JP7687990B2 - Insulating resin composition for power cables and power cables - Google Patents

Description

This disclosure relates to an insulating resin composition for power cables and a power cable.

Cross-linked polyethylene (XLPE), which has been used in conventional power cables, can develop water trees when used as power cable insulation in a humid environment. Water trees are dendritic defects that arise from foreign objects or air bubbles (voids) in the resin.

Water trees grow when the water in the resin concentrates at the interfaces of foreign objects and voids due to dielectrophoresis of water caused by passing electricity through a power cable. Water trees can cause the destruction of power cables, so there is a need to suppress the growth of water trees.

To suppress the growth of water trees, hydrophilic molecules, such as polyethylene glycols, are added to the hydrophobic polyethylene in XLPE resin. Hydrophilic molecules such as polyethylene glycols disperse water evenly in the XLPE resin, preventing the concentration of water at foreign objects and void interfaces, suppressing the growth of water trees.

Patent Document 1 also discloses a polyolefin composition for electrical insulation that contains a polyolefin or crosslinked polyolefin and a small amount of high molecular weight polyethylene glycol. It also discloses a technology that shows that low hydrophilic or hydrophobic materials such as polypropylene glycol do not prevent the formation of water trees in the insulator.

In this way, the addition of hydrophilic molecules suppresses the growth of water trees. However, the addition of hydrophilic molecules increases the dielectric loss tangent (tan δ) of the resin, reducing the power transmission efficiency of the power cable.

U.S. Pat. No. 4,305,849

The objective of this disclosure is to provide an insulating resin composition for power cables and a power cable that can suppress the growth of water trees and reduce the dielectric loss tangent.

[1] An insulating resin composition for power cables, comprising components (a), (b), (c), and (d), wherein the component (a) is polyethylene, the component (b) is at least one resin selected from resins (b1) and (b2), the resin (b1) is a resin grafted with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof, the resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, the component (c) is a water tree inhibitor, and the component (d) is a crosslinking agent.
[2] The insulating resin composition for a power cable according to the above [1], wherein the component (c) is at least one compound selected from polyalkylene glycols and derivatives thereof, polyglycerin, glycerin fatty acid esters, and sorbitol esters.
[3] The insulating resin composition for a power cable according to the above [1] or [2], wherein a crosslinked product obtained by crosslinking the insulating resin composition for a power cable has a dielectric loss tangent at 30 kV/mm at 90°C of 1.0% or less.
[4] The insulating resin composition for a power cable according to any one of the above [1] to [3], wherein the resin (b1) is formed by adding at least one modified monomer selected from an unsaturated dicarboxylic acid, an unsaturated dicarboxylic anhydride, and an unsaturated dicarboxylic acid derivative to at least one resin selected from a polypropylene, a polyethylene, and an olefin-based copolymer.
[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) 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 on the outside of the conductor and surrounding the conductor; an insulating layer disposed on the outside of the internal semiconductive layer and surrounding the internal semiconductive layer, the insulating layer being formed by crosslinking the insulating resin composition for a power cable according to any one of [1] to [5] above; and an external semiconductive layer disposed on the outside of the insulating layer and surrounding the insulating layer.

According to the present disclosure, it is possible to provide an insulating resin composition for power cables and a power cable that can suppress the growth of water trees and reduce the dielectric loss tangent.

FIG. 1 is a cross-sectional view showing an example of a power cable to which the insulating resin composition for a power cable according to an embodiment of the present invention is applied. FIG. 2 is a schematic diagram showing a water tree test.

The following provides a detailed explanation based on the embodiment.

As a result of extensive research, the inventors have discovered that even if a water tree inhibitor is included to inhibit the growth of water trees, the inclusion of a specific component can suppress the increase in dielectric loss tangent caused by the water tree inhibitor, and have completed the present disclosure based on this knowledge.

The insulating resin composition for power cables (hereinafter, simply referred to as the insulating resin composition) of the embodiment includes components (a), (b), (c), and (d), where 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 derivatives thereof, 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.

The insulating resin composition of the embodiment includes components (a), (b), (c), and (d). The cross-linked product (hereinafter simply referred to as the cross-linked product) obtained by cross-linking the insulating resin composition is an insulator and is suitable for use in the insulating layer of a power cable.

