A bifunctional modified environment-friendly ester-based insulating oil based on nano-alumina and a preparation method and application thereof

Abstract

The application provides a kind of based on nano-alumina bifunctional modified environment-friendly ester-based insulating oil and its preparation method and application, belongs to electrical insulation material technical field.The application is based on the bifunctional surface modification technology of nano-alumina, first surface functionalization modification is carried out to nano-alumina in PEI solution, and the second surface functionalization modification is carried out to nano-alumina after PEI modification using silane coupling agent;The bifunctional modified nano-alumina particles are dispersed in ester-based insulating oil, and the bifunctional modified environment-friendly ester-based insulating oil is obtained.The application can significantly improve the power frequency breakdown voltage, thermal conductivity and reduce acid value of environment-friendly ester-based insulating oil by adjusting the ratio and content of bifunctional modified nano-alumina, combined with the synergistic effect of PEI and silane coupling agent, which fills the gap of prior art.

Description

Technical Field

[0001] This invention relates to the field of electrical insulation materials technology, and in particular to a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina, its preparation method, and its application. Background Technology

[0002] Environmentally friendly ester-based insulating oils, including natural and synthetic ester insulating oils, have gained widespread attention in recent years for their high biodegradability, high flash point, and strong flame retardancy, making them an environmentally friendly insulating medium. However, environmentally friendly ester-based insulating oils still face some technical challenges in practical applications: they are prone to absorbing moisture in humid environments, leading to a significant drop in breakdown voltage and affecting the operational safety of high-voltage equipment; their electrical insulation strength under high electric fields is still slightly inferior to optimized mineral oils; the unsaturated fatty acid chains in environmentally friendly ester oils are prone to oxidative decomposition, producing acidic substances and deposits, resulting in a decline in the quality of the insulating oil and a shortened service life of the equipment; simultaneously, they easily generate trace amounts of moisture and acidic products during operation, which not only accelerate oil aging but may also corrode the metal parts of the equipment, further affecting insulation performance and operational stability.

[0003] To improve the dielectric properties of environmentally friendly ester-based insulating oils during long-term operation and extend their service life, researchers have proposed several technical methods for modifying these oils, including chemical modification, the addition of functional additives, and the introduction of nanomaterials. Among these, nanomaterials, due to their unique physicochemical properties such as high specific surface area, high dielectric constant, and excellent thermal conductivity, are an effective method for improving the performance of environmentally friendly ester-based insulating oils. Surface modification methods can modify the surface of nanomaterials, for example, by introducing coupling agents or functional polymers, improving their compatibility with environmentally friendly ester-based insulating oils. This method can significantly improve the dispersibility and stability of nanoparticles.

[0004] However, while existing methods for modifying environmentally friendly ester-based insulating oils have made some progress, several problems remain, such as insufficient dispersion of nanomaterials, unstable modification effects, and high process costs. These issues, to some extent, limit the large-scale application of environmentally friendly ester-based insulating oils in power equipment. Therefore, developing a simple, efficient, and economically feasible modification method to significantly improve the electrical properties, thermal conductivity, and anti-aging capabilities of environmentally friendly ester-based insulating oils is of paramount importance. Summary of the Invention

[0005] The purpose of this invention is to provide a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina, its preparation method and application, which can significantly improve the electrical properties, thermal conductivity and anti-aging ability of the environmentally friendly ester-based insulating oil, and the method is simple, efficient and economical.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina, comprising the following steps:

[0008] Nano-alumina was first ultrasonically mixed with a polyethyleneimine solution to perform the first modification, resulting in PEI-modified nano-alumina.

[0009] PEI-modified nano-alumina was mixed with a silane coupling agent solution by a second ultrasonic process to perform a second modification, resulting in double-modified nano-alumina.

[0010] The dual-modified nano-alumina was mixed with ester-based insulating oil to obtain a dual-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

[0011] Preferably, before mixing the nano-alumina with the polyethyleneimine solution, the nano-alumina is ultrasonically dispersed in an alcohol solvent and then subjected to vacuum filtration and drying in sequence.

