Highly insulating thermoplastic sleeve and method for producing the same based on a co-extrusion process

Abstract

This invention relates to the field of high-voltage electrical equipment technology, specifically disclosing a high-insulation-performance thermoplastic bushing and a method for preparing it based on a co-extrusion process. The invention wraps an insulating material around the surface of a nonlinear layer to prevent corona discharge. Furthermore, it uses the same thermoplastic material as both the insulating layer (including a first thermoplastic layer and a second thermoplastic layer) and the matrix material of the nonlinear layer, eliminating the obvious interface between them and preventing charge accumulation. This avoids insulation failure, improves the bushing's insulation withstand voltage performance, and enhances operational reliability.

Description

Technical Field

[0001] This invention relates to the field of high-voltage electrical equipment technology, specifically to a high-insulation thermoplastic bushing and a method for preparing it based on a co-extrusion process. Background Technology

[0002] High-voltage direct current (HVDC) bushings, as key insulation equipment in converter stations, provide crucial 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 conduit, an insulating core, a coaxial flange grounding conductor, and external insulating skirts. 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. Summary of the Invention

[0003] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, one objective of this invention is to provide a high-insulation thermoplastic bushing that effectively solves the problem of insulation failure, improves the bushing's insulation withstand voltage performance, and enhances operational reliability.

[0004] Specifically, the first aspect of the present invention provides a high-insulation thermoplastic sleeve, comprising: a conductor, and a first thermoplastic layer, a nonlinear layer, a grounding layer and a second thermoplastic layer disposed on the conductor;

[0005] The first thermoplastic layer is in contact with the surface of the conductor;

[0006] The nonlinear layer and the ground layer are disposed between and surrounded by the first thermoplastic layer and the second thermoplastic layer, and the nonlinear layer is in contact with the ground layer;

[0007] The material of the first thermoplastic layer includes a first thermoplastic material, the material of the second thermoplastic layer includes a second thermoplastic material, and the material of the nonlinear layer includes a first thermoplastic matrix material and a nonlinear functionalized filler; the first thermoplastic material, the second thermoplastic material and the first thermoplastic matrix material are the same.

[0008] In related technologies, the surface of the nonlinear layer in bushings is either not covered by insulating material or is not completely covered by insulating material, leading to corona discharge on the surface of the nonlinear layer. This causes the bushing's insulation performance to deteriorate after prolonged operation, and may even result in insulation failure. Furthermore, in existing bushings, the insulating layer and the nonlinear layer are made of different materials. The insulating layer typically uses epoxy resin (impregnated paper) or oil-impregnated paper composite material as the insulating material, while the nonlinear layer typically uses silicone rubber or similar materials as the matrix material. These materials have significantly different dielectric constants and conductivity, resulting in a distinct interface between the insulating and nonlinear layers. Over time, charge accumulates at this interface, causing changes and distortions in the internal electric field, ultimately leading to insulation failure. To address these problems, this invention wraps the surface of the nonlinear layer with insulating material, thus preventing corona discharge. Simultaneously, it uses the same thermoplastic material as both the insulating layer (including the first and second thermoplastic layers) and the matrix material of the nonlinear layer, eliminating the distinct interface between the insulating and nonlinear layers, preventing charge accumulation, avoiding insulation failure, improving the bushing's insulation withstand voltage performance, and enhancing operational reliability.

[0009] According to some embodiments of the present invention, the first thermoplastic material, the second thermoplastic material, and the first thermoplastic matrix material are all polypropylene materials, polyethylene materials, polyethylene terephthalate, or thermoplastic polyurethane, preferably polypropylene materials; preferably, the polypropylene materials include polypropylene grafts 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 thermoplastic layer and the second thermoplastic layer to the thickness of the nonlinear layer is 20:1-80:1.

[0011] According to some embodiments of the present invention, the thickness of the first thermoplastic layer is 30mm-100mm; the thickness of the second thermoplastic layer is 8mm-15mm; and the thickness of the nonlinear layer is 2mm-9mm.

[0012] According to some embodiments of the present invention, the high-insulation thermoplastic sleeve further includes: a shielding layer, the shielding layer being sleeved on the conductor and located between the conductor and the first thermoplastic layer; the material of the shielding layer includes a second thermoplastic matrix material and a conductive filler, the second thermoplastic matrix material being the same as the first thermoplastic material; 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.

[0013] According to some embodiments of the present invention, the grounding layer is disposed inside the nonlinear layer; the nonlinear layer is embedded in the first thermoplastic layer; and the second thermoplastic layer covers the surfaces of the first thermoplastic layer and the nonlinear layer.

[0014] According to some embodiments of the present invention, the nonlinear layer is disposed on the surface of the first thermoplastic layer; the ground layer is disposed on the surface of the nonlinear layer; and the second thermoplastic layer covers the surfaces of the first thermoplastic layer, the nonlinear layer, and the ground layer.

