Resin composition, power cable, and method for manufacturing a power cable

Abstract

The resin composition is a resin composition that constitutes the insulating layer of a power cable, comprising a base resin containing polyolefin and an inorganic filler, wherein the resin composition satisfies formula (1) with respect to the dispersibility of the inorganic filler in the resin composition, 600D + 1.6 × 10 4 ≤A ≤ 16000D + 3.0 × 10 6 ...(1). Here, D is the volume-average particle size of the inorganic filler, with units of nm, and A is the average Voronoi area, which is the area of ​​the region surrounding the target particle in the cross-section of the resin composition by the perpendicular bisectors of the lines connecting the target particle and other adjacent particles of the inorganic filler, with units of nm. 2 That is the case.

Description

Technical Field

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

Background Art

[0002] An inorganic filler may be added to a resin composition constituting an insulating layer of a power cable (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] According to one aspect of the present disclosure, there is provided a resin composition constituting an insulating layer of a power cable, including a base resin containing a polyolefin and an inorganic filler, wherein the resin composition satisfies the following formula (1) with respect to the dispersibility of the inorganic filler in the resin composition: 600D + 1.6×10 4 ≦A≦16000D + 3.0×10 6 ···(1), where D is the volume average particle diameter of the inorganic filler, with the unit being nm, and A is the average value of the Voronoi area, which is the area of the region surrounding a target particle by the perpendicular bisector of the straight line connecting the target particle of the inorganic filler and another particle adjacent to the target particle in the cross-section of the resin composition, with the unit being nm 2 . A resin composition is provided.

Brief Description of the Drawings

[0005] [Figure 1] 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. [Figure 2] FIG. 2 is a schematic cross-sectional view of an insulating layer of a power cable according to an embodiment of the present disclosure. [Figure 3]Figure 3 is a flowchart showing a method for manufacturing a power cable according to one embodiment of the present disclosure. [Figure 4] Figure 4 is a schematic cross-sectional view of an extruder according to one embodiment of the present disclosure. [Figure 5A] Figure 5A is a schematic diagram showing the measurement of the AC tree starting electric field strength. [Figure 5B] Figure 5B is a schematic cross-sectional view of the tip of the needle electrode during the measurement of the AC tree starting electric field strength. [Figure 6] Figure 6 is a table showing the results for samples A1 through A12. [Figure 7] Figure 7 is a table showing the results for samples B1 through B12. [Figure 8] Figure 8 is a table showing the results for samples C1 through C12. [Figure 9] Figure 9 is a table showing the results for samples D1 through D12. [Figure 10] Figure 10 shows the relationship between the volume-average particle size D of the inorganic filler and the Voronoi area A for samples A1 to D12. [Figure 11] Figure 11 is a table showing the results for samples A3-1 to A3-8. [Modes for carrying out the invention]

[0006] The purpose of this disclosure is to make it less likely for the characteristics of power cables to deteriorate due to thermal aging to occur.

[0007] According to this disclosure, it is possible to make it less likely for the characteristics of power cables to deteriorate due to thermal aging to occur.

[0008] <Knowledge gained by the inventors, etc.> First, we will explain the knowledge gained by the inventors, etc.

[0009] By dispersing inorganic fillers within the insulating layer of a power cable, space charges generated within the insulating layer can be trapped within each inorganic filler. This suppresses the accumulation of localized space charges within the insulating layer, thereby improving the insulation performance of the power cable.

[0010] Conventionally, it was thought that it was sufficient for the inorganic filler to be uniformly dispersed in the insulating layer.

[0011] However, even if the inorganic filler is uniformly dispersed in the insulating layer, it was found that new problems arise in the thermally aged insulating layer when the inorganic filler is dispersed in states (i) and (ii). (i) When the inorganic filler is sparsely dispersed by the conventional manufacturing method 1 (ii) When the inorganic filler is densely dispersed by the conventional manufacturing method 2

[0012] In this context, "a state in which inorganic fillers are sparsely dispersed" means a state in which at least some of the inorganic fillers aggregate (form clumps), resulting in long interparticle distances, that is, a state in which the Voronoi area, as described later, is large.

[0013] On the other hand, "a state in which inorganic fillers are densely dispersed" means a state in which the inorganic fillers are dispersed individually without agglomerating, and the distance between particles is short, that is, a state in which the Voronoi area, which will be described later, is small.

[0014] (i) When the inorganic filler is sparsely dispersed by the conventional manufacturing method 1 In conventional manufacturing method 1, for example, first, without forming the masterbatch described later, the entire amount of base resin and inorganic filler was melted and mixed in an extruder to form a pellet-shaped mixture. Furthermore, by impregnating this mixture with a crosslinking agent, pellets containing all the compounding agents were formed. Subsequently, the insulating layer was extruded by feeding these pellets into an extruder.

[0015] In this case, the inorganic filler was mixed with the molten base resin only twice (pellet extrusion and insulation layer extrusion). As a result, although the inorganic filler was uniformly dispersed in the insulation layer, at least some of the inorganic filler aggregated and formed clumps. In other words, the inorganic filler was uniformly dispersed in the insulation layer, but also sparsely dispersed.

[0016] Normally, even if an electric tree is generated in the insulating layer due to a high electric field, the uniformly dispersed inorganic filler in the insulating layer can prevent the propagation of the electric tree.

[0017] However, when the insulating layer ages due to heat, at least a portion of the base resin deteriorates. This deterioration of the base resin can occur, for example, due to the breaking of the main chain of the base resin, resulting in a partial decrease in molecular weight, or due to the deterioration of the electrical properties of the base resin caused by oxygen-based functional groups generated by oxidation reactions of the base resin. As a result, electrical treeing becomes more easily propagated in the deteriorated parts of the base resin.

[0018] As described above, when the inorganic filler was sparsely dispersed in the insulating layer and at least a portion of the base resin deteriorated due to thermal aging, electrical trees easily propagated through the deteriorated areas of the base resin that formed in broad regions between the sparsely dispersed inorganic fillers. Therefore, the inorganic filler could not adequately prevent the propagation of electrical trees. As a result, when the insulating layer deteriorated due to thermal aging, the dielectric breakdown strength of the insulating layer decreased excessively.

[0019] (ii) When the inorganic filler is densely dispersed by the conventional manufacturing method 2 In conventional manufacturing method 2, for example, a masterbatch was first formed by melt-mixing a small amount of base resin with a high concentration of inorganic filler. After the masterbatch was formed, polyolefin was added to the masterbatch and melt-mixed with an extruder. This diluted the masterbatch so that the inorganic filler content reached a predetermined level, forming a pellet-shaped mixture. Furthermore, a crosslinking agent was impregnated into the mixture to form pellets containing all the compounding agents. Subsequently, the pellets were fed into an extruder to extrude and mold the insulating layer.

[0020] In this case, there were three instances where the inorganic filler was mixed with the molten base resin (masterbatch formation, dilution mixing, and insulation layer extrusion). As a result, the inorganic filler was sufficiently separated and uniformly dispersed. That is, the inorganic filler was uniformly dispersed within the insulation layer, but densely dispersed.

[0021] Thus, when inorganic fillers are densely dispersed, the insulating layer tends to harden. However, under normal circumstances, the presence of a base resin between the inorganic fillers provides the desired elasticity to the insulating layer as a whole.

[0022] However, as mentioned above, when the insulating layer ages due to heat, at least a portion of the base resin deteriorates. As a result, the deteriorated portion of the base resin hardens.

[0023] As described above, when inorganic fillers are densely dispersed in the insulating layer, and at least a portion of the base resin deteriorates due to thermal aging, the deteriorated portion of the base resin hardens in the narrow regions between the densely dispersed inorganic fillers, meaning that the original flexibility of the base resin is lost. As a result, when the insulating layer undergoes thermal aging, the elasticity of the insulating layer as a whole increases excessively.

[0024] In response to the aforementioned new challenges, the inventors diligently considered various options and subsequently invented the novel manufacturing method (two-stage material introduction extrusion method) described below. This successfully reduced the likelihood of deterioration of power cable characteristics caused by thermal aging.

[0025] This disclosure is based on the aforementioned findings discovered by the inventors.

[0026] <Embodiments of this disclosure> Next, embodiments of this disclosure will be listed and described.

[0027] [1] A resin composition according to one aspect of the present disclosure is A resin composition that constitutes the insulating layer of a power cable, A base resin containing polyolefin, Inorganic fillers and, Includes, The resin composition satisfies formula (1) with respect to the dispersibility of the inorganic filler in the resin composition. 600D + 1.6 × 10 4 ≤A ≤ 16000D + 3.0 × 10 6 ...(1) Here, D is the volume-average particle size of the inorganic filler, with units of nm. A is the average value of the Voronoi area, which is the area of ​​the region surrounding a target particle in a cross-section of the resin composition, defined by the perpendicular bisectors of the straight lines connecting the target particle and other adjacent particles of the inorganic filler, and the unit is nm. 2 That is the case. This configuration makes it less likely for the characteristics of power cables to deteriorate due to thermal aging.

[0028] [2] Power cables relating to other aspects of the present disclosure are: A conductor and An insulating layer covering the outer circumference of the conductor, Equipped with, The aforementioned insulating layer is A base resin containing polyolefin, Inorganic fillers and, Includes, The insulating layer satisfies formula (1) with respect to the dispersibility of the inorganic filler in the insulating layer. 600D + 1.6 × 10 4≦A≦16000D + 3.0×10 6 ···(1) Here, D is the volume average particle diameter of the inorganic filler, with the unit of nm, A is the average value of the Voronoi area, which is the area of the region surrounding the target particle by the perpendicular bisector of the straight line connecting the target particle among the inorganic fillers and another particle adjacent to the target particle in the cross-section of the insulating layer, with the unit of nm 2 is as follows. According to this configuration, it is possible to make it difficult for the characteristics of the power cable to deteriorate due to thermal aging.

