Power cable, method of producing power cable, method of examining power cable

Abstract

A power cable includes: a conductor, an internal semiconductive layer, an insulating layer, and an external semiconductive layer, in this order from a central axis toward an outer circumference of the conductor, wherein the insulating layer includes a resin composition containing polyolefin, and when a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor is observed at 25°C with near-infrared light, the insulating layer has no streak defect having a length of 5 mm or more in the axial direction of the conductor.

Description

POWER CABLE, METHOD OF PRODUCING POWER CABLE, METHOD OF EXAMINING POWER CABLE

[0001] The present disclosure relates to a power cable, a method of producing a power cable, and a method of examining a power cable.

[0002] Foreign matters that get mixed in an insulating layer of a power cable will degrade insulation of the insulating layer. For this reason, after production of the power cable, a sample with a predetermined length is sometimes collected from an insulating layer and examined for the presence or absence of foreign matters in the sample of the insulating layer (for example, Patent Literature 1).

[0003] PTL. 1: Japanese Patent Laid-Open Publication No. H7-122134

[0004] According to an aspect of the present disclosure, 　　　there is provided a power cable, including: a conductor, an internal semiconductive layer, an insulating layer, and an external semiconductive layer, in this order from a central axis toward an outer circumference of the conductor, 　　　wherein the insulating layer includes a resin composition containing polyolefin, and 　　　when a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor is observed at 25°C with near-infrared light, the insulating layer has no streak defect having a length of 5 mm or more in the axial direction of the conductor.

[0005] FIG. 1 is a schematic cross-sectional view orthogonal to an axial direction of a power cable according to an embodiment of the present disclosure.FIG. 2 is a diagram illustrating a region having a predetermined length of the insulating layer according to an embodiment of the present disclosure, as observed with near-infrared light.FIG. 3 is a schematic configuration diagram of an extruder used in a method of producing of a power cable according to an embodiment of the present disclosure.FIG. 4 is a schematic cross-sectional diagram for explaining the shape of a screw flight.FIG. 5 is a schematic cross-sectional diagram for explaining a retention prevention member.FIG. 6 is a flow chart illustrating a method of producing a power cable according to an embodiment of the present disclosure.FIG. 7 is a diagram illustrating a case where a scorch adheres to the vicinity of a crosshead.FIG. 8 is a diagram illustrating a region of an insulating layer having a predetermined length as observed with near-infrared light, in a comparative example to which a conventional production method is applied.Problem to be Solved by the Disclosure

[0006] An object of the present disclosure is to ensure the absence of a protrusion of the semiconductive layer throughout the power cable.Advantageous Effect of the Present Disclosure

[0007] According to the present disclosure, the absence of the protrusion of the semiconductive layer throughout the power cable can be ensured by examining only the vicinity of the end of the insulating layer.Description of Embodiment of the Present Disclosure

[0008] <Knowledges Obtained by the Inventors> 　　　First, an overview of findings obtained by the inventors will be described.

[0009] (Regarding Degradation in Insulation due to Scorch During Extrusion Step) 　　　An internal semiconductive layer, an insulating layer, and an external semiconductive layer, which constitute a power cable, are extrusion-molded in this order from a central axis toward an outer circumference of a conductor.

[0010] In a step of forming an insulating layer of a power cable, when a resin composition for the insulating layer is extruded over a long time, the resin composition is retained in the cylinder of an extruder. When the resin composition is retained, the viscosity of the resin composition increases, possibly resulting in a scorch (hereinafter also referred to as scorch SC).

[0011] The term “scorch SC” used herein means a foreign matter which is a resin composition with increased viscosity. Specific examples of the scorch include early crosslinked products of the base resin in cases where the resin composition contains a crosslinking agent, decomposition products of the base resin, etc., and carbonized products of each of the materials constituting the resin composition.

[0012] When the above-mentioned scorch SC of the resin composition is generated, the following problem may arise. As an example, a vicinity of a portion (hereinafter, referred to as a “junction”) of a crosshead (hereinafter referred to as a crosshead CH) where a semiconductive resin composition for an internal semiconductive layer 120 and a resin composition for an insulating layer 130 join together, will be described below with reference to FIG. 7. In addition, a similar problem may arise in the vicinity of a junction where the semiconductive resin composition for the external semiconductive layer and the resin composition for the insulating layer 130 join together.

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

[0014] As illustrated in FIG. 7(ii), as the resin composition is extruded, the scorch SC gradually moves toward the junction of the crosshead CH. When the scorch SC reaches the junction of the crosshead CH, the scorch SC bites into the semiconductive resin composition, and the extrusion of the semiconductive resin composition is restricted by the scorch SC. For this reason, a streak defect (hereinafter also referred to as a streak defect SD) is generated at the interface between the internal semiconductive layer 120 and the insulating layer 130 at the downstream of the junction of the crosshead CH.

[0015] The term “streak defect SD” used herein refers to at least one of a concave portion and a convex portion that continuously extends along the axial direction of the conductor. A foreign matter such as a scorch SC may be present at the end of the streak defect SD.

[0016] At this time, the semiconductive resin composition flows into a gap between the junction of the crosshead CH and the scorch SC adhering to the junction. For this reason, an origin of the protrusion (described as “origin of protrusion P” in the figure) of the semiconductive resin composition may be generated.

[0017] Thereafter, as illustrated in FIG. 7(iii), the scorch SC that has adhered to the junction of the crosshead CH becomes detached, and the scorch SC moves downstream from the junction of the crosshead CH. At this time, a protrusion P of the internal semiconductive layer 120 may be generated in the vicinity of the rear end (upstream end) of the scorch SC that is detached from the junction of the crosshead CH.

[0018] In the power cable produced through the above process, the presence of a streak defect SD in the insulating layer 130 means that a scorch SC has been generated due to retention of the resin composition during extrusion. When a streak defect SD and a scorch SC are present in the insulating layer 130 of the power cable after production, the streak defect SD and the scorch SC themselves do not affect the insulation of the insulating layer 130.

[0019] However, in a case where the insulating layer 130 has a streak defect SD, that is, in a case where a scorch SC has been generated during the extrusion step of the insulating layer 130, a protrusion P of the semiconductive layer (internal semiconductive layer 120 or external semiconductive layer) may have been generated in at least a part of the power cable. An electric field is more likely to be concentrated at the protrusion P where the semiconductive layer mentioned above protrudes toward the insulating layer 130. Therefore, local insulation may be degraded at the protrusion P.

[0020] Even when only the above-mentioned streak defect SD is generated within a region having a predetermined length from the extruded rearmost end of the insulating layer 130, a protrusion P of the semiconductive layer may have been generated in an extruded intermediate region of the insulating layer 130 (that is, a region between the front end and the rear end of the insulating layer 130 which is continuously extruded) due to the intermittent generation and discharge of the scorch SC during the extrusion step.

[0021] Therefore, there has been a demand for a power cable which can be ensured to have no protrusion P of the semiconductive layer throughout the power cable.

[0022] (Regarding Novel Findings in Examination Step) 　　　Conventionally, an insulating layer of an AC power cable contains crosslinked polyethylene, but does not contain an additive having a refractive index different from that of the crosslinked polyethylene. Therefore, when the insulating layer is heated to a temperature equal to or more than the melting point of the polyethylene (e.g., 120°C or more), the insulating layer becomes transparent to visible light.