The component (a) contained in the insulating resin composition is polyethylene. The polyethylene can be produced by either a low-pressure process or a high-pressure process, but low-density polyethylene (LDPE) produced by a high-pressure process is preferred.

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

Resin (b1) is a resin to which at least one modified monomer selected from unsaturated organic acids and their derivatives has been grafted (hereinafter, also referred to simply as a grafted resin).

The functional group introduced into the resin by grafting the modified monomer is preferably a functional group having a C=O bond. Among these, a carbonyl group, a carboxyl group, an ester group, an acid anhydride, an amide group, or an imide group is preferable. The water tree inhibitor, which is component (c), is restrained by hydrogen bonds with the C=O in the molecule grafted to the resin, thereby suppressing molecular motion due to an electric field. In this way, the C=O in the molecule grafted to the resin suppresses the motion of the water tree inhibitor, and therefore the dielectric loss tangent of the crosslinked product can be efficiently reduced. In addition, the grafted resin can improve the dispersibility of the water tree inhibitor and suppress bleed-out of the crosslinked product.

Preferred modified monomers are unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides, and unsaturated dicarboxylic acid derivatives. As the resin (b1) to which such modified monomers are grafted, it is preferable that 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. As the polyethylene, it is preferable that it is low-density polyethylene produced by a high-pressure process.

As the unsaturated dicarboxylic acid, maleic acid, fumaric acid and itaconic acid are preferable. As the unsaturated dicarboxylic acid 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 acid monoamide, maleimide, N-phenylmaleimide and N-cyclohexylmaleimide are preferable. These can be used alone or in combination of two or more. Among them, maleic anhydride, which is a five-membered ring cyclic acid anhydride, is preferable.

The graft amount of the modified monomer is preferably 0.25 wt% or more and 2.00 wt% or less in the grafted resin. When the modified monomer is grafted in 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.

When the modified monomer is maleic anhydride, the content of the modified monomer 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 in this range, the dielectric loss tangent in the system can be uniformly suppressed. In particular, when maleic anhydride is contained in an amount exceeding 0.50 wt%, the graft resin may adhere to the metal inner walls of the kneading device or extrusion device, reducing the uniformity of the resin composition. As a result, the growth of water trees may be promoted.

The graft resin can be prepared, for example, by the method described in paragraph [0098] of the specification of Japanese Patent No. 6205032, in which the raw materials polyethylene, antioxidant, modified monomer and organic peroxide are mixed in an extruder, heated and reacted.

Resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers.

The ethylene-acrylate copolymer is preferably an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, or an ethylene-butyl acrylate copolymer.

The lower limit of the content of component (b) in the insulating resin composition 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 of component (b) is within the above range, the dielectric loss tangent of the cross-linked product of the insulating resin composition can be sufficiently reduced.

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

Component (c) contained in the insulating resin composition is a water tree inhibitor. Component (c), the water tree inhibitor, is a hydrophilic molecule and can impart hygroscopicity to the crosslinked product of the insulating resin composition. By imparting hygroscopicity to the crosslinked product, moisture in the crosslinked product is uniformly dispersed, suppressing the concentration of moisture at the interfaces of foreign objects and voids, and suppressing the growth of water trees in the crosslinked product.

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.

The polyalkylene glycol is preferably polyethylene glycol, polypropylene glycol, a block copolymer of polyethylene glycol-polypropylene glycol, or an alkyl ether or carboxylate ester thereof. The purpose of these water tree inhibitors is to impart moisture absorption to the crosslinked product of the insulating resin composition by hydrogen bonding with water molecules, and the polyalkylene glycol derivatives exhibit the same effect as polyalkylene glycol. When polyalkylene glycol and its derivatives are used by being absorbed into resin pellets during dry blending, they are preferably liquid at 60°C. Polyglycerin is a compound in which multiple glycerins are polymerized by ether bonds, and can have a linear, cyclic, or branched structure. In addition, by alkylating or esterifying these polyglycerins into a carboxylate, the dispersibility in the insulating resin composition of the present disclosure is improved. In particular, it is preferable to introduce the polyglycerin into the insulating resin composition by fatty acid esterification in terms of the manufacturing stability of the compound. Sorbitol is a polyhydric alcohol derived from glucose, and a sorbitol ester is a compound in which one or more hydroxyl groups of sorbitol are esterified with a fatty acid. Sorbitol can be dispersed in the insulating resin composition by esterification.

