Process for the preparation of high performance polypropylene sleeves

Abstract

This invention relates to the field of high-voltage electrical equipment technology, specifically disclosing a method for preparing a high-performance polypropylene bushing. This method uses a segmented casting method to prepare a segmented nonlinear layer structure. From the middle to both ends of the nonlinear layer, the doping concentration gradient of the nonlinear functionalized filler is controlled to decrease, thereby reducing the conductivity difference between the nonlinear layer and the polypropylene insulation layer. This reduces the occurrence of electric field concentration at both ends of the nonlinear layer, improving the insulation and operational reliability of the bushing.

Description

Technical Field

[0001] This invention relates to the field of high-voltage electrical equipment technology, and specifically to a method for preparing a high-performance polypropylene bushing. Background Technology

[0002] High-voltage direct current (HVDC) bushings are critical insulation devices in converter stations, providing essential insulation and support for high-voltage conductors penetrating transformer casings, converter station walls, and other grounding conductors. The basic structure of an HVDC bushing includes a high-voltage current-carrying conductor, an insulating core, a coaxial flange grounding conductor, and an outer insulating skirt. In practical engineering applications, bushings need to withstand the combined effects of operating voltage, load current, short-term fault overvoltages and overcurrents, as well as long-term electrical, thermal, and mechanical stresses. This places extremely high demands on the bushing's insulation performance. However, current bushings generally suffer from poor insulation performance, or even insulation failure, leading to poor reliability. Summary of the Invention

[0003] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, one objective of this invention is to propose a method for preparing a high-performance polypropylene bushing, employing a segmented casting method to prepare a nonlinear layer with a gradient variation in filler concentration, thereby reducing the phenomenon of electric field concentration at both ends of the nonlinear layer and improving the insulation and operational reliability of the bushing.

[0004] Specifically, the present invention provides a method for preparing a high-performance polypropylene sleeve, comprising the following steps: A first polypropylene layer is coated onto the surface of the conductor; A grounding layer is laid on the surface of the first polypropylene layer; Multiple nonlinear segments are formed in steps along the length direction on the outer side of the grounding layer. The multiple nonlinear segments are symmetrically arranged and constitute a nonlinear layer. The nonlinear layer is in contact with the grounding layer. The doping concentration of the nonlinear functionalized filler in adjacent nonlinear segments is different, and along the length direction of the nonlinear layer, from the middle to both ends of the nonlinear layer, the doping concentration of the nonlinear functionalized filler in the nonlinear segment decreases in a gradient. A second polypropylene layer is formed to cover the nonlinear layer.

[0005] Traditional nonlinear bushings experience a rapid increase in conductivity of the nonlinear material when operating at high voltages (above 200 kV), resulting in a significant difference in conductivity between the nonlinear material and the insulating layer. This leads to electric field concentration at the interface between the two, particularly at the ends of the nonlinear layer, deteriorating the bushing's insulation performance and reducing operational reliability. This invention employs a segmented casting method to fabricate a segmented nonlinear layer structure. From the middle to both ends of the nonlinear layer, the doping concentration gradient of the nonlinear functionalized filler is controlled to decrease, reducing the conductivity difference between the nonlinear layer and the polypropylene insulating layer. This reduces the occurrence of electric field concentration at the ends of the nonlinear layer, improving the bushing's insulation and operational reliability.

[0006] According to some embodiments of the present invention, the difference in doping concentration of the nonlinear functionalized filler in adjacent nonlinear segments is 5%-10% by volume.

[0007] According to some embodiments of the present invention, the nonlinear layer includes 3-9 nonlinear segments. Preferably, the nonlinear layer includes: one first nonlinear segment, two second nonlinear segments, and two third nonlinear segments arranged along the length direction; the nonlinear segments are arranged in the following order along the length direction: third nonlinear segment, second nonlinear segment, first nonlinear segment, second nonlinear segment, and third nonlinear segment.

[0008] According to some embodiments of the present invention, the doping concentration of the nonlinear functionalized filler in the first nonlinear segment is 30%-35% by volume; the doping concentration of the nonlinear functionalized filler in the second nonlinear segment is 25%-30% by volume; and the doping concentration of the nonlinear functionalized filler in the third nonlinear segment is 20%-25% by volume.

[0009] According to some embodiments of the present invention, the material of the nonlinear layer includes a first polypropylene matrix material and the nonlinear functionalized filler; the material of the first polypropylene layer, the material of the second polypropylene layer and the first polypropylene matrix material are the same, and all are polypropylene-based materials; preferably, the polypropylene-based material includes a polypropylene graft containing anhydride groups; optionally, the nonlinear functionalized filler includes at least one of micron-sized silicon carbide powder, nano-sized silicon carbide powder, micron-sized zinc oxide powder, and nano-sized zinc oxide powder.

[0010] According to some embodiments of the present invention, the ratio of the total thickness of the first polypropylene layer and the second polypropylene layer to the thickness of the nonlinear layer is 20:1-80:1; optionally, the thickness of the first polypropylene layer is 30mm-100mm; optionally, the thickness of the second polypropylene layer is 8mm-15mm; optionally, the thickness of the nonlinear layer is 2mm-9mm.

[0011] According to some embodiments of the present invention, before coating the conductor surface with the first polypropylene layer, the preparation method further includes: coating the conductor surface with a shielding layer; coating the shielding layer surface with the first polypropylene layer; the shielding layer is made of a second polypropylene matrix material and a conductive filler, wherein the second polypropylene matrix material is the same as the material of the first polypropylene layer; optionally, the conductive filler includes at least one of conductive carbon black, graphite, graphene, and carbon nanotubes; the thickness of the shielding layer is 0.2 mm to 1 mm.

[0012] According to some embodiments of the present invention, the shielding layer and the first polypropylene layer are coated on the surface of the conductor by co-extrusion.

[0013] According to some embodiments of the present invention, the nonlinear segment is prepared by casting.

[0014] According to some embodiments of the present invention, forming the second polypropylene layer includes: placing the sample obtained after forming the nonlinear layer in a preheated mold; injecting molten polypropylene material into the mold, annealing, and cooling to form the second polypropylene layer; the temperature of the preheated mold is 140°C-180°C.

[0015] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0016] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is an exemplary process flow diagram of the preparation method of the high-performance polypropylene sleeve of the present invention.

[0017] Figure 2 This is another exemplary process flow diagram of the preparation method of the high-performance polypropylene sleeve of the present invention.

[0018] Figure 3 This is a schematic diagram of the cross-section of a high-performance polypropylene bushing at the grounding layer.

[0019] Figure label: 1000, High-performance polypropylene sleeve; 100, Conductor; 200, First polypropylene layer; 300, Grounding layer; 400, Nonlinear layer; 410, First nonlinear segment; 420, Second nonlinear segment; 430, Third nonlinear segment; 500, Second polypropylene layer; 600, Shielding layer; 700, First mold; 800, Second mold. Detailed Implementation

[0020] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0021] In the description of the embodiments of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention. In addition, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0022] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention based on the specific circumstances.

[0023] In this invention, unless otherwise expressly specified and limited, "above or below" a first feature may include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on" the first feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the first feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0024] High-voltage direct current (HVDC) bushings are critical insulation devices in converter stations, providing essential insulation and support for high-voltage conductors penetrating transformer casings, converter station walls, and other grounding conductors. The basic structure of an HVDC bushing includes a high-voltage current-carrying conductor, an insulating core, a coaxial flange grounding conductor, and an outer insulating skirt. In practical engineering applications, bushings must withstand the combined effects of operating voltage, load current, short-term fault overvoltages and overcurrents, as well as long-term electrical, thermal, and mechanical stresses. This places extremely high demands on the bushing's insulation performance.

[0025] When traditional nonlinear bushings operate at high voltages (above 200 kV), the conductivity of the nonlinear material increases rapidly, resulting in a significant difference in conductivity between the nonlinear material and the insulating layer. This leads to electric field concentration at the interface between the two, especially at both ends of the nonlinear layer, which deteriorates the insulation performance of the bushing and reduces its operational reliability.

[0026] To address the above problems, this invention proposes a method for preparing high-performance polypropylene bushings. The method employs a segmented casting process to prepare a segmented nonlinear layer structure. From the middle to both ends of the nonlinear layer, the doping concentration gradient of the nonlinear functionalized filler is controlled to decrease, thereby reducing the conductivity difference between the nonlinear layer and the polypropylene insulating layer. This reduces the occurrence of electric field concentration at both ends of the nonlinear layer, improving the insulation and operational reliability of the bushing.

