Resin composition and power cable

Abstract

A resin composition included in an insulating layer of a power cable includes polyethylene, and has a nitrogen content of 2.0 ppm or more and 9.0 ppm or less.

Description

RESIN COMPOSITION AND POWER CABLE

[0001] The present disclosure relates to a resin composition and a power cable.

[0002] For example, polyethylene has been used as a base resin included in an insulating layer of a power cable (e.g., PTL. 1).

[0003] In the insulating layer, from the viewpoint of reducing voltage degradation, an insulating oil has been added to its base resin (e.g., PTL. 2). In addition, a wide variety of insulating oils having improved oxidation resistance are disclosed (e.g., PTL. 3 and PTL. 4).

[0004] PTL. 1: WO 2019 / 202870 PTL. 2: Japanese Patent Laid-Open Publication No. H11-224542 PTL. 3: EP 0091249 PTL. 4: WO 02 / 099820

[0005] According to an aspect of the present disclosure, there is provided a resin composition included in an insulating layer of a power cable, including polyethylene, wherein a content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less.

[0006] [Fig. 1] FIG. 1 is a schematic cross-sectional view orthogonal to an axial direction of a power cable according to an embodiment of the present disclosure. [Fig. 2] FIG. 2 is a flow chart illustrating a method of manufacturing a power cable according to an embodiment of the present disclosure. [Fig. 3] FIG. 3 is a schematic cross-sectional view illustrating Evaluation 1. [Fig. 4A] FIG. 4A is a schematic cross-sectional view illustrating Evaluation 2. [Fig. 4B] FIG. 4B is an enlarged schematic cross-sectional view of a tip of a needle electrode in Evaluation 2.Problem to be Solved by the Disclosure

[0007] An object of the present disclosure is to improve an insulation of the insulating layer. Advantageous Effect of the Disclosure

[0008] According to the present disclosure, the insulation of the insulating layer can be improved. Description of Embodiment of Disclosure

[0009] The insulation of the insulating layer of the power cable is evaluated on DC breakdown electric field strength, as is known, for example, in Japanese Patent Laid-Open Publication No. 2019-189842 or US 2021 / 032434A. In the conventional evaluations, flat electrodes (plate electrodes) have been used.

[0010] However, it is found that the insulation of the insulating layer is degraded in some cases at an electric field strength lower than the DC breakdown electric field strength of the insulating layer, in the power cable. In the power cable with degraded insulation, electrical treeing locally occurs in a part of the insulating layer. The occurrence of the local electrical treeing also occurred in some cases in the insulating layer, which was produced based on PTL. 2 to PTL. 4.

[0011] The present inventors have studied to reproduce the occurrence of the above-described electrical treeing, and have found that the electrical treeing can be observed by measuring the DC breakdown electric field strength using the needle electrode. The needle electrode can be used to simulate a condition where a local electric field is applied, and reproduce occurrence of the local electrical treeing.

[0012] The inventors have reproduced the occurrence of the electrical treeing for the various resin compositions different in composition by the above-described tests, and have also performed elemental analysis of the resin compositions. As a result, it is found that when the resin composition contains nitrogen atoms and their content is within a predetermined range, the local electrical treeing can be stably reduced. The nitrogen atom in the resin composition is derived from a nitrogen compound contained as an impurity in a lubricating oil, for example. By adjusting the content of nitrogen atoms by the type and addition amount of the lubricating oil, the electrical treeing can be stably reduced. Embodiments of Disclosure

[0013] Next, embodiments of the present disclosure will be listed and described.

[0014] [1] A resin composition according to an aspect of the present disclosure is: a resin composition included in an insulating layer of a power cable, including polyethylene, wherein a content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less. With this configuration, the occurrence of the local electrical treeing can be reduced.

[0015] [2] The resin composition according to [1], further including a lubricating oil containing a nitrogen compound, wherein the nitrogen atom in the resin composition is derived from the nitrogen compound. With this configuration, the occurrence of the local electrical treeing can be stably reduced.