[0015] A second aspect of the present invention provides a method for preparing a high-insulation thermoplastic sleeve based on a co-extrusion process, comprising the following steps:

[0016] A first thermoplastic layer and a nonlinear layer are coated on the surface of a conductor using a co-extrusion method, such that the first thermoplastic layer is formed between the conductor and the nonlinear layer; wherein, the material of the first thermoplastic layer includes a first thermoplastic material, and the material of the nonlinear layer includes a first thermoplastic matrix material and a nonlinear functionalized filler, wherein the first thermoplastic matrix material is the same as the first thermoplastic material;

[0017] Remove the two ends of the nonlinear layer;

[0018] A grounding layer is laid on the surface of the nonlinear layer;

[0019] A second thermoplastic layer is coated on the surface of the first thermoplastic layer, the nonlinear layer, and the ground layer; wherein the material of the second thermoplastic layer includes a second thermoplastic material, which is the same as the first thermoplastic material.

[0020] Existing dry epoxy resin-based bushings are difficult to process. Epoxy resin-based bushings employ vacuum casting to maintain insulation strength, resulting in extremely long production cycles (over six months) for high-voltage products. Furthermore, 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 thermoplastic material used in this invention has the characteristics of softening upon heating and re-curing upon cooling. Co-extrusion and melt-re-curing methods can be used to prepare the various layers of the bushing (e.g., the shielding layer, the first thermoplastic layer, and the nonlinear layer). Compared to related technologies that use vacuum casting to prepare resin-impregnated paper bushings, the production cycle is significantly shortened.

[0021] According to some embodiments of the present invention, the high-insulation thermoplastic sleeve further includes a shielding layer, and the method includes: coating the shielding layer, the first thermoplastic layer and the nonlinear layer on the surface of the conductor by co-extrusion, such that the shielding layer is formed between the conductor and the first thermoplastic layer and the first thermoplastic layer is formed between the shielding layer and the nonlinear layer.

[0022] According to some embodiments of the present invention, coating the surface of the first thermoplastic layer, the nonlinear layer and the grounding layer with a second thermoplastic layer includes: placing a sample obtained after applying the grounding layer in a preheated mold; injecting molten second thermoplastic material into the mold, annealing, and cooling to form the second thermoplastic layer; the temperature of the preheated mold is 140°C-180°C.

[0023] 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

[0024] 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:

[0025] Figure 1 This is a partial axial cross-sectional schematic diagram of an exemplary high-insulation thermoplastic sleeve of the present invention.

[0026] Figure 2 This is a partial cross-sectional view of another exemplary high-insulation thermoplastic sleeve of the present invention in the axial direction.

[0027] Figure 3 yes Figure 2 A schematic diagram of the cross-section of a high-insulation thermoplastic bushing at the grounding layer.

[0028] Figure 4 This is a partial cross-sectional view of another exemplary high-insulation thermoplastic sleeve of the present invention in the axial direction.

[0029] Figure 5 This is a partial cross-sectional view of another exemplary high-insulation thermoplastic sleeve of the present invention in the axial direction.

[0030] Figure 6 This is an exemplary process flow diagram of the preparation method of the high-insulation thermoplastic sleeve of the present invention.

[0031] Figure 7 This is another exemplary process flow diagram of the method for preparing the high-insulation thermoplastic sleeve of the present invention.

[0032] Figure label:

[0033] 1000, High-insulation thermoplastic sleeve; 100, Conductor; 200, First thermoplastic layer; 300, Nonlinear layer; 400, Grounding layer; 500, Second thermoplastic layer; 600, Shielding layer. Detailed Implementation

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] High-voltage direct current (HVDC) bushings, as key insulation equipment in converter stations, provide crucial 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 conduit, an insulating core, a coaxial flange grounding conductor, and external insulating skirts. 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.

[0039] Research has found that in bushings using related technologies, the surface of the nonlinear layer is not covered by insulating material or is not completely covered by insulating material, leading to corona discharge on the surface of the nonlinear layer. After long-term operation, the insulation performance of the bushing deteriorates, and even insulation failure occurs. In addition, the materials of the insulating layer and the nonlinear layer in existing bushings 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 other materials 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 on this interface, causing changes and distortions in the internal electric field, which in turn leads to insulation failure.

[0040] To address the aforementioned issues, this invention encapsulates the surface of the nonlinear layer with an insulating material to prevent corona discharge. Furthermore, it employs the same thermoplastic material as both the insulating layer (comprising a first thermoplastic layer and a second thermoplastic layer) and the matrix material of the nonlinear layer, eliminating the need for a distinct interface between them and preventing charge accumulation. This avoids insulation failure, improves the bushing's insulation withstand voltage performance, and enhances operational reliability.

[0041] Specifically, refer to Figure 1 The first aspect of the present invention provides a high-insulation thermoplastic sleeve 1000, comprising: a conductor 100, and a first thermoplastic layer 200, a nonlinear layer 300, a grounding layer 400 and a second thermoplastic layer 500 disposed on the conductor 100.

[0042] The first thermoplastic layer 200 is in contact with the surface of the conductor 100;

[0043] The nonlinear layer 300 and the ground layer 400 are disposed between and surrounded by the first thermoplastic layer 200 and the second thermoplastic layer 500, and the nonlinear layer 300 is in contact with the ground layer 400.

