Resin composition and power cable

Abstract

To improve insulation properties of an insulating layer.SOLUTION: The resin component constitutes an insulating layer of a power cable, and contains polyethylene, and the nitrogen content in the resin component is 2. 0ppm or more and 9. 0ppm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] For example, polyethylene has been used as a base resin constituting the insulating layer of a power cable (for example, Patent Document 1).

[0003] In order to suppress voltage degradation in the insulating layer, insulating oil has been added to the base resin (for example, Patent Document 2). Also, various insulating oils with improved oxidation resistance have been disclosed (for example, Patent Documents 3 and 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 202870 [Patent Document 2] Japanese Patent Application Publication No. 11-224542 [Patent Document 3] European Patent Application Publication No. 0091249 [Patent Document 4] International Publication No. 02 / 099820 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present disclosure is to improve the insulating properties of an insulating layer. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, A resin composition constituting an insulating layer of a power cable, Contains polyethylene, The content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less. A resin composition is provided. [Effects of the Invention]

[0007] According to the present disclosure, the insulating properties of the insulating layer can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view perpendicular to the axial direction of a power cable according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a flowchart illustrating a method for manufacturing a power cable according to one embodiment of the present disclosure. [Figure 3] FIG. 3 is a schematic cross-sectional view showing Evaluation 1. [Figure 4A] FIG. 4A is a schematic cross-sectional view showing Evaluation 2. [Figure 4B] FIG. 4B is a schematic cross-sectional view showing an enlarged view of the tip of the needle electrode in Evaluation 2. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Description of the embodiments of the present disclosure] The insulation properties of the insulation layer of a power cable are evaluated by DC breakdown field strength, as known in, for example, JP 2019-189842 A or US 2021 / 032434 A. Previous evaluations used flat electrodes (plate electrodes).

[0010] However, it has been found that the insulation properties of power cables may be reduced at electric field strengths lower than the DC breakdown field strength of the insulation layer. In power cables with reduced insulation properties, electrical trees may be locally generated in parts of the insulation layer. This local generation of electrical trees may also occur in insulation layers manufactured based on Patent Documents 2 to 4.

[0011] The inventors have conducted research to reproduce the occurrence of the above-mentioned electrical tree and have found that electrical trees can be observed by measuring the DC breakdown field strength using a needle electrode. The needle electrode creates a pseudo-condition in which a local electric field is applied, making it possible to reproduce the occurrence of local electrical trees.

[0012] The inventors reproduced the occurrence of electrical treeing using the above-mentioned test for resin compositions with various different compositions and also performed elemental analysis of the resin compositions. As a result, they found that the resin composition may contain nitrogen atoms, and that localized electrical treeing can be stably suppressed when the content is within a predetermined range. The nitrogen atoms in the resin composition are derived from nitrogen compounds contained as impurities in lubricating oil, for example. By adjusting the nitrogen atom content by the type and amount of lubricating oil, electrical treeing can be stably suppressed.

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

[0014] [1] A resin composition according to one embodiment of the present disclosure comprises: A resin composition constituting an insulating layer of a power cable, Contains polyethylene, The content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less. Resin composition. This configuration makes it possible to suppress local occurrence of electrical trees.

[0015] [2] In the resin composition according to the above [1], Further containing a lubricating oil containing a nitrogen compound, The nitrogen atoms in the resin composition are derived from the nitrogen compound. According to this configuration, it is possible to more stably suppress the occurrence of local electrical trees.

[0016] [3] In the resin composition according to the above [2], The 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. According to this configuration, it is possible to more stably suppress the occurrence of local electrical trees.

[0017] [4] In the resin composition according to any one of [1] to [3] above, The DC breakdown field strength measured using a needle electrode with a tip having a curvature radius of 5 μm is 100 kV / mm or more. This configuration makes it possible to stably suppress dielectric breakdown of the insulating layer due to localized occurrence of electrical trees during actual use.

[0018] [5] A power cable according to one embodiment of the present disclosure includes: A conductor; an insulating layer provided to cover the outer periphery of the conductor; Equipped with The insulating layer contains the resin composition according to any one of the above [1] to [4]. This configuration makes it possible to suppress local occurrence of electrical trees.

