Cable connection structure, coupled power cable, and cable connection structure manufacturing method

Abstract

This cable connection structure comprises: a first power cable and a second power cable each having a conductor, a cable inner semiconductive layer, a cable insulating layer, and a cable outer semiconductive layer in this order from the central axis of the conductor toward the outer periphery; a cylindrical sleeve that connects the conductor of the first power cable and the conductor of the second power cable; and an insulating unit that is configured as a cylindrical member, is provided so as to cover the outer periphery of a region including the sleeve, and maintains the insulating properties of the periphery of the sleeve. The first power cable and the second power cable each have an insulation exposure portion in which the cable insulating layer is exposed, and the insulation exposure portion has an outer peripheral surface satisfying formula (1). (1): 1.1 ≤ Raa / Rac ≤ 6, where Raa is the arithmetic mean roughness of the outer peripheral surface of the insulation exposure portion in the axial direction of the conductor and has a unit of μm, and Rac is the arithmetic mean roughness of the outer peripheral surface of the insulation exposure portion in the circumferential direction of the conductor and has a unit of μm.

Description

Cable connection structure, connected power cable, and method for manufacturing cable connection structure 【0001】 The present disclosure relates to a cable connection structure, a connected power cable, and a method for manufacturing a cable connection structure. 【0002】 In a cable connection structure, a cylindrical insulating unit containing rubber may be provided so as to cover the conductor connection portions of a pair of power cables (for example, Patent Document 1). 【0003】 Japanese Patent Application Laid-Open No. 2015-216807 【0004】 According to one aspect of the present disclosure, a first power cable and a second power cable each having a conductor, a cable internal semiconductive layer, a cable insulating layer, and a cable external semiconductive layer in this order from the central axis of the conductor toward the outer periphery, a cylindrical sleeve connecting the conductor of the first power cable and the conductor of the second power cable, and an insulating unit configured as a cylindrical member and provided so as to cover the outer periphery of a region including the sleeve and maintaining insulation around the sleeve are provided. Each of the first power cable and the second power cable has an insulation exposed portion where the cable insulating layer is exposed, and the insulation exposed portion has an outer peripheral surface satisfying Formula (1): 1.1 ≤ Raa / Rac ≤ 6... (1) Here, Raa is the arithmetic mean roughness of the outer peripheral surface of the insulation exposed portion in the axial direction of the conductor, and the unit is μm, and Rac is the arithmetic mean roughness of the outer peripheral surface of the insulation exposed portion in the circumferential direction of the conductor, and the unit is μm. A cable connection structure is provided. 【0005】Figure 1 is a schematic cross-sectional view showing a power cable. Figure 2 is a schematic cross-sectional view along the axial direction of a conductor showing a cable connection structure according to one embodiment of the present disclosure. Figure 3A is a diagram showing the cross-sectional profile of the insulation exposed portion in the axial direction of the conductor in sample C. Figure 3B is a diagram showing the cross-sectional profile of the insulation exposed portion in the circumferential direction of the conductor in sample C. Figure 4 is a diagram showing the cross-sectional profile of the insulation exposed portion in the axial direction of the conductor in sample D. Figure 5 is a diagram showing the cross-sectional profile of the insulation exposed portion in the axial direction of the conductor in sample B. Figure 6 is a diagram showing the cross-sectional profile of the insulation exposed portion in the axial direction of the conductor in sample A. Figure 7 is a diagram showing the cross-sectional profile of the insulation exposed portion in the axial direction of the conductor in sample F. Figure 8 is a diagram showing the discharge charge amount as a function of void radius. Figure 9 is a schematic cross-sectional view showing the interface between the insulation exposed portion and the insulation unit where the cable insulation layer of the power cable is exposed in the case of a conventional mirror finish. 【0006】 [Problems this disclosure aims to solve] The purpose of this disclosure is to obtain stable insulation for cable connection structures. 【0007】 [Effects of this disclosure] According to this disclosure, stable insulation of the cable connection structure can be obtained. 【0008】 [Description of Embodiments in this Disclosure] <Inventor's Knowledge> First, the inventor's knowledge will be explained. 【0009】 In the cable connection structure, an insulating unit is provided to cover the outer circumference of the power cable, which has been stripped in stages along the axial direction of the conductor. When positioning the insulating unit, insulating oil is applied to the diameter-expanding pipe that expands the diameter of the insulating unit, the inner surface of the insulating unit, and the exposed portion where the cable insulation layer of the power cable is exposed (hereinafter also referred to as the "insulated exposed portion"). 【0010】In the cable connection structure described above, when the exposed insulation portion of the power cable's cable insulation layer is brought into close contact with the insulation unit, voids containing air or areas where insulating oil accumulates locally (hereinafter also referred to as "oil reservoirs") may occur between them. Voids may also be present within these oil reservoirs. In these voids and oil reservoirs, the dielectric constant was lower than that of the cable insulation layer and the insulation unit's insulation layer. Therefore, there was a risk of partial discharge occurring due to voids or oil reservoirs formed at the interface between the exposed insulation portion and the insulation unit. 【0011】 Here, when spherical voids exist within a concentric insulator, the discharge charge Q (in pC) can be calculated using the following formula. 【0012】 【0013】 Here, r0 is the inner radius of the insulator (in mm), R0 is the outer radius of the insulator (in mm), R is the distance from the central axis of the insulator to the void (in mm), a is the radius of the void (in μm), ε0 is the permittivity of vacuum (in F / m), ε1 is the relative permittivity of the insulator, ε2 is the relative permittivity of the void, and V is the voltage applied to the insulator (in kV). 【0014】 As shown in Figure 8, the discharge charge tends to increase monotonically as the void radius increases. Even if an oil reservoir occurs instead of an air-filled void, the discharge charge can be determined by changing the relative permittivity ε² of the void to the relative permittivity of the insulating oil. In this case as well, the discharge charge tends to increase monotonically as the radius of the oil reservoir increases. Note that Figure 8 is an approximation, and the discharge charge may differ from the actual value. 【0015】Therefore, conventionally, it has been considered preferable to "mirror-finish" or "sand-finish" the outer surface of the exposed insulation portion of a power cable to prevent the formation of air-containing voids and oil accumulations between the exposed insulation portion and the insulation unit. "Mirror-finish" here refers to a finish that involves a first process of cutting the outer semiconducting layer of the power cable to expose the cable insulation layer, a second process of covering the outer surface of the exposed insulation portion treated in the first process with an insulating heat-shrinkable tube, and a third process of removing the heat-shrinkable tube after the second process. "Sand-finish" here refers to a finish that involves a first process similar to the first process of "mirror-finish," and a second process of polishing the outer surface of the exposed insulation portion treated in the first process with sandpaper. Conventionally, the surface roughness of the exposed insulation portion was uniformly reduced by the "mirror-finish" or "sand-finish" described above. 【0016】 However, the inventors' studies revealed that if the outer surface of the exposed insulation portion is given a mirror finish or a sanded finish, as in the conventional method, the following new problems arise. 【0017】 In conventional mirror-finish designs, voids or oil deposits that formed between the exposed insulation portion of the power cable's insulation layer and the insulation unit tended to move along the smooth, mirror-finished outer surface of the exposed insulation portion in the axial direction of the conductor. 【0018】 Therefore, conventionally, as shown in Figure 9, voids or oil reservoirs were locally aggregated at the interface between the insulation exposed portion and the insulation unit. As a result, voids or oil reservoirs were locally concentrated and formed along the interface between the insulation exposed portion and the insulation unit. 【0019】 However, conventional cable connection structures were applied at low voltages. Therefore, as shown in the 100kV case in Figure 8, for example, even if voids or oil reservoirs occurred, the amount of discharged charge was small. As a result, in conventional cable connection structures, voids or oil reservoirs caused by the mirror finish did not pose an insulation problem. 