Cable connection structure, coupled power cable, and method for producing cable connection structure

Abstract

This cable connection structure comprises: a conductor connection part that connects respective conductors of a pair of power cables; an internal semiconductive layer that is provided so as to cover the outer periphery of the conductor connection part and has semiconductivity; an insulating layer that is provided so as to cover the outer periphery of the internal semiconductive layer, has insulating properties, and has an inner peripheral surface facing the internal semiconductive layer and an outer peripheral surface opposite to the inner peripheral surface; an external semiconductive layer that is provided so as to cover the outer periphery of the insulating layer and has semiconductivity; and an external semiconductive layer that is provided so as to cover the outer peripheral surface of the external semiconductive layer and has semiconductivity. The insulating layer comprises a base polymer including a propylene unit, a modified polymer including a propylene unit and modified by at least one selected from an unsaturated organic acid and derivatives thereof, and a thermoplastic elastomer. A ratio of a storage elastic modulus of an inner-side sample of the insulating layer to a storage elastic modulus of an outer-side sample of the insulating layer is 1.1 to 2.5, and a ratio of a volume resistivity of the inner-side sample to a volume resistivity of the outer-side sample is 1.0 to 1.5.

Description

Cable connection structure, linked power cable, and method for manufacturing a cable connection structure

[0001] This disclosure relates to a cable connection structure, a connected power cable, and a method for manufacturing a cable connection structure.

[0002] When manufacturing power cables to be laid over long distances, it is sometimes possible to produce connected power cables of the desired length by connecting multiple power cables within the factory. This cable connection structure is called a "Factory Joint (FJ)" (for example, Patent Document 1).

[0003] Japanese Patent Application Publication No. 9-56039

[0004] According to one aspect of the present disclosure, a conductor connection portion connecting the respective conductors of a pair of power cables; an internal semiconducting layer provided to cover the outer circumference of the conductor connection portion and having semiconductivity; an insulating layer provided to cover the outer circumference of the internal semiconducting layer and having insulating properties, having an inner surface facing the internal semiconducting layer and an outer surface opposite to the inner surface; and an external semiconducting layer provided to cover the outer circumference of the insulating layer and having semiconductivity, wherein the insulating layer comprises a base polymer containing propylene units, a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives, and a thermoplastic elastomer, and the storage modulus of the insulating layer relative to the storage modulus of the outer sample of the insulating layer The ratio of the storage modulus of the inner sample is 1.1 or more and 2.5 or less, and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample is 1.0 or more and 1.5 or less. Herein, the outer sample of the insulating layer is taken from a position 0.3 mm from the outer peripheral surface toward the inner semiconducting layer, and the inner sample of the insulating layer is taken from a position 0.3 mm from the inner peripheral surface toward the outer peripheral surface, the storage modulus of the outer sample and the storage modulus of the inner sample are measured at 25°C by dynamic viscoelasticity measurement, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under the conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. A cable connection structure is provided.

[0005] Figure 1 is a schematic cross-sectional view showing a cable connection structure according to one embodiment of the present disclosure. Figure 2 is a flowchart showing a method for manufacturing a connected power cable according to one embodiment of the present disclosure. Figure 3 is a schematic diagram showing a second heating step. Figure 4 shows the temperature at the sampling location of the outer sample and the temperature at the sampling location of the inner sample during the second heating step of sample A3. Figure 5 shows the temperature at the sampling location of the outer sample and the temperature at the sampling location of the inner sample during the second heating step of sample B1.

[0006] [Problems this disclosure aims to solve] The purpose of this disclosure is to improve the flexibility, strength, and insulation properties of the insulating layer in cable connection structures.

[0007] [Effects of this disclosure] According to this disclosure, the flexibility, strength, and insulation properties of the insulating layer in a cable connection structure can be improved.

[0008] [Description of Embodiments in this Disclosure] <Knowledge Obtained by the Inventors, etc.> First, we will briefly explain the knowledge obtained by the inventors, etc.

[0009] In a cable connection structure configured as an FJ, an insulating layer is formed by wrapping insulating tape around the outer circumference of the internal semiconducting layer. In such an insulating layer, if voids occur between the multiple layers of insulating tape, the insulating properties of the insulating layer may decrease due to these voids. Therefore, a heating process (the second heating process described later) is performed after the insulating layer formation process. This allows the multiple layers of insulating tape in the insulating layer to fuse together through heating. As a result, it becomes possible to suppress the generation of voids in the insulating layer.

[0010] Here, the inventors are considering polypropylene as the base polymer constituting the insulating layer of the cable connection structure. With an insulating layer containing polypropylene, good insulation can be obtained even in a non-crosslinked state.

[0011] However, as a result of investigations by the inventors, it was found that the insulating layer of a cable connection structure containing polypropylene as the base polymer presents the following new problems.

[0012] Polypropylene has a higher melting point and higher thermal resistance than polyethylene. Therefore, the temperature inside the insulating layer does not rise easily during the heating process after the insulating layer formation process.

[0013] Therefore, the inventors considered increasing the temperature at which the heating device's heater heats the insulating layer from the outside in order to melt the insulating tape all the way to the inside of the insulating layer.

[0014] However, when the heating device's heater heated the insulating layer from the outside to an excessively high temperature, the outer surface of the insulating layer became excessively heated, leading to thermal degradation. As a result, the elasticity and volume resistivity on the outer surface of the insulating layer became excessively low. Consequently, the required insulating and mechanical properties for the cable connection structure could not be obtained.

[0015] On the other hand, the inventors considered heating the conductor connection portion by induction heating in addition to heating by the heater of the heating device during the heating process after the insulating layer formation process. As a result, it was possible to heat the inside of the insulating layer without excessively increasing the heating temperature by the heater of the heating device. Consequently, the temperature difference between the outside and inside of the insulating layer was reduced.

[0016] However, when the temperature difference between the outside and inside of the insulating layer was reduced, the cooling after heating was carried out evenly and slowly on both sides of the insulating layer. As a result, the degree of crystallinity of the base polymer on the outside of the insulating layer and the degree of crystallinity of the base polymer on the inside of the insulating layer were similarly high. Due to this high degree of crystallinity, the elasticity of the insulating layer was uniformly high in the thickness direction. In this case, it became difficult to ensure sufficient flexibility of the connected power cable when the connected power cable with a cable connection structure was bent. Consequently, there were limitations on the reel diameter when winding the connected power cable onto a reel.

[0017] Therefore, as a result of diligent research, the present inventors have succeeded in improving the resin component constituting the insulating layer and improving the second heating step as a new manufacturing method, thereby obtaining a desired distribution of elasticity in the thickness direction of the insulating layer of the cable connection structure and making the insulating properties of the insulating layer uniform in the thickness direction of the insulating layer.

[0018] This disclosure is based on the aforementioned findings discovered by the inventors.

[0019] <Embodiments of the Disclosure> Next, embodiments of the Disclosure will be described by listing them.

[0020] [1] A cable connection structure according to one aspect of the present disclosure comprises: a conductor connection portion connecting the respective conductors of a pair of power cables; an internal semiconducting layer provided to cover the outer circumference of the conductor connection portion and having semiconductivity; an insulating layer provided to cover the outer circumference of the internal semiconducting layer, having insulating properties, and having an inner surface facing the internal semiconducting layer and an outer surface opposite to the inner surface; and an external semiconducting layer provided to cover the outer circumference of the insulating layer and having semiconductivity, wherein the insulating layer comprises: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, wherein the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample of the insulating layer is 1.1 or more and 2.5 or less; and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample is 1.0 or more and 1.5 or less. The outer sample of the insulating layer is taken from a position 0.3 mm from the outer peripheral surface toward the inner semiconducting layer, and the inner sample of the insulating layer is taken from a position 0.3 mm from the inner peripheral surface toward the outer peripheral surface. The storage modulus of the outer sample and the storage modulus of the inner sample are measured at 25°C by dynamic viscoelasticity measurement, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. This configuration makes it possible to improve the flexibility, strength, and insulating properties of the insulating layer.

[0021] [2] In the cable connection structure described in [1] above, when the total content of the base polymer, the modified polymer, and the thermoplastic elastomer in the insulating layer is 100 parts by mass, the content of the modified polymer in the insulating layer is 1 part by mass or more and 10 parts by mass or less, and the content of the thermoplastic elastomer in the insulating layer is 10 parts by mass or more and 45 parts by mass or less. With this configuration, it is possible to stably achieve both a desired distribution of elasticity and uniformity of insulation in the thickness direction of the insulating layer.

[0022] [3] In the cable connection structure described in [1] or [2] above, the thermoplastic elastomer includes a styrene-based elastomer. This configuration allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0023] [4] In the cable connection structure described in any one of [1] to [3] above, the thermoplastic elastomer includes an olefin-based elastomer. This configuration allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0024] [5] In the cable connection structure described in any one of [1] to [4] above, the storage modulus of the inner sample of the insulating layer is 650 MPa or more and 900 MPa or less, and the storage modulus of the outer sample of the insulating layer is 280 MPa or more and 670 MPa or less. This configuration makes it possible to achieve both rigidity and flexibility in the cable connection structure.

[0025] [6] In the cable connection structure described in any one of [1] to [5] above, the thickness of the insulating layer is 3 mm or more. With this configuration, even if the connected power cable equipped with the cable connection structure is applied to high-voltage applications, it is possible to stably achieve both the flexibility and insulating properties of the insulating layer.

[0026] [7] A connected power cable according to one aspect of the present disclosure comprises at least one cable connection structure described in any one of [1] to [6]. This configuration improves the flexibility, strength, and insulation of the insulating layer.

