A nonlinear gradient epoxy impregnated paper core and method of making the same

Abstract

The application belongs to the technical field of high-voltage direct-current power equipment insulation, and discloses a kind of nonlinear gradient epoxy impregnated paper core body and preparation method thereof.The nonlinear gradient epoxy impregnated paper core body comprises, from inside to outside, a first nonlinear conductive layer, a common insulation layer, a second to sixth nonlinear conductive layer and a grounding screen, each nonlinear conductive layer is formed by micron zinc oxide modified insulation paper with different mass fractions through epoxy resin vacuum impregnation and curing, and the zinc oxide filler is uniformly doped during the preparation of the insulation paper pulp.The present application sets up nonlinear conductive layers with different zinc oxide contents in a radial gradient, which makes the electric field distribution in the bushing core uniform, and under the condition of 800kV direct-current voltage and load current, the maximum electric field strength is reduced from 16kV / mm to below 6kV / mm, which significantly improves the insulation reliability of the bushing.

Description

Technical Field

[0001] This invention belongs to the field of insulation technology for high-voltage DC power equipment, specifically relating to a nonlinear gradient epoxy impregnated paper core and its preparation method. Background Technology

[0002] High-voltage DC epoxy-impregnated paper bushings are key insulation devices in power systems, responsible for power transmission and mechanical support, and operate under complex conditions of high voltage and high current for extended periods. Because the bushing's insulation structure is cylindrical, its internal electric field tends to be unevenly distributed radially under DC voltage.

[0003] During actual operation, the central conductor rod generates heat due to current flow, and its temperature is typically significantly higher than the ambient temperature, creating a radial temperature gradient that gradually decreases from the inside to the outside of the bushing. This temperature gradient significantly affects the electrical conductivity of the insulation material, thereby exacerbating the non-uniformity of the electric field distribution. Excessively high local electric field strength may induce partial discharge, which, over the long term, will reduce the reliability of the insulation system and threaten the safe and stable operation of power equipment.

[0004] Therefore, how to effectively control the radial electric field distribution inside the epoxy-impregnated paper sleeve and suppress the electric field concentration phenomenon caused by temperature gradient is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a nonlinear gradient epoxy impregnated paper core and its preparation method, which solves the problem that the insulation reliability of existing epoxy impregnated paper sleeves is reduced due to uneven internal electric field distribution and excessively high local electric field intensity under the combined action of DC high voltage and radial temperature gradient.

[0006] This invention is achieved through the following technical solution: This invention discloses a nonlinear gradient epoxy impregnated paper core, which is coaxially disposed on the outside of a central guide rod. The epoxy impregnated paper core comprises, from the inside out, a first nonlinear conductive layer, a common insulating layer, a second nonlinear conductive layer, a third nonlinear conductive layer, a fourth nonlinear conductive layer, a fifth nonlinear conductive layer, a grounding shield, and a sixth nonlinear conductive layer. The first to sixth nonlinear conductive layers are micron-modified ZnO insulating papers with different mass fractions, and the ZnO filler in the micron-modified ZnO insulating paper is uniformly doped during the pulp preparation stage of the insulating paper; the epoxy-impregnated paper core is formed by integral vacuum impregnation and curing of epoxy resin after each layer is wound.

[0007] Furthermore, the mass fraction of ZnO in the second nonlinear conductive layer is the first mass fraction; The mass fraction of ZnO in the first and third nonlinear conductive layers is the second mass fraction. The mass fraction of ZnO in the fourth nonlinear conductive layer is the same as the third mass fraction. The mass fraction of ZnO in the fifth and sixth nonlinear conductive layers is the fourth mass fraction; Among them, the first quality score < the second quality score < the third quality score < the fourth quality score.

[0008] Furthermore, the ZnO mass fraction in the first nonlinear conductive layer is 33.3 wt%, and the radial thickness is 40 mm; The radial thickness of the ordinary insulating layer is 65 mm; The second nonlinear conductive layer has a ZnO mass fraction of 20 wt% and a radial thickness of 20 mm. The third nonlinear conductive layer has a ZnO mass fraction of 33.3 wt% and a radial thickness of 40 mm. The fourth nonlinear conductive layer has a ZnO mass fraction of 50 wt% and a radial thickness of 40 mm. The fifth nonlinear conductive layer contains 60 wt% ZnO and is wound to a core diameter of 345 mm. The sixth nonlinear conductive layer contains 60 wt% ZnO, and the final diameter of the core reaches 371 mm after winding.

[0009] Furthermore, the particle size range of the micron-sized ZnO is 1~10 μm.

[0010] Furthermore, the radial conductivity of the epoxy-impregnated paper core satisfies a nonlinear relationship: ; in, σ The electrical conductivity of the material; σ 1 represents the stable conductivity in the low electric field region; E The electric field strength that the material withstands; E a The activation energy of the material; E b For switching field strength; γ These are nonlinear coefficients; k Boltzmann's constant; T The temperature at which the material is located.

[0011] Furthermore, the central guide rod is a double-conductor copper structure with an air gap between the inner and outer conductors.