In order to suppress bleed-out in the crosslinked 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. In addition, from the viewpoint of ease of handling, the number average molecular weight of the water tree inhibitor is preferably 20000 or less.

Regarding the content of the water tree inhibitor 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 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. Furthermore, when the content of the water tree inhibitor is 1.0 wt% or less, the bleeding out of the crosslinked product can be further suppressed.

Component (d) contained in the insulating resin composition is a crosslinking agent, which crosslinks the polyethylene of component (a). The crosslinking agent of component (d) is preferably di-t-hexyl peroxide (Perhexyl D manufactured by NOF Corp.), dicumyl peroxide (Percumyl D manufactured by NOF Corp.), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Perhexa 25B manufactured by NOF Corp.), α,α'-di(t-butylperoxy)diisopropylbenzene (Perbutyl P manufactured by NOF Corp.), t-butylcumyl peroxide (Perbutyl C manufactured by NOF Corp.), or di-t-butyl peroxide (Perbutyl D manufactured by NOF Corp.). These may be used alone or in combination of two or more. Of these, dicumyl peroxide is preferred.

The lower limit of the crosslinking agent content in the insulating resin composition 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 crosslinking agent content is within the above range, the insulating resin composition can be crosslinked well.

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 hindered phenol-based antioxidant, the main purpose of which is to capture radicals, in combination with a phosphorus-based antioxidant or a sulfur-based antioxidant, the main purpose of which is to decompose peroxides.

The lower limit of the content of the antioxidant in the insulating resin composition 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 of the antioxidant is within the above range, oxidation deterioration in the cross-linked product of the insulating resin composition can be effectively suppressed.

In addition to the above components, the insulating resin composition may contain various substances, so long as they do not inhibit the growth of water trees and the decrease in the dielectric loss tangent in the crosslinked product of the insulating resin composition. Examples of the various substances include stabilizers, lubricants, inorganic fillers, surface treatment agents, flame retardants, acid scavengers, and voltage stabilizers.

The insulating resin composition can be obtained by dry blending component (a) with components (b), (c), and (d) using a Henschel mixer or the like, followed by melt kneading. As a melt kneading device, a single-screw or twin-screw extruder, a Banbury, a kneader, or other kneading machine can be used. In particular, it is preferable to attach a metal mesh filter with a mesh size of 100 μm or less to a single-screw or twin-screw extruder capable of continuous processing in order to remove foreign matter, and perform resin extrusion. After adding component (d), the insulating resin composition is preferably kneaded at 120°C or higher and 135°C or lower.

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

The dielectric loss tangent at 30 kV/mm at 90°C of the crosslinked product obtained by crosslinking the insulating resin composition 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, the dielectric loss tangent of a power cable in which the crosslinked product of the insulating resin composition is applied to the insulating layer is small, and therefore the power transmission efficiency can be improved.

The dielectric loss tangent of the cross-linked product is a value measured at 90°C and 30 kV/mm in accordance with JIS C 2138. Note that the dielectric loss tangent is dependent on temperature and applied electric field, and when the cross-linked product of the insulating resin composition is measured at 23°C and 10 kV/mm, the dielectric loss tangent of the cross-linked product is 0.1% or less.

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 moisture absorption 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. In addition, 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.

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, more preferably 0.5 g/10 min or more and 3.0 g/10 min or less. If 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 for a long period of time.

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 the insulating layer, the growth of water trees in the insulating layer can be suppressed and the dielectric loss tangent can be reduced, so that the power cable can transmit electricity efficiently even when used at high voltage or ultra-high voltage. Such a power cable is preferably used as an underground cable or a submarine cable. When the power cable is an AC power cable, the effects of suppressing the growth of water trees and reducing the dielectric loss tangent are even more effectively exhibited.

Figure 1 is a cross-sectional view showing an example of a power cable to which an insulating resin composition according to an embodiment of the present invention is applied.

