Power cable, method for manufacturing power cable, and method for inspecting power cable

Abstract

This power cable comprises a conductor, an internal semiconductive layer, an insulating layer, and an external semiconductive layer in this order from the center axis of the conductor to the outer circumference. The insulating layer includes a resin composition that includes a polyolefin. The insulating layer has no stripe defects with a length of 5 mm or greater in the axial direction of the conductor when a region that is 10 mm long in the axial direction of the conductor from an end section of the insulating layer is observed using near-infrared light at 25°C.

Description

Power cable, power cable manufacturing method, and power cable inspection method

[0001] The present disclosure relates to a power cable, a method for manufacturing a power cable, and a method for inspecting a power cable.

[0002] Foreign matter mixed into the insulation layer of a power cable reduces the insulating properties of the insulation layer, so after manufacturing the power cable, a sample of the insulation layer having a predetermined length is taken and inspected for the presence or absence of foreign matter (see, for example, Patent Document 1).

[0003] Japanese Unexamined Patent Publication No. 7-122134

[0004] According to one aspect of the present disclosure, there is provided a power cable comprising a conductor, an inner semiconductive layer, an insulating layer, and an outer semiconductive layer, in this order from a central axis of the conductor toward an outer periphery, the insulating layer comprising a resin composition containing polyolefin, and the insulating layer having no line defects with a length of 5 mm or more in the axial direction of the conductor when a region 10 mm long from an end of the insulating layer in the axial direction of the conductor is observed with near-infrared light at 25°C.

[0005] Fig. 1 is a schematic cross-sectional view perpendicular to the axial direction of a power cable according to an embodiment of the present disclosure. Fig. 2 is a diagram of a region of a predetermined length of an insulating layer according to an embodiment of the present disclosure, observed using near-infrared light. Fig. 3 is a schematic configuration diagram of an extruder used in a method for manufacturing a power cable according to an embodiment of the present disclosure. Fig. 4 is a schematic cross-sectional view for explaining the shape of a screw flight. Fig. 5 is a schematic cross-sectional view for explaining a retention suppression member. Fig. 6 is a flowchart showing a method for manufacturing a power cable according to an embodiment of the present disclosure. Fig. 7 is a diagram showing a case where discoloration has adhered near a crosshead. Fig. 8 is a diagram of a region of a predetermined length of an insulating layer, observed using near-infrared light, in a comparative example in which a conventional manufacturing method was applied.

[0006] Problems to be Solved by the Present Disclosure The objective of the present disclosure is to ensure that there are no protrusions of the semiconductive layer throughout the entire power cable.

[0007] Effect of the Present Disclosure According to the present disclosure, it is possible to ensure that no protrusions are present in the semiconductive layer throughout the entire power cable by inspecting only the vicinity of the end of the insulating layer.

[0008] [Explanation of Embodiments of the Present Disclosure] <Insights Obtained by the Inventors> First, an outline of insights obtained by the inventors will be described.

[0009] (Regarding Deterioration of Insulation Properties Due to Burning During Extrusion Process) The inner semiconductive layer, insulating layer, and outer semiconductive layer that constitute the power cable are extruded outward from the central axis of the conductor in this order.

[0010] In a process for forming an insulating layer of a power cable, if a resin composition for the insulating layer is extruded for a long period of time, the resin composition will remain in the cylinder of the extruder. When the resin composition remains, the viscosity of the resin composition increases. As a result, discoloration of the resin composition (hereinafter, sometimes referred to as discolored SC) may occur.

[0011] The term "scorched SC" as used herein refers to foreign matter resulting from an increase in the viscosity of the resin composition. Specific examples of scorched SC include premature crosslinking of the base resin when the resin composition contains a crosslinking agent, decomposition products of the base resin, and carbonized products of the materials that make up the resin composition.

[0012] When the above-described resin composition discoloration SC occurs, the following problems may occur. As an example, the vicinity of a portion (hereinafter also referred to as a "junction") where the semiconductive resin composition for the inner semiconductive layer 120 and the resin composition for the insulating layer 130 join together in a crosshead (hereinafter sometimes referred to as a crosshead CH) will be described below with reference to Fig. 7. Note that the same problems as those described below may also occur near the junction where the semiconductive resin composition for the outer semiconductive layer and the resin composition for the insulating layer 130 join together.

[0013] As described above, the discoloration SC generated in the extruder enters the crosshead CH together with the flow of the resin composition, as shown in FIG. 7(i).

[0014] 7(ii), as the resin composition is extruded, the discoloration SC gradually moves toward the crosshead CH junction. When the discoloration SC reaches the crosshead CH junction, the discoloration SC bites into the semiconductive resin composition, restricting the extrusion of the semiconductive resin composition. As a result, a line defect (hereinafter sometimes referred to as a line defect SD) occurs at the interface between the internal semiconductive layer 120 and the insulating layer 130 downstream of the crosshead CH junction.

[0015] The "line defect SD" here refers to at least one of a concave portion and a convex portion that continuously extends along the axial direction of the conductor. Foreign matter such as burnt marks SC may be present at the end of the line defect SD.

[0016] At this time, the semiconductive resin composition flows into the gap between the junction of the crosshead CH and the burnt part SC attached to the junction, which may result in the generation of the origin of the protrusion of the semiconductive resin composition (indicated as "Origin of protrusion P" in the figure).

[0017] 7(iii), the discoloration SC adhering to the junction of the crosshead CH becomes detached and moves downstream of the junction of the crosshead CH. At this time, a protrusion P of the internal semiconductive layer 120 may occur near the rear end (upstream end) of the discoloration SC that has become detached from the junction of the crosshead CH.

[0018] In a power cable manufactured through the above process, if a line defect SD is present in the insulating layer 130, this means that a scorch SC has occurred due to retention of the resin composition during extrusion. Even if line defects SD and scorch SC are present in the insulating layer 130 of the manufactured power cable, the line defects SD and scorch SC themselves do not affect the insulating properties of the insulating layer 130.

[0019] However, if the insulating layer 130 has a line defect SD, i.e., if a burn mark SC has occurred during the extrusion process of the insulating layer 130, there is a risk that a protrusion P of the semiconductive layer (the inner semiconductive layer 120 or the outer semiconductive layer) may have been present in at least a part of the power cable. An electric field tends to concentrate at the protrusion P, where the semiconductive layer protrudes toward the insulating layer 130. Therefore, there is a risk that the insulating properties of the protrusion P may be locally reduced.

[0020] Even if only the above-mentioned streak defect SD occurs within a region of a predetermined length from the final extrusion end of the insulating layer 130, there is a possibility that a protrusion P of the semiconductive layer may have occurred in the extrusion intermediate region of the insulating layer 130 due to the intermittent generation and outflow of burn marks SC during the extrusion process.

[0021] Therefore, there has been a demand for a power cable that can ensure that no protrusions P of the semiconductive layer are present throughout the entire power cable.

[0022] (Regarding new findings in the inspection process) Conventionally, the insulation layer of an AC power cable contains cross-linked polyethylene, but does not contain an additive with a refractive index different from that of the cross-linked polyethylene. Therefore, when the insulation layer is heated to a temperature equal to or higher than the melting point of polyethylene (e.g., 120°C or higher), the insulation layer becomes transparent to visible light.

[0023] Therefore, for conventional AC power cables, the inspection process is carried out as follows: First, an inspection sample is taken from the end of the insulating layer, including a region of a predetermined length in the axial direction of the conductor. Next, the inspection sample is heated to a temperature equal to or higher than the melting point of polyethylene. After the insulating layer has become transparent through heating, the inspection sample is visually observed. Through these steps, the presence or absence of the above-mentioned line defects or protrusions in the semiconducting layer is inspected within the inspection sample of the insulating layer.

[0024] However, for example, in a DC power cable, it has been difficult to inspect the insulating layer for the presence or absence of line defects even when the insulating layer is heated.

[0025] The insulation layer of the DC power cable contained an additive that suppresses the accumulation of space charge within the insulation layer. The additive had a refractive index different from that of cross-linked polyethylene. Examples of such additives include inorganic fillers or modified polymers. As a result, even when the insulation layer was heated to a temperature above the melting point of polyethylene, it remained opaque to visible light.

