Insulating resin composition for power cables and power cables

Abstract

To provide an insulating resin composition for a power cable and a power cable which suppress growth of a water tree and can lower a dielectric loss tangent.SOLUTION: An insulating resin composition for a power cable contains a component (a), a component (b), a component (c) and a component (d), wherein the component (a) is polyethylene, the component (b) is at least one resin selected from a resin (b1) and a resin (b2), the resin (b1) is a resin grafted with at least one modified monomer selected from an unsaturated organic acid and its derivative, the resin (b2) is at least one ethylenic copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer and an ethylene-vinyl acetate vinyl copolymer, the component (c) is a water tree inhibitor, and the component (d) is a crosslinking agent.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an insulating resin composition for a power cable and a power cable.

Background Art

[0002] Crosslinkable polyethylene (XLPE) that has been conventionally used for power cables generates water trees when used as an insulator for power cables in an environment with a large amount of moisture. A water tree is a dendritic defect that occurs from foreign substances or air bubbles (voids) in the resin.

[0003] Due to the dielectrophoresis of water generated by passing electricity through a power cable, moisture in the resin concentrates on the interface of foreign substances or voids, and thus the water tree grows. Since the water tree causes the destruction of the power cable, it is required to suppress the growth of the water tree.

[0004] In order to suppress the growth of water trees, in XLPE resins, hydrophilic molecules such as polyethylene glycols are added to hydrophobic polyethylene. Due to hydrophilic molecules such as polyethylene glycols, moisture is uniformly dispersed in the XLPE resin, so that the concentration of moisture at the interface of foreign substances or voids is suppressed, and the growth of water trees is suppressed.

[0005] Further, Patent Document 1 discloses a polyolefin composition for electrical insulation containing polyolefin or crosslinked polyolefin and a small amount of high molecular weight polyethylene glycol. Further, a technique is also disclosed that a material with low hydrophilicity or hydrophobicity such as polypropylene glycol does not prevent the generation of water trees in the insulator.

[0006] Thus, the growth of water trees is suppressed by adding hydrophilic molecules. However, the addition of hydrophilic molecules increases the dielectric loss tangent (tan δ) of the resin, so that the power transmission efficiency of the power cable decreases.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] U.S. Patent No. 4,305,849 [Summary of the Invention] [Problems to be Solved by the Invention]

[0008] An object of the present disclosure is to provide an insulating resin composition for a power cable and a power cable that can suppress the growth of water trees and reduce the dielectric loss tangent. [Means for Solving the Problems]

[0009] [1] An insulating resin composition for a power cable, comprising component (a), component (b), component (c), and component (d), wherein component (a) is polyethylene, component (b) is at least one resin selected from resin (b1) and resin (b2), resin (b1) is a resin grafted with at least one modified monomer selected from unsaturated organic acids and their derivatives, resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, component (c) is a water tree inhibitor, and component (d) is a crosslinking agent. [2] The insulating resin composition for a power cable according to [1] above, wherein component (c) is at least one compound selected from polyalkylene glycols and their derivatives, polyglycerin, glycerin fatty acid esters, and sorbitol esters. [3] The insulating resin composition for a power cable according to [1] or [2] above, wherein the dielectric loss tangent of the crosslinked product obtained by crosslinking the insulating resin composition for a power cable is 1.0% or less at 90°C and 30 kV / mm. [4] The insulating resin composition for a power cable according to any one of [1] to [3] above, wherein at least one modified monomer selected from unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides, and unsaturated dicarboxylic acid derivatives is added to at least one resin selected from polypropylene, polyethylene, and olefin copolymers. [5] The insulating resin composition for a power cable according to any one of [1] to [4] above, wherein the content ratio of the component (b) contained in the insulating resin composition for a power cable is 3.0 wt% or more and 25.0 wt% or less. [6] A power cable comprising a conductor, an internal semiconductive layer disposed outside the conductor and surrounding the conductor, an insulating layer formed by crosslinking the insulating resin composition for a power cable according to any one of [1] to [5] above and disposed outside the internal semiconductive layer and surrounding the internal semiconductive layer, and an external semiconductive layer disposed outside the insulating layer and surrounding the insulating layer. [Advantages of the Invention]

[0010] According to the present disclosure, there can be provided an insulating resin composition for a power cable and a power cable that can suppress the growth of water trees and reduce the dielectric loss tangent. [Brief Description of the Drawings]

[0011]

Figure 1

Figure 2

[0012] Hereinafter, a detailed description will be given based on the embodiments.

