Power cable and method for manufacturing power cable

Abstract

[Question] To reduce uneven wall thickness of the insulation layer or the occurrence of voids or delamination due to shrinkage, and to provide a power cable with excellent dielectric breakdown strength and a method for manufacturing the power cable. [Solution] According to the power cable 1 of the present invention, the insulation layer 13 contains a propylene resin within a specific range, and the relationship between the cooling rate X at the interface portion of the insulation layer 13 with the inner semiconductive layer 12 during manufacturing and the cooling rate Y at the center portion of the insulation layer 13 during manufacturing is set to a specific relationship. This allows not only the surface of the insulation layer 13 but also the interior of the insulation layer 13, the interface portion of the insulation layer 13 with the inner semiconductive layer 12, and the interior of the insulation layer 13 to be reliably cooled and hardened. As a result, the metal conductor 11 is prevented from deviating from the center of the power cable 1 due to its own weight, wall thickness unevenness is less likely to occur, and deformation caused by cooling shrinkage is dispersed, thereby suppressing the occurrence of voids due to shrinkage, resulting in a power cable 1 with excellent dielectric breakdown strength.

Description

Technical Field

[0001] The present invention relates to a power cable and a method for manufacturing the power cable. More specifically, the present invention relates to a power cable having a structure in which an inner semiconducting layer, an insulating layer, and an outer semiconducting layer are sequentially laminated on the outer periphery of a metal conductor by extrusion molding, and a method for manufacturing the power cable. Background Art

[0002] Generally speaking, power cables are made by extruding a metal conductor and sequentially coating it with an inner semiconducting layer, an insulating layer, and an outer semiconducting layer. Cross-linked polyethylene is commonly used as the insulating layer of such power cables.

[0003] However, cross-linked polyethylene has a melting point of about 110°C (T m ), so when conducting a large current, the insulation layer may be deformed and electrically broken down (insulation breakdown) due to heat. To prevent such electrical breakdown, it is necessary to control the amount of current flowing below a certain value or to design the cross-sectional area of the metal conductor of the power cable to be large.

[0004] To address these issues, research has been conducted on resin compositions that constitute insulating layers. For example, resin compositions comprising propylene homopolymers, copolymers of ethylene and propylene, or copolymers of α-olefins other than propylene and propylene, and dielectric liquids have been studied as insulating resin compositions capable of achieving an operating temperature higher than the operating temperature of polyethylene-based insulation (70°C), exceeding 90°C (see Patent Document 1).

[0005] In addition, as another example, regarding a method for producing a polymer composition consisting of a controlled-cooling-heating mixing system of polypropylene and a propylene-α-olefin copolymer, a method for producing a composition having high dielectric strength by combining uncontrolled cooling and controlled cooling of an insulator has been studied (see Patent Document 2, etc.).

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Patent No. 4875282

[0009] Patent Document 2: Patent No. 6189927 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] Herein, the insulating layer using a resin composition containing a propylene resin is formed by melting the resin composition, extruding it onto a metal conductor, and cooling the extruded resin composition to below the crystallization temperature by air cooling or water cooling (liquid cooling).

[0012] On the other hand, because the insulation layer of a high-voltage power cable is tens of millimeters thick, the temperature difference between the insulation layer surface (the surface facing the outer semiconductive layer) and the interior of the insulation layer can reach tens of degrees Celsius. Consequently, when each layer is cooled and solidified through external cooling from the outside of the cable, even if the insulation layer surface solidifies, the interior of the insulation layer has not yet solidified and remains soft. Consequently, the metal conductor deviates from the center of the cable due to its own weight, resulting in a defect known as uneven wall thickness.

[0013] When the metal conductor deviates from the center of the cable, resulting in uneven wall thickness, the insulation layer becomes thinner, reducing dielectric breakdown strength. Furthermore, the occurrence of voids or delamination caused by shrinkage (shrinkage) on the cable surface during cooling also contributes to reduced dielectric breakdown strength. These occur near the last solidified conductor or at the interface between different materials. However, the above-mentioned technologies do not fully address these issues.

[0014] In view of the above problems, an object of the present invention is to provide a power cable and a method for manufacturing a power cable having an excellent dielectric breakdown strength and with reduced occurrence of voids or peeling caused by uneven insulation thickness or shrinkage.

[0015] Means of solving problems

[0016] To solve the above problems, the power cable of the present invention is a power cable having a structure in which an inner semiconductive layer, an insulating layer, and an outer semiconductive layer are sequentially stacked on the outer periphery of a metal conductor, and is characterized in that:

[0017] The insulating layer contains 15% by mass or more of a propylene resin having a melting point of 110° C. or more relative to the entire layer.

[0018] The relationship between the cooling rate X [°C / min] during the production of the interface portion of the insulating layer with the inner semiconductive layer and the cooling rate Y [°C / min] during the production of the central portion of the insulating layer is expressed by the following formula (Z):

[0019] X≥Y×0.8……(Z).

[0020] The power cable of the present invention is characterized in that, in the above invention, the propylene-based resin is obtained by multi-stage gas phase polymerization.

[0021] The power cable of the present invention is characterized in that, in the above invention, the propylene-based resin is an olefin-based thermoplastic elastomer.

[0022] The power cable of the present invention is characterized in that, in the above invention, the insulating layer contains a thermoplastic resin having a melting point lower than 110° C. in an amount greater than 0% by mass and not more than 75% by mass relative to the entire insulating layer.

[0023] The power cable of the present invention is characterized in that, in the aforementioned invention, the aforementioned cooling rate X and the aforementioned cooling rate Y are cooling rates confirmed based on the relationship between the aforementioned cooling rate and the aforementioned shoulder temperature, obtained for a target sample collected from a corresponding portion, based on the shoulder temperature [°C] measured on the low temperature side of the melting peak observed during the heating process using DSC (differential scanning calorimetry) and the cooling rate [°C / minute] during cooling after the heating process.

[0024] The power cable of the present invention is characterized in that, in the above invention, the cooling rate Y is 0.1° C. / min to 30° C. / min.

[0025] The power cable of the present invention is characterized in that, in the aforementioned invention, the thickness of the insulating layer is 5 mm or more.

[0026] The method for manufacturing a power cable of the present invention is characterized in that:

[0027] The insulating layer contains 15% by mass or more of a propylene resin having a melting point of 110° C. or more relative to the entire layer, and is formed in a manner represented by the following formula (Z):

[0028] The relationship between the cooling rate X [°C / min] during the production of the interface portion of the insulating layer with the inner semiconductive layer and the cooling rate Y [°C / min] during the production of the central portion of the insulating layer is expressed by the following formula (Z):

[0029] X≥Y×0.8……(Z).

[0030] The power cable manufacturing method of the present invention is characterized in that, in the aforementioned invention, the aforementioned cooling rate X and the aforementioned cooling rate Y are determined using DSC (differential scanning calorimetry), based on the shoulder temperature [°C] measured on the low temperature side of the melting peak observed during the heating process and the cooling rate [°C / minute] during cooling after the aforementioned heating process, and the relationship between the aforementioned cooling rate and the aforementioned shoulder temperature is obtained for the target sample collected from the corresponding part, and the cooling rate is confirmed based on this relationship.