[0023] Then, the examination step is performed on the conventional AC power cable as follows. First, a sample for examination is collected, which includes a region having a predetermined length from the end of the insulating layer in the axial direction of the conductor. Next, the sample for examination is heated to a temperature equal to or more than the melting point of the polyethylene. The sample for examination is visually observed in a state where the insulating layer has become transparent upon such heating. The above steps have been used to examine the presence or absence of the above-mentioned streak defect, protrusion of the semiconductive layer, or the like, in the sample for examination of the insulating layer.

[0024] However, for example, in a case of a DC power cable, it was difficult to examine the presence or absence of a streak defect or the like, even in a state where the insulating layer is heated as mentioned above.

[0025] This is because the insulating layer of the DC power cable contains an additive which suppresses the accumulation of space charge in the insulating layer. The additive has a refractive index different from that of the crosslinked polyethylene. Examples of the additive include inorganic fillers and modified polymers. Therefore, even when the insulating layer is heated to a temperature equal to or more than the melting point of the polyethylene, the insulating layer is opaque to visible light.

[0026] As a result, for the DC power cable, even when the examination step was performed in a state where the insulating layer was heated as mentioned above, it was impossible to detect the presence of the streak defect, protrusion of the semiconductive layer, or the like in the sample for examination of the insulating layer.

[0027] For a sample for examination, prepared by thinly slicing an insulating layer of a DC power cable to a thickness of about 1 mm in an axial direction of a conductor, the inside of the insulating layer can be observed with visible light in some cases. In this case, the presence or absence of the foreign matter, protrusion, or the like in the sample for examination has been examined by observing the sample for examination having a thickness of about 1 mm with visible light. Even in this case, however, it was impossible to detect the presence of a streak defect extending along the axial direction of the conductor because of the sample for examination being too thin.

[0028] As mentioned above, it was conventionally difficult to detect a streak defect in the DC power cable, and it was impossible to reliably determine whether or not a scorch had been generated during the extrusion step of the insulating layer. As a result, it was conventionally difficult to ensure the absence of the protrusion of the semiconductive layer throughout the power cable.

[0029] Therefore, the present inventors have studied the examination light in the examination step. Specifically, the present inventors noted that the near-infrared light transmittance of the insulating layer is high even when the insulating layer includes an additive having a refractive index different from that of the crosslinked polyethylene.

[0030] As a result of intensive study, the present inventors have conceived a new examination method that uses near-infrared light to observe the insulating layer including the above-mentioned additive across a predetermined length in the axial direction of the conductor. It has been confirmed that the new examination method makes it possible to examine a sample of an insulating layer for the presence or absence of a streak defect SD or the like without heating the insulating layer, even when the insulating layer includes the above-mentioned additive.

[0031] Specifically, as illustrated in a comparative example of FIG. 8 below, it is found that the presence of a streak defect SD can be detected at the interface between the insulating layer 130 and the external semiconductive layer 140. The presence of the streak defect SD can also be detected at the interface between the internal semiconductive layer 120 and the insulating layer 130 as mentioned above.

[0032] (Regarding New Production Method Which Suppresses Generation of Scorch) 　　　The present inventors further studied the configuration of an extruder as a new production method which suppresses the generation of a scorch SC. The present inventors examined the insulating layer which was extrusion-molded by the extruder using the new examination method mentioned above for each time of improving the configuration of the extruder. As a result, the present inventors have found a configuration of an extruder that can suppress the generation of the scorch SC in the insulating layer.

[0033] Specifically, the inside of the extruder is adjusted to be an inert gas atmosphere. Furthermore, at least one of the screw and the discharge part of the extruder is configured to prevent the retention of the resin composition. It is found that this configuration can suppress the generation of the scorch SC of the resin composition.

[0034] The present disclosure is based on the above-mentioned knowledges found by 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 an embodiment of the present disclosure includes: a conductor, an internal semiconductive layer, an insulating layer, and an external semiconductive layer, in this order from a central axis toward an outer circumference of the conductor, 　　　wherein the insulating layer includes a resin composition containing polyolefin, and 　　　when a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor is observed at 25°C with near-infrared light, the insulating layer has no streak defect having a length of 5 mm or more in the axial direction of the conductor. 　　　According to this configuration, the absence of the protrusion of the semiconductive layer throughout the power cable can be ensured by examining only the vicinity of the end of the insulating layer.

[0037] [2] In the power cable described above in [1], 　　　when the region having a length of 10 mm from the end of the insulating layer in the axial direction of the conductor is observed at 25°C with near-infrared light, the insulating layer has no streak defect having a length of 5 mm or more in the axial direction of the conductor at an interface between the internal semiconductive layer and the insulating layer and at an interface between the insulating layer and the external semiconductive layer. 　　　According to this configuration, the absence of the protrusion of the semiconductive layer throughout the power cable can be ensured by examining only the vicinity of the end of the insulating layer.

[0038] [3] In the power cable described above in [1] or [2], 　　　a length of the power cable in the axial direction of the conductor is 4 km or more. 　　　According to this configuration, the absence of protrusion of the semiconductive layer can be ensured over a long distance in the axial direction of the conductor.

[0039] [4] In the power cable described above in any one of [1] to [3], 　　　when a sheet of the insulating layer having a thickness of 2.0 mm is produced, light transmittance at a wavelength of 500 nm through the sheet is 50% or less as measured at a temperature equal to or more than a melting point of the polyolefin. 　　　According to this configuration, a power cable can be obtained, which can be ensured to have no protrusion of the semiconductive layer even when the configuration of the power cable is such that it is difficult to detect a streak defect with visible light.

[0040] [5] A method of producing a power cable according to another aspect of the present disclosure includes: 　　　preparing a resin composition containing polyolefin; and 　　　forming an internal semiconductive layer, an insulating layer including the resin composition, and an external semiconductive layer, in this order from a central axis toward an outer circumference of a conductor, so as to cover the outer circumference of the conductor, 　　　wherein in the formation of the insulating layer, 　　　the insulating layer is extrusion-molded so that when a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor is observed at 25°C with near-infrared light, no streak defect having a length of 5 mm or more in the axial direction of the conductor is generated. 　　　According to this configuration, the absence of the protrusion of the semiconductive layer throughout the power cable can be ensured by examining only the vicinity of the end of the insulating layer.

[0041] [6] A method of examining a power cable according to another aspect of the present disclosure includes: 　　　preparing a power cable including a conductor, an internal semiconductive layer, an insulating layer containing polyolefin, and an external semiconductive layer in this order from a central axis toward an outer circumference of the conductor; 　　　collecting a sample for examination of the insulating layer including a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor; and 　　　examining the sample for examination of the insulating layer for the presence or absence of a streak defect having a length of 5 mm or more in the axial direction of the conductor by observing the sample for examination at 25°C with near-infrared light. 　　　According to this configuration, the presence of a streak defect can be detected even when the configuration of the power cable is such that it is difficult to detect a streak defect with visible light.Details of the Embodiment of the Present Disclosure

[0042] Next, an embodiment of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these illustrations, but intended to be indicated by claims and encompass all the changes which fall within the meaning and scope equivalent to claims.

[0043] <One Embodiment of the Present Disclosure> (1) Resin Composition 　　　The resin composition of this embodiment is a material constituting an insulating layer 130 of a power cable 10 described later. The resin composition includes, for example, a base resin, an inorganic filler, a modified polymer, a crosslinking agent, and other additives.

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

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

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

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

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

[0049] (2) Power Cable 　　　Next, with reference to FIG. 1, the power cable of this embodiment will be described.