[0012] Preferably, the mass ratio of the nano-alumina to the alcohol solvent is 1:16-20; the ultrasonic dispersion power is 600-800W, and the time is 25-35min;

[0013] The conditions for vacuum filtration include: a vacuum degree of 0.08–0.1 MPa, a filter membrane pore size of 0.2–0.45 μm, and a filtration time of 15–35 min.

[0014] The drying conditions include: vacuum drying, temperature of 50–90°C, and time of 3–5 hours.

[0015] Preferably, the polyethyleneimine solution has a mass concentration of 5-8%, and the solvent is anhydrous ethanol;

[0016] The mass ratio of the nano-alumina to the polyethyleneimine solution is 1:15-25;

[0017] The ultrasonic power of the first ultrasonic hybrid is 800-1400W, and the duration is 30-60min;

[0018] The stirring speed for the first modification was 140–200 rpm, and the time was 2–4 h.

[0019] Preferably, the silane coupling agent solution comprises a silane coupling agent, anhydrous ethanol, and water, wherein the volume ratio of KH-550, anhydrous ethanol, and water is 2–5:80:1–2; and the silane coupling agent includes KH550, KH560, KH570, or KH602.

[0020] Preferably, the mass ratio of the PEI-modified nano-alumina to the silane coupling agent solution is 1:15-25;

[0021] The power of the second ultrasonic mixing is 800-1400W, and the time is 30-60min;

[0022] The stirring speed for the second modification is 140-200 rpm, and the time is 2-4 h.

[0023] Preferably, the ester-based insulating oil includes rapeseed oil, soybean oil, trimethylolpropane ester oil, or pentaerythritol-based synthetic ester oil; the mass of the double-modified nano-alumina is 0.01 to 0.16 wt% of the total mass of the ester-based insulating oil.

[0024] Preferably, the step of mixing the dual-modified nano-alumina with the ester-based insulating oil includes:

[0025] The ester-based insulating oil was heated to 60°C, modified nano-alumina was added, and ultrasonic dispersion was performed. The resulting mixture was then subjected to high-shear stirring and allowed to stand to degas.

[0026] The ultrasonic dispersion power is 800-1400W, and the time is 30-60min;

[0027] The high-shear stirring speed is 2000-2500 rpm, and the stirring time is 1-1.5 h; the settling and degassing time is 12-15 h.

[0028] This invention provides a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina prepared by the preparation method described in the above technical solution.

[0029] This invention provides the application of the above-described bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina in high-voltage equipment.

[0030] This invention provides a method for preparing a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina. The method utilizes a bifunctional surface modification technique based on nano-alumina, involving first surface functionalization of nano-alumina in a PEI solution, followed by a second surface functionalization using a silane coupling agent. The bifunctional modified nano-alumina particles are then dispersed in ester-based insulating oil to obtain the bifunctional modified environmentally friendly ester-based insulating oil. The bifunctional modified nano-alumina exhibits high dielectric constant and high thermal conductivity, enabling it to effectively enhance dielectric strength and mitigate oil degradation at high temperatures in the environmentally friendly ester-based insulating oil. The PEI-modified nano-alumina surface can capture and neutralize free radicals generated by oxidation, effectively improving the antioxidant capacity of the environmentally friendly ester oil. The coupling groups of the silane coupling agent can form a more stable chemical environment with the environmentally friendly ester oil molecules, reducing the impact of acidic byproducts on oil performance. The bifunctional modification layer also improves the uniform distribution of nanoparticles in the oil, avoiding localized electric field concentration caused by particle agglomeration in the electric field. This invention can significantly improve the power frequency breakdown voltage (electrical performance), thermal conductivity (thermal performance), and reduce the acid value (anti-aging ability) of environmentally friendly ester-based insulating oil by adjusting the ratio and content of bifunctional modified nano-alumina and combining the synergistic effect of PEI and silane coupling agent, thus filling the gap in the prior art.

[0031] The method of this invention is simple, low-cost, and produces materials with high stability, making it suitable for large-scale industrial applications. Detailed Implementation

[0032] Unless otherwise specified, all raw materials or reagents used in this invention are commercially available products well known in the art.