[0026] As a result, in the case of DC power cables, even when the inspection process was performed with the above-mentioned insulating layer heated, it was not possible to detect the presence of line defects or protrusions in the semiconducting layer in the inspection sample of the insulating layer.

[0027] In the case of an inspection sample obtained by slicing the insulation layer of a DC power cable thinly to a thickness of about 1 mm in the axial direction of the conductor, it was sometimes possible to observe the inside of the insulation layer using visible light. In such cases, the presence or absence of foreign matter or protrusions within the inspection sample was inspected by observing the inspection sample with a thickness of about 1 mm using visible light. However, even in this case, the presence of a line defect extending along the axial direction of the conductor could not be detected because the inspection sample was thin.

[0028] As described above, it has been difficult to detect line defects in DC power cables in the past, making it impossible to reliably determine whether or not burn marks have occurred during the extrusion process of the insulating layer. As a result, it has been difficult to ensure that no protrusions exist in the semiconducting layer throughout the entire power cable.

[0029] Therefore, the present inventors have studied the inspection light used in the inspection process, and have noticed that even if the insulating layer contains an additive having a refractive index different from that of cross-linked polyethylene, the insulating layer has high transmittance for near-infrared light.

[0030] As a result of extensive research, the inventors have discovered a new inspection method for observing an insulating layer containing the additive described above over a predetermined length in the axial direction of a conductor using near-infrared light. They have confirmed that the new inspection method can be used to inspect a sample of an insulating layer for the presence or absence of line defects SD, etc., without heating the insulating layer, even if the insulating layer contains the additive described above.

[0031] 8, which will be described later, it has been found that the presence of a line defect SD can be detected at the interface between the insulating layer 130 and the outer semiconductive layer 140. Note that the presence of a line defect SD can also be detected at the interface between the inner semiconductive layer 120 and the insulating layer 130, in the same manner as described above.

[0032] (Regarding a new manufacturing method that suppresses the occurrence of discoloration) The present inventors further investigated the configuration of the extruder as a new manufacturing method that suppresses the occurrence of discoloration SC. Each time the present inventors improved the configuration of the extruder, they inspected the insulating layer extruded by the extruder using the above-mentioned new inspection method. As a result, the present inventors discovered an extruder configuration that can suppress the occurrence of discoloration SC in the insulating layer.

[0033] Specifically, the inside of the extruder was adjusted to an inert gas atmosphere. Furthermore, at least one of the screw and the discharge part of the extruder was configured to suppress retention of the resin composition. It was found that such a configuration can suppress the occurrence of discoloration (SC) of the resin composition.

[0034] The present disclosure is based on the above-mentioned findings of the inventors.

[0035] <Embodiments of the Present Disclosure> Next, embodiments of the present disclosure will be listed and described.

[0036] [1] A power cable according to one aspect of the present disclosure comprises a conductor, an inner semiconductive layer, an insulating layer, and an outer semiconductive layer, in this order from the central axis of the conductor toward the outer periphery, the insulating layer containing a resin composition containing polyolefin, and the insulating layer does not have a line defect having a length of 5 mm or more in the axial direction of the conductor when observed with near-infrared light at 25°C in a region 10 mm long from an end of the insulating layer in the axial direction of the conductor. With this configuration, it is possible to ensure that no protrusions of the semiconductive layer are present throughout the power cable by inspecting only the vicinity of the end of the insulating layer.

[0037] [2] The power cable according to the above [1], wherein the insulating layer does not have a line defect with a length of 5 mm or more in the axial direction of the conductor at the interface between the inner semiconductive layer and the insulating layer, and at the interface between the insulating layer and the outer semiconductive layer, when observed with near-infrared light at 25° C. in a region 10 mm long from the end of the insulating layer in the axial direction of the conductor. With this configuration, it is possible to ensure that no protrusions in the semiconductive layer are present throughout the entire power cable by inspecting only the vicinity of the end of the insulating layer.

[0038] [3] The power cable according to [1] or [2], wherein the length of the power cable in the axial direction of the conductor is 4 km or more. With this configuration, it is possible to ensure that no protrusions of the semiconducting layer are present over a long distance in the axial direction of the conductor.

[0039] [4] In the power cable according to any one of [1] to [3] above, when a sheet of the insulating layer having a thickness of 2.0 mm is prepared, the light transmittance of the sheet at a wavelength of 500 nm measured at a temperature equal to or higher than the melting point of the polyolefin is 50% or less. With this configuration, it is possible to obtain a power cable in which the absence of protrusions in the semiconducting layer can be guaranteed even if the structure makes it difficult to detect line defects using visible light.

[0040] [5] A method for producing a power cable according to another aspect of the present disclosure includes the steps of: preparing a resin composition containing a polyolefin; and forming an inner semiconductive layer, an insulating layer containing the resin composition, and an outer semiconductive layer in this order from the central axis of the conductor toward the outer periphery so as to cover the outer periphery of the conductor, wherein in the step of forming the insulating layer, the insulating layer is extrusion-molded so that no stripe defects having a length of 5 mm or more in the axial direction of the conductor are formed when a region 10 mm long from an end of the insulating layer in the axial direction of the conductor is observed with near-infrared light at 25° C. With this configuration, it is possible to ensure that no protrusions in the semiconductive layer are present throughout the power cable by inspecting only the vicinity of the end of the insulating layer.

[0041] [6] A power cable inspection method according to another aspect of the present disclosure includes the steps of: preparing a power cable including a conductor, an inner semiconductive layer, an insulating layer containing polyolefin, and an outer semiconductive layer, in this order from the central axis of the conductor toward the outer periphery; taking an inspection sample of the insulating layer including a region 10 mm long from an end of the insulating layer in the axial direction of the conductor; and inspecting the inspection sample of the insulating layer with near-infrared light at 25° C. for the presence or absence of a line defect having a length of 5 mm or more in the axial direction of the conductor. This configuration makes it possible to detect the presence of a line defect even in a configuration in which it is difficult to detect a line defect with visible light.

[0042] [Details of the embodiment of the present disclosure] Next, one embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0043] <One embodiment of the present disclosure> (1) Resin composition The resin composition of this embodiment is a material that constitutes the insulating layer 130 of the power cable 10 described below. The resin composition contains, for example, a base resin, an inorganic filler, a modified polymer, a crosslinking agent, and other additives.

[0044] (Base Resin) The base resin (base polymer) refers to a resin component that constitutes the main component of a resin composition. The base resin of this embodiment contains, for example, a polyolefin. Examples of polyolefins that constitute the base resin include polyethylene and polypropylene. The base resin of this embodiment is, for example, polyethylene. Polyethylene as a base resin is disclosed in WO2022 / 163197, etc. The base resin of this embodiment is, for example, low-density polyethylene (LDPE).

[0045] (Inorganic Filler) The inorganic filler includes, for example, magnesium oxide (MgO). MgO as an inorganic filler is disclosed in WO2022 / 163197 and the like.

[0046] (Modified Polymer) In this embodiment, the resin composition may contain, for example, a modified polymer. The modified polymer in this embodiment is, for example, a resin containing an olefin unit and modified with at least one selected from unsaturated organic acids and derivatives thereof. The modified polymer in this embodiment may be, for example, unsaturated carboxylic acid-modified polyethylene. Modified polymers are disclosed in WO2022 / 163197 and the like.

[0047] (Crosslinking Agent) The crosslinking agent is, for example, an organic peroxide. Organic peroxides are disclosed in WO2022 / 163197 and the like.

[0048] (Other Additives) The resin composition may further contain, for example, an antioxidant and a lubricant.

[0049] (2) Power Cable Next, the power cable of this embodiment will be described with reference to FIG.

[0050] The power cable 10 of this embodiment is configured as a so-called solid insulated power cable. The power cable 10 may be used for, for example, direct current.