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

[0051] Specifically, the power cable 10 includes, for example, a conductor 110, an internal semiconductive layer 120, an insulating layer 130, an external semiconductive layer 140, a shielding layer 150, and a sheath 160, in this order from a central axis toward an outer circumference of the conductor 110. Hereinafter, the internal semiconductive layer 120 and the external semiconductive layer 140 may be simply referred to as “semiconductive layer”.

[0052] (Conductor (Conductive part)) 　　　The conductor 110 is configured by twisting together a plurality of conductor core wires (conductive core wires) including, for example, pure copper, copper alloy, aluminum, aluminum alloy, or the like.

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

[0054] (Insulating Layer) 　　　The insulating layer 130 is provided so as to cover the outer circumference of the internal semiconductive layer 120. For example, the insulating layer 130 is formed through extrusion molding using the resin composition mentioned above.

[0055] In this embodiment, a novel production method described below suppresses the generation of a scorch SC in a cable core formation step S300 to provide an insulating layer 130 having no streak defect SD. This point will be described in detail below.

[0056] In this embodiment, the insulating layer 130 is crosslinked as a result of heating the above-mentioned resin composition after extrusion molding, for example. That is, for example, at least a part of polyethylene used as the base resin in the insulating layer 130 becomes crosslinked polyethylene.

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

[0058] (Shielding Layer) 　　　The shielding layer 150 is provided so as to cover the outer circumference of the external semiconductive layer 140. The shielding layer 150 is, for example, configured by winding a copper tape, or configured as a wire shield formed by winding a plurality of soft copper wires. A tape including rubberized cloth or the like as a raw material may be wound inside or outside the shielding layer 150.

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

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

[0061] In this embodiment, the insulating layer 130 contains an inorganic filler as mentioned above. The inorganic filler has a refractive index different from that of the polyethylene as the base resin at a wavelength of 500 nm. The insulating layers 130 may include, for example, inorganic fillers whose particle size is close to, or equal to or more than the wavelength of the visible light.

[0062] For this reason, the insulating layer 130 has low visible light transmittance even in a state where the insulating layer 130 is heated to a temperature equal to or more 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, light transmittance at a wavelength of 500 nm through the sheet is, for example, 50% or less as measured at a temperature equal to or more than a melting point of the polyolefin (e.g., 120°C or more). As a result, it becomes difficult to examine the presence or absence of the streak defect SD or the like in the insulating layer 130 of this embodiment with visible light.

[0063] Therefore, in the examination step S440 of this embodiment described below, the insulating layer 130 is observed with near-infrared light. Near-infrared light, for example, has a wavelength longer than the volume average particle size of the inorganic filler, and therefore easily passes through the inorganic filler. As a result, even when the insulating layer 130 includes the above-mentioned additive, it is possible to examine the presence or absence of the streak defect SD or the like in the insulating layer 130 without heating the insulating layer 130.

[0064] Here, a comparison will be made between the insulating layer 130 of the comparative example to which the conventional production method is applied and the insulating layer 130 of this embodiment to which a new production method described later is applied.

[0065] FIG. 8 is a diagram illustrating a region having a length of 10 mm from the end of the insulating layer 130 (an extruded rearmost end of the insulating layer 130, that is, a rearmost end where the insulating layer 130 which is extruded terminates) in the axial direction of the conductor 110 as observed with near-infrared light, in the comparative example to which the conventional production method is applied. In FIG. 8, the sample for examination of the insulating layer 130 is observed from a direction inclined with respect to the axial direction of the conductor 110 (the axial direction of the insulating layer 130 having a cylindrical shape).

[0066] In the comparative example of FIG. 8, a streak defect SD is generated at the interface between the insulating layer 130 and the external semiconductive layer 140. Note that, the streak defect SD may be generated at the interface between the internal semiconductive layer 120 and the insulating layer 130.

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

[0068] In the vicinity of the streak defect SD, there may be a scorch SC generated during extrusion, or a protrusion P where the internal semiconductive layer 120 or the external semiconductive layer 140 protrudes toward the insulating layer 130.

[0069] In the comparative example in which such a streak defect SD is generated, a scorch SC is generated during the extrusion step of the insulating layer 130, and there is a possibility that a protrusion P of the semiconductive resin composition is present in at least a part of the power cable 10. At the protrusion P where the internal semiconductive layer 120 or the external semiconductive layer 140 protrudes toward the insulating layer 130, the local insulation may be degraded.

[0070] In contrast, FIG. 2 is a diagram illustrating a region having a length of 10 mm from the end of the insulating layer 130 (an extruded rearmost end of the insulating layer 130, that is, a rearmost end where the insulating layer 130 which is extruded terminates) in the axial direction of the conductor 110 as observed with near-infrared light, in this embodiment, according to the same examination method as in the comparative example.

[0071] In this embodiment, since a new production method described below is applied, no streak defect SD is generated in the insulating layer 130 as observed with near-infrared light.

[0072] Specifically, for example, when a region having a length of 10 mm from an end of the insulating layer 130 in an axial direction of the conductor 110 (in an axial direction of the insulating layer 130 having a cylindrical shape) is observed at 25°C with near-infrared light, the insulating layer 130 has no streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110, as illustrated in FIG. 2. Thus, the fact that the insulating layer 130 has no streak defect SD means that no retention of the resin composition has occurred and no scorch SC of the resin composition has been generated throughout the extrusion step of the insulating layer 130. Accordingly, the absence of the protrusion P of the semiconductive layer throughout the power cable 10 can be ensured by examining only the vicinity of the end of the insulating layer 130.

[0073] In this embodiment, since a novel production method described below is applied, the insulating layer 130 has no streak defect SD mentioned above at the interface between the insulating layer 130 and the adjacent semiconductive layer.

[0074] Here, in the conventional production method, streak defects SD are likely to be generated 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 the scorch SC adhering to the crosshead CH.

[0075] In contrast, in this embodiment, the insulating layer 130 has no streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110, particularly at the interface between the internal semiconductive layer 120 and the insulating layer 130 and at an interface between the insulating layer 130 and the external semiconductive layer 140. Accordingly, the absence of the protrusion P of the semiconductive resin generated at the interface between the semiconductive layer and the insulating layer 130 can be ensured throughout the power cable 10.

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

[0077] In the conventional production method, as mentioned above, when the insulating layer 130 of the power cable 10 which is long is extruded over a long time, the resin composition is likely to be retained in the extruder. Therefore, a streak defect SD is likely to be generated in the insulating layer 130.

[0078] In contrast, in this embodiment, since a novel production method described later is applied, no streak defect SD is generated in a region having a length of 10 mm from the extruded rearmost end of the insulating layer 130 in the axial direction of the conductor 110, even when the length of the power cable 10 is 4 km or more in the axial direction of the conductor 110. It means that no retention of the resin composition has occurred and no scorch SC of the resin composition has been generated throughout the extrusion step of the insulating layer 130 for a long time. As a result, the absence of the protrusion P of the semiconductive layer can be ensured over a long distance in the axial direction of the conductor 110.

[0079] (4) Extruder 　　　Next, the extruder 200 used in the method of producing the power cable of this embodiment will be illustrated with reference to FIG. 3 to FIG. 5.

[0080] The extruder 200 of this embodiment is configured so that the insulating layer 130 can be extrusion-molded in an inert gas atmosphere, for example.

[0081] In the extruder 200 of this embodiment, for example, at least some of the abutment surfaces on which the resin composition abuts are parallel or inclined to the extrusion direction of the resin composition (axial direction of the cylinder 210). That is, at least some of the abutment surfaces of the resin composition is not perpendicular to the extrusion direction of the resin composition.

