ELECTRIC CABLE WITH IMPROVED THERMOPLASTIC INSULATING LAYER

Abstract

An electric cable for high-voltage applications is disclosed which comprises a core surrounded by an electrically insulating layer made of a composition based on a thermoplastic polymeric material charged with boron nitride powder in an amount up to 20 wt % with respect to the weight of the insulating composition, the boron nitride powder having a particle size distribution D50 up to 15 m. Such a cable has improved thermal conductivity property as well as good dielectric resistance and workability in particular through extrusion processes.

Claims

1-13. (canceled)

14: An electric cable, comprising: a core surrounded by an electrically insulating layer comprising an insulating composition, wherein the insulating composition is based on a thermoplastic polymeric material charged with boron nitride powder in an amount of up to 20 wt % with respect to a total weight of the insulating composition, and the boron nitride powder has a particle size distribution D50 of up to 15 m.

15: The electric cable according to claim 14, wherein the amount of the boron nitride powder is at least 10 wt % with respect to the total weight of the insulating composition.

16: The electric cable according to claim 14, wherein the amount of the boron nitride powder is less than 20 wt % with respect to the total weight of the insulating composition

17: The electric cable according to claim 14, wherein the particle size distribution D50 of the boron nitride powder is up to 10 m.

18: The electric cable according to claim 14, wherein the particle size distribution D50 of the boron nitride powder is at least 0.1 m.

19: The electric cable according to claim 14, wherein boron nitride of the boron nitride powder has a hexagonal form.

20: The electric cable according to claim 19, wherein boron nitride particles of the boron nitride powder are uncoated.

21: The electric cable according to claim 14, wherein the boron nitride powder has a particle size distribution D100 of lower than 50 m.

22: The electric cable according to claim 21, wherein the boron nitride powder has a particle size distribution D100 of lower than 40 m.

23: The electric cable according to claim 14, wherein the thermoplastic polymer material is selected from the group consisting of: a copolymer (i) of propylene with an olefin co-monomer selected from the group consisting of ethylene and an -olefin other than propylene, wherein the copolymer (i) has a melting point of at least 130 C. and a melting enthalpy of from 20 J/g to 90 J/g, a blend of the copolymer (i) with a copolymer (ii) of ethylene with an -olefin, wherein the copolymer (ii) has a melting enthalpy of from 0 J/g to 120 J/g, and a blend of a propylene homopolymer with either the copolymer (i) or the copolymer (ii); and at least one of the copolymer (i) and the copolymer (ii) is a heterophasic copolymer.

24: The electric cable according to claim 14, wherein the thermoplastic polymeric material forming the electrically insulating layer comprises a dielectric fluid.

25: The electric cable according to claim 24, wherein the dielectric fluid is a synthetic or mineral oil of low or high viscosity.

26: The electric cable according to claim 25, wherein the dielectric fluid is a mineral oil, which is selected from the group consisting of a naphthenic oil, an aromatic oil, and a paraffinic oil.

27: The electric cable according to claim 14, comprising an inner and/or an outer semiconductive layer made of a composition comprising a thermoplastic polymeric composition charged with a conductive filler and with boron nitride powder in an amount of up to 20 wt % with respect to a total weight of the thermoplastic polymeric composition, wherein the boron nitride powder has a particle size distribution D50 of up to 15 m.

Description

[0079] Further details will be illustrated in the following detailed description, with reference to the accompanying drawing, in which

[0080] FIG. 1 shows a cable according to the present disclosure.

[0081] FIG. 1 shows a cable 10 according to the disclosure, suitable for transport medium or high voltage current. Cable 10 is a single core cable comprising a conductor 11 sequentially surrounded by an inner layer semiconducting layer 12, an electrically insulating layer 13 and an outer semiconducting layer 14. The conductor 11 and the inner layer semiconducting layer 12 constitutes the cable core.

[0082] 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.

[0083] An outer sheath 18 is the outermost layer.

[0084] 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. The electrically insulating layer 13 and inner and outer semiconductive layers 12 and 14 are made of a thermoplastic composition according to the present disclosure.

[0085] 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.

[0086] The outer sheath 18 is generally made of thermoplastic polyethylene, for example high density polyethylene (HDPE) or medium density polyethylene (MDPE). The outer sheath 18 can be made of a material having low-smoke zero halogen flame-retardant properties.

[0087] 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.

Example 1

Measures of Thermal Conductivity

[0088] A thermoplastic heterophasic ethylene-propylene copolymer (PP) having a melting temperature of 163 C. and a melting enthalpy of 26 J/g has been used alone or in admixture with boron nitride powder at different amounts and particle size distribution to create test samples of insulating compositions for cables.