[0044] The material of the first thermoplastic layer 200 includes a first thermoplastic material, the material of the second thermoplastic layer 500 includes a second thermoplastic material, and the material of the nonlinear layer 300 includes a first thermoplastic matrix material and a nonlinear functionalized filler; the first thermoplastic material, the second thermoplastic material and the first thermoplastic matrix material are the same.

[0045] In this invention, the first thermoplastic layer 200, the nonlinear layer 300, the grounding layer 400, and the second thermoplastic layer 500 are all sleeved on the conductor 100.

[0046] When the materials of the first thermoplastic layer 200, the second thermoplastic layer 500, and the matrix material of the nonlinear layer 300 of the present invention are the same, the following advantages can be brought about: 1. During operation, the bushing will bear a certain load current, resulting in a temperature gradient inside the bushing. The part closer to the conductor has a higher temperature, and the part farther from the conductor has a lower temperature. If different thermoplastic materials are used for each layer, the coefficient of expansion will be different, resulting in different degrees of expansion. Therefore, thermal stress will be generated, which will lead to a deterioration in the mechanical properties of the bushing. However, since the present invention uses the same thermoplastic material, the mechanical strength and reliability of the bushing are improved; 2. Using the same thermoplastic material can also ensure that the layers are tightly bonded, thereby improving the mechanical strength and reliability of the bushing.

[0047] In some embodiments, the first thermoplastic material, the second thermoplastic material, and the first thermoplastic matrix material are all polypropylene, polyethylene, polyethylene terephthalate, or thermoplastic polyurethane, preferably polypropylene. The breakdown field strength of polypropylene materials can easily reach over 210 kV / mm. As a specific example, the polypropylene material includes polypropylene grafts containing anhydride groups. Its breakdown field strength at 90°C can reach 210 kV / mm-800 kV / mm. These materials still possess strong insulation properties 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 approximately 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 dimensions, improving operational reliability. Because the bushing of the present 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.

[0048] 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).

[0049] 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.

[0050] The alkenyl-containing polymeric monomer is selected from at least one monomer having the structure shown in Formula I.

[0051] Formula I

[0052] 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-C 20 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, substituted or unsubstituted C3-C 12The 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;

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] More preferably, the grafting site is initiated by a peroxide-based free radical initiator and the grafting reaction is further carried out.

[0058] 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%.

[0059] 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%.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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%.

[0064] 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.

[0065] In some embodiments, the preparation method of the polypropylene graft containing anhydride groups can be selected from one of the following methods:

[0066] Method 1, the preparation method includes the following steps:

[0067] a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas;

[0068] b. Add the free radical initiator, maleic anhydride monomer, and alkenyl-containing polymerizable monomer to the closed reactor and stir to mix;

[0069] c. Optionally add an interfacial agent and optionally swell the reaction system;

[0070] d. Optionally add a dispersant, raise the temperature of the reaction system to the grafting reaction temperature, and carry out the grafting reaction;

[0071] 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.

[0072] More specifically, the preparation method includes the following steps:

[0073] a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas;

[0074] 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;

[0075] c. Add 0-30 parts of interface agent, and optionally allow the reaction system to swell at 20-60°C for 0-24 hours;

[0076] 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;

[0077] 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.

[0078] Method 2, the preparation method includes the following steps:

[0079] a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas;

[0080] b. Mix the organic solvent and the free radical initiator, and add the mixture to the closed reactor;

[0081] c. Remove the organic solvent;

[0082] d. Add maleic anhydride monomer and alkenyl-containing polymeric monomer, optionally add an interfacial agent, and optionally swell the reaction system;

[0083] e. Optionally add a dispersant, heat the reaction system to the grafting reaction temperature, and carry out the grafting reaction;

[0084] 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.

[0085] More specifically, the preparation method includes the following steps:

[0086] a. Place the copolymerized polypropylene in a closed reactor and replace it with an inert gas;

[0087] b. Mix the organic solvent and free radical initiator to prepare a solution, and add it to a closed reactor containing copolymer polypropylene;

[0088] c. The organic solvent is removed by purging with an inert gas or by vacuum.

[0089] 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;

[0090] 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;

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] In some embodiments, the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 20:1 to 80:1. The first thermoplastic layer 200 and the second thermoplastic layer 500 together constitute the insulating layer of the bushing. The second thermoplastic layer 500 encloses the nonlinear layer 300 internally to prevent corona discharge and a decrease in surface insulation strength caused by the nonlinear layer 300 being exposed externally. Changing the thickness ratio of the insulating layer to the nonlinear layer 300 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.

[0096] In this paper, the first thermoplastic layer 200, the second thermoplastic layer 500, the nonlinear layer 300, and the grounding layer 400 each have a cylindrical structure. The cylindrical structure is symmetrical about the conductor 100 as a central axis. For example, the first thermoplastic layer 200, the second thermoplastic layer 500, and the nonlinear layer 300 are all cylindrical. In this paper, the thickness of each layer refers to the wall thickness of the cylindrical structure, such as a cylinder, i.e., the radial distance from the outer wall to the inner wall.