[0013] As a result of intensive research, the inventors have found that even when a water tree inhibitor is contained to suppress the growth of water trees, by containing specific components, an increase in the tangent of dielectric loss caused by the water tree inhibitor can be suppressed, and based on such findings, the present disclosure has been completed.

[0014] The insulating resin composition for a power cable of the embodiment (hereinafter, also simply referred to as an insulating resin composition) contains component (a), component (b), component (c), and component (d). Component (a) is polyethylene, component (b) is at least one resin selected from resin (b1) and resin (b2). Resin (b1) is a resin grafted with at least one modified monomer selected from unsaturated organic acids and their derivatives, and resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers. Component (c) is a water tree inhibitor, and component (d) is a crosslinking agent.

[0015] The insulating resin composition of the embodiment includes component (a), component (b), component (c), and component (d) as constituent elements. A crosslinked product obtained by crosslinking the insulating resin composition (hereinafter, also simply referred to as a crosslinked product) is an insulator and is suitably used for the insulating layer of a power cable.

[0016] Component (a) contained in the insulating resin composition is polyethylene. Polyethylene can be produced by either a low-pressure process or a high-pressure process, but is preferably low-density polyethylene (LDPE) produced by a high-pressure process.

[0017] Component (b) contained in the insulating resin composition is at least one resin selected from resin (b1) and resin (b2). Component (b) suppresses an increase in the tangent of dielectric loss in the crosslinked product of the insulating resin composition caused by the water tree inhibitor, which is component (c).

[0018] The resin (b1) is a resin grafted with at least one modified monomer selected from unsaturated organic acids and their derivatives (hereinafter also simply referred to as a graft resin).

[0019] As the functional group introduced into the resin by grafting of the modified monomer, it is preferably a functional group having a C=O bond. Among them, a carbonyl group, a carboxyl group, an ester group, an acid anhydride, an amide group, and an imide group are preferable. The water tree inhibitor as the component (c) is restrained by a hydrogen bond with the C=O in the molecule grafted to the resin, so that the molecular motion by an electric field is suppressed. Thus, the C=O in the molecule grafted to the resin can efficiently reduce the dielectric loss tangent of the crosslinked product in order to suppress the motion of the water tree inhibitor. Further, the graft resin can improve the dispersibility of the water tree inhibitor and suppress the bleed-out of the crosslinked product.

[0020] Preferred modified monomers are unsaturated dicarboxylic acids, unsaturated dicarboxylic acid anhydrides, and unsaturated dicarboxylic acid derivatives. As the resin (b1) grafted with such a modified monomer, it is preferable that at least one modified monomer selected from unsaturated dicarboxylic acids, unsaturated dicarboxylic acid anhydrides, and unsaturated dicarboxylic acid derivatives is added to at least one resin selected from polypropylene, polyethylene, and olefin copolymers. As the polyethylene, low-density polyethylene produced by a high-pressure process is preferable.

[0021] As the unsaturated dicarboxylic acid, maleic acid, fumaric acid and itaconic acid are preferable. As the unsaturated dicarboxylic anhydride, maleic anhydride and itaconic anhydride are preferable. As the unsaturated dicarboxylic acid derivative, monomethyl maleate, monoethyl maleate, diethyl maleate, monomethyl fumarate, dimethyl fumarate, diethyl fumarate, maleic monoamide, maleimide, N-phenyl maleimide and N-cyclohexyl maleimide are preferable. These can be used alone or in combination of two or more. Among them, maleic anhydride which is a five-membered cyclic acid anhydride is preferable.

[0022] The graft amount of the modified monomer is preferably 0.25 wt% or more and 2.00 wt% or less in the graft resin. When the modified monomer is grafted within this range, the modified monomer is uniformly dispersed in the insulating resin composition, so that the dielectric loss tangent in the system can be uniformly reduced.

