Insulating composite foil for dry-type transformer and preparation method of insulating composite foil

Abstract

The invention belongs to the technical field of insulating materials, and particularly relates to an insulating composite foil for a dry-type transformer and a preparation method of the insulating composite foil. The preparation method of the insulating composite foil for the dry-type transformer comprises the following steps: (1) coating the upper surface and the lower surface of the polyimide film with an adhesive, and then respectively covering the upper surface and the lower surface of the polyimide film with a layer of non-woven fabric to obtain a composite material; and (2) respectively coating the surface of the non-woven fabric in the step (1) with epoxy resin composite coating liquid, and carrying out curing treatment, the epoxy resin coating liquid is prepared from the following raw materials in parts by weight: 40 to 50 parts of H epoxy resin, 10 to 20 parts of high-temperature epoxy resin, 5 to 15 parts of polyfunctional group resin, 2 to 8 parts of phosphorus-free flame retardant, 3 to 7 parts of modified nano hollow cage-shaped carbon microspheres, 5 to 10 parts of curing agent and 1 to 5 parts of filler. The preparation method is simple in step and easy to produce, and the prepared insulating composite foil has excellent flame retardant property, thermal conductivity, high temperature resistance and electrical property and can meet the use requirements of H-grade insulating composite materials.

Description

Technical Field

[0001] The invention belongs to the technical field of insulating materials, and particularly relates to an insulating composite foil for a dry-type transformer and a preparation method thereof. Background Art

[0002] Transformers are important electrical equipment, primarily used for voltage conversion, power transmission, and distribution within power systems. Transformers are categorized as dry-type and oil-immersed based on their cooling methods. Dry-type transformers rely on air convection for cooling and use insulating materials such as epoxy resin for winding insulation. Due to their advantages such as strong short-circuit resistance, minimal maintenance, high operating efficiency, compact size, and low noise, they are widely used in locations such as local lighting, high-rise buildings, airports, and CNC machinery at ports.

[0003] With increasing grid power requirements and advancements in transformer technology, dry-type transformers are rapidly developing in my country. This continued growth has placed higher demands on insulation materials, which directly impact their lifespan, particularly composite materials used as primary insulation. Existing composite materials for dry-type transformers suffer from poor thermal conductivity and flame retardancy. Against this backdrop, providing a composite material with both excellent thermal conductivity and flame retardancy is a challenge facing those skilled in the art. Summary of the Invention

[0004] In order to overcome the deficiencies of the prior art, the first object of the present invention is to provide a method for preparing an insulating composite foil for a dry-type transformer, which has simple steps and is easy to produce.

[0005] The second object of the present invention is to provide an insulating composite foil for dry-type transformers, which has excellent flame retardancy, thermal conductivity, high temperature resistance and electrical properties and can meet the use requirements of H-class insulating composite materials.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing an insulating composite foil for a dry-type transformer comprises the following steps:

[0008] (1) coating an adhesive on the upper and lower surfaces of a polyimide film and then covering them with a layer of nonwoven fabric to obtain a composite material;

[0009] (2) coating the surface of the nonwoven fabric in step (1) with epoxy resin composite coating liquid, and curing the surface;

[0010] The epoxy resin composite coating liquid comprises the following raw materials in parts by weight: 40-50 parts of H epoxy resin, 10-20 parts of high-temperature epoxy resin, 5-15 parts of multifunctional resin, 2-8 parts of phosphorus-free flame retardant, 3-7 parts of modified nano hollow cage-shaped carbon microspheres, 5-10 parts of curing agent, and 1-5 parts of filler.

[0011] Preferably, the H-class epoxy resin is epoxy resin E51; the high-temperature epoxy resin is epoxy resin E19; the multifunctional resin is selected from one or more of bisphenol A novolac epoxy resin, o-cresol novolac epoxy resin, triglycidyl isocyanurate, dicyclopentadiene epoxy dioxide, TDE-85, and AG-80; the phosphorus-free flame retardant is selected from one or more of melamine cyanurate, melamine, melamine cyanurate, melamine methanol condensate, polyamide, polyimide, guanidine salt, and guanidine condensate.

