CN115286855A - High-voltage direct-current semiconductive shielding material and preparation method thereof - Google Patents
High-voltage direct-current semiconductive shielding material and preparation method thereof Download PDFInfo
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- 238000000034 method Methods 0.000 claims description 5
- OKKRPWIIYQTPQF-UHFFFAOYSA-N Trimethylolpropane trimethacrylate Chemical group CC(=C)C(=O)OCC(CC)(COC(=O)C(C)=C)COC(=O)C(C)=C OKKRPWIIYQTPQF-UHFFFAOYSA-N 0.000 claims description 4
- HXIQYSLFEXIOAV-UHFFFAOYSA-N 2-tert-butyl-4-(5-tert-butyl-4-hydroxy-2-methylphenyl)sulfanyl-5-methylphenol Chemical group CC1=CC(O)=C(C(C)(C)C)C=C1SC1=CC(C(C)(C)C)=C(O)C=C1C HXIQYSLFEXIOAV-UHFFFAOYSA-N 0.000 claims description 3
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- NLZUEZXRPGMBCV-UHFFFAOYSA-N Butylhydroxytoluene Chemical compound CC1=CC(C(C)(C)C)=C(O)C(C(C)(C)C)=C1 NLZUEZXRPGMBCV-UHFFFAOYSA-N 0.000 description 2
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/04—Homopolymers or copolymers of ethene
- C08L23/06—Polyethene
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B9/00—Power cables
- H01B9/02—Power cables with screens or conductive layers, e.g. for avoiding large potential gradients
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
- C08K2201/00—Specific properties of additives
- C08K2201/001—Conductive additives
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L2203/00—Applications
- C08L2203/20—Applications use in electrical or conductive gadgets
- C08L2203/202—Applications use in electrical or conductive gadgets use in electrical wires or wirecoating
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L2207/00—Properties characterising the ingredient of the composition
- C08L2207/06—Properties of polyethylene
- C08L2207/066—LDPE (radical process)
Abstract
The invention provides a high-voltage direct-current semiconductive shielding material and a preparation method thereof, and relates to the field of cable materials. The preparation method comprises the following steps: uniformly mixing ethylene-vinyl acetate copolymer, low-density polyethylene and 30-35 parts of conductive carbon black by weight to obtain a first mixed product, wherein the ethylene-vinyl acetate copolymer and the low-density polyethylene account for 100 parts in total, and the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.5-3: 4; fully mixing the first mixed product with 0.3-0.5 part of antioxidant, 0.2-0.8 part of crosslinking assistant and 0.5-2.0 parts of vulcanizing agent to form a second mixed product; and processing the second mixed product at 115-180 ℃ to obtain the high-voltage direct current semi-conductive shielding material. The high-voltage direct-current semi-conductive shielding material disclosed by the invention has good mechanical property and thermal extension property while reducing the resistivity.
Description
Technical Field
The invention relates to the field of cable materials, in particular to a high-voltage direct-current semi-conductive shielding material and a preparation method thereof, a high-voltage direct-current transmission cable and a high-voltage power transmission system.
Background
The semi-conductive shielding layer in the high-voltage direct-current cable can prevent partial discharge from occurring in a gap between the conductive wire core and the insulating layer. The semi-conductive shielding layer can play a role of a uniform electric field and reduce the electrical stress in the insulating layer, and the semi-conductive shielding layer can inhibit charge injection and growth of electrical branches in the insulating layer to a certain extent, so that the service life of the cable is prolonged.
With the development of high-voltage direct-current cables, the usage amount of the semiconductive shielding material used by the high-voltage direct-current cables is increased. At present, due to the imperfect formula and processing technology system, the semiconductive shielding material for the high-voltage direct-current cable only can be completely imported. Import from abroad has a series of defects of limited supply quantity, long supply period, high cost and the like.