[0023] When the modified monomer is maleic anhydride, the content ratio of the modified monomer contained in the insulating resin composition is preferably 0.01 wt% or more and 0.50 wt% or less. When maleic anhydride is contained in the insulating resin composition within this range, the dielectric loss tangent in the system can be uniformly suppressed. In particular, when the content of maleic anhydride exceeds 0.50 wt%, the graft resin may adhere to the metal inner wall of the kneading device or the extrusion device, reducing the uniformity of the resin composition. As a result, the growth of water trees may be promoted.

[0024] As a method for preparing the graft resin, for example, it can be obtained by mixing and heating and reacting a raw material polyethylene, an antioxidant, a modified monomer and an organic peroxide in an extruder by the method described in paragraph

[0098] of Japanese Patent No. 6205032.

[0025] The resin (b2) is at least one ethylene-based copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer and an ethylene-vinyl acetate copolymer.

[0026] As the ethylene-acrylate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, or an ethylene-butyl acrylate copolymer is preferable.

[0027] Regarding the content ratio of component (b) contained in the insulating resin composition, the lower limit is preferably 3.0 wt% or more, more preferably 4.0 wt% or more, and the upper limit is preferably 25.0 wt% or less, more preferably 15.0 wt% or less. When the content ratio of component (b) is within the above range, the dielectric loss tangent of the crosslinked product of the insulating resin composition can be sufficiently reduced.

[0028] Regarding component (b), resin (b1) can more efficiently reduce the dielectric loss tangent of the crosslinked product compared to resin (b2). Therefore, it is preferable that the insulating resin composition contains resin (b1).

[0029] Component (c) contained in the insulating resin composition is a water tree inhibitor. The water tree inhibitor as component (c) is a hydrophilic molecule and can impart hygroscopicity to the crosslinked product of the insulating resin composition. By imparting hygroscopicity to the crosslinked product, the moisture in the crosslinked product is uniformly dispersed, so that the concentration of moisture at the foreign matter or void interface can be suppressed, and the growth of water trees in the crosslinked product can be suppressed.

[0030] The water tree inhibitor is preferably at least one compound selected from polyalkylene glycols and their derivatives, polyglycerin, glycerin fatty acid esters, and sorbitol esters.

[0031] As the polyalkylene glycol, polyethylene glycol, polypropylene glycol, a block copolymer of polyethylene glycol - polypropylene glycol, and their alkyl ethers or carboxylic acid esters are preferred. The purpose of these water tree inhibitors is to impart hygroscopicity to the cross - linked product of the insulating resin composition by hydrogen bonding with water molecules, and the polyalkylene glycol derivatives exhibit the same effects as the polyalkylene glycols. When the polyalkylene glycol and its derivatives are absorbed by the resin pellets during dry blending and used, they are preferably liquid at 60 °C. Polyglycerin is a compound in which a plurality of glycerins are polymerized by ether bonds and can have a linear, cyclic, or branched structure. Also, by alkyl - etherifying or carboxylic - acid - esterifying these polyglycerins, the dispersibility in the insulating resin composition of the present disclosure is improved. In particular, introduction into the insulating resin composition by fatty acid esterification is preferable from the viewpoint of the production stability of the compound. Sorbitol is a polyhydric alcohol derived from glucose, and sorbitol ester is a compound in which one or more hydroxyl groups of sorbitol are esterified with fatty acids. Sorbitol can be dispersed in the insulating resin composition by esterification.

[0032] In order to suppress the bleed - out in the cross - linked product of the insulating resin composition, the number - average molecular weight of the water tree inhibitor is preferably 1000 or more, more preferably 3000 or more, and even more preferably 9000 or more. On the other hand, in order to improve the dispersibility of the water tree inhibitor during melt - kneading, the number - average molecular weight of the water tree inhibitor is preferably 30000 or less. Also, from the viewpoint of ease of handling, the number - average molecular weight of the water tree inhibitor is preferably 20000 or less.