[0012] Preferably, the adhesive coating has a thickness of 1-5 μm; the epoxy resin composite coating liquid has a thickness of 5-10 mm.

[0013] Preferably, the preparation method of the filler is as follows:

[0014] A: Add nano-silica to an ethanol aqueous solution of a silane coupling agent and react by stirring to obtain silane coupling agent-modified nano-silica;

[0015] B: Add the silane coupling agent modified nano-silica in step A to DMF, then add 4-dimethylaminopyridine and maleic anhydride, and heat to react to obtain anhydride-modified nano-silica;

[0016] C: Add the anhydride-treated nano-silica in step B to DMF, then add methacrylic acid phosphate and azobisisobutyronitrile, and heat to react to obtain a filler.

[0017] Preferably, in step A, the nano-silica, silane coupling agent, and ethanol aqueous solution are used in a ratio of 1 g: (1-2) mL: (10-12) mL; the silane coupling agent is γ-aminopropyltriethoxysilane; the particle size of the nano-silica is 10-100 nm; the stirring reaction temperature is 100-120° C., and the stirring reaction time is 15-20 h.

[0018] Preferably, in step B, the amount ratio of the silane coupling agent modified nano-silica, DMF, 4-dimethylaminopyridine, and maleic anhydride is 1 g: (10-15) mL: (0.001-0.005) g: (0.1-0.3) g; the temperature of the heating reaction is 65-75° C., and the heating reaction time is 3-4 h.

[0019] Preferably, the amount ratio of the anhydride-treated nano-silica, DMF, methyl methacrylate phosphate, and azobisisobutyronitrile in step C is 1 g: (10-15) mL: (0.1-0.3) g: (0.006-0.012) g; the heating reaction temperature is 80-90° C., and the heating reaction time is 5-7 h.

[0020] Preferably, the preparation process of the modified nano hollow cage-like carbon microspheres is as follows:

[0021] The nano hollow cage-like carbon microspheres are added to an ethanol aqueous solution, and then vinyltriethoxysilane is added, and the modified nano hollow cage-like carbon microspheres are obtained through heating reaction.

[0022] Preferably, the dosage ratio of the nano hollow cage-like carbon microspheres, ethanol aqueous solution, and silane coupling agent is 1g: (10-20) mL: (0.1-0.3) g; the outer diameter of the nano hollow cage-like carbon microspheres is 310nm-350nm, the inner diameter is 230nm-255nm, and the mesopore diameter is 30nm-38nm; the concentration of the ethanol aqueous solution is 60-75%; the temperature of the heating reaction is 55-65°C, and the heating reaction time is 3-4h.

[0023] Preferably, the curing in step (2) is to first bake and cure at 70-135° C. for 5-10 minutes, and then cure at 140-150° C. for 5.5-6.5 hours.

[0024] Preferably, the non-woven fabric is made of alkali-free glass fiber non-woven fabric, the adhesive is an organic silicone adhesive, and the curing agent is isomerized methyltetrahydrophthalic anhydride or isomerized methylhexahydrophthalic anhydride.

[0025] An insulating composite foil for a dry-type transformer is prepared by adopting the above-mentioned method for preparing the insulating composite foil for a dry-type transformer.

[0026] Compared with the prior art, the beneficial effects of the present invention are mainly:

[0027] 1. The present invention provides a method for preparing an insulating composite foil for dry-type transformers, which involves improvements to epoxy resin coating liquids. Specifically, a new filler and modified nano-hollow cage-like carbon microspheres are added to the epoxy resin coating liquid. The present invention uses a silane coupling agent and anhydride treatment to modify the nano-silica, improving its stability and uniform distribution in the epoxy resin. This improves the interfacial compatibility between the epoxy resin coating layer and the nonwoven fabric, and enhances the insulation and stability of the material. Furthermore, the introduction of methacrylate phosphate facilitates the formation of a "ceramic" protective layer, enhancing the flame retardancy of the insulating composite foil. The addition of modified nano-hollow cage-like carbon microspheres also improves the thermal conductivity of the insulating composite foil. This is likely due to the hollow structure of the carbon microspheres forming heat conduction channels, thereby improving the thermal conductivity of the material. Modifying the surface of the hollow cage-like carbon microspheres with a silane coupling agent reduces carbon microsphere aggregation in the epoxy resin, ensuring uniform distribution and preventing localized heat accumulation. In addition, the silane coupling agent can also form chemical bonds between the carbon microspheres and the epoxy resin, thereby improving the interfacial heat transfer efficiency.