【0020】 In contrast, the applicable voltage for cable connection structures has been increasing in recent years. As a result, as shown in the case of 200kV in Figure 8, for example, even if the voids or oil reservoirs are minute, the amount of discharged charge tends to be large. Therefore, if voids or oil reservoirs are locally concentrated and formed along the interface between the insulation exposed portion and the insulation unit due to the mirror finish, the insulation performance of the cable connection structure may decrease. 【0021】 On the other hand, in the case of conventional paper finishing, in the second process in which the outer surface of the exposed insulation portion of the power cable's cable insulation layer is polished with sandpaper, dust (shavings) is generated due to the polishing with sandpaper, and there is a possibility that this dust will get mixed in between the exposed insulation portion and the insulation unit. Even if small dust particles get mixed in between the exposed insulation portion and the insulation unit, it does not cause any problems with the insulation performance of the cable connection structure. However, if a large amount of dust gets mixed in between the exposed insulation portion and the insulation unit, there is a risk that voids or oil reservoirs will be formed locally along the interface between the exposed insulation portion and the insulation unit via the dust. For this reason, as mentioned above, under the circumstances where the applied voltage of cable connection structures has been increasing in recent years, if voids or oil reservoirs are formed locally due to dust, there is a risk that the insulation performance of the cable connection structure will decrease. 【0022】 Therefore, in the case of paper finishing, a cleaning process to remove dust was required after the second treatment of sanding with sandpaper. As a result, the processing time was long in the conventional paper finishing method. 【0023】 In response to the novel challenges described above, the inventors conducted thorough research and ultimately invented a cable connection structure that intentionally introduces directional dependence in the surface roughness of the outer periphery of the exposed insulation portion. This successfully achieved stable insulation performance in the cable connection structure. 【0024】 The following disclosure is based on the above findings discovered by the Disclosing Party, etc. 【0025】<Embodiments of the Disclosure> Next, embodiments of the Disclosure will be described by listing them. 【0026】 [1] A cable connection structure according to one aspect of the present disclosure comprises: a first power cable and a second power cable, each having a conductor, an internal semiconducting layer, a cable insulating layer, and an external semiconducting layer in this order from the central axis of the conductor toward the outer circumference; a cylindrical sleeve connecting the conductor of the first power cable and the conductor of the second power cable; and an insulating unit configured as a cylindrical member, provided to cover the outer circumference of the area including the sleeve, and maintaining the insulating properties around the sleeve, wherein each of the first power cable and the second power cable has an insulating exposed portion where the cable insulating layer is exposed, and the insulating exposed portion has an outer surface that satisfies formula (1): 1.1 ≤ Raa / Rac ≤ 6 ... (1) where Raa is the arithmetic mean roughness of the outer surface of the insulating exposed portion in the axial direction of the conductor, and its unit is μm; and Rac is the arithmetic mean roughness of the outer surface of the insulating exposed portion in the circumferential direction of the conductor, and its unit is μm. This configuration allows for stable insulation of the cable connection structure. 【0027】 [2] In the cable connection structure described in [1] above, the insulating exposed portion has a plurality of recesses that are recessed toward the conductor, and a plurality of protrusions that protrude radially outward from each of the plurality of recesses toward the conductor, wherein the plurality of recesses and the plurality of protrusions are alternately provided in the axial direction of the conductor and are arranged spirally along the axial direction of the conductor. With this configuration, voids and oil reservoirs can be dispersed along the plurality of spiral recesses. 【0028】[3] In the cable connection structure described in [2] above, the plurality of protrusions have a first protrusion and a second protrusion adjacent to each other with one of the plurality of recesses in between, and each of the plurality of recesses has a deepest part that is recessed the deepest toward the conductor, a first inclined surface provided between the first protrusion and the deepest part and inclined with respect to the axial direction of the conductor, and a second inclined surface provided between the deepest part and the second protrusion and inclined opposite to the first inclined surface with respect to the deepest part in between, wherein the first inclined surface is longer than the second inclined surface in the axial direction of the conductor. With this configuration, even if the depth of the deepest part is shallow, voids and oil reservoirs can be dispersed in an appropriate amount near the deepest part. 【0029】 [4] In the cable connection structure described in any one of [1] to [3] above, the outer surface of the exposed insulation portion satisfies equation (2): 1.5 ≤ Raa / Rac ≤ 5.5 ... (2). This configuration makes it possible to stably suppress a decrease in the insulation performance of the cable connection structure. 【0030】 [5] In the cable connection structure described in any one of [1] to [4] above, Raa is 15 μm or less. This configuration makes it possible to stably suppress the decrease in the insulating properties of the cable connection structure. 【0031】 [6] A connected power cable according to another aspect of the present disclosure comprises at least one of the cable connection structures described in any one of [1] to [5] above. This configuration makes it possible to obtain stable insulation of the cable connection structure. 【0032】[7] A method for manufacturing a cable connection structure according to yet another aspect of the present disclosure comprises the steps of: preparing a first power cable and a second power cable, each having a conductor, an internal semiconducting layer, a cable insulating layer, and an external semiconducting layer in this order from the central axis of the conductor toward the outer circumference; connecting the conductor of the first power cable and the conductor of the second power cable with a cylindrical sleeve; and arranging an insulating unit, configured as a cylindrical member and maintaining the insulating properties around the sleeve, so as to cover the outer circumference of the region including the sleeve, wherein the step of preparing the first power cable and the second power cable includes the step of forming an insulating exposed portion in which the cable insulating layer is exposed, and in the step of forming the insulating exposed portion, the insulating exposed portion is formed such that it has an outer surface satisfying formula (1): 1.1 ≤ Raa / Rac ≤ 6 ... (1) where Raa is the arithmetic mean roughness of the outer surface of the insulating exposed portion in the axial direction of the conductor, and its unit is μm. Rac is the arithmetic mean roughness of the outer surface of the insulating exposed portion in the circumferential direction of the conductor, and its unit is μm. With this configuration, stable insulation of the cable connection structure can be obtained. 【0033】 [Details of Embodiments of the Disclosure] Next, one embodiment of the Disclosure will be described below with reference to the drawings. However, the Disclosure is not limited to these examples, and is intended to include all modifications within the meaning and scope of the equivalents of the claims, as indicated by the claims. 【0034】 <One Embodiment of the Present Disclosure> (1) Connected Power Cable and Cable Connection Structure The schematic configuration of a connected power cable 10 and a cable connection structure (cable connection part) 20 according to one embodiment of the present disclosure will be described with reference to Figures 1 and 2. 【0035】 In Figure 2, the lower half of the cable connection structure 20 is omitted. In Figure 2, the power cable 100, which has been stripped in stages, is shown from the side. 【0036】As shown in Figure 2, the connected power cable 10 of this embodiment includes, for example, a plurality of power cables 100 and at least one cable connection structure 20. 【0037】 In the following, the "axial direction" of the power cable 100 refers to the direction along the central axis of the power cable 100, and can be rephrased as the longitudinal direction of the power cable 100. The "radial direction" of the power cable 100 refers to the direction from the central axis of the power cable 100 toward the outer circumference. The "circumferential direction" of the power cable 100 refers to the direction along the outer circumference of the power cable 100. The same terminology as for the power cable 100 is used for other cylindrical members constituting the cable connection structure 20. "Suppression" of a predetermined phenomenon means preventing the occurrence of a predetermined phenomenon or making it difficult for a predetermined phenomenon to occur. 