[0019] [Details of the embodiments of the present disclosure] Next, one embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0020] [One embodiment of the present disclosure] (1) Resin composition The resin composition of this embodiment is a material that constitutes the insulating layer 130 of the power cable 10, which will be described later. The resin composition of this embodiment is composed of, for example, a base resin, a lubricating oil, and, as necessary, other additives. The content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less. Herein, the nitrogen atom content refers to the amount of nitrogen atoms (μg) contained per 1 g of the resin composition. Hereinafter, the nitrogen atom content will also be simply referred to as the nitrogen content.

[0021] (base resin) The resin composition includes a base resin that constitutes the main component. The base resin includes, for example, polyethylene. Examples of polyethylene include low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE). These polyethylenes may be, for example, linear or branched. In addition to polyethylene, the base resin may also include modified polyolefins in which polar groups are grafted onto polyolefins. The base resin is disclosed, for example, in WO2019 / 202870 or US2021 / 032434A.

[0022] (lubricating oil) Lubricating oils contain nitrogen compounds and introduce the nitrogen compounds into the resin composition. Examples of lubricating oils include mineral oils and synthetic oils derived from petroleum refining. Mineral oils contain impurities such as nitrogen compounds and contain nitrogen atoms derived from the nitrogen compounds. On the other hand, synthetic oils are substantially free of impurities such as nitrogen compounds due to their manufacturing process. From the viewpoint of adjusting the nitrogen content in the resin composition to a predetermined range, it is preferable to use mineral oils derived from petroleum refining. As the mineral oil, it is preferable to use, for example, paraffinic mineral oils or naphthenic mineral oils. When using synthetic oils as lubricating oils, it is preferable to add nitrogen compounds to adjust the content of the nitrogen compounds.

[0023] The nitrogen compounds contained in the lubricating oil are heterocyclic aromatic amine compounds, such as pyridine and pyrrole. The content of the nitrogen compounds in the lubricating oil is not particularly limited, but from the viewpoint of adjusting the nitrogen content in the resin composition to the above range, it is preferable that the content of nitrogen atoms in the lubricating oil is 50 ppm or more and 500 ppm or less.

[0024] The mechanism by which nitrogen compounds suppress localized electrical tree generation is unclear, but is speculated as follows. Nitrogen compounds have polarization due to the inclusion of nitrogen atoms in their chemical structure, and tend to exhibit electrostatic charge. Therefore, it is speculated that nitrogen compounds can alleviate a locally applied electric field and suppress the penetration of electric charges into the resin composition. Furthermore, by mixing a lubricating oil into the resin composition, the nitrogen compounds can be widely distributed throughout the resin composition as the lubricating oil disperses. This makes it possible to stably suppress electrical tree generation in the resin composition.

[0025] (Other additives) The resin composition may further contain at least one of other additives, such as a crosslinking agent, an inorganic filler, a crosslinking agent, an antioxidant, and a lubricant. From the viewpoint of crosslinking the resin composition, it is preferable to further contain a crosslinking agent. Crosslinking agents, inorganic fillers, crosslinking agents, antioxidants, and lubricants are disclosed in, for example, JP 2020-132819, US 2020 / 279672A, JP 2020-132818, US 2020 / 273598A, JP 2020-132817, US 2020 / 270426A, JP 2019-189842, and US 2021 / 032434A, and therefore, description thereof will be omitted.

[0026] As the other additives, it is preferable to use those which do not substantially contain nitrogen compounds and do not increase the nitrogen content in the resin composition.