[0028] 2. The preparation method of the insulating composite foil of the present invention has simple steps and is easy to produce.

[0029] 3. The insulating composite foil prepared by the present invention has excellent flame retardancy, thermal conductivity, high temperature resistance and electrical properties, and can meet the use requirements of H-class insulating composite materials. DETAILED DESCRIPTION

[0030] The technical solution of the present invention will be further described below in conjunction with specific embodiments. However, it should be understood by those skilled in the art that the following examples are only intended to illustrate the present invention and should not be construed as limiting the present invention. Specific conditions not specified in the examples are to be followed according to conventional conditions or the conditions recommended by the manufacturer. All reagents or instruments used, unless otherwise specified, are conventional products obtained from commercial channels.

[0031] In the following examples, the outer diameter of the nano hollow cage-like carbon microspheres is 310 nm-350 nm, the inner diameter is 230 nm-255 nm, and the mesopore diameter is 30 nm-38 nm; the adhesive is an organic silicone adhesive; the material of the non-woven fabric is a glass fiber non-woven fabric; the H-grade epoxy resin is epoxy resin E51; and the high-temperature epoxy resin is epoxy resin E19.

[0032] (1) Preparation Example

[0033] Preparation Example 1

[0034] This preparation example provides a filler, and the specific preparation method is as follows:

[0035] A: Nano-silica (particle size 40 nm) was added to an ethanol solution of γ-aminopropyltriethoxysilane in the ratio of 1 g:1.5 mL:11 mL. The mixture was stirred at 110°C for 18 h. After completion of the reaction, the mixture was centrifuged, washed with ethanol, and dried to obtain silane coupling agent-modified nano-silica.

[0036] B: According to the amount ratio of silane coupling agent modified nano-silica, DMF, 4-dimethylaminopyridine, and maleic anhydride of 1g:12mL:0.003g:0.2g, the silane coupling agent modified nano-silica of step A was added to DMF, and then 4-dimethylaminopyridine and maleic anhydride were added. The mixture was heated at 70°C for 3.5h. After the reaction was completed, the mixture was centrifuged, washed with water, and dried to obtain anhydride-modified nano-silica;

[0037] C: According to the usage ratio of acid-anhydride nano-silica, DMF, methacrylate phosphate, and azobisisobutyronitrile (ABI) of 1 g:12 mL:0.2 g:0.009 g, the acid-anhydride nano-silica prepared in step B was added to DMF, followed by methacrylate phosphate and ABI. The mixture was heated at 85°C for 6 h. After the reaction was completed, the mixture was filtered, washed, and dried to obtain a filler.

[0038] Preparation Example 2

[0039] This preparation example provides a filler, and the specific preparation method is as follows:

[0040] A: Nano-silica (particle size 100 nm) was added to an ethanol solution of γ-aminopropyltriethoxysilane at a ratio of 1 g:1 mL:10 mL to 60% ethanol aqueous solution. The mixture was stirred at 120°C for 15 h. After completion of the reaction, the mixture was centrifuged, washed with ethanol, and dried to obtain silane coupling agent-modified nano-silica.

[0041] B: According to the amount ratio of silane coupling agent modified nano-silica, DMF, 4-dimethylaminopyridine, and maleic anhydride being 1g:15mL:0.005g:0.3g, the silane coupling agent modified nano-silica prepared in step A was added to DMF, and then 4-dimethylaminopyridine and maleic anhydride were added. The mixture was heated at 75°C for 3h. After the reaction was completed, the mixture was centrifuged, washed with water, and dried to obtain anhydride-modified nano-silica.