[0033] Regarding the content ratio of the water tree inhibitor contained in the insulating resin composition, the lower limit is preferably 0.1 wt% or more, more preferably 0.2 wt% or more, and the upper limit is preferably 2.0 wt% or less, more preferably 1.0 wt% or less. When the content ratio of the water tree inhibitor is within the above range, the growth of water trees in the crosslinked product of the insulating resin composition can be sufficiently suppressed. Further, when the content ratio of the water tree inhibitor is 1.0 wt% or less, the bleed-out of the crosslinked product can be further suppressed.

[0034] Component (d) contained in the insulating resin composition is a crosslinking agent, which crosslinks polyethylene as component (a). The crosslinking agent as component (d) is preferably di-t-hexyl peroxide (Perhexy D manufactured by NOF Corporation), dicumyl peroxide (Perkyl D manufactured by NOF Corporation), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Perhexy 25B manufactured by NOF Corporation), α,α'-di(t-butylperoxy)diisopropylbenzene (Perbutyl P manufactured by NOF Corporation), t-butylcumyl peroxide (Perbutyl C manufactured by NOF Corporation), di-t-butyl peroxide (Perbutyl D manufactured by NOF Corporation). These can be used alone or in combination of two or more. Among them, dicumyl peroxide is preferable.

[0035] Regarding the content ratio of the crosslinking agent contained in the insulating resin composition, the lower limit is preferably 0.1 wt% or more, more preferably 0.5 wt% or more, and the upper limit is preferably 5.0 wt% or less, more preferably 3.0 wt% or less. When the content ratio of the crosslinking agent is within the above range, the insulating resin composition can be crosslinked well.

[0036] In addition, the insulating resin composition may contain an antioxidant in addition to the above components (a), (b), (c) and (d). As the antioxidant, it is preferable to use a combination of a hindered phenol-based antioxidant mainly for radical scavenging and a phosphorus-based antioxidant or a sulfur-based antioxidant mainly for peroxide decomposition.

[0037] Regarding the content ratio of the antioxidant contained in the insulating resin composition, the lower limit is preferably 0.01 wt% or more, more preferably 0.20 wt% or more, and the upper limit is preferably 1.00 wt% or less, more preferably 0.60 wt% or less. When the content ratio of the antioxidant is within the above range, oxidative degradation in the crosslinked product of the insulating resin composition can be suppressed well.

[0038] Also, as long as it does not inhibit the growth suppression of water trees and the reduction of the dielectric loss tangent in the crosslinked product of the insulating resin composition, the insulating resin composition may contain various substances in addition to the above components. Examples of the various substances include stabilizers, lubricants, inorganic fillers, surface treatment agents, flame retardants, acid scavengers, voltage stabilizers, and the like.

[0039] The insulating resin composition can be obtained by melt-kneading component (b), component (c), component (d), etc. with respect to component (a) after dry blending using a Henschel mixer or the like. As the melt-kneading apparatus, a single-screw or twin-screw extruder, a kneader such as a Banbury or a kneader can be used. In particular, it is preferable to attach a metal mesh filter with an opening of 100 μm or less for the purpose of removing foreign substances to a single-screw or twin-screw extruder capable of continuous processing and perform resin extrusion. The insulating resin composition after adding component (d) is preferably kneaded at 120°C or higher and 135°C or lower.

[0040] The specific gravity of the insulating resin composition is preferably 0.91 or more and 0.93 or less.

[0041] Also, the dielectric loss tangent of the crosslinked product obtained by crosslinking the insulating resin composition at 90°C and 30 kV / mm is preferably 1.0% or less, more preferably 0.5% or less. When the dielectric loss tangent of the crosslinked product is within the above range, for a power cable in which the crosslinked product of the insulating resin composition is applied to the insulating layer, since the dielectric loss tangent is small, the power transmission efficiency can be improved.

[0042] The dielectric loss tangent of the crosslinked product is the value measured under the conditions of 90 °C and 30 kV / mm in accordance with JIS C 2138. Note that the dielectric loss tangent depends on temperature and applied electric field. When the crosslinked product of the above insulating resin composition is measured under the conditions of 23 °C and 10 kV / mm, the dielectric loss tangent of the crosslinked product is 0.1% or less.