[0031] Effects of the Invention

[0032] The power cable of the present invention contains a propylene-based resin within a specific range as the insulation layer, and maintains a specific relationship between the cooling rate X at the interface with the inner semiconductive layer and the cooling rate Y at the center of the insulation layer during manufacturing. This ensures that not only the surface of the insulation layer but also the interior of the insulation layer, and the interface with the inner semiconductive layer and the interior of the insulation layer, are reliably cooled and hardened. Consequently, the metal conductor is prevented from deviating from the center of the power cable due to its own weight, and uneven wall thickness is less likely to occur. Furthermore, deformation caused by cooling shrinkage is dispersed, thereby suppressing the formation of voids caused by shrinkage, resulting in a power cable with excellent dielectric breakdown strength.

[0033] Furthermore, according to the method for manufacturing a power cable of the present invention, a power cable achieving the aforementioned effects can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A cross-sectional view showing an example of a power cable according to the present invention.

[0035] Figure 2 Explanatory diagram showing the interface portion between the insulating layer and the internal semiconductive layer and the central portion of the insulating layer.

[0036] Figure 3 A diagram showing an example of measurement of shoulder temperature.

[0037] Figure 4 A diagram showing an example of the relationship between the cooling rate and the shoulder temperature.

[0038] Explanation of symbols

[0039] 1...Power cable

[0040] 11...Metal conductor

[0041] 12...Inner semi-conductive layer

[0042] 13...Insulation layer

[0043] 14...External semiconductive layer

[0044] 21...Interface

[0045] 22...Central Line

[0046] a1, a2...arrow

[0047] L1, L2... tangent DETAILED DESCRIPTION

[0048] (1) Structure of power cable 1:

[0049] An embodiment of the present invention is described below. Figure 1A cross-sectional view showing an example of a power cable 1 according to the present invention. Figure 1 In the figure, 1 represents a power cable, 11 represents a metal conductor, 12 represents an inner semiconductive layer, 13 represents an insulating layer, and 14 represents an outer semiconductive layer. Furthermore, although metal conductor 11 is described as a single wire having a circular cross-section, this is merely an example and also encompasses metal conductors such as those formed from stranded wires, as described later.

[0050] like Figure 1 As shown, the power cable 1 according to the present invention has a structure in which an inner semiconductive layer 12, an insulating layer 13, and an outer semiconductive layer 14 are sequentially stacked on the outer periphery of a metal conductor 11. Each layer will be described below.

[0051] (Metal conductor 11)

[0052] The metal conductor 11 can be made of a metal material such as copper, a copper alloy, aluminum, or an aluminum alloy. The metal conductor 11 can have a circular or rectangular cross-sectional shape. Generally, copper or aluminum is preferably used as the material for the metal conductor 11, and a circular shape is preferred.

[0053] Note that a metal conductor 11 made of the aforementioned metal material may be plated with tin, silver, or the like as the metal conductor 11. The metal conductor 11 may be a single wire or a twisted wire.

[0054] There is no particular limitation on the cross-sectional area or shape of the metal conductor 11, which can be appropriately determined according to the voltage level of the power cable 1 or the laying conditions, but the cross-sectional area of the metal conductor 11 is preferably 60 mm 2 -5000mm 2 , particularly preferably 100 mm 2 -3500mm 2 In the case of a single wire having a circular cross section, the outer diameter is preferably 10 mm to 40 mm.

[0055] Regarding the structure of the metal conductor 11, in the case of a stranded wire, it is preferable to have the structure and shape used in a conventional power cable 1. As for the number of strands / single wire diameter (number of strands / single wire diameter), for example, a structure of 7 strands / 0.6 mm, 7 strands / 0.8 mm, 7 strands / 1.0 mm, 7 strands / 1.2 mm, 19 strands / 2.6 mm, etc. can be used. Alternatively, the number of strands / single wire diameter of the structure described in JIS C3105 hard copper stranded wire or JIS C3102 soft copper stranded wire can be used.

[0056] (Insulation layer 13)

[0057] The resin constituting the insulating layer 13 in the present invention includes a propylene resin having a melting point of 110°C or higher (hereinafter sometimes referred to as "propylene resin"). The propylene resin refers to a resin or resin composition containing a propylene component, and is not particularly limited. For example, polypropylene includes homopolypropylene, random polypropylene, block polypropylene, and atactic polypropylene. Furthermore, the propylene resin includes propylene polymers (such as the aforementioned polypropylene, or a random copolymer of propylene and, for example, 5% by mass or less of an α-olefin other than propylene).

[0058] Examples of random polypropylene include resins formed from random copolymers of α-olefins (preferably olefins having 2 to 4 carbon atoms) and propylene, and examples of block polypropylene include compositions containing homopolypropylene and ethylene-α-olefin (e.g., ethylene-propylene) copolymers. Propylene-based resins include propylene polymers (e.g., propylene-α-olefin copolymers, maleic anhydride-modified polypropylene, etc.).

[0059] Furthermore, in the present invention, the propylene-based resin having a melting point of 110°C or higher is preferably a resin obtained by multi-stage gas-phase polymerization, generally known as an olefin-based thermoplastic elastomer. Resins obtained by multi-stage gas-phase polymerization are known to contain polypropylene and have improved impact strength (particularly low-temperature impact strength) while maintaining rigidity and heat resistance, making them suitable for use as a constituent material for the insulating layer 13 of the power cable 1.

[0060] Propylene resins obtained through multi-stage gas-phase polymerization are generally referred to as heterophasic copolymers, impact copolymers, reactor-produced alloys, reactor-produced thermoplastic elastomers, and reactor TPOs (olefin-based thermoplastic elastomers). Reactor TPOs (olefin-based thermoplastic elastomers) are common.

[0061] Such a resin is a resin composition comprising a crystalline propylene polymer (polypropylene, or a crystalline random copolymer of propylene and, for example, 5% by mass or less of an α-olefin other than propylene) and an ethylene-α-olefin copolymer rubber (or a propylene-α-olefin copolymer rubber. Hereinafter, an ethylene-α-olefin copolymer is sometimes used as an example.), and is generally a resin having a high content of ethylene-α-olefin copolymer rubber.

[0062] From the viewpoint of heat resistance and flexibility, for the containing ratio of propylene polymer and ethylene-α-olefin copolymer rubber, preferably propylene polymer is 20 mass %-70 mass %, and ethylene-α-olefin copolymer rubber is 30 mass %-80 mass %.As the ethylene-α-olefin copolymer rubber contained in the propylene resin (reactor TPO, olefin-based thermoplastic elastomer) obtained by multi-stage gas phase polymerization, ethylene-propylene copolymer rubber, ethylene-butene copolymer rubber, ethylene-hexene copolymer rubber, ethylene-octene copolymer rubber etc. can be listed.As propylene-α-olefin copolymer rubber, in addition to the aforementioned ethylene-propylene copolymer rubber, propylene-butene copolymer rubber, propylene-hexene copolymer rubber, propylene-octene copolymer rubber etc. can also be.

[0063] Typically, the production of heterophasic copolymers by multi-stage gas-phase polymerization requires at least two reactors. For example, the polymerization process includes the following steps: a first-stage process for producing a propylene polymer (polypropylene) alone or a crystalline random copolymer of propylene and an α-olefin other than propylene in an amount of 5% by mass or less (hereinafter referred to as a propylene polymer); and second and subsequent stages for producing a random copolymer elastomer (copolymer rubber, rubber-like copolymer, etc.) with an ethylene-α-olefin (or propylene-α-olefin copolymer) such as ethylene or butene. The highly crystalline propylene polymer is produced in the first stage, followed by a second stage for producing a polymer rubber with an ethylene-α-olefin (or propylene-α-olefin copolymer).