[0042] C: According to the usage ratio of acid-anhydride nano-silica, DMF, methacrylate phosphate, and azobisisobutyronitrile (ABI) of 1 g:15 mL:0.3 g:0.012 g, the acid-anhydride nano-silica prepared in step B was added to DMF, followed by methacrylate phosphate and ABI. The mixture was heated at 90°C for 5 h. After the reaction was completed, the mixture was filtered, washed, and dried to obtain a filler.

[0043] Preparation Example 3

[0044] This preparation example provides a filler, and the specific preparation method is as follows:

[0045] A: Nano-silica (particle size 10 nm) was added to an ethanol solution of γ-aminopropyltriethoxysilane at a ratio of 1 g: 2 mL: 12 mL to 75% ethanol aqueous solution. The mixture was stirred at 100°C for 20 h. After the reaction was complete, the mixture was centrifuged, washed with ethanol, and dried to obtain silane coupling agent-modified nano-silica.

[0046] B: According to the amount ratio of silane coupling agent modified nano-silica, DMF, 4-dimethylaminopyridine, and maleic anhydride of 1g:10mL:0.001g:0.1g, the silane coupling agent modified nano-silica prepared in step A was added to DMF, and then 4-dimethylaminopyridine and maleic anhydride were added. The mixture was heated at 65°C for 4h. After the reaction was completed, the mixture was centrifuged, washed with water, and dried to obtain anhydride-modified nano-silica.

[0047] C: According to the usage ratio of acid-anhydride nano-silica, DMF, methacrylate phosphate, and azobisisobutyronitrile (ABI) of 1g:10mL:0.1g:0.006g, the acid-anhydride nano-silica prepared in step B was added to DMF, followed by methacrylate phosphate and ABI. The mixture was heated at 80°C for 7h. After the reaction was completed, the mixture was filtered, washed, and dried to obtain a filler.

[0048] Preparation Example 4

[0049] The difference between this preparation example and preparation example 1 is that step C is omitted, and the rest is the same as preparation example 1.

[0050] Preparation Example 5

[0051] The difference between this preparation example and preparation example 1 is that step B and step C are omitted, and the rest is the same as preparation example 1.

[0052] Preparation Example 6

[0053] This preparation example provides a modified nano hollow cage-like carbon microsphere, and the specific preparation process is as follows:

[0054] According to the dosage ratio of nano hollow cage-like carbon microspheres, ethanol aqueous solution and vinyltriethoxysilane of 1g:15mL:0.2g, the nano hollow cage-like carbon microspheres were added to a 70% ethanol aqueous solution, and then vinyltriethoxysilane was added. The mixture was heated at 60°C for 3.5h. After the reaction was completed, the modified nano hollow cage-like carbon microspheres were obtained by filtration, washing and drying.

[0055] Preparation Example 7

[0056] This preparation example provides a modified nano hollow cage-like carbon microsphere, and the specific preparation process is as follows:

[0057] According to the dosage ratio of nano hollow cage-like carbon microspheres, ethanol aqueous solution and vinyltriethoxysilane of 1g:20mL:0.3g, the nano hollow cage-like carbon microspheres were added to 75% ethanol aqueous solution, and then vinyltriethoxysilane was added. The mixture was heated at 65°C for 3h to react. After the reaction was completed, the modified nano hollow cage-like carbon microspheres were obtained by filtration, washing and drying.

[0058] Preparation Example 8

[0059] This preparation example provides a modified nano hollow cage-like carbon microsphere, and the specific preparation process is as follows:

[0060] According to the dosage ratio of nano hollow cage-like carbon microspheres, ethanol aqueous solution and vinyltriethoxysilane of 1g:10mL:0.1g, the nano hollow cage-like carbon microspheres were added to the ethanol aqueous solution with a concentration of 60%, and then vinyltriethoxysilane was added. The mixture was heated at 55°C for 4h. After the reaction was completed, the modified nano hollow cage-like carbon microspheres were obtained by filtration, washing and drying.