Disclosure of Invention
The inventor analyzes and finds that: the more the conductive carbon black is added, the lower the resistivity is, however, if the resistivity is reduced only by adding the conductive carbon black, other properties such as mechanical property and thermal elongation property of the semiconductive shielding material are reduced in practical application. Therefore, the inventor hopes to research a high voltage dc semi-conductive shielding material which can reduce the resistivity without reducing other properties of the semi-conductive shielding material.
The invention aims to provide a high-voltage direct-current semi-conductive shielding material and a preparation method thereof.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
the first aspect of the present invention provides a method for preparing a high voltage dc semiconductive shielding material, which may comprise the steps of:
uniformly mixing ethylene-vinyl acetate copolymer, low-density polyethylene and 30-35 parts of conductive carbon black by weight to obtain a first mixed product, wherein the ethylene-vinyl acetate copolymer and the low-density polyethylene account for 100 parts in total, and the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.5-3: 4; fully mixing the first mixed product with 0.3-0.5 part of antioxidant, 0.2-0.8 part of crosslinking assistant and 0.5-2.0 parts of vulcanizing agent to form a second mixed product; and processing the second mixed product at 115-180 ℃ to obtain the high-voltage direct-current semi-conductive shielding material.
A second aspect of the invention provides a high voltage dc semiconducting shielding material obtainable by any one of the preparation methods as described above.
A third aspect of the invention provides a high voltage direct current transmission cable comprising a high voltage direct current semiconducting shield material as described above.
A fourth aspect of the invention provides a high voltage power transmission system comprising a high voltage direct current transmission cable as described above.
Compared with the prior art, the beneficial effects of the invention comprise one or more of the following:
1. the high-voltage direct-current semi-conductive shielding material is added into a gap between a conductive wire core and an insulating layer to form a semi-conductive shielding layer, so that partial discharge is prevented, and the aim of homogenizing an electric field is fulfilled;
2. the high-voltage direct-current semiconductive shielding material and the preparation method thereof are provided, the requirement of a processing technology is reduced, and a series of defects of limited supply quantity, long supply period, high cost and the like are overcome;
3. compared with the shielding material for the common cable, the high-voltage direct-current semi-conductive shielding material has the advantages that the resistivity is tested at the thickness of 0.1cm, and the mechanical property and the thermal extension property of the semi-conductive shielding material are not reduced while the resistivity is reduced when the mechanical and thermal extension tests are carried out at the thickness of 0.2 cm. For example, in the range of 25-90 ℃, the slope of the resistivity of the high-voltage direct current semi-conductive shielding material relative to the temperature is not higher than 1.3, and the degree of linear fitting R 2 Not less than 0.85.
4. Compared with the shielding material for the common cable, the high-voltage direct-current semi-conductive shielding material has good high-temperature performance when the resistivity test is carried out on the shielding material with the thickness of 0.1 cm. For example, when used at 70 to 90 ℃, the resistivity ranges from 85 to 210 Ω · m.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. In the drawings:
FIG. 1 is a schematic diagram showing the relationship between the resistivity and the temperature in the embodiment of the invention
FIG. 2 is a graph showing a linear regression analysis of resistivity versus temperature in an embodiment of the present invention
Detailed Description
In order to more clearly explain the overall concept of the invention, the following detailed description is given by way of example in conjunction with the accompanying drawings.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those specifically described herein, and therefore the scope of the present invention is not limited by the specific embodiments disclosed below.
In addition, in the description of the present invention, it is to be understood that the terms "top," "bottom," "inner," "outer," "axial," "radial," "circumferential," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the present invention and simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed in a particular orientation, and be operated, and thus are not to be construed as limiting the present invention.