[0082] Specifically, the abutment surface of the resin composition, which is adjacent to the pore part of the discharge part 250 through which the resin composition is discharged, is parallel or inclined to the axial direction of the cylinder 210. In addition, the abutment 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 circumferential surface of the screw body part 231 along the axial direction of the cylinder 210.

[0083] Hereinafter, the extruder 200 of this embodiment will be explained in detail.

[0084] As illustrated in FIG. 3, the extruder 200 includes: a cylinder 210 having a cylindrical shape, into which materials for the resin composition are supplied; a hopper 220 for supplying materials into the cylinder 210; a screw 230 which is inserted from the first end (on the left in FIG. 3) in the axial direction of the cylinder 210 and rotatably arranged; a rotary drive mechanism 240 which rotates the screw 230; a discharge part 250 which is attached to the second end (on the right in FIG. 3), opposite to the first end, of the cylinder 210 and has a pore part through which the resin composition is discharged; and an atmosphere control part 260 which controls the inside of the cylinder 210 to be an inert gas atmosphere. Hereinafter, a region close to the first end in the axial direction of the cylinder 210 is referred to as “upstream”, and a region close to the second end in the axial direction of the cylinder 210 is referred to as “downstream”.

[0085] The cylinder 210 having a cylindrical shape includes a space for housing and mixing materials therein. In the cylinder 210 having a cylindrical shape, materials supplied in its inner space are mixed with a screw 230. The screw 230 is inserted from the first end in the axial direction of the cylinder 210, and placed at the center of the radial direction of the cylinder 210. The screw 230 is connected to and rotatably supported by the rotary drive mechanism 240. The screw 230 is configured to be rotated by the rotary drive mechanism 240 to extrude the materials toward the discharge part 250 while mixing them. The screw 230 may be uniaxial or biaxial. In FIG. 3, two screws 230 configured as biaxial are arranged side by side in the depth direction of the paper, and one of the screws 230 is depicted. As the rotary drive mechanism 240, for example, a known rotary motor can be used.

[0086] In the screw 230, a screw flight 232 is helically arranged on the outer circumferential surface of the screw body part 231. The cross-sectional shape of the screw flight 232 may be tapered, as illustrated in FIG. 4. FIG. 4 is a schematic diagram for explaining the shape of a screw flight, and is a cross-sectional diagram along the axial direction of the screw 230. In FIG. 4, a region close to the tip of the screw 230 (a region close to the second end in the axial direction of the cylinder 210) is illustrated on the right, and a region close to an end (base end) of the screw 230 (a region close to the first end in the axial direction of the cylinder 210) is illustrated on the left. When the screw flight 232 has a rectangular shape, as illustrated by the dashed line in FIG. 4, the resin composition is more likely to be retained in the vicinity of the abutment surface of the screw flight 232 close to the end of the screw 230, and the resin composition that is retained there may be decomposed by heating in some cases. Therefore, as for the cross-sectional shape of the screw flight 232, the side surface of the screw flight 232 close to the end of the screw 230 may be tapered, as illustrated in FIG. 4, from the viewpoint of preventing the retention of the resin composition in the vicinity of the abutment surface of the screw 230. In other words, the angle between the abutment surface of the resin composition on the screw flight 232 facing upstream of the cylinder 210 and the outer circumferential surface of the screw body part 231 may be obtuse. For example, the angle may be 120° or more and 145° or less.

[0087] The discharge part 250 is arranged at the second end in the axial direction of the cylinder 210. The discharge part 250 is configured, for example, to have a plurality of pores that penetrate in the thickness direction of the discharge part 250 (in the axial direction of the cylinder 210) to extrude the resin composition that has been mixed in the cylinder 210 to the outside. For example, a breaker plate can be used as the discharge part 250. A mesh or the like may be placed between the cylinder 210 and the discharge part 250 for the purpose of removing foreign matter contained in the resin composition.

[0088] As illustrated in FIG. 5, a retention prevention member 270 may be placed at the second end in the axial direction of the cylinder 210, in contact with the discharge part 250. FIG. 5 is a schematic diagram for explaining a retention prevention member, and is a cross-sectional diagram of the second end in the axial direction of the cylinder 210. As illustrated in FIG. 4, the retention prevention member 270 prevents the resin composition from being retained in the cylinder 210 and promotes extrusion through the discharge part 250. The retention prevention member 270 has a plurality of tapered pore parts 271 that penetrate the retention prevention member 270 in the thickness direction and gradually decrease in diameter in the thickness direction. The retention prevention member 270 is arranged so that the small-diameter opening of the tapered pore part 271 is communicated with the pore part 251 of the discharge part 250. In this configuration, the abutment surface of the resin composition at the tapered pore part 271 of the retention prevention member 270 adjacent to the pore part of the discharge part 250 is parallel or inclined to the axial direction of the cylinder 210. Thus, the resin composition is prevented from being retained at the edge of the pore part 251 of the discharge part 250 and at the corner between the inner wall of the cylinder 210 and the discharge part 250. The retention prevention member 270 may be made of the same material as the breaker plate, for example.

[0089] An atmosphere control part 260 that controls the inside of the cylinder 210 to be an inert gas atmosphere is connected to the cylinder 210. The atmosphere control part 260 is configured to supply an inert gas to the inside of the cylinder 210. Although air may be introduced inside the cylinder 210 along with materials supplied from the hopper 220, the atmosphere control part 260 can control the inside of the cylinder 210 to be an inert gas atmosphere. Thus, materials to be mixed or the resulting resin composition can be prevented from being oxidized by air and being thermally decomposed.

[0090] The inert gas is not particularly limited. For example, nitrogen gas or argon gas may be used as the inert gas.

[0091] The extruder 200 may include, for example, a heating part (not shown) for heating the inside of the cylinder 210. As the heating part, a conventionally known one can be used.

[0092] (5) Method of Producing Power Cable 　　　Next, with reference to FIG. 6, a method of producing a power cable of this embodiment will be described.

[0093] As illustrated in FIG. 6, the method of producing the power cable of this embodiment includes a resin composition preparation step S100, a conductor preparation step S200, a cable core formation step S300, a sample for examination collection step S420, an examination step S440, a shielding layer formation step S500, and a sheath formation step S600, for example. The method of producing a power cable of this embodiment includes a method of examining a power cable.

[0094] (S100: Resin Composition Preparation Step) 　　　First, the resin composition constituting 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 mixed material.

[0096] After the mixed material is formed, the mixed material is granulated by an extruder. As a result, pellet-shaped resin compositions constituting the insulating layer 130 are formed. An extruder 200 illustrated in FIG. 3 may be used.

[0097] (S200: Conductor Preparation Step) 　　　Meanwhile, a conductor 110 is prepared which is formed by twisting a plurality of conductor core wires.

[0098] (S300: Cable Core Formation Step (Insulating Layer Formation Step)) 　　　After the resin composition preparation step S100 and the conductor preparation step S200 are completed, the insulating layer 130 is formed so that the outer circumference of the conductor 110 is covered with the resin composition mentioned above in the cable core formation step S300.

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

[0100] Specifically, for example, a resin composition for the internal semiconductive layer is charged into an extruder A of the three-layer co-extruder, the extruder A forming the internal semiconductive layer 120.