[0027] [8] A method for manufacturing a cable connection structure according to one aspect of the present disclosure comprises: forming a conductor connection portion by connecting the respective conductors of a pair of power cables; forming a semiconducting internal semiconducting layer so as to cover the outer periphery of the conductor connection portion; forming an insulating layer so as to cover the outer periphery of the internal semiconducting layer; forming a semiconducting external semiconducting layer so as to cover the outer periphery of the insulating layer; and heating the insulating layer and the external semiconducting layer, and then cooling them, wherein the step of forming the insulating layer comprises: preparing an insulating tape containing a resin composition; wrapping the insulating tape around the outer periphery of the internal semiconducting layer to form an inner circumferential surface facing the internal semiconducting layer and an outer circumferential surface opposite to the inner circumferential surface, wherein the step of preparing the insulating tape comprises, as the resin composition, a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, and prepares the insulating tape, In the step of heating and then cooling the insulating layer and the outer semiconducting layer, the insulating layer is heated and then cooled such that the ratio of the storage modulus of the inner sample of the insulating layer to the storage modulus of the outer sample of the insulating layer is 1.1 or more and 2.5 or less, and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample is 1.0 or more and 1.5 or less. Here, the outer sample of the insulating layer is taken from a position 0.3 mm from the outer peripheral surface toward the inner semiconducting layer, and the inner sample of the insulating layer is taken from a position 0.3 mm from the inner peripheral surface toward the outer peripheral surface. The storage modulus of the outer sample and the storage modulus of the inner sample are measured at 25°C by dynamic viscoelasticity measurement, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. This configuration makes it possible to improve the flexibility, strength, and insulating properties of the insulating layer.

[0028] [9] In the method for manufacturing the cable connection structure described in [8] above, the insulating layer is heated from a region close to the outer surface of the insulating layer while the conductor connection portion is heated by electromagnetic induction, thereby heating the insulating layer from a region close to the inner surface of the insulating layer. With this configuration, thermal degradation on the outside of the insulating layer can be suppressed while the insulating tape on the inside of the insulating layer can be stably fused.

[0029] [Details of Embodiments of the Disclosure] Next, one embodiment of the Disclosure will be described below with reference to the drawings. However, the Disclosure is not limited to these examples and is intended to include all modifications within the meaning and scope of the equivalents of the claims, as indicated by the claims.

[0030] <One Embodiment of the Present Disclosure> (1) Outline of the Linked Power Cable and Cable Connection Structure The linked power cable 10 and cable connection structure (cable connection part) 20 according to the first embodiment of the present disclosure will be described with reference to Figure 1. In Figure 1, the power cable 100 is shown with its side stripped in stages. The lower part of Figure 1 is omitted.

[0031] As shown in Figure 1, the connected power cable 10 of this embodiment is configured, for example, as a submarine cable laid on the seabed, and comprises a plurality of power cables 100 and at least one cable connection structure 20.

[0032] (Power Cable) The power cable 100 is configured as a solid-insulated cable, which is a high-voltage power transmission cable.

[0033] The power cable 100 has, for example, a conductor 110, an internal semiconducting layer 120, a cable insulation layer 130, an external semiconducting layer 140, a water-absorbing layer (not shown), a cable metal tube 150, and a cable sheath 160, arranged from the central axis of the conductor 110 toward the outer circumference of the power cable 100. The base polymer contained in the cable insulation layer 130 is, for example, polypropylene.

[0034] One of the pair of power cables 100 may be referred to as the "first power cable 100a", and the other power cable 100 may be referred to as the "second power cable 100b".

[0035] (Cable connection structure) As shown in FIG. 1, the cable connection structure 20 includes, for example, the first power cable 100a, the second power cable 100b, a conductor connection portion 210, an internal semiconductive layer 220, an insulating layer 230, an external semiconductive layer 240, a water-absorbing tape layer 242, a metal pipe (protective pipe) 250, and an anticorrosion layer (connection portion sheath) 260 in this order from a region near the conductor connection portion 210 toward the outside.

[0036] In the conductor connection portion 210, the conductor 110 of the first power cable 100a and the conductor 110 of the second power cable 100b are connected.

[0037] The internal semiconductive layer 220 is provided so as to cover the outer periphery of the conductor connection portion 210. The internal semiconductive layer 220 has semiconductive properties.

[0038] The insulating layer 230 is provided so as to cover the outer periphery of the internal semiconductive layer 220. The insulating layer 230 has insulating properties. The insulating layer 230 has an inner peripheral surface facing the internal semiconductive layer 220 and an outer peripheral surface opposite to the inner peripheral surface.

[0039] The external semiconductive layer 240 is provided so as to cover the outer periphery of the insulating layer 230. The external semiconductive layer 240 has semiconductive properties.

[0040] In the present embodiment, the base polymer in each of the internal semiconductive layer 220, the insulating layer 230, and the external semiconductive layer 240 is, for example, polypropylene.

[0041] In the cable connection structure 20, parts other than the internal semiconductive layer 220, the insulating layer 230, and the external semiconductive layer 240 can be configured, for example, as a general configuration of FJ as described in Japanese Unexamined Patent Application Publication No. 2023-065809. Note that the metal pipe 250 and the anticorrosion layer 260 may not be provided.

[0042] (Specific dimensions, etc.) As specific dimensions in the cable connection structure 20, although not particularly limited, for example, the diameter of the conductor connection portion 210 is 5 mm or more and 60 mm or less, the thickness of the internal semiconductive layer 220 is 0.5 mm or more and 3 mm or less, the thickness of the insulating layer 230 is 3 mm or more and 35 mm or less, and the thickness of the external semiconductive layer 240 is 0.5 mm or more and 3 mm or less. The DC voltage applied to the connecting power cable 10 of the present embodiment is, for example, 20 kV or more.

[0043] (2) Resin composition constituting the insulating tape The insulating layer 230 is formed, for example, by winding an insulating tape around the outer circumference of the internal semiconductive layer 220. The insulating tape contains a resin composition. The resin composition of the present embodiment has, for example, a base polymer (A), a modified polymer (B), a thermoplastic elastomer (C), and other additives.

[0044] Hereinafter, the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) are also referred to as "resin components".

[0045] (Base polymer (A)) The base polymer (base resin) (A) refers to a resin component that constitutes the main component of the resin composition. "Main component" means the component with the highest content.

[0046] The base polymer (A) of the present embodiment contains, for example, at least a propylene unit as a monomer unit.

[0047] That is, by analyzing the resin composition of the present embodiment with a nuclear magnetic resonance (NMR) apparatus, a propylene unit is detected as a monomer unit derived from the base polymer (A).

[0048] The base polymer (A) is composed of, for example, polypropylene (also referred to as a propylene-based resin, PP) having a propylene unit in the main chain. Examples of polypropylene include homopolypropylene (homo-PP), random polypropylene (random-PP), block polypropylene (block-PP), and the like.

[0049] NMR analysis of the resin composition of this embodiment reveals, for example, that if the base polymer (A) is random PP or block PP, propylene units and ethylene units derived from the random PP or block PP are detected. If the base polymer (A) is homo PP, propylene units derived from the homo PP are detected.

[0050] From the viewpoint of obtaining high insulation properties in the insulating layer 230, the base polymer (A) may be random PP. Homo PP has a higher crystalline content compared to random PP and can obtain high insulation properties. However, insulating layer 230 containing homo PP may cause cracking within or between crystals. For this reason, homo PP may not be able to obtain its inherent insulation properties. In contrast, random PP contains ethylene units, resulting in a lower crystalline content. However, insulating layer 230 containing random PP is less prone to cracking due to coarse crystallization. As a result, random PP can obtain higher insulation properties compared to homo PP.

[0051] When the base polymer (A) is random PP, the ethylene unit content in the random PP may be, for example, 0.5% by mass or more and 15% by mass or less. By setting the ethylene unit content to 0.5% by mass or more, the growth of coarse spherulites can be suppressed. On the other hand, by setting the ethylene unit content to 15% by mass or less, the decrease in the melting point can be suppressed, and stable use in non-crosslinked or semi-crosslinked forms can be achieved.

[0052] In this embodiment, the stereoregularity of polypropylene is not particularly limited, but may be isotactic, for example. Here, if the stereoregularity of polypropylene is atactic, the polypropylene will not crystallize. In contrast, isotactic polypropylene is polymerized with a Ziegler-Natta catalyst and is generally available. By having isotactic stereoregularity, a decrease in the melting point can be suppressed in the composition. As a result, a predetermined crystallinity can be easily obtained, and high insulating properties can be obtained.

[0053] In this embodiment, the storage modulus of the base polymer (A) alone is, for example, 800 MPa or more and 1700 MPa or less. The measurement method and measurement conditions for the "storage modulus of the base polymer (A) alone" are the same as those for the storage modulus of the insulating layer 230, which will be described later.

[0054] The melt flow rate (MFR) of the base polymer (A) is not particularly limited, but the MFR of the base polymer (A) may be, for example, 0.1 g / 10 min or more and 5.0 g / 10 min or less, or 0.1 g / 10 min or more and 2.0 g / 10 min or less. Here, "MFR" refers to the value measured in accordance with JIS K7210 at a temperature of 190°C and a load of 2.16 kg. By setting the MFR of the base polymer (A) within the above range, the phase structure described later can be easily formed in the insulating layer 230.

[0055] The melting point of the base polymer (A) is not particularly limited, but it may be, for example, 130°C to 165°C. This allows for the easy formation of the phase structure described later when the base polymer (A) is mixed with at least one of the modified polymer (B) and the thermoplastic elastomer (C).

[0056] (Modified polymer (B)) Modified polymer (B) contains propylene units as its main chain and is modified with at least one selected from unsaturated organic acids and their derivatives.

[0057] The inclusion of propylene units in the modified polymer (B) improves the compatibility when the base polymer (A) and the modified polymer (B) are mixed.

[0058] By modifying the modified polymer (B) with an unsaturated organic acid containing polar groups, polar groups can be introduced into the resin composition. This allows space charges to be trapped by the dispersed polar groups in the insulating layer 230 formed by the insulating tape containing the resin composition. In other words, local accumulation of space charges in the insulating layer 230 can be suppressed. As a result, the insulating properties of the insulating layer 230 can be improved.