[0097] In some specific embodiments, the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 20:1, 30:1, 40:1, 50:1, 60:1, 70:1 or 80:1.

[0098] In some embodiments, the thickness of the first thermoplastic layer 200 is 30mm-100mm, for example, 30mm, 40mm, 50mm, 60mm, 70mm, 80mm, 90mm, or 100mm. The thickness of the second thermoplastic layer 500 is 8mm-15mm, for example, 8mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm, or 15mm. The thickness of the first thermoplastic layer 200 can be selected according to the voltage rating of the sleeve application. The thickness of the second thermoplastic layer 500 should not be too thick to avoid making the equipment bulky and increasing production costs.

[0099] In some embodiments, the thickness of the nonlinear layer 300 is 2mm-9mm, for example, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, or 9mm. Optimizing the thickness of the nonlinear layer 300, within the scope of this invention, is beneficial for better homogenization of 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 edge effects in the electric field distribution, resulting in 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 concentrated electric field lines 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 employs 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.

[0100] The nonlinear layer 300 of the present invention comprises a first thermoplastic 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.

[0101] The material of the nonlinear layer 300 of the present invention can be prepared by uniformly mixing a nonlinear functionalized filler with a first thermoplastic 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 thermoplastic 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 thermoplastic matrix material to remove residual impurities. During cleaning, the temperature of the internal mixer is 30℃-50℃ (e.g., 30℃, 35℃, 40℃, 45℃ followed by 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 thermoplastic matrix material can be (20-30):(70-80), e.g., 20:80, 25:75, or 30:70.

[0102] In some embodiments, reference Figure 2-3 The high-insulation thermoplastic bushing 1000 further includes a shielding layer 600. The shielding layer 600 is sleeved on the conductor 100 and located between the conductor 100 and the first thermoplastic layer 200. The material of the shielding layer 600 includes a second thermoplastic matrix material and conductive filler. The second thermoplastic matrix material is the same as the first thermoplastic material, thereby avoiding a significant interface between the shielding layer 600 and the first thermoplastic layer 200, thus preventing charge accumulation at the interface, preventing insulation failure, and improving the insulation performance of the bushing. The shielding layer 600 is connected to a conductor, which is grounded, thereby shielding the distorted electric field of the external flange of the bushing.

[0103] 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.

[0104] 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 always cylindrical. In this document, the thickness of the shielding layer 600 refers to the wall thickness of the cylindrical structure, such as a cylinder, i.e., the radial distance from the outer wall to the inner wall.

[0105] In some specific embodiments, the conductive filler includes at least one of conductive carbon black, graphite, graphene, and carbon nanotubes.

[0106] 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.

[0107] In some embodiments, the grounding layer 400 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 400 is less than the length of the nonlinear layer 300, and the length of the nonlinear layer 300 is less than the length of the first thermoplastic layer 200 and the second thermoplastic layer 500. The grounding layer 400 is disposed in the middle of the nonlinear layer 300. In this document, the length of a layer refers to the length of the layer structure extending along the bushing axial direction.

[0108] In other embodiments, such as Figure 2-3 As shown, the nonlinear layer 300 is sleeved on the surface of the first thermoplastic layer 200; the grounding layer 400 is sleeved on the surface of the nonlinear layer 300; and the second thermoplastic layer 500 covers the surfaces of the first thermoplastic layer 200, the nonlinear layer 300, and the grounding layer 400. The production of the sleeve structure is relatively simple, without an embedding process, and can effectively utilize the radial thickness of the nonlinear layer.

[0109] In some embodiments, such as Figure 4-5 As shown, the grounding layer 400 is disposed inside the nonlinear layer 300; the nonlinear layer 300 is embedded within the first thermoplastic layer 200; and the second thermoplastic layer 500 covers the surfaces of the first thermoplastic layer 200 and the nonlinear layer 300. In the bushing structure, the grounding layer 400 is wrapped by the nonlinear layer 300, resulting in better mechanical strength, increased insulation thickness of the outer polypropylene layer, and better prevention of corona discharge.

[0110] A second aspect of this invention provides a method for preparing a high-insulation thermoplastic sleeve based on a co-extrusion process, with reference to... Figure 6 The method includes the following steps:

[0111] A first thermoplastic layer 200 and a nonlinear layer 300 are coated on the surface of the conductor 100 using a co-extrusion method, such that the first thermoplastic layer 200 is formed between the conductor 100 and the nonlinear layer 300; wherein, the material of the first thermoplastic layer 200 includes a first thermoplastic material, and the material of the nonlinear layer 300 includes a first thermoplastic matrix material and a nonlinear functionalized filler, wherein the first thermoplastic matrix material is the same as the first thermoplastic material;

[0112] Remove the two ends of the nonlinear layer 300;

[0113] A grounding layer 400 is laid on the surface of the nonlinear layer 300;

[0114] A second thermoplastic layer 500 is covered on the surfaces of the first thermoplastic layer 200, the nonlinear layer 300, and the ground layer 400; wherein the material of the second thermoplastic layer 500 includes a second thermoplastic material, which is the same as the first thermoplastic material.

