A high-frequency harmonic resistant electro-thermal combined action DC voltage divider composite insulating material and a preparation method thereof

Abstract

This invention relates to a composite insulating material for DC voltage dividers resistant to the combined electro-thermal effects of high-frequency harmonics and its preparation method. The insulating material comprises an adaptive core layer, a gradient transition layer, and a high-purity insulating layer. The adaptive core layer is located at the innermost layer, the gradient transition layer consists of two layers symmetrically positioned at both ends of the adaptive core layer, and the high-purity insulating layer consists of two layers positioned outside the two gradient transition layers. Through innovative material and structural design, a highly efficient synergy between high electric field strength tolerance and excellent heat dissipation is achieved, significantly suppressing the electro-thermal synergistic breakdown phenomenon under high frequencies. The preparation method includes pretreatment of the adaptive core layer, gradient transition layer, and high-purity insulating layer; assembly of the adaptive core layer, gradient transition layer, and high-purity insulating layer to obtain a composite preform structure; vacuum pressure impregnation and interface fusion strengthening of the composite preform structure to obtain a primary preform; and curing and stabilization treatment of the primary preform to obtain the DC voltage divider composite insulating material.

Description

Technical Field

[0001] This invention belongs to the field of high voltage and insulation technology, specifically relating to a composite insulation material for DC voltage dividers that is resistant to the combined electro-thermal effects of high-frequency harmonics and its preparation method. Background Technology

[0002] Flexible DC transmission technology, as a representative of the next generation of power transmission technology, has been widely used in long-distance, high-capacity power transmission, grid interconnection, and offshore wind power transmission due to its advantages such as strong controllability, no need for phase commutation, and suitability for new energy integration. DC voltage dividers, as key core equipment for voltage measurement in flexible DC transmission systems, directly affect the accuracy and safety of the entire system's control and protection systems through their measurement accuracy and operational reliability. However, in flexible DC converter stations, the fully controlled power devices such as insulated-gate bipolar transistors in the voltage source converters typically operate at switching frequencies of several kilohertz or even higher. Their rapid turn-on and turn-off processes generate a large amount of high-frequency harmonic voltage on the DC side.

[0003] While single-component polypropylene (PP) film possesses high intrinsic electrical strength and excellent breakdown resistance, it suffers from severe dielectric loss and heat generation under high-frequency harmonics. Coupled with the polymer's poor thermal conductivity, this heat is difficult to dissipate, leading to excessively high localized temperatures and accelerating material thermal degradation. Experiments show that as the frequency increases from 8kHz to 20kHz, the breakdown field strength decreases by up to 27.5%, easily triggering thermo-electrical synergistic breakdown. On the other hand, while single-component insulating paper exhibits good thermal stability and a porous fiber structure, facilitating impregnation with insulating oil and improving heat dissipation, its inherent dielectric strength is significantly lower than that of PP, resulting in weaker resistance to high-frequency electric field stress. At high frequencies, its breakdown field strength decreases by as much as 33.1%.

[0004] In summary, under the dual harsh conditions of "high electric field strength" and "high-frequency thermal stress" faced by DC voltage dividers in flexible DC transmission systems, existing single-component insulation materials—whether relying on high electrical strength polypropylene or on good heat dissipation insulating paper—are insufficient to independently meet all requirements. The heat dissipation difficulties of polypropylene and the low voltage withstand capability of insulating paper have become key technical bottlenecks restricting the development of DC voltage divider insulation systems towards higher voltage levels, more compact designs, and higher operational reliability. Therefore, there is an urgent need to develop a novel composite insulation structure that effectively combines the advantages of both polypropylene and insulating paper, achieving synergistic and complementary electrothermal performance, to fundamentally improve the insulation life and operational safety of DC voltage dividers under complex stress conditions. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing single-component insulating materials that suffer severe performance degradation under the combined stress of high-frequency harmonics and high temperature, and to provide a composite insulating material for DC voltage dividers that is resistant to the combined electro-thermal effects of high-frequency harmonics. Through innovative design of materials and structure, it achieves efficient synergy between high electric field strength tolerance and excellent heat dissipation, and significantly suppresses the electro-thermal synergistic breakdown phenomenon under high frequency.

[0006] Another objective of this invention is to provide a method for preparing a composite insulation structure for a DC voltage divider that is resistant to the combined electro-thermal effects of high-frequency harmonics.

[0007] A composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects, comprising an adaptive core layer, a gradient transition layer, and a high-purity insulating layer;

[0008] The adaptive core layer is located at the innermost layer, the gradient transition layer consists of two layers and is symmetrically disposed at both ends of the adaptive core layer, and the high-purity insulation layer consists of two layers and is disposed outside the two gradient transition layers.

[0009] The high-purity insulating layer is a surface-treated polypropylene film;

[0010] The gradient transition layer is a polypropylene composite film filled with aluminum nitride nanoparticles, which are distributed in a gradient within the polypropylene composite film.

[0011] The adaptive core layer is a porous polyimide film impregnated with a temperature-sensitive, high thermal conductivity impregnating agent.

[0012] The surface treatment process includes the following steps:

[0013] For materials with a nominal thickness of 0.005 mm to 0.010 mm and a dielectric loss factor lower than [missing information], use materials with a nominal thickness of 0.005 mm to 0.010 mm and a dielectric loss factor lower than [missing information]. Electrical grade biaxially oriented polypropylene films are treated in a low-pressure plasma device with an oxygen-argon mixture, with a treatment power of 300 W to 500 W and a treatment time of 60 s to 90 s, in order to introduce polar groups on their surface and form a nanoscale rough structure.

[0014] The mass fraction of nano-aluminum nitride in the polypropylene composite film exhibits a continuous gradient distribution, increasing linearly from 1% to 10% from the side of the film facing the outer layer to the side facing the inner layer.

[0015] The gradient transition layer is coated with a silane coupling agent interface layer on the side near the adaptive core layer.

[0016] A transition bonding layer with a thickness of 0.008~0.012mm is inserted between the silane coupling agent interface layer and the gradient transition layer. The transition bonding layer is an aramid fiber paper layer impregnated with highly thermally conductive silicone oil.

[0017] A method for preparing a composite insulating material for a DC voltage divider resistant to the combined electro-thermal effects of high-frequency harmonics includes the following steps:

[0018] Pretreatment preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials;

[0019] A composite preform structure was obtained by assembling an adaptive core layer, a gradient transition layer, and a high-purity insulating layer.

[0020] Vacuum pressure impregnation and interface fusion strengthening of the composite preform structure were performed to obtain a primary preform.

[0021] The primary preform is cured, shaped, and stabilized to obtain a composite insulating material for DC voltage dividers that is resistant to the combined electro-thermal effects of high-frequency harmonics.

[0022] The pretreatment and preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials specifically includes:

[0023] Preparation of high-purity insulating layer:

[0024] Select dielectric materials with a nominal thickness of 0.005 mm to 0.010 mm and a dielectric loss factor lower than [value missing]. Electrical grade biaxially oriented polypropylene film was used as the insulating medium. The film was placed in a low-pressure plasma device with oxygen-argon mixed gas for treatment. The treatment power was 300 W to 500 W and the treatment time was 60 s to 90 s to introduce polar groups on its surface and form a nanoscale rough structure, thereby obtaining a polypropylene film with high surface energy and high wettability. After the treatment, the film was stored in a dry nitrogen atmosphere for later use.