[0027] Specifically, this invention provides a method for preparing a high-performance polypropylene sleeve 1000, the process flow diagram of which is shown below. Figure 1 As shown, the preparation method includes the following steps: A first polypropylene layer 200 is coated on the surface of conductor 100; A grounding layer 300 is laid on the surface of the first polypropylene layer 200; Multiple nonlinear segments are formed in steps along the length direction on the outer side of the grounding layer 300. The multiple nonlinear segments are symmetrically arranged and constitute a nonlinear layer 400, which is in contact with the grounding layer 300. The doping concentration of the nonlinear functional filler in adjacent nonlinear segments is different, and along the length direction of the nonlinear layer 400, from the middle to both ends of the nonlinear layer 400, the doping concentration of the nonlinear functional filler in the nonlinear segments decreases in a gradient. A second polypropylene layer 500 is formed, which covers the nonlinear layer 400.

[0028] In this invention, "along the length direction" refers to the direction in which the guide rod extends. The "length direction of the nonlinear layer" is the same as the direction in which the guide rod extends.

[0029] In this invention, the doping concentration of the nonlinear functionalized filler in the nonlinear segment refers to the proportion of the volume of the nonlinear functionalized filler to the volume of the nonlinear segment (i.e., the total volume of the first polypropylene matrix material and the nonlinear functionalized filler).

[0030] In this invention, multiple nonlinear segments contact each other to form a whole, together constituting a nonlinear layer 400.

[0031] The polypropylene layer used in this invention possesses high insulation performance. The breakdown field strength of polypropylene material can easily reach over 210 kV / mm, especially for polypropylene grafts containing anhydride groups, whose breakdown field strength at 90℃ can reach 210 kV / mm-800 kV / mm. These materials maintain strong insulation performance even under high electric field strength conditions, which is beneficial for manufacturing high-insulation bushings and reducing the occurrence of insulation failure. Currently, bushings both domestically and internationally generally use epoxy resin (impregnated paper) or oil-impregnated paper composite materials as insulation materials, with a long-term operating design field strength of around 12 kV / mm. The dielectric insulation strength of polypropylene materials is much higher than that of epoxy resin-based impregnated paper materials. Using polypropylene materials to manufacture bushings, compared to traditional impregnated paper bushings, can achieve the same or higher insulation performance with reduced size, improving operational reliability. Because the bushing of this invention uses an insulating material with extremely high breakdown field strength, it abandons the complex capacitive voltage equalization structure of traditional bushings, does not include a capacitor screen, but includes a grounding layer and ports.

[0032] Furthermore, existing dry epoxy resin-based sleeves are difficult to process. Epoxy resin-based sleeves employ vacuum casting to maintain insulation strength, resulting in extremely long production cycles (over six months) for high-voltage products. Moreover, issues such as internal stress cracking, delamination, and air bubbles at the adhesive-paper interface caused by uneven temperature and pressure lead to highly complex quality control. The polypropylene material used in this invention has the characteristic of softening upon heating and re-curing upon cooling. It allows for the preparation of the sleeve's layers using co-extrusion and melt-re-curing methods. Compared to related technologies that use vacuum casting to prepare resin-impregnated paper sleeves, this significantly shortens the production cycle.

[0033] In some embodiments, the length of the nonlinear layer 400 is less than the length of the first polypropylene layer 200. The length of the first polypropylene layer 200 is equal to the length of the second polypropylene layer 500. The second polypropylene layer 500 completely covers the nonlinear layer 400, preventing the nonlinear layer 400 from being exposed, thereby preventing corona discharge from occurring on the surface of the nonlinear layer 400, which could lead to a deterioration in the insulation performance of the bushing after long-term operation, or even insulation failure.

[0034] In some embodiments, the difference in doping concentration of the nonlinear functionalized filler in adjacent nonlinear segments is 5%-10% by volume. Optimizing the filler concentration difference between adjacent nonlinear segments can reduce the conductivity difference between the nonlinear layer and the polypropylene insulation layer while ensuring the homogenization of the extremely non-uniform electric field of the bushing. This reduces the phenomenon of electric field concentration at both ends of the nonlinear layer, thereby improving the insulation and operational reliability of the bushing.

[0035] In some specific embodiments, the difference in doping concentration of the nonlinear functionalized filler in adjacent nonlinear segments is 5 vol%, 6 vol%, 7 vol%, 8 vol%, 9 vol%, or 10 vol%.

[0036] In some embodiments, the nonlinear layer 400 includes 3-9 nonlinear segments, such as 3, 5, or 7 nonlinear segments; preferably, it includes 5 nonlinear segments. The doping concentration of the nonlinear functionalized filler in the multiple nonlinear segments is different from each other and decreases in a gradient from the middle to both ends of the nonlinear layer. Too many nonlinear segments will lead to complex manufacturing processes and increased production costs.

[0037] In some embodiments, the nonlinear layer 400 includes: one first nonlinear segment 410, two second nonlinear segments 420, and two third nonlinear segments 430 arranged along the length direction. The nonlinear segments are arranged in the following order along the length direction: third nonlinear segment 430, second nonlinear segment 420, first nonlinear segment 410, second nonlinear segment 420, and third nonlinear segment 430. The doping concentrations of the nonlinear functionalized fillers in the first nonlinear segment 410, second nonlinear segment 420, and third nonlinear segment 430 are different from each other and decrease in a gradient.

[0038] In some embodiments, the doping concentration of the nonlinear functionalized filler in the first nonlinear segment 410 is 30%-35% by volume. The doping concentration of the nonlinear functionalized filler in the second nonlinear segment 420 is 25%-30% by volume. The doping concentration of the nonlinear functionalized filler in the third nonlinear segment 430 is 20%-25% by volume. Optimizing the doping concentration of the nonlinear functionalized filler in each nonlinear segment is beneficial for homogenizing the highly non-uniform electric field of the bushing, while reducing the phenomenon of electric field concentration at both ends of the nonlinear layer 400.

[0039] In some specific embodiments, the doping concentration of the nonlinear functionalized filler in the first nonlinear segment 410 is 30 vol%, 31 vol%, 32 vol%, 33 vol%, 34 vol%, or 35 vol%.

[0040] In some specific embodiments, the doping concentration of the nonlinear functionalized filler in the second nonlinear segment 420 is 25 vol%, 26 vol%, 27 vol%, 28 vol%, 29 vol%, or 30 vol%.

[0041] In some specific embodiments, the doping concentration of the nonlinear functionalized filler in the third nonlinear segment 430 is 20 vol%, 21 vol%, 22 vol%, 23 vol%, 24 vol%, or 25 vol%.

[0042] In some embodiments, the material of the nonlinear layer 400 includes a first polypropylene matrix material and the nonlinear functionalized filler.

[0043] The materials of the first polypropylene layer 200, the second polypropylene layer 500, and the first polypropylene matrix material are the same, and all are polypropylene-based materials. Preferably, the polypropylene-based material includes a polypropylene graft containing anhydride groups. In existing bushings, the materials of the insulating layer and the nonlinear layer are different. The insulating layer usually uses epoxy resin (impregnated paper) or oil-impregnated paper composite material as the insulating material, while the nonlinear layer usually uses silicone rubber or the like as the matrix material. These materials have significantly different dielectric constants and conductivity, resulting in a clear interface between the insulating layer and the nonlinear layer. After long-term operation, charge accumulates at this interface, causing changes and distortions in the internal electric field, which in turn leads to insulation failure. This invention uses the same polypropylene-based material as the insulating layer (i.e., including the first and second polypropylene layers) and the matrix material of the nonlinear layer, so that there is no clear interface between the insulating layer and the nonlinear layer, avoiding charge accumulation, thereby preventing insulation failure, improving the insulation withstand voltage performance of the bushing, and enhancing operational reliability.

[0044] In this document, the polypropylene graft containing anhydride groups includes structural units derived from copolymer polypropylene, structural units derived from maleic anhydride monomers, and structural units derived from alkenyl-containing polymeric monomers. Based on the weight of the polypropylene graft containing anhydride groups, the content of structural units derived from maleic anhydride monomers and alkenyl polymeric monomers in the grafted state in the polypropylene graft containing anhydride groups is 0.1 wt% to 5 wt% (e.g., 0.1 wt%, 0.3 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, or 5 wt%), preferably 0.4 to 3 wt%; and the molar ratio of structural units derived from maleic anhydride monomers to structural units derived from alkenyl polymeric monomers in the polypropylene graft containing anhydride groups is 1:(1 to 20) (e.g., 1:1, 1:5, 1:10, 1:15, or 1:20), preferably 1:(1 to 10).