[0101] As the extruder B for forming the insulating layer 130, an extruder 200 illustrated in FIG. 3 is used. The pellet-shaped resin composition mentioned above is charged into the extruder 200. At this time, the set temperature of the extruder B is set to a temperature higher than the melting point of the base resin by 10°C or more and 80°C or less. The set temperature is adjusted appropriately based on the linear velocity and extrusion pressure.

[0102] At this time, the insulating layer 130 is extrusion-molded in an inert gas atmosphere, in this embodiment. The term “inert gas atmosphere” used herein means that the partial pressure of the inert gas is 90% or more, or 95% or more, or 100% of the total pressure.

[0103] Specifically, as illustrated in FIG. 3, an inert gas is supplied inside the cylinder 210 by the atmosphere control part 260, thereby suppressing generation of the scorch SC due to thermal decomposition of the resin composition and the like.

[0104] At this time, in this embodiment, the insulating layer 130 is extrusion-molded in a state where at least some of the abutment surfaces on which the resin composition abuts are parallel or inclined to the extrusion direction of the resin composition (axial direction of the cylinder 210) in the extruder 200.

[0105] Specifically, as illustrated in FIG. 5, the insulating layer 130 is extrusion-molded in a state where the abutment surface of the resin composition adjacent to the pore part of the discharge part 250 through which the resin composition is discharged is parallel or inclined to the axial direction of the cylinder 210. In addition, the insulating layer 130 is extrusion-molded in a state where the abutment 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 circumferential surface of the screw body part 231 along the axial direction of the cylinder 210, as illustrated in FIG. 4.

[0106] Thus, the retention of the resin composition in the extruder 200 can be prevented. As a result, the generation of the scorch SC of the resin composition can be suppressed.

[0107] In addition, a resin composition for the external semiconductive layer is charged into an extruder C forming the external semiconductive layer 140, the resin composition including materials similar to those of the resin composition for the internal semiconductive layer charged into the extruder A.

[0108] Next, the respective extrudates from the extruders A to C are guided to a crosshead CH, and the internal semiconductive layer 120, the insulating layer 130, and the external semiconductive layer 140, from the central axis toward the outer circumference of the conductor, are simultaneously extruded on the outer circumference of the conductor 110. Accordingly, an extruded material that is to be a cable core is formed.

[0109] Thereafter, the insulating layer 130 is crosslinked by heating with radiation from an infrared heater or by heat transfer through a heating medium such as nitrogen gas or silicone oil at a high temperature, in a crosslinking tube pressurized with nitrogen gas or the like. Thus, a cable core constituted by the conductor 110, the internal semiconductive layer 120, the insulating layer 130, and the external semiconductive layer 140 is formed.

[0110] (S420: Sample for Examination Collection Step) 　　　After the cable core formation step S300 is completed, a sample for examination is collected.

[0111] Specifically, a region including a region having a length of 10 mm from the extruded rearmost end of the cable core in the axial direction of the conductor 110 is cut using a cable slicer. After the cable core is cut, the conductor 110 is removed from the cut piece. The cut piece is in a state where the internal semiconductive layer 120, the insulating layer 130, and the external semiconductive layer 140 still remain therein. Thus, a sample for examination of the insulating layer 130 including a region having a length of 10 mm from an end (extruded rearmost end) of the insulating layer 130 in an axial direction of the conductor 110 is collected.

[0112] (S440: Examination Step) 　　　After the sample for examination is collected, the sample for examination of the insulating layer 130 is examined for the presence or absence of the streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110 in the sample for examination by observing the sample for examination at 25°C with near-infrared light without heating.

[0113] Note that the term “axial direction of the conductor 110” used herein in the context of the sample for examination collected, when the conductor 110 is removed from the sample for examination, means the axial direction of the region where the conductor 110 was present. The “axial direction of the conductor 110” can also be rephrased as the “axial direction of the insulating layer 130” having a cylindrical shape.

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

[0115] For example, an indium gallium arsenide (InGaAs) camera with high photosensitivity in the near-infrared light wavelength range is used as an imaging camera. The lens to be attached to the imaging camera is a dedicated lens with high near-infrared transmittance. The imaging camera is arranged opposite to the source of near-infrared light across the sample for examination.

[0116] The sample for examination may be immersed in a liquid having a refractive index that is closer to the refractive index of the base resin than to the refractive index of air at the wavelength of near-infrared light. As the liquid, for example, liquid paraffin may be used. After the sample for examination is immersed in the above liquid, the sample for examination is removed from the liquid. Thus, a liquid having a refractive index close to the refractive index of the base resin is applied to the surface of the sample for examination. Application of such a liquid makes it possible to suppress deterioration in image quality caused by unevenness that occurs on the surface of the sample for examination when the sample for examination is cut.

[0117] At that time, in this embodiment, the sample for examination of the insulating layer 130 is observed from a direction inclined with respect to the axial direction of the conductor 110 (the axial direction of the insulating layer 130 having a cylindrical shape). Thus, the vicinity of the outer circumferential surface of the internal semiconductive layer 120 or the vicinity of the inner circumferential surface of the external semiconductive layer 140 can be observed. As a result, the presence or absence of the streak defect SD can be detected at the interface between each of the semiconductive layers and the insulating layer 130.

[0118] According to the examination step S440, the presence of a streak defect SD can be detected even when the configuration of the power cable 10 is such that it is difficult to detect the streak defect SD with visible light.

[0119] As a result of the examination step S440, when the insulating layer 130 of the sample for examination has a streak defect SD, as in the comparative example illustrated in FIG. 8, the cable core from which the sample for examination is collected is evaluated as defective.

[0120] On the other hand, as a result of the examination step S440, when the insulating layer 130 of the sample for examination has no streak defect SD, as in this embodiment illustrated in FIG. 2, the cable core from which the sample for examination is collected is evaluated as non-defective. The cable core evaluated as non-defective is used for the next step.

[0121] (S500: Shielding Layer Formation Step) 　　　After the examination step S440 is completed, the shielding layer 150 is formed outside the external semiconductive layer 140, for example, by winding a copper tape therearound, using a cable core evaluated as non-defective.

[0122] (S600: Sheath Formation Step) 　　　After the shielding layer 150 is formed, vinyl chloride is charged into an extruder and extruded from the extruder, to form a sheath 160 on the outer circumference of the shielding layer 150.

[0123] As described above, the power cable 10 as the solid insulation power cable is produced.

[0124] (6) Summary of this Embodiment 　　　According to this embodiment, one or more effects described below are achieved.

[0125] (a) In this embodiment, the insulating layer 130 is extrusion-molded in an inert gas atmosphere, in the insulating layer formation step. Further, the insulating layer 130 is extrusion-molded in a state where at least some of the abutment surfaces on which the resin composition abuts are parallel or inclined to the extrusion direction of the resin composition (axial direction of the cylinder 210), in the extruder 200. Thus, the retention of the resin composition in the extruder 200 can be prevented. As a result, the generation of the scorch SC of the resin composition can be suppressed.

[0126] (b) In this embodiment, the insulating layer 130 obtained by the above-mentioned novel production method has no streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110, when a region having a length of 10 mm from an end of the insulating layer 130 in an axial direction of the conductor 110 is observed at 25°C with near-infrared light.

[0127] The fact that the insulating layer 130 has no streak defect SD means that no retention of the resin composition has occurred and no scorch SC of the resin composition has been generated throughout the extrusion step of the insulating layer 130. That is, it corresponds to the absence of the protrusion P of the internal semiconductive layer 120 and the external semiconductive layer 140 throughout the power cable 10 including not only the region in the vicinity of the extruded rearmost end from which the sample for examination is collected but also an extruded intermediate region (that is, a region between the front end and the rear end of the insulating layer 130 which is continuously extruded).