[0029] [3] In the power cable according to [2] above, After performing the first acceleration treatment on the test piece collected from the insulating layer, the storage elastic modulus of the test piece at 25°C measured by dynamic viscoelasticity measurement is 800 MPa or less, Here, the first acceleration treatment is a step of heating the test piece for 200 hours under the condition of a temperature of 145°C in a nitrogen atmosphere, a step of performing 5 heat cycles including a high-temperature step of heating the test piece at a temperature of 100°C for 1 hour and a low-temperature step of cooling the test piece at a temperature of 0°C for 1 hour, has. According to this configuration, even when the insulating layer is thermally aged, the flexibility of the power cable can be maintained.

[0030] [4] In the power cable according to [2] or [3] above, After performing the first acceleration treatment on the test piece collected from the insulating layer, by applying an alternating electric field to the test piece at a temperature of 90°C using a needle-shaped electrode having a tip with a curvature radius of 5 μm, when an electric tree is generated in the test piece, the alternating electric field strength is 600 kV / mm or more, Here, the first acceleration treatment is a step of heating the test piece for 200 hours under the condition of a temperature of 145°C in a nitrogen atmosphere, A process of performing a heat cycle five times, which includes a high-temperature step of heating the test specimen at a temperature of 100°C for 1 hour, and a low-temperature step of cooling the test specimen at a temperature of 0°C for 1 hour. It holds. This configuration allows the insulation properties of the power cable to be maintained even if the insulating layer undergoes thermal aging.

[0031] [5] In any one of the power cables described in [2] to [4] above, In the cross-section of the insulating layer, the standard deviation of the Voronoi area, which is the area of ​​the region surrounding the target particle in the inorganic filler by the perpendicular bisector of the straight line connecting the target particle and other adjacent particles, is 3.0 × 10⁻⁶. 7 nm 2 The following applies: This configuration makes it less likely for the characteristics of the power cable to deteriorate, even if the insulating layer undergoes thermal aging under more severe temperature conditions.

[0032] [6] In any one of the power cables described in [2] to [5] above, The inorganic filler contains silicon dioxide, The volume-average particle size D of the inorganic filler is 1 nm or more and 1 μm or less. The average value A of the Voronoi area in the cross-section of the insulating layer is 1.6 × 10 4 nm 2 The above 1.9 × 10 7 nm 2 The following applies: This configuration reliably prevents excessive increases in elasticity and excessive decreases in dielectric breakdown strength in the insulating layer, even if the insulating layer undergoes thermal aging.

[0033] [7] In any one of the power cables described in [2] to [5] above, The inorganic filler contains magnesium oxide, The volume-average particle size D of the inorganic filler is 100 nm or more and 5 μm or less. The average value A of the Voronoi area in the cross-section of the insulating layer is 7.6 × 104 nm 2 The above 8.3 × 10 7 nm 2 The following applies: This configuration reliably prevents excessive increases in elasticity and excessive decreases in dielectric breakdown strength in the insulating layer, even if the insulating layer undergoes thermal aging.

[0034] [8] In any one of the power cables described in [2] to [7] above, The base resin further comprises a modified polyolefin having polar groups. This configuration allows for stable dispersibility of inorganic fillers.

[0035] [9] In any one of the power cables described in [2] to [7] above, The base resin does not contain a modified polyolefin having polar groups. With this configuration, by implementing a new manufacturing method, dispersibility of the inorganic filler in the insulating layer can be obtained even if the base resin does not contain modified polyolefin.

[0036]

[10] Methods for manufacturing power cables according to other aspects of the present disclosure are: A step of preparing a resin composition comprising a base resin containing polyolefin and an inorganic filler, The process involves forming an insulating layer so as to cover the outer circumference of a conductor by extrusion molding of the resin composition, Equipped with, The step of preparing the resin composition is: A step of preparing a masterbatch containing the base resin and the inorganic filler, A step of preparing a filler-free compound that includes the base resin and does not contain the inorganic filler, It has, The step of forming the insulating layer is, A step of introducing a portion of the masterbatch and the filler-free compound from the first inlet of the extruder, A step of introducing the remaining portion of the master batch from a second inlet located downstream of the first inlet of the extruder, It holds. This configuration makes it less likely for the characteristics of power cables to deteriorate due to thermal aging.

[0037] Next, an embodiment of the present disclosure will be described below with reference to the drawings. It should be understood that the embodiments disclosed herein are illustrative and not restrictive in all respects. At least one configuration or feature described in each embodiment and example can be combined with other embodiments and examples, or modified in various ways. The scope of the present disclosure is defined by the claims, and all modifications within the meaning and scope of the claims are intended to be included.

[0038] <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, which will be described later, and includes, for example, a base resin and an inorganic filler. The resin composition may further include a crosslinking agent and other additives.

[0039] (Base resin) The base resin (base polymer) refers to the resin component that constitutes the main component of the resin composition. In this embodiment, the base resin includes, for example, polyolefin. Examples of polyolefins included in the base resin include polyethylene, polypropylene, ethylene-α-olefin copolymer, and thermoplastic elastomers obtained by dispersing or copolymerizing ethylene-propylene rubber in polypropylene. Among these, the base resin may be at least one of polyethylene or polypropylene. Two or more of these may be used in combination.

[0040] Examples of polyethylene included in the base resin include low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE). Furthermore, these polyethylenes may be in either a linear or branched form.

[0041] (Modified polyolefin) The base resin may further contain a modified polyolefin having polar groups.

[0042] In modified polyolefins, for example, polar groups are grafted onto the polyolefin. By including such modified polyolefins, the compatibility (adhesion) between the polar inorganic filler and the base resin can be improved, and the dispersibility of the inorganic filler, as described later, can be stably obtained.

[0043] The polyolefin constituting the modified polyolefin may be, for example, the same as the polyolefin contained in the base resin.

[0044] Modified polyolefins are composed, for example, of acid-modified polyolefins. The polar groups grafted onto the acid-modified polyolefin may be, for example, at least one of maleic anhydride and phthalic anhydride.

[0045] The modification rate (copolymerization rate) of polar groups to the polyolefin in the modified polyolefin is not particularly limited, but may be, for example, 0.1% by mass or more and 0.5% by mass or less.

[0046] The content of modified polyolefin in the resin composition is not particularly limited, but for example, it may be 0.5 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the base resin.

[0047] On the other hand, in this embodiment, the base resin does not need to contain a modified polyolefin having polar groups. By implementing the novel manufacturing method (two-stage material introduction extrusion method) described later, the dispersibility of the inorganic filler described later can be obtained in the insulating layer 130 even if the base resin does not contain a modified polyolefin. This makes it possible to reduce manufacturing costs.

[0048] (Inorganic fillers) The inorganic filler traps space charges in the insulating layer 130 and acts to suppress the local accumulation of space charges in the insulating layer 130. This improves the insulating properties of the insulating layer 130.

[0049] Examples of inorganic fillers include magnesium oxide (MgO), silicon dioxide (SiO2, also referred to as "silica" or "nanosilica"), titanium dioxide, zinc oxide, and aluminum oxide. Two or more of these may be used in combination as inorganic fillers.

[0050] The inorganic filler may contain, for example, MgO. Methods for forming MgO as an inorganic filler include, for example, a gas-phase method in which Mg vapor is brought into contact with oxygen, or a seawater method in which it is formed from seawater as a raw material. In this embodiment, the method for forming the inorganic filler may be either the gas-phase method or the seawater method.

[0051] The inorganic filler may, for example, contain only magnesium oxide. This can improve the insulating properties of the insulating layer 130.

[0052] Alternatively, the inorganic filler may include, for example, silicon dioxide. While not limited to, the silicon dioxide used as the inorganic filler could be at least one of fumed silica, colloidal silica, precipitated silica, or deflagration silica. Among these, fumed silica is preferred.

[0053] The inorganic filler may, for example, contain only silicon dioxide. This can improve the insulating properties of the insulating layer 130.

[0054] In this embodiment, the content of the inorganic filler in the resin composition is adjusted according to the target Voronoi area described later and is not particularly limited. However, the content of the inorganic filler in the resin composition may be, for example, 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the base resin. By setting the inorganic filler content to 0.1 parts by mass or more, space charges can be sufficiently trapped with respect to the inorganic filler. On the other hand, by setting the inorganic filler content to 5 parts by mass or less, the moldability of the resin composition can be improved and the dispersibility of the inorganic filler in the insulating layer 130 described later can be stably obtained.

[0055] In this embodiment, the mean volume diameter D of the inorganic filler is not particularly limited, but may be, for example, 10 μm or less, 5 μm or less, 1 μm or less, 700 nm or less, 300 nm or less, or 100 nm or less.

[0056] The "volume-average particle size (MV)" referred to here can be calculated using the following formula, where di is the particle diameter and Vi is the particle volume. D = Σ(Vidi) / ΣVi A dynamic light scattering particle size and particle size distribution analyzer is used to measure the volume-average particle size.

[0057] By setting the volume-average particle size D of the inorganic filler to 10 μm or less, it becomes easier to uniformly disperse the inorganic filler in the insulating layer 130. Furthermore, by setting the volume-average particle size of the inorganic filler to 5 μm or less, or 1 μm or less, or 700 nm or less, or 300 nm or less, or 100 nm or less, it becomes even easier to uniformly disperse the inorganic filler in the insulating layer 130.

[0058] There are no particular limitations on the lower limit of the volume-average particle size of the inorganic filler. However, from the viewpoint of stably forming the inorganic filler, the volume-average particle size of the inorganic filler may be, for example, 1 nm or more, preferably 5 nm or more.