[0027] (nitrogen content) The resin composition of this embodiment has a nitrogen atom content of 2.0 ppm or more and 9.0 ppm or less. The nitrogen atoms are mainly derived from the nitrogen compounds contained in the lubricating oil. In other words, the nitrogen atom content corresponds to the nitrogen compound content in the resin composition. The nitrogen content in the resin composition can be appropriately adjusted by the nitrogen compound content in the lubricating oil used and the amount of lubricating oil added. By setting the nitrogen content to 2.0 ppm or more, localized electrical tree generation can be suppressed in the insulating layer formed from the resin composition. On the other hand, if the nitrogen atom content is excessively high, the influence of the conductivity due to the nitrogen compounds increases, and the insulating properties of the resin composition tend to decrease. Furthermore, if the nitrogen atom content is high, the amount of lubricating oil added also increases, which softens the resin composition and tends to reduce heat resistance. In this regard, by setting the nitrogen content to 9.0 ppm or less, localized electrical tree generation can be suppressed to maintain high insulating properties, while the amount of lubricating oil added can be kept within an appropriate range, thereby maintaining high heat resistance of the resin composition. From the viewpoint of achieving high levels of both suppression of localized electrical tree generation and heat resistance, the nitrogen content is preferably 3.0 ppm or more and 7.0 ppm or less, and 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, a total nitrogen automatic analyzer (TN).

[0029] The resin composition of this embodiment containing the above materials is fed as pellets into an extruder when the insulating layer 130 of the power cable 10 is extrusion-molded.

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

[0031] By ensuring that the nitrogen content in the resin composition is within a predetermined range, the resin composition sheet can suppress a decrease in insulation properties at electric field strengths lower than the DC breakdown field strength. As a result, localized electrical tree generation can be suppressed. Specifically, as shown in the examples, the DC breakdown field strength measured using a needle electrode with a tip having a curvature radius of 5 μm can be made 100 kV / mm or more.

[0032] Furthermore, the resin composition sheet can have a DC breakdown field strength of 250 kV / mm or more as measured using a flat electrode.

[0033] Furthermore, since the resin composition can maintain high heat resistance, fluctuations in tensile strength and elongation can be suppressed to a minimum when subjected to a heat aging test. Specifically, as shown in the examples, when a heat aging test is performed based on IEC 60811-1-2, the fluctuation rates of tensile strength and elongation before and after heating are ±25% or less.

[0034] (3) Method for producing resin composition The resin composition described above may be prepared, for example, by adding a lubricating oil together with the other additives described above to a heat-melted base resin, kneading the mixture, and then adding a crosslinking agent. Alternatively, the resin composition may be prepared by adding the other additives described above to a heat-melted base resin, kneading the mixture, and then adding a lubricating oil together with a crosslinking agent. The nitrogen content of the lubricating oil may be measured in advance, and a lubricating oil may be selected such that, when a predetermined amount is added, the nitrogen content of the resin composition falls within the above range. Specifically, a lubricating oil having a nitrogen atom content of 50 ppm to 500 ppm may be used. The amount of lubricating oil added may be adjusted appropriately depending on the nitrogen atom content; for example, it may be 0.6 parts by mass to 2.5 parts by mass per 100 parts by mass of polyethylene.

[0035] In the method for producing a resin composition, as described above, the base resin, additives, and lubricating oil may be mixed, and then the mixture may be kneaded while heated. Alternatively, the mixture of the base resin and additives may be kneaded while heated, 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 preferable to knead the base resin, additives, and lubricating oil while heating. Heating and kneading can uniformly disperse the lubricating oil in the resin composition. As a result, nitrogen compounds derived from the lubricating oil can be uniformly dispersed in the resin composition, and localized electrical tree generation in the resin composition can be more reliably suppressed.

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

[0037] The power cable 10 of this embodiment has a conductor 110, an inner semiconductive layer 120, an insulating layer 130, an outer semiconductive layer 140, a shielding layer 150, and a sheath 160. The conductor 110, the inner semiconductive layer 120, the insulating layer 130, the outer semiconductive layer 140, the shielding layer 150, the sheath 160, and their respective dimensions are disclosed in WO2019 / 202870 or US2021 / 032434, and therefore will not be described here.

[0038] The insulating layer 130 is formed by extrusion molding the resin composition of the present embodiment described above. Note that the properties of the resin composition of the present embodiment described above can also be obtained in a sheet cut out from the insulating layer 130 after extrusion molding.

[0039] The method for manufacturing a 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. The resin composition preparation step S100 is described in the above-mentioned method for manufacturing a resin composition. 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 in, for example, JP 2020-132819 A or US 2020 / 279672 A, and therefore detailed description thereof will be omitted.

