ACCESSORY FOR HIGH VOLTAGE DIRECT CURRENT ENERGY CABLES

Abstract

The present invention relates to an accessory for high-voltage direct-current (HVDC) energy cables comprising: at least one element made from a crosslinked elastomeric polymer material, and at least one scavenging layer comprising zeolite particles. The zeolite particles are able to scavenge, very efficiently and irreversibly, the by-products deriving from the cross-linking reaction, so as to avoid space charge accumulation in the element during the accessory lifespan. Moreover, the zeolite particles can prevent the crosslinking by-products present in the element of a non-degassed accessory from migrating towards the insulating layer of the energy cable on which the accessory is mounted.

Claims

1. Accessory (100, 300) for high voltage direct current (HVDC) energy cables comprising: at least one element made from a crosslinked elastomeric polymer material (13, 15a, 15b, 33, 35), and at least one scavenging layer (16, 36) comprising zeolite particles.

2. Accessory according to claim 1, wherein said scavenging layer (16, 36) is applied in a radial external position with respect to the element made from a crosslinked elastomeric polymer material (13, 15a, 15b, 33, 35).

3. Accessory according to claim 2, wherein the element made from a crosslinked elastomeric polymer material (13, 15a, 15b, 33, 35) is a least one of an insulating element (13, 33) and a stress-relief cone (15a, 15b, 35).

4. Accessory according to claim 3, wherein said scavenging layer (16, 36) is applied in direct contact with the insulating element (13, 33).

5. Accessory according to claim 3, wherein an outer semiconducting layer (14) is interposed between the insulating element (13, 33) and the scavenging layer (16, 36).

6. Accessory according to claim 1, wherein said scavenging layer (16, 36) is in a form selected from a polymeric sheath or a tape.

7. Accessory according to claim 6, wherein said tape is a self-amalgamating tape or a hygroscopic tape.

8. Accessory 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.

9. Accessory 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.

10. Accessory 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 .

11. Accessory 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.

12. Accessory according to claim 1, wherein said scavenging layer (16, 36) comprises a polymer material selected from elastomeric copolymers of the ethylene with at least one alpha-olefin, silicon polymers, polymers or copolymers of butane, butyl rubbers and mixture thereof.

13. Accessory according to claim 1, wherein said scavenging layer (16, 36) comprises a polymer material having a 200% elastic modulus of from 2 MPa and 8 MPa according to ASTM D638-10.

14. Accessory according to claim 1, said accessory being an electrical cable joint.

15. Accessory according to claim 1, said accessory being an electrical cable termination.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0058] Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawings, in which:

[0059] FIG. 1 is a partial longitudinal cross section of a joint, particularly suitable for HVDC cables, according to the present invention;

[0060] FIG. 2 is a partial longitudinal cross section of a termination, particularly suitable for HVDC cables, according to the present invention;

[0061] FIGS. 3, 4 and 5 show some experimental results obtained according to the examples reported hereinbelow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] In FIG. 1, a longitudinal section of a preferred embodiment of a joint (100) mounted on a pair of joined electrical cables (200) according to the present invention is schematically represented.

[0063] Each cable of the joined pair cable (200) comprises a conductor (not shown in the figure) and an insulating layer (21a, 21b) made, for example, from crosslinked polyethylene (XLPE). When the pair of cables (200) is for use for HVDC power transport, the insulating layers (21a, 21b) are degassed to abate the concentration of cross-linking by-products under a predetermined threshold of, for example, 0.5 wt %.

[0064] The joint (100) comprises: an insulating element (13), made of, e.g., cross-linked and non-degassed EPDM, encircling and being in contact with the cable insulating layer (21a, 21b) of the electrical cable (200). A semiconductive electrode (12) which is embedded in said electrically insulating element (13) encircles the cable (200) where the cable insulating layers (21a, 21b) are removed to bare the cable conductors then joined, and can encircle a limited portion of the insulating layers (21a, 21b) in the vicinity of the removal zone.