[0064] As the reaction catalyst used when performing multi-stage gas phase polymerization, there can be mentioned conventionally known Ziegler-Natta catalysts or metallocene catalysts, etc. In addition, the monomers used when performing multi-stage gas phase polymerization include monomers of α-olefins such as propylene, ethylene, and butene.

[0065] Resins produced through multi-stage gas-phase polymerization are prepared by blending the finely powdered resin components produced in each stage in the polymerization reactor. By melt-kneading these components, a resin having a microscopic phase-separated structure of finer islands of the elastomer component (copolymer rubber component) and a sea of propylene polymer components, such as polypropylene, can be obtained, compared to conventional methods of melt-kneading micropellets of polypropylene and elastomer. The average particle size of the elastomer component used in this invention is preferably 5 μm or less, particularly preferably 1 μm or less.

[0066] Commercially available propylene resins obtained through such multi-stage gas-phase polymerization are a series of resins known as catalyst alloy process resins, including, for example, Adflex, Hifax, Softell, and Adsyl (all registered trademarks) (all manufactured by Liondel Buser Co., Ltd.). Also applicable are Tafmar PN (registered trademark) (manufactured by Mitsui Chemicals, Inc.), Tafselene (registered trademark) (manufactured by Sumitomo Chemical Co., Ltd.), Niucon (registered trademark) (manufactured by Nippon Polypro Co., Ltd.), and Planum TPO (registered trademark) (manufactured by Planum Polypro Co., Ltd.).

[0067] Furthermore, the propylene resins having a melting point of 110°C or higher also include resin compositions finely dispersed in the following manner: a propylene polymer (the aforementioned polypropylene, or a random copolymer of propylene and α-olefins other than propylene in an amount of less than 5% by mass, etc.) is mixed with at least one selected from ethylene-α-olefin copolymers and propylene-α-olefin copolymers, for example, melt-kneaded until islands having a diameter of less than 5 μm are formed, and an island component such as an ethylene-α-olefin copolymer is formed in the sea component of the propylene polymer.

[0068] The propylene resin used in the insulating layer 13 preferably has appropriate melt fluidity, which can be expressed as the melt flow rate (MFR) (230°C / 2.16 kgf) specified in ISO 1133 (JIS K7210). From the perspective of productivity during extrusion molding, the MFR is preferably 0.1 g / 10 min or higher, more preferably 0.5 g / 10 min or higher. Furthermore, from the perspective of avoiding uneven wall thickness, the MFR is preferably 10 g / 10 min or lower, more preferably 5 g / 10 min or lower.

[0069] The propylene resin having a melting point of 110°C or higher, which constitutes insulating layer 13, is present in an amount of 15% by mass or greater relative to the total weight of insulating layer 13. This 15% by mass or greater propylene resin content in insulating layer 13 allows for a resin that can withstand heat generation caused by high currents. The propylene resin preferably comprises 15% to 100% by mass, and particularly preferably 20% to 100% by mass, relative to the total weight of insulating layer 13.

[0070] It should be noted that when a thermoplastic resin having a melting point lower than 110° C. or other resins described later is added to the insulating layer 13 , the content of the propylene resin is preferably 20% to 99% by mass, particularly preferably 25% to 99% by mass, relative to the entire insulating layer 13 .

[0071] In terms of insulation properties, the thickness of the insulating layer 13 is preferably 2 mm or more, more preferably 5 mm or more, and particularly preferably 10 mm or more. In terms of installation workability, it is preferably 50 mm or less, and particularly preferably 40 mm or less.

[0072] As the resin constituting the insulating layer 13 , other than the aforementioned propylene resin, a known thermoplastic resin used for the insulating layer 13 of the power cable 1 , such as a polyolefin resin such as ethylene-α-olefin copolymer or polyethylene, can be used.

[0073] In order to achieve lower elasticity and improve resistance to low-temperature embrittlement of the insulating layer 13, a thermoplastic resin having a melting point below 110°C can be added as an optional component. Such thermoplastic resins include, for example, styrene plastic elastomers, ethylene-α-olefin copolymers (e.g., ethylene-butene copolymers) or propylene-α-olefin copolymers produced by methods other than multi-stage gas phase polymerization. By adding these resins, lower elasticity and resistance to low-temperature embrittlement are improved.

[0074] As styrene-based thermoplastic elastomers, hydrogenated (referring to hydrogenation, the same applies hereinafter) styrene-based thermoplastic elastomers are also included, for example, styrene-butadiene block copolymer (SBR), styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS) or (hydrogenated) styrene-butadiene rubber (HSBR) obtained by hydrogenating a butadiene polymer block and an isoprene polymer block, (hydrogenated) styrene-ethylene-butylene-styrene block copolymer (SEBS), (hydrogenated) styrene-ethylene-propylene-styrene block copolymer (SEPS), (hydrogenated) styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS), styrene-ethylene-butylene-olefin crystalline block copolymer (SEBC), styrene-butadiene block copolymer (SBR) and other styrene-based thermoplastic elastomers can be used alone or in combination of two or more.

[0075] When added, these resins are preferably contained in a range of greater than 0 mass % and 80 mass % or less, and particularly preferably greater than 0 mass % and 75 mass % or less, relative to the entire insulating layer 13 .

[0076] As an optional component for achieving low elasticity, the insulating layer 13 may contain alkylated aromatics such as polyisobutylene and linear alkylbenzene, or silicone oil in an amount ranging from 0% to 10% by mass relative to the entire insulating layer 13 . As optional components used to achieve improved heat resistance, organic peroxides such as di-tert-hexyl peroxide (Parhexyl D manufactured by NOF Corporation), diisopropylphenyl peroxide (Parkmil D manufactured by NOF Corporation), 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (Parhexyl 25B manufactured by NOF Corporation), α,α'-di(tert-butylperoxy)diisopropylbenzene (Parbcil P manufactured by NOF Corporation), tert-butylisopropylphenyl peroxide (Parbcil C manufactured by NOF Corporation), and di-tert-butyl peroxide (Parbcil D manufactured by NOF Corporation) can be added in a range of 0% by mass to 5% by mass relative to the entire insulating layer 13.

[0077] (Inner semiconducting layer 12 and outer semiconducting layer 14)

[0078] The materials constituting the inner semiconductive layer 12 and the outer semiconductive layer 14 (hereinafter sometimes referred to as "the inner semiconductive layer 12, etc.") are not particularly limited. Materials used for the inner semiconductive layer 12, etc. in conventional power cables 1 can be used. Examples include resin compositions comprising a thermoplastic resin such as a polyolefin resin as a matrix material and an added conductive material. Alternatively, the resin composition may be crosslinked. The amount of each component in the resin composition can be appropriately determined depending on the selected components and the intended purpose.

[0079] The thermoplastic resin constituting the inner semiconductive layer 12 and the like is not particularly limited. In the present invention, examples thereof include the aforementioned materials for the insulating layer 13, such as propylene resins, vinyl polymers such as ethylene-vinyl acetate copolymer (EVA) and ethylene-ethyl acrylate (ethyl acrylate) copolymer (EEA), which have good dispersion properties for conductive carbon, and other thermoplastic resins such as polyolefin resins. These resins may be used alone or in combination of two or more. Of these thermoplastic resins, propylene resins are preferred.