【0038】 (First power cable and second power cable) As shown in Figure 1, the power cable 100 is configured as a solid-insulated cable that is a high-voltage power transmission cable on land, underground, underwater, or on the seabed (seabed). The power cable 100 is configured as, for example, a CV cable (cross-linked polyethylene insulated vinyl sheath cable, also called XLPE cable). 【0039】 Specifically, the power cable 100 has, for example, a conductor 110, an internal semiconducting layer 120, a cable insulation layer 130, an external semiconducting layer 140, a water-absorbing layer (not shown), a cable metal shielding layer 150, and a cable sheath 160, in this order from the central axis of the conductor 110 toward the outer circumference of the power cable 100. 【0040】 The conductor 110 has, for example, a plurality of conducting strands (not shown in the diagram). The conducting strands include, for example, at least one of copper and aluminum. 【0041】The cable insulation layer 130 contains polyolefin. Examples of polyolefin include polyethylene or polypropylene. The polyethylene may be cross-linked. On the other hand, the polypropylene may be non-cross-linked or slightly cross-linked. 【0042】 The cable metal shielding layer 150 is, for example, a metal cover or a winding layer of copper wire or copper tape. 【0043】 As shown in FIG. 2, a plurality of power cables 100 are provided. Among the plurality of power cables 100, a pair of power cables 100 are abutted with the axes of their respective conductors 110 aligned. Hereinafter, one of the pair of power cables 100 may be referred to as the "first power cable 100a", and the other power cable 100 may be referred to as the "second power cable 100b". 【0044】 As shown in FIG. 2, each power cable 100 is gradually peeled off from the tip in the axial direction of the conductor 110 in the opposite direction (so-called "step peeling"). That is, the conductor 110, the cable internal semiconductive layer 120, the cable insulation layer 130, the cable external semiconductive layer 140, the cable metal shielding layer 150, and the cable sheath 160 are exposed in this order from the tip in the axial direction of the conductor 110 in the opposite direction. With such a configuration, the power cables 100 can be connected in order from a region close to the central axis toward the outer periphery. 【0045】 In the present embodiment, as shown in FIG. 2, the power cable 100 that is gradually peeled off has, for example, an insulation exposed portion 132, a slope portion 135, and an external semiconductive exposed portion 142. 【0046】 In the insulation exposed portion 132, for example, the cable insulation layer 130 is exposed with a first diameter along the axial direction of the conductor 110. That is, in the insulation exposed portion 132, the cable insulation layer 130 extends in a straight cylindrical shape. The first diameter of the insulation exposed portion 132 has a predetermined error due to the surface roughness of the outer peripheral surface of the insulation exposed portion 132 described later. 【0047】The slope portion 135 is inclined with respect to the axial direction of the conductor 110, for example, by gradually increasing the diameter from the insulation exposed portion 132 toward the external semi-conductive exposed portion 142 described later in the axial direction of the conductor 110. 【0048】 The slope portion 135 has an innermost end (small-diameter end, lower end) IE and an outermost end (large-diameter end, upper end) OE. The innermost end IE is in contact with the insulation exposed portion 132 and has the smallest diameter within the slope portion 135. The outermost end OE is in contact with the external semi-conductive exposed portion 142 described later at a position opposite to the innermost end IE and has the largest diameter within the slope portion 135. 【0049】 Here, the slope portion 135 is inclined at a small angle with respect to the axial direction of the conductor 110. Specifically, the inclination angle of the slope portion 135 with respect to the conductor 110 is, for example, more than 0° and 10° or less, or may be 0.1° or more and 8° or less, or may be 0.1° or more and 5° or less. 【0050】 The external semi-conductive exposed portion 142 is in contact with, for example, the end portion (outermost end OE) of the slope portion 135 opposite to the insulation exposed portion 132. In the external semi-conductive exposed portion 142, for example, the cable external semi-conductive layer 140 is exposed with a constant second diameter larger than the first diameter along the axial direction of the conductor 110. That is, in the external semi-conductive exposed portion 142, the cable external semi-conductive layer 140 extends in a straight cylindrical shape. 【0051】 (Cable connection structure) As shown in FIG. 2, the cable connection structure 20 is configured to connect a pair of power cables 100, for example, as an intermediate connection portion. FIG. 2 shows a case where the cable connection structure 20 is applied to, for example, an insulation connection portion of a three-phase alternating current. 【0052】 Specifically, the cable connection structure 20 has, for example, the pair of power cables 100 described above, a sleeve (conductor connection tube) 200, and an insulation unit (insulation cylinder, rubber connection cylinder, rubber unit) 300. 【0053】(Sleeve) The sleeve 200 is configured, for example, as a cylindrical metal tube, and is provided to connect the conductors 110 of a pair of power cables 100 and to surround the ends of each of the conductors 110. The cylindrical sleeve 200 may have a partition wall (not shown) in the center of the hollow portion. 【0054】 On the outside of the sleeve 200, for example, a semiconductive tape layer (not shown) may be provided to fill the step between the outer circumference of the pair of cable insulation layers 130 and the outer circumference of the sleeve 200. Furthermore, a sleeve cover (not shown) may be provided to surround the outer circumference of the sleeve 200 and to secure each end of the pair of cable insulation layers 130. These measures can suppress uneven surface pressure of the insulation unit 300, which will be described later. 【0055】 (Insulation Unit) The insulation unit 300 is configured, for example, as a cylindrical member having a hollow portion (not shown). A pair of power cables 100 are inserted through the hollow portion of the insulation unit 300. The insulation unit 300 is provided to cover the outer circumference of the area including the sleeve 200. 【0056】 The insulating unit 300 is configured, for example, as a so-called room-temperature shrinkable type or a factory-expandable type. That is, the insulating unit 300 has an elastic material that is integrally molded and is designed to shrink elastically at room temperature to closely adhere to the connection portion of the power cable 100. 【0057】 The insulating unit 300 is configured, for example, to maintain insulation around the sleeve 200 while mitigating the electric field around the sleeve 200. Specifically, the insulating unit 300 includes, for example, an internal semiconducting layer 320, an insulating layer 340, and an external semiconducting layer 360. 【0058】The internal semiconducting layer 320 of the insulating unit is configured in a cylindrical shape to cover the outer circumference of the sleeve 200. The internal semiconducting layer 320 of the insulating unit contains, for example, semiconducting rubber. Examples of the base polymer of the semiconducting rubber include ethylene propylene rubber and silicone rubber. Examples of fillers contained in the semiconducting rubber include carbon black. The internal semiconducting layer 320 of the insulating unit is at the same potential as the sleeve 200. 【0059】 The insulating layer 340 of the insulating unit is provided so as to cover the outer circumference of the internal semiconducting layer 320 of the insulating unit, a portion of the first power cable 100a, and a portion of the second power cable 100b. The insulating layer 340 of the insulating unit includes, for example, insulating rubber. Examples of insulating rubber include ethylene propylene rubber and silicone rubber. 【0060】 The insulating unit's external semiconducting layer 360 is provided in the area outside the insulating unit 300 and is provided so as to cover the outer circumference of the insulating unit's insulating layer 340, a portion of the first power cable 100a, and a portion of the second power cable 100b. The insulating unit's external semiconducting layer 360, like the insulating unit's internal semiconducting layer 320, contains semiconducting rubber. 【0061】 The external semiconducting layer 360 of the insulating unit has, for example, a stress cone portion 380. The stress cone portion 380 is provided near both ends in the axial direction of the insulating layer 340 of the insulating unit. The stress cone portion 380 has a cone shape (with an inner circumferential surface 382) that gradually widens toward the axial center of the insulating layer 340 of the insulating unit. 【0062】 A portion of the inner circumferential surface 382 of the stress cone portion 380 is exposed to the hollow portion of the insulating unit 300 and is in contact with the outer circumferential surface of the power cable 100, which has been peeled off in stages. 