[0051] Specifically, the power cable 10 has, for example, a conductor 110, an inner semiconductive layer 120, an insulating layer 130, an outer semiconductive layer 140, a shielding layer 150, and a sheath 160, in this order from the central axis toward the outer periphery of the conductor 110. Hereinafter, the inner semiconductive layer 120 and the outer semiconductive layer 140 may be simply referred to as "semiconductive layers."

[0052] (Conductor (Conductive Portion)) The conductor 110 is formed by twisting together a plurality of conductor core wires (conductive core wires) containing, for example, pure copper, copper alloy, aluminum, or aluminum alloy.

[0053] (Internal Semiconductive Layer) The internal semiconductive layer 120 is provided so as to cover the outer periphery of the conductor 110. The internal semiconductive layer 120 contains a semiconductive resin composition, and is configured to suppress electric field concentration near the surface of the conductor 110.

[0054] (Insulating Layer) The insulating layer 130 is provided so as to cover the outer periphery of the internal semiconducting layer 120. The insulating layer 130 is formed by extrusion molding using the above-mentioned resin composition, for example.

[0055] In this embodiment, the generation of scorch marks SC is suppressed in the cable core forming step S300 by a new manufacturing method described later, so that the insulating layer 130 does not have line defects SD. This point will be described in detail later.

[0056] In this embodiment, the insulating layer 130 is cross-linked, for example, by heating the resin composition after extrusion molding. That is, for example, at least a portion of the polyethylene serving as the base resin in the insulating layer 130 is cross-linked polyethylene.

[0057] (Outer Semiconductive Layer) The outer semiconductive layer 140 is provided so as to cover the outer periphery of the insulating layer 130. The outer semiconductive layer 140 has semiconductivity and is configured to suppress electric field concentration between the insulating layer 130 and the shielding layer 150. The outer semiconductive layer 140 contains, for example, the same semiconductive resin composition as the inner semiconductive layer 120.

[0058] (Shielding Layer) The shielding layer 150 is provided so as to cover the outer periphery of the outer semiconducting layer 140. The shielding layer 150 is formed, for example, by winding copper tape, or is formed as a wire shield wound with a plurality of annealed copper wires or the like. Tape made of a material such as rubberized cloth may be wound inside or outside the shielding layer 150.

[0059] (Sheath) The sheath 160 is provided so as to cover the outer periphery of the shielding layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0060] (3) Characteristics of the Insulating Layer of the Present Embodiment Next, the characteristics of the insulating layer 130 of the present embodiment will be described.

[0061] In this embodiment, as described above, the insulating layer 130 contains an inorganic filler. The inorganic filler has a refractive index at a wavelength of 500 nm that is different from the refractive index of the polyethylene base resin. For example, the insulating layer 130 may contain inorganic fillers whose particle diameter is close to the wavelength of visible light or greater than the wavelength of visible light.

[0062] For this reason, the insulating layer 130 has low transmittance to visible light even when heated to a temperature equal to or higher than the melting point of the base resin. Specifically, when a sheet of the insulating layer 130 having a thickness of 2.0 mm is produced, the light transmittance of the sheet at a wavelength of 500 nm measured at a temperature equal to or higher than the melting point of the polyolefin base resin (e.g., 120°C or higher) is, for example, 50% or less. As a result, it is difficult to inspect the insulating layer 130 of this embodiment for the presence or absence of line defects SD using visible light.

[0063] Therefore, in the inspection step S440 of this embodiment, which will be described later, the insulating layer 130 is observed using near-infrared light. Near-infrared light has a wavelength longer than the volume average particle diameter of the inorganic filler, for example, and therefore easily passes through the inorganic filler. This makes it possible to inspect the insulating layer 130 for the presence or absence of line defects SD and the like without heating the insulating layer 130, even if the insulating layer 130 contains the additives described above.

[0064] Here, the insulating layer 130 of the comparative example, which is manufactured by a conventional manufacturing method, will be compared with the insulating layer 130 of the present embodiment, which is manufactured by a new manufacturing method described below.

[0065] 8 is a diagram showing a comparative example in which a conventional manufacturing method was applied, in which a region 10 mm long from the end (final extrusion end) of the insulating layer 130 in the axial direction of the conductor 110 was observed using near-infrared light. In Fig. 8, the inspection sample of the insulating layer 130 was observed from a direction tilted with respect to the axial direction of the conductor 110 (the axial direction of the cylindrical insulating layer 130).

[0066] 8, a line defect SD occurs at the interface between the insulating layer 130 and the outer semiconductive layer 140. Note that a line defect SD may also occur at the interface between the inner semiconductive layer 120 and the insulating layer 130.

[0067] The line defect SD extends continuously, for example, along the axial direction of the conductor 110 (the axial direction of the cylindrical insulating layer 130). The line defect SD has a length of, for example, 5 mm or more in the axial direction of the conductor 110. The line defect SD has a height of, for example, 300 μm or more in the radial direction of the insulating layer 130 and a width of, for example, 100 μm or more in the circumferential direction of the insulating layer 130.

[0068] In the vicinity of the line defect SD, there may be a burn mark SC formed during extrusion, a protrusion P where the inner semiconductive layer 120 or the outer semiconductive layer 140 protrudes toward the insulating layer 130, or the like.

[0069] In the comparative example in which such a streak defect SD occurred, a burn mark SC occurred during the extrusion process of the insulating layer 130, and there is a possibility that a protrusion P of the semiconductive resin composition was present in at least a part of the power cable 10. At the protrusion P where the inner semiconductive layer 120 or the outer semiconductive layer 140 protrudes toward the insulating layer 130, there is a possibility that the insulation properties may be locally reduced.

[0070] In contrast, Figure 2 shows an observation of a region 10 mm long from the end (final extrusion end) of the insulating layer 130 in the axial direction of the conductor 110 using near-infrared light in this embodiment, using an inspection method similar to that used in the comparative example.

[0071] In this embodiment, by applying a new manufacturing method described later, no line defects SD are generated in the insulating layer 130 when observed with near-infrared light.

[0072] Specifically, as shown in Fig. 2, when a region 10 mm long from the end of the insulating layer 130 in the axial direction of the conductor 110 (the axial direction of the tubular insulating layer 130) is observed with near-infrared light at 25°C, the insulating layer 130 does not have a line defect SD with a length of 5 mm or more in the axial direction of the conductor 110. Thus, the absence of line defects SD in the insulating layer 130 means that no retention of the resin composition occurred throughout the extrusion process of the insulating layer 130, and no discoloration SC of the resin composition occurred. This makes it possible to ensure that no protrusions P of the semiconductive layer are present throughout the power cable 10 by inspecting only the vicinity of the end of the insulating layer 130.

[0073] In this embodiment, by applying a novel manufacturing method described below, the insulating layer 130 does not have the above-mentioned line defects SD at the interface between the insulating layer 130 and the adjacent semiconducting layer.

[0074] Here, in the conventional manufacturing method, line defects SD were likely to occur at the interface between the internal semiconductive layer 120 and the insulating layer 130, and at the interface between the insulating layer 130 and the external semiconductive layer 140, due to the influence of burn marks SC adhering to the crosshead CH.

[0075] In contrast, in this embodiment, the insulating layer 130 does not have any line defects SD with a length of 5 mm or more in the axial direction of the conductor 110, particularly at the interface between the inner semiconductive layer 120 and the insulating layer 130 and at the interface between the insulating layer 130 and the outer semiconductive layer 140. This makes it possible to ensure that no protrusions P of the semiconductive resin occur at the interfaces between the semiconductive layers and the insulating layer 130 throughout the entire power cable 10.

[0076] In this embodiment, the length of the power cable 10 in the axial direction of the conductor 110 is, for example, 4 km or more. That is, the extrusion process of the insulating layer 130 is carried out over a long period of time.

[0077] In the conventional manufacturing method, as described above, when the insulating layer 130 of the long power cable 10 is extruded for a long period of time, the resin composition tends to remain in the extruder, which tends to cause a streak defect SD in the insulating layer 130.

