POWER CABLE

Abstract

Power cable having an insulation system comprising at least one layer made of a thermoplastic material based on a polypropylene matrix admixed with a dielectric fluid, the thermoplastic material having a melting enthalpy of from 15 to 50 J/g and the polypropylene matrix being made of a material selected from: a heterophasic ethylene-propylene copolymer (a) having a melting enthalpy of from 15 to 50 J/g; or an intimate admixture of (a) and a propylene homopolymer or an ethylene propylene copolymer (b) having a melting enthalpy greater than 50 J/g. The cable is particularly suitable for current transport at high voltage or extra high voltage.

Claims

1. Power cable having an insulation system comprising at least one layer made of a thermoplastic material based on a polypropylene matrix admixed with a dielectric fluid, the thermoplastic material having a melting enthalpy of from 15 to 50 J/g and the polypropylene matrix being made of a propylene material selected from: a heterophasic ethylene-propylene copolymer (a) having a melting enthalpy of from 15 to 50 J/g; or an intimate admixture of (a) and a propylene homopolymer or an ethylene propylene copolymer (b) having a melting enthalpy greater than 50 J/g.

2. Power cable according to claim 1 suitable for current transport at high voltage or extra high voltage.

3. Power cable according to claim 1 wherein the insulating system comprises an inner semiconducting layer, an insulating layer and an outer semiconducting layer, and at least the insulating layer is made of the thermoplastic material as described in claim 1.

4. Power cable according to claim 2 wherein the inner semiconducting layer and an outer semiconducting layer are made of the thermoplastic material as described in claim 1.

5. Power cable according to claim 1 wherein the thermoplastic material has a melting enthalpy of from 20 to 45 J/g.

6. Power cable according to claim 1 wherein the heterophasic ethylene-propylene copolymer (a) comprises an elastomeric phase in an amount of from 45 to 85 wt % with respect to the total weight of the copolymer.

7. Power cable according to claim 1 wherein the heterophasic ethylene-propylene copolymer (a) has a melting enthalpy of from 20 to 45 J/g.

8. Power cable according to claim 1 wherein the propylene homopolymer or the random ethylene propylene copolymer (b) has a melting enthalpy greater than 60 J/g.

9. Power cable according to claim 1 wherein (b) is a random ethylene propylene copolymer.

10. Power cable according to claim 1 wherein the thermoplastic material comprises from 1 wt % to 10 wt % of dielectric fluid.

11. Power cable according to claim 9 wherein the thermoplastic material comprises from 3 wt % to 7 wt % of dielectric fluid.

12. Power cable according to claim 1 wherein the thermoplastic material has a melt flow rate of from 0.4 to 5 g/10 min at 2.16 kg/230° C.

13. Power cable according to claim 1 wherein the thermoplastic material has a flexural modulus of from 80 to 400 MPa.

14. Power cable according to claim 1 wherein the thermoplastic material has a melting peak greater than 140° C.

15. Power cable according to claim 14 wherein the thermoplastic material has a melting peak greater than 150° C.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0046] Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawing, in which:

[0047] FIG. 1 is a perspective view of an electric cable according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] FIG. 1 shows a cable (10) according to the invention, suitable for transport high voltage or extra high voltage current. Cable (10) is a single core cable comprising a conductor (11) sequentially surrounded by an inner layer semiconducting layer (12), an insulating layer (13) and an outer semiconducting layer (14), these three layers constituting the insulating system.

[0049] The outer semiconducting layer (14) is surrounded by metal screen (15) which is surrounded, in turn, by a metal water barrier (17). Between the metal screen (15) and the metal water barrier (17), a semiconducting tape (16) is interposed having cushioning and, preferably, or water-absorbent properties.

[0050] An outer sheath (18) is the outermost layer.

[0051] The conductor (11) generally consists of metal wires, preferably of copper or aluminium, stranded together by conventional methods, or of a solid aluminium or copper rod. At least one of insulating layer (13) and inner and outer semiconductive layers (12) and (14) is made of a thermoplastic material according to the invention as heretofore defined.

[0052] The metal screen (15) is generally made of electrically conducting wires or tapes helically wound, while the metal water barrier (17) is generally made of aluminium or copper, preferably in form of a foil longitudinally wound around the metal screen (15).