[0012] This invention also discloses a method for preparing a nonlinear gradient epoxy impregnated paper core, comprising the following steps: Step S1: Prepare insulating paper with different mass fractions of micron-sized ZnO: Micron-sized ZnO filler is ultrasonically dispersed in distilled water to form a suspension. The suspension is mixed with pulped insulating paper pulp, stirred, ultrasonically dispersed, and then vacuum filtered, hot-pressed, and dried to obtain modified insulating paper with a first mass fraction, a second mass fraction, a third mass fraction, and a fourth mass fraction, wherein the first mass fraction < the second mass fraction < the third mass fraction < the fourth mass fraction. Step S2: The outer side of the central guide rod is wound in layers radially from the inside out to form a core prototype with a radial conductivity gradient. The specific winding sequence is as follows: A modified insulating paper with a second mass fraction is wound to form a first nonlinear conductive layer; Unmodified insulating paper is wound around the outside of the first nonlinear conductive layer to form a common insulating layer; A second nonlinear conductive layer is formed by winding modified insulating paper with a first mass fraction around the outside of the ordinary insulating layer. A third nonlinear conductive layer is formed by winding modified insulating paper with a second mass fraction around the outside of the second nonlinear conductive layer. A fourth nonlinear conductive layer is formed by winding modified insulating paper with a third mass fraction around the outside of the third nonlinear conductive layer. A fifth nonlinear conductive layer is formed by winding modified insulating paper with a fourth mass fraction around the outside of the fourth nonlinear conductive layer. A grounding shield is rolled onto the surface of the fifth nonlinear conductive layer; The modified insulating paper with a fourth mass fraction is continued to be wound around the outside of the grounding screen to form a sixth nonlinear conductive layer, thus completing the core winding. S3. The wound core is vacuum dried, then impregnated with epoxy resin, and then cured by heating to obtain epoxy-impregnated paper core.

[0013] Furthermore, in step S1, the freeness of the insulating paper pulp after beating is 40~60°SR.

[0014] Furthermore, the grounding screen is a metal foil layer; in step S2, the fifth nonlinear conductive layer and the sixth nonlinear conductive layer are made of modified insulating paper with the same mass fraction and are respectively disposed on the inner and outer sides of the grounding screen.

[0015] Furthermore, in step S3, the vacuum drying temperature is 80~120℃, the vacuum degree is less than 50 Pa, and the drying time is 24~48 hours; During vacuum impregnation, the epoxy resin temperature is 50~70℃, and the impregnation time is 8~12 hours; The curing process uses a segmented curing process: 80℃ for 2 hours, 100℃ for 2 hours, and 130℃ for 8 hours.

[0016] Compared with the prior art, the present invention has the following beneficial technical effects: This invention discloses a nonlinear gradient epoxy impregnated paper core, comprising, from the inside out, a first nonlinear conductive layer, a common insulating layer, second to sixth nonlinear conductive layers, and a grounding shield. Each nonlinear conductive layer is composed of insulating paper modified with micron-sized ZnO at different mass fractions, and the ZnO filler is uniformly doped during the insulating paper pulp preparation stage. The core is formed by integrally vacuum impregnating and curing epoxy resin after each layer is wound. This structure, by setting multiple nonlinear conductive layers with different ZnO mass fractions in the radial direction, forms a gradually changing conductivity gradient distribution from the inside out. This allows the conductivity at different radial positions within the core to automatically adjust according to the actual electric field strength at that location, thereby achieving adaptive control of the DC electric field distribution. Simultaneously, uniformly doping the ZnO filler during the pulp preparation stage effectively avoids the sedimentation and agglomeration of the filler in the epoxy resin matrix due to density differences, improving the dispersion stability of the filler and the consistency of material properties. By controlling the winding thickness of each layer and the ZnO mass fraction, the maximum electric field strength inside the bushing core was reduced from 16 kV / mm to below 6 kV / mm under 800kV DC voltage and 0~5000A load current conditions. Furthermore, the overall vacuum impregnation and curing method ensured tight interfacial bonding between layers, avoiding insulation weaknesses caused by delamination or interfacial defects, and guaranteeing the overall insulation performance of the core.

[0017] Furthermore, the ZnO mass fraction of the second nonlinear conductive layer is defined as the first mass fraction, the first and third nonlinear conductive layers as the second mass fraction, the fourth nonlinear conductive layer as the third mass fraction, and the fifth and sixth nonlinear conductive layers as the fourth mass fraction, with the first mass fraction < the second mass fraction < the third mass fraction < the fourth mass fraction. This feature causes the radial conductivity of the core to exhibit a non-monotonic gradient distribution of "low—higher—low—higher—higher—highest—highest," that is, high nonlinear conductive layers are set near the central guide rod and near the flange, while a lower concentration transition layer is set between them. This achieves separate control of the initial high field strength region of the central guide rod and the high field strength region induced by the temperature gradient of the flange, while avoiding the overall decrease in electrical resistance caused by using high-concentration filler throughout the entire radial range, thus balancing the dual requirements of electric field homogenization and insulation strength.