[0061] (2) Example

[0062] Example 1

[0063] This embodiment provides a method for preparing an insulating composite foil for a dry-type transformer, which is specifically as follows:

[0064] (1) coating an adhesive with a thickness of 3 μm on the upper and lower surfaces of a polyimide film and then covering each surface with a layer of nonwoven fabric to obtain a composite material;

[0065] (2) coating the surface of the nonwoven fabric in step (1) with an epoxy resin composite coating liquid having a thickness of 8 mm, baking and curing the nonwoven fabric at 120° C. for 8 minutes, and then placing the nonwoven fabric in a vacuum curing oven at 145° C. under vacuum for 5.5-6.5 hours;

[0066] The above-mentioned epoxy resin composite coating liquid is composed of the following raw materials in parts by weight: 45 parts of epoxy resin E51, 15 parts of epoxy resin E19, 8 parts of bisphenol A novolac epoxy resin, 4 parts of melamine, 6 parts of modified nano hollow cage carbon microspheres obtained in Preparation Example 6, 8 parts of isomerized methylhexahydrophthalic anhydride, and 3 parts of filler obtained in Preparation Example 1.

[0067] This embodiment also provides an insulating composite foil for a dry-type transformer, which is prepared using the above preparation method.

[0068] Example 2

[0069] This embodiment provides a method for preparing an insulating composite foil for a dry-type transformer, which is specifically as follows:

[0070] (1) coating an adhesive with a thickness of 1 μm on the upper and lower surfaces of a polyimide film and then covering each surface with a layer of nonwoven fabric to obtain a composite material;

[0071] (2) coating the surface of the nonwoven fabric in step (1) with a 5 mm thick epoxy resin composite coating liquid, baking and curing at 135° C. for 5 min, and then placing in a vacuum curing furnace at 140° C. for 6.5 h;

[0072] The above-mentioned epoxy resin coating liquid is composed of the following raw materials in parts by weight: 50 parts of epoxy resin E51, 10 parts of epoxy resin E19, 3 parts of o-cresol epoxy resin, 2 parts of AG-80, 2 parts of polyamide, 7 parts of modified nano hollow cage carbon microspheres obtained in Preparation Example 7, 10 parts of isomerized methyltetrahydrophthalic anhydride, and 5 parts of filler obtained in Preparation Example 2.

[0073] This embodiment also provides an insulating composite foil for a dry-type transformer, which is prepared using the above preparation method.

[0074] Example 3

[0075] This embodiment provides a method for preparing an insulating composite foil for a dry-type transformer, which is specifically as follows:

[0076] (1) coating an adhesive with a thickness of 5 μm on the upper and lower surfaces of a polyimide film and then covering each surface with a layer of nonwoven fabric to obtain a composite material;

[0077] (2) coating the surface of the nonwoven fabric in step (1) with an epoxy resin coating liquid having a thickness of 10 mm, baking and curing the nonwoven fabric at 70° C. for 10 min, and then placing the nonwoven fabric in a vacuum curing furnace at 150° C. for 5.5 h;

[0078] The epoxy resin coating liquid is composed of the following raw materials in parts by weight: 40 parts of epoxy resin E51, 20 parts of epoxy resin E19, 15 parts of triglycidyl isocyanurate, 4 parts of melamine cyanurate, 2 parts of melamine, 3 parts of modified nano hollow cage carbon microspheres obtained in Preparation Example 8, 5 parts of isomerized methylhexahydrophthalic anhydride, and 1 part of the filler obtained in Preparation Example 3.

[0079] This embodiment also provides an insulating composite foil for a dry-type transformer, which is prepared using the above preparation method.

[0080] (3) Comparative Example

[0081] Comparative Example 1

[0082] The difference between this comparative example and Example 1 is that nano-silicon dioxide is used instead of the filler obtained in Preparation Example 1, and the rest is the same as Example 1.

[0083] Comparative Example 2

[0084] The difference between this comparative example and Example 1 is that the filler obtained in Preparation Example 4 is used instead of the filler obtained in Preparation Example 1, and the rest is the same as Example 1.

[0085] Comparative Example 3

[0086] The difference between this comparative example and Example 1 is that the filler obtained in Preparation Example 5 is used instead of the filler obtained in Preparation Example 1, and the rest is the same as Example 1.