[0025] Preparation of the gradient transition layer: Using polypropylene as the matrix material and aluminum nitride nanoparticles as the thermally conductive filler, the proportion of aluminum nitride added at each position along the film thickness direction was first determined according to the mass fraction gradient model. Based on this proportion, polypropylene melts with different aluminum nitride concentrations were prepared. Using a multi-layer co-extrusion casting process, the polypropylene melts with different aluminum nitride concentrations were fed into the corresponding flow channels of the co-extrusion die. The flow rate of each layer of melt was controlled so that the extruded film exhibited a linear gradient distribution of aluminum nitride mass fraction from 1% to 10% along the thickness direction. While the film was being extruded, a linear electric field orientation process was used to apply an online directional electric field to guide the aluminum nitride particles to oriented along the film thickness direction. After co-extrusion was completed and the gradient film was cooled and shaped, a silane coupling agent interface layer was coated on the surface of the side with the high filler concentration to enhance the interfacial bonding force between the gradient transition layer and the adaptive core layer, thus completing the preparation of the gradient transition layer.

[0026] Fabrication of the adaptive core layer: A polyimide porous film with a thickness controlled between 0.008 mm and 0.015 mm, a porosity maintained between 30% and 40%, and a permeable microporous structure was selected as the substrate. The substrate was surface-modified via in-situ polymerization, introducing alumina particles into the polyimide matrix to form… The substrate is prepared by using a temperature-sensitive, high thermal conductivity impregnating agent. This agent consists of 85-90 parts by weight of dimethyl silicone oil, 5-8 parts by weight of surface-modified boron nitride nanosheets, and 3-7 parts by weight of phase change microcapsules. The phase change microcapsules have a melting point of 70-75°C. The impregnating agent is then completely filled into the substrate using a vacuum impregnation process. The impregnating agent is initially gelled in the micropores of the composite substrate by heat treatment at 80-90℃ for 30-90 minutes to obtain the adaptive thermal management core layer.

[0027] The quality score gradient model is as follows:

[0028]

[0029] in, The mass fraction of aluminum nitride in the inner layer of the gradient transition layer; The mass fraction of the outer aluminum nitride layer; The total thickness of the gradient film; This represents the distance from the inner layer at any position along the film thickness direction.

[0030] The adaptive core layer, gradient transition layer, and high-purity insulating layer are assembled to obtain a composite preform structure, specifically including:

[0031] The adaptive core layer, gradient transition layer, and high-purity insulation layer were pre-assembled. The electric and thermal field coupling distribution of the pre-assembled overall structure was simulated using multiphysics simulation software. With the goal of smooth electric field transition and efficient thermal diffusion, the assembly sequence, alignment accuracy, and bonding requirements were determined. The adaptive core layer, gradient transition layer, and high-purity insulation layer were then assembled according to the determined assembly sequence, alignment accuracy, and bonding requirements. During assembly, a transition bonding layer was inserted on one side of the gradient transition layer coated with a silane coupling agent interface layer. After assembly, preliminary pressing was performed to tightly bond the high-purity insulation layer, gradient transition layer, transition bonding layer, and adaptive core layer into a whole, resulting in a composite preform structure with complete structure and good matching of each layer.

[0032] Vacuum pressure impregnation and interface fusion strengthening of the composite preform structure yields a primary preform, specifically including:

[0033] The composite preform is placed in a vacuum impregnation tank, and a vacuum is drawn until the absolute pressure is below 3-5 Pa and maintained for at least 5-6 hours to completely remove gas from the interlayer and micropores of the material. Under the condition of maintaining high vacuum, preheated to 60-70°C and deeply degassed dimethyl silicone oil is injected until the composite structure is completely submerged. After closing the oil injection valve, dry nitrogen is introduced into the tank to gradually increase the pressure to 0.7-0.9 MPa, and the pressure is maintained at this pressure for 9-10 hours. The high pressure drives the low viscosity impregnating agent to force penetration, ensuring that the base impregnating agent fully wets the aramid paper layer and fills the pretreated interface gaps between the layers, realizing the tight bonding of solid and liquid media at the molecular scale and the densification of the overall structure, thus obtaining a primary preform.

[0034] The primary preform undergoes curing, shaping, and performance stabilization treatments to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined effects, specifically including:

[0035] After the pressure impregnation is completed, the pressure inside the impregnation tank is slowly released to atmospheric pressure at a rate of 0.1 MPa per hour. The primary preform is then removed and the surface oil is drained. It is then transferred to an oven for step-by-step curing under inert gas protection.

[0036] First, maintain at 55-60°C for 1.5-2 hours, then at 90-100°C for 4.5-5 hours, and finally at 110-120°C for 1.5-2 hours.

[0037] After heat treatment, the material is slowly cooled to room temperature in the oven to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined action.

[0038] A composite insulating material for DC voltage dividers that is resistant to high-frequency harmonic electro-thermal combined effects, prepared using the above method.

[0039] The innovative design of the gradient transition layer in this invention achieves a synchronous and continuous spatial variation of the material's equivalent dielectric constant and thermal conductivity through a linear gradient distribution of aluminum nitride filler from 1% to 10%. Regarding dielectric properties, a multi-layer co-extrusion process combined with electric field orientation technology enables the filler to form an ordered arrangement along the thickness direction within the polypropylene matrix. This structure ensures that the electric field intensity distribution meets certain requirements. The smooth transition law, as shown in the simulation, reduces the interfacial electric field concentration factor from 3.8 to below 1.2 compared to the traditional step dielectric interface, effectively suppressing space charge injection and accumulation at the interlayer interface. In terms of thermal management, the directional arrangement of aluminum nitride particles constructs a preferential heat flow channel from the low-pressure side to the high-pressure side. Experimental measurements show that the in-plane thermal conductivity of the gradient structure continuously increases from 2.5 W / (m·K) to 8.6 W / (m·K), while the corresponding value for traditional homogeneous composite materials is only 4.2 W / (m·K). This bidirectional gradient characteristic allows the insulation structure to ensure both ideal linear attenuation of the electric field distribution and efficient directional heat dissipation when subjected to high-frequency electric fields, resolving the fundamental contradiction in traditional designs where increasing thermal conductivity inevitably sacrifices insulation, and strengthening insulation inevitably worsens heat dissipation.

[0040] The first level of protection in this invention is achieved through the solid-liquid phase transition of the phase change microcapsules. When the local temperature reaches a preset threshold, the microcapsule shell softens and ruptures, and the core material melts and absorbs a large amount of latent heat. This process can stabilize the hot spot temperature near the phase transition point within milliseconds, preventing thermal runaway. The second level of protection is achieved through the recombination of nanofillers. The release of the fluidity of the phase change material causes the boron nitride nanosheets to realign, forming a high-density thermally conductive network in the direction of heat flow, rapidly diffusing the accumulated heat to the entire surface area. The third level of protection is achieved through the self-regulation of dielectric properties. The mixed liquid of silicone oil and phase change material generates microflow under the action of an electric field, continuously flushing and homogenizing the charge distribution at the electrode interface. Experimental data shows that this microflow can reduce the local electric field distortion rate by more than 40%. The three-layer protection mechanism works in concert to autonomously suppress temperature fluctuations and electric field distortion when encountering abnormal operating conditions such as short-term overload and cooling system failure, significantly extending the safe operating time of the equipment under non-standard conditions.

[0041] This invention's seven-layer symmetrical heterogeneous structure, through careful material selection and spatial arrangement, generates a systematic gain effect of "1+1>2". In terms of electric field management, the high-purity insulating layer, gradient transition layer, and adaptive core layer arranged sequentially from the outside in constitute a three-stage adjustment sequence of dielectric constant from 2.2 to 4.5 and then to 3.8, representing a "low-high-medium" adjustment. This design allows the electric field intensity along the thickness direction to exhibit a three-stage optimization curve: rapid attenuation, gradual transition, and finally uniform distribution. Finite element analysis shows that under 10kHz harmonic superposition, the electric field non-uniformity coefficient inside the structure is optimized from 2.8 in the traditional design to 1.5. In terms of thermal management, the innovative insertion of the aramid fiber paper layer establishes a dedicated in-plane heat diffusion channel. After impregnation with silicone oil, its radial thermal conductivity reaches 12 W / (m·K), enabling rapid dissipation of local hotspot heat in the planar direction. Combined with the longitudinal heat conduction of the gradient layer, this forms a three-dimensional heat dissipation system. Attached Figure Description

[0042] Figure 1 SEM images of the surface morphology of the 120-A / PI modified film;

[0043] Figure 2 For 30-A / PI, 60-A / PI, and 120-A / PI SEM cross-sectional characterization of the composite layer. Detailed Implementation

[0044] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the present invention is not limited to these embodiments. These embodiments are provided to fully and completely disclose the present invention and to fully convey the scope of the present invention to those skilled in the art. The raw materials and reagents used in the embodiments are all conventional commercial products or can be prepared by methods commonly used in the art.