[0128] As described above, according to this embodiment, the absence of the protrusion P of the internal semiconductive layer 120 and the external semiconductive layer 140 throughout the power cable 10 can be ensured by examining only the vicinity of the end of the insulating layer 130. The absence of the protrusion P of the semiconductive layer throughout the power cable 10 can suppress degradation in insulation due to the protrusion P.

[0129] (c) In the configuration in which the insulating layer 130 includes an inorganic filler, the retention of the resin composition is likely to occur during the extrusion step of the insulating layer 130 and a scorch SC may be easily generated in some cases. In addition, such a configuration makes it difficult to detect the presence of the streak defect SD with visible light, in the insulating layer 130 having a predetermined length in the axial direction of conductor 110, even when the insulating layer 130 is heated, as mentioned above.

[0130] In contrast, in this embodiment, even when the resin composition constituting the insulating layer 130 contains an inorganic filler, the generation of the scorch SC of the resin composition can be suppressed by the above-mentioned new production method. Accordingly, observation with near-infrared light confirms that no streak defect SD is generated in the insulating layer 130 of this embodiment.

[0131] As described above, according to this embodiment, a power cable 10 can be obtained, which can be ensured to have no protrusion P of the semiconductive layer even when the configuration of the power cable 10 is such that a scorch SC of the resin composition are easily generated and it is difficult to detect a streak defect SD with visible light.

[0132] (7) Modified Examples of this Embodiment 　　　The embodiment described above can be modified as in the following modified examples, if necessary. Hereinafter, only the elements different from the above-mentioned embodiment will be described, and the elements substantially the same as the elements described in the above-mentioned embodiment are represented by the same reference numerals and the description thereof will be omitted.

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

[0134] In the modified example 1, the resin composition may include no modified polymer. The additives in the modified example 1, for example, is the same as those in the embodiment mentioned above.

[0135] (Power Cable) 　　　When a region having a length of 10 mm from the end of the insulating layer 130 in the axial direction of the conductor 110 (in the axial direction of the insulating layer 130 having a cylindrical shape) is observed at 25°C with near-infrared light, the insulating layer 130 of the power cable 10 of the modified example 1 has no streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110, as with the embodiment mentioned above.

[0136] (Method of Producing Power Cable) 　　　The method of producing a power cable of the modified example 1 is the same as that in the embodiment mentioned above.

[0137] (Effect of Modified Example 1) 　　　In the modified example 1, the insulating layer 130 includes silicon dioxide as an inorganic filler. This configuration is such that it may be difficult to detect the presence of the streak defect SD with visible light in the insulating layer 130 having a predetermined length in the axial direction of conductor 110, even when the insulating layer 130 is heated. In contrast, in the insulating layer 130 of the modified example 1 obtained by the novel production method mentioned above, observation of the unheated state with near-infrared light confirms that no streak defect SD is generated. As described above, according to the modified example 1, a power cable 10 can be obtained, which can be ensured to have no protrusion P of the semiconductive layer even when the configuration of the power cable is such that it is difficult to detect a streak defect SD with visible light.

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

[0139] (Base Resin) 　　　Examples of polypropylene in the base resin include homopolypropylene (homo PP), random polypropylene (random PP), and block polypropylene (block PP). Polypropylene as the base resin is disclosed, for example, in WO2023 / 017562.

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

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

[0142] The styrene-based elastomer is, for example, a copolymer containing styrene units as a hard segment, and at least one type of monomer units of ethylene units, propylene units, butylene units, isoprene units as a soft segment.

[0143] The olefin-based elastomer is a copolymer, for example, containing two types of olefin units. The olefin-based elastomer is a copolymer, for example, containing ethylene units and α-olefin units having 3 or more carbon atoms, a copolymer containing propylene units and α-olefin units having 4 or more carbon atoms, or the like.

[0144] The styrene-based polymer and the olefin-based elastomer as the thermoplastic elastomers mentioned above are disclosed, for example, in WO2023 / 017562.

[0145] On the other hand, in the modified example 2, the resin composition may include no thermoplastic elastomer.

[0146] (Inorganic Filler) 　　　In the modified example 2, the resin composition may include no inorganic filler.

[0147] On the other hand, in the modified example 2, the resin composition may include a small amount of inorganic filler as that in the above mentioned embodiment or modified example 1.

[0148] (Crosslinking Agent, Other Additives) 　　　In the modified example 2, the resin component constituting the insulating layer 130 may be non-crosslinked from the viewpoint of recycling. In this case, the resin composition may include no crosslinking agent.

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

[0150] Other additives in the modified example 2, for example, are the same as those in the embodiments mentioned above.

[0151] (Power Cable) 　　　In the power cable 10 of the modified example 2, the insulating layer 130 may be non-crosslinked as mentioned above, or the insulating layer 130 may be slightly crosslinked, for example, so that the decomposition residue of the crosslinking agent is less than 300 ppm. Thus, with the insulating layer 130 being non-crosslinked or having reduced degree of crosslinking, the recyclability can be improved.

[0152] When a region having a length of 10 mm from the end of the insulating layer 130 in the axial direction of the conductor 110 (in the axial direction of the insulating layer 130 having a cylindrical shape) is observed at 25°C with near-infrared light, the insulating layer 130 of the power cable 10 of the modified example 2 has no streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110, as with the embodiment mentioned above, for example.

[0153] (Method of Producing Power Cable) 　　　The method of producing a power cable of the modified example 2 is the same as that in the embodiment mentioned above, except that the insulating layer 130 is not crosslinked during the cable core formation step S300. The method of producing a power cable of the modified example 2 in which the insulating layer 130 is slightly crosslinked is the same as that in the embodiment mentioned above.

[0154] (Effect of Modified Example 2) (a) In the modified example 2, the insulating layer 130 containing polypropylene is non-crosslinked or slightly crosslinked. In this configuration, when the insulating layer 130 is heated, the shape of the insulating layer 130 collapses, and therefore cannot be observed with visible light in a state where the insulating layer is heated as in the past. In contrast, in the modified example 2, it is possible to examine the presence or absence of a streak defect SD in a sample for examination of the insulating layer 130 without heating the insulating layer 130 by observing the sample for examination collected from the insulating layer 130 with near-infrared light.

[0155] (b) In the modified example 2, the insulating layer 130 includes an organic additive having a refractive index different from that of the polyolefin (polypropylene) as the base resin at a wavelength of 500 nm. In this configuration, the retention of the resin composition may be likely to occur and scorch SC may be easily generated during the extrusion step of the insulating layer 130 in some cases.

[0156] In contrast, in the modified example 2, even when the resin composition constituting the insulating layer 130 includes the organic additive mentioned above, the generation of the scorch SC of the resin composition can be suppressed by the above-mentioned new production method. Accordingly, observation with near-infrared light confirms that no streak defect SD is generated in the insulating layer 130 of the modified example 2.

[0157] As described above, according to the modified example 2, a power cable 10 can be obtained, which can be ensured to have no protrusion P of the semiconductive layer even when the configuration of the power cable 10 is such that a scorch SC of the resin composition may be easily generated and it may be difficult to detect a streak defect SD with visible light.

[0158] (7-3) Modified Example 3 　　　In the resin composition, power cable 10, and method of producing the power cable 10 of the modified example 3, the configurations not described below are essentially the same as those in the modified example 2.