【0063】In this embodiment, for example, the inner circumferential surface 382 of the stress cone portion 380 exposed in the hollow portion of the insulating unit 300 is positioned to cover the entire slope portion 135 over a wider area than the slope portion 135 in both the first power cable 100a and the second power cable 100b. In other words, the inner circumferential surface 382 of the stress cone portion 380 exposed in the hollow portion of the insulating unit 300 is in close contact with the entire slope portion 135 over a wider area than the slope portion 135, in accordance with the change in diameter near the slope portion 135. This makes it possible to suppress instability in the electric field near the insulating unit 300. 【0064】 The stress cone portion 380 having the above configuration allows equipotential lines to be evenly distributed along the cone shape, thereby suppressing electric field concentration. As a result, the insulating unit 300 can electrically shield the area around the sleeve 200 while mitigating the electric field at the tip of the cable's outer semiconducting layer 140. 【0065】 (Configuration outside the insulating unit) Outside the insulating unit 300, spacers (not shown) may be provided, for example, to surround the outer circumference of each of the pair of power cables 100 and to be in contact with the axial ends of the insulating unit 300. The spacers, for example, gradually decrease in diameter in the direction away from the axial ends of the insulating unit 300 along the conductor 110. This makes it possible to form a gently sloping slope (inclined surface, conical surface) from the outer circumference of the insulating unit 300 toward the outer circumference of the power cable 100. 【0066】 Furthermore, protective parts (not shown) may be provided to cover the outer circumference of the insulating unit 300, the spacer, a portion of the first power cable 100a, and a portion of the second power cable 100b, for example. 【0067】 The protective section may include, for example, a metal tube (not shown) and a filling section (not shown). The metal tube is arranged to surround the outer circumference of the insulating unit 300, the spacer, a portion of the first power cable 100a, and a portion of the second power cable 100b. The filling section contains an insulating compound and is filled between the metal tube and the insulating unit 300. 【0068】 Alternatively, the protective portion may include, for example, a heat-shrinkable tube with a water-blocking layer. The heat-shrinkable tube with a water-blocking layer includes a metal sheet and a resin layer, and the resin layer shrinks upon heating to cover the outer circumference of the insulating unit 300, the spacer, a portion of the first power cable 100a, and a portion of the second power cable 100b. 【0069】 (2) Outer surface of the exposed insulation portion Next, the outer surface of the exposed insulation portion 132 of the power cable 100 of this embodiment, in which the cable insulation layer 130 is exposed, will be described. 【0070】 The exposed insulation portion 132 and the slope portion 135 are formed, for example, using a rotary cutting tool. Specifically, the exposed insulation portion 132 and the slope portion 135 are formed by using a cutting tool and rotating the blade in the circumferential direction of the power cable 100 while applying the blade to the outer circumference of the power cable 100. 【0071】 In this embodiment, mirror finishing and sanding are not performed after the above-described cutting. As a result, in this embodiment, a directional dependence is intentionally introduced in the surface roughness of the outer surface of the insulating exposed portion 132. The arithmetic mean roughness of the outer surface of the insulating exposed portion 132 in the axial direction of the conductor 110 is greater than, for example, the arithmetic mean roughness of the outer surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110. 【0072】 Specifically, the insulating exposed portion 132 has an outer surface that satisfies, for example, equation (1): 1.1 ≤ Raa / Rac ≤ 6 ... (1) 【0073】 Here, Raa is the arithmetic mean roughness of the outer surface of the insulation exposed portion 132 in the axial direction of the conductor 110, and its unit is μm. Rac is the arithmetic mean roughness of the outer surface of the insulation exposed portion 132 in the circumferential direction of the conductor 110, and its unit is μm. Note that Rac is the arithmetic mean roughness of the outer surface of the insulation exposed portion 132 measured using a two-dimensional measuring instrument while rotating the power cable 100 in the circumferential direction of the conductor 110. 【0074】Here, if the outer surface of the exposed insulation portion 132 is given a mirror finish or a sanded finish, Raa and Rac will be approximately equal and smaller. In the case of such a mirror finish or sanded finish, even if Raa and Rac are slightly different, Raa / Rac < 1.1. When Raa / Rac < 1.1 due to a mirror finish, if voids or oil reservoirs occur between the exposed insulation portion 132 and the insulation unit 300, these voids or oil reservoirs may be formed locally along the interface between the exposed insulation portion 132 and the insulation unit 300. When Raa / Rac < 1.1 due to a sanded finish, dust may be mixed in between the exposed insulation portion 132 and the insulation unit 300. If a large amount of dust is mixed in between the exposed insulation portion 132 and the insulation unit 300, it may be formed locally along the interface between the exposed insulation portion 132 and the insulation unit 300 via the dust. As a result, when Raa / Rac < 1.1, the insulation performance of the cable connection structure 20 may decrease depending on the condition after mirror finishing or paper finishing. 【0075】 In contrast, in this embodiment, by setting Raa / Rac ≥ 1.1, irregularities are formed on the outer circumferential surface of the insulating exposed portion 132 in the axial direction of the conductor 110, while the outer circumferential surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110 is smooth. By forming irregularities on the outer circumferential surface of the insulating exposed portion 132 in the axial direction of the conductor 110, the movement of voids and oil accumulations along the outer circumferential surface of the insulating exposed portion 132 in the axial direction of the conductor 110 can be restricted. On the other hand, by making the outer circumferential surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110 smooth, voids and oil accumulations can be dispersed along the outer circumferential surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110. As a result, the formation of voids and oil accumulations that accumulate locally along the interface between the insulating exposed portion 132 and the insulating unit 300 can be suppressed. 【0076】On the other hand, when Raa / Rac > 10, excessively large irregularities are formed on the outer surface of the insulation exposed portion 132 in the axial direction of the conductor 110, compared to the outer surface of the insulation exposed portion 132 in the circumferential direction of the conductor 110. In this case, large voids or oil reservoirs are formed in the excessively large irregularities of the insulation exposed portion 132. As a result, the insulation performance of the cable connection structure 20 is reduced. 【0077】 In contrast, in this embodiment, by setting Raa / Rac ≤ 6, excessively large irregularities are not formed on the outer surface of the insulating exposed portion 132 in the axial direction of the conductor 110, compared to the outer surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110. This makes it possible to suppress the formation of large voids and oil reservoirs caused by excessively large irregularities in the insulating exposed portion 132. As a result, a decrease in the insulation performance of the cable connection structure 20 can be suppressed. 【0078】 In this embodiment, the outer surface of the insulating exposed portion 132 may satisfy, for example, equation (2): 1.5 ≤ Raa / Rac ≤ 5.5 ... (2) 【0079】 By setting Raa / Rac ≥ 1.5, the formation of locally accumulated voids and oil pools along the interface between the insulation exposed portion 132 and the insulation unit 300 can be stably suppressed. On the other hand, by setting Raa / Rac ≤ 5.5, the formation of large voids and oil pools caused by excessively large irregularities in the insulation exposed portion 132 can be stably suppressed. As a result, the deterioration of the insulation performance of the cable connection structure 20 can be stably suppressed. 【0080】 In this embodiment, as described above, a directional dependence is intentionally introduced in the surface roughness of the outer circumferential surface of the insulating exposed portion 132, but it is preferable that the absolute value of the surface roughness is not excessively large. 【0081】 Specifically, Raa may be, for example, 15 μm or less, or 10 μm or less. This makes it possible to stably suppress the formation of large voids and oil reservoirs caused by excessively large irregularities in the insulation exposed portion 132. As a result, it is possible to stably suppress the deterioration of the insulation performance of the cable connection structure 20. 【0082】 The lower limit of Raa is not limited as long as it satisfies the above equation (1). However, Raa may be 3 μm or larger. 【0083】 Since Rac is smaller than Raa, it is naturally 15 μm or less. 