[0045] The copolymerized polypropylene can be any commercially available polypropylene powder suitable for this invention, or it can be produced by commonly used polymerization processes described in the literature.

[0046] The alkenyl-containing polymeric monomer is selected from at least one monomer having the structure shown in Formula I. Formula I Wherein, R1, R2, and R3 are each independently selected from H, substituted or unsubstituted C1-C6 alkyl groups, preferably, R1, R2, and R3 are each independently selected from H, substituted or unsubstituted C1-C3 alkyl groups; R4 is selected from substituted or unsubstituted C1-C6 alkyl groups. 20 Alkyl, substituted or unsubstituted C1-C20 Alkoxy, substituted or unsubstituted C6-C 20 aryl, substituted or unsubstituted C1-C 20 Ester group, substituted or unsubstituted C1-C 20 Carboxyl, substituted or unsubstituted C3-C 20 The substituted group is a cycloalkyl or heterocyclic group, or a cyano group, wherein the substituted group is a halogen, hydroxyl, amino, C1-C6 alkyl, or C3-C6 cycloalkyl; preferably, R4 is selected from substituted or unsubstituted C1-C6 groups. 12 Alkyl, substituted or unsubstituted C1-C 18 Alkoxy, substituted or unsubstituted C6-C 12 aryl, substituted or unsubstituted C1-C 12 Ester group, substituted or unsubstituted C1-C 12 Carboxyl group, substituted or unsubstituted C3-C 12 The substituted group is a cycloalkyl or heterocyclic group, or a cyano group, wherein the substituted group is a halogen, a C1-C6 alkyl group, or a C3-C6 cycloalkyl group; more preferably, R4 is selected from substituted or unsubstituted C1-C6 alkyl groups, substituted or unsubstituted C1-C6 cycloalkyl groups, or C3-C6 cycloalkyl groups. 12 Alkoxy, substituted or unsubstituted C6-C8 aryl, substituted or unsubstituted C1-C6 ester, substituted or unsubstituted C1-C6 carboxyl, substituted or unsubstituted C3-C6 cycloalkyl or heterocyclic, cyano; preferably, the heterocyclic group is selected from imidazole, pyrazol, carbazole, pyrrolidone, pyridinyl, piperidinyl, caprolactam, pyrazinyl, thiazolyl, purine, morpholino, oxazoline; More preferably, the alkenyl-containing polymeric monomer is selected from at least one of vinyl acetate, styrene, α-methylstyrene, (meth)acrylate, vinyl alkyl ether, vinylpyrrolidone, vinylpyridine, vinylimidazolium, and acrylonitrile; the (meth)acrylate is preferably at least one of methyl (meth)acrylate, ethyl (meth)acrylate, and glycidyl (meth)acrylate.

[0047] The polypropylene graft containing anhydride groups can be prepared by a method comprising the following steps: in the presence of an inert gas, a reaction mixture comprising copolymer polypropylene, maleic anhydride monomer and alkenyl polymeric monomer is subjected to a grafting reaction to obtain the polypropylene graft containing anhydride groups.

[0048] The grafting reaction can be carried out using various methods conventional in the art, preferably a solid-phase grafting reaction. For example, active grafting sites can be formed on copolymer polypropylene in the presence of maleic anhydride for grafting and an alkenyl-containing polymeric monomer, or active grafting sites can be formed on copolymer polypropylene first, followed by treatment with grafting monomer. Grafting sites can be formed by treatment with a free radical initiator, or by high-energy ionizing radiation or microwave treatment. Free radicals generated in the polymer as a result of chemical or radiation treatment form grafting sites on the polymer and initiate monomer polymerization at these sites.

[0049] Preferably, the grafting site is initiated by a free radical initiator, and the grafting reaction is further carried out. In this case, the reaction mixture further includes a free radical initiator. More preferably, the free radical initiator is selected from peroxide-based free radical initiators and / or azo-based free radical initiators. The peroxide-based free radical initiator is preferably selected from at least one of benzoyl peroxide, dicumyl peroxide, di-tert-butyl peroxide, lauroyl peroxide, dodecyl peroxide, tert-butyl peroxide, diisopropyl peroxide, tert-butyl peroxide (2-ethylhexanoate), and dicyclohexyl peroxide. The azo-based free radical initiator is preferably azobisisobutyronitrile and / or azobisisoheptanenitrile.

[0050] More preferably, the grafting site is initiated by a peroxide-based free radical initiator and the grafting reaction is further carried out.

[0051] The polypropylene graft containing anhydride groups has at least one of the following characteristics: a melt flow rate of 0.01~30 g / 10min at 230°C and 2.16 kg load, preferably 0.05~20 g / 10min, more preferably 0.1~10 g / 10min, and more preferably 0.2~5 g / 10min; a flexural modulus of 20~900 MPa, more preferably 50~600 MPa; and an elongation at break ≥200%, preferably ≥300%.

[0052] Under the premise of satisfying the above product characteristics, the present invention does not have a particular limitation on the amount of each component in the grafting reaction. Specifically, the mass ratio of the free radical initiator to the total mass of the maleic anhydride monomer and the alkenyl-containing polymeric monomer is (0.1~10):100, preferably (0.5~5):100. The mass ratio of the total mass of the maleic anhydride monomer and the alkenyl-containing polymeric monomer to the copolymer polypropylene is (0.1~8):100, preferably (0.3~5):100. The molar amount of the maleic anhydride monomer can be 5 wt%~100 wt% of the molar amount of the alkenyl-containing polymeric monomer, preferably 10 wt%~100 wt%.

[0053] The present invention does not particularly limit the process conditions for the grafting reaction. Specifically, the temperature of the grafting reaction can be 30℃~130℃, preferably 60℃~120℃; the time can be 0.5 h~10 h, preferably 1 h~5 h.

[0054] In this invention, the "reaction mixture" includes all materials added to the grafting reaction system. The materials can be added all at once or at different stages of the reaction.

[0055] The reaction mixture of the present invention may further include a dispersant. The dispersant is preferably an aqueous solution of water or sodium chloride. The mass amount of the dispersant is preferably 50% to 300% of the mass of the copolymer polypropylene.

[0056] The reaction mixture of the present invention may further include an interface agent. The interface agent is an organic solvent that has a swelling effect on polyolefins, preferably at least one of the following organic solvents that have a swelling effect on copolymer polypropylene: ether solvents, ketone solvents, aromatic solvents, alkane solvents; more preferably at least one of the following organic solvents: chlorobenzene, polychlorinated benzene, alkanes or cycloalkanes with more than C6 carbon atoms, benzene, C1-C4 alkyl-substituted benzene, C2-C6 aliphatic ethers, C3-C6 aliphatic ketones, decahydronaphthalene; even more preferably at least one of the following organic solvents: benzene, toluene, xylene, chlorobenzene, tetrahydrofuran, diethyl ether, acetone, hexane, cyclohexane, decahydronaphthalene, heptane. The mass content of the interface agent is preferably 1% to 30% of the mass of the copolymer polypropylene, more preferably 10% to 25%.

[0057] The reaction mixture of the present invention may further include an organic solvent as a solvent for dissolving the solid free radical initiator. The organic solvent preferably includes at least one of C2-C5 alcohols, C2-C4 ethers, and C3-C5 ketones; more preferably, it includes at least one of C2-C4 alcohols, C2-C3 ethers, and C3-C5 ketones; and most preferably, it includes at least one of ethanol, diethyl ether, and acetone. The mass content of the organic solvent is preferably 1-35% of the mass of the copolymerized polypropylene.

[0058] In some embodiments, the preparation method of the polypropylene graft containing anhydride groups can be selected from one of the following methods: Method 1, the preparation method includes the following steps: a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas; b. Add the free radical initiator, maleic anhydride monomer, and alkenyl-containing polymerizable monomer to the closed reactor and stir to mix; c. Optionally add an interfacial agent and optionally swell the reaction system; d. Optionally add a dispersant, raise the temperature of the reaction system to the grafting reaction temperature, and carry out the grafting reaction; e. After the reaction is complete, optionally filter (in the case of using an aqueous dispersant) and dry to obtain the polypropylene graft containing anhydride groups.