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

[0041] (a) The resin composition of this embodiment contains polyethylene and has a nitrogen content of 2.0 ppm or more and 9.0 ppm or less. This makes it possible to suppress local electrical tree generation in a sheet or insulating layer 130 formed from the resin composition, even when a high electric field is applied locally. Therefore, it is possible to suppress a decrease in insulation properties at an electric field strength lower than the DC breakdown field strength.

[0042] Specifically, when measuring the DC breakdown field strength using a needle electrode with a tip having a curvature radius of 5 μm to reproduce the occurrence of localized electrical trees, the DC breakdown field strength can be made 100 kV / mm or more.

[0043] Furthermore, when the DC breakdown field strength is measured using a plate electrode, the DC breakdown field strength can be set to 250 kV / mm or more, which ensures high insulation properties overall in the resin composition sheet and insulating layer 130.

[0044] (b) In this embodiment, the resin composition may further contain a lubricating oil containing a nitrogen compound, and the nitrogen atoms in the resin composition may be derived from the nitrogen compound. The lubricating oil makes it easy to introduce the nitrogen compound into the resin composition and adjust its content. Therefore, localized electrical tree generation can be more stably suppressed.

[0045] (c) In this embodiment, the content of the lubricating oil is preferably 0.6 parts by mass or more and 2.5 parts by mass or less per 100 parts by mass of polyethylene. By setting the content in this range, it is easy to adjust the nitrogen content in the resin composition to a predetermined range. In addition, it is possible to suppress a decrease in heat resistance due to the addition of the lubricating oil, and to maintain high heat resistance of the resin composition.

[0046] (d) In this embodiment, it is preferable to use a lubricating oil having a nitrogen atom content of 50 ppm or more and 500 ppm or less. By using such a lubricating oil, it is easy to adjust the nitrogen content in the resin composition to a predetermined range, and it is possible to more stably suppress the local generation of electrical trees.

[0047] <Comparison with prior art> A comparison of the present disclosure with the prior art will now be described.

[0048] The aforementioned Patent Document 2 discloses that the material constituting the insulating layer, for example, polyethylene, is cross-linked and then impregnated with insulating oil, and that polybutene oil is used as the insulating oil in the examples. The insulating layer formed by cross-linking may have minute spaces (free volume), which can cause electron avalanches when current is applied, resulting in voltage degradation of the insulating layer. Therefore, Patent Document 2 impregnates the insulating layer with insulating oil, filling the minute spaces with the insulating oil. This suppresses electron avalanches and improves insulation.

[0049] However, Patent Document 2 only discloses that the kinematic viscosity of insulating oil is set within a predetermined range from the viewpoint of filling the insulating oil into a free volume, but does not disclose impurities contained in the insulating oil, nor nitrogen compounds as impurities. Furthermore, it does not disclose that nitrogen compounds have polarization due to the inclusion of nitrogen atoms in their chemical structure, and act to alleviate an electric field applied locally in a resin composition. Furthermore, it does not disclose the content of nitrogen atoms derived from nitrogen compounds in the resin composition. The polybutene oil in the examples is a synthetic oil and does not contain nitrogen compounds as impurities.

[0050] First of all, Patent Document 2 does not disclose the evaluation of the insulating properties of an insulating layer by measurement using a needle electrode, and does not recognize the problem of electrical treeing occurring due to the application of a local electric field in an insulating layer.

[0051] Furthermore, the above-mentioned Patent Document 3 discloses an insulating oil (lubricating oil), and also discloses that the content of non-basic nitrogen compounds is set within a predetermined range in order to improve oxidation resistance.

[0052] However, Patent Document 3 only discloses improving the oxidation resistance of insulating oil, but does not disclose suppressing electrical treeing that occurs in insulating layers due to the application of a localized electric field. Furthermore, Patent Document 3 only discloses adjusting the content of non-basic nitrogen compounds, which are impurities, from the perspective of improving the oxidation resistance of insulating oil, but does not disclose that nitrogen compounds act to alleviate the locally applied electric field in resin compositions. Furthermore, Patent Document 3 does not disclose adjusting the content of nitrogen atoms derived from nitrogen compounds to 2.0 ppm or more and 9.0 ppm or less in resin compositions containing insulating oil.