[0078] In contrast, in this embodiment, by applying a novel manufacturing method described below, even when the length of the power cable 10 in the axial direction of the conductor 110 is 4 km or more, no line defects SD occur in a region 10 mm long from the final extrusion end of the insulating layer 130 in the axial direction of the conductor 110. This means that the resin composition does not stagnate throughout the long extrusion process of the insulating layer 130, and no burn marks SC occur in the resin composition. As a result, it is possible to ensure that no protrusions P of the semiconductive layer exist over a long distance in the axial direction of the conductor 110.

[0079] (4) Extruder Next, the extruder 200 used in the method for manufacturing the power cable of this embodiment will be described with reference to FIGS.

[0080] The extruder 200 of this embodiment is configured to be able to extrude the insulating layer 130 in an inert gas atmosphere, for example.

[0081] In the extruder 200 of this embodiment, for example, at least a portion of the contact surface with which the resin composition comes into contact is parallel to or inclined with respect to the extrusion direction of the resin composition (the axial direction of the cylinder 210). In other words, at least a portion of the contact surface with which the resin composition comes into contact is not perpendicular to the extrusion direction of the resin composition.

[0082] Specifically, the contact surface of the resin composition adjacent to the hole of the discharge section 250 that discharges the resin composition is parallel to or inclined with respect to the axial direction of the cylinder 210. Furthermore, the contact surface of the resin composition on the screw flight 232 facing upstream of the cylinder 210 is inclined at an obtuse angle with respect to the outer peripheral surface of the screw main body 231 that is aligned with the axial direction of the cylinder 210.

[0083] The extruder 200 of this embodiment will be described in detail below.

[0084] 3, the extruder 200 includes a cylindrical cylinder 210 to which a resin composition material is supplied, a hopper 220 for supplying the material into the cylinder 210, a screw 230 inserted from a first axial end (left side in FIG. 3) of the cylinder 210 and rotatably arranged, a rotation drive mechanism 240 for rotating the screw 230, a discharge section 250 attached to a second end (right side in FIG. 3) opposite the first end of the cylinder 210 and provided with a hole for discharging the resin composition, and an atmosphere adjustment section 260 for adjusting the inside of the cylinder 210 to an inert gas atmosphere. Hereinafter, the region near the first axial end of the cylinder 210 will be referred to as "upstream," and the region near the second axial end of the cylinder 210 will be referred to as "downstream."

[0085] The cylindrical cylinder 210 has a space therein for accommodating and mixing the materials. In the cylindrical cylinder 210, the materials supplied to the internal space are mixed by the screw 230. The screw 230 is inserted from a first axial end of the cylinder 210 and disposed at the axial center of the cylinder 210. The screw 230 is connected to a rotation drive mechanism 240 and rotatably supported. The screw 230 is configured to be rotated by the rotation drive mechanism 240 and to extrude the materials toward the discharge section 250 while mixing them. The screw 230 may be a single-shaft or dual-shaft screw. In FIG. 3, two screws 230 are arranged parallel to each other in the depth direction of the page, and only one of the screws 230 is shown. Note that, for example, a known rotary motor or the like can be used as the rotation drive mechanism 240.

[0086] The screw 230 has screw flights 232 arranged in a spiral shape on the outer peripheral surface of the screw body 231. The cross-sectional shape of the screw flights 232 may be tapered, as shown in FIG. 4 . FIG. 4 is a schematic diagram for explaining the shape of the screw flights, and is a cross-sectional view taken along the axial direction of the screw 230. In FIG. 4 , the right side is a region close to the tip of the screw 230 (a region close to the second axial end of the cylinder 210), and the left side is a region close to the end (base end) of the screw 230 (a region close to the first axial end of the cylinder 210). When the screw flights 232 have a rectangular shape as indicated by the dashed line in FIG. 4 , the resin composition tends to remain near the contact surface of the screw flight 232 close to the end of the screw 230, and the remaining resin composition may be thermally decomposed. Therefore, from the viewpoint of suppressing retention of the resin composition near the contact surface of the screw 230, it is preferable that the cross-sectional shape of the screw flight 232 has a tapered side surface of the screw flight 232 near the end of the screw 230, as shown in Fig. 4. In other words, it is preferable that the angle formed between the contact surface of the resin composition on the screw flight 232 facing upstream of the cylinder 210 and the outer circumferential surface of the screw main body 231 is an obtuse angle. The angle is preferably, for example, between 120° and 145°.

[0087] The discharge section 250 is disposed at a second axial end of the cylinder 210. The discharge section 250 has, for example, a plurality of holes penetrating the thickness direction of the discharge section 250 (the axial direction of the cylinder 210) and is configured to push out the resin composition mixed inside the cylinder 210 to the outside. For example, a breaker plate can be used as the discharge section 250. Note that a mesh or the like may be disposed between the cylinder 210 and the discharge section 250 in order to remove foreign matter contained in the resin composition.

[0088] As shown in FIG. 5 , a retention suppression member 270 may be disposed at a second axial end of the cylinder 210 in a position in contact with the discharge section 250. FIG. 5 is a schematic diagram illustrating the retention suppression member, showing a cross-sectional view of the second axial end of the cylinder 210. As shown in FIG. 4 , the retention suppression member 270 suppresses the resin composition from accumulating within the cylinder 210 and promotes extrusion from the discharge section 250. The retention suppression member 270 has a plurality of tapered holes 271 that penetrate the retention suppression member 270 in the thickness direction and whose diameters gradually decrease toward the thickness direction. The retention suppression member 270 is disposed such that the small-diameter openings of the tapered holes 271 communicate with the holes 251 of the discharge section 250. In this configuration, the contact surfaces of the tapered holes 271 of the retention suppression member 270 adjacent to the holes of the discharge section 250, with which the resin composition abuts, are parallel to or inclined with respect to the axial direction of the cylinder 210. This makes it possible to prevent the resin composition from accumulating at the edge of the hole 251 of the discharge portion 250 and at the corner between the inner wall of the cylinder 210 and the discharge portion 250. The accumulating prevention member 270 may be made of the same material as the breaker plate, for example.

[0089] An atmosphere adjustment unit 260 that adjusts the interior of the cylinder 210 to an inert gas atmosphere is connected to the cylinder 210. The atmosphere adjustment unit 260 is configured to supply an inert gas to the interior of the cylinder 210. Air may be mixed into the interior of the cylinder 210 as the materials are supplied from the hopper 220, but the atmosphere adjustment unit 260 can adjust the interior of the cylinder 210 to an inert gas atmosphere. This makes it possible to prevent the materials to be mixed or the resulting resin composition from being oxidized and thermally decomposed by the air.

[0090] The inert gas is not particularly limited, but it is preferable to use, for example, nitrogen gas or argon gas.

[0091] The extruder 200 may include a heating section (not shown) for heating the inside of the cylinder 210. Any conventionally known heating section may be used.

[0092] (5) Method for Manufacturing the Power Cable Next, a method for manufacturing the power cable of this embodiment will be described with reference to FIG.

[0093] 6 , the power cable manufacturing method of this embodiment includes, for example, a resin composition preparing step S100, a conductor preparing step S200, a cable core forming step S300, an inspection sample taking step S420, an inspection step S440, a shielding layer forming step S500, and a sheath forming step S600. The power cable manufacturing method of this embodiment also includes a power cable inspection method.

[0094] (S100: Resin Composition Preparation Step) First, a resin composition that constitutes the insulating layer 130 of this embodiment is prepared.

[0095] In this embodiment, a base resin containing polyethylene, an inorganic filler, a modified polymer, a crosslinking agent, and other additives are mixed (kneaded) in a mixer to form a mixture.

[0096] After the mixture is prepared, the mixture is granulated by an extruder, thereby forming a pellet-shaped resin composition that constitutes the insulating layer 130. The extruder 200 shown in FIG. 3 may be used.

[0097] (S200: Conductor Preparation Step) On the other hand, the conductor 110 formed by twisting together a plurality of conductor core wires is prepared.

[0098] (S300: Cable Core Forming Process (Insulating Layer Forming Process)) After the resin composition preparing process S100 and the conductor preparing process S200 are completed, in the cable core forming process S300, the insulating layer 130 is formed using the above-mentioned resin composition so as to cover the outer periphery of the conductor 110.