[0033] This invention provides a method for preparing a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina, comprising the following steps:

[0034] Nano-alumina was first ultrasonically mixed with a polyethyleneimine solution to perform the first modification, resulting in PEI-modified nano-alumina.

[0035] PEI-modified nano-alumina was mixed with a silane coupling agent solution by a second ultrasonic process to perform a second modification, resulting in double-modified nano-alumina.

[0036] The dual-modified nano-alumina was mixed with ester-based insulating oil to obtain a dual-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

[0037] In this invention, nano-alumina is first ultrasonically mixed with a polyethyleneimine solution to perform a first modification, thereby obtaining PEI-modified nano-alumina.

[0038] In this invention, before mixing the nano-alumina with the polyethyleneimine solution, it is preferable to ultrasonically disperse the nano-alumina in an alcohol solvent, and then perform vacuum filtration and drying in sequence.

[0039] In this invention, the alcohol solvent is preferably anhydrous ethanol; the mass ratio of the nano-alumina to the alcohol solvent is preferably 1:16-20, more preferably 1:16-18; the power of the ultrasonic dispersion is preferably 600-800W, more preferably 600-650W, and the time is preferably 25-35min, more preferably 30min.

[0040] The vacuum filtration conditions preferably include: a vacuum degree of 0.08–0.1 MPa, more preferably 0.1 MPa; a filter membrane pore size of 0.2–0.45 μm, more preferably 0.3–0.4 μm; and a filtration time of 15–35 min, more preferably 25–30 min. The drying conditions preferably include: vacuum drying; a temperature of 50–90 °C, more preferably 60–80 °C, more preferably 65–70 °C; and a drying time of 3–5 h, more preferably 4 h.

[0041] This invention removes physically adsorbed moisture, impurities, or other contaminants that may exist on the surface of nano-alumina by ultrasonically dispersing it in an alcohol solvent, followed by vacuum filtration and drying. This process dissociates agglomerates between particles into dispersed particles, thereby improving the modification effect.

[0042] In this invention, dried nano-alumina is added to a polyethyleneimine (PEI) solution, ultrasonically mixed, and then subjected to a first modification under magnetic stirring.

[0043] In this invention, the mass concentration of the polyethyleneimine solution is preferably 5-8%, more preferably 5-6%, and the solvent is preferably anhydrous ethanol; the mass ratio of the nano-alumina to the polyethyleneimine solution is preferably 1:15-25, more preferably 1:18-21, and even more preferably 1:19-20. This invention does not impose any special limitations on the preparation process of the polyethyleneimine solution; the materials can be mixed evenly according to a process well-known in the art.

[0044] In this invention, the ultrasonic power of the first ultrasonic mixing is preferably 800-1400W, more preferably 1000-1200W, and the time is preferably 30-60min, more preferably 40-50min; the stirring speed of the first modification is preferably 140-200rpm, more preferably 170-180rpm, and the time is preferably 2-4h, more preferably 2.5-3h.

[0045] After the first modification is completed, the product is preferably subjected to centrifugation and vacuum drying in sequence to obtain PEI-modified nano-alumina. The centrifugation speed is preferably 5000-8000 rpm, more preferably 6500-7000 rpm; the time is preferably 15-30 min, more preferably 22-25 min; the vacuum drying temperature is preferably 60-80℃, more preferably 70℃, and the time is preferably 4-6 h, more preferably 5 h.

[0046] In this invention, to enable the environmentally friendly ester-based insulating oil with added nano-alumina material to have better node performance and antioxidant capacity, PEI solution is used to modify the surface of nano-alumina. The PEI-modified nano-alumina surface can capture and neutralize free radicals generated by oxidation, slow down the oxidative decomposition reaction of the environmentally friendly ester oil, enhance the oleophilicity of nano-alumina, and reduce the van der Waals forces between particles, thereby effectively preventing particle agglomeration.

[0047] After obtaining PEI-modified nano-alumina, the present invention performs a second ultrasonic mixing of PEI-modified nano-alumina with a silane coupling agent solution to carry out a second modification, thereby obtaining double-modified nano-alumina.

