Gradient dielectric constant dc bushing core and method of manufacturing the same

Abstract

This invention discloses a gradient dielectric constant DC bushing core and its preparation method. The core includes a central conductive rod and a composite insulator covering it. The composite insulator is formed by winding and curing a continuous composite base tape with multiple dielectric constant partitions along its length. The preparation method involves a segmented sequential impregnation process. First, the entire roll of reinforcing material base tape is impregnated with a resin with the lowest dielectric constant. Then, resins with higher dielectric constants are used to partially impregnate the reinforcing material base tape from low to high. This selectively coats and pre-cures only the base tape segments, thereby forming gradient partitions with varying dielectric constants along the length of a single base tape. After the base tape is wound and cured in the partitioned sequence, an insulating core with radially decreasing dielectric constant is formed. This invention achieves precise gradient control of the dielectric properties of the insulating material, can actively optimize the electric field distribution of the DC bushing, and improve its insulation strength, reliability, and compactness.

Description

Technical Field

[0001] This invention relates to the field of insulation technology for high-voltage electrical equipment, and more specifically, to a gradient dielectric constant DC bushing core and its preparation method. Background Technology

[0002] DC bushings are core insulation devices in critical parts of UHVDC transmission projects, such as the through-wall of the converter valve hall and the valve-side output of the converter transformer. Unlike AC bushings, the electric field distribution of DC bushings under steady-state operation is mainly determined by the conductivity of the material. However, conductivity is extremely sensitive to temperature and electric field strength, which leads to a highly uneven electric field distribution. In particular, dangerous areas of concentrated electric field strength are easily formed near the grounding flange and high-voltage electrode, which are the main causes of insulation failure. At present, epoxy resin impregnated dry bushings are becoming a development trend due to their advantages such as being oil-free, fireproof, and environmentally friendly. To improve the electric field distribution, conventional technical means mainly rely on optimizing the electrode shape (such as using stress cones) or arranging capacitor screens in the insulation. However, these methods mainly optimize the electric field at the edge of the electrode, and have limited optimization of the electric field distribution in the main insulation area inside the insulator, while increasing the structural complexity and manufacturing difficulty.

[0003] In recent years, research has proposed using functionally graded materials (FJTs) to fabricate insulating components. This involves changing the material composition to alter the dielectric or conductivity parameters along a specific direction, thereby actively controlling the electric field. However, existing technologies mostly focus on concepts or laboratory fabrication. There is a lack of practical and feasible process solutions for achieving large-scale, continuous, and repeatable industrial production of such graded materials. For example, if multiple rolls of prepregs with different formulations are prepared separately and then alternately wound, problems such as numerous interlayer interfaces, cumbersome processes, discontinuous gradients, and low production efficiency will arise. Therefore, developing a method for accurately, efficiently, and continuously fabricating large-size graded dielectric constant insulating cores is of great significance for promoting the technological advancement of high-end DC bushings. Summary of the Invention

[0004] The present invention aims to solve the technical problems of uneven electric field distribution in existing DC bushing insulating cores and the difficulty in accurately and continuously preparing large-size gradient dielectric constant insulating cores through industrial methods.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A gradient dielectric constant DC bushing core includes a central conductive rod and a solid cylindrical composite insulator covering the central conductive rod. The composite insulator is made of an insulating composite material, and its material composition changes continuously or in a stepwise manner along the radial direction, such that the equivalent relative dielectric constant of the composite insulator gradually decreases from the inner layer to the outer layer along the radial direction. The composite insulator is formed by winding a continuous composite base strip with multiple dielectric constant partitions along its length. Different dielectric constant partitions on the continuous composite base strip correspond to different positions along the radial direction of the composite insulator after winding. The continuous composite base strip consists of a reinforcing material skeleton and a filling and fixing layer. The resin matrix incorporated in the reinforcing material skeleton and the functional fillers dispersed in the resin matrix together constitute the composite material. The functional fillers include high dielectric constant fillers and low dielectric constant fillers. Along the length direction of the continuous composite base tape, the type and / or content of the functional fillers vary in segments to form the multiple dielectric constant partitions. That is, along the length direction of the continuous composite base tape, after winding, they correspond to different partitions of the composite insulator radially outward. In the different partitions radially outward, the volume fraction of the high dielectric constant filler in the corresponding continuous composite base tape gradually decreases, and / or the volume fraction of the low dielectric constant filler in the corresponding continuous composite base tape gradually increases.