[0059] Specifically, the modified polymer (B) may be, for example, an unsaturated carboxylic acid-modified polypropylene, in which polypropylene is modified with at least one of the unsaturated carboxylic acids and their derivatives. Examples of unsaturated carboxylic acids and their derivatives include acrylic acid, methacrylic acid, crotonic acid, maleic acid, cinnamic acid, itaconic acid, citraconic acid, fumaric acid, and their anhydrides.

[0060] Among these, the modified polymer (B) may be modified with, for example, maleic anhydride. Maleic anhydride has a large number of polar groups per unit molecular weight. As a result, even if the amount of maleic anhydride in the modified polymer (B) is small, a sufficient number of polar groups can be ensured in the modified polymer (B).

[0061] The amount of modification (hereinafter simply referred to as "amount of modification") of at least one of the unsaturated carboxylic acid and its derivatives in the modified polymer (B) is not particularly limited, but the amount of modification may be, for example, 0.1% by mass or more and 10% by mass or less. Here, "amount of modification" means the copolymerization ratio (content) of at least one of the unsaturated carboxylic acid and its derivatives in the modified polymer (B). By setting the amount of modification to 0.1% by mass or more, the accumulation of space charge in the insulating layer 230 can be stably suppressed. On the other hand, by setting the amount of modification to 10% by mass or less, the compatibility between the modified polymer (B) and the base polymer (A) can be improved.

[0062] In this embodiment, the storage modulus of the modified polymer (B) alone is, for example, 1000 MPa or more and 1900 MPa or less. The measurement method and measurement conditions for the "storage modulus of the modified polymer (B) alone" are the same as those for the storage modulus of the insulating layer 230, which will be described later.

[0063] The MFR of modified polymer (B) is not particularly limited, but may be, for example, 0.1 g / 10 min to 500 g / 10 min, or 1 g / 10 min to 300 g / 10 min. The measurement conditions for the MFR of modified polymer (B) here are the same as those described for the MFR of base polymer (A). By having the MFR of modified polymer (B) within the above range, the phase structure described later can be easily formed when base polymer (A) and modified polymer (B) are mixed.

[0064] The melting point of the modified polymer (B) is not particularly limited, but it may be, for example, 130°C to 165°C. This allows for the easy formation of the phase structure described later when the base polymer (A) and the modified polymer (B) are mixed.

[0065] (Thermoplastic elastomer (C)) The thermoplastic elastomer (C) has lower crystallinity compared to the base polymer (A) which has propylene units. The thermoplastic elastomer (C) can suppress excessive crystal growth of the base polymer (A) and impart flexibility to the insulating layer 230.

[0066] Examples of thermoplastic elastomers (C) include amide-based, ester-based, olefin-based, styrene-based, urethane-based, PVC-based, and fluorine-based elastomers. Among these, thermoplastic elastomer (C) may be at least one of styrene-based polymers and olefin-based elastomers.

[0067] (Styrene-based elastomer) The thermoplastic elastomer (C) may contain, for example, a styrene-based elastomer. This allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0068] In this embodiment, the styrene-based elastomer is a copolymer comprising, for example, styrene units as hard segments and at least one monomer unit from among ethylene units, propylene units, butylene units, and isoprene units as soft segments.

[0069] Examples of styrene-based elastomers include styrene-butadiene-styrene block copolymer (SBS), hydrogenated styrene-butadiene-styrene block copolymer, styrene-ethylene-styrene block copolymer (SES), styrene-ethylene-propylene copolymer (SEP), styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS), styrene-ethylene-propylene-styrene block copolymer (SEPS), styrene-isoprene-styrene copolymer (SIS), hydrogenated styrene-isoprene-styrene copolymer, hydrogenated styrene-butadiene rubber, hydrogenated styrene-isoprene rubber, and styrene-ethylene-butylene-olefin crystal block copolymer. Two or more of these may be used in combination.

[0070] In this context, "hydrogenated" means that hydrogen has been added to the double bond. For example, "hydrogenated styrene-butadiene-styrene block copolymer" refers to a polymer in which hydrogen has been added to the double bond of styrene-butadiene-styrene block copolymer. Hydrogen is not added to the double bond of the aromatic ring of styrene. "Hydrogenated styrene-butadiene-styrene block copolymer" can be rephrased as styrene-ethylene-butylene-styrene block copolymer (SEBS).

[0071] The styrene-based elastomer may be a hydrogenated material, for example, one that does not contain double bonds in its chemical structure, excluding the aromatic ring. When a non-hydrogenated material is used, the resin components may degrade due to heat during molding of the resin composition. This may lead to a decrease in the properties of the resulting insulating layer 230. In contrast, using a hydrogenated material can improve resistance to heat degradation. This allows the properties of the insulating layer 230 to be maintained at a higher level.

[0072] Furthermore, the styrene-based elastomer may contain, for example, styrene units and at least one of propylene units and butene units. This allows for a stable improvement in the compatibility between polypropylene as the base polymer (A) and the styrene-based elastomer. As a result, polypropylene and the styrene-based elastomer can be uniformly mixed.

[0073] The styrene unit content (content rate; hereinafter simply referred to as "styrene unit content") in the styrene-based elastomer is not particularly limited, but the styrene unit content may be, for example, 5% by mass or more and 35% by mass or less. By setting the styrene unit content to 5% by mass or more and 35% by mass or less, it is possible to stably suppress the material from becoming excessively hard. This makes it possible to stably suppress the separation and cracking of polypropylene and thermoplastic elastomer (C).

[0074] (Olefin-based elastomer) The thermoplastic elastomer (C) may include, for example, an olefin-based elastomer. This allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0075] Olefin-based elastomers as thermoplastic elastomers (C) are, for example, copolymers containing two olefin units. Examples of olefin-based elastomers include copolymers containing an ethylene unit and an α-olefin unit having 3 or more carbon atoms, and copolymers containing a propylene unit and an α-olefin unit having 4 or more carbon atoms.

[0076] Specifically, examples of olefin-based elastomers include ethylene propylene rubber (EPR), very low-density polyethylene (VLDPE), and propylene-1-butene copolymer (propylene-1-butene rubber (PBR)). Ethylene-1-butene copolymer (ethylene-1-butene rubber (EBR)) is a type of VLDPE.

[0077] The olefin-based elastomer may contain, for example, at least one of propylene units and butene units. This allows for a stable improvement in the compatibility between polypropylene as the base polymer (A) and the olefin-based elastomer. As a result, polypropylene and the olefin-based elastomer can be uniformly mixed.

[0078] When the olefin-based elastomer contains ethylene units, the content of ethylene units in the olefin-based elastomer is not particularly limited, but the content of ethylene units may be, for example, 5% by mass or more and 90% by mass or less. This makes it possible to stably obtain the softening effect and crystallization inhibition effect of the olefin-based elastomer.

[0079] (Characteristics of Thermoplastic Elastomer) In this embodiment, the storage modulus of the thermoplastic elastomer (C) alone is, for example, 20 MPa or more and 500 MPa or less. The measurement method and measurement conditions for the "storage modulus of the thermoplastic elastomer (C) alone" are the same as those for the storage modulus of the insulating layer 230, which will be described later.

[0080] The MFR of the thermoplastic elastomer (C) is not particularly limited, but may be, for example, 0.1 g / 10 min to 5.0 g / 10 min, or 0.1 g / 10 min to 2.0 g / 10 min. The measurement conditions for the MFR of the thermoplastic elastomer (C) here are the same as those described for the MFR of the base polymer (A). By having the MFR of the thermoplastic elastomer (C) within the above range, the phase structure described later can be easily formed when the base polymer (A) and the thermoplastic elastomer (C) are mixed.

[0081] The thermoplastic elastomer (C) may, for example, have no melting point or have a melting point of less than 165°C. This allows for the easy formation of the phase structure described later when the base polymer (A) and the thermoplastic elastomer (C) are mixed.

[0082] (Content of Modified Polymer (B) and Thermoplastic Elastomer (C)) The content of modified polymer (B) and thermoplastic elastomer (C) will be explained below. Here, the total content of the resin components composed of base polymer (A), modified polymer (B), and thermoplastic elastomer (C) is set to 100 parts by mass. Of the resin components, the remainder other than modified polymer (B) and thermoplastic elastomer (C) is base polymer (A).

[0083] In this embodiment, the content of the modified polymer (B) in the resin composition (i.e., the insulating layer 230) may be, for example, 1 part by mass or more and 10 parts by mass or less. By setting the content of the modified polymer (B) to 1 part by mass or more, a decrease in the insulating properties of the insulating layer 230 can be suppressed. On the other hand, by setting the content of the modified polymer (B) to 10 parts by mass or less, a decrease in the moldability of the insulating layer 230 can be suppressed. This makes it possible to suppress a decrease in the insulating properties of the insulating layer 230 caused by a decrease in moldability.

[0084] The content of thermoplastic elastomer (C) in the resin composition (i.e., the insulating layer 230) may be, for example, 10 parts by mass or more and 45 parts by mass or less. By setting the content of thermoplastic elastomer (C) to 10 parts by mass or more, it is possible to suppress the elasticity of the insulating layer 230 from becoming excessively high and to impart flexibility to the insulating layer 230. On the other hand, by setting the content of thermoplastic elastomer (C) to 45 parts by mass or less, that is, by suppressing the excessive incorporation of amorphous thermoplastic elastomer (C), it is possible to suppress the decrease in insulating properties and mechanical properties that are inherently required of polypropylene having a high melting point as the base polymer (A).

[0085] Furthermore, by keeping the content of the modified polymer (B) and the thermoplastic elastomer (C) within the above-mentioned range, the phase structure described later can be easily formed.

[0086] In this way, by keeping the content of the modified polymer (B) and thermoplastic elastomer (C) within the above-mentioned range, it is possible to stably achieve both the desired distribution of elasticity and uniformity of insulation in the thickness direction of the insulating layer 230.

[0087] (Inorganic Filler) In this embodiment, by including the modified polymer (B) in the resin composition, the space charge trapping effect of the modified polymer (B) can be uniformly obtained in the resin composition, as described above. As a result, high insulation properties can be stably obtained without adding an inorganic filler. Therefore, in this embodiment, the resin composition does not need to contain an inorganic filler.