[0048] In this invention, the components of the silane coupling agent solution are preferably silane coupling agent, anhydrous ethanol and water, and the volume ratio of KH-550, anhydrous ethanol and water is preferably 2-5:80:1-2, more preferably 3-4:80:1.2-1.6.

[0049] In this invention, the silane coupling agent preferably includes KH550, KH560, KH570 or KH602, and more preferably KH550.

[0050] In this invention, the mass ratio of the PEI-modified nano-alumina to the silane coupling agent solution is preferably 1:15-25, more preferably 1:18-21, and even more preferably 1:19-20.

[0051] In this invention, PEI-modified nano-alumina is added to a silane coupling agent solution, and after a second ultrasonic mixing, a second modification is performed under magnetic stirring conditions.

[0052] In this invention, the power of the second ultrasonic mixing is preferably 800-1400W, more preferably 1000-1200W; the time is preferably 30-60min, more preferably 40-50min; the stirring speed of the second modification is preferably 140-200rpm, more preferably 160-170rpm; and the time is preferably 2-4h, more preferably 2.5-3h.

[0053] After completing the second modification, the present invention preferably centrifuges and vacuum dries the obtained product to obtain double-modified nano-alumina. The centrifugation speed is preferably 5000-8000 rpm, more preferably 6500-7000 rpm; the time is preferably 15-30 min, more preferably 22-25 min; the vacuum drying temperature is preferably 60-80℃, more preferably 70℃, and the time is preferably 4-6 h, more preferably 5 h.

[0054] In this invention, to enable the environmentally friendly ester-based insulating oil with added nano-alumina material to better slow down the rate of acid value increase and improve stability, a silane coupling agent is used to modify the surface of the PEI-modified nano-alumina. The coupling groups of the silane coupling agent can form a more stable chemical environment with the environmentally friendly ester oil molecules, reducing the impact of acidic byproducts on the oil performance, while further improving the interfacial compatibility between particles and oil, and significantly enhancing the dispersion stability of particles in oil.

[0055] After obtaining the dual-modified nano-alumina, the present invention mixes the dual-modified nano-alumina with ester-based insulating oil to obtain a dual-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

[0056] In this invention, the ester-based insulating oil preferably includes rapeseed oil, soybean oil, trimethylolpropane ester oil, or pentaerythritol-based synthetic ester oil; the trimethylolpropane ester oil is preferably trimethylolpropane trioctanoate; and the pentaerythritol-based synthetic ester oil is preferably pentaerythritol monooleate.

[0057] The ester-based insulating oil described in this invention is an environmentally friendly ester-based insulating oil; this invention does not have any special limitations on the source of the ester-based insulating oil, and any commercially available product well known in the art is acceptable.

[0058] In this invention, the mass of the dual-modified nano-alumina is preferably 0.01 to 0.16 wt% of the total mass of the ester-based insulating oil, more preferably 0.03 to 0.12 wt%, and even more preferably 0.05 to 0.09 wt%.

[0059] In this invention, the step of mixing the dual-modified nano-alumina with the ester-based insulating oil preferably includes:

[0060] The ester-based insulating oil was heated to 60°C, modified nano-alumina was added, and ultrasonic dispersion was performed. The resulting mixture was then subjected to high-shear stirring and allowed to stand to degas.

[0061] The power of the ultrasonic dispersion is preferably 800-1400W, more preferably 1000-1200W; the time is preferably 30-60min, more preferably 40-50min.

[0062] The high-shear stirring speed is preferably 2000-2500 rpm, more preferably 2200-23000 rpm; the stirring time is preferably 1-1.5 h, more preferably 1.2-1.5 h; and the settling and degassing time is preferably 12-15 h, more preferably 12 h.

[0063] This invention provides a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina prepared by the preparation method described in the above technical solution.

[0064] This invention provides the application of the above-described bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina in high-voltage equipment. This invention does not specifically limit the method of application; any method well-known in the art can be used.

[0065] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0066] Unless otherwise specified, all reagents or instruments used in the embodiments of this application are conventional reagent products that can be obtained commercially.