[0045] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0046] Example 1

[0047] This embodiment provides a composite insulating material for a DC voltage divider resistant to the combined electro-thermal effects of high-frequency harmonics. It includes an innermost adaptive core layer, a transition bonding layer sequentially disposed on both sides of the adaptive core layer, a gradient transition layer, and a high-purity insulating layer. The high-purity insulating layer is a surface-treated polypropylene film. The surface treatment process involves applying a polypropylene film with a nominal thickness of 0.005 mm and a dielectric loss factor lower than [missing value]. Electrical grade biaxially oriented polypropylene film was treated in a low-pressure plasma device with an oxygen-argon mixed gas at a power of 300 W for 60 s to introduce polar groups on its surface and form a nanoscale rough structure. The gradient transition layer was a polypropylene composite film filled with aluminum nitride nanoparticles. The mass fraction of aluminum nitride nanoparticles in the polypropylene composite film showed a continuous gradient distribution from 1% to 10% linearly from the side of the film facing the outer layer to the side facing the inner layer. The transition bonding layer was an aramid fiber paper layer impregnated with highly thermally conductive silicone oil with a thickness of 0.010 mm.

[0048] The preparation method includes the following steps:

[0049] Pretreatment preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials:

[0050] The high-purity insulating layer is prepared using materials with a nominal thickness of 0.005 mm and a dielectric loss factor lower than [missing value]. Electrical-grade biaxially oriented polypropylene film is used as the outermost pure insulating medium for both the high-voltage and low-voltage sides. The film is treated in a low-pressure plasma environment of oxygen-argon mixed gas at a power of 300 W for 60 s to introduce polar groups and form a nanoscale rough structure on its surface, thereby obtaining a functionalized polypropylene film with high surface energy and high wettability. After treatment, the film is stored in a dry nitrogen atmosphere for later use. This layer serves as a key barrier to withstand the highest electric field intensity and control the heat source of loss.

[0051] The gradient transition layer was prepared using polypropylene as the matrix material and aluminum nitride nanoparticles as the thermally conductive filler. First, based on a mass fraction gradient model, the proportion of aluminum nitride added at each position along the film thickness direction was determined. Based on this proportion, polypropylene melts with different aluminum nitride concentrations were prepared. A multi-layer co-extrusion casting process was used, feeding the prepared polypropylene melts with different aluminum nitride concentrations into the corresponding flow channels of the co-extrusion die. The melt flow rate of each layer was controlled to ensure that the extruded film exhibited a linear gradient distribution of aluminum nitride mass fraction from 1% to 10% along the thickness direction. Simultaneously with film extrusion, a linear electric field orientation process was employed to apply an online directional electric field, guiding the aluminum nitride particles to oriented along the film thickness direction. After co-extrusion, the gradient transition layer was prepared. After the film is cooled and shaped, a silane coupling agent interface layer is coated on the surface of the side with high filler concentration to enhance the interfacial bonding between the gradient transition layer and the adaptive core layer, thus completing the preparation of the gradient transition layer. The total nominal thickness of this gradient film is 0.020 mm. It is prepared by multi-layer co-extrusion casting process and online electric field orientation process to ensure that the aluminum nitride nanoparticles are oriented in the polypropylene matrix to form an effective thermal conduction path. After this gradient film is cut, a thin silane coupling agent interface layer is coated on the surface of the side with high filler concentration to enhance its chemical bonding ability with the interface of adjacent materials. This gradient layer is one of the core functional layers for achieving smooth electric field transition and directional heat conduction.

[0052] The main purpose of constructing a gradient transition layer is to ensure a smooth electric field transition and effective heat conduction. Aluminum nitride nanoparticles are selected as fillers, and polypropylene is used as the matrix material. The distribution and arrangement of aluminum nitride particles are precisely controlled through multilayer co-extrusion casting process and online electric field orientation process. The design of the gradient film requires that the mass fraction of aluminum nitride nanoparticles increase linearly from the inner layer to the outer layer along the thickness direction of the film, from 1% to 10%, to ensure that the thermal conductivity is gradually enhanced. To achieve this, the following mass fraction gradient model is used to determine the proportion of aluminum nitride added at each position along the film thickness direction.

[0053] The quality fraction gradient model is

[0054]

[0055] in, Indicates distance from the inner layer of the membrane The mass fraction of aluminum nitride at the location, It is the mass fraction of aluminum nitride in the inner layer of the thin film. It is the mass fraction of aluminum nitride in the outer layer of the thin film. It is the total thickness of the film. This is the distance from the inner layer. This model can accurately describe the linear distribution characteristics of aluminum nitride particles in a polypropylene matrix from the inner to the outer layer.

[0056] In this embodiment, the mass fraction of the inner aluminum nitride layer in the mass fraction gradient model is first set. =1%, outer layer =10% and total film thickness Parameters, set , where is the total thickness of the gradient film. To determine the distance from the inner layer of the thin film, the core calculation involves identifying the aluminum nitride mass fraction at any position along the film thickness direction to achieve a linear distribution, thus determining the distance from the inner layer. Taking the middle position of the film thickness as an example, substitute the values ​​into the formula to calculate:

[0057]

[0058] Similarly, when =0mm (inner layer) =1%, =0.020mm (outer layer) =10%, this calculation allows for precise control of the filler addition amount at each location. Combined with multi-layer co-extrusion casting and online electric field orientation processes, it enables a linear distribution of aluminum nitride mass fraction along the film thickness from 1% to 10%.

[0059] To ensure the thermal conductivity of the aluminum nitride particles, an online electric field orientation process oriented the particles within the polypropylene matrix, forming effective thermal conduction channels. This arrangement creates an excellent thermally conductive network within the film, effectively improving thermal conductivity. To further enhance the interfacial properties of the composite film, a thin silane coupling agent interfacial layer is coated on the high filler concentration side of the gradient film. This layer strengthens the chemical bonding between the aluminum nitride particles and the polypropylene matrix, further improving the material's mechanical properties and thermal conductivity. The construction of the silane coupling agent interfacial layer is described using the following interfacial binding energy model:

[0060]

[0061] in, It is the interface integration capability. It is the surface energy of the silane coupling agent. This is the surface energy of the polypropylene matrix. The formula shows that by rationally selecting the surface energy of the silane coupling agent and polypropylene, the interfacial bonding force can be improved, interfacial delamination and stress concentration reduced, thereby enabling the gradient layer to maintain stable thermal conductivity under a high-frequency electric field.

[0062] For the fabrication of the adaptive core layer, a 0.008 mm thick polyimide porous film with a through-pore structure was selected as the substrate for the adaptive functional layer, with its porosity controlled at 30%. The polyimide porous film was first surface-modified, and then alumina particles were introduced into the polyimide matrix via in-situ polymerization to form… Composite substrates are used to enhance their thermal conductivity and mechanical stability. A temperature-sensitive, high-thermal-conductivity impregnating agent is formulated, consisting of dimethyl silicone oil and surface-modified boron nitride nanosheets, with added phase change microcapsules containing a specific melting point (70°C). The weight ratio of dimethyl silicone oil, surface-modified boron nitride nanosheets, and phase change microcapsules is 85:8:7. The temperature-sensitive impregnating agent is then completely filled into the substrate using a vacuum impregnation process. In the pores of the composite substrate, heat treatment at 80 degrees Celsius for 30 minutes causes it to initially gel, forming a smart adaptive film that is stable under normal conditions and can significantly enhance thermal conductivity and electric field homogenization when local overheating occurs.