[0059] More specifically, the preparation method includes the following steps: a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas; b. Dissolve the free radical initiator in maleic anhydride monomer and alkenyl-containing polymeric monomer to prepare a solution, add it to a closed reactor containing copolymer polypropylene, and stir to mix; c. Add 0-30 parts of interface agent, and optionally allow the reaction system to swell at 20-60°C for 0-24 hours; d. Add 0-300 parts of dispersant, heat the system to the graft polymerization temperature of 30℃-130℃, and react for 0.5-10 hours; e. After the reaction is complete, optionally filter (in the case of using an aqueous dispersant) and dry to obtain the polypropylene graft containing anhydride groups.

[0060] Method 2, the preparation method includes the following steps: a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas; b. Mix the organic solvent and the free radical initiator, and add the mixture to the closed reactor; c. Remove the organic solvent; d. Add maleic anhydride monomer and alkenyl-containing polymeric monomer, optionally add an interfacial agent, and optionally swell the reaction system; e. Optionally add a dispersant, heat the reaction system to the grafting reaction temperature, and carry out the grafting reaction; f. After the reaction is complete, optionally filter (in the case of using an aqueous dispersant) and dry to obtain the polypropylene graft containing anhydride groups.

[0061] More specifically, the preparation method includes the following steps: a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas; b. Mix the organic solvent and free radical initiator to prepare a solution, and add it to a closed reactor containing copolymer polypropylene; c. The organic solvent is removed by purging with an inert gas or by vacuum. d. Add maleic anhydride monomer and alkenyl-containing polymeric monomer, add 0-30 parts of interfacial agent, and optionally allow the reaction system to swell at 20-60 °C for 0-24 hours; e. Add 0-300 parts of dispersant, heat the system to the graft polymerization temperature of 30℃-130℃, and react for 0.5-10 hours; f. After the reaction is complete, optionally filter (in the case of using an aqueous dispersant) and dry to obtain the polypropylene graft containing anhydride groups.

[0062] In some embodiments, if volatile components are present in the system after the reaction, the method preferably includes a step of removing the volatile components, which can be carried out by any conventional method, including vacuum extraction at the end of the grafting process or using a stripping agent. Suitable stripping agents include, but are not limited to, inert gases.

[0063] As described above, the "polypropylene graft containing anhydride groups" of the present invention includes both the product (crude product) directly obtained by grafting copolymerized polypropylene and maleic anhydride monomer and alkenyl-containing polymeric monomer through a grafting reaction, and the grafted modified polypropylene pure product obtained by further purification of the product. Therefore, the preparation method may optionally include a step of purifying the crude product. The purification can be carried out using various methods conventional in the art, such as extraction.

[0064] This invention does not impose a particular limitation on the grafting efficiency of the grafting reaction, but a higher grafting efficiency is more advantageous for obtaining the desired polypropylene graft containing anhydride groups through a one-step grafting reaction. Therefore, it is preferable to control the grafting efficiency of the grafting reaction to be 20% to 100%, and more preferably 25% to 80%. The concept of grafting efficiency is well known to those skilled in the art, referring to the ratio of the total amount of maleic anhydride monomer and alkenyl-containing polymeric monomer on the graft to the total amount of maleic anhydride monomer and alkenyl-containing polymeric monomer in the reaction feed.

[0065] In some embodiments, a nonlinear segment with a higher doping concentration of the nonlinear functionalized filler can be formed first, followed by a nonlinear segment with a lower doping concentration of the nonlinear functionalized filler.

[0066] In some embodiments, the ratio of the total thickness of the first polypropylene layer 200 and the second polypropylene layer 500 to the thickness of the nonlinear layer 400 is 20:1 to 80:1. The first polypropylene layer 200 and the second polypropylene layer 500 together constitute the insulating layer of the bushing. The second polypropylene layer 500 encloses the nonlinear layer 400 internally to prevent corona discharge and a decrease in surface insulation strength caused by the nonlinear layer 400 being exposed externally. Changing the thickness ratio of the insulating layer to the nonlinear layer 400 while keeping the overall bushing dimensions constant will affect the bushing's insulation withstand voltage performance. A thickness ratio within the range of this invention is beneficial for improving the bushing's insulation withstand voltage performance.

[0067] In this paper, the first polypropylene layer 200, the second polypropylene layer 500, the nonlinear layer 400, and the grounding layer 300 each have a cylindrical structure. The cylindrical structure is symmetrical about the conductor 100 as a central axis. For example, the first polypropylene layer 200, the second polypropylene layer 500, and the nonlinear layer 400 are all cylindrical. In this paper, the thickness of each layer refers to the wall thickness of the cylindrical structure, i.e., the radial distance from the outer wall to the inner wall. The length of each layer refers to the length of the cylindrical structure along the extension direction of the conductor 100.

[0068] In some specific embodiments, the ratio of the total thickness of the first polypropylene layer 200 and the second polypropylene layer 500 to the thickness of the nonlinear layer 400 is 20:1, 30:1, 40:1, 50:1, 60:1, 70:1 or 80:1.

[0069] In some embodiments, the thickness of the first polypropylene layer 200 is 30mm-100mm, for example, 30mm, 40mm, 50mm, 60mm, 70mm, 80mm, 90mm, or 100mm. The thickness of the second polypropylene layer 500 is 8mm-15mm, for example, 8mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm, or 15mm. The thickness of the first polypropylene layer 200 can be selected according to the voltage rating of the bushing application. The thickness of the second polypropylene layer 500 should not be too thick to avoid making the equipment bulky and increasing production costs.

[0070] In some embodiments, the thickness of the nonlinear layer 400 is 2mm-9mm, for example, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, or 9mm. Optimizing the thickness of the nonlinear layer 400 within the scope of this invention is beneficial for better homogenizing the extremely non-uniform electric field of the bushing. The huge size difference between the high-voltage end and the grounding electrode of a conventional bushing leads to an edge effect in the electric field distribution, which manifests as an extremely non-uniform, strong vertical component electric field distribution during operation. Conventional capacitive bushings typically achieve electric field homogenization by placing capacitor plates with decreasing lengths layer by layer inside, thereby dispersing the electric field lines concentrated in the grounding layer. However, as the bushing voltage level increases, the manufacturing difficulty of this approach increases significantly, and the uniformity of the electric field strength will decrease significantly once the operating environment of the bushing changes. This invention uses a nonlinear layer with electric field self-adaptation to homogenize the extremely non-uniform electric field of the bushing, simplifying the bushing structure and the bushing manufacturing process.

[0071] The nonlinear layer 400 of the present invention comprises a first polypropylene matrix material and a nonlinear functionalized filler. In some embodiments, the nonlinear functionalized filler comprises at least one of micron-sized silicon carbide powder, nano-sized silicon carbide powder, micron-sized zinc oxide powder, and nano-sized zinc oxide powder. The nonlinear functionalized filler of the present invention is commercially available or prepared. Taking micron-sized zinc oxide powder as an example, based on the total weight of the micron-sized zinc oxide powder, its formulation is: 95% mol ZnO, 1.0% mol Bi2O3, 0.5% mol MnO2, 0.4% mol Cr2O3, 1.0% mol Co2O3, 1.0% mol Sb2O3, 0.1% mol Al2O3, and 1.0% mol Si2O3. The preparation process is as follows: all raw materials (purity ≥99.9%) are added to anhydrous ethanol and ground using a ball mill for 8 h. Subsequently, an organic binder (e.g., polyvinyl alcohol, polyvinylpyrrolidone) is added and the mixture is spray-dried to form micron-sized spherical particles. After sieving, the particles were calcined at 1000℃ for 4.5 h, with a heating rate of 0.55℃ / min and a cooling rate of 1.6℃ / min. Following further dispersion and sieving, particles with a diameter range of 20-80 μm and a density of 5.01 g / cm³ were finally obtained. 3 The ZnO pressure-sensitive microspheres have a voltage gradient of 400 V / mm.

