conductor and the inner semiconducting layer (SCI) only was of 0.0059 g/cm.sup.3. The amount of zeolite particles tape placed between the conductor and the inner semiconducting layer (SCI) and also around the outer semiconducting layer (SCE), the amount of zeolite particles in the cable was of about 0.011 g/cm.sup.3 (0.0059 g/cm.sup.3 between the conductor and the inner semiconducting layer+0.0059 g/cm.sup.3 around the outer semiconducting layer). One of the tested cables contained no zeolite particles.

[0076] The concentrations of cross-linking by-products were measured by column gas chromatography of a sample of cross-linked insulating system material.

[0077] The tests were carried out on cables kept at 70 C.). The results are reported in Table 1.

TABLE-US-00001 TABLE 1 Zeolite tape Cross-linking by-products concentration (%) Example position day 0 day 26 day 45 1 (*) (none) 0.76 0.28 2 SCI 0.79 0.40 3 SCI+/SCE 0.78 0.37

TABLE-US-00002 TABLE 2 Zeolite tape Cumyl alcohol concentration (%) Example position day 0 day 26 day 45 1 (*) (none) 0.47 0.20 2 SCI 0.50 0.23 3 SCI + SCE 0.48 0.21 The example marked with an asterisk (*) is comparative. SCI = tape with zeolite placed between the conductor and the inner semiconducting layer (amount 0.0059 g/cm.sup.3) SCI + SCE = tape with zeolite placed between the conductor and the inner semiconducting layer and around the outer semiconducting layer (amount 0.011 g/cm.sup.3)

[0078] From the data reported in Table 1, it is apparent that in the Example 2 and 3 according to the invention the zeolites are able to reduce the cross-linking by-products concentration and, in particular, the cumyl alcohol concentration in substantially shorter time than the known degassing procedure even when used in reduced amount The additional presence of zeolite particles in the outer semiconducting layer (cable of Example 3) improves the reduction of cross-linking by-products, but its effect seems to be less significant than that of the presence of zeolite particles placed between the conductor and the inner semiconducting layer.

EXAMPLE 4

[0079] The insulating system of a cable analogous to that of Example 1 was analyzed after about 20 days at 70 C. from the manufacturing and the overall cross-linking by-products content was found to be reduced from 1.3% down to 0.37% (the cumyl alcohol content was found to be reduced from 0.82% down to 0.22%). After about one year (spent at room temperature) another analysis was carried out and the cross-linking by-products content was found to be further reduced to substantially 0%.

[0080] From these data, we can derive that the zeolite particles placed in the vicinity of the insulating system of an energy cable are able to reduce, down to substantial elimination, the crosslinking by-products not only during degassing heating but also during storage of the cable at ambient temperature.

[0081] The reduction of the cumyl alcohol concentration in the insulating system implies the compound diffusion radially towards both the inside of the insulating system (where it is adsorbed by the zeolite particles) and outside the insulating system (where it can be dispersed in the atmosphere). The diffusion time is important and is expected to depend on the insulating system thickness.

[0082] In past estimations, for a 25 mm insulating thickness an amount of at least 70 g/m zeolite particles (which corresponds to about 0.01 g/cm.sup.3 for a 2000 mm.sup.2 conductor cable) was contemplated to reach a final target of 0.32 wt % of cumyl alcohol content in the insulating system after a 25 days degassing period at 70 C., while for a 15 mm insulating thickness an amount of at least 27 g/m zeolite particles (which corresponds to about 0.005 g/cm.sup.3 for a 1100 mm.sup.2 conductor cable) was contemplated to reach the same final target above.

[0083] These values were considered for taking into account the different length of the cumyl alcohol diffusion path to reach either the absorbing material or the external atmosphere.

[0084] Surprisingly, it has been found that even with a significantly high thickness, a relatively low amount of zeolite particles is sufficient to reach and exceed the desired residual cumyl alcohol concentration, as confirmed by Example 2 and 3 above.