[0099] In this embodiment, for example, a three-layer co-extruder is used to simultaneously form the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140 in this order from the central axis of the conductor 110 toward the outer periphery so as to cover the outer periphery of the conductor 110.

[0100] Specifically, among the three-layer co-extruders, extruder A that forms the inner semiconductive layer 120 is charged with, for example, a resin composition for the inner semiconductive layer.

[0101] The extruder 200 shown in Fig. 3 is used as the extruder B for forming the insulating layer 130. The above-described pellet-shaped resin composition is fed into the extruder 200. At this time, the set temperature of the extruder B is set to a temperature that is 10°C to 80°C higher than the melting point of the base resin. The set temperature is adjusted appropriately based on the linear velocity and extrusion pressure.

[0102] In this embodiment, the insulating layer 130 is extruded in an inert gas atmosphere. The "inert gas atmosphere" here means that the partial pressure of the inert gas is 90% or more, 95% or more, or 100% of the total pressure.

[0103] 3, an inert gas is supplied to the inside of the cylinder 210 by the atmosphere adjusting unit 260. This makes it possible to suppress the occurrence of scorching SC due to thermal decomposition of the resin composition.

[0104] At this time, in this embodiment, the insulating layer 130 is extruded in the extruder 200 with at least a portion of the contact surface with which the resin composition comes into contact being parallel to or inclined with respect to the extrusion direction of the resin composition (the axial direction of the cylinder 210).

[0105] Specifically, as shown in Fig. 5, the insulating layer 130 is extrusion molded in a state where the contact surface of the resin composition adjacent to the hole of the discharge portion 250 that discharges the resin composition is parallel to or inclined with respect to the axial direction of the cylinder 210. Furthermore, as shown in Fig. 4, the insulating layer 130 is extrusion molded in a state where the contact surface of the resin composition on the screw flight 232 facing upstream of the cylinder 210 is inclined at an obtuse angle with respect to the outer peripheral surface of the screw main body portion 231 that is aligned with the axial direction of the cylinder 210.

[0106] This makes it possible to prevent the resin composition from remaining in the extruder 200. As a result, it is possible to prevent the occurrence of scorching SC of the resin composition.

[0107] Furthermore, the same resin composition for the outer semiconductive layer as that for the inner semiconductive layer charged into extruder A is charged into extruder C which forms the outer semiconductive layer 140 .

[0108] Next, the extrudates from the extruders A to C are introduced into the crosshead CH, where the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140 are simultaneously extruded from the central axis of the conductor toward the outer periphery of the conductor 110. This forms an extruded material that will become the cable core.

[0109] Thereafter, the insulating layer 130 is cross-linked by heating it by radiation from an infrared heater or by heat transfer through a heat medium such as high-temperature nitrogen gas or silicone oil in a bridging tube pressurized with nitrogen gas or the like, thereby forming a cable core composed of the conductor 110, the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140.

[0110] (S420: Inspection Sample Collection Step) After the cable core formation step S300 is completed, an inspection sample is collected.

[0111] Specifically, a region including a 10 mm long region from the final extrusion end of the cable core is cut in the axial direction of the conductor 110 using a cable slicer. After the cable core is cut, the conductor 110 is removed from the cut piece. The cut piece remains with the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140. In this way, an inspection sample of the insulating layer 130 including a 10 mm long region from the end (final extrusion end) of the insulating layer 130 in the axial direction of the conductor 110 is obtained.

[0112] (S440: Inspection process) After the inspection sample is taken, the inspection sample of the insulating layer 130 is observed with near-infrared light at 25°C without heating to inspect the inspection sample for the presence or absence of a line defect SD having an axial length of 5 mm or more in the conductor 110.

[0113] It should be noted that the "axial direction of the conductor 110" in the collected test sample means the axial direction of the region where the conductor 110 was present when the conductor 110 is removed from the test sample. The "axial direction of the conductor 110" can also be rephrased as the "axial direction of the cylindrical insulating layer 130."

[0114] In the inspection step S440, the near-infrared light used as the inspection light is, for example, light having a wavelength of 800 nm or more and 2500 nm or less. A halogen lamp or a light-emitting diode (LED) lamp, for example, is used as a light source of the near-infrared light.

[0115] The imaging camera is, for example, an indium gallium arsenide (InGaAs) camera with high light-receiving sensitivity in the near-infrared wavelength range. The lens attached to the imaging camera is a dedicated lens with high transmittance for near-infrared light. The imaging camera is placed opposite the near-infrared light source, sandwiching the inspection sample.

[0116] The test sample may be immersed in a liquid having a refractive index close to that of the base resin at the wavelength of near-infrared light, such as liquid paraffin. This prevents degradation of image quality due to unevenness on the surface of the test sample when it is cut.

[0117] At this time, in this embodiment, the inspection sample of the insulating layer 130 is observed from a direction tilted with respect to the axial direction of the conductor 110 (the axial direction of the cylindrical insulating layer 130). This makes it possible to observe the vicinity of the outer circumferential surface of the inner semiconductive layer 120 or the vicinity of the inner circumferential surface of the outer semiconductive layer 140. As a result, it is possible to inspect the presence or absence of a line defect SD at the interface between each semiconductive layer and the insulating layer 130.

[0118] By the inspection step S440 described above, even in a configuration where it is difficult to detect a line defect SD using visible light, the presence of the line defect SD can be detected.

[0119] As a result of the inspection process S440, if the insulating layer 130 of the inspection sample has a line defect SD, as in the comparative example shown in Figure 8, the cable core from which the inspection sample was taken is determined to be defective.

[0120] On the other hand, if the result of the inspection step S440 shows that the insulating layer 130 of the inspection sample does not have a line defect SD, as in the present embodiment shown in Figure 2, the cable core from which the inspection sample was taken is determined to be a non-defective product. The cable core determined to be a non-defective product is used to carry out the next step.

[0121] (S500: Shielding Layer Forming Step) After the inspection step S440 is completed, the shielding layer 150 is formed on the outside of the outer semiconductive layer 140 by using a cable core determined to be a non-defective product and winding, for example, copper tape.

[0122] (S600: Sheath Forming Step) After the shielding layer 150 is formed, the sheath 160 is formed around the outer periphery of the shielding layer 150 by feeding vinyl chloride into an extruder and extruding it.

[0123] In this manner, the power cable 10 is manufactured as a solid insulated power cable.

[0124] (6) Summary of the Present Embodiment According to the present embodiment, one or more of the following effects can be achieved.

[0125] (a) In this embodiment, in the insulating layer forming step, the insulating layer 130 is extrusion-molded in an inert gas atmosphere. Furthermore, in the extruder 200, the insulating layer 130 is extrusion-molded in a state in which at least a portion of the contact surface with which the resin composition contacts is parallel to or inclined with respect to the extrusion direction of the resin composition (the axial direction of the cylinder 210). This makes it possible to suppress retention of the resin composition in the extruder 200. As a result, it is possible to suppress the occurrence of discoloration (SC) of the resin composition.

[0126] (b) In this embodiment, the insulating layer 130 obtained by the above-mentioned new manufacturing method does not have a line defect SD having a length of 5 mm or more in the axial direction of the conductor 110 when observed using near-infrared light at 25°C in a region 10 mm long from the end of the insulating layer 130 in the axial direction of the conductor 110.

[0127] The absence of streak defects SD in the insulating layer 130 means that the resin composition did not stagnate and no discoloration SC occurred in the resin composition throughout the entire extrusion process of the insulating layer 130. In other words, this corresponds to the absence of protrusions P in the inner semiconductive layer 120 and the outer semiconductive layer 140 throughout the entire power cable 10, including not only the region near the final end of the extrusion where the inspection sample was taken, but also the intermediate region of the extrusion.

[0128] As described above, according to this embodiment, it is possible to ensure that no protrusions P are present in the inner semiconductive layer 120 and the outer semiconductive layer 140 throughout the entire power cable 10 by inspecting only the vicinity of the end of the insulating layer 130. The absence of protrusions P in the semiconductive layers throughout the entire power cable 10 makes it possible to suppress degradation of insulation caused by the protrusions P.