[0018] Furthermore, the ZnO mass fraction of the first nonlinear conductive layer is defined as 33.3 wt% and the radial thickness as 40 mm, the radial thickness of the ordinary insulating layer is 65 mm, the ZnO mass fraction of the second nonlinear conductive layer is 20 wt% and the radial thickness is 20 mm, the ZnO mass fraction of the third nonlinear conductive layer is 33.3 wt% and the radial thickness is 40 mm, the ZnO mass fraction of the fourth nonlinear conductive layer is 50 wt% and the radial thickness is 40 mm, and the ZnO mass fraction of the fifth and sixth nonlinear conductive layers is 60 wt%, respectively wound to a core diameter of 345 mm and a final diameter of 371 mm. The aforementioned parameters constitute an optimized gradient structure suitable for 800kV DC voltage levels: a 33.3wt% concentration and 40mm thickness near the central conductor can suppress electric field concentration near the conductor during the initial operation phase; a 20wt% low-concentration transition layer in the middle provides an electric field buffer for the outer high-concentration layer while avoiding excessively high conductivity that could lead to increased leakage current; and high-concentration layers of 50wt% and 60wt% concentrated near the flange effectively compensate for conductivity distribution changes caused by radial temperature gradients, suppressing electric field concentration near the flange after the electric field migrates outward during long-term operation. The thickness and diameter parameters match the insulation size design requirements of 800kV bushings, ensuring the synergistic effect of each layer electrically.

[0019] Furthermore, the particle size range of micron-sized ZnO was defined as 1–10 μm. ZnO fillers within this particle size range can achieve good dispersion within the pores of the pulp fiber network. This avoids particle agglomeration or clogging of paper pores due to excessively small particle size, which would affect the uniformity of subsequent epoxy resin impregnation. Conversely, excessively large particle size would not reduce the specific surface area and doping uniformity of the effective nonlinear conductivity filler per unit volume, ensuring stable nonlinear conductivity response characteristics of the modified insulating paper under different electric field strengths. Simultaneously, this particle size range matches the porosity between insulating paper fibers, allowing ZnO particles to be firmly embedded in the pulp fiber network, preventing them from easily detaching or migrating during subsequent winding and impregnation processes, thus ensuring the long-term stability of the material properties of each layer.

[0020] This invention also discloses a method for preparing the nonlinear gradient epoxy impregnated paper core, comprising three main steps: preparation of ZnO modified insulating paper, layered winding, and vacuum impregnation and curing. The method first uniformly incorporates ZnO filler into the insulating paper during the pulp preparation stage, then winds it in layers according to a preset sequence to form a core prototype, and finally solidifies it through overall vacuum impregnation and curing. Compared with existing technologies that directly dope ZnO filler into epoxy resin, this preparation method achieves ZnO doping at the pulp stage, fully utilizing the three-dimensional network structure of pulp fibers to physically retain and disperse ZnO particles, avoiding the sedimentation and agglomeration of filler in liquid epoxy resin, and ensuring the accuracy and consistency of ZnO concentration distribution in each layer. Simultaneously, the process sequence of winding followed by overall impregnation allows the epoxy resin to be impregnated only as an adhesive and filler in the final stage, eliminating the need to prepare epoxy mixtures of different concentrations for each layer, simplifying the preparation process, improving production efficiency, and ensuring the continuity and bonding strength of the resin at the interfaces between layers after curing.

[0021] Furthermore, the beating degree of the insulating paper pulp after beating is limited to 40~60°SR. This beating degree range allows the pulp fibers to achieve appropriate fibrillation, ensuring that the insulating paper has sufficient mechanical strength and flexibility to withstand tension and bending deformation during winding, while also ensuring that the paper has appropriate porosity to accommodate ZnO filler and allow epoxy resin to fully penetrate during subsequent impregnation. This avoids the problems of ZnO particles accumulating and unevenly distributing on the paper surface due to too low beating, or the paper being too dense due to too high beating, making it difficult for epoxy resin to penetrate. Thus, it comprehensively ensures the uniformity of each layer of insulating paper and the impregnation quality. Attached Figure Description

