WO2014112682A1

WO2014112682A1 - Power cable

Info

Publication number: WO2014112682A1
Application number: PCT/KR2013/001628
Authority: WO
Inventors: Ho Souk Cho; Hyun Seok Kim; Jin Ho Nam; Young Eun Cho; Ung Kim; Ik Hyun Ryu
Original assignee: Ls Cable & System Ltd.
Priority date: 2013-01-21
Filing date: 2013-02-28
Publication date: 2014-07-24
Also published as: KR102003565B1; KR20140094097A

Abstract

The present invention relates to a power cable. More specifically, the present invention relates to a power cable comprising an insulating layer made from an insulating composition which is recyclable and environmentally friendly, shows excellent flexibility, heat resistance, mechanical and electrical properties and the like and has low production cost.

Description

POWER CABLE

A general power cable comprises a conductor and an insulating layer surrounding the conductor, and a high voltage or ultrahigh voltage power cable may further comprise an inner semiconductive layer between the conductor and the insulating layer, an outer semiconductive layer surrounding the insulating layer, and a sheath layer surrounding the outer semiconductive layer.

In recent years, the temperature at which high-voltage power cables are used was increased from about 90 ℃ to about 110 ℃, and thus it was required to provide an insulating material for preparing an insulating layer which is not deteriorated at the increased temperature or more while maintaining its mechanical and electrical properties. In the prior art, as the base resin of an insulating material for a power cable which is used at 90 ℃, a cross-linked polyolefin, such as polyethylene, an ethylene/propylene rubber (EPR), copolymer or an ethylene/propylene/diene monomer (EPDM) copolymer, has generally been used.

However, polymers, such as cross-linked polyethylene (XLPE), which have been used as the base resin of the insulating material, are difficult to recycle, because they are cross-linked polymer. In addition, if the sheath layer is made of polyvinyl chloride (PVC), it is difficult to separate from the cross-linked polyethylene (XLPE) of the insulating material and produces toxic chlorinated substances when it is incinerated, and thus it is not environmentally friendly.

Also, non-crosslinked high-density polyethylene (HDPE) or low-density polyethylene (LDPE) is recyclable and environmentally friendly. However, the temperature at which this polymer can be used is lower than that of cross-linked polyethylene (XLPE), and thus the use thereof is significantly limited.

Meanwhile, it is known to use an environmentally friendly polypropylene having a melting point of 160 ℃ or higher as the base resin so that the service temperature of the power cable can be increased to about 110 ℃. As used herein, the term “polypropylene” refers to a highly crystalline isotactic polypropylene which is a thermoplastic material having excellent mechanical properties.

However, the above polypropylene has insufficient flexibility due to its high rigidity, and thus a cable comprising an insulating layer made therefrom shows poor workability, and the use of the polypropylene is limited.

In connection with this, it is known to mix the base resin polypropylene with insulating oil having an aromatic structure in order to improve the flexibility of the insulating layer made from the polypropylene. However, the insulating oil having the aromatic hydrocarbon structure is expensive, thus increasing the production cost of the cable.

Accordingly, there is a need to provide an insulating material for a power cable, which is recyclable and environmentally friendly, shows excellent flexibility, heat resistance and mechanical and electrical properties and has low production cost, and a power cable comprising an insulating layer prepared therefrom.

It is an object of the present invention to provide a power cable comprising an insulating layer made from a recyclable, environmentally friendly insulating material which has excellent heat resistance so as to be able to be used as the insulating material without needing to be cross-linked.

Another object of the present invention is to provide a power cable comprising an insulating layer made from an insulating material which has improved flexibility and, at the same time, can maintain its heat resistance, mechanical and electrical properties, etc.

Still another object of the present invention is to provide a power cable comprising an insulating layer made from an insulating material having a low production cost.