[0089] The boron nitride powders tested, which are all hexagonal structures, are shown in Table 1.

TABLE-US-00001 TABLE 1 Boron nitride powders Particle size distribution Mean D50 Mean D100 Boron Nitride (m) (m) BN 1 7 30 BN 2 4 40 BN 3 12 60 BN 4 16 60

[0090] In the preparation of the test samples according to the disclosure, the propylene copolymer, optionally previously intimately admixed with a dielectric fluid DF (dibenzyltoluene in an amount of 6 wt %) in a mixer, in form of granules was mixed with a preset amount of boron nitride in form of powder. The resulting dry mixture powder used was fed into a twin-screw extruder operated at about 200 C. to give a compound in form of plate. The plates, at least 3-4 mm thick, had the amounts and types of boron nitride fillers as indicated in Table 2 below. As reference, unfilled samples were also produced by extruding the thermoplastic heterophasic ethylene-propylene copolymer, optionally admixed with the above mentioned dielectric fluid DF without any boron nitride filler.

[0091] None of the tested composition comprised compatibilizers.

[0092] Measures of thermal conductivity (TC) were than performed on the samples so produced. The TC measurements were done at 70 C. using DTC-300 (TA Instruments) according to the method ASTM E1530-11. Three pieces for each sample were used for the TC measurements and measurements were done before and after calibration with respect to reference unfilled samples.

[0093] The results are shown as a mean of the measurements for each type of samples in the following Table 2.

TABLE-US-00002 TABLE 2 BN TC TC increase Composition Polymer (wt %) [W/(m .Math. K)] (%) 1* PP + DF 0.179 2 PP + DF BN 3 (2) 0.184 3 3 PP + DF BN 3 (5) 0.186 4 4 PP + DF BN 1 (10) 0.200 12 5 PP + DF BN 1 (20) 0.236 32 6 PP + DF BN 2 (10) 0.201 12 7 PP + DF BN 2 (20) 0.235 31 8* PP + DF BN 4 (20) 0.156 13 9* PP 0.188 10* PP BN 4 (20) 0.151 20 *comparative

[0094] From the above results it can be seen an increase of the thermal conductivity of the electrically insulating composition due to the addition of boron nitride according to the disclosure to the thermoplastic polymeric material compared to the thermoplastic polymeric material as such (comparative compositions 1 and 9). On the contrary, the addition of boron nitride with a D50 particle size greater than 15 m (comparative composition 8 and 10) caused a thermal conductivity decrease in the insulating composition.

[0095] Additional compositions prepared as indicated above, but by adding a greater amount of boron nitride (more than 20%) to the thermoplastic polymeric material have shown that the viscosity of the polymeric composition becomes noticeably higher and renders difficult the extrusion by conventional extrusion processes.

Example 2

Measures of Electric Properties

[0096] Samples prepared according to the Example 1 were also tested for their electric properties namely electrical permittivity and, for some samples, electrical conductivity .

[0097] The measurement of permittivity was performed according to IEC 60250 (1969) and ASTM D150-92 (2004) on one sample for each composition, the samples having dimensions 200 mm200 mm and 0.5 mm thick. The samples were subjected to a voltage of 0.5 kV and the measurements were done through a Shering bridge. The samples were varnished before the tests.

[0098] The results of the above electric measures are reported in Table 3 below.

TABLE-US-00003 TABLE 3 Composition 1* 1.9 2 2.1 3 2.2 5 2.5 7 2.6 8* 2.5 9* 2.5 10* 2.9 *comparative

[0099] From the above results, it can be seen that the permittivity of the insulating composition increased with increasing amounts of boron nitride. In the case of the comparative composition 10 containing BN 4 (D50 particle size greater than 15 m), permittivity resulted over the value suitable for an electrically insulating layer, especially for high voltage cables.

[0100] The measurements of electrical conductivity at 10 kV were performed according to IEC 60093 (1980) on one sample having dimensions 200 mm200 mm and 1 mm thick for each composition. Voltage drop (Shunt characteristic) was measured using a picoammeter and a measurement cell provided with guard ring and put at a pressure of 20 bar.

[0101] While the tested compositions according to the present disclosure maintained electrical conductivity in the order of 10.sup.161/m, the comparative composition 8 containing BN 4 (D50 particle size greater than 15 m) had a value of the order of 10.sup.151/m which is unsuitable for the electrically insulating layer of a cable, especially for high voltage cables.