[0022] Figure 1 Simulation structural diagram of an 800kV DC adhesive-impregnated paper bushing; The components include: 1. Central guide rod; 2. Epoxy-impregnated paper core; 3. Flange; 4. Silicone rubber insulator; 5. Equalizing ring; 6. Transformer oil; Figure 2 This is a gradient distribution diagram of the adhesive-impregnated core insulation. Among them, 11, first nonlinear conductive layer; 12, ordinary insulating layer; 13, second nonlinear conductive layer; 14, third nonlinear conductive layer; 15, fourth nonlinear conductive layer; 16, fifth nonlinear conductive layer; 17, grounding shield; 18, sixth nonlinear conductive layer; Figure 3 The conductivity of ZnO-impregnated paper with different mass fractions under different electric field strengths; Figure 4 Radial temperature distribution of the 800kV DC bushing core; Figure 5a The radial electric field distribution curve of a common epoxy-impregnated paper core is shown. Figure 5b This is a graph showing the radial electric field distribution of a nonlinear gradient impregnated paper core. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0024] The detailed description of the embodiments of the present invention provided in the following drawings is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the drawings and embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0025] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0026] To reduce the local electric field intensity inside the bushing, this invention introduces micron-sized ZnO filler into the epoxy impregnated paper, giving the insulating material nonlinear conductivity characteristics. Under a high electric field, the conductivity of this material increases exponentially, meaning that the conductivity significantly increases in areas with high electric field intensity, thereby enhancing local charge migration ability, weakening the electric field concentration effect, and achieving adaptive homogenization control of the electric field distribution. Compared to traditional insulation structures, this invention has the following technical advantages: 1) Through a nonlinear conductivity control mechanism, the highest electric field intensity inside the bushing can be significantly reduced, improving the uniformity of the electric field distribution; 2) Addressing the issue that high-concentration filler may lead to a decrease in material breakdown voltage, this invention adopts a gradient adjustment method, introducing a high-conductivity control layer only in areas of electric field concentration, thereby improving the local electric field distribution while avoiding a decrease in overall dielectric strength; 3) Overcoming the shortcomings of traditional gradient materials, which have fixed conductivity and limited adaptability.

[0027] The nonlinear conductive material used in this invention can automatically adjust its conductivity according to changes in electric field strength, and can achieve dynamic electric field optimization under different temperature environments and load conditions.

[0028] like Figure 1As shown, the epoxy-impregnated paper bushing includes: a bushing body, through which a central guide rod 1 for carrying current passes. The central guide rod 1 adopts an inner and outer double-conductor copper structure, and its upper and lower ends are connected to external equipment through hardware. An air gap is set between the inner and outer double-conductor copper to enhance heat dissipation and reduce operating temperature rise; the bushing body is filled with SF6 gas; and an epoxy-impregnated paper core 2 is disposed outside the central guide rod 1 to provide insulation and mechanical support. The epoxy-impregnated paper core 2 is made by winding insulating paper and then vacuum impregnating and curing it with epoxy resin. To achieve uniform electric field inside the bushing, such as... Figure 2 As shown, an insulating layer and a gradient voltage equalization layer are provided in the epoxy-impregnated paper core 2; an aluminum alloy voltage equalization ring 5 is provided near the bushing body access end to optimize the electric field distribution at the head of the bushing body and reduce the phenomenon of local electric field concentration; a flange 3 is used for assembly and fixing; and a silicone rubber insulator 4 is provided on the outside of the bushing body. The outer surface of the silicone rubber insulator 4 forms a multi-level umbel structure to extend the creepage distance and suppress external insulation flashover.

[0029] like Figure 2 As shown, the epoxy-impregnated paper core 2 includes a central equalizing layer, a common insulating layer 12, and an outer equalizing layer, which are sequentially wrapped from the inside out. To achieve uniform electric field control throughout the entire lifespan of the bushing, the core of this invention is designed with two main functional zones in the radial direction: A central equalization layer is used to suppress the initial electric field concentration near the central conductor 1; An external equalization layer is used to compensate for the outward migration of the electric field caused by the temperature gradient and to suppress the rise of the electric field near flange 3.

[0030] The central equalizing layer serves as the first nonlinear conductive layer 11. The ordinary insulating layer 12 is composed of ordinary epoxy resin impregnated paper. The conductivity of ordinary epoxy resin impregnated paper does not change with the electric field, but is affected by temperature. The higher the temperature, the greater the conductivity. The relationship between conductivity and temperature is: (1) in, σ The electrical conductivity of the material; σ 0 represents the initial conductivity; E a The activation energy of the material; k Let Boltzmann's constant be 1.3806505 × 10⁻⁶. -23 J / K; T The temperature at which the material is located.

[0031] The external equalizing layer is composed of epoxy resin impregnated paper with different mass fractions of nonlinear conductivity. Specifically, the external equalizing layer includes a second nonlinear conductive layer 13, a third nonlinear conductive layer 14, a fourth nonlinear conductive layer 15, a fifth nonlinear conductive layer 16, and a sixth nonlinear conductive layer 18. A grounding shield 17 is also provided between the fifth nonlinear conductive layer 16 and the sixth nonlinear conductive layer 18.