[0159] (Resin Composition) 　　　The resin composition constituting the insulating layer 130 of the modified example 3 includes, for example, a base resin containing polypropylene, a thermoplastic elastomer, and other additives.

[0160] The thermoplastic elastomer of the modified example 3 is the same as that in the case where the resin composition of the modified example 2 includes a thermoplastic elastomer.

[0161] The resin composition of the modified example 3 does not include a modified polymer as used in the modified example 2.

[0162] (Power Cable) 　　　When a region having a length of 10 mm from the end of the insulating layer 130 in the axial direction of the conductor 110 (in the axial direction of the insulating layer 130 having a cylindrical shape) is observed at 25°C with near-infrared light, the insulating layer 130 of the power cable 10 of the modified example 3 has no streak defect SD having a length of 5 mm or more in the axial direction of the conductor 110, as with the embodiment mentioned above.

[0163] (Effect of Modified Example 3) 　　　In the modified example 3, the resin composition includes a thermoplastic elastomer, but does not include a modified polymer. Even in such a configuration, the insulating layer 130 may satisfy a predetermined insulation for direct current. In the modified example 3, the same effects as those in the modified example 2 can be obtained.

[0164] <Other Embodiments of the Present Disclosure> 　　　Although the embodiment of the present disclosure has been specifically described, the present disclosure is not limited to the embodiment described above, and various modifications can be made without departing from the gist of the present disclosure.

[0165] In the above-mentioned embodiment, the case in which the resin composition for the insulating layer 130 includes polyethylene and an inorganic filler has been described, but the present disclosure is not limited thereto. The resin composition for the insulating layer 130 may include polypropylene and an inorganic filler.

[0166] In the above-mentioned embodiment, an explanation is given for a case where the sample for examination collection step S420 and the examination step S440 are performed between the cable core formation step S300 and the shielding layer formation step S500, but the present disclosure is not limited to the case. For example, the sample for examination collection step S420 and the examination step S440 may be performed after the sheath formation step S600.

[0167] In the above-mentioned embodiment, three layers are extruded simultaneously in the cable core formation step S300, but they may be individually extruded.

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

[0169] (1) Preparation of Power Cable 　　　A power cable of each of the samples A1 to A6 and B1 to B6 was produced as follows.

[0170] <Sample A1> 　　　For the sample A1, a resin composition including the following materials was mixed in a Banbury mixer and granulated into pellets with 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 Amount of maleic anhydride-modification: 0.5 mass%

[0173] (Inorganic Filler) Magnesium oxide (MgO): 1 part by mass Method of producing MgO: Vapor phase method Volume average particle size: 50 nm Surface treatment with silane coupling agent: Applied Silane coupling agent: Vinyltrimethoxysilane

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

[0175] Next, a conductor having a cross-sectional area of 2500 mm2, formed by twisting conductor core wires having a diameter of 14 mm and made of a dilute copper alloy, was prepared. After the conductor was prepared, a resin composition for an internal semiconductive layer including an ethylene-ethyl acrylate copolymer, the above-mentioned resin composition, and a resin composition for an external semiconductive layer including a material similar to the resin composition for the internal semiconductive layer were respectively charged into extruders A to C.

[0176] For the sample A, the new production method of the embodiment mentioned above was applied. That is, an insulating layer was extrusion-molded using as the extruder B the extruder of the embodiment mentioned above. Specifically, an inert gas was supplied into a cylinder by an atmosphere control part, and the insulating layer was extrusion-molded in an inert gas atmosphere. An abutment surface of the resin composition adjacent to a pore part of a discharge part through which the resin composition was discharged was parallel or inclined to an axial direction of the cylinder. In addition, the abutment 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 circumferential surface of the screw body part along the axial direction of the cylinder. The set temperature of the extruder B was set to a temperature higher than the melting point of the base resin by 10°C or more and 80°C or less. The set temperature was adjusted appropriately based on the linear velocity and extrusion pressure.

[0177] The respective extrudates from the extruders A to C were guided to a crosshead, and the internal semiconductive layer, the insulating layer, and the external semiconductive layer, from the central axis toward the outer circumference of the conductor, were extruded on the outer circumference of the conductor. At this time, the thicknesses of the internal semiconductive layer, the insulating layer, and the external semiconductive layer were 0.5 mm, 25.0 mm and 0.5 mm, respectively.

[0178] The duration of the extrusion step mentioned above was 100 hours. Thereafter, the above-mentioned extrusion-molded product was heated at about 250°C to crosslink the resin composition for the insulating layer.

[0179] As described above, the power cable of the sample A1 having a length of 6 km in the axial direction of the conductor was produced.

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

[0181] (Inorganic Filler) Silicon dioxide: 1 part by mass Fumed silica (referred to as nanosilica in Table 1 below) Volume average particle size: 12 nm Surface treatment with silane coupling agent: Applied Silane coupling agent: Vinyltrimethoxysilane

[0182] (Modified Polymer) 　　　The resin composition of the sample A2 includes no modified polymer.

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

[0184] (Base Resin) Random polypropylene (r-PP): 70 parts by mass Tacticity: Isotactic Density: 0.9 g / ml

[0185] (Modified Polymer) Maleic anhydride-modified polypropylene (MAH-PP): 5 parts by mass Amount of maleic anhydride-modification: 5 mass%

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

[0187] (Inorganic Filler, Crosslinking Agent) 　　　The resin composition of the sample A3 includes no inorganic filler and no crosslinking agent.

[0188] (Production Step) 　　　For the sample A3, the set temperature of the extruder B was set to a temperature higher than the melting point of the base resin by 10°C or more and 80°C or less. The set temperature was adjusted appropriately based on the linear velocity and extrusion pressure. For the sample A3, the crosslinking step was not performed and the base resin was non-crosslinked.

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

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

[0191] <Sample A5> 　　　For sample A5, a power cable was produced in the same manner as the 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 　　　Note that the resin composition of the sample A5 includes no modified polymer.

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

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

[0195] That is, for the samples B1 to B6, a conventional extruder was used as the extruder B for extrusion molding of the insulating layer. Specifically, no inert gas was supplied into the cylinder. The abutment surface of the resin composition adjacent to the pore part of the discharge part through which the resin composition was discharged was perpendicular to the axial direction of the cylinder. In addition, the abutment 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 part along the axial direction of the cylinder.

[0196] (2) Evaluation (Collecting Sample for Examination) 　　　For each sample, a region including a region having a length of 10 mm from the extruded rearmost end of the cable core in the axial direction of the conductor was cut using a cable slicer. After the cable core was cut, the conductor was removed from the cut piece. Thus, a sample for examination of the insulating layer including a region having a length of 10 mm from the extruded rearmost end of the insulating layer in an axial direction of the conductor was collected.

[0197] (Observation with Near-infrared Light) 　　　After a sample for examination was collected, the sample for examination was examined for the presence or absence of a streak defect having a length of 5 mm or more in the axial direction of the conductor by observing the sample for examination of the insulating layer at 25°C with near-infrared light.

[0198] The examination conditions were as follows: 　　　Examination light: Near-infrared light including wavelengths of 800 nm or more and 2500 nm or less 　　　Light source: Halogen lump 　　　Imaging camera: InGaAs camera 　　　The sample for examination was immersed in liquid paraffin.

[0199] (3) Results 　　　With reference to the following Table 1 and Table 2, the evaluation results of each sample will be described. In each table, the unit of the numerical values for the compounding agent is “parts by mass”.