In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a power cable comprising one or more conductors, an insulating layer covering each of the conductors, and the outermost sheath layer, wherein the insulating layer is made from an insulating composition which comprises a base resin comprising a non-crosslinked propylene homopolymer, a non-crosslinked propylene copolymer or a combination thereof, and based on 100 parts by weight of the base resin, 2.5-10 parts by weight of insulating oil comprising a naphthenic hydrocarbon which has a formula of CnH2n (n = an integer ranging from 20 to 60) and in which a cyclic hydrocarbon and an acyclic hydrocarbon are alternately arranged.

The naphthenic hydrocarbon may be a monocyclic naphthenic hydrocarbon.

The monocyclic naphthenic hydrocarbon may have a 5- or 6-membered ring structure.

The base resin may comprise the non-crosslinked homopolymer and the non-crosslinked propylene copolymer at a mixing ratio of 80:20 to 50:50.

The non-crosslinked propylene copolymer may be a copolymer of a propylene monomer with an ethylene monomer.

The content of the ethylene monomer may be 15 mole% or less based on the total moles of the monomers constituting the non-crosslinked propylene copolymer.

The insulating composition may comprise, based on 100 parts by weight of the base resin, 0.2-5 parts by weight of an amine-, dialkylester-, thioester- or phenol-based antioxidant.

The sheath layer may be made from a composition comprising the same base resin as that of the insulating composition.

The power cable may further comprise an inner semiconductive layer disposed between the conductors and the insulating layer, and an outer insulating layer covering the insulating layer.

The inner semiconductive layer, the outer semiconductive layer, or both may comprise the same base resin as that of the insulating composition.

The base resin of the insulating layer of the power cable according to the present invention comprises a non-crosslinked polypropylene resin which has excellent heat resistance so as to be able to be used as the insulating material without needing to be cross-linked, and thus the insulating layer is recyclable and environmentally friendly.

Further, the insulating layer of the power cable according to the present invention comprises a specific insulating oil, and thus has improved flexibility and, at the same time, can maintain its heat resistance, mechanical and electrical properties , etc., even though it comprises, as its base resin, a polypropylene resin having insufficient polypropylene due to its high rigidity.

In addition, the power cable according to the present invention does not comprise an expensive insulating oil having an aromatic hydrocarbon structure, and thus has a low production cost compared to a power cable which comprises an insulating layer made from a conventional insulating material comprising the aromatic insulating material.

FIG. 1 is a cross-sectional view schematically showing the cross-sectional structure of a power cable according to the present invention.

FIG. 2 is a longitudinal sectional view schematically showing the sectional structure of a power cable according to the present invention.

FIGS. 1 and 2 show an embodiment of a power cable according to the present invention.

As shown in FIGS. 1 and 2, the power cable according to the present invention may comprise: a conductor 1 made of a conductive material, such as copper or aluminum; an insulating layer 3 made of an insulating polymer; an inner semiconductive layer 2 disposed between the conductor 1 and the insulating layer 3 and functioning to suppress partial electrical discharge at the interface between the conductor 1 and the insulating layer 3 and eliminate an air gap between the conductor 1 and the insulating layer 3 and also mitigate local electric field concentration; an outer semiconductive layer 4 functioning to shield the cable and apply a uniform electric field to the insulating material; and a sheath layer 5 for protecting the cable.

The inner or outer

semiconductive layer

2 or 4 of the power cable can be made by a conventional method. Preferably, it may be made from a composition, which comprises the same base resin as that of the insulating layer 3 and can ensure excellent adhesion to the insulating layer 3, so as to prevent exfoliation between the

semiconductive layer

2 or 4 and the insulating layer 3, which can shorten the service life of the power cable. Further, the inner or outer

semiconductive layer

2 or 4 may comprise a conductive filler such as carbon black in order to ensure semiconductive properties. Moreover, the sheath layer 5 of the power cable may also be made from a composition comprising the same base resin as that of the insulating layer 3.

The specifications of the conductor 1, the insulating layer 3, the

semiconductive layers

2 and 4 and the sheath layer 5 may vary depending on the intended use of the cable, the voltage flowing through the cable, etc., and the insulating layer 3, the

semiconductive layers

2 and 4 and the sheath layer 5 may be made of the same or different materials.