[0115] Existing dry epoxy resin-based bushings are difficult to process. Epoxy resin-based bushings employ vacuum casting to maintain insulation strength, resulting in extremely long production cycles (over six months) for high-voltage products. Furthermore, 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 thermoplastic material used in this invention has the characteristics of softening upon heating and re-curing upon cooling. Co-extrusion and melt-re-curing methods can be used to prepare the various layers of the bushing (e.g., the shielding layer, the first thermoplastic layer, and the nonlinear layer). Compared to related technologies that use vacuum casting to prepare resin-impregnated paper bushings, the production cycle is significantly shortened.

[0116] In some embodiments, the high-insulation thermoplastic sleeve further includes a shielding layer 600, referenced. Figure 7 The method includes: coating a shielding layer 600, a first thermoplastic layer 200 and a nonlinear layer 300 onto the surface of a conductor 100 using a co-extrusion method, such that the shielding layer 600 is formed between the conductor 100 and the first thermoplastic layer 200 and the first thermoplastic layer 200 is formed between the shielding layer 600 and the nonlinear layer 300.

[0117] 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, the first thermoplastic layer 200, and the nonlinear layer 300, significantly shortening the production cycle and simplifying the production process.

[0118] In this article, "removing the two ends of the nonlinear layer" refers to removing the two ends of the nonlinear layer along its length.

[0119] In some embodiments, a mechanical stripping method can be used to remove the two ends of the nonlinear layer, thereby adjusting the nonlinear layer to a predetermined length. The mechanical stripping method includes processes such as turning.

[0120] In some embodiments, laying a grounding layer 400 on the surface of the nonlinear layer 300 includes: covering the outside of the nonlinear layer 300 with a grounding material such as low-resistivity semiconductor tape or copper mesh.

[0121] In some embodiments, covering the surface of the first thermoplastic layer 200, the nonlinear layer 300 and the ground layer 400 with the second thermoplastic layer 500 includes: placing the sample obtained after applying the ground layer 400 in a preheated mold; injecting molten second thermoplastic material into the mold, annealing, and cooling to form the second thermoplastic layer 500.

[0122] The temperature of the preheated mold is 140℃-180℃, for example, 140℃, 145℃, 150℃, 155℃, 160℃, 170℃ or 180℃.

[0123] 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.

[0124] 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).

[0125] In the preparation method provided in the second aspect of the present invention, the description of the material type, thickness, thickness ratio, etc. of each layer can be found in the detailed description in the first aspect of the present invention, and will not be repeated here.

[0126] The third aspect of the present invention also provides, for example Figure 4 The method for preparing the high-insulation thermoplastic sleeve shown includes the following steps:

[0127] A first thermoplastic layer 200 is coated on the surface of the conductor 100, and the material of the first thermoplastic layer 200 includes a first thermoplastic material;

[0128] A groove structure is formed on the surface of the first thermoplastic layer 200;

[0129] A nonlinear material is filled into the tank structure and a grounding layer 400 is embedded in the nonlinear material to form a nonlinear layer 300; wherein, the nonlinear material includes a first thermoplastic matrix material and a nonlinear functionalized filler; the first thermoplastic matrix material is the same as the first thermoplastic material;

[0130] A second thermoplastic layer 500 is coated on the surfaces of the first thermoplastic layer 200 and the nonlinear layer 300. The material of the second thermoplastic layer 500 includes a second thermoplastic material, which is the same as the first thermoplastic material.

[0131] In some embodiments, the preparation method includes the following steps:

[0132] A shielding layer 600 and a first thermoplastic layer 200 are simultaneously formed on the surface of conductor 100 using a co-extrusion method;

[0133] A groove structure is formed on the surface of the first thermoplastic layer 200;

[0134] A portion of the nonlinear material in the form of adhesive tape is laid in the tank structure; after laying a grounding layer 400 on the nonlinear material, another portion of the nonlinear material in the form of adhesive tape is laid in the tank structure; heating is used to melt and bond the nonlinear material to form a nonlinear layer 300.

[0135] A second thermoplastic material in the form of tape is laid on the surface of the first thermoplastic layer 200 and the nonlinear layer 300; the resulting sample is placed in a preheated mold to form the second thermoplastic layer 500.

[0136] In other embodiments, the preparation method includes the following steps:

[0137] A first thermoplastic layer 200 is formed on the surface of conductor 100 by extrusion.

[0138] A groove structure is formed on the surface of the first thermoplastic layer 200;

[0139] A portion of the nonlinear material in the form of adhesive tape is laid in the tank structure; after laying a grounding layer 400 on the nonlinear material, another portion of the nonlinear material in the form of adhesive tape is laid in the tank structure; heating is used to melt and bond the nonlinear material to form a nonlinear layer 300.

[0140] A second thermoplastic material in the form of tape is laid on the surface of the first thermoplastic layer 200 and the nonlinear layer 300; the resulting sample is placed in a preheated mold to form the second thermoplastic layer 500.