[0200]

[0201]

[0202] <Samples B1 to B6> 　　　As shown in Table 2, in samples B1 to B6, at least one streak defect was generated throughout the region having a length of 10 mm from the end of the insulating layer in the axial direction of the conductor. The streak defect was found at the interface between the internal semiconductive layer and the insulating layer, or at the interface between the insulating layer and the external semiconductive layer.

[0203] In the samples B1 to B6, since the new production method was not applied, the retention of the resin composition occurred in the extruder, and the scorch was generated. Accordingly, it is believed that the streak defect was generated in the insulating layer due to the scorch adhering to the crosshead. Therefore, in the samples B1 to B6, there was a risk that the protrusion of the semiconductive layer was present in at least a part of the power cable.

[0204] <Samples A1 to A6> 　　　As shown in Table 1, in samples A1 to A6, no streak defect having a length of 5 mm or more in the axial direction of the conductor was generated.

[0205] For the samples A1 to A6, since the new production method was applied, the retention of the resin composition in the extruder was prevented. Accordingly, the generation of the scorch of the resin composition was suppressed throughout the extrusion step of the insulating layer. This can be confirmed by the above results that the insulating layers of the samples A1 to A6 had no streak defect. As a result, in the samples A1 to A6, it is possible to ensure the absence of the protrusion of the semiconductive layer throughout the power cable.

[0206] The above results of the samples A1 to A6 confirm that a power cable was obtained, which can be ensured to have no protrusion P of the semiconductive layer, even when the configuration of the power cable was such that it was difficult to detect a streak defect with visible light.

[0207] <Supplementary Descriptions> 　　　Hereinafter, supplementary descriptions of the aspects of the present disclosure will be given. An aspect referenced by a number in brackets from which the following supplementary description depends corresponds to the aspect described in <Embodiments of the Present Disclosure>.

[0208] [7] The power cable described above in any one of [1] to [4], 　　　wherein the insulating layer includes an inorganic filler.

[0209] [8] The power cable described above in [7], 　　　wherein the resin composition constituting the insulating layer includes: 　　　　　　a base resin containing polyethylene, and 　　　　　　the inorganic filler containing magnesium oxide.

[0210] [9] The power cable described above in [7] or [8], 　　　wherein the resin composition constituting the insulating layer includes: 　　　　　　a base resin containing polyethylene, and 　　　　　　the inorganic filler containing silicon dioxide.

[0211]

[0010] The power cable described above in 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]

[0011] The power cable described above in

[0010] , 　　　wherein the resin composition constituting the insulating layer includes: 　　　　　　a base resin containing polypropylene, and 　　　　　　a modified polymer that contains propylene units and is modified with at least one selected from unsaturated organic acids and derivatives thereof.

[0213]

[0012] The power cable described above in

[0011] , 　　　wherein the resin composition constituting the insulating layer further includes a thermoplastic elastomer.

[0214]

[0013] The power cable described above in

[0010] , 　　　wherein the resin composition constituting the insulating layer includes: 　　　　　　a base resin containing polypropylene, and 　　　　　　a thermoplastic elastomer.

[0215]

[0014] The power cable described above in

[0012] or

[0013] , 　　　wherein the thermoplastic elastomer contains a styrene-based elastomer.

[0216]

[0015] The power cable described above in any one of

[0012] to

[0014] , 　　　wherein the thermoplastic elastomer contains an olefin-based elastomer.

[0217]

[0016] The method of producing a power cable described above in [5], 　　　wherein in the formation of the insulating layer, 　　　the insulating layer is extrusion-molded in an inert gas atmosphere.

[0218]

[0017] The method of producing a power cable described above in [5] or

[0016] , 　　　wherein in the formation of the insulating layer, 　　　the insulating layer is extrusion-molded in a state where at least some of abutment surfaces on which the resin composition abuts is parallel or inclined to an extrusion direction of the resin composition, in the extruder which extrudes the resin composition.

[0219]

[0018] The method of producing a power cable described above in

[0017] , 　　　wherein in the formation of the insulating layer, 　　　the extruder is used which includes a cylinder having a cylindrical shape into which the resin composition is supplied, and a discharge part having a pore part through which the resin composition is discharged from the cylinder, and 　　　the abutment surface of the resin composition adjacent to the pore part of the discharge part is in a state of being parallel or inclined to an axial direction of the cylinder.

[0220]

[0019] The method of producing a power cable described above in

[0017] or

[0018] , 　　　wherein in the formation of the insulating layer, 　　　the extruder is used which includes a cylinder having a cylindrical shape into which the resin composition is supplied, and a screw which is provided in the cylinder along an axial direction of the cylinder and has a helical screw flight, and 　　　the abutment surface of the resin composition on the screw flight facing upstream of the cylinder is in a state of being inclined at an obtuse angle with respect to an outer circumferential surface of a screw body part along the axial direction of the cylinder.

[0221] 10 Power cable 110 Conductor 120 Internal semiconductive layer 130 Insulating layer 140 External semiconductive layer 150 Shielding layer 160 Sheath 200 Extruder 210 Cylinder 220 Hopper 230 Screw 231 Screw body part 232 Screw flight 240 Rotary drive mechanism 250 Discharge part 251 Pore part 260 Atmosphere control part 270 Retention prevention member 271 Tapered pore part CH Crosshead P Protrusion SC Scorch SD Streak defect

Claims

1. A power cable, comprising: a conductor, an internal semiconductive layer, an insulating layer, and an external semiconductive layer, in this order from a central axis toward an outer circumference of the conductor, 　　　wherein the insulating layer comprises a resin composition containing polyolefin, and 　　　when a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor is observed at 25°C with near-infrared light, the insulating layer has no streak defect having a length of 5 mm or more in the axial direction of the conductor.

2. The power cable according to claim 1, 　　　wherein when a region having a length of 10 mm from the end of the insulating layer in the axial direction of the conductor is observed at 25°C with near-infrared light, the insulating layer has no streak defect having a length of 5 mm or more in the axial direction of the conductor at an interface between the internal semiconductive layer and the insulating layer and at an interface between the insulating layer and the external semiconductive layer.

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

4. The 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, a transmittance of light at a wavelength of 500 nm through the sheet is 50% or less as measured at a temperature equal to or more than the melting point of the polyolefin.

5. A method of producing a power cable, comprising: 　　　preparing a resin composition containing polyolefin; and 　　　forming an internal semiconductive layer, an insulating layer including the resin composition, and an external semiconductive layer, in this order from a central axis toward an outer circumference of a conductor, so as to cover an outer circumference of the conductor 　　　wherein in the formation of the insulating layer, 　　　the insulating layer is extrusion-molded so that when a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor is observed at 25°C with near-infrared light, no streak defect having a length of 5 mm or more in the axial direction of the conductor is generated.

6. A method of examining a power cable, comprising: 　　　preparing a power cable including a conductor, an internal semiconductive layer, an insulating layer containing polyolefin, and an external semiconductive layer in this order from a central axis toward an outer circumference of the conductor; 　　　collecting a sample for examination of the insulating layer including a region having a length of 10 mm from an end of the insulating layer in an axial direction of the conductor; and 　　　examining the sample for examination of the insulating layer for the presence or absence of a streak defect having a length of 5 mm or more in the axial direction of the conductor by observing the sample for examination at 25°C from the direction inclined with respect to the axial direction of the insulating layer having a cylindrical shape, using an imaging camera with near-infrared light from the near-infrared light source in a state where the imaging camera is arranged on the opposite side of the light source across the sample for examination.

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