[0032] The epoxy-impregnated paper core 2 is composed of ZnO-modified insulating paper and epoxy resin. Specifically, different mass fractions of micron-sized ZnO filler are doped into the insulating paper. Compared to doping the epoxy resin with filler, modification in the insulating paper can effectively improve the dispersibility and stability of the filler, avoiding its deposition and agglomeration. First, the pulp is beaten using a Walley beater to obtain pulp with a freeness of 40-60°SR. The micron-sized ZnO filler is then ultrasonically dispersed in distilled water to form a suspension. ZnO suspensions of different concentrations are mixed with the pulp, and after stirring and ultrasonic dispersion, ZnO pulp is obtained. Then, through vacuum filtration, hot pressing, and drying processes, ZnO nonlinear insulating paper, i.e., micron-sized ZnO-modified insulating paper, is prepared. Subsequently, according to… Figure 2 The structure shown involves sequentially winding insulating paper with corresponding ZnO mass fractions around the outer periphery of the central guide rod 1; after epoxy resin impregnation and curing, a nonlinear conductivity epoxy-impregnated paper core 2 is obtained. The relationship between its conductivity and electric field strength and temperature is as follows: (2) in, σ The electrical conductivity of the material; σ 1 represents the stable conductivity in the low electric field region; E The electric field strength that the material withstands; E b For switching field strength; γ These are nonlinear coefficients.

[0033] The conductivity of nonlinear epoxy impregnated paper with different mass fractions of ZnO is as follows: Figure 3 As shown, the conductivity of the unmodified insulating paper remains consistently around 2.0 × 10⁻⁶ kV / mm within the range of 0.2–10 kV / mm. -12 The conductivity (S / m) remains unchanged with increasing electric field strength. The conductivity of ZnO-modified insulating paper exhibits a non-linear increase with increasing electric field strength: in the low electric field region (≤1 kV / mm), the conductivity of each modified sample is essentially the same as that of the unmodified sample, maintaining insulation; when the electric field strength exceeds 2~3 kV / mm, the conductivity begins to increase significantly; in the high electric field region (≥5 kV / mm), the conductivity increases exponentially rapidly, reaching 1.0×10⁻¹⁰ S / m for all modified samples at 10 kV / mm. -2The conductivity is on the order of S / m. Furthermore, under the same electric field strength, the sample with a higher ZnO mass fraction has a greater conductivity and a more significant nonlinear conductivity characteristic. The conductivity-electric field strength curves of the four modified samples show a clear gradient distribution.

[0034] In the initial short period of operation, due to the cylindrical structure of the bushing, the electric field strength tends to concentrate near the guide rod. As the operating time increases, the temperature of the central guide rod 1 gradually increases, and the conductivity of the central material increases rapidly due to temperature, causing the electric field strength to gradually shift outwards. Eventually, the electric field strength near flange 3 increases, resulting in a significant degree of non-uniformity in the electric field inside the bushing. Therefore, to ensure the uniformity of the electric field during both the initial and long-term operation, non-linear conductive insulating layers are introduced near the central guide rod 1 and flange 3, respectively, and arranged in a gradient according to the degree of non-linearity, thereby controlling the electric field strength inside the bushing.

[0035] Specifically, the mass fraction of ZnO in the second nonlinear conductive layer 13 is the first mass fraction; the mass fraction of ZnO in the first nonlinear conductive layer 11 and the third nonlinear conductive layer 14 is the second mass fraction; the mass fraction of ZnO in the fourth nonlinear conductive layer 15 is the third mass fraction; and the mass fraction of ZnO in the fifth nonlinear conductive layer 16 and the sixth nonlinear conductive layer 18 is the fourth mass fraction; wherein, the first mass fraction < the second mass fraction < the third mass fraction < the fourth mass fraction.

[0036] The manufacturing process of the nonlinear gradient impregnated paper sleeve core is illustrated below with a specific embodiment: Select a center guide rod 1 that meets the design requirements for length and diameter, and polish, grind and clean its surface to remove oxide layer, burrs and impurities, so as to ensure that the surface of the guide rod is smooth and flat, so as to avoid defects from adversely affecting the subsequent core winding quality and insulation performance.

[0037] Unmodified insulating paper and insulating paper modified with micron-sized ZnO of different mass fractions were pre-dried to remove moisture from the paper and reduce the risk of air and moisture content during subsequent rolling and impregnation processes.

[0038] To achieve gradient insulation control in the vicinity of the central guide rod 1 and the flange 3 area, different types of insulating paper are selected for layered winding according to the preset radial thickness zones during the insulating paper winding process.

[0039] First, a 33.3wt% ZnO-modified insulating paper is uniformly wound onto the outer surface of the central guide rod 1 to form the first nonlinear conductive layer 11. During the winding process, the tension, overlap rate, and interlayer position of the insulating paper are strictly controlled to ensure that each layer is tightly and uniformly bonded until the thickness of this section reaches 40mm.

[0040] Then, the unmodified insulating paper is replaced and the winding continues until the thickness of the section reaches 65mm, forming a common insulating layer 12.

[0041] Replace the insulating paper with ZnO modified insulating paper with a mass fraction of 20wt%, and continue winding until the thickness of the partition reaches 20mm, forming the second nonlinear conductive layer 13.

[0042] The insulating paper is replaced with ZnO modified insulating paper with a mass fraction of 33.3 wt%, and the winding continues until the thickness of the section reaches 40 mm, forming the third nonlinear conductive layer 14.

[0043] Replace the insulating paper with ZnO modified insulating paper with a mass fraction of 50wt%, and continue winding until the thickness of this section reaches 40mm, forming the fourth nonlinear conductive layer 15.