[0141] In the preparation method provided in the third aspect of the present invention, the description of the material type, thickness, thickness ratio, etc. of each layer can be found in the detailed description in the first aspect of the present invention, and will not be repeated here.

[0142] 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.

[0143] Example 1: Preparation of polypropylene graft containing anhydride groups

[0144] 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%.

[0145] The test method for copolymer polypropylene in Example 1 is as follows:

[0146] 1. Determination of copolymer content in copolymer polypropylene:

[0147] 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.

[0148] 2. Determination of xylene-soluble content in copolymer polypropylene:

[0149] 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.

[0150] 3. Determination of the weight-average molecular weight of copolymer polypropylene:

[0151] 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.

[0152] 4. Determination of Melt Flow Rate (MFR):

[0153] 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.

[0154] 5. Determination of melting temperature Tm:

[0155] 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.

[0156] 6. Determination of grafting efficiency GE and parameter M1:

[0157] 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.

[0158] The formulas for calculating M1 and GE are as follows:

[0159] M1=

[0160]

[0161] 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.

[0162] Preparation of shielding layer materials

[0163] Example 2-1

[0164] 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.

[0165] Example 2-2

[0166] Prepare a polypropylene-based shielding layer material, which was purchased from Wanma Polymer Materials Co., Ltd., model number: WMP-1206_S.

[0167] Example 2-3

[0168] Prepare a polyethylene-based shielding layer material, which was purchased from Wanma Polymer Materials Co., Ltd., model WMP-1206_S.

[0169] Materials for fabricating nonlinear layers (hereinafter referred to as nonlinear materials)

[0170] Example 3-1: Preparation of polypropylene graft-based nonlinear materials

[0171] 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.

[0172] (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 ;

[0173] (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.

[0174] (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.

[0175] Example 3-2: Preparation of polypropylene-based nonlinear materials

[0176] 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 .

[0177] Example 3-3: Preparation of Polyethylene-based Nonlinear Materials

[0178] The nonlinear material was prepared according to the method described in Example 3-1, except that polyethylene (the same polyethylene used in Examples 2-3) was used instead of the polypropylene graft containing anhydride groups. The density of the polyethylene was 918 ± 1.5 kg / m³. 3 .

[0179] Preparation of polypropylene graft-based nonlinear tape

[0180] The polypropylene graft-based nonlinear material granules obtained in Example 3-1 were melted in an extruder at 180°C and extruded into a thin film sheet. Then, the extruded thin film sheet was rapidly cooled on a cooling roller to form a primary sheet. Subsequently, the primary sheet was subjected to longitudinal stretching (MD) and transverse stretching (TD) in sequence, with a stretching ratio of 4 times, to improve the strength, transparency, and dimensional stability of the sheet. Next, it was heat-treated at 150°C under tension to fix the stretched structure and prevent shrinkage. Finally, the resulting tape was wound into a master roll, which can be subsequently cut into smaller rolls of the required width.

[0181] Preparation of polypropylene grafted tape

[0182] The polypropylene graft particles containing anhydride groups prepared in Example 1 were melted in an extruder at 180°C and extruded into a thin film sheet. Then, the extruded thin film sheet was rapidly cooled on a cooling roller to form a primary sheet. Subsequently, the primary sheet was subjected to longitudinal stretching (MD) and transverse stretching (TD) in sequence, with a stretching ratio of 4 times, to improve the strength, transparency, and dimensional stability of the sheet. Next, it was heat-treated at 150°C under tension to fix the stretched structure and prevent shrinkage. Finally, the resulting tape was wound into a master roll, which can be subsequently cut into smaller rolls of the required width.

[0183] Preparation of sleeve

[0184] Example 4-1

[0185] according to Figure 7 The process flow diagram shown illustrates the fabrication of the sleeve, with the specific steps as follows:

[0186] (1) Three extruders were configured to heat and plasticize the polypropylene graft containing anhydride groups prepared in Example 1, the shielding layer material prepared in Example 2-1, and the polypropylene graft-based nonlinear material prepared in Example 3-1 to a molten state.

[0187] (2) Three 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, a first thermoplastic layer 200 with a thickness of 65 mm, and a nonlinear layer 300 with a thickness of 4 mm.

[0188] (3) Remove the two ends of the nonlinear layer 300 by turning and process it to a predetermined length (1 / 2 of the sleeve length).

[0189] (4) A grounding layer 400 and a grounding lead connected to the grounding layer 400 are laid on the surface of the nonlinear layer 300 (not shown in the figure).

[0190] (5) Place the structure obtained in step 4 into a preheated mold at 150°C. The mold size is just 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, the vacuum state inside the mold is continuously maintained by a mechanical pump. After molding and demolding, a second thermoplastic layer 500 with a thickness of 15 mm is formed (the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 20:1). The thermoplastic sleeve core is thus obtained. The length of the grounding layer 400 is less than the length of the nonlinear layer 300, and the length of the nonlinear layer 300 is less than the length of the first thermoplastic layer 200 and the second thermoplastic layer 500.

[0191] (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.

[0192] Example 4-2

[0193] The sleeve was prepared according to the method in Example 4-1, except that...