[0080] As the conductive material, any conventionally known material can be used without particular limitation, but conductive carbon is generally preferred. Examples of conductive carbon include carbon black, acetylene black, furnace black, Ketjen black, thermal black, and graphite. These materials may be used alone or in combination. The conductive material is preferably incorporated into the inner semiconductive layer 12 and the like in an amount ranging from 20 to 100 parts by mass relative to 100 parts by mass of the thermoplastic resin.

[0081] The thickness of the inner semiconductive layer 12 and the outer semiconductive layer 14 can be appropriately determined based on the voltage level and installation conditions of the power cable 1 and is not particularly limited. However, from the perspective of insulation properties, the thickness is preferably 0.1 mm or greater, more preferably 0.5 mm or greater. Furthermore, from the perspective of electrical conductivity, the thickness is preferably 5 mm or less, more preferably 3 mm or less, and even more preferably 2 mm or less.

[0082] It should be noted that, as the materials constituting the inner semiconductive layer 12, the insulating layer 13, and the outer semiconductive layer 14, inorganic fillers such as magnesium oxide, aluminum oxide, zirconium oxide, titanium oxide, magnesium hydroxide, aluminum hydroxide, silicon carbide, and mica, plasticizers such as fatty acids such as stearic acid and oleic acid or their metal salts, softeners such as mineral oils, waxes, and paraffins, antioxidants such as esters, amides, and 4,4'-thiobis(3-methyl-6-tert-butylphenol), hindered phenol-based antioxidants, thioether-based antioxidants, colorants, antioxidants, ultraviolet absorbers, lubricants, stabilizers, cross-linking agents, and other previously known additives added to the materials constituting the power cable 1 may be added as needed, within the scope that does not hinder the achievement of the purpose of the present invention.

[0083] It should be noted that although Figure 1 Although not shown, it is common to form a shield layer and a sheath (outer cover) as described below around the cable core of the power cable 1 having the above-mentioned structure.

[0084] (Shielding layer)

[0085] The shielding layer may be made of a metal strip or extrusion. For example, copper or aluminum strips, or aluminum, lead, or stainless steel (SUS) may be extruded around the outer periphery of the cable core. Copper strips are preferably used for the shielding layer. It should be noted that a strip made of rubber-coated fabric or the like may also be wound around the inside or outside of the shielding layer.

[0086] The thickness of the water-blocking layer varies according to the voltage level or installation conditions of the power cable 1 and is not particularly limited. For example, if the water-blocking layer is in a strip shape, the thickness of the water-blocking layer is preferably less than 1 mm, more preferably 0.05 mm to 0.5 mm.

[0087] (Sheath (outer skin))

[0088] The main purpose of the sheath is to protect the cable core including the insulation layer and to isolate moisture. In the present invention, in addition to the simple sheath, it also includes functional layers such as an anti-corrosion layer or a waterproof layer. Examples of materials constituting the sheath include polyvinyl chloride (PVC) resin and polyethylene resin.

[0089] The thickness of the sheath is not particularly limited and can be appropriately determined according to the voltage level or installation conditions of the power cable 1. The thickness of the sheath is preferably 0.1 mm to 3 mm, particularly preferably 0.5 mm to 2 mm.

[0090] (II) Manufacturing of power cable 1:

[0091] The power cable 1 is manufactured by sequentially laminating an inner semiconductive layer 12, an insulating layer 13, and an outer semiconductive layer 14 on the outer periphery of a metal conductor 11 by extrusion molding, and the relationship between the cooling rate X [°C / min] of the interface portion of the insulating layer 13 with the inner semiconductive layer 12 during manufacture and the cooling rate Y [°C / min] of the central portion of the insulating layer 13 during manufacture is formed as expressed by the formula (Z) described below. Figure 1 The structure of the aforementioned shielding layer and sheath, not shown, is conventional.

[0092] In an example of a method for manufacturing a power cable 1 using extrusion molding, a metal conductor 11 is continuously supplied from a roller (not shown) around which the metal conductor 11 is wound to a resin extrusion port (not shown), and the inner semiconductive layer 12, the insulating layer 13, and the outer semiconductive layer 14 are coated using the resin extrusion port. The three layers 12, 13, and 14 can be extruded, coated, and stacked simultaneously, or they can be coated and stacked sequentially.

[0093] It should be noted that the metal conductor 11 is heated by the heat transferred from the previously coated resin (including the resin composition, the same below). Considering that the cooling rate of the resin near the metal conductor 11 slows down, it is preferred to cool the temperature of the metal conductor 11 to a state of 1°C-100°C (more preferably 5°C-100°C) before supplying the resin to the extrusion port.

[0094] The resin (resin composition) constituting inner semiconductive layer 12, insulating layer 13, and outer semiconductive layer 14 is laminated by extruding the resin from a resin extruder around metal conductor 11. The resin temperature during lamination extrusion is preferably above the melting point of the resin, specifically above 110°C, preferably above 140°C. Furthermore, to improve adhesion between the resin layers, the temperature is preferably above 160°C, preferably above 180°C, and more preferably above 200°C. Furthermore, to prevent resin sagging during extrusion, the temperature is preferably below 270°C, and to prevent thermal degradation of the resin, the temperature is preferably below 240°C.

[0095] As a production line for the resin (resin composition) used in the manufacture of the present invention, any of the following cable manufacturing lines can be used: a vertical extrusion line (VCV) that supplies a conductor vertically downward, coats the conductor with resin, and then hardens and cools it; or a horizontal extrusion line that extrudes the resin horizontally and hardens and cools it while the cable remains horizontal; or a catenary extrusion line (CCV) that extrudes the resin in an inclined downward manner and hardens and cools it like a hanging curve. The resin used in the manufacture of the present invention is particularly useful because it can reduce the occurrence of uneven wall thickness when used in a horizontal extrusion line or a catenary extrusion line, which are susceptible to uneven wall thickness due to gravity.

[0096] (III) Cooling rates X, Y and their confirmation methods:

[0097] In the power cable 1 of the present invention, the relationship between the cooling rate X [°C / min] of the interface portion of the insulating layer 13 with the inner semiconductive layer 12 during production and the cooling rate Y [°C / min] of the central portion of the insulating layer 13 during production is expressed by the following formula (Z).

[0098] X≥Y×0.8……(Z)

[0099] Figure 2 The diagram shows the interface portion of the insulating layer 13 with the inner semiconductive layer 12 and the central portion of the insulating layer 13. When viewed from the cross section of the power cable 1, the "interface portion of the insulating layer 13 with the inner semiconductive layer 12" refers to the portion of the insulating layer 13 with the inner semiconductive layer 12. Figure 2 The portion of the interface 21 between the inner semiconductive layer 12 and the insulating layer 13 shown by the thick line is closer to the insulating layer 13 side, that is, within about 1 mm.

[0100] In addition, the "central portion of the insulating layer 13" refers to the Figure 2 The portion within the range of approximately ±1 mm around the center line 22 that divides the thickness of the insulating layer into 1 / 2 is shown by the dotted line. Figure 2 The lengths of arrows a1 and a2 are equal.