[0063] Figure 1 Image (a) shows the SEM image of the surface morphology of the 120-A / PI modified film. The SEM image clearly shows the alumina. The dispersion state of particles in a polyimide (PI) matrix. Figure 1 Figures (b)-(d) show the element distribution results. Figure 1 In (b), the red color indicates that element C corresponds to the polyimide matrix. Figure 1 In (c), the purple color represents Al. Figure 1 In (d), the green element represents O, corresponding to alumina. The matching distribution of the three elements confirms that alumina has been successfully loaded and dispersed in the polyimide matrix through the in-situ polymerization process in step three. This directly verifies the effectiveness of the polyimide-alumina modification process in step three, providing better thermal conductivity and mechanical stability for subsequent temperature-sensitive impregnation agent filling. Composite substrate;

[0064] Figure 2 In the middle, 30-A / PI The composite layer thickness was 317 nm, with 412 nm corresponding to 60-A / PI and 788 nm to 120-A / PI. These thickness differences correspond to the effects of different process parameters in the alumina modification step in step three. Simultaneously, the cross-sectional morphology showed that each composite layer had a tight interface with the polyimide matrix and no obvious delamination defects. This provides a microscopic and structural basis for selecting suitable modification parameters in step three, ensuring the fabrication of layers with the required thickness and stable interfacial properties. Composite substrates support the goal of enhancing the thermal conductivity and mechanical properties of the adaptive thermal management core layer.

[0065] To ensure that the temperature-sensitive, high thermal conductivity impregnating agent can completely penetrate into the pores of the polyimide porous film and to guarantee its uniformity of penetration, the following experiments were conducted to verify the penetration effect and uniformity of the impregnating agent under different conditions.

[0066]

[0067] The above experiments focused on the permeation effect of temperature-sensitive high thermal conductivity impregnating agents in polyimide porous films. By controlling four variables—impregnation time, impregnation temperature, impregnating agent concentration, and film porosity—the permeation depth, permeation uniformity, and absorption amount of the impregnating agent under different conditions were tested. The results showed that: the longer the impregnation time, the better the permeation effect, reaching the optimal permeation state at 120 minutes; increasing the impregnation temperature can accelerate the permeation rate, with the fastest permeation at 60℃ and a more balanced effect at 40℃; too low an impregnating agent concentration (5%) leads to uneven permeation, while medium and high concentrations of 10% and 20% can achieve uniform permeation; the higher the film porosity, the more conducive it is to impregnating agent permeation, with the optimal permeation effect at 40% porosity. Overall, it can be concluded that all four variables affect the permeation performance of the impregnating agent, and reasonable control of these conditions can improve the permeation effect and uniformity of the impregnating agent in polyimide porous films.

[0068] To test the changes in thermal conductivity of the adaptive thermal management core layer under ambient and overheating conditions, the following experimental groups were set up to evaluate the changes in thermal conductivity of the temperature-sensitive impregnating agent under different temperature conditions. The experiments covered the performance changes under ambient and overheating conditions, and focused on testing the impact of the phase change process of the temperature-sensitive impregnating agent on thermal conductivity:

[0069]

[0070] The above experiments focused on testing the thermal conductivity of the temperature-sensitive high thermal conductivity impregnating agent in the core layer of adaptive thermal management. Using temperature as a variable, the correlation between the change in thermal conductivity of the impregnating agent and its phase transition state at different temperatures was investigated. The results showed that in the 25℃-60℃ stage without phase transition, the thermal conductivity of the impregnating agent showed a slow upward trend, increasing from 0.42 W / (m·K) to 0.47 W / (m·K). 70℃ was the critical phase transition temperature, at which point the impregnating agent began to undergo a phase transition, and the thermal conductivity significantly increased to 0. At 60℃, the impregnating agent undergoes a complete phase change, reaching a maximum thermal conductivity of 0.75 W / (m·K). Subsequently, during the high-temperature phase change stabilization stage at 100℃-150℃, the thermal conductivity continues to increase slowly and gradually stabilizes, eventually reaching 0.90 W / (m·K). Overall, this demonstrates that the thermal conductivity of the temperature-sensitive impregnating agent increases with increasing temperature, and the phase change process is the key node for its significant enhancement of thermal conductivity, maintaining stable high thermal conductivity characteristics at high temperatures.

[0071] To test the stability of the adaptive thermal management layer under long-term high-temperature and thermal cycling conditions, experiments need to be designed to simulate the material's operation under long-term high-temperature environments and its performance changes during thermal cycling (i.e., repeated heating and cooling). The experimental group should cover different numbers of thermal cycles and temperatures to test the material's durability and stability under various conditions.

[0072]

[0073] The above experiments focused on testing the stability of the adaptive thermal management core layer. By setting different conditions such as room temperature rest, long-term high temperature, and thermal cycling, the effects of test time on the thermal conductivity, mechanical strength, and morphology of the layer were investigated. The results showed that: after 24 hours of rest at room temperature, there were no changes in performance or morphology (baseline group); under long-term high-temperature conditions of 24 hours of rest at 100°C, 500 hours of rest at 80°C, and 1000 hours of rest at 100°C, the thermal conductivity showed a slight increasing trend, and microcracks only appeared at the 1000-hour high-temperature condition, indicating good long-term stability; under 10 and 20 thermal cycles, the thermal conductivity increased to 0.44 W / m·K and 0.47 W / m·K, respectively, while the mechanical strength decreased to 79 MPa and 75 MPa, with only minor to surface microcracks appearing, and the cycling performance remained good. Overall, the performance degradation of the core layer was gradual under different temperature conditions, and the stability was excellent.

[0074] Based on the assembly of the adaptive core layer, gradient transition layer, and high-purity insulating layer, a composite preform structure is obtained:

[0075] Based on multiphysics simulation, a heterogeneous multilayer shielding structure was precisely assembled. The adaptive core layer, gradient transition layer, and high-purity insulation layer were pre-assembled and used in a DC voltage divider. The electric and thermal fields of the DC voltage divider under the target high-frequency harmonic spectrum were jointly simulated using multiphysics simulation software. Based on the results, the assembly sequence, alignment accuracy, and bonding requirements were determined with the goal of smooth electric field transition and efficient heat diffusion. The adaptive core layer, gradient transition layer, and high-purity insulation layer were assembled according to the determined assembly sequence, alignment accuracy, and bonding requirements. During assembly, a transition bonding layer was inserted on one side of the gradient transition layer coated with a silane coupling agent interface layer. After assembly, preliminary pressing was performed to tightly bond the high-purity insulation layer, gradient transition layer, transition bonding layer, and adaptive core layer into a whole, resulting in a composite preform structure with complete structure and good matching of each layer.

[0076] After determining the thickness and spatial arrangement of each functional layer, we decided on the specific configuration of each functional layer based on the simulation results. In particular, the adaptive thermal management core layer, located at the core of the structure, possesses high thermal conductivity and electric field regulation capabilities. Therefore, it is placed at the center of the structure, symmetrically stacked with gradient dielectric-thermal conductive transition layers on both sides. This gradient transition layer has a higher filler concentration on the core layer side, gradually decreasing towards the outside, ensuring that heat can be effectively directed to the core layer and preventing excessive electric field and heat loss. The outer pure polypropylene functionalized film mainly serves to control the electric field and isolate it, preventing high-frequency harmonic leakage. Between the transition layer and the polypropylene layer, a 0.010 mm thick aramid fiber paper layer impregnated with highly thermally conductive silicone oil is inserted as an auxiliary heat diffusion channel, further optimizing the heat transfer path. This layer design ensures that heat flow can diffuse uniformly and reduces the thermal gradient, avoiding performance degradation due to localized overheating.