[0016] [3] In the resin composition according to [2] a content of the lubricating oil is 0.6 parts by mass or more and 2.5 parts by mass or less with respect to 100 parts by mass of the polyethylene. With this configuration, the occurrence of the local electrical treeing can be stably reduced.

[0017] [4] In the resin composition according to any one of [1] to [3], DC breakdown electric field strength is 100 kV / mm or more as measured using a needle electrode having a tip with a radius of curvature of 5 μm. With this configuration, an insulation breakdown of the insulating layer caused by the occurrence of the local electrical treeing at the time of practical use can be stably reduced.

[0018] [5] A power cable according to an aspect of the present disclosure includes: a conductor, and an insulating layer provided to cover an outer circumference of the conductor; wherein the insulating layer includes the resin composition according to any one of [1] to [4]. With this configuration, the occurrence of the local electrical treeing can be reduced. Detailed description of the disclosure

[0019] Next, an embodiment of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these illustrations, but intended to be indicated by claims and encompass all the changes which fall within the meaning and scope equivalent to claims. One Embodiment of Disclosure

[0020] (1) Resin composition The resin composition of this embodiment is a material included in an insulating layer 130 of a power cable 10 described later. The resin composition of this embodiment includes, for example, a base resin, a lubricating oil, and other additives as desired. A content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less. Here, the content of nitrogen atoms represents an amount (μg) of the nitrogen atoms contained in 1 g of the resin composition. The content of nitrogen atoms is hereinafter also referred to simply as a nitrogen content.

[0021] (Base resin) The resin composition includes a base resin included in its main component. The base resin includes, for example, polyethylene. Examples of the polyethylene include low density polyethylene (LDPE), medium density polyethylene (MDPE), and high density polyethylene (HDPE). The polyethylene may be either linear or branched, for example. Further, the base resin may include, in addition to polyethylene, a modified polyolefin which is a polyolefin grafted with a polar group. The base resin is disclosed, for example, in WO 2019 / 202870 or US 2021 / 032434A.

[0022] (Lubricating oil) The lubricating oil contains a nitrogen compound and introduces the nitrogen compound into the resin composition. Examples of the lubricating oil include mineral oils derived from petroleum refining and synthetic oils. The mineral oil contains impurity such as a nitrogen compound, and thus contains a nitrogen atom derived from the nitrogen compound. On the other hand, the synthetic oil contains substantially no impurity such as a nitrogen compound due to its production method. Mineral oil derived from petroleum refining may be used from a viewpoint of adjusting the nitrogen content in the resin composition within a predetermined range. As the mineral oil, for example, paraffinic mineral oil or naphthenic mineral oil may be used. When the synthetic oil is used as the lubricating oil, the nitrogen compound may be added to adjust the content of the nitrogen compound.

[0023] The nitrogen compound contained in the lubricating oil is a heterocyclic aromatic amine compound. Examples include pyridine and pyrrole. The content of the nitrogen compound in the lubricating oil is not particularly limited, but from the viewpoint of adjusting the nitrogen content in the resin composition within the above range, the content of nitrogen atoms in the lubricating oil may be 50 ppm or more and 500 ppm or less.

[0024] The mechanism by which the nitrogen compound reduces the occurrence of the local electrical treeing is not clear, but it is speculated as follows. Since the nitrogen compound contains at least one nitrogen atom in its chemical structure, it has polarization and tends to exhibit electrification characteristic. Therefore, it is assumed that the nitrogen compound can reduce the locally applied electric field and the entrance of the charge into the resin composition. Moreover, when the lubricating oil is mixed in the resin composition, the nitrogen compound can be widely distributed in the resin composition in accordance with the dispersion of the lubricating oil. As a result, the occurrence of the electrical treeing in the resin composition can be stably reduced.