[0129] (c) In a configuration in which the insulating layer 130 contains an inorganic filler, the resin composition is likely to stagnate and burn marks SC are likely to occur during the extrusion process of the insulating layer 130. Furthermore, in such a configuration, as described above, even if the insulating layer 130 is heated, it becomes difficult to detect the presence of line defects SD using visible light in the insulating layer 130 having a predetermined length in the axial direction of the conductor 110.

[0130] In contrast, in the present embodiment, even if the resin composition constituting the insulating layer 130 contains an inorganic filler, the above-described novel manufacturing method can suppress the occurrence of scorching SC in the resin composition. As a result, it has been confirmed by near-infrared light observation that the insulating layer 130 of the present embodiment does not have line defects SD.

[0131] Thus, according to this embodiment, even if the resin composition is prone to fading SC and it is difficult to detect line defects SD using visible light, a power cable 10 can be obtained that can guarantee that there are no protrusions P in the semiconductive layer.

[0132] (7) Modifications of the Present Embodiment The above-described embodiment can be modified as necessary as shown in the following modifications. Only elements different from the above-described embodiment will be described below, and elements that are substantially the same as the elements described in the above-described embodiment will be assigned the same reference numerals and their description will be omitted.

[0133] (7-1) Modification 1 (Resin Composition) The resin composition constituting the insulating layer 130 of Modification 1 contains, for example, a base resin containing polyethylene, an inorganic filler containing silicon dioxide (nanosilica), a crosslinking agent, and other additives. Silicon dioxide is disclosed in WO2022 / 163198, etc.

[0134] In Modification 1, the resin composition may not contain a modified polymer. The additives in Modification 1 may be the same as those in the above-described embodiment, for example.

[0135] (Power cable) As in the above-described embodiment, the insulating layer 130 of the power cable 10 of the first modified example does not have a line defect SD having a length of 5 mm or more in the axial direction of the conductor 110 when a region 10 mm long from the end of the insulating layer 130 in the axial direction of the conductor 110 (axial direction of the tubular insulating layer 130) is observed with near-infrared light at 25°C.

[0136] (Method of Manufacturing Power Cable) The method of manufacturing the power cable of the first modification is the same as that of the above-described embodiment.

[0137] (Effects of Modification 1) In Modification 1, the insulating layer 130 contains silicon dioxide as an inorganic filler. In such a configuration, even if the insulating layer 130 is heated, it may be difficult to detect the presence of a line defect SD using visible light in the insulating layer 130 having a predetermined length in the axial direction of the conductor 110. In contrast, in the insulating layer 130 of Modification 1 obtained by the above-described new manufacturing method, it has been confirmed by observation with near-infrared light in an unheated state that no line defect SD occurs. Thus, according to Modification 1, it is possible to obtain a power cable 10 that can be guaranteed to have no protrusions P in the semiconductive layer, even if the configuration makes it difficult to detect line defects SD using visible light.

[0138] (7-2) Modification 2 (Resin Composition) The resin composition constituting the insulating layer 130 of Modification 2 contains, for example, a base resin containing polypropylene, a modified polymer, and other additives. The resin composition may further contain a thermoplastic elastomer as described below.

[0139] (Base Resin) Examples of polypropylene base resins include homopolypropylene (homoPP), random polypropylene (randomPP), block polypropylene (blockPP), etc. Polypropylene base resins are disclosed in WO2023 / 017562 and the like.

[0140] (Modified Polymer) In Modified Example 2, the resin composition contains at least a modified polymer. The modified polymer of Modified Example 2 is a resin containing propylene units as a main chain and modified with at least one selected from unsaturated organic acids and their derivatives. The modified polymer may be, for example, unsaturated carboxylic acid-modified polypropylene. Modified polymers are disclosed in WO2023 / 017562 and the like.

[0141] (Thermoplastic elastomer) In Modification 2, the resin composition may further include, for example, a thermoplastic elastomer. The thermoplastic elastomer may be at least one of a styrene-based polymer and an olefin-based elastomer.

[0142] The styrene elastomer is, for example, a copolymer containing styrene units as hard segments and at least one monomer unit selected from ethylene units, propylene units, butylene units, and isoprene units as soft segments.

[0143] The olefin elastomer is, for example, a copolymer containing two olefin units, such as a copolymer containing an ethylene unit and an α-olefin unit having 3 or more carbon atoms, or a copolymer containing a propylene unit and an α-olefin unit having 4 or more carbon atoms.

[0144] The styrene-based polymers and olefin-based elastomers as the thermoplastic elastomers mentioned above are disclosed in WO2023 / 017562 and the like.

[0145] On the other hand, in Modification 2, the resin composition does not need to contain a thermoplastic elastomer.

[0146] (Inorganic Filler) In Modification 2, the resin composition may not contain an inorganic filler.

[0147] On the other hand, in Modification 2, the resin composition may contain a small amount of inorganic filler as in the above embodiment or Modification 1.

[0148] (Cross-linking Agent and Other Additives) In Modification 2, the resin component constituting the insulating layer 130 may be non-cross-linked from the viewpoint of recycling. In this case, the resin composition may not contain a cross-linking agent.

[0149] On the other hand, the resin component constituting the insulating layer 130 may be slightly crosslinked. In this case, the resin composition may contain a small amount of a crosslinking agent, as in the above-described embodiment.

[0150] Other additives in Modification 2 are, for example, the same as those in the above-described embodiment.

[0151] (Power Cable) In the power cable 10 of the second modification, the insulating layer 130 may be non-crosslinked as described above, or may be slightly crosslinked so that the decomposition residue of the crosslinking agent is less than 300 ppm. In this way, by making the insulating layer 130 non-crosslinked or with a low degree of crosslinking, it is possible to improve recyclability.

[0152] As in the above-mentioned embodiments, the insulating layer 130 of the power cable 10 of variant example 2 does not have a line defect SD with a length of 5 mm or more in the axial direction of the conductor 110 when observed with near-infrared light at 25°C in a region 10 mm long from the end of the insulating layer 130 in the axial direction of the conductor 110 (axial direction of the tubular insulating layer 130).

[0153] (Manufacturing Method of Power Cable) The manufacturing method of the power cable of Modification 2 is the same as that of the above-described embodiment, except that the insulating layer 130 is not cross-linked in the cable core forming step S300. The manufacturing method of the power cable in Modification 2 when the insulating layer 130 is slightly cross-linked is the same as that of the above-described embodiment.

[0154] (Effects of Modification 2) (a) In Modification 2, the insulating layer 130 containing polypropylene is non-crosslinked or slightly crosslinked. With this configuration, when the insulating layer 130 is heated, the shape of the insulating layer 130 is distorted, making it impossible to observe the insulating layer 130 in a heated state as in the conventional method using visible light. In contrast, in Modification 2, by observing an inspection sample taken from the insulating layer 130 using near-infrared light, it is possible to inspect the presence or absence of line defects SD in the inspection sample of the insulating layer 130 without heating the insulating layer 130.

[0155] (b) In Modification 2, the insulating layer 130 contains an organic additive having a refractive index different from that of the polyolefin (polypropylene) base resin at a wavelength of 500 nm. In this configuration, the resin composition is likely to stagnate during the extrusion process of the insulating layer 130, which may lead to the occurrence of discoloration (SC).

[0156] In contrast, in Modification 2, even if the resin composition constituting the insulating layer 130 contains the organic additive, the occurrence of discoloration SC in the resin composition can be suppressed by the above-mentioned new manufacturing method. As a result, it has been confirmed by near-infrared light observation that the insulating layer 130 of Modification 2 does not have line defects SD.

[0157] Thus, according to variant example 2, even if the resin composition is prone to fading SC and it is difficult to detect line defects SD using visible light when heated, a power cable 10 can be obtained that can guarantee that no protrusions P are present in the semiconductive layer.

[0158] (7-3) Modification 3 In the resin composition, the power cable 10, and the method for manufacturing the power cable 10 according to Modification 3, the configurations not described below are basically the same as those in Modification 2.