[0067] Example 1

[0068] (1) Nano alumina and anhydrous ethanol were mixed at a mass ratio of 1:16 and ultrasonically dispersed at room temperature for 30 min with a power of 800 W. Then, vacuum filtration was performed with a vacuum degree of 0.1 MPa, a filter membrane pore size of 0.3 μm, and a filtration time of 25 min. The filtered nano alumina was then vacuum dried at a temperature of 70 °C for 4 h for later use.

[0069] (2) The dried nano-alumina from step (1) was mixed with a 5% PEI solution (solvent is anhydrous ethanol) at a mass ratio of 1:20 and ultrasonically dispersed at 1000W at room temperature for 40 min; then magnetically stirred at 170 rpm for 3 h; then centrifuged at 6500 rpm for 22 min; the centrifuged nano-alumina was vacuum dried at 70℃ for 5 h to obtain PEI-modified nano-alumina.

[0070] (3) KH-550, anhydrous ethanol, and deionized water were mixed in a volume ratio of 4:80:1.6. The resulting KH-550 solution was mixed with the dried PEI-modified nano-alumina from step (2) in a mass ratio of PEI-modified nano-alumina to KH-550 solution of 1:20. The mixture was ultrasonically dispersed at room temperature for 40 min at a power of 1000 W. Then, magnetic stirring was performed at a speed of 170 rpm for 3 h. The mixture was then centrifuged at a speed of 6500 rpm for 22 min. The centrifuged nano-alumina was then vacuum dried at a temperature of 70 °C for 5 h to obtain double-modified nano-alumina.

[0071] (4) The obtained dual-modified nano alumina was added to FR3 refined soybean oil (Cargill, China) heated to 60°C, wherein the nano alumina accounted for 0.05 wt% of FR3 refined soybean oil. The mixture was ultrasonically dispersed at 1000W for 40 min, and then high-speed sheared and stirred at 2200 rpm for 1.5 h. After standing and degassing for 12 h, the dual-function modified environmentally friendly ester-based insulating oil was obtained.

[0072] Example 2

[0073] Prepared according to the method of Example 1, the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.07% of the total mass of the environmentally friendly ester-based insulating oil.

[0074] Example 3

[0075] Prepared according to the method of Example 1, the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.09% of the total mass of the environmentally friendly ester-based insulating oil.

[0076] Example 4

[0077] Prepared according to the method of Example 1, the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.11% of the total mass of the environmentally friendly ester-based insulating oil.

[0078] Example 5

[0079] Prepared according to the method of Example 1, the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.12% of the total mass of the environmentally friendly ester-based insulating oil.

[0080] Example 6

[0081] Prepared according to the method of Example 1, the only difference from Example 1 is that the mass of the double-modified nano alumina is 0.09% of the total mass of the environmentally friendly ester-based insulating oil; the mass ratio of the dried nano alumina to the PEI solution in step (2) is 1:19.

[0082] Example 7

[0083] The method of Example 1 was used for preparation, except that the mass of the double-modified nano-alumina was 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mass ratio of the dried nano-alumina to the PEI solution in step (2) was 1:21.

[0084] Example 8

[0085] The method of Example 1 was used for preparation, except that the mass of the double-modified nano alumina was 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mass ratio of PEI-modified nano alumina to KH-550 solution in step (3) was 1:19.

[0086] Example 9

[0087] The method of Example 1 was used for preparation, except that the mass of the double-modified nano alumina was 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mass ratio of the PEI-modified nano alumina to the KH-550 solution in step (3) was 1:21.

[0088] Comparative Example 1

[0089] FR3 refined soybean oil without nano-alumina (source same as in Example 1).

[0090] Comparative Example 2

[0091] Karamay #25 mineral oil without nano-alumina.

[0092] Comparative Example 3

[0093] Prepared according to the method of Example 1, the only difference from Example 1 is that FR3 refined soybean oil is replaced with Karamay #25 mineral oil.

[0094] Test case

[0095] The power frequency breakdown voltage, thermal conductivity at 25℃, and acid value of the insulating oils prepared in Examples 1-9 and Comparative Examples 1-3 were tested using the methods described in national standards GB / T 507-2002, GB / T 29313-2012, and GB / T 28552-2012. The performance test data are shown in Table 1 below:

[0096] Table 1. Test data of insulating oil performance in Examples 1-9 and Comparative Examples 1-3.