[0059] In this embodiment, at least a portion of the inorganic filler may be surface-treated with a silane coupling agent. In other words, at least a portion of the surface of the inorganic filler may have, for example, silyl groups containing a predetermined organic substituent. This can improve the adhesion of the interface between the inorganic filler and the base resin, and improve the mechanical properties and insulating properties of the insulating layer 130. Examples of silane coupling agents include materials described in International Publication Nos. 2022-163197 and International Publication Nos. 2022-163198. The silane coupling agent may be, for example, vinyltrimethoxysilane (VTMS).

[0060] (Crosslinking agent) The crosslinking agent is, for example, an organic peroxide. Examples of organic peroxides include dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and 1,3-bis(t-butylperoxyisopropyl)benzene. Two or more of these may be used in combination.

[0061] (Other additives) The resin composition may further contain, for example, an antioxidant and a lubricant. Examples of antioxidants and lubricants include materials described in International Publication Nos. 2022-163197 and International Publication Nos. 2022-163198. The resin composition may further contain, for example, a colorant.

[0062] (2) Power cables Next, with reference to Figure 1, the power cable 10 of this embodiment will be described.

[0063] The power cable 10 in this embodiment is configured as a so-called solid-insulated power cable and has a conductor 110 and an insulating layer 130. The power cable 10 may have, for example, a conductor 110, an internal semiconducting layer 120, an insulating layer 130, an external semiconducting layer 140, a shielding layer 150, and a sheath 160, in this order from the central axis of the conductor 110 toward the outer circumference of the power cable 10.

[0064] (Insulating layer) The insulating layer 130 covers the outer periphery of the conductor 110 (internal semiconducting layer 120). The insulating layer 130 is made of the resin composition of this embodiment described above.

[0065] In this embodiment, by implementing the novel manufacturing method (two-stage material introduction extrusion method) described later, the inorganic filler is uniformly dispersed in the insulating layer 130 without becoming excessively sparse or excessively dense. This point will be explained later.

[0066] Furthermore, in this embodiment, the insulating layer 130 may be crosslinked, for example, by heating after the resin composition of this embodiment has been extruded. That is, for example, the polyolefin as the base resin in the resin composition constituting the insulating layer may be a crosslinked polyolefin. Note that the resin composition may also contain uncrosslinked polyolefin.

[0067] In this embodiment, if the inorganic filler is surface-treated with a vinylsilane coupling agent, at least some of the organic substituents on the silyl groups bonded to the surface of the inorganic filler may be crosslinked (bonded) to the base resin, for example. Furthermore, vinyl groups may remain on some of the other silyl groups bonded to the surface of the inorganic filler.

[0068] The conductor 110, the internal semiconducting layer 120, the external semiconducting layer 140, the shielding layer 150, and the sheath 160 may have the same configuration as, for example, the configuration described in International Publication Nos. 2022-163197 and International Publication Nos. 2022-163198.

[0069] (Specific dimensions, etc.) The specific dimensions of the power cable 10 are not particularly limited, but for example, the diameter of the conductor 110 may be 5 mm or more and 60 mm or less, the thickness of the internal semiconducting layer 120 may be 1 mm or more and 3 mm or less, the thickness of the insulating layer 130 may be 1 mm or more and 35 mm or less, the thickness of the external semiconducting layer 140 may be 1 mm or more and 3 mm or less, the thickness of the shielding layer 150 may be 1 mm or more and the thickness of the sheath 160 may be 1 mm or more. The voltage applied to the power cable 10 of this embodiment may be, for example, 20 kV or more.

[0070] (3) Dispersion state of inorganic filler Referring to Figure 2, the dispersion state of the inorganic filler in the insulating layer 130 (resin composition) of this embodiment will be described. Figure 2 shows a schematic diagram of a cross-section of the insulating layer 130 according to this embodiment, observed, for example, by a scanning electron microscope (SEM).

[0071] As shown in Figure 2, the insulating layer 130 of this embodiment includes a base resin 132 shown in colorless form and a plurality of inorganic fillers 134 shown as black dots. "(3) Dispersion state of inorganic fillers" is described by assigning reference numerals to the base resin 132 and the inorganic fillers 134.

[0072] Here, the dispersion state of the inorganic filler 134 in the insulating layer 130 can be defined, for example, by the area of ​​the Voronoi region VR (hereinafter also simply referred to as the "Voronoi area").

[0073] The "Voronoi region VR" is the region in the cross-section of the insulating layer 130 that surrounds the target particle 134a of the inorganic filler 134 by the perpendicular bisector PB of the line connecting the target particle 134a and other adjacent particles 134b. The area of ​​this Voronoi region VR is the Voronoi area.

[0074] A larger Voronoi area (VR) means that the region of the base resin 132 surrounding the target particles 134a as inorganic filler 134 is wider. Conversely, a smaller Voronoi area means that the region of the base resin 132 surrounding the target particles 134a as inorganic filler 134 is narrower.

[0075] The Voronoi area also includes the content of the inorganic filler 134 in the insulating layer 130. When the content of the inorganic filler 134 in the insulating layer 130 is low, the Voronoi area tends to be large. On the other hand, when the content of the inorganic filler 134 in the insulating layer 130 is high, the Voronoi area tends to be small.

[0076] In this embodiment, the insulating layer 130 satisfies, for example, formula (1) with respect to the dispersibility of the inorganic filler 134 in the insulating layer 130. 600D + 1.6 × 10 4 ≤A ≤ 16000D + 3.0 × 10 6 ...(1)

[0077] Here, D is the volume-average particle size of the inorganic filler 134, with units of nm. A is the average value of the Voronoi area, which is the area of ​​the Voronoi region VR described above, with units of nm. 2 In equation (1), the unit of the coefficient of D is nm, and the unit of the constant that forms the A-intercept is nm. 2 That is the case.

[0078] In equation (1), 600D + 1.6 × 10 4 By satisfying ≤A, the region of base resin 132 surrounding each inorganic filler 134 is not excessively narrowed. As a result, even if at least a portion of the base resin 132 deteriorates due to thermal aging of the insulating layer 130, an undegraded portion of the base resin 132 can be left in the region between the uniformly dispersed inorganic fillers 134. Consequently, even if the insulating layer 130 undergoes thermal aging, an excessive increase in the overall elasticity of the insulating layer 130 can be prevented.

[0079] On the other hand, in equation (1), A ≤ 16000D + 3.0 × 10 6 By satisfying this condition, the area of ​​base resin 132 surrounding each inorganic filler 134 is not excessively large. As a result, even if at least a portion of the base resin 132 deteriorates due to thermal aging of the insulating layer 130, and electrical trees occur in the deteriorated portion of the base resin 132, the electrical trees are more easily directed to the inorganic filler, and the inorganic filler 134 can sufficiently prevent further propagation of the electrical trees. Consequently, even if the insulating layer 130 undergoes thermal aging, an excessive decrease in the dielectric breakdown strength of the insulating layer 130 can be prevented.

[0080] For example, if the inorganic filler 134 contains silicon dioxide and the volume-average particle size D of the inorganic filler 134 is 1 nm or more and 1 μm or less, then the average value A of the Voronoi area in the cross-section of the insulating layer 130 is 1.6 × 10⁻⁶ 4 nm 2 The above 1.9 × 10 7 nm 2 The following is also possible. This makes it possible to reliably prevent an excessive increase in elasticity and an excessive decrease in dielectric breakdown strength in the insulating layer 130, even if the insulating layer 130 undergoes thermal aging.

[0081] For example, if the inorganic filler 134 contains magnesium oxide and the volume-average particle size D of the inorganic filler 134 is 100 nm or more and 5 μm or less, the average value A of the Voronoi area in the cross-section of the insulating layer 130 is 7.6 × 10⁻⁶. 4 nm 2 The above 8.3 × 10 7 nm 2 The following is also possible. This makes it possible to reliably prevent an excessive increase in elasticity and an excessive decrease in dielectric breakdown strength in the insulating layer 130, even if the insulating layer 130 undergoes thermal aging.

[0082] Furthermore, in this embodiment, the standard deviation σ of the Voronoi area in the cross-section of the insulating layer 130 is, for example, 3.0 × 10 7 nm 2The following is also possible. In this case, the variation in the Voronoi area is reduced. That is, neither an insufficient Voronoi region VR nor an excessive Voronoi region VR is formed within the insulating layer 130. As a result, even if the insulating layer 130 undergoes thermal aging under more severe temperature conditions, it is possible to prevent an excessive increase in the elasticity of the insulating layer 130 due to the deterioration of the base resin 132 in the insufficient Voronoi region VR. Furthermore, even if the insulating layer 130 undergoes thermal aging under the more severe conditions described above, it is possible to prevent an excessive decrease in the dielectric breakdown strength of the insulating layer 130 due to the deterioration of the base resin 132 in the excessive Voronoi region VR.

[0083] The lower limit of the standard deviation σ of the Voronoi area is not restricted, but for example, the standard deviation σ of the Voronoi area is 3.0 × 10⁻⁶. 4 nm 2 That's fine too.

[0084] (4) Characteristics of power cables In the power cable 10 of this embodiment, the insulating layer 130 satisfies equation (1) relating to the dispersibility of the inorganic filler described above. For example, even after a first acceleration treatment that accelerates the thermal aging of the insulating layer 130, the elasticity of the insulating layer 130 does not increase excessively, and the dielectric breakdown strength does not decrease excessively.

[0085] Here, prior to the first accelerated treatment, a test specimen, in the form of a sheet or block, is taken from the insulating layer 130. The "first accelerated treatment" after taking the test specimen includes, for example, a step of heating the test specimen taken from the insulating layer 130 for 200 hours under conditions of 145°C in a nitrogen atmosphere, and a step of performing five heat cycles, each including a high-temperature step of heating the test specimen at 100°C for 1 hour and a low-temperature step of cooling the test specimen at 0°C for 1 hour. In the heat cycle step, the rate of heating and cooling between the high-temperature step and the low-temperature step is, for example, 5°C / min or more and 20°C / min or less.