[0101] Formula (Z) indicates that the cooling rate X of the interface portion of insulating layer 13 with inner semiconductive layer 12 (inside the central portion of insulating layer 13, near metal conductor 11) during manufacturing is at least 80% of the cooling rate Y of the central portion of insulating layer 13 during manufacturing. Thus, external cooling, which is performed from the outside of power cable 1, reliably cools the interface portion of insulating layer 13 with inner semiconductive layer 12 even when cooling and solidifying each layer. This ensures that not only the surface of insulating layer 13 (the interface with outer semiconductive layer 14) but also the inner portion (the side near metal conductor 11) are reliably cooled and solidified.

[0102] Therefore, it can be considered that the metal conductor 11 will not deviate from the center of the power cable 1 due to its own weight, uneven wall thickness is unlikely to occur, and the deformation caused by cooling shrinkage is dispersed, thereby suppressing the occurrence of gaps caused by shrinkage, forming a power cable 1 with good insulation breakdown strength.

[0103] The relationship between the cooling rate X [°C / min] during production of the interface portion of the insulating layer 13 with the internal semiconductive layer 12 and the cooling rate Y [°C / min] during production of the central portion of the insulating layer 13 is preferably the relationship expressed by the following formula (Z').

[0104] Y×3≥X≥Y×0.8……(Z')

[0105] In particular, it is preferred that the cooling rate X [°C / min] of the interface portion of insulating layer 13 with internal semiconductive layer 12 during production is 0.9 times or more the cooling rate Y [°C / min] of the central portion of insulating layer 13 during production.

[0106] Regarding the cooling rates X and Y [°C / minute] as parameters, if actual measurement is difficult, such as by inserting a thermocouple into the insulating layer 13, DSC (differential scanning calorimetry) can be used. Based on the shoulder temperature [°C] measured on the low-temperature side of the melting peak observed during the heating process and the cooling rate [°C / minute] during cooling after the heating process, the relationship between the cooling rate and the shoulder temperature can be determined for samples collected from the relevant portions (the interface with the inner semiconductive layer 12 in the insulating layer 13, or the central portion of the insulating layer 13). This relationship can be used to confirm the relationship. The specific confirmation method is as follows.

[0107] It can be considered that when the resin or resin composition constituting the insulating layer 13 in the power cable 1 of the present invention is subjected to a melting point measurement by DSC by increasing the temperature, for example, the shoulder peak position (called shoulder peak temperature) measured on the low temperature side of the melting peak observed in the range of 120°C-200°C is correlated with the cooling rate when the resin is melted and cooled and solidified just before the measurement. When the cooling rate is very slow during the recrystallization of the measured object such as a resin by cooling and solidification, the crystallizable polymer chain becomes a stable crystal. However, when the cooling rate is fast, the solidification proceeds faster than when the polymer chain is completely folded and crystallized, resulting in a portion that has not become a stable crystal. When the melting point is measured by DSC by increasing the temperature, this incomplete portion becomes a shoulder peak that appears on the low temperature side of the melting peak.

[0108] Therefore, by using a calibration curve with the logarithm of the cooling rate [°C / min] as the horizontal axis and the shoulder temperature [°C] as the vertical axis, the cooling rate of the resin constituting the insulating layer 13 when it is cooled and solidified at the crystallization temperature during the manufacture of the power cable 1 can be obtained.

[0109] First, collect the above Figure 2 The sample (target sample) corresponds to the indicated portion (interface portion, central portion).

[0110] (1) Using DSC, the shoulder peak temperature of such a target sample is confirmed in a temperature increase process of, for example, 120°C to 200°C (hereinafter referred to as "temperature increase process". The melting point can be confirmed using such a temperature increase process).

[0111] (2) After the heating process, cool the sample at a predetermined cooling rate (e.g., 1°C / min) to solidify it. Perform a second melting point measurement on the cooled and solidified sample to confirm the shoulder peak temperature during the heating process. This operation confirms the shoulder peak temperature at a cooling rate of 1°C / min.

[0112] (3) Cool the sample at a different cooling rate (e.g., from 1°C / min to 3°C / min), perform a third melting point measurement, and confirm the shoulder peak temperature during the heating process during this melting point measurement. This procedure confirms the shoulder peak temperature at a cooling rate of 3°C / min.

[0113] (4) Repeat the operation of (3) and confirm the specified cooling rate (e.g., 10°C / min, 20°C / min, etc.) and the shoulder temperature during the heating process during the subsequent melting point measurement (confirm the shoulder temperature at 1°C / min, 3°C / min, 10°C / min, and 20°C / min). It should be noted that the heating rate during the heating process (e.g., 10°C / min, etc.) does not change during the operations of (1) to (4) and remains constant.

[0114] Figure 3 This is an example of a method for confirming the shoulder temperature. Following the steps (2) to (4), Figure 3 Melting curves were recorded for cooling rates of 1°C / min, 3°C / min, 10°C / min, and 20°C / min.

[0115] The shoulder peak temperature is preferably determined by the following methods (i) and (ii) with reference to JIS K7121 (Determination of Transition Temperatures of Plastics).

[0116] (i) Shoulder peak temperature (Ts) [° C.] is the temperature of the top of the shoulder peak.

[0117] (ii) When the shoulder peak temperature (Ts) [°C] is gentle and the apex is difficult to determine, the shoulder peak temperature (Ts) is the temperature at the intersection of the tangent line drawn at the point of maximum slope on the low-temperature side of the shoulder peak curve and the tangent line drawn at the point of minimum slope on the high-temperature side of the shoulder peak curve.

[0118] Figure 3The description shows an example of drawing tangent lines in this manner. For example, for a melting curve with a cooling rate of 1°C / minute, the temperature at the intersection (point indicated by the arrow) of the tangent line L1 drawn at the point of maximum slope on the low-temperature side of the shoulder peak and the tangent line L2 drawn at the point of minimum slope on the high-temperature side of the shoulder peak is approximately 150.9°C. This also applies to cooling rates of 3°C / minute, 10°C / minute, and 20°C / minute.

[0119] (5) The relationship between the cooling rate and the shoulder temperature obtained by (2)-(4) is plotted as the shoulder temperature [°C] (vertical axis) versus the logarithm of the cooling rate [°C / min] (horizontal axis) to create a standard curve (generally a linear function (straight line)). Figure 4 A diagram showing an example of the relationship between the cooling rate and the shoulder temperature.

[0120] (6) Next, the shoulder peak temperature confirmed in (1) is applied to the prepared calibration curve to determine the corresponding cooling rate. The determined cooling rate is estimated to be the cooling rate during the production of the target sample.

[0121] It should be noted that the temperature range of the heating process is not particularly limited and can be widely used in the range of 0°C-200°C. However, reading the melting peak from 120°C-200°C as mentioned above can reliably cover the melting point of the propylene resin and can also exclude the peak when adding a resin with a melting point of less than 110°C, so it is preferred.

[0122] Furthermore, the cooling rate Y during the manufacture of the central portion of the insulating layer 13 is preferably approximately 0.05°C / min to 40°C / min, and more preferably 0.1°C / min to 30°C / min. When the cooling rate Y is within this range, the dielectric breakdown strength (insulation performance) is reliably good. On the other hand, if the cooling rate Y is less than 0.1°C / min, the cooling efficiency is poor, and some uneven wall thickness may occur. If it exceeds 30°C / min, the cooling and solidification proceed too quickly, and microscopic voids may be formed locally due to stress deformation. A cooling rate Y of 0.1°C / min to 15°C / min is particularly preferred.

[0123] On the other hand, the cooling rate X at the interface portion between the insulating layer 13 and the internal semiconductive layer 12 during production is preferably about 0.04° C. / min to 40° C. / min, particularly preferably 0.1° C. / min to 30° C. / min.