As shown in FIG. 1, the power cable 1 includes a conductor 2, an internal semiconductive layer 3 disposed on the outside of the conductor 2, an insulating layer 4 disposed on the outside of the internal semiconductive layer 3 and formed by cross-linking the insulating resin composition described above, and an external semiconductive layer 5 disposed on the outside of the insulating layer 4. The internal semiconductive layer 3 surrounds the conductor 2. The insulating layer 4 surrounds the internal semiconductive layer 3. The external semiconductive layer 5 surrounds the insulating layer 4. Thus, in the power cable 1, the internal semiconductive layer 3, the insulating layer 4, and the external semiconductive layer 5 are laminated in this order on the conductor 2 made of a metal such as copper or aluminum.

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

The power cable 1 may further include a metal shielding layer (not shown) that is disposed outside the outer semiconductive layer 5 and surrounds the outer semiconductive layer 5. The power cable 1 may further include a sheath (not shown) that is disposed outside the metal shielding layer and surrounds the metal shielding layer.

The conductor 2 is continuously fed to the resin extrusion port, where it is coated with the internal semiconductive layer 3, the insulating resin composition layer, and the external semiconductive layer 5. These three layers may be extruded and coated simultaneously, or sequentially. If the conductor is heated by heat transfer from the previously coated resin during coating, the cooling rate of the resin near the conductor will be slow, so it is preferable to adjust the temperature of the conductor by cooling to between 1°C and 100°C before feeding the conductor to the resin extrusion port.

The insulating resin composition is coated by extruding it onto the conductor 2 (onto the internal semiconductive layer 3) from the resin extrusion port of a resin extruder equipped with a metal mesh filter with an opening of 100 μm or less for the purpose of removing foreign matter. The temperature of the insulating resin composition during extrusion is preferably equal to or higher than the melting point of the insulating resin composition, more preferably 110° C. or higher, and more preferably 120° C. or higher. In order to suppress scorch, the temperature of the insulating resin composition during extrusion is preferably 140° C. or lower.

After the conductor 2 is thus coated with the insulating resin composition layer, a crosslinking reaction of the insulating resin composition layer is caused by applying pressure and heat, and the insulating resin composition is crosslinked to form the insulating layer 4. In this way, the power cable 1 is obtained. The power cable 1 is then cooled in a cooling tube or a cooling water tank, and a metal shielding layer or sheath (not shown) is formed by a normal method, if necessary.

When forming an exterior covering such as a metal shielding layer or sheath on the cooled power cable 1, it is preferable that the temperature of the insulating layer 4 be 300°C or less to prevent deformation of the insulating layer 4.

From the viewpoint of insulating properties, the thickness of the insulating layer 4 is preferably 2 mm or more, more preferably 5 mm or more, and even more preferably 10 mm or more, and from the viewpoint of installation workability, it is preferably 50 mm or less, and even more preferably 40 mm or less.

In addition, from the viewpoint of insulating properties, the thickness of the inner semiconductive layer 3 and the thickness of the outer semiconductive layer 5 are both preferably 0.1 mm or more, and more preferably 0.5 mm or more, and from the viewpoint of conductive properties, they are both preferably 5.0 mm or less, and more preferably 3.0 mm or less, and even more preferably 2.0 mm or less.

According to the embodiment described above, even if the component (c) is a water tree inhibitor to suppress the growth of water trees, the inclusion of component (b) can suppress the increase in dielectric loss tangent caused by the water tree inhibitor. Furthermore, by applying a cross-linked product of an insulating resin composition that can suppress the growth of water trees and reduce the dielectric loss tangent to an insulating layer, the power transmission efficiency of the power cable can be improved.

Although the embodiments have been described above, the present invention is not limited to the above embodiments, but includes all aspects included in the concept and claims of this disclosure, and can be modified in various ways within the scope of this disclosure.

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

The raw materials used in the examples and comparative examples are as follows:

Component (a): Polyethylene (CE1559; LDPE manufactured by Sumitomo Chemical Co., Ltd., MFR 0.8)
Resin (b1-1): Maleic anhydride grafted resin (resin obtained by grafting 2.00 wt% of maleic anhydride onto CE1559)
Resin (b1-2): Maleic anhydride grafted resin (resin obtained by grafting 0.25 wt% of maleic anhydride onto CE1559)
Resin (b1-3): Maleic anhydride grafted resin (resin obtained by grafting 1.00 wt% of maleic anhydride onto CE1559)
Resin (b2-1): Ethylene-based copolymer (NUC-6520; Ethylene ethyl acrylate copolymer manufactured by Eneos NUC, 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 20,000)
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 (Percumyl D; dicumyl peroxide manufactured by NOF Corporation)
Antioxidant (Irganox 1010; a hindered phenol-based antioxidant manufactured by BASF)

(Examples 1 to 8 and Comparative Examples 1 and 2)
The raw materials were dry-blended using a Henschel mixer, and then extruded using a single-screw extruder (L/D=24, 120° C.) equipped with a plain-woven mesh having an opening of 0.091 mm to obtain pellet-shaped insulating resin compositions (hereinafter also referred to as resin pellets) having the composition shown in Table 1.