In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; the connection can be mechanical connection, electrical connection or communication; either directly or indirectly through intervening media, either internally or in any other relationship. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
In the present invention, unless expressly stated or limited otherwise, the first feature "on" or "under" the second feature may be directly contacting the second feature or the first and second features may be indirectly contacting each other through intervening media. In the description of the specification, reference to the description of "one embodiment," "some embodiments," "an example," "a specific example," or "some examples" or the like means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The preparation method of the high-voltage direct current semi-conductive shielding material comprises the following steps: uniformly mixing ethylene-vinyl acetate copolymer, low-density polyethylene and 30-35 parts of conductive carbon black by weight to obtain a first mixed product, wherein the ethylene-vinyl acetate copolymer and the low-density polyethylene account for 100 parts in total, and the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.5-3: 4; fully mixing the first mixed product with 0.3-0.5 part of antioxidant, 0.2-0.8 part of crosslinking assistant and 0.5-2.0 parts of vulcanizing agent to form a second mixed product; and processing the second mixed product at 115-180 ℃ to obtain the high-voltage direct-current semi-conductive shielding material.
The ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.6-2.8: 4, the parts of the conductive carbon black are 32 to 34, the parts of the antioxidant are 0.35 to 0.45, the parts of the crosslinking assistant are 0.3 to 0.5, and the parts of the vulcanizing agent are 0.8 to 1.6.
The content of vinyl acetate in the ethylene-vinyl acetate copolymer is 15-18 wt%; second stepThe melt flow index of the alkene-vinyl acetate copolymer is 2-4 g/10min, and the melting point is 80-100 ℃; the melt flow index of the low-density polyethylene is 2 to 3g/10min, and the density is 920.5kg/m 3 The tensile strength is more than or equal to 8MPa, and the elongation at break is more than or equal to 400 percent.
Here, the vulcanizing agent may be dicumyl peroxide (DCP); the antioxidant can be one or the combination of more than two of 4,4' -thiobis (6-tertiary butyl-3-methylphenol), tetra [ beta- (3, 5-di-tertiary butyl-4-hydroxyphenyl) propionic acid ] pentaerythritol ester, 2, 6-di-tertiary butyl-p-cresol and 1,3, 5-tri (3, 5-di-tertiary butyl-4-hydroxybenzyl) isocyanuric acid, and the crosslinking assistant can be jlz2, trimethylolpropane trimethacrylate and gamma-aminopropyltriethoxysilane.
The preparation of the high voltage dc semi-conductive shielding material is detailed as follows:
example 1
According to the mixture ratio in the table 1, after drying treatment is carried out on the ethylene-vinyl acetate copolymer, the low-density polyethylene and the conductive carbon black, the content of vinyl acetate in the ethylene-vinyl acetate copolymer is 16wt%, the melt flow index of the ethylene-vinyl acetate copolymer is 2-4 g/10min, and the melting point is 80-90 ℃; the low-density polyethylene melt with the flow index of 2-3 g/10min is added into a double-track open mill and mixed for 20min at 120 ℃ to obtain a first mixed product, the first mixed product is further mixed with an antioxidant 4,4' -thiobis (6-tert-butyl-3-methylphenol), a crosslinking assistant trimethylolpropane trimethacrylate and a vulcanizing agent dicumyl peroxide (DCP) by an internal mixer at 120 ℃ to obtain a second mixed product, a flat vulcanizing machine is preheated at 120 ℃ and 0MPa, the second mixed product is subjected to sheet forming and exhaust treatment at 120 ℃ and 10MPa, then is subjected to crosslinking at 160 ℃, and finally is cooled to room temperature to obtain the sheet-shaped semiconductive shielding material. The purpose of firstly carrying out the milling and then carrying out the banburying is to avoid the influence of the overlarge temperature difference between the temperature and the rubber material in the banbury mixer on the mixing uniformity on the one hand, and to carry out the banburying afterwards on the other hand, the additive loss can be avoided, and the rubber material and the additive can be fully mixed more efficiently.
Here, the present invention may form the high voltage dc semiconductive shielding material by a press vulcanizer or an extensional rheology extruder, however, the present invention may not be limited thereto.
Here, the second kneaded product may be kneaded, melted, and granulated before being subjected to a press vulcanizer to form a granular high-voltage dc semiconductive shielding material. In addition, the second kneaded product may be subjected to kneading, granulation treatment after melting, and wire rod molding treatment before being subjected to a press vulcanizer, to obtain a linear high-voltage dc semiconductive shielding material.