[0044] The insulating paper was replaced with ZnO modified insulating paper with a mass fraction of 60 wt% and the winding continued until the core diameter reached 345 mm, forming the fifth nonlinear conductive layer 16. Subsequently, a grounding screen 17 is rolled onto the surface of the fifth nonlinear conductive layer 16; 60 wt% ZnO modified insulating paper is continued to be wound around the outside of the grounding screen 17 until the final diameter of the core reaches 371 mm, forming the sixth nonlinear conductive layer 18, thus completing the core winding.

[0045] The rolled core is placed in a vacuum drying chamber for heating, drying, and vacuuming to fully remove residual moisture and air from the core and improve the quality of subsequent impregnation.

[0046] The dried core is placed in the casting tank. After the vacuum level in the tank reaches the preset threshold and is maintained for a period of time, the epoxy resin mixture from the mixing tank is injected into it. After the epoxy resin has fully impregnated the core, the temperature is raised to cure. The entire process ensures that the tank is in a vacuum state.

[0047] After curing, the resin-impregnated paper core is removed from the mold, the core surface is initially polished to remove excess resin and uneven parts, and the end screen lead wire is welded at position 17 of the grounding screen.

[0048] The core is precision machined and finely polished to meet design dimensions and surface quality requirements; then epoxy paint is sprayed onto the core surface to form a protective layer and improve surface insulation performance.

[0049] The processed core is fitted with a sheath, flange 3, base, silicone rubber insulator 4 and equalizing ring 5, and then filled with SF6 gas.

[0050] Specifically, the pulp freeness is 40~60°SR; the hot pressing temperature after vacuum filtration is 80~120℃, and the pressure is 5~15MPa; the drying temperature is 90~110℃, and the drying time is 2~6 hours.

[0051] The vacuum drying temperature is 80~120℃, the vacuum degree is less than 50 Pa, and the drying time is 24~48 hours; the epoxy resin temperature during vacuum impregnation is 50~70℃, and the impregnation time is 8~12 hours; the curing adopts a segmented curing process: 80℃ for 2 hours, 100℃ for 2 hours, and 130℃ for 8 hours.

[0052] To verify the actual effect of the nonlinear conductivity gradient equipotential layer, a structure was constructed as follows: Figure 1 The finite element model of the 800kV DC impregnated paper bushing shown is obtained through electro-thermal field co-simulation. By setting different current-carrying temperatures of the bushing, the electric and thermal field distributions inside the bushing are calculated. Based on formulas (1) and (2), the conductivity distribution inside the impregnated paper core is derived, and the electric field distribution of the core is recalculated. Through repeated iterations, the difference between the two electric field distributions meets the accuracy requirements. The constructed finite element model is as follows: Figure 1 As shown, the internal material structure of the impregnated paper core is as follows: Figure 2 As shown. The total length of the sleeve is 15.58m, and the longest radius is 0.6525m. The epoxy-impregnated paper core 2 inside the sleeve is 11.588m long, with an inner diameter of 0.08m and an outer diameter of 0.291m. The central guide rod 1 has two layers, an inner guide rod with an inner diameter of 0.04m and an outer diameter of 0.06m, and an outer guide rod with an inner radius of 0.0675m and an outer diameter of 0.08m.

[0053] Before the simulation, the electrical, thermal, and basic parameters of the bushing for different materials were set, and the parameter information is shown in Table 1: Table 1

[0054] Temperature field settings: Based on the bushing's structure and operating environment, the heat source within the bushing mainly consists of three parts: 1. Joule heat generated by current flowing through the conductor; 2. Resistive resistance losses generated within the insulating medium; 3. Temperature conduction from the converter transformer's interior to the bushing. Therefore, the bushing heat source equation can be expressed as: (3) in, Q 1 represents the heat generated by the central guide rod 1; I The effective value of the current carrying capacity through the central guide rod 1. R The resistance value of the central guide rod 1; ρ 铜 The resistivity of the copper alloy; l The length of the central guide rod 1,S The current-carrying area of ​​the central guide rod 1; Q 2 represents the heat generated by dielectric loss. E The electric field strength experienced by the medium. ρ 介质 The dielectric conductivity is used. Simultaneously, the upper half of the bushing is designed for thermal convection with air, and the air region and transformer oil region 6 are designed as isothermal zones.

[0055] Electric field setup: Assuming the bushing operates under pure DC electric field conditions, the electric field distribution adopts the static form of Maxwell's equations: (4) Where is the electric displacement vector; and is the electric field intensity. is the electric potential; is the conduction current density; σ is the electrical conductivity.

[0056] The central conductor 1 is set to 800kV, and the current flowing through the conductor is set to 0A, 1000A, 2000A, 3000A, 4000A and 5000A respectively to simulate the electrothermal field of the bushing under different loads. The grounding screen 17 is set to zero potential, and the infinite distance of the geometric region is set as the zero point.