[0088] On the other hand, the resin composition may contain trace amounts of inorganic fillers.

[0089] Specifically, the resin composition contains an inorganic filler, and the amount of the inorganic filler may be, for example, less than 1 part by mass when the total amount of resin components is 100 parts by mass. The lower limit of the inorganic filler content is not limited as long as an inorganic filler can be added.

[0090] Examples of inorganic fillers include magnesium oxide (MgO), silicon dioxide, zinc oxide, aluminum oxide, titanium oxide, zirconium oxide, carbon black, and mixtures of two or more of these.

[0091] The mean volume diameter (MV) of the inorganic filler is not particularly limited, but may be, for example, 1 μm or less, or 700 nm or less, or 100 nm or less. Here, "mean volume diameter (MV)" refers to the particle size of the particles d ｉ , particle volume V ｉ In this case, it can be calculated using the following formula: MV = Σ(V ｉ d ｉ ) / ΣV ｉ Furthermore, a dynamic light scattering particle size and particle size distribution analyzer is used to measure the volume-average particle size.

[0092] There are no particular limitations on the lower limit of the volume-average particle size of the inorganic filler. However, from the viewpoint of stably forming the inorganic filler, the volume-average particle size of the inorganic filler may be, for example, 1 nm or more, or 5 nm or more.

[0093] At least a portion of the inorganic filler may be surface-treated with a silane coupling agent. This can improve the adhesion of the interface between the inorganic filler and the base polymer (A), thereby improving the mechanical properties and insulating properties of the insulating layer 230.

[0094] (Crosslinking agent) In this embodiment, the resin component constituting the insulating layer 230 may be non-crosslinked from the viewpoint of recycling. In this case, the resin composition may not contain a crosslinking agent.

[0095] On the other hand, the resin composition may contain a small amount of crosslinking agent so as to reduce the gel fraction (degree of crosslinking). Specifically, for example, the resin composition may contain a crosslinking agent in such a quantity that the amount of decomposition residue remaining in the insulating layer 230 due to the decomposition of the crosslinking agent in the insulating layer 230 is less than 300 ppm. When dicumyl peroxide is used as the crosslinking agent, the decomposition residue may be, for example, cumyl alcohol or α-methylstyrene.

[0096] (Other Additives) The resin composition may contain other additives as needed. Other additives include antioxidants, lubricants, and colorants.

[0097] (3) Characteristics of the insulating layer In this embodiment, the insulating layer 230 formed by the insulating tape containing the above-described resin composition does not have voids due to fusion by the second heating step S244 described later. The insulating layer 230 of this embodiment has the following characteristics.

[0098] (3-1) Phase Structure In this embodiment, the insulating layer 230 has a predetermined phase structure by mixing a base polymer (A), a modified polymer (B), and a thermoplastic elastomer (C).

[0099] Specifically, the inclusion of the modified polymer (B) in the insulating layer 230 creates a structure in which the base polymer (A) and the modified polymer (B) are compatible, or a sea-island structure in which the modified polymer (B) is finely dispersed within the base polymer (A). Here, "compatible structure" means, for example, that when the phase structure is observed with a transmission electron microscope, no phase separation can be confirmed, and each component is uniformly dispersed.

[0100] The more finely the modified polymer (B) is dispersed in the insulating layer 230, the more effectively the accumulation of space charge can be suppressed. From this viewpoint, a structure in which the resin components are compatible may be formed in the insulating layer 230. Alternatively, if a sea-island structure is formed in the insulating layer 230, the diameter of the island phase formed from the modified polymer (B) may be less than 0.5 μm.

[0101] Furthermore, by including not only the base polymer (A) and the modified polymer (B) but also the thermoplastic elastomer (C) in the insulating layer 230, a sea-island structure is formed in which the thermoplastic elastomer (C) is finely dispersed within the two-component phase structure of the insulating layer 230, which includes the base polymer (A) and the modified polymer (B).

[0102] By forming the above-described phase structure in the insulating layer 230, the polar groups in the modified polymer (B) can be uniformly dispersed within the insulating layer 230. This allows for a uniform trapping effect of space charge within the insulating layer 230.

[0103] Furthermore, by forming the above-described phase structure in the insulating layer 230, the thermoplastic elastomer (C) can be uniformly dispersed in the insulating layer 230. This makes it possible to stably obtain an elastic distribution of the insulating layer 230 according to the difference in cooling rate in the thickness direction of the insulating layer 230.

[0104] (Non-crosslinked or slightly crosslinked) In this embodiment, the insulating layer 230 is non-crosslinked or slightly crosslinked. Even if the insulating layer 230 is slightly crosslinked, the gel fraction (degree of crosslinking) is low. In this case, the insulating layer 230 is slightly crosslinked, for example, by having less than 300 ppm of decomposition residue remaining in the insulating layer 230 due to the decomposed crosslinking agent. In this way, the recyclability can be improved by making the insulating layer 230 non-crosslinked or slightly crosslinked.

[0105] (3-3) Characteristics In this embodiment, the insulating layer 230 has the following characteristics.

[0106] In the following, "outer sample" refers to a sheet taken from a position 0.3 mm from the outer surface of the insulating layer 230 in the radial direction of the conductor connection portion 210 toward the inner semiconducting layer 220. "Inner sample" refers to a sheet taken from a position 0.3 mm from the inner surface of the insulating layer 230 in the radial direction of the conductor connection portion 210 toward the outer surface of the insulating layer 230. Here, "radial direction of the conductor connection portion 210" refers to the direction from the central axis of the conductor connection portion 210 toward the outer circumference.

[0107] (Elasticity) In this embodiment, by performing the second heating step S244 which has been improved as a new manufacturing method described later, the elasticity of the insulating layer 230 differs in the thickness direction of the insulating layer 230.

[0108] Specifically, the ratio of the storage modulus of the inner sample of the insulating layer 230 to the storage modulus of the outer sample of the insulating layer 230 (hereinafter also referred to as the "modulus ratio") is, for example, 1.1 or more and 2.5 or less.

[0109] The "storage modulus" referred to here is measured by dynamic mechanical analysis (DMA) in accordance with JIS K7244-4:1999.

[0110] The dynamic viscoelasticity measurement is performed under the following conditions: Measurement mode: Tensile mode Strain: 0.08% Frequency: 10 Hz Temperature range: 0°C to 200°C Heating rate: 10°C / min

[0111] The "storage modulus of the outer sample" and "storage modulus of the inner sample" in the above-mentioned specification of the elastic modulus ratio are values ​​measured at 25°C by the dynamic viscoelasticity measurement. In the dynamic viscoelasticity measurement, the thickness of each sample is 0.5 mm.

[0112] In this embodiment, by setting the elastic modulus ratio to 1.1 or higher, the outer side of the insulating layer 230 can be made relatively soft while the inner side of the insulating layer 230 is made relatively hard. As a result, in this embodiment, both the flexibility and strength of the insulating layer 230 can be achieved.

[0113] On the other hand, in this embodiment, by setting the elastic modulus ratio to 2.5 or less, it is possible to suppress an excessive decrease in elasticity caused by thermal degradation on the outside of the insulating layer 230. This makes it possible to suppress the occurrence of cracks in the insulating layer 230 caused by impacts during the installation of the connected power cable 10. Furthermore, by setting the elastic modulus ratio to 2.5 or less, it is possible to suppress the occurrence of voids caused by stress differences within the insulating layer 230. This makes it possible to suppress a decrease in the dielectric breakdown strength (e.g., DC breakdown field strength) of the insulating layer 230.

[0114] The elastic modulus of the insulating layer 230 may gradually decrease from the inner circumferential surface of the insulating layer 230 toward the outer circumferential surface of the insulating layer 230 in the radial direction toward the conductor connection portion 210. By suppressing abrupt changes in the elasticity of the insulating layer 230 in this way, the occurrence of cracks within the insulating layer 230 caused by differences in elastic modulus can be suppressed.

[0115] The storage modulus of the inner sample of the insulating layer 230 is, for example, 650 MPa or more and 900 MPa or less. By setting the storage modulus of the inner sample to 650 MPa or more, the rigidity of the cable connection structure 20 can be ensured. On the other hand, by setting the storage modulus of the inner sample to 900 MPa or less, a decrease in the flexibility of the cable connection structure 20 can be suppressed.

[0116] The storage modulus of the outer sample of the insulating layer 230 is, for example, 280 MPa or more and 670 MPa or less. By setting the storage modulus of the outer sample to 280 MPa or more, it is possible to suppress the outer surface of the insulating layer 230 from becoming excessively soft due to thermal degradation on the outside of the insulating layer 230. This makes it possible to suppress the occurrence of cracks in the insulating layer 230 caused by impacts during the laying of the connected power cable 10 equipped with the cable connection structure 20. On the other hand, by setting the storage modulus of the outer sample to 670 MPa or less, the flexibility of the cable connection structure 20 can be ensured.

[0117] (Insulation) In this embodiment, the insulating layer 230 contains a modified polymer (B), which allows for a uniform trapping effect of space charge by the modified polymer (B) within the insulating layer 230. As a result, the insulating properties of the insulating layer 230 are uniform in the thickness direction of the insulating layer 230.

[0118] Specifically, the ratio of the volume resistivity of the inner sample of the insulating layer 230 to the volume resistivity of the outer sample of the insulating layer 230 (hereinafter, also referred to as "volume resistivity ratio") is, for example, 1.0 or more and 1.5 or less.

[0119] The "volume resistivity of the outer sample" and the "volume resistivity of the inner sample" referred to here are measured under the conditions of a temperature of 90°C and a DC electric field of 80 kV / mm.

[0120] In the present embodiment, by setting the volume resistivity ratio to 1.0 or more and 1.5 or less, sufficient insulation can be obtained for the entire insulating layer 230 of the cable connection structure 20.