[0087] Comparative Example 4

[0088] The difference between this comparative example and Example 1 is that the modified nano hollow cage-like carbon microspheres are omitted, and the rest are the same as Example 1.

[0089] Comparative Example 5

[0090] The difference between this comparative example and Example 1 is that nano hollow cage-like carbon microspheres are used instead of the modified nano hollow cage-like carbon microspheres obtained in Preparation Example 6, and the rest is the same as Example 1.

[0091] (IV) Experimental Examples

[0092] The flame retardancy, breakdown voltage, thermal conductivity and high temperature resistance of the materials obtained in Examples 1-3 and Comparative Examples 1-5 were tested, and the results are shown in Table 1.

[0093] Table 1

[0094] Material Flame retardant grade Thermal conductivity (W / m·K) Breakdown voltage (kV) High temperature resistance (220℃, 5min) Example 1 94VTM-0 0.67 18.6 No stratification or foaming Example 2 94VTM-0 0.63 18.1 No stratification or foaming Example 3 94VTM-0 0.60 17.4 No stratification or foaming Comparative Example 1 94VTM-2 0.57 10.8 Serious stratification and blistering Comparative Example 2 94VTM-1 0.61 13.7 Slight stratification and bubbling Comparative Example 3 94VTM-1 0.58 12.5 Slight delamination and bubbling Comparative Example 4 94VTM-0 0.32 16.1 No stratification or foaming Comparative Example 5 94VTM-0 0.45 16.9 No stratification or foaming

[0095] As can be seen from Table 1, the insulating composite foil prepared in the present invention has excellent flame retardancy, thermal conductivity, high temperature resistance and electrical properties, and can meet the use requirements of H-class insulating composite materials.

[0096] Compared with Example 1, the flame retardancy and insulation properties of the materials prepared by using nano-silica in place of the filler obtained in Preparation Example 1, using anhydride-treated nano-silica in place of the filler obtained in Preparation Example 1, and using silane coupling agent-modified nano-silica in place of the filler obtained in Preparation Example 1 in Comparative Example 1, are all reduced. The above results indicate that the filler prepared by closely matching the steps of the present invention can improve the flame retardancy and insulation properties of the material. This may be because the modification of silica by using a silane coupling agent and anhydride treatment improves its stability, makes it evenly distributed in the epoxy resin, improves the interfacial compatibility between the epoxy resin coating layer and the non-woven fabric, and thus improves the insulation and stability of the material. Furthermore, the introduction of methyl methacrylate phosphate is conducive to the formation of a "ceramic" protective layer, thereby improving the flame retardancy of the insulating composite foil.

[0097] Compared with Example 1, Comparative Example 4 omits the modified nano hollow cage-like carbon microspheres, and Comparative Example 5 uses nano hollow cage-like carbon microspheres instead of the modified nano hollow cage-like carbon microspheres obtained in Preparation Example 6, and the thermal conductivity of the material obtained is reduced. The above results show that the modified nano hollow cage-like carbon microspheres prepared by the present invention can improve the thermal conductivity of the material. This may be because the hollow structure of the carbon microspheres can form a heat conduction channel, thereby improving the thermal conductivity of the material. Modifying the surface of the hollow cage-like carbon microspheres by a silane coupling agent can reduce the agglomeration of the carbon microspheres in the epoxy resin, make them evenly distributed, and avoid local heat accumulation. In addition, the silane coupling agent can also form a chemical bond between the carbon microspheres and the epoxy resin, thereby improving the interfacial heat transfer efficiency.

[0098] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the same. The basic principles and main features of the present invention have been described above using specific implementation schemes. Modifications or substitutions may be made based on the present invention, but such modifications or substitutions do not detract from the essence of the corresponding technical solutions from the scope of protection claimed by the present invention.