An insulating material forming the insulating layer 3 of the power cable according to the present invention comprises non-crosslinked polypropylene resin as the base resin.

The non-crosslinked polypropylene resin may comprise a propylene homopolymer and/or a copolymer of propylene with a comonomer selected from among ethylene and C4-C12 α-olefins, for example, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and combinations thereof, preferably ethylene. This is because the copolymer of propylene with ethylene has strong and flexible properties. Herein, the mixing ratio of the propylene homopolymer to the propylene copolymer may be, for example, 80:20 to 50:50.

The content of the comonomer in the propylene copolymer may be 15 mole% or less, and preferably 10 mole% or less, based on the total moles of the monomers of the propylene copolymer. Particularly, a propylene/ethylene copolymer is preferable. The propylene copolymer may be a random copolymer or block copolymer of propylene with ethylene and/or α-olefin. In addition, the polypropylene may comprise a mixture of polyolefins, including low-density polyethylene and linear low-density polyethylene.

The propylene homopolymer or copolymer preferably has a peak crystallization temperature of 110 to 125 ℃ as measured by differential scanning calorimetry (DSC). If the peak crystallization temperature is lower than 110 ℃, the propylene homopolymer and/or copolymer can be melted under the aging test conditions (135 or 150℃) provided in IEC international standards or cannot satisfy a continuous service temperature of 90 ℃. If the peak crystallization temperature is higher than 125 ℃, the crystallization rate of the propylene homopolymer or copolymer during cooling will be fast, and thus the tensile elongation at room temperature will be insufficient.

Further, the propylene homopolymer or copolymer preferably has a weight-average molecular weight (Mw) of 200,000-450,000. If the weight-average molecular weight (Mw) is less than 200,000, the mechanical properties before and after heating will be insufficient, and if it is more than 450,000, the processability of the propylene homopolymer or copolymer can be reduced due to high viscosity.

Moreover, the propylene homopolymer or copolymer preferably has a molecular weight distribution (Mw/Mn) of 2-8. If the molecular weight distribution (Mw/Mn) of the propylene homopolymer or copolymer is less than 2, the processability can be reduced due to high viscosity, and if it is more than 8, the mechanical properties before and after heating can be insufficient.

Meanwhile, the propylene homopolymer or copolymer may, for example, have a melt index of 0.01-1,000 dg/min as measured in accordance with ASTM D-1238, a melting point (Tm) of 140 to 175 ℃ as measured by differential scanning calorimetry (DSC), a melt enthalpy of 30-85 J/g as measured by differential scanning calorimetry (DSC), and a flexural modulus of 30-1,400 MPa, preferably 60-1,000 MPa, as measured in accordance with ASTM D790-00.

As described above, the propylene homopolymer or copolymer in the present invention has a high melting point, so that an insulating layer made from the propylene homopolymer or copolymer shows improved properties at increased temperature, and thus can provide a power cable which can be used at increased temperature. In addition, the propylene homopolymer or copolymer is a non-crosslinked polymer, and thus is recyclable and environmentally friendly.

Unlike the non-crosslinked propylene homopolymer or copolymer according to the present invention, a crosslinked polymer according to the prior art is difficult to recycle and is not environmentally friendly. In addition, if premature crosslinking or scorching occurs in a process of making an insulating layer, the uniform production of the insulating material cannot be ensured and the extrudability can be reduced.

The non-crosslinked homopolymer or copolymer can be prepared by a conventional method. Specifically, it can be prepared by homopolymerization of propylene or copolymerization of propylene with ethylene or α-olefin(s) other than propylene in the presence of a Ziegler-Natta catalyst having low stereospecificity.

Details for the method for preparing the non-crosslinked propylene homopolymer or copolymer are described, for example, in Albizzati et al. in "Polypropylene Handbook", Chapter 2, page 11 onwards (Hanser Publisher, 1996).