[0194] In step 1, the polypropylene (the same polypropylene used in Example 2-2), the shielding layer material prepared in Example 2-2, and the polypropylene-based nonlinear material prepared in Example 3-2 are heated and plasticized to a molten state.

[0195] In step 5, the polypropylene (the same polypropylene used in Example 2-2) is melted and injected into a preheated mold at a temperature of 180°C.

[0196] Example 4-3

[0197] The sleeve was prepared according to the method in Example 4-1, except that...

[0198] In step 1, polyethylene (the same polyethylene used in Examples 2-3), the shielding layer material prepared in Examples 2-3, and the polyethylene-based nonlinear material prepared in Examples 3-3 are heated and plasticized to a molten state;

[0199] In step 5, polyethylene (the same polyethylene used in Examples 2-3) is melted and injected into a preheated mold at a temperature of 160°C.

[0200] Example 4-4

[0201] 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 thermoplastic layer 200 with a thickness of 66.95 mm, a nonlinear layer 300 with a thickness of 2.05 mm, and a second thermoplastic layer 500 with a thickness of 15 mm are formed, wherein the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 40:1.

[0202] Examples 4-5

[0203] 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 thermoplastic layer 200 with a thickness of 66.29 mm, a nonlinear layer 300 with a thickness of 2.71 mm, and a second thermoplastic layer 500 with a thickness of 15 mm are formed, wherein the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 30:1.

[0204] Examples 4-6

[0205] 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 thermoplastic layer 200 with a thickness of 61.36 mm, a nonlinear layer 300 with a thickness of 7.64 mm, and a second thermoplastic layer 500 with a thickness of 15 mm are formed, wherein the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 10:1.

[0206] Examples 4-7

[0207] (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.

[0208] (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 simultaneously extruded and coated on the copper conductor 100. 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 thermoplastic layer 200 with a thickness of 65 mm.

[0209] (3) A groove structure is formed on the surface of the first thermoplastic layer 200 by turning.

[0210] (4) The prepared polypropylene graft-based nonlinear tape is laid in the tank structure and heated to 150°C by an automatic winding machine to melt and bond it into the tank structure, forming a tightly bonded nonlinear layer 300 (thickness of 4 mm). When the tape is laid to half its thickness, the grounding layer 400 (metallic copper mesh with conductivity of 6*10) is laid.6 The S / m (thickness of 0.4 mm) and the grounding lead (not shown in the figure) connected to the grounding layer 400 are buried therein.

[0211] (5) The prepared polypropylene grafted tape is laid on the surface of the first thermoplastic layer 200 and the nonlinear layer 300, and placed in a preheated mold (150°C) so that the mold just holds the sleeve core. During this process, a vacuum is continuously pumped through a mechanical pump to maintain the internal vacuum state. After molding and demolding, a second thermoplastic layer 500 with a thickness of 15 mm is formed (the ratio of the total thickness of the first thermoplastic layer 200 and the second thermoplastic layer 500 to the thickness of the nonlinear layer 300 is 20:1). Thus, the thermoplastic sleeve core is obtained, and its cross-sectional view is shown in the figure. Figure 5 As shown.

[0212] (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.

[0213] Comparative Example 1: Preparation of Silicone Rubber-Based Nonlinear Materials

[0214] 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 .

[0215] Comparative Example 2

[0216] The sleeve was prepared according to the method described in Example 4-1, except that in step 1, the silicone rubber-based nonlinear material prepared in Comparative Example 1 was used instead of the polypropylene graft-based nonlinear material prepared in Example 3-1.

[0217] Comparative Example 3

[0218] The sleeve is prepared according to the method described in Example 4-1, except that step 5 is not performed, the second thermoplastic layer 500 is not formed, and the nonlinear layer 300 and the grounding layer 400 are exposed to the air.

[0219] Comparative Example 4

[0220] The sleeve is prepared according to the method described in Example 4-1, except that step 5 is not performed, the second thermoplastic layer 500 is not formed, and the nonlinear layer 300 and the grounding layer 400 are exposed to the air; and the thickness of the first thermoplastic layer 200 is adjusted to 80 mm.

[0221] Comparative Example 5

[0222] The sleeve is prepared according to the method described in Example 4-1, except that the length of the second thermoplastic layer 500 is shortened so that the length of the second thermoplastic layer 500 is greater than the length of the ground layer 400 but less than the length of the nonlinear layer 300, so that the ground layer 400 is surrounded by the second thermoplastic layer 500, but part of the surface of the nonlinear layer 300 is exposed.

[0223] Casing performance test

[0224] Power frequency initiation partial discharge voltage test:

[0225] Power frequency voltage: refers to AC voltage that is the same as the power grid frequency, usually 50 Hz.

[0226] 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).

[0227] 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.

[0228] The purpose of the test is to verify the insulation strength and reliability of the bushing under power frequency voltage.

[0229] 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.

[0230] Table 1

[0231]

[0232] Results and Discussion:

[0233] Comparing Example 4-1 with Comparative Example 2, it can be seen that in the sleeve of Example 4-1, the materials of the first thermoplastic layer, the second thermoplastic layer, 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.