[0086] In this first acceleration treatment, the step of heating the test specimen under conditions of 145°C can accelerate the thermal aging of the base resin in the test specimen taken from the insulating layer 130.

[0087] Furthermore, in the first acceleration treatment, a heat cycle process including high-temperature and low-temperature steps can be performed to repeatedly induce thermal expansion and contraction in the base resin of the test specimen due to temperature changes. In other words, after the aforementioned thermal aging of the base resin, mechanical fatigue can be induced in the base resin, making the effects of thermal aging of the base resin apparent.

[0088] In this embodiment, after performing a first accelerated treatment on a test piece taken from the insulating layer 130, the storage modulus of the test piece at 25°C, measured by dynamic viscoelasticity measurement, is, for example, 800 MPa or less. In this embodiment, the insulating layer 130 satisfies equation (1) relating to the dispersibility of the inorganic filler described above, that is, the inorganic filler is uniformly dispersed in the insulating layer 130 without becoming excessively dense, so that even if the insulating layer 130 undergoes thermal aging, an undegraded portion of the base resin can be left intact. For the undegraded portion of the base resin, the effect of the inorganic filler partially restraining the base resin (the so-called pinning effect of the inorganic filler) can be stably obtained. As a result, even if a heat cycle occurs after the thermal aging of the insulating layer 130, mechanical fatigue, including thermal expansion and contraction, applied to the undegraded portion of the base resin can be reduced. In this way, an excessive increase in the overall elasticity of the insulating layer 130 can be prevented. As a result, even if the insulating layer 130 undergoes thermal aging, the flexibility of the power cable 10 can be maintained.

[0089] The "storage modulus" referred to here is measured at 25°C by dynamic mechanical analysis (DMA) in accordance with JIS K7244-4:1999. Detailed measurement conditions for the storage modulus will be explained in the examples.

[0090] In this embodiment, after performing a first accelerated treatment on a test piece taken from the insulating layer 130, an AC electric field is applied to the test piece at a temperature of 90°C using a needle-shaped electrode with a tip having a radius of curvature of 5 μm. When an AC electric field strength occurs in the test piece, the AC electric field strength is, for example, 600 kV / mm or more. In this embodiment, the insulating layer 130 satisfies equation (1) relating to the dispersibility of the inorganic filler described above, that is, the inorganic filler is uniformly dispersed without becoming excessively sparse, making it difficult for the propagation of electrical trees via deteriorated parts of the base resin to occur in the regions between the uniformly dispersed inorganic fillers. Furthermore, because the inorganic filler is dispersed to a certain extent, the pinning effect of the inorganic filler described above can be stably obtained. As a result, even if a heat cycle occurs after thermal aging of the insulating layer 130, mechanical fatigue including thermal expansion and contraction applied to the base resin can be reduced, and deterioration of electrical properties can also be suppressed. As a result, even if the insulating layer 130 undergoes thermal aging, the insulation properties of the power cable 10 can be maintained.

[0091] The method and conditions for measuring the AC electric field strength when an electrical tree occurs in the test specimen will be explained in the examples.

[0092] Since the insulating layer 130 satisfies the above-mentioned storage modulus and AC electric field strength that generates electrical trees even after the first acceleration treatment, it naturally satisfies the above-mentioned storage modulus and AC electric field strength that generates electrical trees even in the initial state before the first acceleration treatment.

[0093] Furthermore, in this embodiment, the standard deviation σ of the Voronoi area in the cross-section of the insulating layer 130 is 3.0 × 10 7 nm 2 In the following cases, for example, even after a second accelerated treatment under temperature conditions more severe than those after the first accelerated treatment, the elasticity of the insulating layer 130 does not increase excessively, and the dielectric breakdown strength does not decrease excessively.

[0094] Here, the "second accelerated treatment" includes, for example, a step of heating a test specimen taken from the insulating layer 130 for 200 hours under conditions of 160°C in a nitrogen atmosphere, and a step of performing a heat cycle five times, which includes a high-temperature step of heating the test specimen at 100°C for 1 hour and a low-temperature step of cooling the test specimen at 0°C for 1 hour. In the heat cycle step, the rate of heating and cooling between the high-temperature step and the low-temperature step is set to, for example, 5°C / min or more and 20°C / min or less. This second accelerated treatment can further accelerate the thermal aging of the insulating layer 130.

[0095] In this embodiment, in the cross-section of the insulating layer 130, the standard deviation σ of the Voronoi area is 3.0 × 10 7 nm 2 In the following cases, the storage modulus of the test specimen at 25°C, measured by dynamic viscoelasticity measurement after the second accelerated treatment has been performed on the test specimen taken from the insulating layer 130, may be, for example, 800 MPa or less. This ensures that the flexibility of the power cable 10 can be maintained even if the insulating layer 130 undergoes thermal aging under more severe temperature conditions.

[0096] In this embodiment, in the cross-section of the insulating layer 130, the standard deviation σ of the Voronoi area is 3.0 × 10 7 nm 2 In the following cases, after performing a second acceleration treatment on a test piece taken from the insulating layer 130, an AC electric field is applied to the test piece at a temperature of 90°C using a needle-shaped electrode with a tip having a radius of curvature of 5 μm. The AC electric field strength when an electrical tree is generated in the test piece may be, for example, 600 kV / mm or more. This makes it possible to maintain the insulation properties of the power cable 10 even if the insulating layer 130 undergoes thermal aging under more severe temperature conditions.

[0097] The measurement methods and conditions for the storage modulus and AC electric field strength when electric trees occur after the second acceleration treatment are the same as those for the storage modulus and AC electric field strength when electric trees occur after the first acceleration treatment.

[0098] (5) Method of manufacturing power cables Next, with reference to Figures 3 and 4, the manufacturing method of the power cable 10 of this embodiment will be described. Hereinafter, steps will be abbreviated as "S".

[0099] As shown in Figure 3, the manufacturing method of the power cable 10 of this embodiment includes, for example, a resin composition preparation step S100, a conductor preparation step S200, a cable core formation step S300, and an external formation step S400.

[0100] (Resin composition preparation step S100) First, a resin composition is prepared that includes a base resin containing polyolefin and an inorganic filler.

[0101] In the resin composition preparation step S100 of this embodiment, a masterbatch and a filler-free compound are prepared as the resin composition. That is, the resin composition preparation step S100 includes, for example, a masterbatch preparation step S120 and a filler-free compound preparation step S140.

[0102] (Masterbatch preparation process S120) In the masterbatch preparation step S120, a masterbatch containing a base resin and an inorganic filler is prepared.

[0103] A polyolefin is prepared as the base resin. If the inorganic filler contains magnesium oxide, the base resin may further contain a modified polyolefin having polar groups. Conversely, the base resin does not have to contain a modified polyolefin having polar groups.

[0104] As an inorganic filler, at least one of magnesium oxide and silicon dioxide is prepared. At least a portion of the inorganic filler may be surface-treated with a silane coupling agent.

[0105] When surface-treating inorganic fillers, the amount of moisture in the atmosphere (absolute humidity, mass of water per unit volume) is, for example, 0.1 g / m 3 The following may also be used. This makes it difficult for the silane coupling agents to bond with each other in the atmosphere. As a result, it is possible to avoid reducing the coverage rate of the silane coupling agent on the inorganic filler.

[0106] After preparing the base resin and inorganic filler, the inorganic filler is melted and mixed into the base resin using a mixer such as a banba mixer or kneader, and the mixture is then granulated using an extruder. Alternatively, a twin-screw extruder with high kneading capabilities may be used to perform the mixing and granulation processes in a single step. This forms a masterbatch.

[0107] In this case, the masterbatch contains, for example, an inorganic filler in a higher concentration than the inorganic filler content in the final insulating layer 130. By melting and mixing the masterbatch as described above, the inorganic filler can be uniformly and densely dispersed in the melted masterbatch without excessive aggregation of the inorganic filler.

[0108] (Preparation process for filler-free compound S140) On the other hand, a filler-free compound is prepared that contains a base resin but does not contain inorganic fillers.

[0109] The filler-free compound may be impregnated with other additives besides inorganic fillers, such as a crosslinking agent. The filler-free compound may also contain other additives, such as antioxidants and lubricants.

[0110] The apparatus for mixing and granulating the filler-free compound may be, for example, the same apparatus used for mixing and granulating the masterbatch described above.

[0111] The masterbatch preparation step S120 and the filler-free compound preparation step S140 can be performed in either order.

[0112] (Conductor preparation process S200) On the other hand, a conductor 110 is prepared by twisting together multiple conductor cores.

[0113] (Cable core formation process (extrusion process) S300) After preparing a masterbatch and a filler-free compound as the resin composition, an insulating layer 130 is formed to cover the outer circumference of the conductor 110 by extrusion molding of the resin composition.

[0114] In this embodiment, for example, a three-layer simultaneous extruder is used to extrude a cable core having an internal semiconductive layer 120, an insulating layer 130, and an external semiconductive layer 140. The three-layer simultaneous extruder includes, for example, an extruder A for forming the internal semiconductive layer 120, an extruder 200 described later as an extruder B for forming the insulating layer 130, and an extruder C for forming the external semiconductive layer 140.

[0115] (Extruder A) In a three-layer simultaneous extruder, an internal semiconducting layer resin composition, for example, in which ethylene-ethyl acrylate copolymer and conductive carbon black are pre-mixed, is introduced into extruder A, which forms the internal semiconducting layer 120.

[0116] (Extruder B) The insulating layer 130 is extruded by introducing the above-mentioned resin composition into the extruder B that forms the insulating layer 130.