[0025] (Other Additives) The resin composition may further include at least one of a cross-linking agent, an inorganic filler, an antioxidant, and a lubricant as other additives. From a viewpoint of cross-linking the resin composition, the cross-linking agent may be further included. The cross-linking agent, the inorganic filler, the antioxidant, and the lubricant are disclosed, for example, in Japanese Patent Laid-Open Publication No. 2020-132819, US 2020 / 279672A, Japanese Patent Laid-Open Publication No. 2020-132818, US 2020 / 273598A, Japanese Patent Laid-Open Publication No. 2020-132817, US 2020 / 270426A, Japanese Patent Laid-Open Publication No. 2019-189842, US 2021 / 032434A, so the description thereof is omitted here.

[0026] As the other additives, those that are substantially free from the nitrogen compound and prevent the nitrogen content in the resin composition from increasing may be used.

[0027] (Nitrogen content) In the resin composition of this embodiment, the content of nitrogen atoms is 2.0 ppm or more and 9.0 ppm or less. The nitrogen atom is mainly derived from the nitrogen compound contained in the above-described lubricating oil. That is, the content of nitrogen atoms corresponds to the content of the nitrogen compound contained in the resin composition. The nitrogen content in the resin composition can be appropriately adjusted by the content of the nitrogen compound contained in the lubricating oil to be used and the addition amount of the lubricating oil. By adjusting the nitrogen content to 2.0 ppm or more, the occurrence of the local electrical treeing can be reduced in the insulating layer formed from the resin composition. On the other hand, when the content of nitrogen atoms is excessively high, the influence of the nitrogen atom on the conductivity tends to increase, and the insulation in the resin composition tends to be degraded. In addition, when the content of nitrogen atoms is higher, the addition amount of the lubricating oil is also higher, softening the resin composition, which tends to degrade heat-resistance. In this regard, by adjusting the nitrogen content to 9.0 ppm or less, the addition amount of the lubricating oil falls within an appropriate range, and heat-resistance of the resin composition can be maintained high while reducing occurrence of the local electrical treeing and maintaining insulation high. From a viewpoint of achieving a high level of compatibility between the inhibited occurrence of the local electrical treeing and the heat-resistance, the nitrogen content may be 3.0 ppm or more and 7.0 ppm or less, or may be 4.0 ppm or more and 6.0 ppm or less.

[0028] The nitrogen content in the resin composition and the nitrogen content in the lubricating oil may be measured using, for example, an automatic total nitrogen analyzer (TN).

[0029] The resin composition of this embodiment including the above-described materials is charged into an extruder as pellets when extruding the insulating layer 130 of the power cable 10.

[0030] (2) Characteristics of resin composition The resin composition of this embodiment exhibits the following characteristics when the nitrogen content in the resin composition falls within a predetermined range.

[0031] In a sheet of the resin composition, the nitrogen content in the resin composition falling within the predetermined range can reduce degradation of the insulation at the electric field strength lower than DC breakdown electric field strength. As a result, the occurrence of the local electrical treeing can be reduced. Specifically, the DC breakdown electric field strength can be 100 kV / mm or more as measured using a needle electrode having a tip with a radius of curvature of 5 μm, as shown in Examples.

[0032] Further, in the sheet of the resin composition, the DC breakdown electric field strength can be 250 kV / mm or more as measured using a plate electrode.

[0033] In addition, since the resin composition can maintain a high heat-resistance, it is possible to keep variations in tensile strength and elongation small when a heat-aging test is performed. Specifically, as shown in Examples, rates of variation in tensile strength and elongation before and after heating is ± 25% or less, when a heat-aging test is performed based on IEC 60811-1-2.

[0034] (3) Method of producing resin composition The above-described resin composition may be prepared, for example, by adding the lubricating oil together with the other additives described above to the base resin that has been heated and melted, followed by kneading and addition of the cross-linking agent. Alternatively, the resin composition may be prepared, for example, by adding the other additives described above to the base resin that has been heated and melted, followed by addition of the lubricating oil together with the cross-linking agent. As for the lubricating oil, the nitrogen content may be measured in advance, and the one that, when added in a predetermined amount, results in the nitrogen content satisfying the above-described range in the resin composition may be selected. Specifically, the lubricating oil in which the content of nitrogen atoms is 50 ppm or more and 500 ppm or less may be used. In addition, the addition amount of the lubricating oil may be changed appropriately depending on the content of nitrogen atoms. For example, it may be 0.6 parts by mass or more and 2.5 parts by mass or less with respect to 100 parts by mass of polyethylene.