[0007] The core of the gradient dielectric constant DC bushing core described in this invention lies in the fact that the dielectric constant of the solid cylindrical composite insulator covering the central conductive rod gradually decreases from the inside out. This characteristic is achieved through a continuous composite base tape winding structure with multiple dielectric constant partitions along its length. The base tape itself is a continuous composite material tape, and its special feature is that multiple partitions with different dielectric constants are pre-manufactured along its length, and the dielectric constant of the partitions changes gradient along its length. During winding, the partition with the highest dielectric constant is started, and the partitions with gradually decreasing dielectric constants are gradually wound, so that the required gradient distribution is naturally formed in the radial direction of the solid cylindrical composite insulator formed after winding.

[0008] Furthermore, the reinforcing material skeleton is a continuous fiber paper or fiber cloth;

[0009] Furthermore, the resin matrix is ​​a solid polymer phase formed by pre-curing the liquid epoxy resin, phenolic resin or polyester resin adhesive used during impregnation.

[0010] Furthermore, the high dielectric constant filler is selected from at least one nanoparticle selected from barium titanate, barium strontium titanate, or titanium dioxide, and the low dielectric constant filler is selected from at least one nanoparticle selected from silicon dioxide, aluminum oxide, or boron nitride.

[0011] The present invention also provides a method for preparing a gradient dielectric constant DC bushing core as described above, specifically comprising the following steps:

[0012] S1. Provide a continuous baseband based on a reinforced material skeleton;

[0013] S2. Along the length of the baseband, a segmented sequential impregnation process is used to form multiple continuous composite material partitions with different dielectric constants, thereby obtaining a continuous composite baseband. The segmented sequential impregnation process specifically includes:

[0014] Configure N kinds of resin solutions with different dielectric constants, N≥2, each resin solution contains a resin matrix, a curing agent, and functional fillers composed of high dielectric constant fillers and low dielectric constant fillers in different proportions;

[0015] The baseband is divided into N dielectric constant partitions along its length.

[0016] First, the entire roll of baseband is impregnated with the resin with the lowest dielectric constant, that is, all N dielectric constant zones are impregnated and pre-cured. Then, resins with higher dielectric constants are used sequentially from low to high to selectively impregnate and pre-cur the N dielectric constant zones along the length of the baseband. The Mth impregnation corresponds to impregnating a total of N-(M-1) dielectric constant zones along the length of the baseband, where M∈[1,...,N]. Finally, a continuous composite baseband with a decreasing dielectric constant distribution along the length is obtained.

[0017] S3. The continuous composite base tape is wound onto the central conductive rod according to the correspondence between the continuous composite material partitioning order with different dielectric constants and the radial position of the core, that is, according to the order of the dielectric constant of the continuous composite material partitioning from high to low, to form a core blank.

[0018] S4. The core blank is heated and pressurized as a whole to form the solid cylindrical composite insulator.

[0019] Furthermore, the curing agent is an acid anhydride-based curing agent.

[0020] The present invention also provides a DC bushing, which includes a DC bushing core with gradient dielectric constant as described above.