Example 2
According to the mixture ratio in the table 1, after drying treatment is carried out on the ethylene-vinyl acetate copolymer, the low-density polyethylene and the conductive carbon black, the content of vinyl acetate in the ethylene-vinyl acetate copolymer is 17wt%, the melt flow index of the ethylene-vinyl acetate copolymer is 2-4 g/10min, and the melting point is 80-90 ℃; the low-density polyethylene melt has a flow index of 2-3 g/10min, is added into a double-track open mill and is mixed for 20min at 130 ℃ to obtain a first mixed product, the first mixed product is further mixed with antioxidant 2, 6-di-tert-butyl-p-cresol, crosslinking assistant gamma-aminopropyl triethoxysilane and vulcanizing agent bis-di-five by an internal mixer at 130 ℃ to obtain a second mixed product, a flat vulcanizing machine is preheated at 135 ℃ and 0MPa, the second mixed product is subjected to sheet forming and exhaust treatment at 135 ℃ and 15MPa, then is crosslinked at 175 ℃, and finally is cooled to room temperature to obtain the sheet high-voltage direct current semiconductive shielding material.
Example 3
According to the proportion in the table 1, after drying the ethylene-vinyl acetate copolymer, the low-density polyethylene and the conductive carbon black, the content of vinyl acetate in the ethylene-vinyl acetate copolymer is 18wt%, the melt flow index of the ethylene-vinyl acetate copolymer is 2-4 g/10min, and the melting point is 85-95 ℃; the melt flow index of the low-density polyethylene is 2-3 g/10min, the low-density polyethylene is added into a double-track open mill and mixed for 18min at 140 ℃ to obtain a first mixed product, the first mixed product is further mixed with antioxidant tetra [ beta- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionic acid ] pentaerythritol ester, crosslinking aid trimethylolpropane trimethacrylate and vulcanizing agent DCP by an internal mixer at 140 ℃ to obtain a second mixed product, and the second mixed product is subjected to melt extrusion granulation by a stretching rheological extruder at 140 ℃ to obtain the granular high-voltage direct current semiconductive shielding material.
Here, the granular high-voltage direct current semi-conductive shielding material can be subjected to a hot press molding process to obtain a sheet-shaped high-voltage direct current semi-conductive shielding material.
Comparative example 1
The difference is that no low density polyethylene is added on the basis of example 1.
Comparative example 2
The difference is that, on the basis of example 1, 45 parts of conductive carbon black are added without addition of low-density polyethylene.
Comparative example 3
Based on example 1, the difference is that the ratio of the ethylene-vinyl acetate copolymer to the low density polyethylene is higher than 2.5 to 3:4.
comparative example 4
Based on example 1, the difference is that the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is less than 2.5-3: 4.
TABLE 1 ingredient table
The high voltage dc semi-conductive shielding materials obtained in example 1, example 2, example 3, comparative example 1 and comparative example 2 were subjected to a resistivity test at a thickness of 0.1cm and a mechanical and thermal elongation test at a thickness of 0.2 cm.
The hot elongation performance test comprises the following specific steps: dumbbell-shaped test pieces having a thickness of about 2.0mm were prepared, and a 20mm mark line was marked in the middle of the test piece to measure the thickness of each test piece. The sample load was 0.2MPa. The elongation under load was measured and calculated after storage in an oven at 200 ℃ for 15 min. The load was removed and after 5min of oven standing the sample was taken out, cooled to room temperature and the permanent set was calculated.
The performance of the HVDC semiconductive shielding material at 25 ℃ is shown in Table 2. The resistivity of the high voltage dc semiconducting shield at different temperatures is shown in table 3.
TABLE 2 Performance of HVDC semiconductive shielding material at 25 deg.C
TABLE 3 resistivity of HVDC semiconductive shielding material at different temperatures
For semiconductive shield materials, the lower the resistivity the better the performance of the semiconductive shield material; the larger the tensile strength and the larger the elongation at break, the better the mechanical properties of the semiconductive shielding material are; furthermore, the thermal elongation properties (elongation under load and permanent set after cooling) of the semiconductive shield material are matched to the thermal elongation properties of the insulating layer material.
