Insulating composite foil for dry-type transformer and method for manufacturing the same

Abstract

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

Description

Technical Field

[0001] This invention belongs to the field of insulating materials technology, specifically relating to an insulating composite foil for dry-type transformers and its preparation method. Background Technology

[0002] Transformers are important electrical equipment, mainly used for voltage transformation, power transmission, and distribution in power systems. Based on their cooling methods, transformers are classified into dry-type transformers and oil-immersed transformers. Dry-type transformers rely on air convection for cooling and use epoxy resin and other insulating materials for their windings. Due to their advantages such as strong short-circuit withstand capability, low maintenance workload, high operating efficiency, small size, and low noise, they are widely used in local lighting, high-rise buildings, airports, docks, and CNC machining equipment.

[0003] With the increasing demands of power grid supply and the advancement of transformer technology, dry-type transformers have developed rapidly in my country. Along with the continuous development of dry-type transformers, higher requirements are placed on insulation materials, especially composite materials used as the main insulation, which directly affect their lifespan. Existing composite materials for dry-type transformers suffer from poor thermal conductivity and flame retardancy. Against this backdrop, providing a composite material that combines excellent thermal conductivity and flame retardancy is a problem that needs to be solved by those skilled in the art. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the primary objective of this invention is to provide a method for preparing insulating composite foil for dry-type transformers, which is simple in steps and easy to produce.

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

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing insulating composite foil for dry-type transformers includes the following steps:

[0008] (1) After coating the upper and lower surfaces of the polyimide film with an adhesive, a layer of nonwoven fabric is covered on each surface to obtain a composite material;

[0009] (2) Apply epoxy resin composite coating liquid to the surface of the nonwoven fabric in step (1), and cure it.

[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-like carbon microspheres, 5-10 parts of curing agent, and 1-5 parts of filler.

[0011] Preferably, the H-grade 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 phenolic epoxy resin, o-cresol phenolic epoxy resin, triglycidyl isocyanurate, dicyclopentadiene dioxide epoxy, TDE-85, and AG-80; the phosphorus-free flame retardant is selected from one or more of melamine cyanurate, melamine, melamine cyanurate ester, melamine methanol condensate, polyamide, polyimide, guanidine salt, and guanidine condensate.

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

[0013] Preferably, the filler is prepared by the following method:

[0014] A: Nano-silica is added to an ethanol-water solution of a silane coupling agent, and the reaction is stirred to obtain silane coupling agent modified nano-silica;

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

[0016] C: The anhydride-treated nano-silica from step B is added to DMF, followed by the addition of phosphate methacrylate and azobisisobutyronitrile, and the filler is obtained by heating and reaction.

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

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

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

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

[0021] Modified hollow cage-like carbon nanospheres were obtained by adding nano-hollow cage-like carbon microspheres to an aqueous ethanol solution, followed by the addition of vinyltriethoxysilane and heating reaction.

[0022] Preferably, the ratio of the nano-hollow cage-like carbon microspheres, the ethanol aqueous solution, and the 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 heating reaction temperature is 55-65℃, and the heating reaction time is 3-4h.

[0023] Preferably, the curing in step (2) specifically involves baking and curing at 70-135℃ for 5-10 minutes, followed by curing at 140-150℃ for 5.5-6.5 hours.

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

[0025] An insulating composite foil for dry-type transformers is prepared using the above-mentioned method for preparing insulating composite foil for dry-type transformers.

[0026] Compared with the prior art, the main advantages of the present invention are:

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

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

[0029] 3. The insulating composite foil prepared by this invention has excellent flame retardant properties, thermal conductivity, high temperature resistance and electrical properties, and can meet the requirements for use of Class H insulating composite materials. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to specific embodiments. However, those skilled in the art should understand that the following embodiments are only for illustrating the present invention and should not be regarded as limiting the present invention. Specific conditions not specified in the embodiments are performed according to conventional conditions or conditions recommended by the manufacturer. Unless otherwise specified, the reagents or instruments used are all conventional products obtained through commercial channels.