[0035] In the method of producing the resin composition, after mixing the base resin, the additive, and the lubricating oil, the mixture may be kneaded with heating, as described above. Alternatively, the mixture of the base resin and the additive may be kneaded with heating, and then the kneaded mixture may be immersed in the lubricating oil. From the viewpoint of uniformly dispersing the lubricating oil in the resin composition, it is recommended the base resin, the additive, and the lubricating oil be kneaded with heating. Kneading with heating can uniformly disperse the lubricating oil in the resin composition. As a result, the nitrogen compound derived from the lubricating oil can be uniformly dispersed in the resin composition, making it possible to more reliably reduce the occurrence of the local electrical treeing in the resin composition.

[0036] (4) Power cable Next, with reference to FIG. 1, the power cable of this embodiment will be described.

[0037] The power cable 10 of this embodiment includes a conductor 110, an internal semiconductive layer 120, an insulating layer 130, an external semiconductive layer 140, a shielding layer 150, and a sheath 160. The conductor 110, the internal semiconductive layer 120, the insulating layer 130, the external semiconductive layer 140, the shielding layer 150, the sheath 160, and their dimensions are disclosed in WO 2019 / 202870 or US 2021 / 032434, so the description thereof is omitted here.

[0038] The insulating layer 130 is formed by extruding the resin composition of this embodiment described above. In the sheet cut from the insulating layer 130 after extrusion, the characteristics of the resin composition of this embodiment described above are also obtained.

[0039] The method of producing the power cable 10 includes a resin composition preparation step S100, a conductor preparation step S200, a cable core formation step S300, a shielding layer formation step S400, and a sheath formation step S500, as shown in FIG. 2, for example. The resin composition preparation step S100 has been explained for the method of producing the resin composition described above. The conductor preparation step S200, the cable core formation step S300, the shielding layer formation step S400, and the sheath formation step S500 are disclosed, for example, in Japanese Patent Laid-Open Publication No. 2020-132819 or US 2020 / 279672A, so detailed description thereof is omitted here.

[0040] (5) Summary of this embodiment According to this embodiment, one or more effects described below are achieved.

[0041] (a) The resin composition of this embodiment includes polyethylene, and has the nitrogen content of 2.0 ppm or more and 9.0 ppm or less. As a result, the occurrence of the local electrical treeing can be reduced in a sheet or an insulating layer 130 formed from the resin composition even when a high electric field is locally applied. Therefore, it is possible to reduce the degradation of the insulation at an electric field strength lower than the DC breakdown electric field strength.

[0042] Specifically, in order to reproduce the occurrence of the local electrical treeing, the DC breakdown electric field strength can be 100 kV / mm or more as measured using the needle electrode having a tip with a radius of curvature of 5 μm.

[0043] Further, the DC breakdown electric field strength can be 250 kV / mm or more as measured using a plate electrode. Therefore, an entirely high insulation can be secured in the sheet or the insulating layer 130 of the resin composition.

[0044] (b) In this embodiment, the resin composition further includes the lubricating oil containing nitrogen atoms, and the nitrogen atom in the resin composition may be derived from the nitrogen compound. By means of the lubricating oil, it is easy to introduce the nitrogen compound into the resin composition and also to adjust its content. Accordingly, the occurrence of the local electrical treeing can be stably reduced.

[0045] (c) In this embodiment, the content of the lubricating oil may be 0.6 parts by mass or more and 2.5 parts by mass or less with respect to 100 parts by mass of polyethylene. With such a content, the nitrogen content in the resin composition is easily adjusted within the predetermined range. Further, the degradation of the heat-resistance due to the addition of the lubricating oil can be reduced, and the heat-resistance of the resin composition can be maintained high.

[0046] (d) In this embodiment, the lubricating oil in which the content of nitrogen atoms is 50 ppm or more and 500 ppm or less may be used. With such a lubricating oil, the nitrogen content in the resin composition can be easily adjusted within the predetermined range, and the occurrence of the local electrical treeing can be more stably reduced.