[0121] The volume resistivity of the outer sample of the insulating layer 230 and the volume resistivity of the inner sample of the insulating layer 230 measured under the conditions of a temperature of 90°C and a DC electric field of 80 kV / mm are, for example, 7.0×10 １４ Ω·cm or more, or may be 3.6×10 １５ Ω·cm or more, or may be 5.0×10 １５ Ω·cm or more, or may be 7.0×10 １５ Ω·cm or more. In the measurement of the volume resistivity, the thickness of each sample is 0.2 mm.

[0122] The DC breakdown electric field strength of the outer sample of the insulating layer 230 and the DC breakdown electric field strength of the inner sample of the insulating layer 230 measured under the condition of a temperature of 90°C are, for example, 160 kV / mm or more, or may be 200 kV / mm or more. In the measurement of the DC breakdown electric field strength, the thickness of each sample is 0.2 mm.

[0123] (4) Method for manufacturing a cable connection structure, method for manufacturing a connected power cable Next, referring to FIGS. 2 and 3, the method for manufacturing the connected power cable 10 of the present embodiment will be described. The method for manufacturing the connected power cable 10 includes the method for manufacturing the cable connection structure 20. Hereinafter, the steps are abbreviated as "S".

[0124] In the manufacturing method of the connected power cable 10 of this embodiment, details of steps not described in this disclosure can be carried out as a general manufacturing method for FJ, for example, as described in Japanese Patent Application Publication No. 2023-065809.

[0125] As shown in Figure 2, the manufacturing method of the connected power cable 10 in this embodiment includes, for example, a preparation step S100 and a cable connection step S200.

[0126] (S100: Preparation process) First, prepare several power cables 100. Strip each power cable 100 in stages, starting from the tip of the conductor 110 and moving in the opposite direction.

[0127] Once the stripping of each section of the pair of power cables 100 is complete, the first power cable 100a is inserted, for example, into the semiconducting tube, metal pipe 250 that constitute the outer semiconducting layer 240, and the tube that constitutes the corrosion-resistant layer 260.

[0128] Furthermore, in this embodiment, the first power cable 100a is pre-inserted through the coil 540 used in the second heating step S244, which will be described later.

[0129] (S200: Cable connection process) Once the preparation process S100 is completed, the cable connection process S200 is performed. The cable connection process S200 in this embodiment includes, for example, a conductor connection process S210, an internal semiconducting layer formation process S220, a first heating process S224, an insulating layer formation process S230, an external semiconducting layer formation process S240, a second heating process S244, and a post-processing step S250.

[0130] (S210: Conductor connection process) A conductor connection portion 210 is formed by connecting the respective conductors 110 of a pair of power cables 100.

[0131] (S220: Internal semiconducting layer formation process) After the conductor connection process S210, a semiconducting tape is wrapped around the outer circumference of the conductor connection portion 210 to form an internal semiconducting layer 220 having semiconductivity.

[0132] (S224: First heating step) After the internal semiconducting layer formation step S220, the internal semiconducting layer 220 is heated. In the first heating step S224, the heating device 50 used in the second heating step S244 described later may be used. By performing the first heating step S224, the multiple layers of semiconducting tape wrapped around the internal semiconducting layer 220 are fused together.

[0133] (S230: Insulating layer formation process) After the first heating process S224, an insulating layer 230 is formed to cover the outer periphery of the internal semiconducting layer 220, the cable internal semiconducting layer 120, and the cable insulating layer 130.

[0134] The insulating layer forming step S230 of this embodiment includes, for example, a tape preparation step S232 and a tape winding step S234.

[0135] (S232: Tape preparation step) First, an insulating tape containing the resin composition of this embodiment is prepared. Note that the tape preparation step S232 may be performed in the preparation step S100.

[0136] In this embodiment, a resin component comprising a base polymer (A), a modified polymer (B), and a thermoplastic elastomer (C), along with other additives (such as antioxidants), is mixed (kneaded) in a mixer to form a mixed material. Examples of mixers include open roll mixers, barbell mixers, pressure kneaders, single-screw mixers, and multi-screw mixers.

[0137] Once the mixture is formed, the mixture is extruded into a tape shape using an extruder equipped with a T-die having a slit-shaped discharge port. This forms an insulating tape containing the resin composition that constitutes the insulating layer 230.

[0138] (S234: Tape wrapping process) Once the tape preparation process S232 is completed, insulating tape is wrapped around the outer circumference of the internal semiconducting layer 220 and the outer circumference of the exposed portion of the cable insulation layer 130. This forms the insulation layer 230. The insulation layer 230 has an inner circumferential surface facing the internal semiconducting layer 220 and an outer circumferential surface opposite to the inner circumferential surface.

[0139] (S240: External semiconducting layer formation process) After the insulating layer formation process S230, an external semiconducting layer 240 is formed so as to cover the outer periphery of the insulating layer 230.

[0140] Specifically, a semiconductive tube, which has been pre-threaded onto the first power cable 100a, is placed over the outer circumference of the cable insulation layer 130. After placing the semiconductive tube, it is heat-shrinked. This forms the outer semiconductive layer 240.

[0141] The outer semiconducting layer 240 may also be formed by wrapping a semiconducting tape around the outer circumference of the insulating layer 230.

[0142] As a result, a cable core is formed in the cable connection structure 20, having a conductor connection portion 210, an internal semiconducting layer 220, an insulating layer 230, and an external semiconducting layer 240.

[0143] (S244: Second heating step) After the external semiconducting layer formation step S240, the insulating layer 230 and the external semiconducting layer 240 are heated and then cooled.

[0144] In this embodiment, the insulating layer 230 is heated and then cooled such that the ratio of the storage modulus of the inner sample of the insulating layer 230 to the storage modulus of the outer sample of the insulating layer 230 is 1.1 or more and 2.5 or less, and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample is 1.0 or more and 1.5 or less.

[0145] Specifically, in the second heating step S244 of this embodiment, for example, the heating device 50 shown in Figure 3 is used.

[0146] The heating device 50 includes, for example, a heating furnace (molding furnace) 510, a heater 520, a gas supply line 530, and a coil 540. The heating furnace 510 is configured as a cylindrical body having a hollow section into which a cable core is inserted. The heater 520 is provided in the heating furnace 510 and is configured to heat the cable core inside the hollow section of the heating furnace 510. The gas supply line 530 is provided inside the hollow section of the heating furnace 510, along the axial direction of the heating furnace 510. The gas supply line 530 is configured to seal a gas such as nitrogen gas or air inside the hollow section of the heating furnace 510 and to pressurize the inside of the hollow section of the heating furnace 510. The coil 540 has the first power cable 100a inserted in advance and is moved to the vicinity of the conductor connection section 210 in the second heating step S244. The coil 540 is wound spirally around the outer circumference near the conductor connection section 210. The coil 540 is configured to allow alternating current to flow through it.

[0147] In the second heating step S244, the outer circumference of the cable core is first covered with a group of retaining tubes. The cable core in this state is then set inside the heating device 50.

[0148] Once the cable core is set in the heating device 50, nitrogen gas or air or other gas is supplied from the gas supply line 530 to the cable core in the hollow part of the heating furnace 510, pressurizing it while the cable core is heated by the heater 520. The insulating layer 230 is heated by the heater 520 starting from the area closest to the outer surface of the insulating layer 230.

[0149] Furthermore, in this embodiment, the conductor connection portion 210 is electromagnetically induced by passing an alternating current through the coil 540 while heating the insulating layer 230 from a region close to the outer surface of the insulating layer 230 with the heater 520 described above.

[0150] Here, "electromagnetic induction heating" refers to heating the conductor connection portion 210 by the heat generated due to eddy current loss and hysteresis loss in the conductor connection portion 210 caused by electromagnetic induction. Electromagnetic induction heating allows for non-contact heating of the conductor connection portion 210 of the cable core. This electromagnetic induction heating heats not only the conductor connection portion 210 but also the conductors 110 surrounding the conductor connection portion 210.

[0151] As described above, electromagnetic induction heating of the conductor connection portion 210 allows the insulating layer 230 to be heated not only in the region near the outer surface of the insulating layer 230, but also in the region near the inner surface of the insulating layer 230. This makes it possible to stably heat the inside of the insulating layer 230 without having to excessively raise the temperature of the region near the outer surface of the insulating layer 230 using the heater 520. By not raising the heating temperature of the heater 520 excessively, excessive heating of the outside of the insulating layer 230 is suppressed, and thermal degradation of the outside of the insulating layer 230 can be suppressed. Furthermore, by stably heating the inside of the insulating layer 230 using electromagnetic induction heating of the conductor connection portion 210, the insulating tape inside the insulating layer 230 can be stably fused.

[0152] As described above, by performing electromagnetic induction heating of the conductor connection portion 210, in principle, the temperature difference between the outside and inside of the insulating layer 230 can be reduced.

[0153] However, during the heating in the second heating step S244 of this embodiment, a temperature difference is deliberately created between the outside and inside of the insulating layer 230, within a temperature range that suppresses thermal degradation of the outside of the insulating layer 230.

[0154] Specifically, during the heating in the second heating step S244 of this embodiment, the temperature at a position 0.3 mm from the outer surface of the insulating layer 230 toward the inner semiconducting layer 220, i.e., the sampling position of the outer sample of the insulating layer 230, is set to, for example, 220°C or higher and 285°C or lower.

[0155] By setting the temperature at the sampling location of the outer sample of the insulating layer 230 to 220°C or higher, the cooling rate on the outside of the insulating layer 230 can be increased during the cooling process described later. In other words, the outside of the insulating layer 230 can be rapidly cooled. This suppresses excessive crystallization on the outside of the insulating layer 230. As a result, the elasticity of the outside of the insulating layer 230 can be reduced.

[0156] On the other hand, by keeping the temperature at the sampling location of the outer sample of the insulating layer 230 below 285°C, thermal degradation of the outer surface of the insulating layer 230 can be suppressed. This prevents excessive reduction in elasticity and excessive reduction in insulating properties on the outer surface of the insulating layer 230. As a result, the insulating properties and mechanical properties required for the cable connection structure 20 can be obtained.