【0084】 Next, with reference to Figures 3A to 5, the specific shape of the outer circumferential surface of the insulating exposed portion 132 in this embodiment will be described. 【0085】 In this embodiment, the outer surface of the exposed insulating portion 132 is left in the state as it was, for example, machined by a cutting tool. In other words, the outer surface of the exposed insulating portion 132 has not undergone either the process of covering the outer surface of the exposed insulating portion with an insulating heat shrink tubing, or the process of polishing the outer surface of the exposed insulating portion 132 with sandpaper. 【0086】 Specifically, the outer surface of the insulating exposed portion 132 has one of the following three shapes, depending on the shape of the cutting tool blade used, for example. 【0087】 (Type 1: Sample C using a ring-shaped blade) Figures 3A and 3B show the cross-sectional profiles of the insulation exposed portion 132 in the axial and circumferential directions of the conductor 110 in Sample C, which will be described later. Sample C is the case when a ring-shaped blade is used. A "ring-shaped blade" means a blade whose cross-section is ring-shaped and whose cutting surface that comes into contact with the power cable 100 is curved in an arc shape. 【0088】 As shown in Figure 3B, in the cross-sectional profile of the insulation exposed portion 132 in the circumferential direction of the conductor 110 in type 1, the outer surface of the insulation exposed portion 132 is smooth. This is because the outer surface of the insulation exposed portion 132 was formed by rotating a ring-shaped blade in the circumferential direction of the power cable 100. 【0089】On the other hand, as shown in Figure 3A, in the cross-sectional profile of the insulation exposed portion 132 in the axial direction of the conductor 110 of type 1, the insulation exposed portion 132 has, for example, a plurality of recesses 133 and a plurality of protrusions 134. The plurality of recesses 133 are recessed toward the conductor 110. The plurality of protrusions 134 project outward radially from each of the plurality of recesses 133 toward the conductor 110. The plurality of recesses 133 and the plurality of protrusions 134 are arranged alternately in the axial direction of the conductor 110. The plurality of recesses 133 and the plurality of protrusions 134 are arranged spirally along the axial direction of the conductor 110 by cutting using a rotary cutting tool. This makes it possible to disperse voids and oil reservoirs along the plurality of spiral recesses 133. 【0090】 The multiple protrusions 134 include, for example, a first protrusion 134a and a second protrusion 134b that are adjacent to each other, with one of the multiple recesses 133 in between. Note that either the first protrusion 134a or the second protrusion 134b may be positioned closer to the tip of the conductor 110. 【0091】 Each of the multiple recesses 133 has, for example, a deepest part 136, a first inclined surface 137, and a second inclined surface 138. The deepest part 136 is the deepest recess toward the conductor 110. The first inclined surface 137 is provided between the first convex part 134a and the deepest part 136 and is inclined with respect to the axial direction of the conductor 110. The second inclined surface 138 is provided between the deepest part 136 and the second convex part 134b and is inclined opposite to the first inclined surface 137 with the deepest part 136 in between. 【0092】 The first inclined surface 137 may be longer than the second inclined surface 138 in the axial direction of the conductor 110, for example. This allows for the dispersion of voids and oil reservoirs in an appropriate amount near the deepest part 136, even if the deepest part 136 is shallow. 【0093】At least one of the first inclined surface 137 and the second inclined surface 138 may be curved concave toward, for example, the conductor 110. In Type 1, both the first inclined surface 137 and the second inclined surface 138 are curved concavely. This allows for the appropriate dispersion of voids and oil reservoirs within the curved recess 133. 【0094】 The lengths of the first inclined surface 137 and the second inclined surface 138 in the axial direction of the conductor 110 may differ among the multiple recesses 133. Furthermore, the depth of the deepest part 136 may differ among the multiple recesses 133. By making the shape of the multiple recesses 133 complex in this way, voids and oil reservoirs can be randomly distributed in each of the complex recesses. 【0095】 (Type 2: Sample D using a J-shaped blade) Figure 4 shows the cross-sectional profile of the insulation exposed portion 132 in the axial direction of the conductor 110 in Sample D, which will be described later. Sample D is the case when a J-shaped blade is used. A "J-shaped blade" means a blade whose cross-section is J-shaped and whose cutting surface that comes into contact with the power cable 100 is curved in an arc shape. 【0096】 In the cross-sectional profile of the insulation exposed portion 132 in the circumferential direction of the conductor 110 in Type 2, the outer surface of the insulation exposed portion 132 is smooth, similar to Type 1. 【0097】 As shown in Figure 4, in the cross-sectional profile of the insulation exposed portion 132 in the axial direction of the conductor 110 of type 2, the insulation exposed portion 132 also has a plurality of recesses 133 and a plurality of protrusions 134. 【0098】 However, in Type 2, the shape of the insulating exposed portion 132 may differ from that of Type 1 in the following respects, for example. Specifically, in Type 2, the second inclined surface 138 is curved concavely toward the conductor 110, while the first inclined surface 137 may be curved slightly convexly toward the radially outward direction of the conductor 110. Even with this shape of Type 2, the same effects as Type 1 can be obtained. 【0099】(Type 3: Sample B using a wide L-shaped blade) Figure 5 shows the cross-sectional profile of the insulation exposed portion 132 in the axial direction of the conductor 110 in Sample B, which will be described later. Sample D is the case when a wide L-shaped blade is used. A "wide L-shaped blade" means a blade whose cross-section is L-shaped and whose cutting surface that comes into contact with the power cable 100 is wide. 【0100】 In the cross-sectional profile of the insulation exposed portion 132 in the circumferential direction of the conductor 110 in Type 3, the outer surface of the insulation exposed portion 132 is smooth, similar to Types 1 and 2. 【0101】 As shown in Figure 5, in the cross-sectional profile of the insulation exposed portion 132 in the axial direction of the conductor 110 of type 3, the insulation exposed portion 132 also has a plurality of recesses 133 and a plurality of protrusions 134. 【0102】 However, in Type 3, the shape of the insulating exposed portion 132 may differ from that of Type 1 in the following respects, for example. Specifically, in Type 3, the length of the first inclined surface 137 in the axial direction of the conductor 110 may be, for example, twice or more the length of the second inclined surface 138, or three times or more. Even with such a shape in Type 3, the same effects as in Type 1 can be obtained. 【0103】 The shape of the outer surface of the insulating exposed portion 132 is not limited to the types 1 to 3 described above. The insulating exposed portion 132 may have an outer surface of a different type from types 1 to 3, as long as the outer surface of the insulating exposed portion 132 satisfies at least formula (1) described above. 【0104】 Since the region from the exposed insulation portion 132 to the slope portion 135 is continuously cut along the axial direction of the conductor 110, the outer surface of the slope portion 135 may have the same surface roughness characteristics as the outer surface of the exposed insulation portion 132 described above, except that it gradually widens in the axial direction of the conductor 110. 【0105】 (3) Method for manufacturing a cable connection structure (method for manufacturing a connected power cable, method for connecting cables) Next, with reference to Figures 1 to 5, the method for manufacturing the cable connection structure 20 according to this embodiment will be described. 【0106】 The manufacturing method for the cable connection structure 20 of this embodiment includes, for example, a preparation step S10, a conductor connection step S20, an insulation unit arrangement step S30, and a protective part formation step S40. 【0107】 (S10: Preparation process) First, the components of each layer of the cable connection structure 20 are prepared: a pair of power cables 100, a sleeve 200, an insulating unit 300, a spacer 500, and the protective part. 【0108】 For example, insulating oil is applied in advance at the factory to the diameter-expanding pipe that expands the diameter of the insulating unit 300 and to the inner circumferential surface of the insulating unit 300. After applying the insulating oil, with the insulating unit 300 supported by its vertical end face, the diameter-expanding pipe is inserted hydraulically into the hollow portion of the insulating unit 300. This expands the diameter of the insulating unit 300. 【0109】 Next, for example, at the power cable 100 installation site, the pair of power cables 100 are peeled off in stages axially from one end of each cable. This exposes the conductor 110, cable insulation layer 130, cable outer semiconducting layer 140, and cable sheath 160 in that order from the tip of the power cable 100. 