[0043] In addition, in order to suppress the growth of water trees in the crosslinked product of the insulating resin composition, the moisture absorption rate of the crosslinked product is preferably 500 ppm or more, and more preferably 800 ppm or more. By imparting hygroscopicity to the crosslinked product, moisture in the crosslinked product is uniformly dispersed, and the growth of water trees in the crosslinked product can be suppressed. Also, from the viewpoint of the dielectric strength of the crosslinked product, the moisture absorption rate of the crosslinked product is preferably 2000 ppm or less.

[0044] Also, the resin fluidity of the crosslinked product measured in accordance with JIS K 7210 at a melting temperature of 190 °C and a load of 2.16 kgf is preferably 0.5 g / 10 min or more and 5.0 g / 10 min or less, and more preferably 0.5 g / 10 min or more and 3.0 g / 10 min or less. When the resin fluidity is within the above range, bleeding out of the tree inhibitor from the crosslinked product can be suppressed, and the shape of the crosslinked product can be maintained over a long period.

[0045] The crosslinked product obtained by crosslinking the insulating resin composition of the embodiment is an insulator. In a power cable in which such a crosslinked product of the insulating resin composition is applied to an insulating layer, the growth of water trees in the insulating layer can be suppressed, and the dielectric loss tangent can be reduced. Therefore, even when a power cable is used at high voltage or extra-high voltage, the power cable can transmit power efficiently. Such a power cable is preferably used for underground cables and submarine cables. When the power cable is an alternating current power cable, the effects of suppressing the growth of water trees and reducing the dielectric loss tangent are more effectively exerted.

[0046] FIG. 1 is a cross-sectional view showing an example of a power cable to which the insulating resin composition of the embodiment is applied.

[0047] As shown in Fig. 1, the power cable 1 includes a conductor 2, an inner semiconductive layer 3 disposed outside the conductor 2, an insulating layer 4 disposed outside the inner semiconductive layer 3 and formed by crosslinking the above-mentioned insulating resin composition, and an outer semiconductive layer 5 disposed outside the insulating layer 4. The inner semiconductive layer 3 surrounds the conductor 2. The insulating layer 4 surrounds the inner semiconductive layer 3. The outer semiconductive layer 5 surrounds the insulating layer 4. Thus, in the power cable 1, the inner semiconductive layer 3, the insulating layer 4, and the outer semiconductive layer 5 are laminated in this order on the conductor 2 made of a metal such as copper or aluminum.

[0048] The inner semiconductive layer 3 and the outer semiconductive layer 5 contain, for example, an ethylene-based copolymer such as ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer, or ethylene-vinyl acetate copolymer, an olefin-based elastomer, and conductive carbon black.

[0049] Further, the power cable 1 may further include a metal shielding layer (not shown) disposed outside the outer semiconductive layer 5 and surrounding the outer semiconductive layer 5. Further, the power cable 1 may further include a sheath (not shown) disposed outside the metal shielding layer and surrounding the metal shielding layer.

[0050] The conductor 2 is continuously supplied to the resin extrusion port, where it is coated with the inner semiconductive layer 3, the insulating resin composition layer, and the outer semiconductive layer 5. These three layers may be coated simultaneously by extrusion or sequentially. When the conductor during coating is heated by heat transfer from the previously coated resin, the cooling rate of the resin near the conductor becomes slow. Therefore, it is preferable to adjust the temperature of the conductor to 1°C or more and 100°C or less by cooling and supply the conductor to the resin extrusion port.

[0051] The coating of the insulating resin composition is carried out by extruding it from the resin extrusion port of a resin extruder equipped with a metal mesh filter having an opening of 100 μm or less for the purpose of removing foreign matter, toward the conductor 2 (on the internal semiconductive layer 3). The temperature of the insulating resin composition during extrusion is preferably equal to or higher than the melting point of the insulating resin composition, specifically preferably 110°C or higher, and more preferably 120°C or higher. Also, for scorch suppression, the temperature of the insulating resin composition during extrusion is preferably 140°C or lower.

[0052] After the conductor 2 is coated with the insulating resin composition layer in this way, a crosslinking reaction of the insulating resin composition layer is carried out by pressure heating to form an insulating layer 4 obtained by crosslinking the insulating resin composition. Thus, the power cable 1 is obtained. Thereafter, the power cable 1 is cooled by a cooling pipe or a cooling water tank, and if necessary, a metal shielding layer or a sheath (not shown) is formed by a normal method.