In view of the above, an ethylene-vinyl acetate copolymer is generally used to prepare a semiconductive shield material, but the resistivity is large, and in order to lower the resistivity of the semiconductive shield material, an excessive amount of conductive carbon black is added. Referring to table 2, it can be seen that the resistivity of comparative example 2 is reduced by more than 50%, the tensile strength is reduced by about 1MPa, the elongation at break is reduced by more than 50%, and the resistivity parameter is improved but the elongation at break and the tensile strength parameter are deteriorated by adding too much conductive carbon black, as compared with the performance data of comparative example 1.
Referring to table 2, it can be seen that the resistivity of the semiconductive shield material of the present invention is reduced by more than 50%, and the tensile strength and elongation at break are comparable, as compared with the performance data of comparative example 3 in example 1; compared with the comparative example 3, the resistivity of the semiconductive shielding material of the present invention is reduced by more than 50% and the tensile strength and the elongation at break are equivalent as can be seen from the comparison of the example 2 with the comparative example 3, and the resistivity of the semiconductive shielding material of the present invention is reduced by more than 50% and the tensile strength and the elongation at break are equivalent as can be seen from the comparison of the example 3 with the comparative example 3, and the ratio of the ethylene-vinyl acetate copolymer of the comparative example 3 to the low density polyethylene is 3.55:4, the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.5-3 beyond the range of 4, and the resistivity parameter of the semiconductive shielding material of the comparative example 3 is deteriorated.
Referring to table 2, in example 1, compared with comparative example 4, it can be seen that the resistivity of the semiconductive shielding material of the present invention is reduced by about 80%, the tensile strength is increased by about 1MPa, the elongation at break is increased by about 25%, and the resistivity, tensile strength, and elongation at break parameters are all improved; compared with the comparative example 4, the resistivity of the semiconductive shielding material is reduced by about 80%, the tensile strength is improved by about 2MPa, the elongation at break is improved by about 25%, and the resistivity, the tensile strength and the elongation at break are improved; compared with the comparative example 4, the resistivity of the semiconductive shielding material is reduced by about 80%, the tensile strength is improved by about 3MPa, the elongation at break is improved by about 25%, and the resistivity, the tensile strength and the elongation at break are improved; the ethylene-vinyl acetate copolymer of comparative example 4 and the low density polyethylene were mixed at a ratio of 1.97:4, the ratio of the ethylene-vinyl acetate copolymer of the invention to the low density polyethylene is lower than the range of 2.5-3, and the resistivity, the tensile strength and the elongation at break of the semiconductive shielding material of comparative example 4 are all deteriorated. Through multiple experiments, the inventor provides a semiconductive shielding material with the ratio of ethylene-vinyl acetate copolymer to low-density polyethylene of 2.5-3, and under the combined action of conductive carbon black, antioxidant, crosslinking assistant and vulcanizing agent, the resistivity of the semiconductive shielding material is reduced without reducing the mechanical property.
Referring to table 2, the thermal elongation of the semiconductive shielding material prepared by the present application is matched with that of a common insulating layer material (e.g., a polyethylene insulating layer material), and under the same conditions, the difference between the elongation of the semiconductive shielding material under load and that of the polyethylene insulating layer material is not more than 15% of the elongation of the polyethylene insulating layer material under load, and the difference between the permanent deformation of the semiconductive shielding material after cooling and that of the polyethylene insulating layer material is not more than 5% of the permanent deformation of the polyethylene insulating layer material after cooling.
Referring to table 3, it can be seen that the resistivity of the high voltage dc semiconductive shielding materials of examples 1 to 3 is 25% or less of that of comparative example 1 at 90 ℃, and that the change rate of the resistivity of comparative example 1 with temperature rise is steeper or greater at 25 to 90 ℃, while the change rate of the resistivity of examples 1 to 3 with temperature rise (i.e., the derivative of the resistivity with respect to temperature, for example, like a slope) is gentler or smaller as shown in fig. 1. In table 3, comparative example 2 has a change rate of resistivity with temperature increase comparable to that of the present application, compared to examples 1 to 3, but comparative example 2 has poor mechanical properties of the semiconductive shield material.