[0072] The material of the nonlinear layer 400 of the present invention can be prepared by uniformly mixing a nonlinear functionalized filler with a first polypropylene matrix material. This mixing can be carried out in an internal mixer. The temperature of the internal mixer is 110°C-130°C (e.g., 110°C, 115°C, 120°C, 125°C, or 130°C), and the rotation speed is 50 r / min-70 r / min (e.g., 50 r / min, 55 r / min, 60 r / min, 65 r / min, or 70 r / min). The mixing time is 20 min-40 min (e.g., 20 min, 25 min, 30 min, 35 min, or 40 min). To ensure uniform mixing, the nonlinear functionalized filler and the first polypropylene matrix material can be added to the internal mixer in batches, alternating between the two. Before adding the raw materials, to ensure that no impurities are introduced into the subsequent mixing, the internal mixer can be cleaned with the first polypropylene matrix material to remove residual impurities. During cleaning, the temperature of the internal mixer is 30℃-50℃ (e.g., 30℃, 35℃, 40℃, 45℃, then 50℃), and the rotation speed is 50r / min-70r / min (e.g., 50r / min, 55r / min, 60r / min, 65r / min, or 70r / min). The stirring time is 10min-20min, e.g., 10min, 15min, or 20min. The volume ratio of the nonlinear functionalized filler to the first polypropylene matrix material can be (20-30):(70-80), e.g., 20:80, 25:75, or 30:70.

[0073] In some embodiments, reference Figure 2 Before coating the conductor 100 with the first polypropylene layer 200, the preparation method further includes: coating the conductor 100 with a shielding layer 600; and coating the shielding layer 600 with the first polypropylene layer 200. The shielding layer 600 is connected to a wire, which is grounded, thereby shielding the distorted electric field of the outer flange of the bushing.

[0074] In some specific embodiments, the material of the shielding layer includes a second polypropylene matrix material and a conductive filler, wherein the second polypropylene matrix material is the same as the material of the first polypropylene layer. This avoids a significant interface between the shielding layer 600 and the first polypropylene layer 200, thereby preventing charge accumulation at the interface, preventing insulation failure, and improving the insulation performance of the bushing.

[0075] In some specific embodiments, the thickness of the shielding layer 600 is 0.2 mm to 1 mm, for example, 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm or 1 mm.

[0076] In this document, the shielding layer 600 has a cylindrical structure. The cylindrical structure is symmetrical about the conductor 100 as a central axis. For example, the shielding layer 600 is cylindrical. In this document, the thickness of the shielding layer 600 refers to the wall thickness of the cylindrical structure, i.e., the radial distance from the outer wall to the inner wall. The length of the shielding layer 600 refers to the length of the cylindrical structure, i.e., the length of the cylinder along the extension direction of the conductor 100.

[0077] In some specific embodiments, the conductive filler includes at least one of conductive carbon black, graphite, graphene, and carbon nanotubes.

[0078] In some embodiments, a shielding layer 600 and a first polypropylene layer 200 are coated on the surface of the conductor 100 using a co-extrusion method.

[0079] When using co-extrusion, multiple extruders are configured according to the number of layers to be extruded. Each extruder heats and plasticizes different materials to a molten state, controlling its own temperature, pressure, and flow rate. The molten material flows converge into a co-extrusion die, which is precisely designed to form a layered structure of different materials within the die cavity. This layered structure is simultaneously extruded and coated onto the conductor 100, and rapidly solidified using a water bath or other cooling system to maintain the required geometric dimensions and interfacial bonding quality. This invention uses co-extrusion to simultaneously form the shielding layer 600 and the first polypropylene layer 200, significantly shortening the production cycle and simplifying the production process.

[0080] In some embodiments, laying a grounding layer 300 on the surface of the first polypropylene layer 200 includes: covering the outside of the first polypropylene layer 200 with a grounding material such as a low-resistivity semiconductor 100 tape or copper mesh.

[0081] In some embodiments, the nonlinear segments are prepared by casting. The nonlinear material is cast in segments to form multiple nonlinear segments.

[0082] In some specific embodiments, the method of forming multiple nonlinear segments in steps along the length direction on the outer side of the ground layer 300 includes the following steps: After the grounding layer 300 is formed, the resulting structure is placed in the preheated first mold 700, which covers the grounding layer 300. The material of the first nonlinear segment 410 is melted and injected into the first mold 700. After annealing, it is gradually cooled to room temperature. During this period, the vacuum state inside the mold can be continuously maintained by a mechanical pump. After molding and demolding, the first nonlinear segment 410 is formed. The resulting structure is placed in a preheated second mold 800, which covers the first nonlinear segment 410. The material of the second nonlinear segment 420 is melted and poured into the second mold 800 (the material of the second nonlinear segment 420 is poured above the first nonlinear segment 410 using gravity). After annealing, it is gradually cooled to room temperature, and a mechanical pump can be used to continuously maintain the vacuum inside the mold. After molding, the second nonlinear segment 420 is formed. After the material of the third nonlinear segment 430 is melted, it is poured into the second mold 800 (using gravity, the material of the second nonlinear segment 420 is poured above the second nonlinear segment 420). After annealing, it is gradually cooled to room temperature. During this period, the vacuum state inside the mold can be maintained continuously by a mechanical pump. After molding and demolding, the third nonlinear segment 430 is formed. The resulting structure is flipped over and the first two steps are repeated to form the second nonlinear segment 420 and the third nonlinear segment 430 at the other end of the first nonlinear segment 410, thus forming a symmetrical nonlinear layer 400.

[0083] The temperature of the preheated mold is 160℃-180℃, for example, 160℃, 170℃ or 180℃.

[0084] In some embodiments, forming the second polypropylene layer 500 includes: placing the sample obtained after forming the nonlinear layer 400 in a preheated mold; injecting molten polypropylene material into the mold, annealing, and cooling to form the second polypropylene layer 500.

[0085] The temperature of the preheated mold is 140℃-180℃, for example, 140℃, 145℃, 150℃, 155℃, 160℃, 170℃ or 180℃.

[0086] The preheating mold can be equipped with an air extraction port, through which a vacuum is drawn inside the mold to maintain the vacuum state inside the mold. This can effectively remove gases, volatiles and moisture from inside the polymer melt, prevent the formation of microbubbles, voids or delamination defects in the product, thereby improving the density and flatness of the structure and enhancing the interfacial bonding.

[0087] In some embodiments, conductor 100 is typically a conductive element made of a metallic material, preferably aluminum, copper, or other alloys, including a metal rod. The DC resistance, current carrying capacity, and dimensions of the rod meet the corresponding application requirements. Conductor 100 is typically cylindrical.

[0088] In some embodiments, the grounding layer 300 comprises a low-resistivity semiconductor tape or copper mesh. This invention does not specifically limit the grounding layer; commonly used grounding materials in the art can be used. In high-voltage bushings, the conductive core rod passes through the grounding flange, forming a "point effect region" with an extremely non-uniform electric field. The grounding layer, by introducing a grounded electrode structure, effectively expands the grounding equipotential surface, mitigates flange electric field distortion, and provides a device detection port. The length of the grounding layer 300 is less than the length of the nonlinear layer 400, and the length of the nonlinear layer 400 is less than the length of the first polypropylene layer 200 and the second polypropylene layer 500. The grounding layer 300 is disposed in the middle of the nonlinear layer 400.

[0089] In some embodiments, the prepared high-insulation thermoplastic sleeve can be placed in an insulating sleeve made of glass fiber or ceramic, and the gap between the sleeve and the sleeve can be filled with SF6 or insulating paste. Finally, the flange and silicone rubber shed external insulation are installed (if a ceramic sleeve is used, it comes with its own external insulating shed).

[0090] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0091] Example 1: Preparation of polypropylene graft containing anhydride groups The basic copolymer polypropylene powder selected has the following characteristics: ethylene content of 18.1 mol%, xylene-soluble content of 48.7 wt%, and weight-average molecular weight of 34.3 × 10⁻⁶. 4 The basic copolymer polypropylene powder, at 230℃ and a load of 2.16 kg, has a melt flow rate (MFR) of 1.21 g / 10 min, a melting temperature (Tm) of 143.4℃, a breakdown electric field (90℃) of 236 kV / mm, and a DC volume resistivity (90℃, 15 kV / mm) of 1.16E13 Ω·m. Fine powder smaller than 40 mesh is removed by sieving. 2.0 kg of the above-mentioned basic copolymer polypropylene powder is weighed and added to a 10 L reactor equipped with a mechanical stirrer. The reaction system is sealed, and nitrogen is used for purging to remove oxygen. A solution of 1.3 g benzoyl peroxide, 10 g maleic anhydride, and 40 g styrene is added, stirred and mixed for 30 min, swelled at 40℃ for 2 hours, then heated to 90℃ and reacted for 4 hours. After the reaction, nitrogen is used for purging and cooling to obtain polypropylene-g-styrene / maleic anhydride material. Among them, the content of structural units derived from maleic anhydride monomers and alkenyl polymer monomers in the grafted state in the polypropylene graft containing anhydride groups is denoted as M1, M1 is 1.27%, and the grafting efficiency GE is 52%.