The insulating composition forming the insulating layer 3 of the power cable according to the present invention may comprise, in addition to the above-described resin, specific insulating oil.

The insulating oil may comprise a naphthenic hydrocarbon. As used herein, the term “naphthene” refers to a collection of saturated hydrocarbons having a structure in which carbons in the molecule are bonded to form a ring. Naphthene is also named "cycloparaffin", because it has properties similar to those of paraffinic hydrocarbons. The insulating oil may comprise a monocyclic naphthenic hydrocarbon having a molecular formula of C_nH_2n wherein n is an integer ranging from 20 to 60, that is, a naphthenic hydrocarbon having a structure of the following Chemistry Figure 1 in which a cyclic hydrocarbon and an acyclic hydrocarbon are alternately arranged. Herein, the cyclic hydrocarbon may be 3 to 6 membered, and preferably 5- or 6-membered.

[Chemistry Figure 1]

wherein the larger circles represent carbon atoms, and the smaller circles represent hydrogen atoms.

Unlike the monocyclic naphthenic oil, an insulating oil having a molecular formula C_nH_2n _-m(n = an integer from 20 to 60, and m = an integer from 4 to 8) and having a structure of the following Chemistry Figure 2 in which cyclic hydrocarbons in the molecule are attached to each other, that is, an insulating oil having a multicyclic structure such as a bicyclic or tricyclic structure, has low insulating oil performance due to its low miscibility with the base resin, and thus an insulating layer comprising the same will have insufficient mechanical and electrical properties:

[Chemistry Figure 2]

wherein the lager circles represent carbon atoms, and the smaller circles represent hydrogen atoms.

The insulating oil that is used in the present invention preferably has 20 to 60 carbon atoms. If the insulating oil has less than 20 carbon atoms, it can be vaporized and decomposed due to its low molecular weight in a process of extruding the insulating layer, and if it has more than 60 carbon atoms, it can flow out due to its high molecular weight in a process of extruding the insulating layer.

The content of the insulating oil in the insulating composition forming the insulating layer 3 of the power cable according to the present invention may be 2.5-10 parts by weight based on 100 parts by weight of the propylene homopolymer or copolymer that is the base resin. If the content of the insulating oil is less than 2.5 parts by weight, an insulating layer made from the insulating composition cannot have sufficient flexibility so that an operation of installing the cable will not be easy, and if the content of the insulating oil is more than 10 parts by weight, the insulating oil can flow out in a process of extruding the insulating layer, so that the manufacture and processing of the cable will be difficult.

As described above, the insulating oil that is used in the present invention has a very high rigidity, and thus improves the flexibility of an insulating layer made from the insulating composition comprising the low-flexibility polypropylene resin as the base resin while maintaining the excellent heat resistance and mechanical and electrical properties of the polypropylene resin. Further, the insulating oil that is used in the present invention is significantly inexpensive, and thus reduces the production cost of the cable, while it provides equal or superior flexibility, heat resistance and mechanical and electrical properties compared to the use of conventional insulating oil having an aromatic hydrocarbon structure.

The insulating composition forming the insulating layer 3 of the power cable according to the present invention may comprise, in addition to the base resin and the insulating oil, other additives such as an antioxidant. The antioxidant that is used in the present invention may be an amine-, dialkylester-, thioester- or phenol-based antioxidant, and examples thereof include distearyl thiopropionate, pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], [3-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propanoyloxy]-2,2-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propanoyloxymethyl]propyl], 3-(3,5-di-t-butyl-4-hydroxyphenyl)propanoate, thioethylene bis(3,5-di-t-butyl-4-hydroxyhydrocinamate), 3,5-bis(1,1-dimethylethyl)-4-hydroxybenzene propionic acid octadecyl ester, 3,3'-thiobis-1,1'-dioctadecyl ester, etc. Herein, the content of the additives, including the antioxidant, may be 0.2-5 parts by weight based on 100 parts by weight of the base resin.