[0077] Based on the joint simulation results of electric and thermal fields, all layers need to be precisely aligned and assembled in a cleanroom environment with constant temperature and humidity to prevent any particulate contamination or the impact of temperature and humidity changes on the structure. An optical alignment system is used during assembly to ensure strict alignment of each layer and guarantee functional matching between each layer.

[0078] The electric field distribution simulation process is as follows:

[0079] To further quantify the effect of heat diffusion, a heat diffusion efficiency model is established, which calculates the total heat flow performance based on the thermal conductivity and thickness of each layer of material.

[0080] Let the first The thermal conductivity of the layer is Thickness is The heat transfer capacity of each layer is then expressed as:

[0081]

[0082] in, It is the first The heat transfer capacity of the layer It is the cross-sectional area. It's a temperature difference. It is the first Thermal conductivity of the layer material;

[0083] For multi-layer structures, the total heat flow This can be obtained through parallel calculations of the heat flow in each layer:

[0084]

[0085] This formula calculates the total heat transfer capacity across multiple material layers, where the thermal conductivity and thickness of each layer affect the total heat transfer capacity. It refers to the number of layers. This is achieved by optimizing the thermal conductivity of each layer's material. and thickness This can optimize the performance of the thermal management system, thereby achieving efficient heat diffusion and electric field control;

[0086] The thermal field distribution simulation process is as follows:

[0087] In a multilayer structure, the electric field strength Distribution and dielectric constant Related to the potential gradient of each layer, let the relationship between the interlayer electric field and potential be expressed as the gradient relationship between the electric field and potential, i.e.:

[0088]

[0089] in It is the electric field strength. It refers to electric potential. Assume that within each layer, the potential distribution is linear, meaning that from the high-voltage side to the low-voltage side, the potential increases with distance. Changes, potential gradient This determines the electric field strength. In an electric field, the dielectric constant... It has a significant impact on the distribution of electric potential;

[0090] In a multilayer medium, assuming the first The dielectric constant of the layer is , No. Layer electric field strength With the Layer dielectric constant The relationship is described by the following formula:

[0091]

[0092] in For the first The potential difference of the layer, For the first The thickness of the layer, For the first The dielectric constant of the layer.

[0093] In the electric field transition layer, the dielectric constant is... The changes gradually occur as the layers change, from the high filler concentration side to the low filler concentration side;

[0094] To ensure a smooth transition of the electric field, the dielectric constant of the gradient transition layer is set. It changes in gradient at different locations, for example from (High filler concentration side) to (On the low filler concentration side) it gradually decreases, and this change is described by the following gradient formula:

[0095]

[0096] in This refers to the distance from the high-voltage side. The thickness of the gradient transition layer, and These are the maximum and minimum dielectric constants of the layer, respectively. Supplementary information: The formulas for the relationship between electric field strength and potential gradient, and the electric field strength formula for multilayer dielectrics, are used to derive the distribution law of electric field strength in multilayer structures. The dielectric constant gradient formula is used to describe the variation characteristics of the dielectric constant of the transition layer from high to low. Through the derivation of these formulas and finite element simulation, the design of the gradient dielectric-thermal conductive transition layer is optimized to achieve a smooth electric field transition and efficient heat diffusion, avoiding insulation failure caused by electric field concentration and heat accumulation. This section serves as the design basis and optimization support for the technical solution.

[0097] The structure was simulated using finite element analysis software, with variations in the dielectric constants of different material layers. The simulation model included layers such as a gradient transition layer, a pure polypropylene layer, and a thermally diffused layer. Appropriate boundary conditions were set during the simulation, and the electric field variation was analyzed through electric field distribution and potential gradient. The simulation results allowed for adjustment of the dielectric constant variation gradient in the transition layer, ensuring a smooth transition of the electric field within the layer. By adjusting the rate of change of the dielectric constant, the smoothness of the electric field intensity could be precisely controlled, avoiding large abrupt changes at the edges of the transition layer. During the simulation, the electric field intensity distribution, especially near the transition layer, was examined to check for drastic fluctuations. The optimization objective was to reduce the nonlinear variation of the electric field in the transition layer, ensuring a gradual decrease in electric field intensity from the high-voltage side to the low-voltage side, and preventing the formation of localized high-electric-field regions. Through this process, the design of the gradient transition layer could be effectively optimized, enabling a smooth transition of the electric field within the layer and avoiding drastic changes in electric field intensity. By precisely controlling the gradient change of the dielectric constant and utilizing multiphysics simulation models, a stable electric field distribution can be achieved, preventing insulation failure or electrical breakdown caused by electric field concentration, while simultaneously improving the overall stability and safety of the structure.

[0098] Vacuum pressure impregnation and interfacial fusion strengthening were applied to the composite preform structure to obtain a primary preform:

[0099] The assembled seven-layer composite preform structure was placed in a high-strength vacuum impregnation tank, and the system was evacuated to an absolute pressure below 3 Pa and maintained for at least 5 hours to completely remove gas from the interlayer spaces and micropores of the material. Under the condition of maintaining high vacuum, dimethyl silicone oil base impregnating agent that had been preheated to 60°C and deeply degassed was injected until the composite structure was completely submerged. After closing the oil injection valve, dry nitrogen was introduced into the tank to gradually increase the pressure to 0.7 MPa, and the pressure was maintained at this level for 9 hours. The high pressure was used to drive the low-viscosity impregnating agent to force penetration, ensuring that the base impregnating agent fully wetted the aramid paper layers and filled the pretreated interfacial gaps between all functional layers, achieving a tight bond between the solid and liquid media at the molecular scale and densification of the overall structure, thus obtaining a primary preform.

[0100] The primary preform is cured, shaped, and stabilized to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined effects.

[0101] After pressure impregnation, the pressure inside the impregnation tank is slowly released to atmospheric pressure at a rate of 0.1 MPa per hour. The composite structure is then removed and the surface oil is drained. It is then transferred to a programmable temperature-controlled oven for step-by-step curing under inert gas protection: first, it is kept at 60°C for 2 hours, then at 100°C for 4.5 hours, and finally at 120°C for 2 hours. This heat treatment process aims to promote the full spreading and cross-linking of the impregnating agent at the interface, stabilize the filler distribution of the gradient layer, and activate the initial properties of the functional materials in the adaptive core layer. After the heat treatment, the structure is slowly cooled to room temperature in the oven, thus obtaining the final stable gradient dielectric-thermal conductive heterogeneous multilayer shielding composite insulation structure. This structure has progressively enhanced thermal management capabilities and adaptive electric field regulation capabilities from the outside to the inside.

[0102] Example 2

[0103] This embodiment provides a method for preparing a composite insulating material for a DC voltage divider resistant to the combined electro-thermal effects of high-frequency harmonics, comprising the following preparation steps:

[0104] Pretreatment preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials:

[0105] The high-purity insulating layer is prepared using materials with a nominal thickness of 0.0075 mm and a dielectric loss factor lower than [missing value]. Electrical-grade biaxially oriented polypropylene film is used as the outermost pure insulating medium for both the high-voltage and low-voltage sides. The film is treated in a low-pressure plasma environment of oxygen-argon mixed gas at a power of 400 W for 75 s to introduce polar groups and form a nanoscale rough structure on its surface, thereby obtaining a functionalized polypropylene film with high surface energy and high wettability. After treatment, the film is stored in a dry nitrogen atmosphere for later use. This layer serves as a key barrier to withstand the highest electric field intensity and control the heat source of loss.