[0021] Beneficial Effects: In summary, this invention provides a gradient dielectric constant DC bushing core and its preparation method. By actively designing and implementing a radially decreasing dielectric constant gradient, this invention effectively homogenizes the electric field distribution inside the DC bushing, thereby improving insulation margin and equipment operational reliability. Furthermore, in the preparation of the gradient dielectric constant DC bushing core, a segmented sequential impregnation process is used to directly form gradient dielectric constant zones on a single baseband, avoiding interface inaccuracies and error accumulation caused by multiple roll changes. By controlling the impregnation length and adhesive formulation, precise control of the gradient distribution can be achieved. Other features and advantages of this invention will be set forth in the following description. Attached Figure Description

[0022] To more clearly illustrate the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic flowchart of a method for preparing a gradient dielectric constant DC bushing core according to an embodiment of the present invention;

[0024] Figure 2 This is a schematic diagram of a gradient dielectric constant DC bushing core fabrication process according to an embodiment of the present invention;

[0025] In the diagram, A is insulating crepe paper; B is the unwinding roller; and C is the impregnation tank. Detailed Implementation

[0026] The present invention will be described below with reference to specific embodiments. It should be noted that the embodiments described below are examples of the present invention and are only used to illustrate the present invention, and are not intended to limit the present invention. Other combinations and various modifications within the scope of the present invention can be made without departing from the spirit or scope of the present invention.

[0027] This embodiment details a gradient dielectric constant DC bushing core and its preparation method. The target core dimensions in this embodiment are: conductive rod diameter 200mm, insulation outer diameter 440mm (insulation thickness 120mm), core length 5800mm, and the design adopts an 8-layer stepped gradient dielectric constant distribution.

[0028] Materials preparation:

[0029] Reinforcing material skeleton: Insulating crepe paper with a basis weight of 100g / m² and a roll width of 6000mm is selected to meet the core length and leave processing allowance;

[0030] Resin matrix: Bisphenol A type epoxy resin with an epoxy equivalent of 210-230 g / mol is selected. The curing agent is methylhexahydrophthalic anhydride, and 2-ethyl-4-methylimidazolium is selected as an accelerator. The reaction between acid anhydride curing agents and epoxy resin is relatively slow at moderate temperatures. Adding a small amount of tertiary amine accelerators such as 2-ethyl-4-methylimidazolium can significantly reduce the reaction activation energy, initiating and accelerating the reaction at relatively low temperatures (such as 80-110℃). During the drying stage after impregnation, under the action of the accelerator, the epoxy resin can partially crosslink at a lower temperature, achieving a gel-like, non-sticky pre-cured state, which facilitates subsequent processing.

[0031] Functional fillers:

[0032] High dielectric constant filler: Barium titanate (BaTiO3) nanoparticles with an average particle size of 100nm are treated with silane coupling agent KH-550. Untreated nano-barium titanate, silica and other inorganic fillers have hydrophilic surfaces, poor compatibility with hydrophobic epoxy resins, and are prone to agglomeration, resulting in uneven dispersion in epoxy resins. The silane coupling agent KH-550 has a molecular structure where one end (ethoxy) can form a strong chemical bond with the hydroxyl groups on the surface of the inorganic filler after hydrolysis, and the other end (amino) can react chemically with the epoxy resin, thereby building a strong molecular bridge between the filler and the epoxy resin. That is, the treatment with silane coupling agent KH-550 improves the interfacial compatibility and bonding force between the inorganic filler and the organic resin matrix.

[0033] Low dielectric constant filler: Fumed silica (SiO2) powder with an average particle size of 50 nm, treated with silane coupling agent KH-570;

[0034] Thermally conductive / regulating filler: Hexagonal boron nitride (h-BN) nanosheets with a thickness of less than 20nm. h-BN has a graphene-like layered structure. The boron and nitrogen atoms in its sheets are bonded by strong covalent bonds, resulting in extremely high in-plane thermal conductivity. When the nanosheets are oriented or semi-oriented in the resin matrix (especially easy to achieve during the winding and lamination process), they can form a highly efficient planar thermal conduction path. When the DC bushing core is in operation, the Joule heat and dielectric loss heat generated by the central conductive rod need to be dissipated radially (i.e., in the direction of the insulation layer thickness). The addition of h-BN can significantly improve the lateral thermal conductivity of the composite material layer, which helps to conduct internal heat to the outer layer and surface more quickly and evenly, thereby reducing the overall operating temperature and radial temperature difference inside the core. This directly improves the long-term thermal stability and load capacity of the product.