[0117] In this embodiment, the insulating layer 130 is formed by applying a two-stage material introduction extrusion method as a new manufacturing method.

[0118] Specifically, as extruder B, we use extruder 200 as shown in Figure 4. As shown in Figure 4, extruder 200 includes, for example, a cylinder 210, a first inlet (first hopper) 222, a second inlet (second hopper) 224, a screw 230, a rotary drive mechanism 240, and a discharge section 250. Hereafter, the region near the first axial end of cylinder 210 will also be referred to as the "upstream" region, and the region near the second axial end of cylinder 210 will also be referred to as the "downstream" region.

[0119] The cylinder 210 is cylindrical in shape. A resin composition is supplied into the cylinder 210. The cylinder 210 has a heater (not shown) that can heat the inside of the cylinder 210.

[0120] The first input port 222 is located near the first axial end of the cylinder 210.

[0121] The second inlet 224 is located downstream of the first inlet 222 of the cylinder 210, that is, at a predetermined distance in the axial direction of the cylinder 210 from the first end of the cylinder 210. The distance between the first inlet 222 and the second inlet 224 may be adjusted according to the dispersibility of the inorganic filler in the resin composition mixed in the cylinder 210.

[0122] The screw 230 is inserted into the cylinder 210 from its first axial end and is configured to be rotatable. The screw 230 has screw flights (not shown) arranged helically on its outer circumferential surface.

[0123] The rotary drive mechanism 240 is provided at the first axial end of the cylinder 210 and is configured to rotate the screw 230.

[0124] The discharge section 250 is provided at the second end of the cylinder 210 opposite to the first end, and is configured to discharge the resin composition.

[0125] The specific procedure involves first mixing a portion of the masterbatch and the filler-free compound in pellet form without melting them, before extruding the insulating layer 130 using the extruder 200.

[0126] After mixing a portion of the masterbatch with the filler-free compound, the temperature inside the cylinder 210 is set to a temperature that is, for example, 10°C to 80°C higher than the melting point of the base resin.

[0127] Once the internal temperature of cylinder 210 reaches a predetermined temperature, a portion of the masterbatch and the filler-free compound are introduced from the first inlet 222 of the extruder 200. This melts and mixes the masterbatch, in which the inorganic filler is uniformly and densely dispersed beforehand, with the filler-free compound, and dilutes the masterbatch with the filler-free compound. In other words, the distance between the inorganic fillers can be increased in the mixture formed by melting the masterbatch and filler-free compound in cylinder 210. As a result, the inorganic filler can be uniformly and sparsely dispersed in the mixture without agglomerating.

[0128] In conjunction with the introduction of material into the first inlet 222 described above, the remaining masterbatch is introduced from the second inlet 224, which is located downstream of the first inlet 222 of the extruder 200. This allows multiple regions in which the inorganic filler is densely dispersed without agglomerating to be randomly scattered from the second inlet 224 into the mixed material in which the inorganic filler introduced from the first inlet 222 is uniformly and loosely dispersed.

[0129] Because the distance from the second inlet 224 to the discharge section 250 is short, the extruded material is discharged from the discharge section 250 before the inorganic filler is completely and uniformly dispersed throughout the entire extruded material. In other words, while the material introduced from the first inlet 222 is thoroughly melted and mixed over a long period of time along the entire axial length of the cylinder 210, the material introduced from the second inlet 224 is only melted and mixed over a short period of time in a limited area of ​​the cylinder 210. This makes it possible to maintain multiple areas where the inorganic filler introduced from the second inlet 224 is densely dispersed in a randomly scattered state.

[0130] By extrusion molding using the extruder 200 described above, an insulating layer 130 can be formed in the common head described later, in which the inorganic filler is uniformly dispersed without being excessively sparse or excessively dense.

[0131] Specifically, for example, the insulating layer 130 can be formed such that the dispersibility of the inorganic filler in the insulating layer 130 satisfies the above-mentioned equation (1). Furthermore, for example, in the cross-section of the insulating layer 130, the standard deviation σ of the above-mentioned Voronoi area is 3.0 × 10 7 nm 2 The insulating layer 130 can be formed as follows.

[0132] The mean value A of the Voronoi area and the standard deviation σ of the Voronoi area in equation (1) are adjustable by, for example, the amount of inorganic filler in the masterbatch, the total amount of inorganic filler in the final insulating layer 130, the amount of a portion of the masterbatch and filler-free compound introduced from the first inlet 222, the amount of the remainder of the masterbatch introduced from the second inlet 224, the ratio of the material introduced from the first inlet 222 to the second inlet, the total axial length of the cylinder 210, the distance from the first inlet 222 to the second inlet 224, the distance from the second inlet 224 to the discharge section 250, the internal temperature of the cylinder 210, the pressure inside the cylinder 210, the rotational speed of the screw 230, and the screw flight period of the screw 230.

[0133] (Extruder C) An external semiconducting layer resin composition, made of the same material as the internal semiconducting layer resin composition used in extruder A, is fed into extruder C to form the external semiconducting layer 140.

[0134] (Common Head) Next, the extruded materials from extruders A to C are guided to the common head, and the internal semiconducting layer 120, insulating layer 130, and external semiconducting layer 140 are simultaneously extruded onto the outer circumference of the conductor 110 from the inside out.

[0135] (bridge) Subsequently, the insulating layer 130 is crosslinked in a crosslinked tube pressurized with nitrogen gas or the like by heating with radiation from an infrared heater or by heat transfer through a heat transfer medium such as high-temperature nitrogen gas or silicone oil. This forms a cable core having a conductor 110, an internal semiconducting layer 120, an insulating layer 130, and an external semiconducting layer 140.

[0136] (External formation process S400) After forming the cable core, the shielding layer 150 and the sheath 160 are formed in this order to cover the outer circumference of the cable core. The method for forming the shielding layer 150 and the sheath 160 may be the same as the method described in, for example, International Publication Nos. 2022-163197 and International Publication Nos. 2022-163198.

[0137] Based on the above, the power cable 10 of this embodiment is manufactured.

[0138] (6) Summary of this embodiment This embodiment provides one or more of the following effects.

[0139] In this embodiment, the novel manufacturing method described above (two-stage material introduction extrusion method) makes it possible to form an insulating layer 130 in which the inorganic filler is uniformly dispersed without being excessively sparse or excessively dense.

[0140] As a result, in this embodiment, the insulating layer 130 satisfies, for example, formula (1) with respect to the dispersibility of the inorganic filler 134 in the insulating layer 130. 600D + 1.6 × 10 4 ≤A ≤ 16000D + 3.0 × 10 6 ...(1)

[0141] In equation (1), 600D + 1.6 × 10 4 By satisfying ≤A, the region of base resin 132 surrounding each inorganic filler 134 is not excessively narrowed. That is, the inorganic fillers 134 are uniformly dispersed in the insulating layer 130 without becoming excessively dense, according to the volume average particle size D of the inorganic fillers 134. As a result, even if at least a portion of the base resin 132 deteriorates due to thermal aging of the insulating layer 130, the region between the uniformly dispersed inorganic fillers 134 leaves an undegraded portion of the base resin 132, maintaining the original flexibility of the base resin 132. Consequently, even if the insulating layer 130 undergoes thermal aging, an excessive increase in the overall elasticity of the insulating layer 130 can be prevented.

[0142] On the other hand, in equation (1), A ≤ 16000D + 3.0 × 10 6 By satisfying this condition, the region of base resin 132 surrounding each inorganic filler 134 is not excessively large. That is, the inorganic filler 134 is uniformly dispersed in the insulating layer 130 without becoming excessively sparse, according to the volume average particle size D of the inorganic filler 134. As a result, even if at least a portion of the base resin 132 deteriorates due to thermal aging of the insulating layer 130, it is possible to make it difficult for electrical trees to propagate through the deteriorated portion of the base resin 132 in the region between the uniformly dispersed inorganic fillers 134. Furthermore, even if electrical trees do occur in the deteriorated portion of the base resin 132, it is possible to make it easier for the electrical trees to reach the inorganic filler, and the inorganic filler 134 can sufficiently prevent further propagation of the electrical trees. As a result, even if the insulating layer 130 undergoes thermal aging, it is possible to prevent an excessive decrease in the dielectric breakdown strength of the insulating layer 130.

[0143] As described above, this embodiment makes it possible to reduce the deterioration of the characteristics of power cables caused by thermal aging.

[0144] <Other embodiments of this disclosure> Although embodiments of this disclosure have been described in detail above, this disclosure is not limited to the embodiments described above, and various modifications are possible without departing from its essence.

[0145] In the above embodiment, the case in which the insulating layer 130 is crosslinked was described, but the insulating layer 130 does not have to be crosslinked. However, the above-described effects of this embodiment can be stably satisfied when the insulating layer 130 is crosslinked. [Examples]

[0146] Next, embodiments relating to the present disclosure will be described. These embodiments are examples of the present disclosure and the present disclosure is not limited to these embodiments. In Figures 6 to 11 of the following embodiments, "aEb" means "a × 10 b It means "...".

[0147] (1) Experiment 1 In Experiment 1, we investigated the dependence of the average value A of the Voronoi area.

[0148] (1-1) Regarding Sample A1 to Sample A4, Sample B1 to Sample B4, Sample C1 to Sample C4, and Sample D1 to Sample D4 Power cables were manufactured using the novel manufacturing method described in the above embodiment, with samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4.

[0149] (Masterbatch preparation process) The following compounding agents were melted and mixed using a Banba mixer, and then granulated using an extruder to form a pellet-shaped masterbatch.