【0110】 In this embodiment, the steps for preparing the power cable 100 include, for example, an insulating exposed portion formation step S12, a slope portion formation step S14, an external semiconducting exposed portion formation step S16, and an insulating oil application step S18. 【0111】 The following steps—insulation exposure formation step S12, slope formation step S14, and external semiconducting exposure formation step S16—are performed, for example, using the rotary cutting tool described above. 【0112】(S12: Insulation Exposure Formation Process) Using a cutting tool, the blade is applied to the outer circumference of the power cable 100 and rotated in the circumferential direction of the power cable 100. At this time, with the rotation diameter of the blade fixed to a first diameter, the blade is moved in the axial direction of the conductor 110 while rotating the blade in the circumferential direction of the power cable 100. This forms an insulation exposure portion 132 in which the cable insulation layer 130 is exposed with a first diameter along the axial direction of the conductor 110. The first diameter is the average value of the outer diameter of the insulation exposure portion 132. There are, for example, two methods for fixing the power cable 100. The first method is, for example, a method of firmly fixing the power cable 100. In the first method, the cross-section of the power cable 100 after cutting becomes close to a perfect circle. On the other hand, the second method is, for example, a method of fixing the power cable 100 via a spring. In the second method, the cross-sectional shape of the power cable 100 after cutting remains close to the cross-sectional shape before cutting. For example, if the cross-section of the power cable 100 before cutting is elliptical, then the cross-section of the power cable 100 after cutting will also be elliptical. 【0113】 In this embodiment, for example, the insulating exposed portion 132 is formed such that it has an outer circumferential surface that satisfies formula (1): 1.1 ≤ Raa / Rac ≤ 6. Furthermore, for example, the insulating exposed portion 132 may be formed such that it has an outer circumferential surface that satisfies formula (2): 1.5 ≤ Raa / Rac ≤ 5.5. 【0114】 In this embodiment, the insulating exposed portion 132 may, for example, have a plurality of recesses 133 that are recessed toward the conductor 110, and a plurality of protrusions 134 that project outward radially from each of the plurality of recesses 133 toward the conductor 110. For example, the plurality of recesses 133 and the plurality of protrusions 134 may be formed alternately in the axial direction of the conductor 110, and may be formed in a spiral shape along the axial direction of the conductor 110. 【0115】 Specifically, for example, the outer surface of the insulating exposed portion 132 may be cut to have one of the shapes described above (types 1 to 3) by using one of the following: a ring-shaped blade, a J-shaped blade, or a wide L-shaped blade. 【0116】In this embodiment, for example, the outer surface of the exposed insulating portion 132 is left as it is after being machined by a cutting tool. In other words, neither the process of covering the outer surface of the exposed insulating portion with an insulating heat-shrinkable tube nor the process of polishing the outer surface of the exposed insulating portion 132 with sandpaper is applied to the outer surface of the exposed insulating portion 132. 【0117】 (S14: Slope Formation Process) Once the insulation exposure process S12 is completed, the blade is rotated in the circumferential direction of the power cable 100 while moving the blade away from the insulation exposure 132 along the axial direction of the conductor 110. At this time, as the blade is moved away from the insulation exposure 132 along the axial direction of the conductor 110, the rotation diameter of the blade is gradually increased from the first diameter. When the rotation diameter of the blade becomes a second diameter which is larger than the first diameter, the change in the rotation diameter of the blade is stopped. As a result, a slope portion 135 that is inclined with respect to the axial direction of the conductor 110 is formed by gradually expanding the diameter away from the insulation exposure 132 in the axial direction of the conductor 110. 【0118】 In this case, the outer surface of the slope portion 135 may also be machined to have the same surface roughness characteristics as the outer surface of the insulating exposed portion 132 described above, except that it gradually widens in the axial direction of the conductor 110. 【0119】 (S16: External Semiconducting Exposed Part Formation Process) In the external semiconducting exposed part formation process S16, no special processing is performed, but a portion of the cable's external semiconducting layer 140 that was exposed as a result of the slope part formation process S14 is left. As a result, an external semiconducting exposed part 142 is formed that is in contact with the end of the slope part 135 and has a second diameter larger than the first diameter along the axial direction of the conductor 110, in which the cable's external semiconducting layer 140 is exposed. 【0120】 (S18: Insulating oil application process) After the insulating exposed portion 132, the slope portion 135, and the external semiconducting exposed portion 142 are formed, insulating oil is applied to the outer surface of the area in which the insulating exposed portion 132, the slope portion 135, and the external semiconducting exposed portion 142 are covered by the insulating unit 300 in the insulating unit placement process S30 described later. 【0121】As described above, once the components constituting the cable connection structure 20 are prepared, the power cable 100 is passed through the insulating unit 300, and the insulating unit 300 is moved to a predetermined position on the power cable 100. 【0122】 (S20: Conductor connection process) Once the preparation process S10 is complete, the pair of power cables 100 are butted together inside the sleeve 200 with the axes of their conductors 110 aligned. After the pair of power cables 100 are butted together, the sleeve 200 is compressed to connect the conductors 110 of the pair of power cables 100. 【0123】 Furthermore, a semiconductive tape layer (not shown) may be provided to fill the step between the outer circumference of the pair of cable insulation layers 130 and the outer circumference of the sleeve 200. In addition, a sleeve cover may be provided on the outside of that. 【0124】 (S30: Insulation unit placement process) Once the conductor connection process S20 is completed, an insulation unit 300 is provided to ensure insulation around the sleeve 200, so as to cover the outer circumference of the area including the sleeve 200. Specifically, the insulation unit 300, which has been expanded by the expansion pipe, is moved to a position where it overlaps with the sleeve 200. Once the insulation unit 300 is in the predetermined position, the expansion pipe is gradually withdrawn from the insulation unit 300, and the insulation unit 300 is gradually reduced in diameter in the axial direction. In this way, the insulation unit 300 is positioned so as to cover the outer circumference of the sleeve 200 and a portion of the outer circumference of each of the pair of power cables 100. 【0125】 In this embodiment, the inner circumferential surface 382 of the stress cone portion 380 exposed in the hollow portion of the insulating unit 300 is arranged in each of the first power cable 100a and the second power cable 100b to cover the entire slope portion 135 over a wider area than the slope portion 135. 【0126】 (S40: Protective section formation process) Once the insulation unit placement process S30 is completed, a protective section is formed to cover the outer circumference of the insulation unit 300, a portion of the first power cable 100a, and a portion of the second power cable 100b. As the protective section, for example, a configuration including a metal pipe or a configuration including a heat shrink tubing with a water-shielding layer may be applied. 【0127】 Based on the above, the cable connection structure 20 and the connecting power cable 10 of this embodiment are manufactured. 【0128】 (4) Summary of this embodiment This embodiment provides one or more of the following effects. 【0129】 (a) In this embodiment, the exposed insulation portion 132 in which the cable insulation layer 130 is exposed has an outer surface that satisfies formula (1): 1.1 ≤ Raa / Rac ≤ 6. 【0130】 In this embodiment, by setting Raa / Rac ≥ 1.1, irregularities are formed on the outer circumferential surface of the insulating exposed portion 132 in the axial direction of the conductor 110, while the outer circumferential surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110 is smooth. By forming irregularities on the outer circumferential surface of the insulating exposed portion 132 in the axial direction of the conductor 110, the movement of voids and oil accumulations along the outer circumferential surface of the insulating exposed portion 132 in the axial direction of the conductor 110 can be restricted. On the other hand, by making the outer circumferential surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110 smooth, voids and oil accumulations can be dispersed along the outer circumferential surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110. This makes it possible to suppress the formation of voids and oil accumulations that accumulate locally along the interface between the insulating exposed portion 132 and the insulating unit 300. 