[0159] (Resin Composition) The resin composition constituting the insulating layer 130 of the third modification contains, for example, a base resin containing polypropylene, a thermoplastic elastomer, and other additives.

[0160] The thermoplastic elastomer of the third modification is the same as that of the second modification in which the resin composition contains a thermoplastic elastomer.

[0161] The resin composition of the third modification does not contain a modified polymer as used in the second modification.

[0162] (Power cable) As in the above-described embodiments, the insulating layer 130 of the power cable 10 of the third modification does not have a line defect SD having a length of 5 mm or more in the axial direction of the conductor 110 when a region 10 mm long from the end of the insulating layer 130 in the axial direction of the conductor 110 (axial direction of the tubular insulating layer 130) is observed with near-infrared light at 25°C.

[0163] (Effects of Modification 3) In Modification 3, the resin composition contains a thermoplastic elastomer but does not contain a modified polymer. Even with this configuration, the insulating layer 130 may satisfy a predetermined insulating property for direct current. Modification 3 can also achieve the same effects as Modification 2.

[0164] <Other Embodiments of the Present Disclosure> Although the embodiments of the present disclosure have been specifically described above, the present disclosure is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the present disclosure.

[0165] In the above embodiment, the resin composition for the insulating layer 130 contains polyethylene and an inorganic filler, but the present disclosure is not limited to this. The resin composition for the insulating layer 130 may contain polypropylene and an inorganic filler.

[0166] In the above embodiment, the case where the test sample collection step S420 and the inspection step S440 are performed between the cable core formation step S300 and the shielding layer formation step S500 has been described, but the present disclosure is not limited to this case. For example, the test sample collection step S420 and the inspection step S440 may be performed after the sheath formation step S600.

[0167] In the above embodiment, three layers are simultaneously extruded in the cable core forming step S300, but each layer may be extruded one by one.

[0168] Next, examples according to the present disclosure will be described. These examples are examples of the present disclosure, and the present disclosure is not limited to these examples.

[0169] (1) Production of Power Cables Power cables of samples A1 to A6 and B1 to B6 were produced as follows.

[0170] <Sample A1> For sample A1, a resin composition containing the following materials was mixed in a Banbury mixer and granulated into pellets in an extruder.

[0171] (Base resin) Low-density polyethylene (LDPE): 100 parts by mass Density: 0.92 g / ml

[0172] (Modified polymer) Maleic anhydride-modified polyethylene (MAH-PE): 2.5 parts by mass Maleic anhydride modification amount: 0.5% by mass

[0173] (Inorganic filler) Magnesium oxide (MgO): 1 part by mass MgO production method: gas phase method Volume average particle size: 50 nm Surface treatment with silane coupling agent: Yes Silane coupling agent: vinyltrimethoxysilane

[0174] (Crosslinking agent) 2,5-dimethyl-2,5-di(t-butylperoxy)hexane: 1.3 parts by mass

[0175] Next, a wire with a cross-sectional area of ​​2500 mm2 was formed by twisting together conductor core wires made of dilute copper alloy with a diameter of 14 mm. ２ After the conductors were prepared, the resin composition for the inner semiconductive layer containing the ethylene-ethyl acrylate copolymer, the above-mentioned resin composition, and the resin composition for the outer semiconductive layer made of the same materials as the resin composition for the inner semiconductive layer were charged into extruders A to C, respectively.

[0176] For sample A, the novel manufacturing method of the above-described embodiment was applied. That is, the extruder of the above-described embodiment was used as extruder B to extrude an insulating layer. Specifically, an inert gas was supplied into the cylinder by the atmosphere adjustment unit, and the insulating layer was extrusion-molded in an inert gas atmosphere. The contact surface of the resin composition adjacent to the hole of the discharge unit from which the resin composition is discharged was parallel to or inclined with respect to the axial direction of the cylinder. Furthermore, the contact surface of the resin composition on the screw flight facing upstream of the cylinder was inclined at an obtuse angle with respect to the outer peripheral surface of the screw body portion along the axial direction of the cylinder. The set temperature of extruder B was set to a temperature 10°C to 80°C higher than the melting point of the base resin. The set temperature was appropriately adjusted based on the linear speed and extrusion pressure.

[0177] The extrudates from each of the extruders A to C were introduced into a crosshead, and the inner semiconductive layer, the insulating layer, and the outer semiconductive layer were simultaneously extruded from the central axis of the conductor toward the outer periphery thereof, with thicknesses of the inner semiconductive layer, the insulating layer, and the outer semiconductive layer set to 0.5 mm, 25.0 mm, and 0.5 mm, respectively.

[0178] The extrusion process was carried out for 100 hours. The extrusion product was then heated at about 250° C. to crosslink the resin composition for the insulating layer.

[0179] In this manner, a power cable sample A1 having a conductor axial length of 6 km was manufactured.

[0180] <Sample A2> For Sample A2, a power cable was manufactured in the same manner as for Sample A1, except for the following differences.

[0181] (Inorganic filler) Silicon dioxide: 1 part by mass Fumed silica (described as nanosilica in Table 1 below) Volume average particle size: 12 nm Surface treatment with silane coupling agent: Yes Silane coupling agent: vinyltrimethoxysilane

[0182] (Modified Polymer) The resin composition of sample A2 does not contain a modified polymer.

[0183] <Sample A3> For sample A3, a power cable was manufactured in the same manner as for sample A1, except for the following differences.

[0184] (Base resin) Random polypropylene (r-PP): 70 parts by mass Stereoregularity: isotactic Density: 0.9 g / ml

[0185] (Modified polymer) Maleic anhydride-modified polypropylene (MAH-PP): 5 parts by mass Maleic anhydride modification amount: 5% by mass

[0186] (Thermoplastic elastomer) Styrene ethylene butylene styrene block copolymer (SEBS): 25 parts by mass Styrene unit content: 25% by mass

[0187] (Inorganic Filler, Crosslinking Agent) The resin composition of sample A3 does not contain an inorganic filler or a crosslinking agent.

[0188] (Manufacturing Process) For Sample A3, the set temperature of Extruder B was set to a temperature 10°C to 80°C higher than the melting point of the base resin. The set temperature was appropriately adjusted based on the linear speed and extrusion pressure. For Sample A3, the crosslinking step was not performed, and the base resin was not crosslinked.

[0189] <Sample A4> For Sample A4, a power cable was manufactured in the same manner as for Sample A3, except for the following differences.

[0190] (Thermoplastic elastomer) Ethylene propylene rubber (EPR): 25 parts by mass Ethylene unit content: 25% by mass

[0191] <Sample A5> For sample A5, a power cable was manufactured in the same manner as for sample A4, except for the following differences.

[0192] Content of base resin r-PP: 75 parts by mass Content of thermoplastic elastomer EPR: 25 parts by mass However, the resin composition of sample A5 does not contain a modified polymer.

[0193] <Sample A6> For sample A6, a power cable was produced in the same manner as for sample A1, except that the base resin was r-PP, similar to that of sample A3, the resin composition did not contain MAH-PE, and the base resin was non-crosslinked, similar to that of sample A3.

[0194] <Samples B1 to B6> Samples B1 to B6 were produced in the same manner as samples A1 to A6, respectively, except that the new production method was not applied.

[0195] That is, for samples B1 to B6, a conventional extruder was used as extruder B to extrude the insulating layer. Specifically, no inert gas was supplied into the cylinder. The contact surface of the resin composition adjacent to the hole of the discharge section that discharges the resin composition was perpendicular to the axial direction of the cylinder. Furthermore, the contact surface of the resin composition on the screw flight facing upstream of the cylinder was perpendicular to the outer circumferential surface of the screw body that was aligned with the axial direction of the cylinder.

[0196] (2) Evaluation (Collection of Inspection Samples) For each sample, a region including a 10 mm long region from the final extrusion end of the cable core was cut in the axial direction of the conductor using a cable slicer. After cutting the cable core, the conductor was removed from the cut piece. In this way, an inspection sample of the insulating layer was collected, including a 10 mm long region from the final extrusion end of the insulating layer in the axial direction of the conductor.