[0065] The joint (100) further comprises an outer semiconducting layer (14) encircling the electrically insulating element (13) and two stress-relief cones (15a, 15b). Two stress-relief cones (15a, 15b), made of polymeric material, for example of cross-linked ethylene propylene rubber are provide each at a side of the electrically insulating element (13).

[0066] A scavenging layer (16) comprising zeolite particles is located in a radial outer position with respect to the insulating element (13), the outer semiconducting layer (14) and the stress-relief cones (15a, 15b). According to an embodiment of the present invention, the scavenging layer (16) can be a polymer layer, for example, from a material of those listed for the insulating element (13) and comprising zeolite particles dispersed therein.

[0067] In FIG. 2, a longitudinal section of a preferred embodiment of a termination (300) mounted on an electrical cable (400) according to the present invention is schematically represented.

[0068] The electrical cable (400) comprises a conductor (not shown in the figure) and an insulating layer (41) made, for example, from crosslinked polyethylene (XLPE), similar to those already disclosed above in connection with the pair of cable (200).

[0069] The termination (300) comprises: an insulating element (33), made for example from cross-linked and non-degassed EPDM, surrounding and in contact with the insulating layer (41) of the electrical cable (400) and a stress-relief cone (35) also surrounding and in contact with the insulating layer (41).

[0070] A scavenging layer (36) comprising zeolite particles is located in a radial outer position with respect to the insulating element (33) and the stress-relief cone (35) and in contact thereto. The scavenging layer (36) is a polymeric layer made from a material of those listed for the insulating element (33) and comprising zeolite particles dispersed therein.

[0071] In both the embodiments shown in FIGS. 1 and 2, the zeolites of the scavenging layer (16, 36) draw and capture the crosslinking by-products present in the insulating element (13, 33) thus preventing their migration towards the insulating layer (21a, 21b, 41) of the cable/pair of cables (400, 200). Without wishing to be bound to such theory, as the zeolites absorb the crosslinking by-products, the concentration of such product decreases in the portion of the insulating element (13, 33) nearer to the scavenging layer (16, 36). This decrease causes a migration of further amounts of crosslinking by-products in the direction of the scavenging layer (16, 36) and, as a consequence, away from the cable insulating layer (21a, 21b, 41).

[0072] FIGS. 1 and 2 show 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.

[0073] The following examples are provided to further illustrate the invention.

EXAMPLES 1-7

[0074] Some tests were carried out to evaluate the ability of different materials to absorb cumyl alcohol, one of the major by-products deriving from crosslinking reaction of polyethylene with cumyl peroxide. Each material in the form of fine particles was placed in an amount of about 0.6 g in little bags made from a non-woven polyester fabric, which are porous so as to allow free migration of cumyl alcohol molecules.

[0075] The little bags of the materials to be tested were placed in a cylinder made from aluminum, having a plurality of recesses to host the samples and a central circular cavity where a glass beaker containing cumyl alcohol was placed. In one of the recesses a fully degassed sample of crosslinked polyethylene (XLPE) was placed to measure the solubility of cumyl alcohol in that material as reference. The cylinder, containing the little bags of the materials to be tested, the sample of XLPE and the cumyl alcohol, was fastened with a closing plate provided with an O-ring to obtain an airtight closure. The amount of cumyl alcohol absorbed by each sample was measured by weighing the sample at regular intervals up to 1460 hours of exposure to cumyl alcohol when asymptotic conditions have been reached at all temperatures in the 40 to 70 C. range. The solubility of cumyl alcohol in each sample was calculated as:

[00001]$C.A..Math.\mathrm{solubility}=\frac{C.A..Math.\%}{p\left(C.A.\right)}$

wherein: [0076] C.A. % is the weight percentage of cumyl alcohol absorbed by the sample with respect to the initial weight of the sample; [0077] p(C.A.) is the vapour tension of cumyl alcohol at the testing temperature (expressed in bar).