[0053] The outer sheath (18) is generally made of thermoplastic polyethylene, for example high density polyethylene (HDPE) or medium density polyethylene (MDPE). Advantageously, le outer sheath (18) can be made of a material having low-smoke zero halogen properties.

[0054] 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.

[0055] The layer or layers of thermoplastic material according to the present invention may be manufactured in accordance with known methods, for example by extrusion. The extrusion 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.

[0056] Three sample cables having the design of cable (10) of FIG. 1 were manufactured. The sample cables were 500 m long, had one conductor made of copper stranded wires and a conductor cross-section area of 1,000 mm.sup.2. The screen was made of aluminium tape sandwiched by two water-swellable tapes. The whole was surrounded by an aluminium water barrier in form of foil longitudinally folded. The outer sheath of all of the sample cables was made of HDPE.

[0057] The three sample cables had the insulating layer made of a thermoplastic material as set forth in Table 1 and a thickness of about 17 mm. All of the three sample cables have the inner and the outer semiconductive layer made of a mixture HPP/RPP 70:30 containing dibenzyltoluene (6 wt %) and conductive carbon black (30 wt %).

TABLE-US-00001 TABLE 1 Thermoplastic material Sample PP matrix Dielectric fluid Melting Melting Cables (ratio) (amount) enthalpy peak S1 HPP Dibenzyltoluene 24 J/g 162° C. 100 (6 wt %) S2 HPP/RPP Dibenzyltoluene 40 J/g 159° C. 75/25 (6 wt %) S3* HPP/RPP Naphthenic oil 56 J/g 154° C. 50/50 (6 wt %) *comparative HPP: heterophasic ethylene-propylene copolymer having a melting enthalpy of 23 J/g and about 70 wt % of elastomeric phase; RPP: random ethylene propylene copolymer having a melting enthalpy of 78 J/g; Dibenzyltoluene: C.sub.ar/C.sub.tot = 0.86 Naphthenic oil: 3 wt % aromatic carbon atoms, 41 wt % naphthenic carbon atoms, 56 wt % paraffinic carbon atoms and 0.1 wt % polar compounds; C.sub.ar/C.sub.tot < 0.04.

[0058] The three sample cables were tested under alternate current (AC) at increasing voltage and electric gradient. In particular, sample cables S1 and S2 according to the invention successfully passed power frequency voltage tests up to 260 kV (21 kV/mm) showing partial discharge level lower than 2 pC at this voltage. Sample cables S1 and S2 had not breakdown when tested at 422 kV (34.2 kV/mm).

[0059] Comparative sample cable S3, under the same test conditions, showed an increasing partial discharge level (60 pC after 5 minutes at 130 kV and 10 kV/mm; 45 pC after 5 minutes at 200 kV and 16.2 kV/mm) then had a breakdown after 2 minutes at 260 kV (21 kV/mm).

[0060] Another sample cable S1 according to the invention was tested under direct current (DC). 50 m of sample was subjected to voltage of 500 kV (30 kV/mm), 550 kV (33 kV/mm) and 600 kV (36 kV/mm) for five cycles per each voltage. The conductor temperature was of 70-75° C. Neither breakdown nor flashover has occurred. No evidence of thermal instability or any other phenomenon which could lead to electrical or thermal degradation during a long term test.

[0061] Also another sample of comparative cable S3 was tested under DC. 60 m of sample was subjected to voltages at the same conditions disclosed above. The conductor temperature was of 70-75° C. During ramping (100_kV/10 min) from 500 kV to 550 kV (at approximately 530 kV) the tested sample broke down.

[0062] Morphological investigations by SEM microscopy were carried out on the insulating layer of the sample cable S3. The results indicate lack of cohesion between the matrix components in the polypropylene matrix (and not in the dielectric fluid) observed. In particular, the lack of cohesion regarded the elastomeric amorphous phase (mainly provided by the heterophase copolymer (a)) and the crystalline phase (mainly provided by the random copolymer (b)), and resulted in microcavities and microcracks.

[0063] Analogous SEM microscopy inspections were carried out on the insulation material of sample cable S1 according to the invention and substantially no microfractures between the amorphous and the crystalline phase of the insulation were detected.

[0064] Cables having at least one layer of the insulation system made of a thermoplastic material according to the invention showed to efficiently perform at extra high voltages.