[0035] Resin Adhesive Formulation: Eight resin adhesive formulations (N=8) were designed, with bisphenol A epoxy resin as 100 parts by weight. Examples of each resin adhesive formulation are as follows:

[0036] Formula 1: 20 parts SiO, 6 parts h-BN, 21 parts BaTiO3;

[0037] Formula 2: 1 part SiO2, 6 parts h-BN, 18 parts BaTiO3;

[0038] Formula 3: 3 parts SiO2, 6 parts h-BN, 15 parts BaTiO3;

[0039] Formula 4: 6 parts SiO2, 6 parts h-BN, 12 parts BaTiO3;

[0040] Formula 5: 9 parts SiO2, 6 parts h-BN, 9 parts BaTiO3;

[0041] Formula 6: 12 parts SiO2, 6 parts h-BN, 6 parts BaTiO3;

[0042] Formula 7: 15 parts SiO2, 6 parts h-BN, 3 parts BaTiO3;

[0043] Formula 8: 18 parts SiO2, 6 parts h-BN, 0 parts BaTiO3;

[0044] All resin solutions were stirred evenly under vacuum at 60°C.

[0045] Segmented sequential impregnation preparation of continuous composite substrate:

[0046] Figure 1 This is a schematic flowchart of a method for preparing a gradient dielectric constant DC bushing core according to an embodiment of the present invention. Figure 2 This is a schematic diagram of a gradient dielectric constant DC bushing core fabrication process according to an embodiment of the present invention, as shown below. Figure 1 and Figure 2 As shown, the required length of the whole roll of insulating crepe paper A is divided into 8 dielectric constant zones along the length direction. For insulation with a small thickness (less than 300mm), the insulating crepe paper A can be divided into 8 dielectric constant zones along the length direction on an average basis.

[0047] The insulating crepe paper A is completely unwound by the unwinding roller B, which can be driven to move up and down. First, the resin solution of Formula 8 is injected into the impregnation tank C and the insulating crepe paper A is completely immersed in the impregnation tank C by driving the unwinding roller B to move up and down, that is, the eight dielectric constant zones are completely impregnated for 60 seconds. Then, the insulating crepe paper A is removed from the impregnation tank C by driving the unwinding roller B to move up and down, and is dried by circulating hot air at 80°C for 3 minutes to pre-cur the resin solution. The resin solution in the impregnation tank C is then replaced with the resin solution of Formula 7.

[0048] The unwinding roller B is driven up and down again to immerse the insulating crepe paper A into the impregnation tank C. The paper is impregnated along the length direction (the first part to contact the impregnation tank C is the starting point of the length direction) in 7 dielectric constant zones for 60 seconds. Then, the unwinding roller B is driven up and down to remove the insulating crepe paper A from the impregnation tank C and dried with hot air circulation at 80°C for 3 minutes to pre-cur the resin. The resin in the impregnation tank C is then replaced with the resin in formula 6.

[0049] The insulating crepe paper A is repeatedly immersed into the impregnation tank C by driving the unwinding roller B to move up and down, and the resin in the impregnation tank C is replaced in turn. The number of dielectric constant zones impregnated along the length direction decreases by one each time until the resin of formula 1 is impregnated, that is, one dielectric constant zone is impregnated along the length direction for 60 seconds. Then, the insulating crepe paper A is removed from the impregnation tank C by driving the unwinding roller B to move up and down, and is dried with hot air circulation at 80°C for 3 minutes to allow the resin to pre-cur.