[0150] Base resin: Low-density polyethylene (LDPE) (density 0.920 g / cm³) 3 , MFR=1g / 10min) 97.5 parts by mass or more and 100 parts by mass or less. Here, the melt flow rate (MFR) is the value measured in accordance with JIS K7210 at a measurement temperature of 190°C and a load of 2.12N.

[0151] Modified polyolefin: Maleic anhydride-modified polyethylene (MAH-PE, modification rate 0.5% by mass) 0 to 2.5 parts by mass.

[0152] Inorganic fillers: Silicon dioxide (nanosilica) or magnesium oxide (MgO). Inorganic filler content: The content in the final insulating layer was adjusted according to the average value A of the Voronoi area, within a range of 0.1 parts by mass to 5 parts by mass. The volume-average particle size D of the inorganic filler is between 10 nm and 2000 nm.

[0153] The inorganic filler was surface-treated using a dry method with a stirring device under the following conditions. Silane coupling agent: Vinyltrimethoxysilane (VTMS) Diluted organic solvent: Ethanol (water concentration 1.5 vol%) Surface treatment method: Dry method (Initial) moisture content of inorganic filler: 0.5% by mass Moisture content in the atmosphere: 0.01 g / m 3 Processing temperature: Room temperature (24℃)

[0154] (Preparation process for filler-free compound) The following compounding agents were melted and mixed using a Banba mixer, and then granulated using an extruder to form a pellet-shaped filler-free compound.

[0155] Lubricant: Oleamide (specified amount) Antioxidant: 4,4′-thiobis(3-methyl-6-t-butylphenol) specified amount

[0156] Subsequently, the pellet-shaped filler-free compound was impregnated with the following crosslinking agent. Dicumyl peroxide 1.3 parts by mass

[0157] (Conductor preparation process) Next, a conductor was prepared by twisting together thin copper alloy conductor cores with a diameter of 14 mm.

[0158] (Cable core formation process) After preparing the conductors, the resin composition for the internal semiconducting layer containing an ethylene-ethyl acrylate copolymer, the resin composition for the insulating layer described above, and the resin composition for the external semiconducting layer made of the same material as the resin composition for the internal semiconducting layer were each fed into extruders A to C.

[0159] At this time, an insulating layer was formed by applying a two-stage material extrusion method as a new manufacturing method. The extruder B used was the extruder 200 described above. The distance from the first inlet 222 to the second inlet 224 in the axial direction of the cylinder 210 of the extruder 200 was set to 1 m.

[0160] First, a portion of the masterbatch and the filler-free compound were mixed in pellet form without melting. Next, the temperature inside the cylinder 210 was set to a temperature that was, for example, 10°C to 80°C higher than the melting point of the base resin.

[0161] Once the internal temperature of cylinder 210 reached a predetermined temperature, a portion of the masterbatch and the filler-free compound were introduced from the first inlet 222 of the extruder 200. In conjunction with the introduction of materials to the first inlet 222 as described above, the remaining masterbatch was introduced from the second inlet 224, which is located downstream of the first inlet 222 of the extruder 200.

[0162] For samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, the insulation layer was formed so that the average value A of the different Voronoi areas within the range satisfying equation (1) above was obtained by adjusting the total content of the inorganic filler in the final insulating layer 130, the amount of a portion of the masterbatch and filler-free compound introduced from the first inlet 222, the amount of the remaining masterbatch introduced from the second inlet 224, and the rotation speed of the screw 230, while keeping the internal temperature of the cylinder 210 the same. 7 nm 2 The above conditions were adjusted as follows:

[0163] The extruded materials from extruders A to C were guided to a common head, and the internal semiconducting layer, insulating layer, and external semiconducting layer were simultaneously extruded onto the outer circumference of the conductor, from the inside outwards. At this time, the thicknesses of the internal semiconducting layer, insulating layer, and external semiconducting layer were set to 1 mm, 3 mm, and 1 mm, respectively. Subsequently, the resin composition for the insulating layer was crosslinked by heating the extruded material at approximately 250°C. As a result, a power cable having a conductor, internal semiconducting layer, insulating layer, and external semiconducting layer arranged from the center outwards was manufactured.

[0164] (1-2) Regarding Sample A5 to Sample A8, Sample B5 to Sample B8, Sample C5 to Sample C8, and Sample D5 to Sample D8 For samples A5 to A8, B5 to B8, C5 to C8, and D5 to D8, power cables were manufactured under substantially the same conditions as one of the following: samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, respectively, except that conventional manufacturing method 1 was applied.

[0165] In conventional manufacturing method 1, first, without forming a masterbatch, the entire amount of base resin and inorganic filler was melt-mixed in an extruder to form a pellet-shaped mixture. Furthermore, a crosslinking agent was impregnated into this mixture to form pellets containing all the compounding agents. The insulating layer was extruded by feeding these pellets into a conventional extruder B (an extruder with one input port). The methods and conditions for processes other than the insulating layer formation process in conventional manufacturing method 1 were the same as those for the new manufacturing method and conditions described above.

[0166] (1-3) Regarding Sample A9 to Sample A12, Sample B9 to Sample B12, Sample C9 to Sample C12, and Sample D9 to Sample D12 For samples A9 to A12, B9 to B12, C9 to C12, and D9 to D12, power cables were manufactured under substantially the same conditions as one of the following: samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, respectively, except that conventional manufacturing method 2 was applied.

[0167] In conventional manufacturing method 2, a masterbatch was first formed by melt-mixing a predetermined amount of base resin and inorganic filler. After the masterbatch was formed, the base resin was added to the masterbatch, and these were melt-mixed using an extruder. This diluted the masterbatch so that the inorganic filler content in the final insulating layer was a predetermined amount, forming a pellet-shaped mixture. Furthermore, a crosslinking agent was impregnated into the mixture to form pellets containing all the compounding agents. The insulating layer was extruded by feeding these pellets into an extruder. The methods and conditions for processes other than the insulating layer formation process in conventional manufacturing method 2 were the same as those for the new manufacturing method and conditions described above.

[0168] (1-4) Evaluation The following evaluations were performed on each sample.

[0169] (1-4-1) Voronoi area Test specimens of the insulating layer were obtained by thinly slicing the insulating layer of each sample in the circumferential direction. The thickness of the test specimens was set to 1 mm. The sampling position for the insulating layer sheet was set to the center in the thickness direction of the insulating layer.

[0170] As described above, the cross-sections of the collected test specimens were observed using a scanning electron microscope (SEM). Cross-sectional images were taken at each of 10 arbitrary locations on the test specimen, with a field of view of 15 μm × 10 μm.

[0171] Next, by converting the cross-sectional image of the test specimen into a black and white binary image, an image was obtained in which the base resin was white and the inorganic filler was black.

[0172] After acquiring a black and white binary image, the image analysis software ImageJ was used to divide the image into Voronoi regions surrounding each inorganic filler, and the area of ​​each Voronoi region was obtained. As a result, the average value A of the Voronoi area and the standard deviation σ of the Voronoi area were calculated for 10 arbitrary locations on the test specimen.

[0173] (1-4-2) Storage modulus before the first acceleration treatment From the insulating layer of each sample, the same test specimens used for evaluating the Voronoi area were taken.

[0174] For each sample specimen prior to the first accelerated treatment, the storage modulus was measured at 25°C by dynamic viscoelasticity measurement in accordance with JIS K7244-4:1999.

[0175] The dynamic viscoelasticity measurement was performed under the following conditions. Measurement mode: Tensile mode Distortion: 0.08% Frequency: 10Hz Temperature range: 0°C to 200°C Heating rate: 10℃ / min The storage modulus was measured at 25°C using the dynamic viscoelasticity measurement method.

[0176] A storage modulus of 800 MPa or less, obtained from the above measurements of the test specimen before the first accelerated treatment, was evaluated as "good."

[0177] (1-4-3) AC tree starting field strength before the first acceleration process The AC tree start field strength for each sample before the first acceleration treatment was evaluated as follows.

[0178] As shown in Figure 5A, rectangular parallelepiped (block-shaped) test specimens TP were taken from the insulating layer of each sample. In the test specimen TP, the length of the first surface S1 was 3 mm, the width of the first surface S1 was 3 mm, and the height from the first surface S1 to the second surface S2 was 4 mm. To volatilize the decomposition residue of the crosslinking agent from the test specimen TP, the test specimen TP was vacuum-dried at 80°C for 10 days.

[0179] After drying as described above, a conductive coating consisting of silver paste was applied to the first surface S1 of the test specimen TP to form the electrode FE, as shown in Figure 5A.

[0180] On the other hand, as shown in Figures 5A and 5B, a metal needle-shaped electrode NE with a tip radius of curvature r of 5 μm was prepared. The tip angle of the needle-shaped electrode NE was set to 30°. The tip of the needle-shaped electrode NE was inserted from the second surface S2 to the first surface S1 of the test specimen TP. The distance d between the tip of the needle-shaped electrode NE and the electrode FE was set to 1 mm.

[0181] The test specimen TP, which had the aforementioned needle-shaped electrode NE pierced into it, was immersed in silicone oil. The temperature of the silicone oil was set to 90°C. The test specimen TP in the silicone oil was placed under an optical microscope.

[0182] In this state, while observing the test specimen TP, a commercial frequency (50 Hz) alternating electric field was applied between the needle electrode NE and electrode FE, and the voltage used to apply the alternating electric field was increased at a rate of 200 V / sec. This made the test specimen TP appear as if an impulse voltage had been applied. By gradually increasing the applied voltage as described above, the voltage V when an electrical tree was generated in the test specimen TP from the tip of the needle electrode NE was measured.

[0183] As a result, the alternating electric field strength E (also called the "AC tree initiation electric field strength E") when an electric tree was generated in the test specimen TP was calculated using Mason's equation (A) below. E=2V / {r×ln(1+4d / r)} ···(A) Here, V is the voltage when an electrical tree occurs in the test specimen TP, d is the distance between the needle electrode NE and electrode FE, and r is the radius of curvature of the tip of the needle electrode NE.