[0057] The temperature distribution of the bushing core along the radial direction of flange 3 under different load current conditions obtained through simulation calculations is as follows: Figure 4 As shown, with the increase of load current, the Joule heat loss of the central conductor 1 and its adjacent area gradually increases, causing a significant rise in the internal temperature of the bushing core and forming a temperature gradient that gradually decreases from the central conductor 1 outwards in the radial direction. When the load current reaches the rated current of 5000A, the temperature near the central conductor 1 can rise to over 140℃, while the temperature of the outer area of ​​the bushing is about 40℃, resulting in a significant temperature difference between the inside and outside of the core. Since the conductivity of epoxy-impregnated paper composite insulation materials typically exhibits a significant temperature dependence, the radial temperature gradient will cause uneven conductivity distribution at different locations in the core, thereby altering the electric field distribution inside the bushing. Especially at the boundary between high-temperature and low-temperature regions, the conductivity difference may lead to distortion of the potential distribution, resulting in localized field concentration.

[0058] Furthermore, the electric field distribution of a conventional epoxy-impregnated paper core under different load current conditions was obtained through simulation calculations, and the results are as follows: Figure 5aAs shown. When the load current is small, for example, in the range of 0-1000A, the temperature rise of the bushing core is low, and the conductivity distribution of the insulation material is relatively uniform. At this time, the electric field distribution is mainly affected by the coaxial cylindrical structure of the bushing. Due to the small radius of the equipotential surface near the central guide rod 1, the electric field lines are more densely distributed. As the radial distance increases, the area of ​​the electric field line distribution gradually increases, and the electric field strength decreases accordingly. Therefore, under low load current conditions, the electric field strength near the central guide rod 1 is relatively large, with a maximum field strength of about 5kV / mm. As the load current further increases, the Joule heat loss at the central guide rod 1 increases, leading to a significant increase in the temperature of the central guide rod 1 and its adjacent area. Since the conductivity of the epoxy-impregnated paper insulation material is temperature-dependent, the increased conductivity in the high-temperature region causes a redistribution of the potential distribution inside the bushing core. The field strength near the central guide rod 1 gradually decreases, while the high field strength region gradually shifts to the outer side of the core and near flange 3. When the load current reaches 5000A, the local electric field strength near flange 3 can increase to approximately 16kV / mm, significantly exceeding the requirements for field strength control in conventional engineering designs for epoxy-impregnated paper insulation structures. This indicates that the ordinary epoxy-impregnated paper core 2 exhibits significant electric field distortion and localized field strength concentration under high-current operating conditions. Therefore, it is necessary to optimize the material conductivity distribution and structural parameters of the impregnated paper bushing core, focusing on controlling the electric field distribution near the central guide rod 1 and flange 3 to suppress the phenomenon of electric field concentration towards the outside under high load current. This will achieve uniformity of the overall electric field of the bushing core and improve the insulation reliability of the impregnated paper bushing under high-current and high-temperature conditions.

[0059] Furthermore, according to Figure 2 The insulation structure shown redesigns the epoxy-impregnated paper core, and simulation calculations are used to obtain the electric field distribution of the nonlinear gradient epoxy-impregnated paper core under different load current conditions. The results are as follows. Figure 5b As shown, by introducing nonlinear conductivity control layers near the central guide rod 1 and flange 3, the electric field distribution inside the bushing core is significantly improved, high field strength regions are effectively suppressed, and the electric field distribution along the radial direction is more uniform. Under different load current conditions, the maximum electric field strength of the nonlinear gradient epoxy-impregnated paper core does not exceed 6 kV / mm. Compared with the maximum field strength of approximately 16 kV / mm near flange 3 of a conventional epoxy-impregnated paper core under a 5000A load current, the maximum field strength is reduced by approximately 62.5%. Furthermore, the optimized field strength is no longer concentrated in localized areas such as near the central guide rod 1 or flange 3, but exhibits a more uniform distribution within the radial range of the core. These results demonstrate that the nonlinear conductivity gradient structure can effectively alleviate the problem of localized field strength concentration under high load conditions, improving the electric field control capability and insulation withstand level of the bushing core.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A nonlinear gradient epoxy impregnated paper core, characterized in that, The epoxy-impregnated paper core (2) is coaxially disposed on the outside of the central guide rod (1). The epoxy-impregnated paper core (2) includes, from the inside out, the following layers in sequence: a first nonlinear conductive layer (11), a common insulating layer (12), a second nonlinear conductive layer (13), a third nonlinear conductive layer (14), a fourth nonlinear conductive layer (15), a fifth nonlinear conductive layer (16), a grounding shield (17), and a sixth nonlinear conductive layer (18). Among them, the first nonlinear conductive layer (11) to the sixth nonlinear conductive layer (18) are micron-modified ZnO insulating papers with different mass fractions, and the ZnO filler in the micron-modified ZnO insulating paper is uniformly doped during the pulp preparation stage of the insulating paper; the epoxy impregnated paper core (2) is formed by overall vacuum impregnation and curing of epoxy resin after each layer is wound.