[0157] During the heating in the second heating step S244 of this embodiment, the temperature at a position 0.3 mm from the inner circumferential surface to the outer circumferential surface of the insulating layer 230, that is, at the sampling position of the inner sample of the insulating layer 230, is set to, for example, more than 170°C and 190°C or less.

[0158] By setting the temperature at the sampling location inside the insulating layer 230 to 170°C or higher, the temperature inside the insulating layer 230 can be set to be above the melting point of the base polymer (A). This allows for stable fusion of the insulating tape inside the insulating layer 230.

[0159] On the other hand, by keeping the temperature at the sampling location of the inner sample of the insulating layer 230 below 190°C, a temperature difference can be created between the outer and inner surfaces of the insulating layer 230. This allows the outer surface of the insulating layer 230 to be rapidly cooled, while the inner surface of the insulating layer 230 can be slowly cooled during the cooling process described later. Slow cooling of the inner surface of the insulating layer 230 allows the base polymer (A) inside the insulating layer 230 to crystallize. As a result, the elasticity of the inner surface of the insulating layer 230 can be made higher than that of the outer surface.

[0160] Furthermore, during the heating in the second heating step S244 of this embodiment, the temperature difference obtained by subtracting the temperature at the sampling location of the outer sample of the insulating layer 230 from the temperature at the sampling location of the outer sample of the insulating layer 230 may be, for example, 40°C or more and 105°C or less. This allows the outside of the insulating layer 230 to be rapidly cooled while the inside of the insulating layer 230 is slowly cooled during the cooling process described later. As a result, the elastic modulus ratio of the insulating layer 230 can be set to 1.1 or more and 2.5 or less, and the volume resistivity ratio of the insulating layer 230 can be set to 1.0 or more and 1.5 or less.

[0161] In this embodiment, the heating time of the second heating step S244 is not particularly limited, but is set to fuse the multiple layers of insulating tape wrapped around the insulating layer 230 together. Specifically, the heating time of the second heating step S244 may be, for example, 1 hour or more and 10 hours or less, or 2 hours or more and 5 hours or less. By heating for 1 hour or more, or 2 hours or more, the insulating tape can be stably fused together. On the other hand, by heating for 10 hours or less, or 5 hours or less, thermal degradation of the insulating layer 230 can be suppressed.

[0162] As a result of the heating in the second heating step S244 described above, the internal semiconducting layer 220, the insulating layer 230, and the external semiconducting layer 240 fuse together and become one unit.

[0163] After heating as described above, the coil 540 is cut and removed, and the cable core is taken out of the heating device 50. Next, the group of retaining tubes that covered the cable core is removed. This allows the cable core to cool naturally, for example, in the atmosphere. Alternatively, the cable core may be cooled using a predetermined cooling means to achieve the cooling rate described later.

[0164] At this time, as described above, during the heating in the second heating step S244, a temperature difference is deliberately created between the outside and inside of the insulating layer 230 within a temperature range that suppresses thermal degradation of the outside of the insulating layer 230. This allows the outside of the insulating layer 230 to be rapidly cooled while the inside of the insulating layer 230 is cooled slowly during cooling.

[0165] Specifically, at the sampling location of the outer sample of the insulating layer 230, the cooling rate (cooling rate) at a temperature of 110°C may be set to, for example, 30°C / min or more, or 75°C / min or more, or 100°C / min or more. This suppresses excessive crystallization on the outside of the insulating layer 230. As a result, the elasticity of the outer surface of the insulating layer 230 can be reduced. The "cooling rate at a temperature of 110°C" here is determined by the absolute value of the temperature gradient with respect to time at a temperature of 110°C.

[0166] At this time, the cooling rate at the sampling position of the outer sample of the insulating layer 230 at a temperature of 110°C may be set to, for example, 310°C / min or less. This makes it possible to suppress excessive variation in the degree of crystallinity in the thickness direction of the insulating layer 230. As a result, it is possible to suppress the occurrence of excessive differences in elasticity in the thickness direction of the insulating layer 230.

[0167] On the other hand, the cooling rate (cooling rate) at the sampling location of the inner sample of the insulating layer 230 is not limited as long as it is slower than the cooling rate at the sampling location of the outer sample of the insulating layer 230. However, the cooling rate at the sampling location of the inner sample of the insulating layer 230 at a temperature of 110°C may be, for example, 1°C / min or more and 10°C / min or less, or 2°C / min or more and 8°C / min or less.

[0168] By cooling after heating in the second heating step S244 described above, the elastic modulus ratio of the insulating layer 230 can be set to 1.1 or more and 2.5 or less, and the volume resistivity ratio of the insulating layer 230 can be set to 1.0 or more and 1.5 or less.

[0169] (S250: Subsequent process) Once the second heating process S244 is completed, the water-absorbing tape layer 242, the metal tube 250, and the corrosion-resistant layer 260 are formed in this order to cover the outer circumference of the outer semiconductive layer 240. After that, a cover portion 270 is formed on the axial end of the corrosion-resistant layer 260.

[0170] Based on the above, the connected power cable 10 of this embodiment is manufactured.

[0171] (5) Summary of this embodiment This embodiment provides one or more of the following effects.

[0172] (a) In this embodiment, during the heating in the second heating step S244, a temperature difference is created between the outside and inside of the insulating layer 230 within a temperature range that suppresses thermal degradation of the outside of the insulating layer 230. This makes it possible to rapidly cool the outside of the insulating layer 230 while slowly cooling the inside of the insulating layer 230 during subsequent cooling.

[0173] Due to this slow cooling inside the insulating layer 230 and rapid cooling outside the insulating layer 230, a predetermined amount of base polymer (A) crystallizes inside the insulating layer 230, while the resin component solidifies in a near-amorphous state outside the insulating layer 230. This makes it possible to make the elasticity of the insulating layer 230 different in the thickness direction of the insulating layer 230.

[0174] On the other hand, by including a predetermined amount of modified polymer (B) in the insulating layer 230, the polar groups in the modified polymer (B) can be uniformly dispersed within the insulating layer 230. This allows for a uniform trapping effect of space charge by the modified polymer (B) within the insulating layer 230. In other words, even if a distribution of crystallinity of the insulating layer 230 occurs in the thickness direction of the insulating layer 230 as described above, variations in insulating properties caused by the distribution of crystallinity can be suppressed. As a result, the insulating properties of the insulating layer 230 can be made uniform in the thickness direction of the insulating layer 230.

[0175] As described above, according to this embodiment, it is possible to obtain a desired distribution of elasticity in the thickness direction of the insulating layer 230 while making the insulating properties of the insulating layer 230 uniform in the thickness direction of the insulating layer 230. As a result, it is possible to obtain a connected power cable 10 equipped with a cable connection structure 20 that improves the flexibility, strength, and insulating properties of the insulating layer 230.

[0176] (b) In this embodiment, the ratio of the storage modulus of the inner sample of the insulating layer 230 to the storage modulus of the outer sample of the insulating layer 230 can be 1.1 or more and 2.5 or less.

[0177] By setting the elastic modulus ratio to 1.1 or higher, the outer surface of the insulating layer 230 can be made relatively soft while the inner surface of the insulating layer 230 is made relatively hard. By making the outer surface of the insulating layer 230 relatively soft, the flexibility of the cable connection structure 20 and the connected power cable 10 can be improved. For example, excessive constraints on the reel diameter when winding the connected power cable 10 onto a reel can be avoided. Furthermore, by making the inner surface of the insulating layer 230 relatively hard, the rigidity of the cable connection structure 20 and the connected power cable 10 can be ensured. In this way, it is possible to achieve both the flexibility and strength of the insulating layer 230.

[0178] On the other hand, by setting the elastic modulus ratio to 2.5 or less, it is possible to suppress an excessive decrease in elasticity caused by thermal degradation on the outside of the insulating layer 230. This makes it possible to suppress the occurrence of cracks in the insulating layer 230 caused by impacts during the laying of the connected power cables 10. Furthermore, by setting the elastic modulus ratio to 2.5 or less, it is possible to suppress the occurrence of voids caused by stress differences within the insulating layer 230. This makes it possible to suppress a decrease in the dielectric breakdown strength (e.g., DC breakdown field strength) of the insulating layer 230.

[0179] (c) In this embodiment, by uniformly dispersing polar groups in the modified polymer (B) that traps space charge in the insulating layer 230 in a predetermined content, the ratio of the volume resistivity of the inner sample of the insulating layer 230 to the volume resistivity of the outer sample of the insulating layer 230 can be set to 1.0 or more and 1.5 or less. For example, even if the degree of crystallinity on the outside of the insulating layer 230 becomes relatively low due to rapid cooling of the outside of the insulating layer 230 during cooling after the temperature difference occurs in the second heating step S244, the decrease in insulating performance on the outside of the insulating layer 230 can be suppressed. As a result, sufficient insulating performance required for the insulating layer 230 of the cable connection structure 20 as a whole can be obtained.

[0180] (d) In this embodiment, even when the thickness of the insulating layer 230 is 3 mm or more, it is possible to obtain a desired distribution of elasticity in the thickness direction of the insulating layer 230 while making the insulating properties of the insulating layer 230 uniform in the thickness direction of the insulating layer 230. Therefore, even if the connected power cable 10 equipped with the cable connection structure 20 of this embodiment is applied to high-voltage applications, it is possible to stably achieve both the flexibility and insulating properties of the insulating layer 230.

[0181] <Other Embodiments of the Disclosure> Although embodiments of the Disclosure have been described in detail above, the Disclosure is not limited to the embodiments described above and can be modified in various ways without departing from its essence.

[0182] In the above-described embodiment, the case in which the connecting power cable 10 is laid on the seabed was explained, but the connecting power cable 10 may also be configured to be laid on land or elsewhere.

[0183] Next, embodiments relating to this disclosure will be described. These embodiments are examples of this disclosure and the disclosure is not limited to these embodiments.