[0047] <Comparison with prior art> Next, a comparison between the present disclosure and prior art will be described.

[0048] The above-mentioned PTL. 2 discloses that the material included in the insulating layer, for example, polyethylene, is cross-linked and then impregnated with the insulating oil, and that the polybutene oil is used as the insulating oil in Examples. The insulating layer formed by cross-linking may contain tiny spaces (free volume), and such tiny spaces cause electron avalanche when energized, resulting in voltage degradation of the insulating layer. Therefore, in PTL. 2, the insulating layer is impregnated with the insulating oil, filling the tiny spaces with the insulating oil. Accordingly, electron avalanche can be reduced and insulation can be improved.

[0049] However, PTL. 2 does not disclose the nitrogen compound as the impurity as well as impurities contained in the insulating oil, but only discloses that the kinetic viscosity of the insulating oil is set within a predetermined range from the viewpoint of filling the free volume. Moreover, PTL. 2 does not disclose that since the nitrogen compound has polarization due to at least one nitrogen atom contained in its chemical structure, and acts to reduce an electric field locally applied in the resin composition. Furthermore, there is no disclosure about the content of nitrogen atoms derived from the nitrogen compound in the resin composition. Note that the polybutene oil in Example is a synthetic oil, and does not contain the nitrogen compound as an impurity.

[0050] PTL. 2 does not disclose that insulation of the insulating layer is evaluated by the measurement using a needle electrode at all. In other words, a problem of the electrical treeing occurring upon local application of an electric field in the insulating layer is not recognized.

[0051] In addition, PTL. 3 mentioned above discloses an insulating oil (lubricating oil), and discloses that a content of a non-basic nitrogen compound is adjusted within a predetermined range from the viewpoint of improving oxidation resistance.

[0052] However, PTL. 3 only discloses the insulating oil enhances the oxidation resistance, and does not disclose the insulating layer reduces the electrical treeing occurring upon local application of the electric field. Moreover, PTL. 3 does not disclose the nitrogen compound acts to reduce the electric field locally applied in the resin composition, but only discloses the content of the non-basic nitrogen compound, which is to be an impurity, is adjusted from the viewpoint of enhancing oxidation resistance of the insulating oil. Furthermore, there is no disclosure about the content of nitrogen atoms derived from the nitrogen compound being adjusted to be 2.0 ppm or more and 9.0 ppm or less in the resin composition containing the insulating oil.

[0053] In addition, PTL. 4 mentioned above discloses the insulating oil, and discloses that, from the viewpoint of increasing the impact breakdown voltage, the insulating oil is configured so as to include a nitrogen-containing heterocyclic compound which contains the nitrogen atom as a heterocyclic constituent atom and in which all bonds between the nitrogen atom and other atoms are single bonds.

[0054] PTL. 4 mentioned above does not disclose reducing an electrical treeing occurring upon local application of an electric field in the insulating layer. Moreover, PTL. 4 does not disclose that the nitrogen compound acts to reduce an electric field locally applied in the resin composition and reduces occurrence of the local electric treeing. Furthermore, there is no disclosure about the content of nitrogen atoms derived from the nitrogen compound being adjusted to be 2.0 ppm or more and 9.0 ppm or less in the resin composition containing the insulating oil.

[0055] As described above, PTL. 2 to PTL. 4 do not describe a problem of reducing an electrical treeing occurring upon local application of an electric field in the insulating layer. In addition, PTL 2 does not recognize the nitrogen compound in the lubricating oil and does not disclose the adjustment of the content of nitrogen atoms in the resin composition within a predetermined range. Furthermore, while PTL. 3 and PTL. 4 disclose the nitrogen compound in the lubricating oil, they do not disclose that the nitrogen compound is added to the resin component so that the content of nitrogen atoms derived from the nitrogen compound in the resin composition falls within a predetermined range.