2. The nonlinear gradient epoxy impregnated paper core according to claim 1, characterized in that, The mass fraction of ZnO in the second nonlinear conductive layer (13) is the first mass fraction; The mass fraction of ZnO in the first nonlinear conductive layer (11) and the third nonlinear conductive layer (14) is the second mass fraction; The mass fraction of ZnO in the fourth nonlinear conductive layer (15) is the third mass fraction; The mass fraction of ZnO in the fifth nonlinear conductive layer (16) and the sixth nonlinear conductive layer (18) is the fourth mass fraction; Among them, the first quality score < the second quality score < the third quality score < the fourth quality score.

3. The nonlinear gradient epoxy impregnated paper core according to claim 2, characterized in that, The first nonlinear conductive layer has a ZnO mass fraction of 33.3 wt% and a radial thickness of 40 mm. The radial thickness of the ordinary insulating layer is 65 mm; The second nonlinear conductive layer has a ZnO mass fraction of 20 wt% and a radial thickness of 20 mm. The third nonlinear conductive layer has a ZnO mass fraction of 33.3 wt% and a radial thickness of 40 mm. The fourth nonlinear conductive layer has a ZnO mass fraction of 50 wt% and a radial thickness of 40 mm. The fifth nonlinear conductive layer contains 60 wt% ZnO and is wound to a core diameter of 345 mm. The sixth nonlinear conductive layer contains 60 wt% ZnO, and the final diameter of the core reaches 371 mm after winding.

4. The nonlinear gradient epoxy impregnated paper core according to claim 1, characterized in that, The particle size range of the micron-sized ZnO is 1~10 μm.

5. The nonlinear gradient epoxy impregnated paper core according to claim 1, characterized in that, The radial conductivity of the epoxy-impregnated paper core (2) satisfies a nonlinear relationship: ； in, σ The electrical conductivity of the material; σ 1 represents the stable conductivity in the low electric field region; E The electric field strength that the material withstands; E a The activation energy of the material; E b For switching field strength; γ These are nonlinear coefficients; k Boltzmann's constant; T The temperature at which the material is located.

6. The nonlinear gradient epoxy impregnated paper core according to claim 1, characterized in that, The central guide rod (1) is a double-conductor copper structure with an air gap between the inner and outer conductors.

7. A method for preparing a nonlinear gradient epoxy impregnated paper core according to any one of claims 1-6, characterized in that, Includes the following steps: Step S1: Prepare insulating paper with different mass fractions of micron-sized ZnO: Micron-sized ZnO filler is ultrasonically dispersed in distilled water to form a suspension. The suspension is mixed with pulped insulating paper pulp, stirred, ultrasonically dispersed, and then vacuum filtered, hot-pressed, and dried to obtain modified insulating paper with a first mass fraction, a second mass fraction, a third mass fraction, and a fourth mass fraction, wherein the first mass fraction < the second mass fraction < the third mass fraction < the fourth mass fraction. Step S2: On the outside of the central guide rod (1), wind layers radially from the inside out to form a core prototype with a radial conductivity gradient. The specific winding sequence is as follows: A modified insulating paper with a second mass fraction is wound to form a first nonlinear conductive layer (11). Unmodified insulating paper is wound around the outside of the first nonlinear conductive layer (11) to form a common insulating layer (12). A modified insulating paper with a first mass fraction is wound around the outside of the ordinary insulating layer (12) to form a second nonlinear conductive layer (13). A third nonlinear conductive layer (14) is formed by winding modified insulating paper with a second mass fraction around the outside of the second nonlinear conductive layer (13). A fourth nonlinear conductive layer (15) is formed by winding a modified insulating paper with a third mass fraction around the outside of the third nonlinear conductive layer (14). A modified insulating paper with a fourth mass fraction is wound around the outside of the fourth nonlinear conductive layer (15) to form a fifth nonlinear conductive layer (16). A grounding screen (17) is rolled onto the surface of the fifth nonlinear conductive layer (16). A modified insulating paper with a fourth mass fraction is continued to be wound around the outside of the grounding screen (17) to form a sixth nonlinear conductive layer (18), thus completing the core winding. S3. The wound core is vacuum dried, then impregnated with epoxy resin, and then heated and cured to obtain epoxy impregnated paper core (2).

8. The preparation method according to claim 7, characterized in that, In step S1, the freeness of the insulating paper pulp after beating is 40~60°SR.

9. The preparation method according to claim 7, characterized in that, The grounding screen (17) is a metal foil layer; in step S2, the fifth nonlinear conductive layer (16) and the sixth nonlinear conductive layer (18) are made of modified insulating paper with the same mass fraction and are respectively disposed on the inner and outer sides of the grounding screen (17).

10. The preparation method according to claim 7, characterized in that, In step S3, the vacuum drying temperature is 80~120℃, the vacuum degree is less than 50 Pa, and the drying time is 24~48 hours. During vacuum impregnation, the epoxy resin temperature is 50~70℃, and the impregnation time is 8~12 hours; The curing process uses a segmented curing process: 80℃ for 2 hours, 100℃ for 2 hours, and 130℃ for 8 hours.

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