[0124] (IV) Effects of the invention:

[0125] According to the power cable 1 of the present invention, the insulating layer 13 contains a propylene-based resin within a specific range. The cooling rate X at the interface between the insulating layer 13 and the inner semiconductive layer 12 during manufacturing is specifically determined, and the cooling rate Y at the center of the insulating layer 13 during manufacturing is specifically determined. This ensures that not only the surface of the insulating layer 13 but also the interior of the insulating layer 13, and the interface with the inner semiconductive layer 12 and the interior of the insulating layer 13, are reliably cooled and hardened. Consequently, the metal conductor 11 is prevented from shifting from the center of the power cable 1 due to its own weight, and uneven wall thickness is less likely to occur. Furthermore, deformation caused by cooling shrinkage is dispersed, thereby suppressing the formation of voids caused by collapse, resulting in a power cable 1 with excellent dielectric breakdown strength.

[0126] Furthermore, according to the method for manufacturing the power cable 1 of the present invention, a power cable 1 that achieves the aforementioned effects can be provided.

[0127] (V) Variations of the specific embodiment:

[0128] It should be noted that the embodiment described above is only one embodiment of the present invention, and the present invention is not limited to the aforementioned embodiment. Of course, variations and improvements within the scope of the structure of the present invention and capable of achieving the purpose and effect are also included in the content of the present invention. In addition, for the specific structure and shape when implementing the present invention, there is no problem with other structures or shapes within the scope of achieving the purpose and effect of the present invention. The present invention is not limited to the above-mentioned embodiments, and variations or improvements within the scope of achieving the purpose of the present invention are included in the present invention.

[0129] For example, in the above (III), four conditions of 1°C / min, 3°C / min, 10°C / min, and 20°C / min are given as examples of the DSC cooling rate. However, the DSC cooling rate is to obtain the Figure 4 The cooling rate conditions used in the DSC measurement and the number of conditions can be arbitrarily determined because the conditions can be conveniently selected based on the relationship shown as an example.

[0130] Furthermore, within the scope of achieving the object of the present invention, the specific structure and shape when implementing the present invention may also be other structures.

[0131] Example

[0132] Hereinafter, the present invention will be described in detail based on Examples and Comparative Examples, but the present invention is not limited thereto.

[0133] [Example 1]

[0134] Manufacturing of power cables:

[0135] The following materials and methods are used to sequentially stack an inner semiconducting layer, an insulating layer, and an outer semiconducting layer on the periphery of a metal conductor to produce a Figure 1 The structure of the power cable is shown.

[0136] As the metal conductor, JIS C3105 hard copper stranded wire (number of strands / strand diameter: 19 strands / 2.6 mm, cross-sectional area: 100 mm) was used. 2 ) copper conductor was extruded using a single-screw extruder (L / D = 24, 200°C, full-flight screw) with Resin A (Hifax CA10A), described below, as the insulating layer material. This, along with the resins constituting the inner and outer semiconductive layers shown in Table 1, extruded using another single-screw extruder (L / D = 24, 200°C, full-flight screw), were then coated around the outer circumference of the copper conductor using a three-layer co-extrusion die (200°C). After coating, the conductor was cooled (air-cooled) by passing through a pressurized cooling pipe, and then cooled (cooled) by passing through a cooling water tank, thereby producing the power cable of Example 1.

[0137] The components of the manufactured power cables are: the cross-sectional area of the copper conductor is 100mm 2 The thickness of the inner semiconductive layer is 0.5 mm, the thickness of the insulating layer is 10 mm, and the thickness of the outer semiconductive layer is 0.5 mm. The constituent materials of the inner and outer semiconductive layers (hereinafter sometimes referred to as "semiconductive layers") are shown in Table 1.

[0138] (Composition of Semiconducting Layer)

[0139] [Table 1]

[0140] parts by mass EV450 (ethylene-vinyl acetate copolymer) (a) 70 Adflex Q200F (olefin-based thermoplastic elastomer) (b) 30 Carbon black (carbon black) 30 ノクラック300 (anti-aging agent) 0.4

[0141] (Note) The total of a and b (both are thermoplastic resins) is 100 parts by mass.

[0142] The semiconductive layers (inner and outer semiconductive layers) were prepared by dry-blending the resin compositions shown in Table 1 using a Henschel mixer, kneading and extruding them through a commercially available single-screw extruder (L / D = 24, 200°C), and pelletizing them. Specifically, the following resins were used.

[0143] (Resin constituting the semiconductive layer)

[0144] EV450 (ethylene-vinyl acetate copolymer (EVA), manufactured by Mitsui Dauphine Chemical Co., Ltd.)

[0145] Adflex Q200F (a resin obtained by multi-stage gas phase polymerization, a thermoplastic elastomer (olefin-based thermoplastic elastomer) composed of a propylene-α-olefin copolymer and polypropylene, manufactured by Sanaroma Co., Ltd., MFR 0.8 g / 10 min (230°C), melting point 162°C)

[0146] Ducon Black (carbon black, manufactured by Ducon Co., Ltd.)

[0147] Nocru 300 (anti-aging agent, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)

[0148] [Example 2-Example 8]

[0149] In Example 1, the resin constituting the insulating layer was changed from Resin A to the resins or resin compositions shown in Table 2 (Examples 2 and 3 were common to Example 1). Power cables of Examples 2 to 8 were manufactured in the same manner as in Example 1, except that the supply rate of the copper conductor serving as the metal conductor and the water temperature of the cooling water tank used for water cooling were adjusted to vary the cooling rate. In Examples 2 and 4, only air cooling was performed, not water cooling.

[0150] [Comparative Example 1]

[0151] A power cable of Comparative Example 1 was produced in the same manner as in Example 1 except that the supply rate of the copper conductor as the metal conductor and the water temperature of the cooling water tank were adjusted to change the cooling rate.

[0152] The insulating layer is made of the following resin or a resin composition obtained by mixing the following resins: Resin A and Resin B correspond to acrylic resins.

[0153] (Resin A)

[0154] Hifax CA10A (a resin obtained by multi-stage gas phase polymerization, a thermoplastic elastomer (olefin-based thermoplastic elastomer) composed of 65% by mass of an ethylene-propylene copolymer and polypropylene: manufactured by Liondel Busser Co., Ltd., MFR 0.6 g / 10 min (230°C), melting point 145°C)

[0155] (Resin B)

[0156] Adflex Q200F (a thermoplastic elastomer (olefin-based thermoplastic elastomer) composed of a propylene-α-olefin copolymer and polypropylene obtained by multi-stage gas phase polymerization, manufactured by Sanaroma Co., Ltd., MFR 0.8 g / 10 min (230°C), melting point 162°C)

[0157] (Resin C)

[0158] Teftec H1041 (hydrogenated styrene-based thermoplastic elastomer (SEBS), manufactured by Asahi Kasei Corporation, MFR 5.0 g / 10 min (230°C), melting point 86°C)

[0159] (Resin D)

[0160] Teflon BL3110M (ethylene-butene copolymer, manufactured by Mitsui Chemicals, Inc., MFR 3 g / 10 min (190°C), melting point 108°C)

[0161] [Test Example 1]

[0162] Lightning impulse test:

[0163] The power cable manufactured by the above method was subjected to a lightning impulse test at a test temperature of 90° C. in accordance with JIS C3005. It should be noted that a lightning impulse test result of 170 kV / mm or higher was considered acceptable, while a result below 170 kV / mm was considered unacceptable.