[0184] (1) Experiment 1 (1-1) Manufacturing of cable connection structures Cable connection structures A5, B6 and B7 were manufactured from samples B1 and A2 as follows.

[0185] <Samples A2 to A5> A pair of power cables were prepared, each having a conductor, an internal semiconducting layer, a cable insulation layer, and an external semiconducting layer. The cross-sectional area of ​​the conductor was 325 mm². ２ The cable insulation layer's base polymer was random PP. The thicknesses of the cable's internal semiconducting layer, cable insulation layer, and cable's external semiconducting layer were set to 0.5 mm, 9 mm, and 0.5 mm, respectively. Each power cable was stripped in stages from the conductor's end toward the opposite side.

[0186] After the preparation process, a conductor connection was formed by connecting the conductors of a pair of power cables. After the conductor connection process, an internal semiconducting layer was formed by wrapping a semiconducting tape containing random PP around the outer circumference of the conductor connection. The thickness of the internal semiconducting layer was set to 0.5 mm.

[0187] After the internal semiconducting layer formation process, the internal semiconducting layer was heated, causing the multiple layers of semiconducting tape to fuse together within the internal semiconducting layer.

[0188] Next, an insulating layer formation process was carried out. An insulating tape containing a resin composition comprising the following materials was prepared.

[0189] (Base polymer (A)) Random polypropylene (r-PP): Stereoregularity: isotactic, Density: 0.9 g / ml, Melting point: 150°C, Heat of fusion: 60 J / g, Storage modulus of element at 25°C measured by dynamic viscoelasticity measurement: 1150 MPa, Content in resin component: 70 parts by mass

[0190] (Modified Polymer (B)) Maleic anhydride-modified polypropylene (MAH-PP): Amount of maleic anhydride modification: 5% by mass, Melting point: 160°C, Storage modulus of the element at 25°C measured by dynamic viscoelasticity measurement: 1300 MPa, Content in resin components: 5 parts by mass

[0191] (Thermoplastic Elastomer (C)) Styrene-ethylene-butylene-styrene block copolymer (SEBS): Styrene unit content: 25% by mass, Melting point: none, Storage modulus of the element at 25°C measured by dynamic viscoelasticity measurement: 120 MPa, Content in resin component: 25 parts by mass

[0192] After the tape preparation process, an insulating layer was formed by wrapping insulating tape around the outer periphery of the internal semiconducting layer and the outer periphery of the exposed portion of the cable insulation layer. The thickness of the insulating layer was set to 9 mm.

[0193] After the insulating layer formation process, an outer semiconducting layer was formed by covering the outer periphery of the insulating layer with a semiconducting tube containing random PP. The thickness of the outer semiconducting layer was set to 0.5 mm.

[0194] After the external semiconducting layer formation process, a second heating process was performed as follows.

[0195] In the second heating step, thermocouples were placed (inserted) at a position 0.3 mm from the outer surface of the insulating layer toward the inner semiconducting layer (i.e., the sampling position for the outer sample) and at a position 0.3 mm from the outer surface of the insulating layer toward the inner semiconducting layer (i.e., the sampling position for the inner sample).

[0196] Next, with the outer circumference of the cable core covered by a group of retaining tubes, the insulating layer and the outer semiconducting layer were heated using the aforementioned heating device under the following conditions.

[0197] Electromagnetic induction heating of conductor connection: Temperature at a position 0.3 mm from the outer surface of the insulating layer toward the inner semiconducting layer (hereinafter also referred to as "outer temperature"): 220°C or higher and 285°C or lower. Temperature at a position 0.3 mm from the inner surface of the insulating layer toward the outer surface (hereinafter also referred to as "inner temperature"): 180°C. Heating time: 4 hours.

[0198] The external and internal temperatures of each sample during the second heating process are shown in Table 1. The heating by the heater of the heating device and the electromagnetic induction heating of the conductor connection by passing AC current through the coil were adjusted to obtain the external and internal temperatures of each sample shown in Table 1.

[0199] After the heating described above, the coil was cut and removed, and the cable core was taken out of the heating device. Next, the retaining tubes that had been covering the cable core were removed. This allowed the cable core to cool naturally in the atmosphere.

[0200] Based on the above, the cable connection structures for samples A2 to A5 were manufactured.

[0201] (Sample B1) The cable connection structure of Sample B1 was manufactured in the same manner as Sample A3, except that the external temperature was set to 180°C during the second heating step.

[0202] (Sample B6) The cable connection structure of Sample B6 was manufactured in the same manner as Sample A3, except that the external temperature was set to 290°C during the second heating step.

[0203] (Sample B7) The cable connection structure of Sample B7 was manufactured in the same manner as Sample A3, except that induction heating was not performed during the second heating step and the external temperature was set to 300°C.

[0204] (1-2) The following evaluations were performed on the cable connection structures of evaluation samples B1, A2 to A5, B6 and B7.

[0205] (Temperature transition) The temperature transition of the second heating process was measured using the thermocouples described above on the outside and inside of the insulating layer of each sample.

[0206] (Sampling after the second heating process) The insulating layer of the cable connection structure of each sample was thinly sliced ​​along the circumferential direction. Here, "circumferential direction" refers to the direction along the outer circumference of the insulating layer. Using the above slicing method, an outer sample of the insulating layer was taken from a position 0.3 mm from the outer surface of the insulating layer toward the conductor. An inner sample of the insulating layer was taken from a position 0.3 mm from the inner surface of the insulating layer toward the outer surface. The size of each sample for dynamic viscoelasticity measurement was 5 mm in length, 42 mm in width, and 0.5 mm in thickness. The size of each sample for volume resistivity and DC breakdown field strength measurement was 50 mm in length, 50 mm in width, and 0.2 mm in thickness.

[0207] (Storage Modulus) The storage modulus of the insulating layer in each sample was measured by dynamic viscoelasticity measurement (DMA) in accordance with JIS K7244-4:1999 under the following conditions: Measurement device: DVA-200 manufactured by IT Measurement Control Co., Ltd. Measurement mode: Tensile mode Strain: 0.08% Frequency: 10 Hz Temperature range: 0°C to 200°C Heating rate: 10°C / min

[0208] At this time, the storage modulus of the outer sample and the modulus of elasticity of the inner sample were measured at 25°C using the DMA described above.

[0209] (Volume Resistivity) The volume resistivity of the insulating layer in each sample was measured as follows. First, a 25 mm diameter flat electrode was placed on the first surface of the sheet, which served as either the outer or inner sample, and on the second surface opposite the first surface. After placing the electrodes, the sheet was immersed in silicone oil at a temperature of 90°C. In this state, a DC electric field of 80 kV / mm was applied to the sheet. The volume resistivity was then measured.

[0210] (DC breakdown field strength) The DC breakdown field strength of the insulating layer in each sample was measured as follows. Specifically, similar to the measurement of volume resistivity, a 25 mm diameter flat electrode was placed on the first surface of the sheet (either the outer or inner sample) and on the second surface opposite to the first surface. After placing the electrodes, the sheet was immersed in silicone oil at a temperature of 90°C. In this state, the applied voltage was increased at a rate of 4 kV / min. Subsequently, the DC breakdown field strength of the sheet was measured when dielectric breakdown occurred. A score of A (good) was given if the DC breakdown field strength was 160 kV / mm or higher in both the outer and inner samples. On the other hand, a score of B (poor) was given if the DC breakdown field strength was less than 160 kV / mm in at least one of the outer or inner samples.

[0211] (1-3) Results: Refer to Figures 4 and 5 and Table 1 below to explain the results of the evaluation of each sample.

[0212]

[0213] <Comparison of Temperature Changes> Refer to Table 1, Figures 4 and 5, and compare the temperature changes of Sample B1 and Sample A3 as examples. In Figures 4 and 5, the start of cooling is defined as "0 s (0 seconds)".

[0214] (Sample B1) As shown in Table 1 and Figure 5, during the second heating step of Sample B1, the heating by the heater of the heating device and the electromagnetic induction heating of the conductor connection part were adjusted to set both the outer and inner temperatures of the insulating layer to 180°C. As a result, during the cooling after heating in the second heating step of Sample B1, the temperature decreased slowly at both the outer and inner temperatures of the insulating layer. Thus, in Sample B1, the rate of cooling of the base polymer on the outside of the insulating layer and the rate of cooling of the base polymer on the inside of the insulating layer were close. Specifically, the cooling rate at 110°C at the sampling location of the outer sample of the insulating layer was approximately 8°C / min, and the cooling rate at 110°C at the sampling location of the inner sample of the insulating layer was approximately 4°C / min.

[0215] (Sample A3) In contrast, as shown in Table 1 and Figure 4, during the second heating step of Sample A3, the heating by the heater of the heating device and the electromagnetic induction heating of the conductor connection part were adjusted to set the temperature outside the insulating layer to 250°C and the temperature inside the insulating layer to 180°C. This resulted in a temperature difference of 70°C between the outside and inside of the insulating layer.

[0216] During the cooling process after heating in the second heating step of sample A3, the temperature inside the insulating layer decreased gradually, while the temperature outside the insulating layer decreased rapidly. As a result, the rate at which the base polymer cooled outside the insulating layer was higher than the rate at which it cooled inside the insulating layer. Specifically, the cooling rate at the sampling location of the sample inside the insulating layer at 110°C was approximately 4°C / min, while the cooling rate at the sampling location of the sample outside the insulating layer at 110°C was approximately 31°C / min.

[0217] <Comparison of each characteristic> (Sample B1) During the second heating step of Sample B1, induction heating was performed as described above, and both the outer and inner temperatures of the insulating layer were 180°C. Therefore, during the cooling after heating in the second heating step of Sample B1, both the outer and inner surfaces of the insulating layer were cooled slowly.

[0218] Therefore, in sample B1, the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was close to 1. However, in sample B1, the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was also close to 1.