[0053] When forming an exterior such as a metal shielding layer or a sheath on the cooled power cable 1, in order to prevent deformation of the insulating layer 4, the temperature of the insulating layer 4 is preferably 300°C or lower.

[0054] Regarding the thickness of the insulating layer 4, from the viewpoint of insulation characteristics, it is preferably 2 mm or more, more preferably 5 mm or more, and even more preferably 10 mm or more. From the viewpoint of laying workability, it is preferably 50 mm or less, and more preferably 40 mm or less.

[0055] Also, regarding the thickness of the internal semiconductive layer 3 and the external semiconductive layer 5, from the viewpoint of insulation characteristics, both are preferably 0.1 mm or more, more preferably 0.5 mm or more. From the viewpoint of conductive characteristics, both are preferably 5.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less.

[0056] According to the embodiments described above, even if the water tree inhibitor as component (c) is contained to suppress the growth of water trees, by containing component (b), an increase in the tangent of dielectric loss caused by the water tree inhibitor can be suppressed. And, by applying a crosslinked product of an insulating resin composition that can suppress the growth of water trees and reduce the tangent of dielectric loss to the insulating layer, the power transmission efficiency of the power cable can be improved.

[0057] As described above, the embodiments have been explained, but the present invention is not limited to the above embodiments, and includes all aspects included in the concept of the present disclosure and the scope of the claims, and can be variously modified within the scope of the present disclosure.

Examples

[0058] Next, examples and comparative examples will be described, but the present invention is not limited to these examples.

[0059] The raw materials used in the examples and comparative examples are as follows.

[0060] · Component (a): Polyethylene (CE1559; LDPE manufactured by Sumitomo Chemical Co., Ltd., MFR 0.8) · Resin (b1-1): Maleic anhydride graft resin (resin obtained by grafting 2.00 wt% of maleic anhydride onto CE1559) · Resin (b1-2): Maleic anhydride graft resin (resin obtained by grafting 0.25 wt% of maleic anhydride onto CE1559) · Resin (b1-3): Maleic anhydride graft resin (resin obtained by grafting 1.00 wt% of maleic anhydride onto CE1559) · Resin (b2-1): Ethylene copolymer (NUC-6520; ethylene-ethyl acrylate copolymer manufactured by Eneos NUC Co., Ltd., acrylate content 24 wt%, MFR 1.6) · Component (c-1): Water tree inhibitor (PEG-20000; polyethylene glycol manufactured by Adeka Corporation, number average molecular weight 20000) · Component (c-2): Water tree inhibitor (polypropylene glycol 4000; polypropylene glycol manufactured by Adeka Corporation, number average molecular weight 4000) · Component (c-3): Water tree inhibitor (polypropylene glycol 400; polypropylene glycol manufactured by Adeka Corporation, number average molecular weight 400) · Component (d): Crosslinking agent (Parkmyl D; dicumyl peroxide manufactured by NOF Corporation) · Antioxidant (Irganox 1010; hindered phenol-based antioxidant manufactured by BASF Corporation)

[0061] (Examples 1 to 8 and Comparative Examples 1 to 2) After dry-blending each raw material using a Henschel mixer, it was extruded with a single-screw extruder (L / D = 24, 120 °C) equipped with a plain-weave mesh with an opening size of 0.091 mm to obtain a pellet-shaped insulating resin composition (hereinafter also referred to as resin pellets) having the composition shown in Table 1.

[0062] [Measurement and Evaluation] For the insulating resin compositions obtained in the above Examples and Comparative Examples, the following measurements and evaluations were performed. The results are shown in Table 1.

[0063] [1] Moisture absorption rate Using the resin pellets, they were molded at 120 °C to form a sample with a length of 100 mm, a width of 150 mm, and a height of 1 mm. The sample was crosslinked at 160 °C, and a test piece obtained by dividing the crosslinked product into six parts was taken out after being stored in a thermo-hygrostat at 70 °C and a relative humidity of 90% for 4 hours, and the water content was measured by the Karl Fischer method of Method B (moisture vaporization method) of JIS K7251 to determine the moisture absorption rate.