[0097]

[0098]

[0099] Table 1 shows that the addition of bifunctional modified nano-alumina material to environmentally friendly ester-based insulating oil significantly improves its power frequency breakdown voltage, increases its thermal conductivity, and reduces its acid value; it also has a good effect on improving the insulation performance of environmentally friendly ester-based insulating oil. When the content of bifunctional modified nano-alumina is 0.12 wt% of the total content of environmentally friendly ester-based insulating oil, the nanoparticles are prone to agglomeration and sedimentation, easily forming conductive channels in the oil, thus weakening the improvement effect on power frequency breakdown voltage and acid value. Compared with environmentally friendly ester-based insulating oil, the improvement effect of bifunctional modified nano-alumina material on mineral oil is not very obvious. This is because mineral oil has fewer polar molecules, resulting in weaker interfacial interactions with particles, and the ability of nano-alumina particles to capture free radicals is limited by the properties of the oil.

[0100] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina, characterized in that, Includes the following steps: Nano-alumina was first ultrasonically mixed with a polyethyleneimine solution to perform the first modification, resulting in PEI-modified nano-alumina. PEI-modified nano-alumina was mixed with a silane coupling agent solution by a second ultrasonic process to perform a second modification, resulting in double-modified nano-alumina. The dual-modified nano-alumina was mixed with ester-based insulating oil to obtain a dual-functional modified environmentally friendly ester-based insulating oil based on nano-alumina. The mass concentration of the polyethyleneimine solution is 5-8%, and the mass ratio of the nano-alumina to the polyethyleneimine solution is 1:15-25; The mass ratio of the PEI-modified nano-alumina to the silane coupling agent solution is 1:15~25; The silane coupling agent solution comprises a silane coupling agent, anhydrous ethanol, and water, wherein the silane coupling agent includes KH550, KH560, KH570, or KH602.

2. The preparation method according to claim 1, characterized in that, Before mixing the nano-alumina with the polyethyleneimine solution, the nano-alumina is ultrasonically dispersed in an alcohol solvent and then subjected to vacuum filtration and drying in sequence.

3. The preparation method according to claim 2, characterized in that, The mass ratio of the nano-alumina to the alcohol solvent is 1:16~20; the ultrasonic dispersion power is 600~800W, and the time is 25~35min; The conditions for vacuum filtration include: a vacuum degree of 0.08~0.1 MPa, a filter membrane pore size of 0.2~0.45 μm, and a filtration time of 15~35 min; The drying conditions include: vacuum drying, temperature of 50~90℃, and time of 3~5h.

4. The preparation method according to claim 1, characterized in that, The solvent for the polyethyleneimine solution is anhydrous ethanol; The ultrasonic power of the first ultrasonic hybrid is 800~1400W, and the duration is 30~60min; The stirring speed for the first modification is 140~200 rpm, and the time is 2~4 h.

5. The preparation method according to claim 1, characterized in that, The volume ratio of KH550, anhydrous ethanol, and water is 2~5:80:1~2.

6. The preparation method according to claim 1 or 5, characterized in that, The power of the second ultrasonic mixing is 800~1400W, and the time is 30~60min; The stirring speed for the second modification is 140~200 rpm, and the time is 2~4 h.

7. The preparation method according to claim 1, characterized in that, The ester-based insulating oil includes rapeseed oil, soybean oil, trimethylolpropane ester oil, or pentaerythritol-based synthetic ester oil; the mass of the double-modified nano-alumina is 0.01~0.16 wt% of the total mass of the ester-based insulating oil.

8. The preparation method according to claim 1, characterized in that, The step of mixing the dual-modified nano-alumina with the ester-based insulating oil includes: The ester-based insulating oil was heated to 60°C, modified nano-alumina was added, and ultrasonic dispersion was performed. The resulting mixture was then subjected to high-shear stirring and allowed to stand to degas. The ultrasonic dispersion power is 800~1400W, and the time is 30~60min; The high-shear stirring speed is 2000~2500 rpm, and the stirring time is 1~1.5h; the settling and degassing time is 12~15h.

9. The bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina as described in claim 9 in high-voltage equipment.

Images