Referring to fig. 2, the linear regression analysis of the change of the resistivity with respect to the temperature of example 1, example 2 and example 3, respectively, for example, the slope of the resistivity with respect to the temperature of the high voltage dc semi-conductive shielding material of the present invention is not higher than 1.3 in the range of 25 to 90 ℃, and the degree of linear fitting R 2 Not less than 0.85. In addition, the high-voltage direct current semi-conductive shielding material can reach 90 ℃ in use temperature, for example, when the high-voltage direct current semi-conductive shielding material is used at 70-90 ℃, the range of the resistivity is 85-210 omega-m, and obviously, the high-voltage direct current semi-conductive shielding material has good high-temperature performance.
The semi-conductive shielding material prepared by the method is added in a gap between a conductive wire core and an insulating layer of the high-voltage direct-current cable to form a semi-conductive shielding layer, so that the effect of homogenizing an electric field is achieved; the semiconductive shielding material prepared by the invention is suitable for 110-220 kV high-voltage cables.
The above description is only an example of the present invention, and is not intended to limit the present invention. Various modifications and alterations to this invention will become apparent to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.
Claims (9)
1. A preparation method of a high-voltage direct current semi-conductive shielding material is characterized by comprising the following steps of:
uniformly mixing ethylene-vinyl acetate copolymer, low-density polyethylene and 30-35 parts of conductive carbon black by weight to obtain a first mixed product, wherein the ethylene-vinyl acetate copolymer and the low-density polyethylene account for 100 parts in total, and the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.5-3: 4; fully mixing the first mixed product with 0.3-0.5 part of antioxidant, 0.2-0.8 part of crosslinking assistant and 0.5-2.0 parts of vulcanizing agent to form a second mixed product; and processing the second mixed product at 115-180 ℃ to obtain the high-voltage direct current semi-conductive shielding material.
2. The preparation method according to claim 1, wherein the ratio of the ethylene-vinyl acetate copolymer to the low-density polyethylene is 2.6-2.8: 4, the part of the conductive carbon black is 32 to 34, the part of the antioxidant is 0.35 to 0.45, the part of the crosslinking assistant is 0.3 to 0.5, and the part of the vulcanizing agent is 0.8 to 1.6.
3. The method according to claim 1, wherein the ethylene-vinyl acetate copolymer has a vinyl acetate content of 15 to 18wt%.
4. The preparation method according to claim 1, wherein the ethylene-vinyl acetate copolymer has a melt flow index of 2 to 4g/10min and a melting point of 80 to 100 ℃; the melt flow index of the low-density polyethylene is 2 to 3g/10min, and the density is 920.5kg/m 3 The tensile strength is more than or equal to 8MPa, and the elongation at break is more than or equal to 400 percent.
5. The method according to claim 1, wherein the crosslinking coagent is trimethylolpropane trimethacrylate, the vulcanizing agent is dicumyl peroxide, and the antioxidant is 4,4' -thiobis (6-t-butyl-3-methylphenol).
6. A high voltage direct current semiconducting shielding material, characterized in that it is obtained by the preparation method according to any of claims 1 to 5.
7. The high-voltage direct current semi-conductive shielding material according to claim 6, wherein the semi-conductive shielding material has a resistivity slope with respect to temperature of not higher than 1.3 and a linear fitting degree R within a range of 25-90 ℃ 2 Not less than 0.85, and has a resistivity in the range of 85-210. Omega. M when used at 70-90 ℃.
8. An HVDC transmission cable, characterized in that the cable comprises a high voltage DC semiconducting shield according to claim 6 or 7.
9. A high voltage power transmission system, characterized in that the power transmission system comprises a high voltage direct current transmission cable according to claim 8.
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