[0184] The AC tree initiation field strength E obtained from the above-described measurement on the test specimen TP before the first acceleration treatment was evaluated as "good" if it was 600 kV / mm or higher.

[0185] (1-4-4) Storage modulus after the first acceleration treatment From the insulating layer of each sample, the same test specimens used for evaluating the Voronoi area were taken.

[0186] After sampling the test specimens, a first accelerated treatment was performed using a rapid temperature rise / fall constant temperature bath. The first accelerated treatment consisted of a step of heating the test specimens taken from the insulating layer 130 for 200 hours under conditions of 145°C in a nitrogen atmosphere, and a step of performing five heat cycles, each including a high-temperature step of heating the test specimens at 100°C for 1 hour and a low-temperature step of cooling the test specimens at 0°C for 1 hour. In the heat cycle process, the rate of heating and cooling between the high-temperature and low-temperature steps was set to 10°C / min.

[0187] After the first accelerated treatment was performed on the test specimens, the storage modulus of the specimens was measured at 25°C by dynamic viscoelasticity measurement in accordance with JIS K7244-4:1999. The measurement method and conditions for the storage modulus after the first accelerated treatment were the same as those for the storage modulus before the first accelerated treatment.

[0188] A storage modulus obtained from the above measurements of the test specimen after the first accelerated treatment was evaluated as "good" if it was 800 MPa or less.

[0189] (1-4-5) AC tree starting field strength after the first acceleration process From the insulating layer of each sample, the same test specimen TP was taken as the test specimen TP used in the evaluation of the AC tree initiation field strength before the first acceleration treatment.

[0190] After sampling the test specimen TP, the first acceleration treatment described above was performed. The method and conditions of the first acceleration treatment in the evaluation of the AC tree starting electric field strength were the same as those of the first acceleration treatment in the evaluation of the storage modulus described above.

[0191] After performing a first acceleration treatment on the specimen TP, the AC electric field strength (AC tree initiation field strength) E was determined when an electric tree was generated in the specimen TP by applying an AC electric field to the specimen TP using a needle-shaped electrode NE with a tip having a radius of curvature of 5 μm. The measurement method and conditions for the AC tree initiation field strength after the first acceleration treatment were the same as those for the AC tree initiation field strength before the first acceleration treatment.

[0192] The AC tree initiation field strength E obtained from the above measurements on the test specimen after the first acceleration treatment was evaluated as "good" if it was 600 kV / mm or higher.

[0193] (1-5)Result The results of the sample evaluation in Experiment 1 are shown in Figures 6 to 10. In Figures 6 to 9, the unit of the compounding agent content is "parts by mass". The horizontal and vertical axes in Figure 10 are on a logarithmic scale.

[0194] (Sample A5 to Sample A8, Sample B5 to Sample B8, Sample C5 to Sample C8, Sample D5 to Sample D8) For samples A5 to A8, B5 to B8, C5 to C8, and D5 to D8, based on the average volume-average particle size D and average Voronoi area A of the inorganic filler shown in Figures 6 to 9, A > 16000D + 3.0 × 10⁻¹⁰6 That was the case.

[0195] Before the first acceleration treatment, the storage modulus was 800 MPa or less for samples A5 to A8, B5 to B8, C5 to C8, and D5 to D8, and the AC tree start field strength E was 600 kV / mm or more.

[0196] However, after the first acceleration treatment, the storage modulus was 800 MPa or less for samples A5 to A8, B5 to B8, C5 to C8, and D5 to D8, but the AC tree start field strength E was less than 600 kV / mm.

[0197] In samples A5 to A8, B5 to B8, C5 to C8, and D5 to D8, conventional manufacturing method 1 was used, resulting in only two instances of mixing the inorganic filler with the molten base resin (pellet extrusion and insulation layer extrusion). Consequently, while the inorganic filler was uniformly dispersed within the insulation layer, it was also sparsely dispersed in the other samples.

[0198] Therefore, in samples A5 to A8, B5 to B8, C5 to C8, and D5 to D8, the inorganic filler was unable to adequately prevent the propagation of electrical trees due to thermal aging during the first accelerated treatment. As a result, it is thought that the dielectric breakdown strength in the insulating layer was excessively reduced when the insulating layer underwent thermal aging.

[0199] (Sample A9 to Sample A12, Sample B9 to Sample B12, Sample C9 to Sample C12, Sample D9 to Sample D12) For samples A9 to A12, B9 to B12, C9 to C12, and D9 to D12, based on the volume-average particle size D and average Voronoi area A of the inorganic filler shown in Figures 6 to 9, A < 600D + 1.6 × 10⁻¹⁰, as shown in Figure 10. 4 That was the case.

[0200] Before the first acceleration treatment, the storage modulus was 800 MPa or less for samples A9 to A12, B9 to B12, C9 to C12, and D9 to D12, and the AC tree start field strength E was 600 kV / mm or more.

[0201] However, after the first acceleration treatment, the AC tree start field strength E was 600 kV / mm or higher in samples A9 to A12, B9 to B12, C9 to C12, and D9 to D12, but the storage modulus was over 800 MPa.

[0202] In samples A9 to A12, B9 to B12, C9 to C12, and D9 to D12, conventional manufacturing method 2 was used, resulting in three instances where the inorganic filler was mixed with the molten base resin (masterbatch formation, dilution mixing, and insulation layer extrusion). Consequently, while the inorganic filler was uniformly dispersed within the insulation layer, it was also densely dispersed.

[0203] Therefore, in samples A9 to A12, B9 to B12, C9 to C12, and D9 to D12, the degraded portion of the base resin hardened in narrow regions between the densely dispersed inorganic fillers, due to thermal aging during the first accelerated treatment. As a result, it is thought that the elasticity of the insulating layer as a whole increased excessively when the insulating layer underwent thermal aging.

[0204] (Sample A1 to Sample A4, Sample B1 to Sample B4, Sample C1 to Sample C4, Sample D1 to Sample D4) In contrast, samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4 satisfied equation (1), as shown in Figure 10, based on the volume-average particle size D and average Voronoi area A of the inorganic fillers shown in Figures 6 to 9. 600D + 1.6 × 10 4 ≤A ≤ 16000D + 3.0 × 10 6 ...(1)

[0205] For samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, the storage modulus was 800 MPa or less, and the AC tree starting field strength E was 600 kV / mm or more, both before and after the first acceleration treatment.

[0206] In samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, by implementing the novel manufacturing method described in the above embodiment, it was possible to form an insulating layer in which the inorganic filler was uniformly dispersed without being excessively sparse or excessively dense.

[0207] As a result, in samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, even if at least a portion of the base resin deteriorated due to thermal aging during the first heat treatment, the original flexibility of the base resin could be maintained. Furthermore, even in the cases described above, the inorganic filler was able to sufficiently prevent further propagation of electrical trees. As a result, it was confirmed that in samples A1 to A4, B1 to B4, C1 to C4, and D1 to D4, satisfying equation (1) made it possible to reduce the deterioration of the characteristics of the power cables due to thermal aging.

[0208] (2) Experiment 2 As Experiment 2, the dependence of the standard deviation σ of the Voronoi area was examined.

[0209] (2-1) For Samples A3-1 to A3-8 Power cables were manufactured under substantially the same conditions as those of Sample A3, except that the extrusion conditions of the insulating layer were adjusted so that the standard deviation σ of the Voronoi area was different for Samples A3-1 to A3-8. Sample A3-2 was the same as Sample A3 in FIG. 6.

[0210] Specifically, by adjusting the introduction amount of a part of the masterbatch from the first inlet 222 and the filler-free compound, the introduction amount of the remainder of the masterbatch from the second inlet 224, and the rotation speed of the screw 230, the average value A of the Voronoi area was approximately 57.0×10 4 nm 2 and the insulating layer was formed so that the standard deviation σ of the Voronoi area was different.

[0211] (2-2) Evaluation For each sample, the following evaluations were performed.

[0212] (Voronoi area, storage elastic modulus after the first acceleration treatment, and AC tree initiation field strength) In Experiment 2 as well, the Voronoi area, the storage elastic modulus after the first acceleration treatment, and the AC tree initiation field strength were obtained by the same method and conditions as in Experiment 1.

[0213] (Storage elastic modulus and AC tree initiation field strength after the second acceleration treatment) In Experiment 2, another test piece different from the test pieces used in the above evaluation was sampled, and the storage elastic modulus and AC tree initiation field strength of each sample after the second acceleration treatment were evaluated.

[0214] The measurement methods and conditions for the storage elastic modulus and the AC tree initiation field strength of each sample after the second acceleration treatment were the same as those for the measurement methods and conditions of the storage elastic modulus and the AC tree initiation field strength after the first acceleration treatment in Experiment 1, except that the second acceleration treatment was performed instead of the first acceleration treatment.

[0215] The second acceleration treatment was assumed to have a step of heating a test piece collected from the insulating layer 130 for 200 hours under the condition of a temperature of 160 °C in a nitrogen atmosphere, and a step of performing a heat cycle thereafter. The step of performing the heat cycle in the second acceleration treatment was the same as the step of performing the heat cycle in the first acceleration treatment.

[0216] (2-3) Results The results of evaluating the samples in Experiment 2 are shown in Fig. 11.

[0217] (Samples A3-6 to A3-8) For samples A3-6 to A3-8, the average value A of the Voronoi area satisfied Equation (1), but the standard deviation σ of the Voronoi area was 3.0×10 7 nm 2 exceeded.

[0218] After the first acceleration treatment, in samples A3-6 to A3-8, the storage elastic modulus was 800 MPa or less, and the AC tree initiation field strength E was 600 kV / mm or more.