[0056] In contrast, the lubricating oil that contains a predetermined amount of the nitrogen compound is firstly selected from a number of lubricating oils in the present disclosure. As described later in Examples, an amount of impurity that may be included in the lubricating oil greatly varies depending on the purification method, and a lubricating oil that enables the content of nitrogen atoms in the resin composition to fall within a predetermined range is selected. Subsequently, an addition amount to the resin composition is determined depending on the content of the nitrogen compound in the selected lubricating oil, which allows the content of nitrogen atoms derived from the nitrogen compound in the resin composition to be adjusted to 2.0 ppm or more and 9.0 ppm or less for the first time. As a result, the lubricating oil can be contained in the resin composition to uniformly disperse the nitrogen compound, so that a locally applied electric field can be reduced and the occurrence of the local electrical treeing can be reduced.

[0057] As described above, the present disclosure provides an effect different from those of the patent literatures. Since the patent literatures do not recognize the object of the present disclosure, they could not have conceived that the content of nitrogen atoms in the resin composition is adjusted, and a person skilled in the art could not have conceived the effect of the present disclosure based on the state of the art at the time of application.

[0058] Next, examples according to the present disclosure will be described. These examples are illustrative of the present disclosure, and the present disclosure is not limited by these examples.

[0059] (1) Preparation of resin composition For samples 1 to 7, first, low density polyethylene (LDPE) as the polyethylene, naphthenic mineral oil derived from petroleum refining as the mineral oil A that is the lubricating oil, and dicumyl peroxide (hereinafter referred to as DCP) as the cross-linking agent were individually prepared and mixed together to prepare the resin composition. Specifically, with respect to 100 parts by mass of LDPE, the addition amount of the mineral oil A was adjusted as shown in Table 1 below, and the addition amount of DCP was set to 1.3 parts by mass, and the samples 1 to 7 of the resin compositions were prepared. For sample 8, the resin composition was prepared in the same manner as for samples 1 to 7, except that the mineral oil A was not added. For sample 9, the resin composition was prepared in the same manner as for sample 3, except that a polybutene oil which was a synthetic oil was added instead of the mineral oil A. A component analysis confirmed that the mineral oil A contained 350 ppm of the nitrogen molecules derived from the nitrogen compound. The nitrogen compound was a heterocyclic aromatic amine compound such as pyridine or pyrrole. It was confirmed that the polybutene oil contained neither nitrogen compound nor nitrogen atom derived from the nitrogen compound.

[0060]

[0061] In Table 1, the content of nitrogen atoms (nitrogen content) contained in the resin composition was measured using an automatic total nitrogen analyzer (TN device). Specifically, a predetermined amount of the resin composition was accommodated in a ceramic boat and weighed on a microbalance. After weighing, the ceramic boat was introduced into the TN device and the sampling frequency was set to twice, and the content of nitrogen atoms derived from the nitrogen compound in the resin composition was measured. As the ceramic boat, one that had been baked in advance was used.

[0062] For samples 3-1 to 3-6, the resin compositions were prepared in the same manner as for sample 3, except that mineral oils B to G that had different contents of nitrogen atoms were used as the naphthene-based mineral oils as shown in the following Table 2 instead of the mineral oil A. The content of nitrogen atoms derived from the nitrogen compound in each of the mineral oils were as follows: mineral oil B, 500 ppm; mineral oil C, 300 ppm; mineral oil D, 200 ppm; mineral oil E, 150 ppm; mineral oil F, 100 ppm; mineral oil G, 50 ppm.

[0063]

[0064] (2) Evaluation In this example, samples 1 to 7 and samples 3-1 to 3-6 of the resin compositions were evaluated by the following method.