Examples

Hereinafter, the present invention will be described in further detail with reference to examples. However, the present invention is not limited to the examples described herein and may also be embodied in other forms. Rather, the examples disclosed herein are provided to enable the disclosure of the present invention to be thoroughly and completely understood and the idea of the present invention to be sufficiently delivered to those skilled in the art.

According to the components and contents shown in Table 1 below, insulating compositions were prepared. Using each of the compositions, a sample sheet having a thickness of 2 mm and a size of 30 cm x 30 cm was prepared using a hot press. Here, the units of the amount shown in Table 1 are parts by weight.

Table 1

Components	Example	Comparative Example
Components	1	1	2	3	4	5
Base resin	100	100	100	100	100	100
Monocyclic naphthenic oil (C₃₀H₆₀)	5	-	-	-	1	13
Monocyclic naphthenic oil (C₁₀H₂₀)	-	5	-	-	-	-
Monocyclic naphthenic oil (C₈₀H₁₆₀)	-	-	5	-	-	-
Multicyclic naphthenic oil (C₃₀H₅₄)	-	-	-	5	-	-

- Base resin: 50:50 mixture of polypropylene (Q200, Basell) and an ethylene-propylene copolymer (R520Y, SK Global Chemical)

- Monocyclic naphthenic oil (C₃₀H₆₀): insulating oil No. 4 (Dongnam Petrochemical)

- Monocyclic naphthenic oil (C₁₀H₂₀): insulating oil No. 1 (Dongnam Petrochemical)

- Monocyclic naphthenic oil (C₈₀H₁₆₀): insulating oil No. 6 (Dongnam Petrochemical)

- Monocyclic naphthenic oil (C₃₀H₅₄): insulating oil No. 35 (Dongnam Petrochemical).

Evaluation of physical properties of sheet

1) Evaluation of mechanical properties at room temperature

In accordance with the IEC-60811-1-1 standard, the tensile strength and tensile elongation of the sample sheet prepared in each of Example 1 and Comparative Examples 1 to 5 were measured at a tensile speed of 250 mm/min at room temperature. The IEC standard requires that the tensile strength be 1.27 kgf/㎟ or more and the tensile elongation be 200% or more.

2) Evaluation of mechanical properties after heating

Each of the sample sheets prepared in Example 1 and Comparative Examples 1 to 5 was aged by heating at 150 ℃ for 168 hours, and then the mechanical properties thereof were measured in the same manner as described in the above section 1). Herein, it is required that each of the tensile strength and the tensile elongation should be 75% or more.

3) Evaluation of flexural strength

In accordance with the IEC 60811-1-1 standard, the flexural strength of each of the sample sheets prepared in Example 1 and Comparative Examples 1 to 5 was measured. Herein, lower values indicate better flexibility and workability (including installation property) of an insulating layer and cable comprising the sample sheet.

4) Evaluation of dielectric breakdown strength

In accordance with the ASTM D149 standard, the dielectric breakdown strength of each of the sample sheets prepared in Example 1 and Comparative Examples 1 to 5 was measured. Herein, higher values indicate better electrical properties.

According to the above-described methods for evaluating the physical properties of the sheets, the flexibility and mechanical and electrical properties of each of the sample sheets prepared in Example 1 and Comparative Examples 1 to 5 were evaluated, and the results of the evaluation are shown in Table 2 below.

Table 2

Components	Example	Comparative Example
Components	1	1	2	3	4	5
Tensile strength (kgf/㎟) at room temperature	2.0	2.4	ND	2.1	2.05	ND
Tensile elongation (%) at room temperature	698	660		605	680
Residual tensile strength (%) after heating	81	88		100	90
Residual elongation (%) after heating	79	58		70	60
Flexural strength (MPa)	30	37		33	42
Dielectric breakdown strength (kV/mm)	65	57		55	55

ND = not determinable.