[0106] The gradient transition layer was prepared using polypropylene as the matrix material and aluminum nitride nanoparticles as the thermally conductive filler. First, based on a mass fraction gradient model, the proportion of aluminum nitride added at each position along the film thickness direction was determined. Based on this proportion, polypropylene melts with different aluminum nitride concentrations were prepared. A multi-layer co-extrusion casting process was used, feeding the prepared polypropylene melts with different aluminum nitride concentrations into the corresponding flow channels of the co-extrusion die. The melt flow rate of each layer was controlled to ensure that the extruded film exhibited a linear gradient distribution of aluminum nitride mass fraction from 1% to 10% along the thickness direction. Simultaneously with film extrusion, a linear electric field orientation process was employed to apply an online directional electric field, guiding the aluminum nitride particles to oriented along the film thickness direction. After co-extrusion, the gradient transition layer was prepared. After the film is cooled and shaped, a silane coupling agent interface layer is coated on the surface of the side with high filler concentration to enhance the interfacial bonding between the gradient transition layer and the adaptive core layer, thus completing the preparation of the gradient transition layer. The total nominal thickness of this gradient film is 0.020 mm. It is prepared by multi-layer co-extrusion casting process and online electric field orientation process to ensure that the aluminum nitride nanoparticles are oriented in the polypropylene matrix to form an effective thermal conduction path. After this gradient film is cut, a thin silane coupling agent interface layer is coated on the surface of the side with high filler concentration to enhance its chemical bonding ability with the interface of adjacent materials. This gradient layer is one of the core functional layers for achieving smooth electric field transition and directional heat conduction.

[0107] The main purpose of the gradient transition layer is to ensure a smooth electric field transition and effective heat conduction. Aluminum nitride nanoparticles are selected as fillers, and polypropylene is used as the matrix material. The distribution and arrangement of aluminum nitride particles are precisely controlled through multilayer co-extrusion casting process and online electric field orientation process. The design of the gradient film requires that the mass fraction of aluminum nitride nanoparticles increases linearly from the inner layer to the outer layer along the thickness direction of the film, from 1% to 10%.

[0108] For the fabrication of the adaptive core layer, a 0.0115 mm thick polyimide porous film with a through-pore structure was selected as the substrate for the adaptive functional layer, with its porosity controlled at 35%. The polyimide porous film was first surface-modified, and then alumina particles were introduced into the polyimide matrix via in-situ polymerization to form… Composite substrates are used to enhance their thermal conductivity and mechanical stability. A temperature-sensitive, high-thermal-conductivity impregnating agent is formulated, consisting of dimethyl silicone oil and surface-modified boron nitride nanosheets, with added phase change microcapsules containing a specific melting point (73°C). The weight ratio of dimethyl silicone oil, surface-modified boron nitride nanosheets, and phase change microcapsules is 87:7:6. The temperature-sensitive impregnating agent is then completely filled into the substrate using a vacuum impregnation process. In the pores of the composite substrate, heat treatment at 85 degrees Celsius for 60 minutes is carried out to induce preliminary gelation, forming a smart adaptive film that is stable under normal conditions and can significantly enhance thermal conductivity and electric field homogenization capabilities when local overheating occurs.

[0109] Based on the assembly of the adaptive core layer, gradient transition layer, and high-purity insulating layer, a composite preform structure is obtained:

[0110] Based on multiphysics simulation, a heterogeneous multilayer shielding structure was precisely assembled. The adaptive core layer, gradient transition layer, and high-purity insulation layer were pre-assembled and used in a DC voltage divider. The electric and thermal fields of the DC voltage divider under the target high-frequency harmonic spectrum were jointly simulated using multiphysics simulation software. Based on the results, the assembly sequence, alignment accuracy, and bonding requirements were determined with the goal of smooth electric field transition and efficient heat diffusion. The adaptive core layer, gradient transition layer, and high-purity insulation layer were assembled according to the determined assembly sequence, alignment accuracy, and bonding requirements. During assembly, a transition bonding layer was inserted on one side of the gradient transition layer coated with a silane coupling agent interface layer. After assembly, preliminary pressing was performed to tightly bond the high-purity insulation layer, gradient transition layer, transition bonding layer, and adaptive core layer into a whole, resulting in a composite preform structure with complete structure and good matching of each layer.

[0111] Vacuum pressure impregnation and interfacial fusion strengthening were applied to the composite preform structure to obtain a primary preform:

[0112] The assembled seven-layer composite preform structure was placed in a high-strength vacuum impregnation tank, and the system was evacuated to an absolute pressure below 4 Pa ​​and maintained for at least 5.5 hours to completely remove gas from the interlayer spaces and micropores of the material. Under the condition of maintaining high vacuum, dimethyl silicone oil base impregnating agent that had been preheated to 65°C and deeply degassed was injected until the composite structure was completely submerged. After closing the oil injection valve, dry nitrogen was introduced into the tank to gradually increase the pressure to 0.8 MPa, and the pressure was maintained at this pressure for 9.5 hours. The high pressure was used to drive the low-viscosity impregnating agent to force penetration, ensuring that the base impregnating agent fully wetted the aramid paper layers and filled the pretreated interfacial gaps between all functional layers, realizing the tight bonding of solid and liquid media at the molecular scale and the densification of the overall structure, thus obtaining a primary preform.

[0113] The primary preform is cured, shaped, and stabilized to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined effects.

[0114] After pressure impregnation, the pressure inside the impregnation tank is slowly released to atmospheric pressure at a rate of 0.1 MPa per hour. The composite structure is then removed and the surface oil is drained. It is then transferred to a programmable temperature-controlled oven for step-by-step curing under inert gas protection: first, it is held at 57°C for 1.7 hours, then at 95°C for 4.7 hours, and finally at 115°C for 1.7 hours. This heat treatment process aims to promote the full spreading and cross-linking of the impregnating agent at the interface, stabilize the filler distribution of the gradient layer, and activate the initial properties of the functional materials in the adaptive core layer. After the heat treatment, the structure is slowly cooled to room temperature in the oven, thus obtaining the final stable gradient dielectric-thermal conductive heterogeneous multilayer shielding composite insulation structure. This structure has progressively enhanced thermal management capabilities and adaptive electric field regulation capabilities from the outside to the inside.

[0115] Example 3

[0116] This embodiment provides a method for preparing a composite insulating material for a DC voltage divider resistant to the combined electro-thermal effects of high-frequency harmonics, comprising the following preparation steps:

[0117] Pretreatment preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials:

[0118] The preparation of the high-purity insulating layer uses materials with a nominal thickness of 0.010 mm and a dielectric loss factor lower than [missing value]. Electrical-grade biaxially oriented polypropylene film is used as the outermost pure insulating medium for both the high-voltage and low-voltage sides. The film is treated in a low-pressure plasma environment of oxygen-argon mixed gas at a power of 500 W for 90 s to introduce polar groups and form a nanoscale rough structure on its surface, thereby obtaining a functionalized polypropylene film with high surface energy and high wettability. After treatment, the film is stored in a dry nitrogen atmosphere for later use. This layer serves as a key barrier to withstand the highest electric field intensity and control the heat source of loss.

[0119] The gradient transition layer was prepared using polypropylene as the matrix material and aluminum nitride nanoparticles as the thermally conductive filler. First, based on a mass fraction gradient model, the proportion of aluminum nitride added at each position along the film thickness direction was determined. Based on this proportion, polypropylene melts with different aluminum nitride concentrations were prepared. A multi-layer co-extrusion casting process was used, feeding the prepared polypropylene melts with different aluminum nitride concentrations into the corresponding flow channels of the co-extrusion die. The melt flow rate of each layer was controlled to ensure that the extruded film exhibited a linear gradient distribution of aluminum nitride mass fraction from 1% to 10% along the thickness direction. Simultaneously with film extrusion, a linear electric field orientation process was employed to apply an online directional electric field, guiding the aluminum nitride particles to oriented along the film thickness direction. After co-extrusion, the gradient transition layer was prepared. After the film is cooled and shaped, a silane coupling agent interface layer is coated on the surface of the side with high filler concentration to enhance the interfacial bonding between the gradient transition layer and the adaptive core layer, thus completing the preparation of the gradient transition layer. The total nominal thickness of this gradient film is 0.020 mm. It is prepared by multi-layer co-extrusion casting process and online electric field orientation process to ensure that the aluminum nitride nanoparticles are oriented in the polypropylene matrix to form an effective thermal conduction path. After this gradient film is cut, a thin silane coupling agent interface layer is coated on the surface of the side with high filler concentration to enhance its chemical bonding ability with the interface of adjacent materials. This gradient layer is one of the core functional layers for achieving smooth electric field transition and directional heat conduction.