【0131】 On the other hand, in this embodiment, by setting Raa / Rac ≤ 6, excessively large irregularities are not formed on the outer surface of the insulating exposed portion 132 in the axial direction of the conductor 110, compared to the outer surface of the insulating exposed portion 132 in the circumferential direction of the conductor 110. This makes it possible to suppress the formation of large voids and oil reservoirs caused by excessively large irregularities in the insulating exposed portion 132. 【0132】 In this way, by suppressing the formation of large voids and oil reservoirs between the insulating exposed portion 132 and the insulating unit 300, it is possible to suppress the occurrence of partial discharge caused by voids and oil reservoirs, even when the applied voltage of the cable connection structure 20 is high. 【0133】As described above, according to this embodiment, it is possible to obtain stable insulation of the cable connection structure 20. 【0134】 (b) In this embodiment, the outer surface of the insulation exposed portion 132 can be left in the state as it is after being cut by the cutting tool. In other words, both the process of covering the outer surface of the insulation exposed portion 132 with an insulating heat shrink tube and the process of polishing the outer surface of the insulation exposed portion 132 with sandpaper can be eliminated. Furthermore, by not polishing the outer surface of the insulation exposed portion 132 in which the cable insulation layer 130 of the power cable 100 is exposed with sandpaper, the mixing of dust between the insulation exposed portion 132 and the insulation unit 300 can be suppressed. As a result, the cleaning process for dust removal that was necessary with sandpaper finishing can also be eliminated. As a result, the manufacturing process of the cable connection structure 20 of this embodiment can be simplified and the time required for the manufacturing process can be shortened. 【0135】 <Other Embodiments of the Disclosure> Although embodiments of the Disclosure have been described in detail above, the Disclosure is not limited to the embodiments described above and can be modified in various ways without departing from its essence. 【0136】 In the above-described embodiment, Figure 2 shows an example where the cable connection structure 20 is applied to an insulating connection section in which the insulating unit's external semiconducting layer 360 has a separation portion. However, the cable connection structure 20 of this embodiment may also be applied to a normal connection section in which the insulating unit's external semiconducting layer 360 does not have a separation portion. 【0137】 In the above-described embodiment, one cable connection structure 20 of the linked power cable 10 was explained. However, the linked power cable 10 may have multiple cable connection structures 20. 【0138】 In the above-described embodiment, the case where the cable connection structure 20 is applied to three-phase AC was shown as an example. However, the cable connection structure 20 may also be applied to DC. 【0139】Next, embodiments relating to this disclosure will be described. These embodiments are examples of this disclosure and the disclosure is not limited to these embodiments. 【0140】 (1) Manufacturing of Cable Connection Structures As cable connection structures for connecting 154kV class power cables, the following sample cable connection structures A to F were manufactured. Ten cable connection structures were manufactured for each sample. 【0141】 (Common configuration of cable connection structure) Conductor cross-sectional area of ​​power cable: 600 mm² ２ Second diameter of the external semiconducting exposed portion of the power cable: 65 mm Angle of the slope portion relative to the axial direction of the conductor: 5° First diameter (average outer diameter) of the insulated exposed portion of the power cable: 63 mm Inner diameter of the insulation unit before installation: 55 mm Outer diameter of the insulation unit: 160 mm 【0142】 (Samples B to D) Samples B to D were manufactured, each containing a cable connection structure that satisfies the configuration of the above-described embodiment. 【0143】 (Sample B) In the preparation process for Sample B, a wide L-shaped blade was used. Specifically, a cutting tool with a wide L-shaped blade was used, and the blade was rotated in the circumferential direction of the power cable while applying the blade to the outer circumference of the power cable. At this time, with the rotation diameter of the blade fixed to the first diameter, the blade was moved in the axial direction of the conductor while rotating the blade in the circumferential direction of the power cable. This formed an insulation exposure portion in which the cable insulation layer was exposed along the axial direction of the conductor at the first diameter. 【0144】 In this case, in sample B, the outer surface of the exposed insulation portion was left in the state it was in after being machined with a cutting tool. In other words, in sample B, neither mirror polishing nor the sanding treatment described later was applied to the outer surface of the exposed insulation portion 132. 【0145】 Furthermore, in the preparation process, the slope portion and the external semiconducting exposed portion were formed according to the embodiment described above. At this stage, surface roughness measurement, as described later, was performed. Subsequently, the conductor connection process, the insulation unit placement process, and the protection portion formation process were performed according to the embodiment described above. As a result, the cable connection structure of sample B was manufactured. 【0146】 (Sample C) In Sample C, the cable connection structure was manufactured in the same way as in Sample B, except that a ring-shaped blade was used in the preparation process. 【0147】 (Sample D) In ​​Sample D, the cable connection structure was manufactured in the same way as Sample B, except that a J-shaped blade was used in the preparation process. 【0148】 (Samples A, E, and F) Samples A, E, and F were manufactured, which are cable connection structures that do not satisfy the configuration of the above-described embodiment. 【0149】 (Sample A) In the preparation process for Sample A, the first treatment of cutting the outer surface of the exposed insulating layer was performed in the same manner as in Sample B, except that an L-shaped blade with a cutting surface width half the width of the cutting surface of the wide L-shaped blade used in Sample B was used. Furthermore, in Sample A, the outer surface of the exposed insulating layer was sanded. In the sanding treatment, the outer surface of the exposed insulating layer that underwent the first treatment was polished with 400 grit sandpaper in the second treatment. The subsequent steps for Sample A were performed in the same manner as in Sample B. 【0150】 (Sample E) In Sample E, the cable connection structure was manufactured in the same way as Sample B, except that a plate-shaped cutting blade was used in the preparation process. 【0151】 (Sample F) In Sample F, the cable connection structure was manufactured in the same way as Sample A, except that paper finishing was not performed in the preparation process. 【0152】 (2) Evaluation of cable connection structure The following evaluations were performed on each sample. 【0153】(Surface Roughness Measurement) After the preparation process for each sample, the surface roughness of the outer surface of the exposed insulation portion was measured. For the surface roughness measurement, an SE600 model manufactured by Kosaka Seisakusho Co., Ltd. was used to measure the cross-sectional profile of the exposed insulation portion in the axial and circumferential directions of the conductor. The measurement length was set to "8 mm" for each direction. As a result of the measurement, the arithmetic mean roughness Raa (in μm) of the outer surface of the exposed insulation portion in the axial direction of the conductor and the arithmetic mean roughness Rac (in μm) of the outer surface of the exposed insulation portion in the circumferential direction of the conductor were determined. At this time, Rac was determined by measuring the arithmetic mean roughness of the outer surface of the exposed insulation portion while rotating the power cable in the circumferential direction of the conductor. Furthermore, the ratio Raa / Rac was determined based on these results. 【0154】 (Partial Discharge Test) A partial discharge test was conducted to evaluate the cable connection structure of each sample. Specifically, in accordance with the Japan Electric Association's power standard "Standard A-266 for 154kVCV Cable Straight Connection Boxes," a 190kV AC voltage was applied for 10 minutes between the conductor of the power cable and the metal shielding layer, and the amount of charge from the partial discharge generated from the cable connection structure was measured. Based on the measurement results, a charge amount of 5pC or less from the partial discharge generated from the cable connection structure was evaluated as "good," and a charge amount exceeding 5pC was evaluated as "poor." 【0155】 For each sample, we determined the number of cable connection structures that were evaluated as "good" out of 10 cable connection structures. 