[0219] In sample B1, the degree of crystallinity of the insulating layer was uniformly high in the thickness direction due to slow cooling both on the outer and inner sides of the insulating layer. Therefore, it is considered that the elasticity of the insulating layer in sample B1 was uniformly high in the thickness direction.

[0220] (Samples B6 and B7) During the second heating step for samples B6 and B7, the internal temperature of the insulating layer was 180°C, but the external temperature of the insulating layer was higher than 285°C.

[0221] Therefore, in samples B6 and B7, the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was greater than 2.5. In samples B6 and B7, the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was greater than 1.5. Furthermore, in samples B6 and B7, the DC breakdown field strength was less than 160 kV / mm.

[0222] In samples B6 and B7, the outer surface of the insulating layer underwent thermal degradation due to excessively high temperatures outside the insulating layer. Therefore, it is believed that in samples B6 and B7, both the elasticity and insulating properties of the outer surface of the insulating layer were excessively reduced.

[0223] (Samples A2 to A5) In contrast, for samples A2 to A5, the outer temperature of the insulating layer was set to 220°C to 285°C, and the inner temperature of the insulating layer was set to 180°C. As a result, during the cooling after heating in the second heating step for samples A2 to A5, the inside of the insulating layer was cooled slowly, while the outside of the insulating layer was rapidly cooled.

[0224] As a result, in samples A2 to A5, the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was between 1.1 and 2.5. In samples A2 to A5, the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was between 1.0 and 1.5. In samples A2 to A5, the DC breakdown field strength was 160 kV / mm or higher.

[0225] From the results of samples A2 to A5 above, it was confirmed that a cable connection structure can be obtained in which the desired distribution of elasticity in the thickness direction of the insulating layer is achieved while uniform insulating properties in the thickness direction of the insulating layer.

[0226] (2) Experiment 2 (2-1) Manufacturing of Cable Connection Structures Cable connection structures of samples A6 to A10 were manufactured. During the second heating step of samples A6 to A10, the inner and outer temperatures of the insulating layer were the same as those of sample A3.

[0227] (Sample A6) The cable connection structure of Sample A6 was manufactured in the same manner as Sample A3, except that the content of r-PP, MAH-PP, and SEBS in the resin component of the insulating layer was 70 parts by mass, 1 part by mass, and 29 parts by mass, respectively.

[0228] (Sample A7) The cable connection structure of Sample A7 was manufactured in the same manner as Sample A3, except that the content of r-PP, MAH-PP, and SEBS in the resin component of the insulating layer was 70 parts by mass, 10 parts by mass, and 20 parts by mass, respectively.

[0229] (Sample A8) The cable connection structure of Sample A8 was manufactured in the same manner as Sample A3, except that the content of r-PP, MAH-PP, and SEBS in the resin component of the insulating layer was 85 parts by mass, 5 parts by mass, and 10 parts by mass, respectively.

[0230] (Sample A9) The cable connection structure of Sample A9 was manufactured in the same manner as Sample A3, except that the content of r-PP, MAH-PP, and SEBS in the resin component was 55 parts by mass, 5 parts by mass, and 40 parts by mass, respectively.

[0231] (Sample A10) The cable connection structure of Sample A10 was manufactured in the same manner as Sample A3, except that the insulating layer contained the following olefin-based elastomer instead of SEBS as the thermoplastic elastomer (C).

[0232] Ethylene propylene rubber (EPR): Ethylene unit content: 25% by mass, Melting point: None, Heat of fusion: None, Storage modulus of the element at 25°C measured by dynamic viscoelasticity measurement: 80 MPa, Content in resin components: 25 parts by mass

[0233] (2-2) The insulating layers in the cable connection structures of the results samples A6 to A10 satisfied the following characteristics.

[0234] Ratio of storage modulus of inner sample to storage modulus of outer sample: 1.1 or more and 2.5 or less Storage modulus of outer sample: 280 MPa or more and 670 MPa or less Storage modulus of inner sample: 650 MPa or more and 900 MPa or less Ratio of volume resistivity of inner sample to volume resistivity of outer sample: 1.0 or more and 1.5 or less Volume resistivity of outer and inner samples: 7.0 × 10 １４ DC breakdown field strength of outer and inner samples with a value of Ω·cm or higher: 160kV / mm or higher

[0235] From the results of samples A6 to A10 described above, it was confirmed that even if the composition of the insulating layer differs from that of sample A3, by slowly cooling the inside of the insulating layer while rapidly cooling the outside of the insulating layer, a cable connection structure can be obtained in which the desired distribution of elasticity in the thickness direction of the insulating layer is obtained, and the insulating properties of the insulating layer are uniform in the thickness direction of the insulating layer.

[0236] <Note> The following are descriptions of the embodiments of this disclosure. The embodiments referred to by the numbers in brackets [] to which the following notes are dependent correspond to the embodiments described in <Embodiments of this Disclosure>.

[0237]

[10] The cable connection structure according to [3] above, wherein the styrene elastomer comprises styrene units and at least one of propylene units and butene units.

[0238]

[11] The cable connection structure according to [4] above, wherein the olefin elastomer comprises at least one of propylene units and butene units.

[0239]

[12] The volume resistivity of the outer sample and the volume resistivity of the inner sample are 7.0 × 10 １４ The volume resistivity is Ω·cm or greater, where the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under the conditions of a temperature of 90°C and a DC electric field of 80kV / mm. The cable connection structure according to any one of [1] to [6],

[10] , or

[11] above.

[0240] 10 Connecting power cable 20 Cable connection structure 50 Heating device 100 Power cable 100a First power cable 100b Second power cable 110 Conductor 120 Cable internal semiconducting layer 130 Cable insulation layer 140 Cable external semiconducting layer 150 Cable metal tube 160 Cable sheath 210 Conductor connection part 220 Internal semiconducting layer 230 Insulation layer 240 External semiconducting layer 242 Water-absorbing tape layer 250 Metal tube 260 Corrosion-resistant layer 270 Cover part 510 Heating furnace 520 Heater 530 Gas supply line 540 Coil

Claims

1. A conductor connection portion connecting the respective conductors of a pair of power cables; an internal semiconducting layer provided to cover the outer circumference of the conductor connection portion and having semiconductivity; an insulating layer provided to cover the outer circumference of the internal semiconducting layer, having insulating properties, and having an inner circumferential surface facing the internal semiconducting layer and an outer circumferential surface opposite to the inner circumferential surface; and an external semiconducting layer provided to cover the outer circumference of the insulating layer and having semiconductivity, wherein the insulating layer comprises: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, wherein the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample of the insulating layer is 1.1 or more and 2.5 or less; and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample is 1.0 or more and 1.5 or less, where the outer sample of the insulating layer is taken from a position 0.3 mm from the outer circumferential surface toward the internal semiconducting layer, The inner sample of the insulating layer is taken from a position 0.3 mm from the inner circumferential surface toward the outer circumferential surface; the storage modulus of the outer sample and the storage modulus of the inner sample are measured at 25°C by dynamic viscoelasticity measurement; and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm in a cable connection structure.

2. The cable connection structure according to claim 1, wherein when the total content of the base polymer, the modified polymer, and the thermoplastic elastomer in the insulating layer is 100 parts by mass, the content of the modified polymer in the insulating layer is 1 part by mass or more and 10 parts by mass or less, and the content of the thermoplastic elastomer in the insulating layer is 10 parts by mass or more and 45 parts by mass or less.

3. The cable connection structure according to claim 1 or claim 2, wherein the thermoplastic elastomer comprises a styrene-based elastomer.

4. The cable connection structure according to any one of claims 1 to 3, wherein the thermoplastic elastomer comprises an olefin-based elastomer.

5. The cable connection structure according to any one of claims 1 to 4, wherein the storage modulus of the inner sample of the insulating layer is 650 MPa or more and 900 MPa or less, and the storage modulus of the outer sample of the insulating layer is 280 MPa or more and 670 MPa or less.

6. The cable connection structure according to any one of claims 1 to 5, wherein the thickness of the insulating layer is 3 mm or more.

7. A connecting power cable comprising at least one cable connection structure according to any one of claims 1 to 6.

8. The process comprises: forming a conductor connection portion by connecting the respective conductors of a pair of power cables; forming a semiconducting internal semiconducting layer so as to cover the outer circumference of the conductor connection portion; forming an insulating layer so as to cover the outer circumference of the internal semiconducting layer; forming a semiconducting external semiconducting layer so as to cover the outer circumference of the insulating layer; and heating the insulating layer and the external semiconducting layer, and then cooling them, wherein the process of forming the insulating layer comprises: preparing an insulating tape containing a resin composition; wrapping the insulating tape around the outer circumference of the internal semiconducting layer to form an inner surface facing the internal semiconducting layer and an outer surface opposite to the inner surface in the insulating layer, wherein the process of preparing the insulating tape comprises, as the resin composition, a base polymer containing propylene units; a modified polymer containing propylene units modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, and prepares the insulating tape. A method for manufacturing a cable connection structure, comprising the steps of heating the insulating layer and the outer semiconducting layer and then cooling them, wherein the insulating layer is heated and then cooled such that the ratio of the storage modulus of the inner sample of the insulating layer to the storage modulus of the outer sample of the insulating layer is 1.1 or more and 2.5 or less, and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample is 1.0 or more and 1.5 or less, wherein the outer sample of the insulating layer is taken from a position 0.3 mm from the outer peripheral surface toward the inner semiconducting layer, the inner sample of the insulating layer is taken from a position 0.3 mm from the inner peripheral surface toward the outer peripheral surface, the storage modulus of the outer sample and the storage modulus of the inner sample are measured at 25°C by dynamic viscoelasticity measurement, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under the conditions of a temperature of 90°C and a DC electric field of 80 kV / mm.

9. The method for manufacturing a cable connection structure according to claim 8, wherein in the step of heating the insulating layer and the outer semiconducting layer and then cooling them, the insulating layer is heated from a region close to the outer surface of the insulating layer while the conductor connection portion is heated by electromagnetic induction, thereby heating the insulating layer from a region close to the inner surface of the insulating layer.

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