[0219] However, after the second acceleration treatment, in samples A3-6 to A3-8, the storage elastic modulus exceeded 800 MPa, and the AC tree initiation field strength E was less than 600 kV / mm.

[0220] For samples A3-6 to A3-8, the standard deviation σ of the Voronoi area was 3.0×10 7 nm 2The variability in the Voronoi area was excessive. In other words, either too small or too large Voronoi regions were formed within the insulating layer. As a result, in samples A3-6 to A3-8, it is thought that the thermal aging of the second accelerated treatment under more severe temperature conditions than the first accelerated treatment resulted in an excessive increase in the elasticity of the insulating layer and an excessive decrease in the dielectric breakdown strength of the insulating layer.

[0221] (Samples A3-1 to A3-5) In samples A3-1 to A3-5, equation (1) is satisfied for the mean value A of the Voronoi area, and the standard deviation σ of the Voronoi area is 3.0 × 10⁻⁶. 7 nm 2 The results were as follows:

[0222] In samples A3-1 to A3-5, the storage modulus was 800 MPa or less, and the AC tree initiation field strength E was 600 kV / mm or more, not only after the first acceleration treatment but also after the second acceleration treatment.

[0223] In samples A3-1 to A3-5, the standard deviation σ of the Voronoi area is 3.0 × 10⁻⁶. 7 nm 2 The following was observed, meaning that the variation in Voronoi area was small. In other words, neither excessively small nor excessively large Voronoi regions were formed within the insulating layer. As a result, it was confirmed that in samples A3-6 to A3-8, even if at least a portion of the base resin deteriorated due to thermal aging in the second accelerated treatment under more severe temperature conditions than the first accelerated treatment, excessive increases in the elasticity of the insulating layer and excessive decreases in the dielectric breakdown strength of the insulating layer were prevented.

[0224] (3) Experiment 3 Experiment 3 investigated the dependence on inorganic fillers.

[0225] (3-1) Regarding Sample E3, Sample F3, and Sample G3 Power cables for samples E3, F3, and G3 were manufactured under substantially the same conditions as those for sample A3 (sample A3-2), except that titanium dioxide, zinc oxide, and aluminum oxide were used as inorganic fillers.

[0226] Specifically, in each of samples E3, F3, and G3, the average value of the Voronoi area A is approximately 57.0 × 10⁻⁶. 4 nm 2 Furthermore, the standard deviation σ of the Voronoi area is 3.0 × 10⁻⁶. 7 nm 2 An insulating layer was formed as follows:

[0227] (3-2) Evaluation In Experiment 3, the same evaluation was performed as in Experiments 1 and 2.

[0228] (3-3) Results In samples E3, F3, and G3, the storage modulus was 800 MPa or less, and the AC tree starting field strength E was 600 kV / mm or more, both before and after the first acceleration treatment.

[0229] Furthermore, in samples E3, F3, and G3, the storage modulus was 800 MPa or less even after the second acceleration treatment, and the AC tree initiation field strength E was 600 kV / mm or more.

[0230] From the results of samples E3, F3, and G3, equation (1) is satisfied for the mean value A of the Voronoi area, and the standard deviation σ of the Voronoi area is 3.0 × 10⁻⁶. 7 nm 2 We confirmed that, regardless of the type of inorganic filler, it was possible to prevent an excessive increase in the elasticity of the insulating layer and an excessive decrease in the dielectric breakdown strength of the insulating layer, even after the first or second heat treatment.

[0231] <Note> The following are aspects of the present disclosure. The aspects referred to by the numbers within [] to which the following appendices are subordinate correspond to the aspects described in <Embodiments of the Present Disclosure>.

[0232]

[11] In the step of forming the insulating layer, with respect to the dispersibility of the inorganic filler in the insulating layer, the insulating layer is formed so as to satisfy formula (1). 600D + 1.6×10 4 ≦A≦16000D + 3.0×10 6 ···(1) Here, D is the volume average particle diameter of the inorganic filler, with the unit being nm. A is the average value of the Voronoi area, which is the area of the region surrounding the target particle by the perpendicular bisector of the straight line connecting the target particle and another particle adjacent to the target particle among the inorganic fillers in the cross-section of the insulating layer, with the unit being nm 2 is The method for manufacturing a power cable according to

[10] .

Explanation of Reference Signs

[0233] 10 Power cable 110 Conductor 120 Inner semiconductive layer 130 Insulating layer 132 Base resin 134 Inorganic filler 134a Target particle 134b Other particle 140 Outer semiconductive layer 150 Shielding layer 160 Sheath 200 Extruder 210 Cylinder 222 First inlet 224 Second inlet 230 Screw 240 Rotation drive mechanism 250 Discharge part PB Perpendicular bisector VR Voronoi region FE Electrode NE needle electrode S1 Page 1 S2 Page 2 TP test film

Claims

1. A resin composition that constitutes the insulating layer of a power cable, A base resin containing polyolefin, Inorganic fillers and Includes, The resin composition satisfies formula (1) with respect to the dispersibility of the inorganic filler in the resin composition. 600D+1.6×10 ４ ≦A≦16000D+3.0×10 ６ ・・・(1) Here, D is the volume-average particle size of the inorganic filler, with units of nm. A is the average value of the Voronoi area, which is the area of ​​the region surrounding a target particle in a cross-section of the resin composition, defined by the perpendicular bisectors of the straight lines connecting the target particle and other adjacent particles of the inorganic filler, and the unit is nm. ２ That is Resin composition.

2. A conductor and An insulating layer covering the outer circumference of the conductor, Equipped with, The aforementioned insulating layer is A base resin containing polyolefin, Inorganic fillers and Includes, The insulating layer satisfies formula (1) with respect to the dispersibility of the inorganic filler in the insulating layer. 600D+1.6×10 ４ ≦A≦16000D+3.0×10 ６ ・・・(1) Here, D is the volume-average particle size of the inorganic filler, with units of nm. A is the average value of the Voronoi area, which is the area of ​​the region surrounding the target particle in the cross-section of the insulating layer, defined by the perpendicular bisectors of the straight lines connecting the target particle and other adjacent particles of the inorganic filler, and the unit is nm. ２ That is Power cable.

3. After performing a first accelerated treatment on a test specimen taken from the insulating layer, the storage modulus of the test specimen at 25°C, as measured by dynamic viscoelasticity measurement, is 800 MPa or less. Here, the first acceleration process is A step of heating the test specimen in a nitrogen atmosphere at a temperature of 145°C for 200 hours, A process of performing a heat cycle five times, which includes a high-temperature step of heating the test specimen at a temperature of 100°C for one hour, and a low-temperature step of cooling the test specimen at a temperature of 0°C for one hour. has The power cable according to claim 2.

4. After performing a first acceleration treatment on a test piece taken from the insulating layer, an alternating electric field is applied to the test piece at a temperature of 90°C using a needle-shaped electrode having a tip with a radius of curvature of 5 μm. The alternating electric field strength when an electrical tree is generated in the test piece is 600 kV / mm or more. Here, the first acceleration process is A step of heating the test specimen in a nitrogen atmosphere at a temperature of 145°C for 200 hours, A process of performing a heat cycle five times, which includes a high-temperature step of heating the test specimen at a temperature of 100°C for one hour, and a low-temperature step of cooling the test specimen at a temperature of 0°C for one hour. has The power cable according to claim 2 or claim 3.

5. In the cross-section of the insulating layer, the standard deviation of the Voronoi area, which is the area of ​​the region surrounding the target particle in the inorganic filler by the perpendicular bisector of the straight line connecting the target particle and other adjacent particles, is 3.0 × 10⁻⁶. ７ nm ２ The following is The power cable according to claim 2 or claim 3.

6. The inorganic filler contains silicon dioxide, The volume-average particle size D of the inorganic filler is 1 nm or more and 1 μm or less. The average value A of the Voronoi area in the cross-section of the insulating layer is 1.6 × 10 ４ nm ２ or more and 1.9 × 10 ７ nm ２ or less The power cable according to claim 2 or claim 3.

7. The inorganic filler contains magnesium oxide, The volume-average particle size D of the inorganic filler is 100 nm or more and 5 μm or less. The average value A of the Voronoi area in the cross-section of the insulating layer is 7.6 × 10 ４ nm ２ The above 8.3 x 10 ７ nm ２ The following is The power cable according to claim 2 or claim 3.

8. The base resin further comprises a modified polyolefin having polar groups. The power cable according to claim 2 or claim 3.

9. The base resin does not contain a modified polyolefin having polar groups. The power cable according to claim 2 or claim 3.

10. A step of preparing a resin composition comprising a base resin containing polyolefin and an inorganic filler, The process involves forming an insulating layer so as to cover the outer circumference of a conductor by extrusion molding of the resin composition, Equipped with, The step of preparing the resin composition is: A step of preparing a masterbatch containing the base resin and the inorganic filler, A step of preparing a filler-free compound that includes the base resin and does not contain the inorganic filler, It has, The step of forming the insulating layer is, A step of introducing a portion of the masterbatch and the filler-free compound from the first inlet of the extruder, A step of introducing the remaining portion of the master batch from a second inlet located downstream of the first inlet of the extruder, It has, In the process of forming the insulating layer, Regarding the dispersibility of the inorganic filler in the insulating layer, the insulating layer is formed such that formula (1) is satisfied. 600D+1.6×10 4 ≦A≦16000D+3.0×10 6 ... (1) Here, D is the volume-average particle size of the inorganic filler, with units of nm. A is the average value of the Voronoi area, which is the area of ​​the region surrounding the target particle in the cross-section of the insulating layer, by the perpendicular bisectors of the straight lines connecting the target particle and other adjacent particles of the inorganic filler, and the unit is nm². A method for manufacturing power cables.

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