[0092] The test method for copolymer polypropylene in Example 1 is as follows: 1. Determination of copolymer content in copolymer polypropylene: According to the method specified in industry standard SH / T 1800-2016 "Analysis of Monomer Content and Sequence Structure of Ethylene-Propylene Copolymer Polypropylene by Carbon-13 Nuclear Magnetic Resonance Spectroscopy", a Bruker AVANCE III 400MHz NMR spectrometer with a 10 mm probe and deuterated o-dichlorobenzene as the solvent was used. Approximately 200 mg of sample / 2.5 ml of solvent was used, and the sample tube was heated in an oil bath at 130–140 °C until the sample dissolved to form a homogeneous solution. The test conditions were: probe temperature 125 °C, 90° pulse, sampling time AQ of 5 seconds, and delay time D1 of 10 seconds.

[0093] 2. Determination of xylene-soluble content in copolymer polypropylene: The tests were performed using a Polymer CharCrystal CRYST-EX instrument. Trichlorobenzene solvent was used; the solution was heated to 150 °C, held at that temperature for 90 min, and then sampled for testing. The solution was then cooled to 35 °C, held at that temperature for 70 min, and then sampled for testing again.

[0094] 3. Determination of the weight-average molecular weight of copolymer polypropylene: High-temperature GPC was used for determination using a Polymer Laboratory PL-GPC 220 gel permeation chromatography system. The sample was dissolved in 1,2,4-trichlorobenzene at a concentration of 1.0 mg / ml. The test temperature was 150 °C, and the flow rate was 1.0 ml / min. A standard curve was established using the molecular weight of polystyrene as an internal reference, and the molecular weight and molecular weight distribution of the sample were calculated based on the elution time.

[0095] 4. Determination of Melt Flow Rate (MFR): The determination was performed using a CEAST 7026 melt flow indexer at 230 °C and a 2.16 kg load, according to the method specified in GB / T 3682-2018.

[0096] 5. Determination of melting temperature Tm: Differential scanning calorimetry (DSC) was used to analyze the melting and crystallization processes of the material. Specifically, under nitrogen protection, 5–10 mg of sample was heated from 20 °C to 200 °C using a three-stage temperature rise and fall measurement method. The change in heat flow reflected the melting and crystallization processes, and the melting temperature Tm was calculated accordingly.

[0097] 6. Determination of grafting efficiency GE and parameter M1: Place 2-4g of the grafted product into a Soxhlet extractor and extract with ethyl acetate for 24 hours to remove unreacted monomers and their homopolymers, obtaining pure grafted product. Dry and weigh the product, and calculate the parameters M1 and grafting efficiency GE.

[0098] The formulas for calculating M1 and GE are as follows: M1=

[0099]

[0100] In the above formula, w0 is the mass of the PP matrix; w1 is the mass of the grafted product before extraction; w2 is the mass of the grafted product after extraction; and w3 is the total mass of the added maleic anhydride monomer and alkenyl-containing polymeric monomer.

[0101] Preparation of shielding layer materials Example 2-1 A shielding layer material based on polypropylene graft containing anhydride groups was prepared. This shielding material was purchased from Zhejiang Wanma Co., Ltd., model number: PSD_WMP-00012.

[0102] Example 2-2 Prepare a polypropylene-based shielding layer material, which was purchased from Wanma Polymer Materials Co., Ltd., model number: WMP-1206_S.

[0103] Materials for fabricating nonlinear layers (hereinafter referred to as nonlinear materials) Example 3-1: Preparation of polypropylene graft-based nonlinear materials (filler doping concentration is 25 vol%) The equipment used in this paper according to the preparation process is a torque rheometer (internal mixer). The torque rheometer was provided by Harbin Hap Electric Technology Co., Ltd., model RM-200C, with a power output speed of 0.1~200rpm, a maximum temperature of 350℃, a maximum torque of 150 N·m, and a volume capacity of 50ml.

[0104] (1) Weighing: The ratio used in this embodiment is 25 vol% zinc oxide pressure-sensitive microspheres (purchased from Xi'an Daoshun Smart Nonmetallic Materials Co., Ltd.) and 75 vol% polypropylene graft containing anhydride groups prepared in Example 1. The torque rheometer (internal mixer) has a volume capacity of 50 mL. The mass of each component is calculated according to the internal mixer volume (50 mL) * volume fraction * density and weighed (taking zinc oxide pressure-sensitive microspheres as an example, its mass is equal to the internal mixer volume (50 mL) * 25 vol% * density); the density of the polypropylene graft containing anhydride groups is approximately 0.91 g / cm³. 3 The density of the zinc oxide pressure-sensitive microspheres is 5.01 g / cm³. 3 ; (2) Cleaning: The internal mixer was cleaned with the polypropylene graft containing anhydride groups prepared in Example 1. The internal mixer temperature was set to 40°C and the speed was set to 60 r / min. The mixer was stirred for 15 minutes to remove other impurities remaining in the internal mixer. (3) Mixing: Under the conditions of 120℃ and 5 r / min in the internal mixer, the polypropylene graft containing anhydride groups and zinc oxide pressure-sensitive microspheres weighed in step (1) are injected into the internal mixer in batches and crosswise. Then, the speed is gradually increased to 60 r / min. After the speed is stabilized, the mixture is stirred for 30 min to obtain polypropylene graft-based nonlinear material granules.

[0105] Example 3-2: Preparation of polypropylene graft-based nonlinear materials (filler doping concentration is 30 vol%) The nonlinear material was prepared according to the method described in Example 3-1, except that the ratio used in this example was 30 vol% zinc oxide pressure-sensitive microspheres (purchased from Xi'an Daoshun Smart Nonmetallic Materials Co., Ltd.) and 70 vol% polypropylene graft containing anhydride groups prepared in Example 1.

[0106] Example 3-3: Preparation of polypropylene graft-based nonlinear materials (filler doping concentration of 35 vol%) The nonlinear material was prepared according to the method described in Example 3-1, except that the ratio used in this example was 35 vol% zinc oxide pressure-sensitive microspheres (purchased from Xi'an Daoshun Smart Nonmetallic Materials Co., Ltd.) and 65 vol% polypropylene graft containing anhydride groups prepared in Example 1.

[0107] Examples 3-4: Preparation of polypropylene graft-based nonlinear materials (filler doping concentration of 20 vol%) The nonlinear material was prepared according to the method described in Example 3-1, except that the ratio used in this example was 20 vol% zinc oxide pressure-sensitive microspheres (purchased from Xi'an Daoshun Smart Nonmetallic Materials Co., Ltd.) and 80 vol% polypropylene graft containing anhydride groups prepared in Example 1.

[0108] Examples 3-5: Preparation of polypropylene graft-based nonlinear materials (filler doping concentration of 50 vol%) The nonlinear material was prepared according to the method described in Example 3-1, except that the ratio used in this example was 50 vol% zinc oxide pressure-sensitive microspheres (purchased from Xi'an Daoshun Smart Nonmetallic Materials Co., Ltd.) and 50 vol% polypropylene graft containing anhydride groups prepared in Example 1.

[0109] Examples 3-6: Preparation of polypropylene-based nonlinear materials (filler doping concentration of 25 vol%) The nonlinear material was prepared according to the method described in Example 3-1, except that polypropylene (the same polypropylene used in Example 2-2) was used instead of the polypropylene graft containing anhydride groups. The density of the polypropylene was 900 kg / m³. 3 .

[0110] Examples 3-7: Preparation of polypropylene-based nonlinear materials (filler doping concentration of 30 vol%) Nonlinear materials were prepared according to the method described in Examples 3-2, except that polypropylene (the same polypropylene used in Examples 2-2) was used instead of the polypropylene graft containing anhydride groups. The density of the polypropylene was 900 kg / m³. 3 .

[0111] Examples 3-8: Preparation of polypropylene-based nonlinear materials (filler doping concentration of 35 vol%) Nonlinear materials were prepared according to the method described in Examples 3-3, except that polypropylene (the same polypropylene used in Examples 2-2) was used instead of the polypropylene graft containing anhydride groups. The density of the polypropylene was 900 kg / m³. 3 .