Claims

1. A method for preparing an insulating composite foil for a dry-type transformer, characterized in that: The following steps are involved: (1) coating an adhesive on the upper and lower surfaces of a polyimide film and then covering them with a layer of nonwoven fabric to obtain a composite material; (2) coating the surface of the nonwoven fabric in step (1) with epoxy resin composite coating liquid, and curing the surface; The epoxy resin composite coating liquid comprises the following raw materials in parts by weight: 40-50 parts of H epoxy resin, 10-20 parts of high-temperature epoxy resin, 5-15 parts of multifunctional resin, 2-8 parts of phosphorus-free flame retardant, 3-7 parts of modified nano hollow cage-shaped carbon microspheres, 5-10 parts of curing agent, and 1-5 parts of filler.

2. The method for preparing the insulating composite foil for dry-type transformer according to claim 1, characterized in that: The preparation method of the filler is as follows: A: Add nano-silica to an ethanol aqueous solution of a silane coupling agent and react by stirring to obtain silane coupling agent-modified nano-silica; B: Add the silane coupling agent modified nano-silica in step A to DMF, then add 4-dimethylaminopyridine and maleic anhydride, and heat to react to obtain anhydride-modified nano-silica; C: Add the anhydride-treated nano-silica in step B to DMF, then add methacrylic acid phosphate and azobisisobutyronitrile, and heat to react to obtain a filler.

3. The method for preparing the insulating composite foil for dry-type transformer according to claim 2, characterized in that: In step A, the nano-silica, silane coupling agent, and ethanol aqueous solution are used in a ratio of 1 g: (1-2) mL: (10-12) mL; the silane coupling agent is γ-aminopropyltriethoxysilane; the particle size of the nano-silica is 10-100 nm; the stirring reaction temperature is 100-120° C., and the stirring reaction time is 15-20 h.

4. The method for preparing the insulating composite foil for dry-type transformer according to claim 2, characterized in that: In step B, the amount ratio of the silane coupling agent-modified nano-silica, DMF, 4-dimethylaminopyridine, and maleic anhydride is 1 g: (10-15) mL: (0.001-0.005) g: (0.1-0.3) g; the heating reaction temperature is 65-75° C., and the heating reaction time is 3-4 h.

5. The method for preparing the insulating composite foil for dry-type transformer according to claim 2, characterized in that: In step C, the ratio of the anhydride-treated nano-silica, DMF, methyl methacrylate phosphate, and azobisisobutyronitrile is 1 g: (10-15) mL: (0.1-0.3) g: (0.006-0.012) g; the heating reaction temperature is 80-90° C., and the heating reaction time is 5-7 h.

6. The method for preparing the insulating composite foil for dry-type transformer according to claim 1, characterized in that: The preparation process of the modified nano hollow cage-like carbon microspheres is as follows: The nano hollow cage-like carbon microspheres are added to an ethanol aqueous solution, and then vinyltriethoxysilane is added, and the modified nano hollow cage-like carbon microspheres are obtained through heating reaction.

7. The method for preparing the insulating composite foil for dry-type transformer according to claim 6, characterized in that: The dosage ratio of the nano hollow cage-shaped carbon microspheres, ethanol aqueous solution, and silane coupling agent is 1g: (10-20)mL: (0.1-0.3)g; the outer diameter of the nano hollow cage-shaped carbon microspheres is 310nm-350nm, the inner diameter is 230nm-255nm, and the mesopore diameter is 30nm-38nm; the concentration of the ethanol aqueous solution is 60-75%; the temperature of the heating reaction is 55-65°C, and the heating reaction time is 3-4h.

8. The method for preparing the insulating composite foil for dry-type transformer according to claim 1, characterized in that The curing in step (2) is specifically to first bake and cure at 70-135° C. for 5-10 minutes, and then cure at 140-150° C. for 5.5-6.5 hours.

9. The method for preparing the insulating composite foil for dry-type transformer according to claim 1, characterized in that: The non-woven fabric is made of glass fiber non-woven fabric, the adhesive is an organic silicone adhesive, and the curing agent is isomerized methyltetrahydrophthalic anhydride or isomerized methylhexahydrophthalic anhydride.

10. An insulating composite foil for a dry-type transformer, characterized in that: The insulating composite foil is prepared by the method for preparing the insulating composite foil for dry-type transformers according to any one of claims 1 to 9.