[0065] (Evaluation 1: DC breakdown electric field strength as measured using plate electrode) For Evaluation 1, measurement of the DC breakdown electric field strength using the plate electrode was performed as shown in FIG. 3. Specifically, each of the resin compositions was extruded in a sheet with a thickness of 0.15 mm, and then the sheet was maintained at 180°C to thereby cross-link the resin composition, and thus the sheets S including respective samples were prepared. The flat electrode FE1 was placed on the first surface S1 of the sheet S. The flat electrode FE2 was placed on the second surface S2 of the sheet. The second surface S2 is opposite to the first surface S1. The flat electrodes FE1 and FE2 are circular with a diameter of 25 mm. The sheet S was immersed in silicone oil O. The temperature of the silicone oil O was 90°C. A voltage was applied to the sheet S using the flat electrodes FE1 and FE2. The voltage was increased at a rate of 4 kV / min. The DC breakdown electric field strength of the sheet S was calculated based on the applied voltage and the thickness of the sheet S at the time of the insulation breakdown of the sheet S. In this example, those in which the DC breakdown electric field strength as measured using the plate electrode is 250 kV / mm or more is determined to have high insulation.

[0066] (Evaluation 2: DC breakdown electric field strength as measured using needle electrode) For Evaluation 2 of the electrical characteristics, the measurement of the DC breakdown electric field strength using the needle electrode was performed as shown in FIG. 4A and FIG. 4B. Specifically, the sheets S including respective samples were prepared in the same manner as in Evaluation 1 of electrical characteristics, except that each of the resin compositions was extruded into a sheet with a thickness of 4 mm. An electrode FE3 was formed by applying a conductive paint including silver paste on the first surface S1 of the sheet S. The electrode FE3 was formed in a circular shape with a diameter of 25 mm. The tip of the needle electrode A (NE) was pierced from the second surface S2 of the sheet S along the thickness direction of the sheet S. The tip of the needle electrode A (NE) has the radius of curvature r of 5 μm. The tip of the needle electrode A (NE) is 1 mm away from the electrode FE3. The sheet S pierced with the needle electrode A (NE) was immersed in the silicone oil O. The temperature of the silicone oil O was 90°C. A voltage was applied between the needle electrode A (NE) and the electrode FE3. The voltage was increased at a rate of 4 kV / min. The DC breakdown electric field strength of the sheet S was calculated based on the applied voltage and the distance between the needle electrode A (NE) and the electrode FE3 at the time of the insulation breakdown of the sheet S. In this example, those having the DC breakdown electric field strength of 100 kV / mm or more as measured using the needle electrode are determined to reduce the occurrence of the local electrical treeing.

[0067] (Evaluation 3: Heat-aging test) The heat-aging test was performed according to IEC 60811-1-2. Specifically, each of the resin compositions was cross-linked simultaneously with press-forming to produce a sheet with a thickness of 0.5 mm. The resulting sheet was heated at 135°C for 168 hours. Then, a tensile test was performed using a dumbbell on the sheet before and after heating to measure tensile strength and elongation. In this example, the sheet in which rates of variation in tensile strength and elongation before and after heating were within ± 25% was evaluated as “A”, having high heat-resistance, whereas the sheet in which rates of variation in tensile strength and elongation before and after heating were out of the range described above was evaluated as “B”, having low heat-resistance.

[0068] (3) Evaluation results The results of each evaluation are summarized in Table 1 and Table 2 above.

[0069] As shown in Table 1, any of samples 1 to 7 had the DC breakdown electric field strength of 250 kV / mm or more as measured using the plate electrode, and the entire insulation of the sheet was observed to be high. However, the DC breakdown electric field strength of any of samples 1 to 6 having the nitrogen content of 2.0 ppm or more, as measured using the needle electrode, was 100 kV / mm or more, whereas that of sample 7 having the nitrogen content of 1.4 ppm was observed to be 90 kV / mm, which is less than 100 kV / mm. Therefore, it is found that by adjusting the nitrogen content to 2.0 ppm or more, the occurrence of the local electrical treeing can be reduced.

[0070] On the other hand, the heat-aging test confirmed that samples 2 to 7 having the nitrogen content of 9.0 ppm or less are less likely to be heat-aged, whereas sample 1 having the nitrogen content of 9.8 ppm is likely to be heat-aged. In sample 1, the addition amount of mineral oil was higher than that in samples 2 to 6, and it is assumed that the mineral oil softens the resin composition, thereby lowering heat-resistance thereof. Therefore, it is found that by adjusting the nitrogen content to 9.0 ppm or less, the resin composition can be prevented from being heat-aged, and thus its heat-resistance can be maintained high.