As can be seen in Table 2 above, because the insulating oil (C₁₀H₂₀) contained in the insulating composition of Comparative Example 1 was vaporized and decomposed due to its low molecular weight (140), the sheet made from the insulating composition showed insufficient residual elongation (58%) after heating, suggesting that it has poor mechanical properties. In addition, because the insulating oil was decomposed during the process of preparing the sample sheet, the sheet showed high flexural strength (37 MPa), suggesting that it has poor flexibility. In addition, it showed low dielectric breakdown strength (57 kV/mm), suggesting that it has insufficient electrical properties.

Also, in the case of the insulating composition of Comparative Example 2 comprising the insulating oil (C₈₀H₁₆₀) having a high molecular weight (1,120) and in the case of the insulating composition of Comparative Example 5 comprising an excessive amount (13 parts by weight) of the insulating oil (C₃₀H₆₀), the insulating oil flowed out, and thus slippage occurred in the extruder during the extrusion process of preparing the sample sheet, so that a sample sheet could not be prepared and the physical properties could not be measured.

Moreover, because the insulating oil (C₃₀H₆₀) contained in the insulating composition of Comparative Example 3 has a multicyclic structure, and thus has insufficient miscibility with the base resin so that it cannot exhibit its function, the residual elongation of the sample sheet after heating was 70% (less than 75%), suggesting that the mechanical properties are insufficient. Also, the sample sheet of Comparative Example 3 showed low dielectric breakdown strength (55 kV/mm), suggesting that it has insufficient electrical properties.

Further, because the content of the insulating oil (C₃₀H₆₀) in the insulating composition of Comparative Example 4 was very low (1 part by weight), the sample sheet of Comparative Example 4 showed the highest flexural strength of 42 MPa, indicating the worst flexibility, and also showed the lowest dielectric breakdown strength (55 kV/mm), indicating the worst electrical properties.

However, the insulating composition of Example 1 showed excellent mechanical properties at room temperature and after heating. In addition, because it contains suitable insulating oil, it showed the lowest flexural strength (30 MPa), indicating the best flexibility, and also showed the highest dielectric breakdown strength (65 kV/mm), indicating the best electrical properties.

Although the preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

A power cable comprising one or more conductors, an insulating layer covering each of the conductors, and the outermost sheath layer,

wherein the insulating layer is made from an insulating composition which comprises a base resin comprising a non-crosslinked propylene homopolymer, a non-crosslinked propylene copolymer or a combination thereof, and based on 100 parts by weight of the base resin, 2.5-10 parts by weight of insulating oil comprising a naphthenic hydrocarbon which has a formula of C_nH_2n (n = an integer ranging from 20 to 60) and in which a cyclic hydrocarbon and an acyclic hydrocarbon are alternately arranged.
The power cable according to claim 1, wherein the naphthenic hydrocarbon is a monocyclic naphthenic hydrocarbon.
The power cable according to claim 1 or 2, wherein the monocyclic naphthenic hydrocarbon has a 5- or 6-membered ring structure.
The power cable according to claim 1 or 2, wherein the base resin comprises the non-crosslinked homopolymer and the non-crosslinked propylene copolymer at a mixing ratio of 80:20 to 50:50.
The power cable according to claim 4, wherein the non-crosslinked propylene copolymer is a copolymer of a propylene monomer with an ethylene monomer.
The power cable according to claim 5, wherein the content of the ethylene monomer is 15 mole% or less based on the total moles of the monomers constituting the non-crosslinked propylene copolymer.
The power cable according to claim 1 or 2, wherein the insulating composition comprises, based on 100 parts by weight of the base resin, 0.2-5 parts by weight of an amine-, dialkylester-, thioester- or phenol-based antioxidant.
The power cable according to claim 1 or 2, wherein the sheath layer is made from a composition comprising the same base resin as that of the insulating composition.
The power cable according to claim 1 or 2, wherein the power cable further comprises an inner semiconductive layer disposed between the conductors and the insulating layer, and an outer insulating layer covering the insulating layer.
The power cable according to claim 9, wherein the inner semiconductive layer, the outer semiconductive layer, or both comprise the same base resin as that of the insulating composition.