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

[0054] However, Patent Document 4 does not disclose the suppression of electrical treeing caused by the application of a local electric field in an insulating layer. It also does not disclose that a nitrogen compound acts to alleviate a locally applied electric field in a resin composition and suppress the occurrence of local electrical treeing. It also does not disclose that the content of nitrogen atoms derived from a nitrogen compound in a resin composition containing insulating oil is adjusted to be 2.0 ppm or more and 9.0 ppm or less.

[0055] Thus, Patent Documents 2 to 4 do not describe the problem of suppressing electrical treeing that occurs in an insulating layer due to the application of a local electric field. Patent Document 2 also does not recognize nitrogen compounds in lubricating oils, and does not disclose adjusting the content of nitrogen atoms in a resin composition to a predetermined range. Furthermore, Patent Documents 3 and 4 disclose nitrogen compounds in lubricating oils, but do not disclose adding the nitrogen compounds to the resin components so that the content of nitrogen atoms derived from the nitrogen compounds in the resin composition falls within a predetermined range.

[0056] In contrast, in the present disclosure, a lubricating oil containing a predetermined amount of nitrogen compounds is first selected from among many lubricating oils. As will be described in the examples, the impurity content of a lubricating oil varies greatly depending on its refining method, but a lubricating oil is selected so that the nitrogen atom content in the resin composition falls within a predetermined range. Next, the amount of lubricating oil added to the resin composition is determined based on the nitrogen compound content of the selected lubricating oil. This allows the content of nitrogen atoms derived from the nitrogen compounds in the resin composition to be adjusted to 2.0 ppm or more and 9.0 ppm or less. As a result, by adding a lubricating oil to the resin composition, the nitrogen compounds can be uniformly dispersed, thereby mitigating the locally applied electric field and suppressing the local occurrence of electrical treeing.

[0057] As described above, the present disclosure can achieve effects different from those of the patent documents. Since the patent documents do not recognize the problems of the present disclosure, it is impossible to conceive of adjusting the content of nitrogen atoms in a resin composition, and the effects of the present disclosure would not be predictable by a person skilled in the art given the state of the art at the time of filing. [Example]

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

[0059] (1) Preparation of resin composition For Samples 1 to 7, low-density polyethylene (LDPE) was used as the polyethylene, naphthenic mineral oil derived from petroleum refining was used as the lubricating oil, and dicumyl peroxide (DCP) was used as the crosslinking agent. These components were then mixed to prepare a resin composition. Specifically, the amount of mineral oil A added was adjusted appropriately as shown in Table 1 below, and 1.3 parts by mass of DCP was added per 100 parts by mass of LDPE to prepare the resin compositions of Samples 1 to 7. For Sample 8, the resin composition was prepared in the same manner as Samples 1 to 7, except that mineral oil A was not added. For Sample 9, the resin composition was prepared in the same manner as Sample 3, except that synthetic polybutene oil was added instead of mineral oil A. Component analysis confirmed that mineral oil A contained 350 ppm of nitrogen molecules derived from nitrogen compounds. The nitrogen compounds were heterocyclic aromatic amine compounds such as pyridine and pyrrole. Furthermore, polybutene oil did not contain any nitrogen compounds, confirming the absence of nitrogen molecules derived from nitrogen compounds.

[0060] [Table 1]

[0061] In Table 1, the content of nitrogen atoms contained in the resin composition (nitrogen content) was measured using a total nitrogen automatic analyzer (TN apparatus). Specifically, a predetermined amount of the resin composition was placed in a ceramic boat and weighed on a microbalance. After weighing, the ceramic boat was introduced into the TN apparatus, and the content of nitrogen atoms derived from nitrogen compounds contained in the resin composition was measured, with two samples taken. The ceramic boat used had been pre-baked.

[0062] For Samples 3-1 to 3-6, resin compositions were prepared in the same manner as Sample 3, except that mineral oils B to G, which had different nitrogen atom contents, were used as naphthenic mineral oils instead of mineral oil A, as shown in Table 2 below. The contents of nitrogen molecules derived from nitrogen compounds in each mineral oil were 500 ppm for mineral oil B, 300 ppm for mineral oil C, 200 ppm for mineral oil D, 150 ppm for mineral oil E, 100 ppm for mineral oil F, and 50 ppm for mineral oil G.