[0234] Comparing Example 4-1 with Comparative Example 3-5, it can be seen that the exposure of the nonlinear layer surface leads to a deterioration in insulation performance.

[0235] Comparing Examples 4-1 with Examples 4-2 and 4-3, it can be seen that Example 4-1, which uses a polypropylene graft containing anhydride groups as the matrix material for the first thermoplastic layer, the second thermoplastic layer, and the nonlinear layer, exhibits better insulation performance than Examples 4-2 and 4-3. Example 4-2, which uses polypropylene, shows better insulation performance than Example 4-3.

[0236] Comparing Examples 4-1 and 4-6, it can be seen that the ratio of the total thickness of the first thermoplastic layer and the second thermoplastic 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.

[0237] 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.

[0238] 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 thermoplastic bushing having high insulating properties, characterized in that include: A conductor, and a first thermoplastic layer, a nonlinear layer, a grounding layer, and a second thermoplastic layer disposed on the conductor; The first thermoplastic layer is in contact with the surface of the conductor; The nonlinear layer and the ground layer are disposed between and surrounded by the first thermoplastic layer and the second thermoplastic layer, and the nonlinear layer is in contact with the ground layer; The material of the first thermoplastic layer includes a first thermoplastic material, the material of the second thermoplastic layer includes a second thermoplastic material, and the material of the nonlinear layer includes a first thermoplastic matrix material and a nonlinear functionalized filler; the first thermoplastic material, the second thermoplastic material and the first thermoplastic matrix material are the same.

2. The high dielectric performance thermoplastic bushing of claim 1, wherein, The first thermoplastic material, the second thermoplastic material, and the first thermoplastic matrix material are all polypropylene, polyethylene, polyethylene terephthalate, or thermoplastic polyurethane. 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.

3. The high dielectric performance thermoplastic bushing of claim 2, wherein, The first thermoplastic material, the second thermoplastic material, and the first thermoplastic matrix material are all polypropylene materials.

4. The high-insulation performance thermoplastic sleeve of claim 1 or 2, wherein, The ratio of the total thickness of the first thermoplastic layer and the second thermoplastic layer to the thickness of the nonlinear layer is 20:1-80:

1.

5. The high dielectric performance thermoplastic bushing of claim 1 or 2, wherein, The thickness of the first thermoplastic layer is 30mm-100mm; The thickness of the second thermoplastic layer is 8mm-15mm; The thickness of the nonlinear layer is 2mm-9mm.

6. The high dielectric performance thermoplastic bushing of claim 1 or 2, wherein, Also includes: A shielding layer is sleeved on the conductor and located between the conductor and the first thermoplastic layer; The material of the shielding layer includes a second thermoplastic matrix material and a conductive filler, wherein the second thermoplastic matrix material is the same as the first thermoplastic material; 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.

7. A high dielectric performance thermoplastic bushing according to claim 1 or 2, wherein, The grounding layer is disposed inside the nonlinear layer; the nonlinear layer is embedded in the first thermoplastic layer; the second thermoplastic layer covers the surfaces of the first thermoplastic layer and the nonlinear layer.

8. The high-insulation thermoplastic sleeve according to claim 1 or 2, characterized in that, The nonlinear layer is sleeved on the surface of the first thermoplastic layer; the grounding layer is sleeved on the surface of the nonlinear layer; the second thermoplastic layer covers the surfaces of the first thermoplastic layer, the nonlinear layer, and the grounding layer.

9. A method for producing a thermoplastic sleeve with high insulation properties based on a co-extrusion process, characterized in that, Includes the following steps: A first thermoplastic layer and a nonlinear layer are coated on the surface of a conductor using a co-extrusion method, such that the first thermoplastic layer is formed between the conductor and the nonlinear layer; wherein, the material of the first thermoplastic layer includes a first thermoplastic material, and the material of the nonlinear layer includes a first thermoplastic matrix material and a nonlinear functionalized filler, wherein the first thermoplastic matrix material is the same as the first thermoplastic material; Remove the two ends of the nonlinear layer; A grounding layer is laid on the surface of the nonlinear layer; A second thermoplastic layer is coated on the surface of the first thermoplastic layer, the nonlinear layer, and the ground layer; wherein the material of the second thermoplastic layer includes a second thermoplastic material, which is the same as the first thermoplastic material.

10. The method of claim 9, wherein, The high-insulation thermoplastic sleeve further includes a shielding layer, and the method includes: coating the shielding layer, the first thermoplastic layer and the nonlinear layer on the surface of the conductor by co-extrusion, such that the shielding layer is formed between the conductor and the first thermoplastic layer and the first thermoplastic layer is formed between the shielding layer and the nonlinear layer.

11. The method according to claim 9 or 10, characterized in that, The process of coating the surface of the first thermoplastic layer, the nonlinear layer, and the grounding layer with a second thermoplastic layer includes: placing the sample obtained after applying the grounding layer into a preheated mold; injecting molten second thermoplastic material into the mold, annealing, and cooling to form the second thermoplastic layer; The temperature of the preheated mold is 140℃-180℃.

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