[0120] The main purpose of constructing a gradient transition layer is to ensure a smooth electric field transition and effective heat conduction. Aluminum nitride nanoparticles are selected as fillers, and polypropylene is used as the matrix material. The distribution and arrangement of aluminum nitride particles are precisely controlled through multilayer co-extrusion casting process and online electric field orientation process. The design of the gradient film requires that the mass fraction of aluminum nitride nanoparticles increases linearly from the inner layer to the outer layer along the thickness direction of the film, from 1% to 10%.

[0121] For the fabrication of the adaptive core layer, a 0.015 mm thick polyimide porous film with a through-pore structure was selected as the substrate for the adaptive functional layer, with its porosity controlled at 40%. The polyimide porous film was first surface-modified, and then alumina particles were introduced into the polyimide matrix via in-situ polymerization to form… Composite substrates are used to enhance their thermal conductivity and mechanical stability. A temperature-sensitive, high-thermal-conductivity impregnating agent is formulated, consisting of dimethyl silicone oil and surface-modified boron nitride nanosheets, with added phase change microcapsules having a specific melting point (75°C). The weight ratio of dimethyl silicone oil, surface-modified boron nitride nanosheets, and phase change microcapsules is 90:6:4. The temperature-sensitive impregnating agent is then completely filled into the substrate using a vacuum impregnation process. In the pores of the composite substrate, a brief heat treatment at 80 degrees Celsius is performed to induce preliminary gelation, forming a smart adaptive film that is stable under normal conditions and can significantly enhance thermal conductivity and electric field homogenization capabilities when local overheating occurs.

[0122] Based on the assembly of the adaptive core layer, gradient transition layer, and high-purity insulating layer, a composite preform structure is obtained:

[0123] Based on multiphysics simulation, a heterogeneous multilayer shielding structure was precisely assembled. The adaptive core layer, gradient transition layer, and high-purity insulation layer were pre-assembled and used in a DC voltage divider. The electric and thermal fields of the DC voltage divider under the target high-frequency harmonic spectrum were jointly simulated using multiphysics simulation software. Based on the results, the assembly sequence, alignment accuracy, and bonding requirements were determined with the goal of smooth electric field transition and efficient heat diffusion. The adaptive core layer, gradient transition layer, and high-purity insulation layer were assembled according to the determined assembly sequence, alignment accuracy, and bonding requirements. During assembly, a transition bonding layer was inserted on one side of the gradient transition layer coated with a silane coupling agent interface layer. After assembly, preliminary pressing was performed to tightly bond the high-purity insulation layer, gradient transition layer, transition bonding layer, and adaptive core layer into a whole, resulting in a composite preform structure with complete structure and good matching of each layer.

[0124] Vacuum pressure impregnation and interfacial fusion strengthening were applied to the composite preform structure to obtain a primary preform:

[0125] The assembled seven-layer composite preform structure was placed in a high-strength vacuum impregnation tank, and the system was evacuated to an absolute pressure of less than 5 Pa and maintained for at least 6 hours to completely remove gas from the interlayer and micropores of the material. Under the condition of maintaining high vacuum, dimethyl silicone oil base impregnating agent that has been preheated to 70°C and deeply degassed was injected until the composite structure was completely submerged. After closing the oil injection valve, dry nitrogen was introduced into the tank to gradually increase the pressure to 0.9 MPa, and the pressure was maintained at this pressure for 10 hours. The low viscosity impregnating agent was forced to penetrate by high pressure to ensure that the base impregnating agent fully wets the aramid paper layer and fills the pretreated interface gaps between all functional layers, so as to achieve a tight bond between solid and liquid media at the molecular scale and densification of the overall structure, thus obtaining a primary preform.

[0126] The primary preform is cured, shaped, and stabilized to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined effects.

[0127] After pressure impregnation, the pressure inside the impregnation tank is slowly released to atmospheric pressure at a rate of 0.1 MPa per hour. The composite structure is then removed and the surface oil is drained. It is then transferred to a programmable temperature-controlled oven for step-by-step curing under inert gas protection: first, it is kept at 55°C for 1.5 hours, then at 90°C for 5 hours, and finally at 110°C for 1.5 hours. This heat treatment process aims to promote the full spreading and cross-linking of the impregnating agent at the interface, stabilize the filler distribution of the gradient layer, and activate the initial properties of the functional materials in the adaptive core layer. After the heat treatment, the structure is slowly cooled to room temperature in the oven, thus obtaining the final stable gradient dielectric-thermal conductive heterogeneous multilayer shielding composite insulation structure. This structure has progressively enhanced thermal management capabilities and adaptive electric field regulation capabilities from the outside to the inside.

[0128] Comparative example:

[0129] The insulation structure of the DC voltage divider is prepared using a conventional method, employing a single electrical grade biaxially oriented polypropylene film (model: BOPP-TD20, nominal thickness 0.010mm) impregnated with conventional dimethyl silicone oil (model: 201-10cs), with a gradient-free structure and adaptive core layer design.

[0130] The composite insulation structures prepared in Examples 1-3 of this invention were compared with those in the comparative examples under the same test conditions. The thermal conductivity was measured according to GB / T 42919.4-2023, and the overall insulation and voltage dividing performance of the DC voltage divider complied with GB / T 19749.4-2023 and GB / T 20840.15-2022. The core insulation indicators, such as breakdown field strength, also met the requirements of test standards such as IEC 60243-1 and GB / T 1408.1-2016. The test results are shown in the table below:

[0131]

[0132] As shown in the table, compared with the comparative example, the electro-thermal integrated performance of the three embodiments of the present invention has been significantly optimized, and Embodiment 2, as an intermediate process solution, achieves the optimal balance between performance and process stability. Specifically: the interface electric field concentration coefficient decreased from 3.8 to 1.0-1.1, the electric field non-uniformity coefficient decreased from 2.8 to 1.4-1.5, and the local electric field distortion rate decreased from 85% to 40%-42%, effectively suppressing interface electric field concentration and local distortion; the thermal conductivity within the gradient transition layer increased from 4.2 W / (m·K) to 8.6-8.7 W / (m·K), and the breakdown field strength retention rate at 20kHz high frequency increased from 65.2% to 80.0%-82.7% under rated operating conditions. The maximum internal hot spot temperature was reduced from 135℃ to 81-84℃, achieving a synergistic improvement in efficient heat diffusion and high-frequency insulation stability. Among them, Example 3 showed the best performance in terms of interface electric field concentration coefficient, Example 2 showed the best performance in core insulation indicators such as local electric field distortion rate and high-frequency breakdown field strength retention rate, and Example 1 had a greater advantage in hot spot temperature control. All three were significantly better than traditional insulation structures, fully verifying the technical advantages of the composite insulation structure of the present invention in resisting high-frequency harmonic electro-thermal combined stress.

[0133] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A composite insulating material for DC voltage dividers resistant to the combined electro-thermal effects of high-frequency harmonics, characterized in that: It includes an adaptive core layer, a gradient transition layer, and a high-purity insulating layer; The adaptive core layer is located at the innermost layer, the gradient transition layer consists of two layers and is symmetrically disposed at both ends of the adaptive core layer, and the high-purity insulation layer consists of two layers and is disposed outside the two gradient transition layers.

2. The composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects as described in claim 1, characterized in that: The high-purity insulating layer is a surface-treated polypropylene film; The gradient transition layer is a polypropylene composite film filled with aluminum nitride nanoparticles, which are distributed in a gradient within the polypropylene composite film. The adaptive core layer is a porous polyimide film impregnated with a temperature-sensitive, high thermal conductivity impregnating agent.