[0063] [Table 2]

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

[0065] (Evaluation 1: DC breakdown field strength with a flat electrode) For Evaluation 1, DC breakdown field strength was measured using flat electrodes as shown in Figure 3. Specifically, each resin composition was extruded into a sheet with a thickness of 0.15 mm, and then the sheet was heated to 180°C to crosslink the resin composition, producing a sheet S consisting of each sample. A flat electrode FE1 was placed on the first surface S1 of the sheet S. A flat electrode FE2 was placed on the second surface S2 of the sheet. The second surface S2 was located opposite the first surface S1. The flat electrodes FE1 and FE2 were circular and 25 mm in diameter. The sheet S was immersed in silicone oil O. The temperature of the silicone oil O was set to 90°C. A voltage was applied to the sheet S using flat electrodes FE1 and FE2. The voltage was increased at a rate of 4 kV / min. The DC breakdown field strength of the sheet S was calculated based on the applied voltage and thickness of the sheet S at the time of dielectric breakdown. In this embodiment, if the DC breakdown field strength due to the flat electrode is 250 kV / mm or more, the insulation is determined to be high.

[0066] (Evaluation 2: DC breakdown field strength using a needle electrode) For electrical property evaluation 2, DC breakdown field strength was measured using a needle-shaped electrode, as shown in Figures 4A and 4B. Specifically, a sheet S consisting of each sample was prepared in the same manner as in electrical property evaluation 1, except that each resin composition was extrusion-molded into a 4 mm-thick sheet. A conductive coating made of silver paste was applied to the first surface S1 of the sheet S to form an electrode FE3. The electrode FE3 was formed into a circular shape with a diameter of 25 mm. The tip of a needle-shaped electrode A (NE) was inserted into the second surface S2 of the sheet S along the thickness direction of the sheet S. The tip of the needle-shaped electrode A (NE) had a curvature radius r of 5 μm. The tip of the needle-shaped electrode A (NE) was spaced 1 mm from the electrode FE3. The sheet S inserted with the needle-shaped electrode A (NE) was immersed in silicone oil O. The temperature of the silicone oil O was set to 90°C. A voltage was applied between the needle-shaped electrode A (NE) and the electrode FE3. The voltage was increased at a rate of 4 kV / min. The DC breakdown field strength of the sheet S was determined based on the applied voltage when the sheet S broke down and the distance between the needle electrode A (NE) and the electrode FE3. In this example, if the DC breakdown field strength of the needle electrode is 100 kV / mm or more, it is determined that local electrical tree generation can be suppressed.

[0067] (Evaluation 3: Heat aging test) The heat aging test was performed based on IEC 60811-1-2. Specifically, each resin composition was press-molded and simultaneously crosslinked to produce a 0.5 mm thick sheet. This sheet was heated at 135°C for 168 hours. A tensile test was then performed using a dumbbell on the sheet before and after heating to measure tensile strength and elongation. In this example, if the tensile strength and elongation after heating were within ±25% of those before heating, the heat resistance was evaluated as high and rated "A." If they were outside this range, the heat resistance was evaluated as low and rated "B."

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

[0069] As shown in Table 1, for Samples 1 to 7, the DC breakdown field strength using a plate electrode was all 250 kV / mm or higher, confirming that the overall insulation of the sheet was high. However, for the DC breakdown field strength using a needle electrode, Samples 1 to 6, which had a nitrogen content of 2.0 ppm or higher, all had a value of 100 kV / mm or higher, while Sample 7, which had a nitrogen content of 1.4 ppm, had a value of 90 kV / mm, which was less than 100 kV / mm. This indicates that by setting the nitrogen content to 2.0 ppm or higher, it is possible to suppress the occurrence of localized electrical trees.