[0164] It should be noted that the lightning impulse test is a test (insulation breakdown test) to confirm the electrical breakdown level (insulation breakdown strength) of the insulation layer. If the lightning impulse test is good, it can be considered that there is no uneven wall thickness in the insulation layer, and no gaps or peeling caused by shrinkage.

[0165] (Measurement of cooling rate, confirmation of cooling rate during production)

[0166] Regarding the sampling in measuring the cooling rate, for the power cables of Examples 1 to 8 and Comparative Example 1, Figure 2 Approximately 100 mg of target samples were collected from the "interface portion between the insulating layer and the inner semiconductive layer" and the "central portion of the insulating layer" corresponding to the indicated portions.

[0167] The measurement was performed using a commercially available DSC apparatus (DSC7020AS-3D: manufactured by Hitachi High-Tech Science & Technology Co., Ltd.) The heating rate was 10°C / min, and the temperature cycle range was 0°C to 200°C.

[0168] Next, according to the confirmation method described in "(III) Cooling rate X, Y and its confirmation method:", after the DSC measurement of the target sample, a calibration curve is then prepared using the sample. With respect to 1°C / min, 3°C / min, 10°C / min, and 20°C / min in (III), the DSC cooling rate [°C / min] is set to 3°C / min, 5°C / min, 10°C / min, and 20°C / min, and heating and cooling are repeated. The shoulder peak temperature [°C / min] that appears on the low temperature side of the melting peak relative to the logarithm of each cooling rate is confirmed, and the following is obtained: Figure 4The relationship shown in the figure is used to make a standard curve. It should be noted that, as shown in (III) (especially (4) (i), (ii)) and Figure 3 As shown, the shoulder peak temperature is obtained from the intersection of two tangent lines relative to the shoulder.

[0169] The shoulder peak temperature confirmed by the first melting point measurement was then applied to the prepared calibration curve to determine the corresponding cooling rate, which was used as the cooling rate during the production of the target sample. The results are shown in Table 2 along with the resin (or resin composition) constituting the insulating layer, etc.

[0170] (Constitution materials and results)

[0171] [Table 2]

[0172]

[0173] It can be confirmed that, as shown in Table 2, the relationship between the cooling rate X [°C / min] during the manufacture of the interface portion of the insulating layer with the internal semiconductive layer and the cooling rate Y [°C / min] during the manufacture of the central portion of the insulating layer has the following formula (Z). In Examples 1 to 8, the lightning impulse test results were all qualified, and the insulation breakdown characteristics were good.

[0174] It should be noted that in Examples 4 and 5, the cooling rate Y during the production of the central portion of the insulating layer was outside the range of 0.1°C / min to 30°C / min, resulting in inferior results compared to Examples 1 to 3. Furthermore, in Example 8, the content of Resin C (hydrogenated styrene-based thermoplastic elastomer) was higher than that of Example 6, which had a similar structure, and the content of Propylene-based resin (Resin B) was correspondingly lower, resulting in inferior results compared to Example 6.

[0175] On the other hand, in Comparative Example 1, the cooling rate X during production of the interface portion was lower than the cooling rate Y during production of the central portion, thus failing the relationship in equation (Z). Consequently, the lightning impulse test result was less than 170 kV / mm, failing the test. It is believed that the cooling rate relationship in Comparative Example 1 caused "shrinkage" in the insulation layer, degrading the dielectric breakdown characteristics (insulation properties).

[0176] Industrial Applicability

[0177] The present invention can be utilized as a means for simply providing a power cable having excellent dielectric breakdown strength (insulation characteristics) and a method for manufacturing the power cable, and has high industrial applicability.

Claims

1. A power cable comprising a structure in which an inner semiconductive layer, an insulating layer, and an outer semiconductive layer are sequentially stacked on the outer periphery of a metal conductor, wherein: The insulating layer contains 15% by mass or more of a propylene resin having a melting point of 110° C. or more relative to the entire insulating layer. The relationship between the cooling rate X of the interface portion of the insulating layer with the inner semiconductive layer during production and the cooling rate Y of the central portion of the insulating layer during production is expressed by the following formula (Z): X ≥ Y × 0.8 … (Z); The interface portion between the insulating layer and the inner semiconductive layer is the portion within 1 mm of the interface between the inner semiconductive layer and the insulating layer on the side closest to the insulating layer; The central portion of the insulating layer is a portion within a range of ±1 mm around a central line dividing the thickness of the insulating layer into 1 / 2.

2. The power cable according to claim 1, characterized in that The insulating layer contains more than 0% by mass and 80% by mass or less of a styrene-based thermoplastic elastomer.

3. The power cable according to claim 1 or 2, characterized in that: The propylene resin is obtained by multi-stage gas phase polymerization.

4. The power cable according to claim 1 or 2, characterized in that: The aforementioned propylene-based resin is an olefin-based thermoplastic elastomer.

5. The power cable according to claim 1 or 2, characterized in that: The insulating layer contains a thermoplastic resin having a melting point lower than 110° C. in an amount ranging from greater than 0% by mass to 75% by mass or less based on the entire insulating layer.

6. The power cable according to claim 4, characterized in that: The insulating layer contains a thermoplastic resin having a melting point lower than 110° C. in an amount ranging from greater than 0% by mass to 75% by mass or less based on the entire insulating layer.

7. The power cable according to claim 1 or 2, characterized in that: The cooling rate X and the cooling rate Y are cooling rates obtained by the following method: (1) Using a differential scanning calorimeter, determine the shoulder peak temperature of the sample collected during the heating process from 120°C to 200°C; the sample collected is the interface portion between the insulating layer and the inner semiconductive layer, and the central portion of the insulating layer; (2) After the heating process, cool the sample at a prescribed cooling rate of 1°C / min to cool and solidify it; perform a second melting point measurement on the cooled and solidified sample to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, confirm the shoulder peak temperature when the cooling rate is 1°C / min; (3) After the heating process, the cooling rate is changed from 1°C / min to 3°C / min to cool the sample to cool and solidify; the melting point of the cooled and solidified sample is measured for the third time to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, the shoulder peak temperature is confirmed when the cooling rate is 3°C / min; (4) Repeat the operation of (3) to confirm the prescribed cooling rate of 10°C / min and 20°C / min and the shoulder peak temperature during the heating process during the subsequent melting point measurement; (5) Plot the relationship between the cooling rate and the shoulder temperature obtained by (2)-(4) as the logarithm of the shoulder temperature on the vertical axis against the cooling rate on the horizontal axis to create a standard curve; (6) Next, the shoulder peak temperature determined in (1) is applied to the prepared calibration curve to determine the corresponding cooling rate.