[0078] The test was carried out at different temperatures (40 C., 60 C. and 70 C.). The results are reported in Table 1.

TABLE-US-00001 TABLE 1 C.A. Solubility (% w/bar) Example Material 40 C. 60 C. 70 C. 1 (*) XLPE 15,804 7,648 5,421 2 (*) Dellite 72T 106,438 24,068 19,322 3 (*) Carbon D 212,452 52,204 35,184 4 (*) Supercarb 241,933 61,572 41,330 5 (*) J550 278,031 20,396 26,292 6 CBV 600 1,154,989 257,433 131,559 7 CBV 712 1,151,132 255,940 128,989 [0079] The examples marked with an asterisk (*) are comparative. [0080] XLPE: polyethylene (Borealis LE 4253) crosslinked by cumyl peroxide (1.45 wt %, preheating at 120 C. for 2 minutes, heating at 150 C. for 15 minutes and final cooling at 20 C.); [0081] Dellite 72T: montmorillonite nanoclay modified with quaternary ammonium salt (Laviosa Chimica Mineraria S.p.A.); [0082] Carbon D: active carbon Carbon Decolorans code 434507 (Carlo Erba, IT) [0083] Supercarb: active carbon Adsorbent 2-4566 (SUPELCO, Bellefonte Pa. USA) [0084] J550: sodium polyacrylate resin Aqua keep 10SH-P (SUMITOMO SEIKA) [0085] CBV 600: Y-type zeolite having: 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 (ZEOLYST); [0086] CBV 712: Y-type zeolite having: specific surface area=730 m.sup.2/g; SiO.sub.2/Al.sub.2O.sub.3 ratio=12; Na.sub.2O %=0.05; dimensionality=3; maximum diffusing sphere diameter=7.35 (ZEOLYST).

[0087] From the data reported in Table 1, it is apparent that in the Example 6 and 7 according to the invention the zeolites are able to absorb cumyl alcohol in large amounts, much greater than those obtainable by means of other absorbing materials, such as montmorillonite nanoclay, carbon particles and sodium polyacrylate resin, a water absorbing material commonly used in energy cables.

EXAMPLES 8-11

[0088] In order to simulate the conditions in an accessory for energy cable, the absorption ability of zeolite CBV 600 was evaluated according to the following method.

[0089] A crosslinkable ethylene/propylene/diene copolymer (EPDM) commonly used as insulating material for DC accessory, was used to produce discs of freshly crosslinked polymer material as disclosed above.

[0090] A disc of freshly crosslinked EPDM (diameter 140 mm, thickness 2.87 mm) was placed in a cylinder similar to that of Examples 1-7, but devoid of recesses and cavity to host samples and container, and three little bags as described in Examples 1-7 containing zeolite CBV 600 were placed on the disc (weight ratio zeolite/EPDM=0.013). The testing device was closed airtight and maintained at the testing temperature (60 C. or 40 C.) for 16 days. In the diagrams of FIGS. 3-4, the amounts of acetophenone and cumyl alcohol in the EPDM disc as such (i.e. just after crosslinking) and after contact with the zeolite are reported. A remarkable reduction of the amount of crosslinking by-products in the insulating material when placed in contact with the zeolite particles is apparent.

EXAMPLES 12-13

[0091] In order to evaluate the ability of the zeolite particles to absorb the crosslinking by-products even during storage at room temperature, a sample of zeolite CBV600 (Example 12) or of zeolite CBV712 (Example 13) was placed into a bag made from Polylam, where also a small container containing cumyl alcohol was placed. The bag was hermetically closed. The amount of cumyl alcohol absorbed by the zeolite sample was measured over time by extracting the zeolite sample from the bag after a certain time. The results are reported in the diagram of FIG. 5, from which it appears that the zeolite continued to absorb cumyl alcohol even after more than 4000 hours of exposure at 23 C. From these data, we can derive that the zeolite particles placed in the vicinity of the insulating layer of a cable accessory should be able to eliminate the crosslinking by-products also at ambient temperature.