[0031] In the following embodiments, 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 adhesive is silicone adhesive; the nonwoven fabric is made of glass fiber nonwoven fabric; the H-grade epoxy resin is epoxy resin E51; and the high-temperature epoxy resin is epoxy resin E19.

[0032] (I) Preparation Example

[0033] Preparation Example 1

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

[0035] A: According to the ratio of nano silica, γ-aminopropyltriethoxysilane, and 65% ethanol aqueous solution 1g:1.5mL:11mL, nano silica (particle size of 40nm) was added to the ethanol aqueous solution of γ-aminopropyltriethoxysilane. The mixture was stirred at 110℃ for 18h. After the reaction was completed, the nano silica was obtained by centrifugation, washing with ethanol, and drying.

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

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

[0038] Preparation Example 2

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

[0040] A: According to the ratio of nano silica, γ-aminopropyltriethoxysilane, and 60% ethanol aqueous solution of 1g:1mL:10mL, nano silica (particle size of 100nm) was added to the ethanol aqueous solution of γ-aminopropyltriethoxysilane and stirred at 120℃ for 15h. After the reaction was completed, the nano silica was obtained by centrifugation, washing with ethanol, and drying.

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

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

[0043] Preparation Example 3

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

[0045] A: According to the ratio of nano silica, γ-aminopropyltriethoxysilane, and 75% ethanol aqueous solution 1g:2mL:12mL, nano silica (particle size of 10nm) was added to the ethanol aqueous solution of γ-aminopropyltriethoxysilane and stirred at 100℃ for 20h. After the reaction was completed, the nano silica was obtained by centrifugation, washing with ethanol, and drying.

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

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

[0048] Preparation Example 4

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

[0052] Preparation Example 6

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

[0054] According to the ratio of nano-hollow cage-like carbon microspheres, ethanol aqueous solution, and vinyltriethoxysilane 1g:15mL:0.2g, the nano-hollow cage-like carbon microspheres were added to a 70% ethanol aqueous solution, followed by the addition of vinyltriethoxysilane. The mixture was heated at 60℃ for 3.5h. After the reaction was completed, the mixture was filtered, washed, and dried to obtain modified nano-hollow cage-like carbon microspheres.

[0055] Preparation Example 7

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

[0057] According to the ratio of nano-hollow cage-like carbon microspheres, ethanol aqueous solution, and vinyltriethoxysilane 1g:20mL:0.3g, the nano-hollow cage-like carbon microspheres were added to a 75% ethanol aqueous solution, followed by the addition of vinyltriethoxysilane. The mixture was heated at 65℃ for 3 hours. After the reaction was completed, the mixture was filtered, washed, and dried to obtain modified nano-hollow cage-like carbon microspheres.

[0058] Preparation Example 8

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

[0060] According to the ratio of nano-hollow cage-like carbon microspheres, ethanol aqueous solution, and vinyltriethoxysilane 1g:10mL:0.1g, the nano-hollow cage-like carbon microspheres were added to a 60% ethanol aqueous solution, followed by the addition of vinyltriethoxysilane. The mixture was heated at 55℃ for 4 hours. After the reaction was completed, the mixture was filtered, washed, and dried to obtain modified nano-hollow cage-like carbon microspheres.

[0061] (II) Implementation Examples

[0062] Example 1

[0063] This embodiment provides a method for preparing insulating composite foil for dry-type transformers, as detailed below:

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

[0065] (2) Coat the surface of the nonwoven fabric in step (1) with an epoxy resin composite coating liquid with a thickness of 8 mm, bake and cure at 120°C for 8 min, and then vacuum stand in a vacuum curing oven at 145°C for 5.5-6.5 h.

[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 phenolic epoxy resin, 4 parts of melamine, 6 parts of modified nano-hollow cage-like 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 dry-type transformers, which is prepared using the above-described preparation method.