[0064] [2] Resin fluidity Using the resin pellets, in accordance with JIS K7210, under the conditions of a melting temperature of 190 °C and a load of 2.16 kgf, the resin fluidity was measured by the amount of extruded resin (g) per 10 minutes.

[0065] [3] Length of water tree First, a small power cable was manufactured. First, using resin pellets, an insulating resin composition was extruded with a single-screw extruder (L / D = 24, 120 °C, full-flight screw) equipped with a plain weave mesh with an opening of 0.091 mm, and then extruded with another single-screw extruder (L / D = 24, 120 °C, full-flight screw) equipped with a plain weave mesh with an opening of 0.091 mm. It was coated on a copper conductor at a three-layer head (120 °C) together with a semiconductive resin (NUCV-9590, a semiconductive resin composition based on an ethylene-ethyl acrylate copolymer manufactured byENEOS NUC). The coated cable was passed through a pressurized cross-linking tube and then sent to a water tank for cooling. Thus, a small power cable was obtained that included a copper conductor with an outer diameter of about 2 mm, an inner semiconductive layer with a thickness of about 0.5 mm, an insulating layer with a thickness of about 2 mm, and an outer semiconductive layer with a thickness of about 0.5 mm.

[0066] Next, as shown in Fig. 2, the small power cable 1 was immersed in a 3.5 wt% NaCl aqueous solution, and a 4 kV, 1000 Hz AC voltage was applied between the conductor and the NaCl aqueous solution for 200 hours to conduct a water tree test. For 10 samples obtained by slicing the power cable after the test into rings with a thickness of 1 mm along the axial direction, they were observed with an optical microscope, and the length of the water tree was measured.

[0067] [4] Dielectric loss tangent Using resin pellets, it was molded at 120 °C for 10 minutes to obtain a film-like sample with a thickness of 0.3 mm. The sample was cross-linked at 160 °C for 30 minutes, and the cross-linked product was measured for dielectric loss tangent at 90 °C under the condition of 30 kV / mm according to JIS C 2138.

[0068]

Table 1

[0069] As shown in Table 1, in Examples 1 to 8, since the insulating resin composition contained component (a), component (b), component (c), and component (d), it was possible to achieve suppression of water tree growth and reduction of the dielectric loss tangent. On the other hand, in Comparative Examples 1 and 2, since the insulating resin composition did not contain at least one of components (a) to (d), it was not possible to achieve suppression of water tree growth and reduction of the dielectric loss tangent.

Explanation of Signs

[0070] 1 Power cable 2 Conductor 3 Inner semiconductive layer 4 Insulating layer 5 Outer semiconductive layer

Claims

1. comprising component (a), component (b), component (c) and component (d), wherein said component (a) is polyethylene, said component (b) is resin (b1), and said resin (b1) is a resin grafted with at least one modified monomer selected from unsaturated organic acids and their derivatives, said component (c) is a water tree inhibitor, which is at least one compound selected from polyalkylene glycols and their derivatives, polyglycerin, glycerin fatty acid esters, and sorbitol esters, said component (d) is a crosslinking agent, an insulating resin composition for power cables.

2. The insulating resin composition for power cables according to claim 1, wherein the crosslinked product obtained by crosslinking the insulating resin composition for power cables has a dielectric loss tangent of 1.0% or less at 90 °C and 30 kV / mm.

3. The insulating resin composition for power cables according to claim 1 or 2, wherein said resin (b1) has at least one modified monomer selected from unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides and unsaturated dicarboxylic acid derivatives added to at least one resin selected from polypropylene, polyethylene and olefin copolymers.

4. The insulating resin composition for power cables according to any one of claims 1 to 3, wherein the content ratio of said component (b) contained in the insulating resin composition for power cables is 3.0 wt% or more and 25.0 wt% or less.

5. a conductor, an internal semiconductive layer disposed outside the conductor and surrounding the conductor, an insulating layer disposed outside the internal semiconductive layer and surrounding the internal semiconductive layer, the insulating layer being formed by crosslinking the insulating resin composition for power cables according to any one of claims 1 to 4, and an external semiconductive layer disposed outside the insulating layer and surrounding the insulating layer A power cable comprising.

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