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

[1] Moisture absorption rate Resin pellets were used to mold a sample having a length of 100 mm, a width of 150 mm, and a height of 1 mm at 120°C. The sample was crosslinked at 160°C. The crosslinked product was divided into 6 test pieces, which were then stored in a thermo-hygrostat at 70°C and a relative humidity of 90% for 4 hours and then taken out. The moisture content was measured by the Karl Fischer method, Method B (moisture evaporation method) of JIS K7251, to determine the moisture absorption rate.

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

[3] Length of water tree First, a small power cable was manufactured. First, an insulating resin composition was extruded using resin pellets 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 a copper conductor was coated with 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 by Eneos NUC Co., Ltd.) 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. The coated cable was passed through a pressurized crosslinking tube and then sent to a water tank for cooling. In this way, a small power cable was obtained that had 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.

Next, the small power cable 1 was immersed in a 3.5 wt% NaCl aqueous solution as shown in Figure 2, and a 1000 Hz AC voltage of 4 kV was applied between the conductor and the NaCl aqueous solution for 200 hours to perform a water tree test. After the test, the power cable was sliced axially into 1 mm thick slices to obtain 10 samples, which were then observed under an optical microscope to measure the length of the water trees.

[4] Dielectric loss tangent The resin pellets were molded into a film-like sample having a thickness of 0.3 mm at 120°C for 10 minutes, and the sample was crosslinked at 160°C for 30 minutes. The dielectric loss tangent of the crosslinked product was measured in accordance with JIS C 2138 at 90°C and 30 kV/mm.

As shown in Table 1, in Examples 1 to 8, the insulating resin composition contained components (a), (b), (c), and (d), and thus suppression of water tree growth and reduction of dielectric loss tangent were achieved. On the other hand, in Comparative Examples 1 and 2, the insulating resin composition did not contain at least one of components (a) to (d), and thus suppression of water tree growth and reduction of dielectric loss tangent were not achieved.

Reference Signs List 1 Power cable 2 Conductor 3 Inner semiconductive layer 4 Insulating layer 5 Outer semiconductive layer

Claims (5)

Contains components (a), (b), (c) and (d),
The component (a) is polyethylene,
The component (b) is a resin (b1 ) , and the resin (b1) is a resin grafted with at least one modifying monomer selected from an unsaturated organic acid and a derivative thereof,
The component (c) is a water tree inhibitor and is at least one compound selected from polyalkylene glycols and derivatives thereof, polyglycerin, glycerin fatty acid esters, and sorbitol esters;
The component (d) is a crosslinking agent.
An insulating resin composition for power cables. 2. The insulating resin composition for a power cable according to claim 1 , wherein a crosslinked product obtained by crosslinking the insulating resin composition for a power cable has a dielectric loss tangent at 30 kV/mm at 90° C. of 1.0% or less. 3. The insulating resin composition for a power cable according to claim 1, wherein the resin (b1) is formed by adding at least one modified monomer selected from an unsaturated dicarboxylic acid, an unsaturated dicarboxylic anhydride, and an unsaturated dicarboxylic acid derivative to at least one resin selected from a polypropylene, a polyethylene, and an olefin-based copolymer. 4. The insulating resin composition for a power cable according to claim 1 , wherein a 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. A conductor;
an inner semiconducting layer disposed outside the conductor and surrounding the conductor;
an insulating layer that is disposed outside the internal semiconductive layer and surrounds the internal semiconductive layer, the insulating layer being formed by crosslinking the insulating resin composition for a power cable according to any one of claims 1 to 4 ;
an outer semiconducting layer disposed outside the insulating layer and surrounding the insulating layer.

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