[0068] Example 2

[0069] This embodiment provides a method for preparing insulating composite foil for dry-type transformers, as detailed below:

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

[0071] (2) Coat the surface of the nonwoven fabric in step (1) with an epoxy resin composite coating liquid with a thickness of 5 mm, bake and cure at 135°C for 5 min, and then vacuum stand in a vacuum curing oven at 140°C for 6.5 h.

[0072] The epoxy resin coating liquid described above 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-like 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 dry-type transformers, which is prepared using the above-described preparation method.

[0074] Example 3

[0075] This embodiment provides a method for preparing insulating composite foil for dry-type transformers, as detailed below:

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

[0077] (2) Coat the surface of the nonwoven fabric in step (1) with an epoxy resin coating liquid with a thickness of 10 mm, bake and cure at 70°C for 10 min, and then let it stand in a vacuum curing oven at 150°C for 5.5 h.

[0078] The epoxy resin coating liquid described above 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 the modified nano-hollow cage-like 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 dry-type transformers, which is prepared using the above-described preparation method.

[0080] (III) Comparative Example

[0081] Comparative Example 1

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

[0083] Comparative Example 2

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

[0085] Comparative Example 3

[0086] The difference between this comparative example and Example 1 is that the packing material obtained in Preparation Example 5 is used instead of the packing material obtained in Preparation Example 1, and the rest is the same as in 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, while the rest is the same as in 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 Example 6, and the rest is the same as in 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 rating Thermal conductivity (W / m·K) Breakdown voltage (kV) High temperature resistance (220℃, 5 min) Example 1 94VTM-0 0.67 18.6 No layering, no bubbling Example 2 94VTM-0 0.63 18.1 No layering, no bubbling Example 3 94VTM-0 0.60 17.4 No layering, no bubbling Comparative Example 1 94VTM-2 0.57 10.8 Severe layering and bubbling Comparative Example 2 94VTM-1 0.61 13.7 Slight layering and bubbling occurred. Comparative Example 3 94VTM-1 0.58 12.5 Slight layering and bubbling occurred. Comparative Example 4 94VTM-0 0.32 16.1 No layering, no bubbling Comparative Example 5 94VTM-0 0.45 16.9 No layering, no bubbling

[0095] As shown in Table 1, the insulating composite foil prepared by the present invention has excellent flame retardant properties, thermal conductivity, high temperature resistance and electrical properties, and can meet the requirements for use of Class H insulating composite materials.

[0096] Compared to Example 1, Comparative Example 1 used nano-silica instead of the filler obtained in Preparation Example 1, Comparative Example 2 used anhydride-modified nano-silica instead of the filler obtained in Preparation Example 1, and Comparative Example 3 used silane coupling agent-modified nano-silica instead of the filler obtained in Preparation Example 1. The flame retardant and insulation properties of the materials prepared in these examples were all reduced. These results demonstrate that the filler obtained by the present invention through the seamless integration and close coordination of each step can improve the flame retardant and insulation properties of the material. This may be because the modification of silica with silane coupling agent and anhydride modification improves its stability, allowing it to be uniformly distributed in the epoxy resin, improving the interfacial compatibility between the epoxy resin coating and the nonwoven fabric, thereby improving the insulation and stability of the material. Furthermore, the introduction of phosphate methacrylate is beneficial for forming a "ceramicized" protective layer, improving the flame retardant properties of the insulating composite foil.

[0097] Compared to Example 1, Comparative Example 4 omitted the modified hollow cage-like carbon microspheres, while Comparative Example 5 used hollow cage-like carbon microspheres instead of the modified hollow cage-like carbon microspheres obtained in Preparation Example 6, resulting in a decrease in the thermal conductivity of the material. These results demonstrate that the modified hollow cage-like carbon microspheres prepared in this invention can improve the thermal conductivity of the material. This may be because the hollow structure of the carbon microspheres can form thermally conductive channels, thereby improving the thermal conductivity of the material. Furthermore, modifying the surface of the hollow cage-like carbon microspheres with a silane coupling agent can reduce the aggregation of carbon microspheres 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, improving interfacial heat transfer efficiency.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. Based on the present invention, some modifications or substitutions can be made, but these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of protection claimed by the present invention.