[0071] Sample 8 containing no lubricating oil added thereto had the DC breakdown electric field strength of 250 kV / mm or more as measured using the plate electrode, and the entire insulation of the sheet was observed to be high. However, the DC breakdown electric field strength was less than 100 kV / mm as measured using the needle electrode, and it was confirmed that the occurrence of the local electrical treeing was not reduced.

[0072] In sample 9 that contained a polybutene oil added thereto as the lubricating oil, but contained no nitrogen compound as an impurity since the polybutene oil was a synthetic oil, and had the content of nitrogen atoms in the resin composition of 0, the DC breakdown electric field strength as measured using the needle electrode was observed to be less than 100 kV / mm. That is, it was confirmed that the occurrence of the local electrical treeing was not reduced.

[0073] As shown in Table 2, in samples 3-1 to 3-6, the content of the impurities in the lubricating oil varies depending on the type of the mineral oil, and the content of nitrogen atoms in the resin composition varies depending on the addition amount of the mineral oil. It was observed that in all of the samples, high heat-resistance was maintained because the addition amount of the lubricating oil was appropriate.

[0074] In sample 3 and samples 3-2 to 3-5, the nitrogen content was 2.0 ppm of more and 9.0 ppm or less, and it was observed that not only the DC breakdown electric field strength as measured using the plate electrode but also the DC breakdown electric field strength as measured using the needle electrode could be higher, thereby reducing the occurrence of the local electrical treeing. Particularly, in sample 3 and samples 3-2 to 3-4, it was confirmed that the nitrogen content set to 3.0 ppm or more and 7.0 ppm or less resulted in the DC breakdown electric field strength of 120 kV / mm or more as measured using the needle electrode, and the nitrogen content set to 4.0 ppm or more and 6.0 ppm or less resulted in the DC breakdown electric field strength of 130 kV / mm or more as measured using the needle electrode, thereby more reliably reducing the occurrence of the local electrical treeing.

[0075] In contrast, in sample 3-6, it was confirmed that the nitrogen content was 1.0 ppm, which is less than 2.0 ppm, and the DC breakdown electric field strength as measured using the needle electrode was 89 kV / mm, which was significantly lower. This is probably because there is an excessively small amount of the nitrogen compound which acts to reduce the locally applied electric field.

[0076] In sample 3-1, it was confirmed that the nitrogen content was 10.0 ppm, which is more than 9.0 ppm, while the DC breakdown electric field strength as measured using the needle electrode was lower, as low as 98 kV / mm. It is probably because excessively high nitrogen content reduces insulation, resulting in reduced DC breakdown electric field strength.

[0077] Based on the above description, by adjusting the nitrogen content in the resin composition within a predetermined range, a desired heat-resistance can be maintained while the occurrence of the local electrical treeing is reduced.

[0078] 10...Power cable 110...Conductor 120...Internal semiconductive layer 130...Insulating layer 140...External semiconductive layer 150...Shielding layer 160...Sheath NE...Needle electrode FE1, FE2...Flat electrode FE3...Electrode O...Silicone oil S...Sheet S1...First surface S2...Second surface

Claims

1. A resin composition included in an insulating layer of a power cable, comprising polyethylene, wherein a content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less.

2. The resin composition according to claim 1, further comprising a lubricating oil containing a nitrogen compound, wherein the nitrogen atom in the resin composition is derived from the nitrogen compound.

3. The resin composition according to claim 2, wherein a content of the lubricating oil is 0.6 parts by mass or more and 2.5 parts by mass or less with respect to 100 parts by mass of the polyethylene.

4. In the resin composition according to any one of claims 1 to 3, wherein DC breakdown electric field strength is 100 kV / mm or more as measured using a needle electrode having a tip with a radius of curvature of 5 μm.

5. A power cable comprising: a conductor, and an insulating layer provided to cover an outer circumference of the conductor, wherein the insulating layer includes the resin composition according to any one of claims 1 to 4.

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