[0112] Examples 3-9: Preparation of silicone rubber-based nonlinear materials (filler doping concentration of 25 vol%) The nonlinear material was prepared according to the method described in Example 3-1, except that silicone rubber (model: Wacker R230, purchased from Wacker Chemicals) was used instead of the polypropylene graft containing anhydride groups. The density of the silicone rubber was 1.10 g / cm³. 3 .

[0113] Examples 3-10: Preparation of silicone rubber-based nonlinear materials (filler doping concentration of 30 vol%) Nonlinear materials were prepared according to the method described in Examples 3-2, except that silicone rubber (model: Wacker R230, purchased from Wacker Chemicals) was used instead of the polypropylene graft containing anhydride groups. The density of the silicone rubber was 1.10 g / cm³. 3 .

[0114] Examples 3-11: Preparation of silicone rubber-based nonlinear materials (filler doping concentration of 35 vol%) Nonlinear materials were prepared according to the method described in Examples 3-3, except that silicone rubber (model: Wacker R230, purchased from Wacker Chemicals) was used instead of the polypropylene graft containing anhydride groups. The density of the silicone rubber was 1.10 g / cm³. 3 .

[0115] Preparation of sleeve Example 4-1 according to Figure 2 The process flow diagram shown illustrates the fabrication of the sleeve, with the specific steps as follows: (1) Two extruders were configured to heat and plasticize the polypropylene graft containing anhydride groups prepared in Example 1 and the shielding layer material prepared in Example 2-1 to a molten state.

[0116] (2) Two molten material flows are gathered into a co-extrusion composite die head, so that different materials form a layered structure in the die cavity. Then the layered structure is extruded and coated on the copper conductor 100 at the same time. After cooling in a water bath, it is quickly shaped to obtain a shielding layer 600 with a thickness of 0.5 mm and a first polypropylene layer 200 with a thickness of 65 mm.

[0117] (3) A grounding layer 300 and a grounding lead connected to the grounding layer 300 are laid on the surface of the first polypropylene layer 200 (not shown in the figure).

[0118] (4) The structure obtained in step 3 is placed in a first mold 700 preheated at 150°C, and the mold is covered with a grounding layer 300. The polypropylene graft-based nonlinear material (with a filler doping concentration of 35 vol%) prepared in Examples 3-3 is melted and injected into the first mold 700. After annealing, it is gradually cooled to room temperature, and a vacuum state inside the mold is continuously maintained by a mechanical pump. After molding and demolding, a first nonlinear segment 410 with a thickness of 4 mm is formed.

[0119] Next, the obtained structure was placed in a second mold 800 preheated to 150°C, with the mold covering the first nonlinear segment 410. The polypropylene graft-based nonlinear material (with a filler doping concentration of 30 vol%) prepared in Examples 3-2 was melted and injected into the mold (the nonlinear material was poured above the first nonlinear segment 410 using gravity). After annealing, the material was gradually cooled to room temperature, while a mechanical pump continuously maintained a vacuum inside the mold. After molding, a second nonlinear segment 420 with a thickness of 4 mm was formed.

[0120] Then, the polypropylene graft-based nonlinear material (with a filler doping concentration of 25 vol%) prepared in Example 3-1 was melted and injected into the second mold 800 (the nonlinear material was poured above the second nonlinear segment 420 using gravity). After annealing, it was gradually cooled to room temperature, and a vacuum state inside the mold was continuously maintained by a mechanical pump. After molding and demolding, a third nonlinear segment 430 with a thickness of 4 mm was formed.

[0121] Next, the resulting structure is flipped over and the first two steps are repeated to form a second nonlinear segment 420 with a thickness of 4 mm and a third nonlinear segment 430 with a thickness of 4 mm at the other end of the first nonlinear segment 410.

[0122] (5) Place the structure obtained in step 4 into a preheated mold at 150°C. The mold size is just large enough to hold the sleeve core. Melt the polypropylene graft containing anhydride groups prepared in Example 1 and inject it into the mold. After annealing, gradually cool it to room temperature. During this period, a mechanical pump is used to maintain a vacuum inside the mold. After molding and demolding, a second polypropylene layer 500 with a thickness of 15 mm is formed (the ratio of the total thickness of the first polypropylene layer 200 and the second polypropylene layer 500 to the thickness of the nonlinear layer 400 is 20:1). Thus, the polypropylene sleeve core is obtained. Its cross-sectional view at the grounding layer is shown in the figure. Figure 3 As shown, the length of the grounding layer 300 is less than the length of the nonlinear layer 400, and the length of the nonlinear layer 400 is less than the length of the first polypropylene layer 200 and the second polypropylene layer 500.

[0123] (6) Place the bushing core into the insulating sleeve made of glass fiber, and fill the gap between the core and the sleeve with SF6. Finally, complete the installation of the flange and the outer insulation of the silicone rubber shed, thus completing the preparation of the bushing.

[0124] Example 4-2 The sleeve was prepared according to the method in Example 4-1, except that... In step 4, the first nonlinear segment 410 is prepared using the polypropylene graft-based nonlinear material prepared in Example 3-3 (with a filler doping concentration of 35 vol%); the second nonlinear segment 420 is prepared using the polypropylene graft-based nonlinear material prepared in Example 3-1 (with a filler doping concentration of 25 vol%); and the third nonlinear segment 430 is prepared using the polypropylene graft-based nonlinear material prepared in Example 3-4 (with a filler doping concentration of 20 vol%).

[0125] Example 4-3 The sleeve was prepared according to the method in Example 4-1, except that... In step 4, the first nonlinear segment 410 is prepared using the polypropylene graft-based nonlinear material prepared in Examples 3-5 (with a filler doping concentration of 50 vol%); the second nonlinear segment 420 is prepared using the polypropylene graft-based nonlinear material prepared in Examples 3-3 (with a filler doping concentration of 35 vol%); and the third nonlinear segment 430 is prepared using the polypropylene graft-based nonlinear material prepared in Examples 3-4 (with a filler doping concentration of 20 vol%).

[0126] Example 4-4 The sleeve was prepared according to the method in Example 4-1, except that... In step 1, the polypropylene (the same polypropylene used in Example 2-2) and the shielding layer material prepared in Example 2-2 are heated and plasticized to a molten state; In step 4, the first nonlinear segment 410 is prepared using the polypropylene graft-based nonlinear material prepared in Examples 3-8 (with a filler doping concentration of 35 vol%); the second nonlinear segment 420 is prepared using the polypropylene graft-based nonlinear material prepared in Examples 3-7 (with a filler doping concentration of 30 vol%); and the third nonlinear segment 430 is prepared using the polypropylene graft-based nonlinear material prepared in Examples 3-6 (with a filler doping concentration of 25 vol%).

[0127] In step 5, polypropylene (the same polypropylene used in Example 2-2) is melted and injected into a preheated mold at a temperature of 180°C.

[0128] Examples 4-5 The sleeve is prepared according to the method of Example 4-1, except that a shielding layer 600 with a thickness of 0.5 mm, a first polypropylene layer 200 with a thickness of 66.95 mm, a nonlinear layer 400 with a thickness of 2.05 mm, and a second polypropylene layer 500 with a thickness of 15 mm are formed, wherein the ratio of the total thickness of the first polypropylene layer 200 and the second polypropylene layer 500 to the thickness of the nonlinear layer 400 is 40:1.

[0129] Examples 4-6 The sleeve is prepared according to the method of Example 4-1, except that a shielding layer 600 with a thickness of 0.5 mm, a first polypropylene layer 200 with a thickness of 66.29 mm, a nonlinear layer 400 with a thickness of 2.71 mm, and a second polypropylene layer 500 with a thickness of 15 mm are formed, wherein the ratio of the total thickness of the first polypropylene layer 200 and the second polypropylene layer 500 to the thickness of the nonlinear layer 400 is 30:1.

[0130] Examples 4-7 The sleeve is prepared according to the method of Example 4-1, except that a shielding layer 600 with a thickness of 0.5 mm, a first polypropylene layer 200 with a thickness of 61.36 mm, a nonlinear layer 400 with a thickness of 7.64 mm, and a second polypropylene layer 500 with a thickness of 15 mm are formed, wherein the ratio of the total thickness of the first polypropylene layer 200 and the second polypropylene layer 500 to the thickness of the nonlinear layer 400 is 10:1.