Claims

1. A method for producing an insulating composite foil for dry-type transformers, characterized by, It comprises the following steps: (1) coating adhesive on the upper and lower surfaces of the polyimide film, and covering a layer of non-woven fabric on each surface respectively to obtain a composite material; (2) coating an epoxy resin composite coating solution on the surface of the non-woven fabric in step (1), and curing to obtain the polyimide film composite material; The epoxy resin composite coating solution comprises the following raw materials 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 carbon microspheres, 5-10 parts of curing agent, and 1-5 parts of filler. The preparation method of the filler is as follows: A: adding nano silicon dioxide into an ethanol aqueous solution of silane coupling agent to obtain silane coupling agent modified nano silicon dioxide through stirring reaction; B: adding the silane coupling agent modified nano silicon dioxide in step A into DMF, and then adding 4-dimethylaminopyridine and maleic anhydride to obtain anhydride modified nano silicon dioxide through heating reaction; C: adding the anhydride modified nano silicon dioxide in step B into DMF, and then adding methyl methacrylate phosphate and azobisisobutyronitrile to obtain the filler through heating reaction.

2. The method for preparing insulating composite foil for dry-type transformers according to claim 1, characterized in that, In step A, the dosage ratio of the nano silicon dioxide, the silane coupling agent and the ethanol aqueous solution is 1g:(1-2)mL:(10-12)mL; the silane coupling agent is γ-aminopropyl triethoxysilane; the particle size of the nano silicon dioxide is 10-100nm; the stirring reaction is carried out at a temperature of 100-120℃ for 15-20h.

3. The method for preparing insulating composite foil for dry-type transformers according to claim 1, characterized in that, In step B, the dosage ratio of the silane coupling agent modified nano silicon dioxide, DMF, 4-dimethylaminopyridine and maleic anhydride is 1g:(10-15)mL:(0.001-0.005)g:(0.1-0.3)g; the heating reaction is carried out at a temperature of 65-75℃ for 3-4h.

4. The method for preparing insulating composite foil for dry-type transformers according to claim 1, characterized in that, In step C, the dosage ratio of the anhydride modified nano silicon dioxide, DMF, methyl methacrylate phosphate and azobisisobutyronitrile is 1g:(10-15)mL:(0.1-0.3)g:(0.006-0.012)g; the heating reaction is carried out at a temperature of 80-90℃ for 5-7h.

5. The method for preparing insulating composite foil for dry-type transformers according to claim 1, characterized in that, The preparation process of the modified nano hollow cage carbon microspheres is as follows: adding nano hollow cage carbon microspheres into an ethanol aqueous solution, and then adding vinyl triethoxysilane to obtain modified nano hollow cage carbon microspheres through heating reaction.

6. The method for preparing insulating composite foil for dry-type transformers according to claim 5, characterized in that, The dosage ratio of the nano hollow cage carbon microspheres, the ethanol aqueous solution and the silane coupling agent is 1g:(10-20)mL:(0.1-0.3)g; the outer diameter of the nano hollow cage carbon microspheres is 310-350nm, the inner diameter is 230-255nm, and the mesopore diameter is 30-38nm; the concentration of the ethanol aqueous solution is 60-75%; the heating reaction is carried out at a temperature of 55-65℃ for 3-4h.

7. The method for preparing insulating composite foil for dry-type transformers according to claim 1, characterized in that... In step (2), the curing is specifically baking at 70-135℃ for 5-10min, and then curing at 140-150℃ for 5.5-6.5h.

8. The method for preparing insulating composite foil for dry-type transformers according to claim 1, characterized in that, The non-woven fabric is a glass fiber non-woven fabric, the adhesive is a silicone adhesive, and the curing agent is isomerized methyl tetrahydrophthalic anhydride or isomerized methyl hexahydrophthalic anhydride.

9. An insulating composite foil for dry-type transformers, characterized by, The insulating composite foil for dry transformers is prepared by the method of any one of claims 1-8.