[0070] On the other hand, in the heat aging test, it was confirmed that Samples 2 to 7, which had a nitrogen content of 9.0 ppm or less, were less susceptible to heat aging, while Sample 1, which had a nitrogen content of 9.8 ppm, was more susceptible to heat aging. Sample 1 contained a larger amount of mineral oil than Samples 2 to 6, and it is presumed that the mineral oil softened the resin composition, thereby reducing its heat resistance. Therefore, it was found that by keeping the nitrogen content at 9.0 ppm or less, heat aging of the resin composition can be suppressed and high heat resistance can be maintained.

[0071] In addition, Sample 8, which did not contain lubricating oil, had a DC breakdown field strength of 250 kV / mm or more using a flat electrode, confirming that the overall insulation of the sheet was high. However, the DC breakdown field strength using a needle electrode was less than 100 kV / mm, confirming that localized electrical tree generation could not be suppressed.

[0072] Furthermore, in Sample 9, in which polybutene oil was added as a lubricant but the polybutene oil was a synthetic oil that did not contain nitrogen compounds as impurities, and the content of nitrogen atoms in the resin composition was 0, it was confirmed that the DC breakdown field strength measured with a needle electrode was less than 100 kV / mm, which confirmed that localized electrical tree generation could not be suppressed.

[0073] As shown in Table 2, the impurity content of the lubricating oil in Samples 3-1 to 3-6 differs depending on the type of mineral oil, and the nitrogen atom content in the resin composition changes depending on the amount of lubricating oil added. It was confirmed that the amount of lubricating oil added was appropriate in all samples, allowing them to maintain high heat resistance.

[0074] In Sample 3 and Samples 3-2 to 3-5, the nitrogen content was 2.0 ppm or more and 9.0 ppm or less, and it was confirmed that not only the DC breakdown field strength with a flat electrode but also the DC breakdown field strength with a needle-like electrode could be increased, and localized electrical tree generation could be suppressed. In particular, in Sample 3 and Samples 3-2 to 3-4, it was confirmed that by setting the nitrogen content to 3.0 ppm or more and 7.0 ppm or less, the DC breakdown field strength with a needle-like electrode could be increased to 120 kV / mm or more, and by setting the nitrogen content to 4.0 ppm or more and 6.0 ppm or less, the DC breakdown field strength with a needle-like electrode could be increased to 130 kV / mm or more, and localized electrical tree generation could be more reliably suppressed.

[0075] In contrast, in Sample 3-6, the nitrogen content was 1.0 ppm, less than 2.0 ppm, and the DC breakdown field strength using a needle electrode was confirmed to be significantly low at 89 kV / mm, which is thought to be due to the excessively low amount of nitrogen compounds that act to alleviate the locally applied electric field.

[0076] In addition, in sample 3-1, the nitrogen content was 10.0 ppm, exceeding 9.0 ppm, and it was confirmed that the DC breakdown field strength measured with a needle electrode was as low as 98 kV / mm. This is thought to be because the excessive nitrogen content reduced the insulating properties, resulting in a reduction in the DC breakdown field strength.

[0077] From the above, by adjusting the nitrogen content in the resin composition to a predetermined range, it is possible to maintain the desired heat resistance while suppressing the occurrence of localized electrical trees. [Explanation of symbols]

[0078] 10 Power Cable 110 Conductor 120 Internal semiconductive layer 130 Insulating layer 140 outer semiconductive layer 150 Shielding layer 160 Sheath NE needle electrode FE1, FE2 flat electrode FE3 electrode Silicone oil S seat S1 1st page S2 side 2

Claims

1. A resin composition constituting an insulating layer of a power cable, Contains polyethylene, The content of nitrogen atoms in the resin composition is 2.0 ppm or more and 9.0 ppm or less. Resin composition.

2. Further containing a lubricating oil containing a nitrogen compound, The nitrogen atoms in the resin composition are derived from the nitrogen compound. The resin composition according to claim 1.

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

4. The DC breakdown field strength measured using a needle electrode having a tip with a curvature radius of 5 μm is 100 kV / mm or more. The resin composition according to claim 1 or claim 2.

5. A conductor; an insulating layer provided to cover the outer periphery of the conductor; Equipped with The insulating layer contains the resin composition according to claim 1 or 2. Power cable.

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