8. The power cable according to claim 4, characterized in that: The cooling rate X and the cooling rate Y are cooling rates obtained by the following method: (1) Using a differential scanning calorimeter, determine the shoulder peak temperature of the sample collected during the heating process from 120°C to 200°C; the sample collected is the interface portion between the insulating layer and the inner semiconductive layer, and the central portion of the insulating layer; (2) After the heating process, cool the sample at a prescribed cooling rate of 1°C / min to cool and solidify it; perform a second melting point measurement on the cooled and solidified sample to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, confirm the shoulder peak temperature when the cooling rate is 1°C / min; (3) After the heating process, the cooling rate is changed from 1°C / min to 3°C / min to cool the sample to cool and solidify; the melting point of the cooled and solidified sample is measured for the third time to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, the shoulder peak temperature is confirmed when the cooling rate is 3°C / min; (4) Repeat the operation of (3) to confirm the prescribed cooling rate of 10°C / min and 20°C / min and the shoulder peak temperature during the heating process during the subsequent melting point measurement; (5) Plot the relationship between the cooling rate and the shoulder temperature obtained by (2)-(4) as the logarithm of the shoulder temperature on the vertical axis against the cooling rate on the horizontal axis to create a standard curve; (6) Next, the shoulder peak temperature determined in (1) is applied to the prepared calibration curve to determine the corresponding cooling rate.

9. The power cable according to claim 5, characterized in that: The cooling rate X and the cooling rate Y are cooling rates obtained by the following method: (1) Using a differential scanning calorimeter, determine the shoulder peak temperature of the sample collected during the heating process from 120°C to 200°C; the sample collected is the interface portion between the insulating layer and the inner semiconductive layer, and the central portion of the insulating layer; (2) After the heating process, cool the sample at a prescribed cooling rate of 1°C / min to cool and solidify it; perform a second melting point measurement on the cooled and solidified sample to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, confirm the shoulder peak temperature when the cooling rate is 1°C / min; (3) After the heating process, the cooling rate is changed from 1°C / min to 3°C / min to cool the sample to cool and solidify; the melting point of the cooled and solidified sample is measured for the third time to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, the shoulder peak temperature is confirmed when the cooling rate is 3°C / min; (4) Repeat the operation of (3) to confirm the prescribed cooling rate of 10°C / min and 20°C / min and the shoulder peak temperature during the heating process during the subsequent melting point measurement; (5) Plot the relationship between the cooling rate and the shoulder temperature obtained by (2)-(4) as the logarithm of the shoulder temperature on the vertical axis against the cooling rate on the horizontal axis to create a standard curve; (6) Next, the shoulder peak temperature determined in (1) is applied to the prepared calibration curve to determine the corresponding cooling rate.

10. The power cable according to claim 6, characterized in that The cooling rate X and the cooling rate Y are cooling rates obtained by the following method: (1) Using a differential scanning calorimeter, determine the shoulder peak temperature of the sample collected during the heating process from 120°C to 200°C; the sample collected is the interface portion between the insulating layer and the inner semiconductive layer, and the central portion of the insulating layer; (2) After the heating process, cool the sample at a prescribed cooling rate of 1°C / min to cool and solidify it; perform a second melting point measurement on the cooled and solidified sample to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, confirm the shoulder peak temperature when the cooling rate is 1°C / min; (3) After the heating process, the cooling rate is changed from 1°C / min to 3°C / min to cool the sample to cool and solidify; the melting point of the cooled and solidified sample is measured for the third time to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, the shoulder peak temperature is confirmed when the cooling rate is 3°C / min; (4) Repeat the operation of (3) to confirm the prescribed cooling rate of 10°C / min and 20°C / min and the shoulder peak temperature during the heating process during the subsequent melting point measurement; (5) Plot the relationship between the cooling rate and the shoulder temperature obtained by (2)-(4) as the logarithm of the shoulder temperature on the vertical axis against the cooling rate on the horizontal axis to create a standard curve; (6) Next, the shoulder peak temperature determined in (1) is applied to the prepared calibration curve to determine the corresponding cooling rate.

11. The power cable according to claim 1 or 2, characterized in that: The cooling rate Y is 0.1°C / min to 30°C / min.

12. The power cable according to claim 4, characterized in that The cooling rate Y is 0.1°C / min to 30°C / min.

13. The power cable according to claim 5, characterized in that The cooling rate Y is 0.1°C / min to 30°C / min.

14. The power cable according to claim 6, characterized in that The cooling rate Y is 0.1°C / min to 30°C / min.

15. The power cable according to claim 7, characterized in that The cooling rate Y is 0.1°C / min to 30°C / min.

16. The power cable according to any one of claims 8 to 10, characterized in that: The cooling rate Y is 0.1°C / min to 30°C / min.

17. The power cable according to claim 1 or 2, characterized in that: The thickness of the insulating layer is 5 mm or more.

18. The power cable according to claim 4, characterized in that The thickness of the insulating layer is 5 mm or more.

19. The power cable according to claim 5, characterized in that The thickness of the insulating layer is 5 mm or more.

20. The power cable according to claim 6, characterized in that The thickness of the insulating layer is 5 mm or more.

21. The power cable according to claim 7, characterized in that The thickness of the insulating layer is 5 mm or more.

22. The power cable according to any one of claims 8 to 10, characterized in that: The thickness of the insulating layer is 5 mm or more.

23. The power cable according to claim 11, characterized in that The thickness of the insulating layer is 5 mm or more.

24. The power cable according to any one of claims 12 to 15, characterized in that: The thickness of the insulating layer is 5 mm or more.

25. The power cable according to claim 16, characterized in that The thickness of the insulating layer is 5 mm or more.

26. A method for manufacturing a power cable according to claim 1, wherein the method comprises a structure in which an inner semiconductive layer, an insulating layer, and an outer semiconductive layer are sequentially laminated on the outer periphery of a metal conductor by extrusion molding, wherein: The insulating layer contains 15% by mass or more of a propylene resin having a melting point of 110° C. or higher relative to the entire insulating layer, and is formed as represented by the following formula (Z): The relationship between the cooling rate X of the interface portion of the insulating layer with the inner semiconductive layer during production and the cooling rate Y of the central portion of the insulating layer during production is expressed by the following formula (Z): X ≥ Y × 0.8 … (Z); The interface portion between the insulating layer and the inner semiconductive layer is the portion within 1 mm of the interface between the inner semiconductive layer and the insulating layer on the side closest to the insulating layer; The central portion of the insulating layer is a portion within a range of ±1 mm around a central line dividing the thickness of the insulating layer into 1 / 2.

27. The method for manufacturing a power cable according to claim 26, characterized in that: The cooling rate X and the cooling rate Y are cooling rates obtained by the following method: (1) Using a differential scanning calorimeter, determine the shoulder peak temperature of the sample collected during the heating process from 120°C to 200°C; the sample collected is the interface portion between the insulating layer and the inner semiconductive layer, and the central portion of the insulating layer; (2) After the heating process, cool the sample at a prescribed cooling rate of 1°C / min to cool and solidify it; perform a second melting point measurement on the cooled and solidified sample to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, confirm the shoulder peak temperature when the cooling rate is 1°C / min; (3) After the heating process, the cooling rate is changed from 1°C / min to 3°C / min to cool the sample to cool and solidify; the melting point of the cooled and solidified sample is measured for the third time to confirm the shoulder peak temperature during the heating process during the melting point measurement; using this operation, the shoulder peak temperature is confirmed when the cooling rate is 3°C / min; (4) Repeat the operation of (3) to confirm the prescribed cooling rate of 10°C / min and 20°C / min and the shoulder peak temperature during the heating process during the subsequent melting point measurement; (5) Plot the relationship between the cooling rate and the shoulder temperature obtained by (2)-(4) as the logarithm of the shoulder temperature on the vertical axis against the cooling rate on the horizontal axis to create a standard curve; (6) Next, the shoulder peak temperature determined in (1) is applied to the prepared calibration curve to determine the corresponding cooling rate.

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