[0197] (Observation using near-infrared light) After the test sample was taken, the test sample of the insulating layer was observed using near-infrared light at 25°C to check whether or not the test sample had a line defect with a length of 5 mm or more in the axial direction of the conductor.

[0198] The inspection conditions were as follows: Inspection light: near-infrared light containing wavelengths of 800 nm or more and 2500 nm or less Light source: halogen lamp Imaging camera: InGaAs camera The inspection sample was immersed in liquid paraffin.

[0199] (3) Results The results of evaluation of each sample are explained with reference to the following Tables 1 and 2. In each table, the unit of the numerical values ​​of the compounding ingredients is "parts by mass."

[0200]

[0201]

[0202] <Samples B1 to B6> As shown in Table 2, in Samples B1 to B6, at least one line defect occurred over the entire region 10 mm long from the end of the insulating layer in the axial direction of the conductor. The line defect was found at the interface between the inner semiconducting layer and the insulating layer, or at the interface between the insulating layer and the outer semiconducting layer.

[0203] In samples B1 to B6, the new manufacturing method was not applied, and therefore the resin composition remained in the extruder, causing discoloration. It is believed that discoloration adhering to the crosshead caused stripe defects in the insulating layer. Therefore, in samples B1 to B6, there was a risk of protrusions in the semiconducting layer being present in at least a portion of the power cable.

[0204] <Samples A1 to A6> As shown in Table 1, in samples A1 to A6, no line defects with a length of 5 mm or more in the axial direction of the conductor occurred.

[0205] In samples A1 to A6, the application of the new manufacturing method made it possible to suppress retention of the resin composition in the extruder. This made it possible to suppress discoloration of the resin composition throughout the entire extrusion process of the insulating layer. This was confirmed by the above results, in which the insulating layers of samples A1 to A6 did not have streak defects. This made it possible to ensure that no protrusions were present in the semiconducting layer throughout the entire power cable for samples A1 to A6.

[0206] From the results of the above samples A1 to A6, it was confirmed that a power cable was obtained that can guarantee the absence of protrusions P in the semiconductive layer, even if the configuration makes it difficult to detect line defects using visible light.

[0207] <Supplementary Notes> The following supplementary notes are provided regarding aspects of the present disclosure. The aspects referenced by the numbers in brackets [ ] to which the supplementary notes below depend correspond to the aspects described in <Embodiments of the present disclosure>.

[0208] [7] The power cable according to any one of [1] to [4], wherein the insulating layer contains an inorganic filler.

[0209] [8] The power cable according to [7], wherein the resin composition constituting the insulating layer comprises: a base resin containing polyethylene; and the inorganic filler containing magnesium oxide.

[0210] [9] The power cable according to [7] or [8], wherein the resin composition constituting the insulating layer comprises: a base resin containing polyethylene; and the inorganic filler containing silicon dioxide.

[0211]

[10] The power cable according to any one of [1] to [4] and [7] to [9], wherein the insulating layer contains an organic additive having a refractive index different from that of the polyolefin at a wavelength of 500 nm.

[0212]

[11] The power cable according to

[10] , wherein the resin composition constituting the insulating layer comprises: a base resin containing polypropylene; and a modified polymer containing a propylene unit and modified with at least one selected from an unsaturated organic acid and a derivative thereof.

[0213]

[12] The power cable according to

[11] , wherein the resin composition constituting the insulating layer further contains a thermoplastic elastomer.

[0214]

[13] The power cable according to

[10] , wherein the resin composition constituting the insulating layer comprises a base resin containing polypropylene and a thermoplastic elastomer.

[0215]

[14] The power cable according to

[12] or

[13] , wherein the thermoplastic elastomer includes a styrene-based elastomer.

[0216]

[15] The power cable according to any one of

[12] to

[14] , wherein the thermoplastic elastomer includes an olefin-based elastomer.

[0217]

[16] The method for manufacturing a power cable according to [5], wherein in the step of forming the insulating layer, the insulating layer is extrusion-molded in an inert gas atmosphere.

[0218]

[17] The method for manufacturing a power cable according to [5] or

[16] , wherein in the step of forming the insulating layer, the insulating layer is extruded in an extruder that extrudes the resin composition, with at least a part of a contact surface with which the resin composition comes into contact being parallel to or inclined with respect to the extrusion direction of the resin composition.

[0219]

[18] The method for manufacturing a power cable according to the above

[17] , wherein the step of forming the insulating layer uses the extruder including a cylindrical cylinder into which the resin composition is supplied and a discharge part having a hole through which the resin composition is discharged from the cylinder, and the contact surface of the resin composition adjacent to the hole in the discharge part is set parallel to or inclined with respect to the axial direction of the cylinder.

[0220]

[19] The method for manufacturing a power cable according to

[17] or

[18] above, wherein the step of forming the insulating layer uses the extruder including: a cylindrical cylinder into which the resin composition is supplied; and a screw provided inside the cylinder along the axial direction of the cylinder and having a helical screw flight; and the abutment surface of the screw flight facing upstream of the cylinder with the resin composition is inclined at an obtuse angle with respect to the outer peripheral surface of the main body of the screw along the axial direction of the cylinder.

[0221] DESCRIPTION OF SYMBOLS 10 Power cable 110 Conductor 120 Inner semiconductive layer 130 Insulating layer 140 Outer semiconductive layer 150 Shielding layer 160 Sheath 200 Extruder 210 Cylinder 220 Hopper 230 Screw 231 Screw main body 232 Screw flight 240 Rotation drive mechanism 250 Discharge section 251 Hole 260 Atmosphere adjustment section 270 Retention suppression member 271 Tapered hole CH Crosshead P Projection SC Burn SD Line defect

Claims

1. A power cable comprising a conductor, an inner semiconductive layer, an insulating layer, and an outer semiconductive layer, in this order from the central axis of the conductor toward the outer periphery, wherein the insulating layer comprises a resin composition containing polyolefin, and wherein the insulating layer does not have a line defect with a length of 5 mm or more in the axial direction of the conductor when a region 10 mm long from the end of the insulating layer in the axial direction of the conductor is observed with near-infrared light at 25°C.

2. The power cable according to claim 1, wherein the insulating layer does not have a line defect with a length of 5 mm or more in the axial direction of the conductor at the interface between the inner semiconductive layer and the insulating layer, and at the interface between the insulating layer and the outer semiconductive layer, when observed with near-infrared light at 25°C in a region 10 mm long from the end of the insulating layer in the axial direction of the conductor.

3. The power cable according to claim 1 or 2, wherein the length of the power cable in the axial direction of the conductor is 4 km or more.

4. A power cable according to any one of claims 1 to 3, wherein when a sheet of the insulating layer having a thickness of 2.0 mm is produced, the light transmittance of the sheet at a wavelength of 500 nm measured at a temperature equal to or higher than the melting point of the polyolefin is 50% or less.

5. A method for manufacturing a power cable, comprising: a step of preparing a resin composition containing polyolefin; and a step of forming an inner semiconductive layer, an insulating layer containing the resin composition, and an outer semiconductive layer in this order from the central axis of the conductor toward the outer periphery so as to cover the outer periphery of the conductor, wherein in the step of forming the insulating layer, the insulating layer is extrusion-molded so that no stripe defects with a length of 5 mm or more in the axial direction of the conductor do not occur when a region 10 mm long from the end of the insulating layer in the axial direction of the conductor is observed with near-infrared light at 25°C.

6. A power cable inspection method comprising the steps of: preparing a power cable comprising a conductor, an inner semiconductive layer, an insulating layer containing polyolefin, and an outer semiconductive layer, in this order from the central axis of the conductor to the outer periphery; taking an inspection sample of the insulating layer including a region 10 mm long from the end of the insulating layer in the axial direction of the conductor; and inspecting the inspection sample of the insulating layer with near-infrared light at 25°C for the presence or absence of a line defect having a length of 5 mm or more in the axial direction of the conductor in the inspection sample.

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