【0156】 (Dust contamination evaluation) After manufacturing the cable connection structure of each sample, the insulation unit was removed from the power cable. Next, the exposed insulation portion of the power cable where the cable insulation layer was exposed was observed, and the presence of dust was checked on the exposed insulation portion. If no dust was found on the exposed insulation portion, it was evaluated as "good" as no dust had been mixed between the exposed insulation portion and the insulation unit. On the other hand, if dust was found on the exposed insulation portion, it was evaluated as "poor" as dust had been mixed between the exposed insulation portion and the insulation unit. 【0157】 (3) Results Refer to Table 1 below and explain the results of the evaluation of each sample. 【0158】 【0159】 (Sample A) In Sample A, as shown in Figure 6, although the outer surface of the insulation exposed portion in the axial direction of the conductor had minute irregularities, it was flat, similar to the outer surface of the insulation exposed portion in the circumferential direction of the conductor. For this reason, Raa / Rac < 1.1 in Sample A. 【0160】 In sample A, although the partial discharge test was successful, dust was found to be mixed in between the exposed insulation portion of the power cable's cable insulation layer and the insulation unit. 【0161】 In Sample A, if a large amount of dust is mixed between the exposed insulation portion of the power cable's insulation layer and the insulation unit, it may accumulate locally along the interface between the exposed insulation portion and the insulation unit via the dust. Therefore, in a paper finish like Sample A, the insulation performance of the cable connection structure may decrease depending on the condition after the paper finish. 【0162】 (Samples E and F) In sample F, as shown in Figure 7, excessively large irregularities were formed on the outer surface of the insulation exposed portion in the axial direction of the conductor. Similarly, in sample E, excessively large irregularities were formed, similar to Figure 7. Therefore, in samples E and F, Raa / Rac > 6. In sample E, eight cable connection structures were evaluated as "defective" as a result of the partial discharge test. In sample F, all ten cable connection structures were evaluated as "defective" as a result of the partial discharge test. 【0163】 In samples E and F, large voids or oil reservoirs were formed in excessively large irregularities in the exposed insulation areas. As a result, it is believed that the insulation performance of the cable connection structure was reduced in samples E and F. 【0164】(Samples B to D) Samples B to D yielded the cross-sectional profiles of the insulation exposed portion in the axial direction of the conductor, as shown in Figures 5, 3A, and 4. On the other hand, sample C yielded the cross-sectional profile of the insulation exposed portion in the circumferential direction of the conductor, as shown in Figure 3B. The cross-sectional profiles of the insulation exposed portion in the circumferential direction of the conductors of samples B and D were similar to those in Figure 3B. 【0165】 Samples B through D satisfied equation (1): 1.1 ≤ Raa / Rac ≤ 6. In samples B through D, no dust was found between the exposed insulation portion of the power cable's insulation layer and the insulation unit. As a result, in the partial discharge test, all 10 cable connection structures in samples B through D were evaluated as "good". 【0166】 In samples B to D, setting Raa / Rac ≥ 1.1 suppressed the formation of locally accumulated voids and oil pools along the interface between the insulation exposure and the insulation unit. On the other hand, setting Raa / Rac ≤ 6 in samples B to D suppressed the formation of large voids and oil pools caused by excessively large irregularities in the insulation exposure. As a result, it was confirmed that stable insulation performance of the cable connection structure could be obtained in samples B to D. 【0167】 In samples B through D, the process of sanding the outer surface of the exposed insulation portion of the power cable's insulation layer with sandpaper was omitted, which suppressed the incorporation of dust between the exposed insulation portion and the insulation unit. As a result, cleaning for dust removal was also unnecessary in samples B through D. Consequently, it was confirmed that the manufacturing process of the cable connection structure was simplified and the time required for the manufacturing process was reduced in samples B through D. 【0168】10 Connecting power cable 20 Cable connection structure 100 Power cable 100a First power cable 100b Second power cable 110 Conductor 120 Cable internal semiconducting layer 130 Cable insulation layer 132 Insulation exposed part 133 Recess 134 Protrusion 134a First protrusion 134b Second protrusion 135 Slope part 136 Deepest part 137 First inclined surface 138 Second inclined surface 140 Cable external semiconducting layer 142 External semiconducting exposed part 150 Cable metal shielding layer 160 Cable sheath 200 Sleeve 300 Insulation unit 320 Insulation unit internal semiconducting layer 340 Insulation unit insulation layer 360 Insulation unit external semiconducting layer 380 Stress cone part 382 Inner surface 400 Protective part 500 Spacer IE Innermost edge OE Outermost edge

Claims

1. A cable connection structure comprising: a first power cable and a second power cable, each having a conductor, an internal semiconducting layer, a cable insulation layer, and an external semiconducting layer in this order from the central axis of the conductor toward the outer circumference; a cylindrical sleeve connecting the conductor of the first power cable and the conductor of the second power cable; and an insulating unit configured as a cylindrical member, provided to cover the outer circumference of the area including the sleeve, and maintaining the insulation around the sleeve, wherein each of the first power cable and the second power cable has an insulation exposed portion where the cable insulation layer is exposed, and the insulation exposed portion has an outer surface that satisfies formula (1): 1.1 ≤ Raa / Rac ≤ 6 ... (1) where Raa is the arithmetic mean roughness of the outer surface of the insulation exposed portion in the axial direction of the conductor, with units of μm, and Rac is the arithmetic mean roughness of the outer surface of the insulation exposed portion in the circumferential direction of the conductor, with units of μm.

2. The cable connection structure according to claim 1, wherein the insulating exposed portion has a plurality of recesses that are recessed toward the conductor, and a plurality of protrusions that project outward radially from each of the plurality of recesses toward the conductor, and the plurality of recesses and the plurality of protrusions are alternately arranged in the axial direction of the conductor and are arranged spirally along the axial direction of the conductor.

3. The plurality of protrusions have a first protrusion and a second protrusion adjacent to one of the plurality of recesses, and each of the plurality of recesses has a deepest part that is recessed the deepest toward the conductor, a first inclined surface provided between the first protrusion and the deepest part and inclined with respect to the axial direction of the conductor, and a second inclined surface provided between the deepest part and the second protrusion and inclined opposite to the first inclined surface with respect to the deepest part, wherein the first inclined surface is longer than the second inclined surface in the axial direction of the conductor, the cable connection structure according to claim 2.

4. The outer surface of the insulated exposed portion satisfies formula (2): 1.5 ≤ Raa / Rac ≤ 5.5 ... (2) The cable connection structure according to any one of claims 1 to 3.

5. The cable connection structure according to any one of claims 1 to 4, wherein Raa is 15 μm or less.

6. A connecting power cable comprising at least one cable connection structure according to any one of claims 1 to 5.

7. The process comprises the steps of: preparing a first power cable and a second power cable, each having a conductor, an internal semiconducting layer, a cable insulation layer, and an external semiconducting layer in this order from the central axis of the conductor toward the outer circumference; connecting the conductor of the first power cable and the conductor of the second power cable with a cylindrical sleeve; and arranging an insulating unit, configured as a cylindrical member and maintaining insulation around the sleeve, so as to cover the outer circumference of the region including the sleeve, wherein the step of preparing the first power cable and the second power cable includes the step of forming an insulation exposed portion in which the cable insulation layer is exposed, and in the step of forming the insulation exposed portion, the insulation exposed portion is formed such that it has an outer surface that satisfies equation (1): 1.1 ≤ Raa / Rac ≤ 6 ... (1) where Raa is the arithmetic mean roughness of the outer surface of the insulation exposed portion in the axial direction of the conductor, and its unit is μm. A method for manufacturing a cable connection structure, wherein Rac is the arithmetic mean roughness of the outer surface of the insulating exposed portion in the circumferential direction of the conductor, and its unit is μm.

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