3. The DC voltage divider composite insulating material according to claim 2, characterized in that: The surface treatment process includes the following steps: An electrician-grade biaxially-stretched polypropylene film with a nominal thickness of 0.005 mm to 0.010 mm and a dielectric loss factor lower than 10-4 is placed in a low-pressure plasma device with oxygen-argon mixed gas, and treated at a power of 300 W to 500 W for 60 s to 90 s to introduce polar groups on the surface and form a nano-scale rough structure. An electrician-grade biaxially-stretched polypropylene film with a nominal thickness of 0.005 mm to 0.010 mm and a dielectric loss factor lower than 10-4 is placed in a low-pressure plasma device with oxygen-argon mixed gas, and treated at a power of 300 W to 500 W for 60 s to 90 s to introduce polar groups on the surface and form a nano-scale rough structure.

4. The composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects as described in claim 2, characterized in that: The mass fraction of nano-aluminum nitride in the polypropylene composite film exhibits a continuous gradient distribution, increasing linearly from 1% to 10% from the side of the film facing the outer layer to the side facing the inner layer.

5. A composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects as described in claim 1 or 2, characterized in that: The gradient transition layer is coated with a silane coupling agent interface layer on the side near the adaptive core layer. A transition bonding layer with a thickness of 0.008~0.012mm is inserted between the silane coupling agent interface layer and the gradient transition layer. The transition bonding layer is an aramid fiber paper layer impregnated with highly thermally conductive silicone oil.

6. A method for preparing a composite insulating material for a DC voltage divider resistant to the combined electro-thermal effects of high-frequency harmonics, characterized in that: Includes the following steps: Pretreatment preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials; A composite preform structure was obtained by assembling an adaptive core layer, a gradient transition layer, and a high-purity insulating layer. Vacuum pressure impregnation and interface fusion strengthening of the composite preform structure were performed to obtain a primary preform. The primary preform is cured, shaped, and stabilized to obtain a composite insulating material for DC voltage dividers that is resistant to the combined electro-thermal effects of high-frequency harmonics.

7. The method for preparing a composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects according to claim 6, characterized in that: The pretreatment and preparation of adaptive core layer, gradient transition layer and high-purity insulating layer materials specifically includes: Preparation of high-purity insulating layer: Select dielectric materials with a nominal thickness of 0.005 mm to 0.010 mm and a dielectric loss factor lower than [value missing]. Electrical grade biaxially oriented polypropylene film was used as the insulating medium. The film was placed in a low-pressure plasma device with oxygen-argon mixed gas for treatment. The treatment power was 300 W to 500 W and the treatment time was 60 s to 90 s to introduce polar groups on its surface and form a nanoscale rough structure, thereby obtaining a polypropylene film with high surface energy and high wettability. After the treatment, the film was stored in a dry nitrogen atmosphere for later use. Preparation of the gradient transition layer: Using polypropylene as the matrix material and aluminum nitride nanoparticles as the thermally conductive filler, the proportion of aluminum nitride added at each position along the film thickness direction was first determined according to the mass fraction gradient model. Based on this proportion, polypropylene melts with different aluminum nitride concentrations were prepared. Using a multi-layer co-extrusion casting process, the polypropylene melts with different aluminum nitride concentrations were fed into the corresponding flow channels of the co-extrusion die. The flow rate of each layer of melt was controlled so that the extruded film exhibited a linear gradient distribution of aluminum nitride mass fraction from 1% to 10% along the thickness direction. While the film was being extruded, a linear electric field orientation process was used to apply an online directional electric field to guide the aluminum nitride particles to oriented along the film thickness direction. After co-extrusion was completed and the gradient film was cooled and shaped, a silane coupling agent interface layer was coated on the surface of the side with the high filler concentration to enhance the interfacial bonding force between the gradient transition layer and the adaptive core layer, thus completing the preparation of the gradient transition layer. Fabrication of the adaptive core layer: A polyimide porous film with a thickness controlled between 0.008 mm and 0.015 mm, a porosity maintained between 30% and 40%, and a permeable microporous structure was selected as the substrate. The substrate was surface-modified via in-situ polymerization, introducing alumina particles into the polyimide matrix to form… Composite substrate; A temperature-sensitive, high thermal conductivity impregnating agent is formulated, comprising 85-90 parts by weight of dimethyl silicone oil, 5-8 parts by weight of surface-modified boron nitride nanosheets, and 3-7 parts by weight of phase change microcapsules. The phase change microcapsules have a melting point of 70-75°C. The impregnating agent is completely filled into the substrate using a vacuum impregnation process. The impregnating agent is initially gelled in the micropores of the composite substrate by heat treatment at 80-90℃ for 30-90 minutes to obtain the adaptive thermal management core layer.

8. The method for preparing a composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects according to claim 7, characterized in that: The quality score gradient model is as follows: ； in, The mass fraction of aluminum nitride in the inner layer of the gradient transition layer; The mass fraction of the outer aluminum nitride layer; The total thickness of the gradient film; This represents the distance from the inner layer at any position along the film thickness direction.

9. The method for preparing a composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects according to claim 6, characterized in that: The adaptive core layer, gradient transition layer, and high-purity insulating layer are assembled to obtain a composite preform structure, specifically including: The adaptive core layer, gradient transition layer, and high-purity insulation layer were pre-assembled. The electric and thermal field coupling distribution of the pre-assembled overall structure was simulated using multiphysics simulation software. With the goal of smooth electric field transition and efficient thermal diffusion, the assembly sequence, alignment accuracy, and bonding requirements were determined. The adaptive core layer, gradient transition layer, and high-purity insulation layer were then assembled according to the determined assembly sequence, alignment accuracy, and bonding requirements. During assembly, a transition bonding layer was inserted on one side of the gradient transition layer coated with a silane coupling agent interface layer. After assembly, preliminary pressing was performed to tightly bond the high-purity insulation layer, gradient transition layer, transition bonding layer, and adaptive core layer into a whole, resulting in a composite preform structure with complete structure and good matching of each layer.

10. The method for preparing a composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects according to claim 6, characterized in that: Vacuum pressure impregnation and interface fusion strengthening of the composite preform structure yields a primary preform, specifically including: The composite preform is placed in a vacuum impregnation tank, and a vacuum is drawn until the absolute pressure is below 3-5 Pa and maintained for at least 5-6 hours to completely remove gas from the interlayer and micropores of the material. Under the condition of maintaining high vacuum, preheated to 60-70°C and deeply degassed dimethyl silicone oil is injected until the composite structure is completely submerged. After closing the oil injection valve, dry nitrogen is introduced into the tank to gradually increase the pressure to 0.7-0.9 MPa, and the pressure is maintained at this pressure for 9-10 hours. The high pressure drives the low viscosity impregnating agent to force penetration, ensuring that the base impregnating agent fully wets the aramid paper layer and fills the pretreated interface gaps between the layers, realizing the tight bonding of solid and liquid media at the molecular scale and the densification of the overall structure, thus obtaining a primary preform.

11. A method for preparing a composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined effects, as described in claim 6 or 10, characterized in that: The primary preform undergoes curing, shaping, and performance stabilization treatments to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined effects, specifically including: After the pressure impregnation is completed, the pressure inside the impregnation tank is slowly released to atmospheric pressure at a rate of 0.1 MPa per hour. The primary preform is then removed and the surface oil is drained. It is then transferred to an oven for step-by-step curing under inert gas protection. First, maintain at 55-60°C for 1.5-2 hours, then at 90-100°C for 4.5-5 hours, and finally at 110-120°C for 1.5-2 hours. After heat treatment, the material is slowly cooled to room temperature in the oven to obtain a DC voltage divider composite insulation material resistant to high-frequency harmonic electro-thermal combined action.

12. A composite insulating material for a DC voltage divider resistant to high-frequency harmonic electro-thermal combined action, prepared by the method described in any one of claims 6-11.

Images