[0050] The insulating crepe paper A, which has been impregnated eight times, is wound along the length of the conductive rod to form a core blank. The wound core blank is then heated and pressurized to solidify it into a solid cylindrical composite insulator, thus obtaining the gradient dielectric constant DC bushing core.

[0051] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A gradient dielectric constant DC bushing core, characterized in that, include: A central conductive rod and a solid cylindrical composite insulator covering the central conductive rod; the composite insulator is composed of an insulating composite material, and its material composition changes continuously or in a stepwise manner along the radial direction, such that the equivalent relative permittivity of the composite insulator gradually decreases from the inner layer to the outer layer along the radial direction; wherein, the composite insulator is formed by winding a continuous composite base tape having multiple dielectric constant partitions along its length, and the different dielectric constant partitions on the continuous composite base tape correspond to different positions along the radial direction of the composite insulator after winding; the continuous composite base tape is composed of a reinforcing material skeleton, a resin matrix filled and cured in the reinforcing material skeleton, and functional fillers dispersed in the resin matrix, and the type and / or content of the functional fillers vary segmentally along the length direction of the continuous composite base tape to form the multiple dielectric constant partitions.

2. The gradient dielectric constant DC bushing core according to claim 1, characterized in that, The reinforcing material skeleton is a continuous fiber paper or fiber cloth; the resin matrix is ​​a solid polymer phase formed by pre-curing the liquid epoxy resin, phenolic resin or polyester resin liquid used during impregnation.

3. The gradient dielectric constant DC bushing core according to claim 2, characterized in that, The functional fillers include high dielectric constant fillers and low dielectric constant fillers; along the length direction of the continuous composite base tape, after winding, they correspond to different radially outward partitions of the composite insulator, and the volume fraction of the high dielectric constant filler in the corresponding continuous composite base tape gradually decreases and / or the volume fraction of the low dielectric constant filler in the corresponding continuous composite base tape gradually increases in the different radially outward partitions.

4. The gradient dielectric constant DC bushing core according to claim 3, characterized in that, The high dielectric constant filler is selected from at least one nanoparticle of barium titanate, barium strontium titanate, or titanium dioxide; the low dielectric constant filler is selected from at least one nanoparticle of silicon dioxide, aluminum oxide, or boron nitride.

5. A method for preparing a gradient dielectric constant DC bushing core as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Provide a continuous baseband based on a reinforced material skeleton; S2. In the length direction of the baseband, form multiple continuous composite material partitions with different dielectric constants through a segmented sequential impregnation process to obtain a continuous composite baseband; S3. Wind the continuous composite baseband onto a central conductive rod according to the correspondence between the sequence of the continuous composite material partitions with different dielectric constants and the radial position of the core, that is, according to the order of the dielectric constants of the continuous composite material partitions from high to low, to form a core blank; S4. The core blank is heated and pressurized as a whole to form the solid cylindrical composite insulator.

6. The method according to claim 5, characterized in that, In step S2, the segmented sequential impregnation process specifically includes: preparing N kinds of resin solutions with different dielectric constants, N≥2, each resin solution containing a resin matrix, a curing agent, and functional fillers composed of high dielectric constant fillers and low dielectric constant fillers in different proportions; dividing the baseband into N dielectric constant partitions along its length; first impregnating the entire roll of baseband with the resin solution with the lowest dielectric constant, that is, impregnating all N dielectric constant partitions and pre-curing it, and then using resin solutions with higher dielectric constants in order from low to high to selectively impregnate and pre-cur the N dielectric constant partitions of the baseband along its length, wherein the Mth impregnation corresponds to impregnating a total of N-(M-1) dielectric constant partitions of the baseband along its length, M∈[1,...,N], and finally obtaining a roll of continuous composite baseband with a decreasing dielectric constant distribution along its length.

7. The method according to claim 6, characterized in that, The curing agent is an acid anhydride-based curing agent.

8. A DC bushing, comprising a gradient dielectric constant DC bushing core as described in any one of claims 1 to 4.

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