[0131] Comparative Example 1 The sleeve was prepared according to the method described in Example 4-1, except that, In step 4 of this comparative example, only the first nonlinear segment 410 is formed, and the length of the first nonlinear segment 410 is equal to the sum of the lengths of the first nonlinear segment 410, the second nonlinear segment 420, and the third nonlinear segment 430 in Example 4-1.

[0132] Comparative Example 2 The sleeve was prepared according to the method described in Example 4-1, except that, In step 4, the first nonlinear segment 410 is prepared using the silicone rubber-based nonlinear material prepared in Examples 3-11 (with a filler doping concentration of 35 vol%); the second nonlinear segment 420 is prepared using the silicone rubber-based nonlinear material prepared in Examples 3-10 (with a filler doping concentration of 30 vol%); and the third nonlinear segment 430 is prepared using the silicone rubber-based nonlinear material prepared in Examples 3-9 (with a filler doping concentration of 25 vol%).

[0133] Comparative Example 3 The sleeve was prepared according to the method described in Example 4-1, except that step 5 was not performed, and the second polypropylene layer 500 was not formed, leaving the nonlinear layer 400 exposed to the air.

[0134] Comparative Example 4 The sleeve was prepared according to the method described in Example 4-1, except that step 5 was not performed, the second polypropylene layer 500 was not formed, and the nonlinear layer 400 was exposed to the air; and the thickness of the first polypropylene layer 200 was adjusted to 80 mm.

[0135] Casing performance test Power frequency initiation partial discharge voltage test: Power frequency voltage: refers to AC voltage that is the same as the power grid frequency, usually 50 Hz.

[0136] Partial discharge (PD): Partial discharge is a minute electrical breakdown phenomenon that occurs in a dielectric within a non-penetrating defect area (such as an air gap, impurities, or interlayer debonding). Current bushing inspection standards GB / T 4109-2022 (Insulating Bushings with AC Voltage Higher Than 1000 V) and GB / T22674 (Bushings for DC Systems) specify the upper limit of partial discharge for bushings during type / factory and special tests (10 pC for adhesive-impregnated paper / oil-impregnated paper bushings, and 20 pC for special type bushings).

[0137] Partial Discharge Inception Voltage (PDIV): The minimum voltage at which the partial discharge of the test sample first exceeds the limit. In this paper, the limit discharge is set to 10pC.

[0138] The purpose of the test is to verify the insulation strength and reliability of the bushing under power frequency voltage.

[0139] Test method: The bushings prepared in the examples and comparative examples were connected to high voltage, with the outer flange grounded. A power frequency voltage (50 Hz) was applied, with the voltage value increasing from 0 at a rate of 10 kV / min, with each 5 kV interval serving as a node. This voltage was maintained for 1 minute, and the real-time discharge quantity of the bushing was recorded. When the discharge quantity first exceeded 10 pC during the test, the power frequency initiation partial discharge voltage was recorded.

[0140] Table 1

[0141] Results and Discussion: Comparing Example 4-1 with Comparative Example 1, it can be seen that in the bushing of Example 4-1, the nonlinear layer is segmented, and the filler concentration decreases gradually from the middle to both ends. This reduces the difference in conductivity between the nonlinear layer and the polypropylene insulation layer, thereby reducing the phenomenon of electric field concentration at both ends of the nonlinear layer and improving the insulation and operational reliability of the bushing.

[0142] Comparing Example 4-1 with Comparative Example 2, it can be seen that in the sleeve of Example 4-1, the first polypropylene layer material, the second polypropylene layer material, and the matrix material of the nonlinear layer are the same, and there is no interfacial charge accumulation, so there is no problem of insulation failure. However, Comparative Example 2 has the problem of insulation failure.

[0143] Comparing Example 4-1 with Comparative Examples 3 and 4, it can be seen that the exposure of the nonlinear layer surface leads to a deterioration in insulation performance.

[0144] Comparing Example 4-1 and Example 4-3, it can be seen that the filler concentration difference between adjacent nonlinear segments in the nonlinear layer affects the bushing insulation performance.

[0145] Comparing Example 4-1 and Example 4-4, it can be seen that using polypropylene grafts containing anhydride groups as the matrix material for the first polypropylene layer, the second polypropylene layer, and the nonlinear layer results in better insulation performance compared to Example 4-2 (which uses ordinary polypropylene).

[0146] Comparing Examples 4-1 and 4-7, it can be seen that the ratio of the total thickness of the first polypropylene layer and the second polypropylene layer to the thickness of the nonlinear layer affects the insulation withstand voltage performance of the bushing. If this thickness ratio is too small, it will have an adverse effect on the withstand voltage performance.

[0147] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0148] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A process for the production of high performance polypropylene casing, characterized in that, Includes the following steps: A first polypropylene layer is coated onto the surface of the conductor; A grounding layer is laid on the surface of the first polypropylene layer; Multiple nonlinear segments are formed in steps along the length direction on the outer side of the grounding layer. The multiple nonlinear segments are symmetrically arranged and constitute a nonlinear layer. The nonlinear layer is in contact with the grounding layer. The doping concentration of the nonlinear functionalized filler in adjacent nonlinear segments is different, and along the length direction of the nonlinear layer, from the middle to both ends of the nonlinear layer, the doping concentration of the nonlinear functionalized filler in the nonlinear segment decreases in a gradient. A second polypropylene layer is formed to cover the nonlinear layer; The material of the nonlinear layer includes a first polypropylene matrix material and the nonlinear functionalized filler; The materials of the first polypropylene layer, the second polypropylene layer, and the first polypropylene matrix are the same, and all are polypropylene-based materials; The length of the grounding layer is less than the length of the nonlinear layer; The grounding layer is located in the middle of the nonlinear layer.

2. The production method according to claim 1, characterized by, The difference in doping concentration of the nonlinear functionalized filler in adjacent nonlinear segments is 5%-10% by volume.

3. The method of claim 1, wherein, The nonlinear layer comprises 3-9 nonlinear segments.

4. The preparation method according to claim 1, characterized in that, The nonlinear layer includes: one first nonlinear segment, two second nonlinear segments, and two third nonlinear segments arranged along the length direction, wherein the arrangement order of the one first nonlinear segment, two second nonlinear segments, and two third nonlinear segments along the length direction is as follows: third nonlinear segment, second nonlinear segment, first nonlinear segment, second nonlinear segment, and third nonlinear segment.

5. The preparation method according to claim 4, characterized in that, The doping concentration of the nonlinear functionalized filler in the first nonlinear segment is 30%-35% by volume; the doping concentration of the nonlinear functionalized filler in the second nonlinear segment is 25%-30% by volume; and the doping concentration of the nonlinear functionalized filler in the third nonlinear segment is 20%-25% by volume.

6. The preparation method according to claim 1, characterized in that, The polypropylene materials include polypropylene grafts containing anhydride groups; The nonlinear functionalized filler includes at least one of micron-sized silicon carbide powder, nano-sized silicon carbide powder, micron-sized zinc oxide powder, and nano-sized zinc oxide powder.

7. The preparation method according to claim 1, characterized in that, The ratio of the total thickness of the first polypropylene layer and the second polypropylene layer to the thickness of the nonlinear layer is 20:1-50:1; The thickness of the first polypropylene layer is 30mm-100mm; The thickness of the second polypropylene layer is 8mm-15mm; The thickness of the nonlinear layer is 2mm-9mm.

8. The preparation method according to claim 1, characterized in that, Before coating the conductor surface with the first polypropylene layer, the preparation method further includes: coating the conductor surface with a shielding layer; coating the shielding layer surface with the first polypropylene layer; the shielding layer is made of a second polypropylene matrix material and a conductive filler, wherein the second polypropylene matrix material is the same as the material of the first polypropylene layer; The conductive filler includes at least one of conductive carbon black, graphite, graphene, and carbon nanotubes. The thickness of the shielding layer is 0.2mm-1mm.

9. The preparation method according to claim 8, characterized in that, The shielding layer and the first polypropylene layer are coated onto the surface of the conductor using a co-extrusion method.

10. The preparation method according to claim 1, characterized in that, The nonlinear segment was prepared by casting.

11. The preparation method according to claim 1, characterized in that, The process of forming the second polypropylene layer includes: placing the sample obtained after forming the nonlinear layer into a preheated mold; injecting molten polypropylene material into the mold, annealing, and cooling to form the second polypropylene layer; The temperature of the preheated mold is 140℃-180℃.

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