Synthetic ester insulating oil and preparation method thereof

Abstract

The invention provides synthetic ester insulating oil and a preparation method thereof, and relates to the field of insulating oil. The preparation method comprises the following steps: S1, esterification reaction: adding C5-C10 saturated fat polyol and C8-C18 saturated fatty acid into a reaction kettle, adding a catalyst, replacing air in the reaction kettle with nitrogen, heating to 200-210 DEG C in a gradient manner, and reacting for 1-1.5 hours; s2, refining: filtering materials in the reaction kettle at 90-100 DEG C, collecting filtrate, performing reduced pressure distillation, collecting middle fraction, decolorizing with activated clay, filtering, adsorbing with a molecular sieve, and filtering to obtain base oil; s3, compounding: heating the base oil to 50-60 DEG C, adding the additive, shearing, stirring and filtering to obtain the insulating oil. The insulating oil has the advantages of high oxidation stability, high thermal stability, low pour point and high viscosity index.

Description

Technical Field

[0001] The present invention relates to the field of insulating oils, and more particularly, to a synthetic ester insulating oil and a preparation method thereof. Background Art

[0002] Insulating oil is an important liquid insulating medium in oil-immersed transformers, mainly playing roles such as insulation, heat dissipation, and arc quenching. The performance of insulating oil determines the environmental protection characteristics, fire prevention ability of the transformer, and affects its service life.

[0003] Traditional insulating oils (mineral oils, silicone oils, ordinary synthetic esters) have certain limitations. For example, their thermal stability is insufficient: they are easily oxidized and decomposed at high temperatures, resulting in a decline in insulation performance; their environmental friendliness is poor: mineral oils have low biodegradability and leakage pollutes the environment; their dielectric properties are limited: indicators such as breakdown voltage and volume resistivity are difficult to meet the requirements of high-voltage equipment; there is a risk of flammability: some synthetic esters have a low flash point, posing a safety hazard. On the other hand, since fossil energy will gradually be depleted, synthetic esters made from biological oils can overcome the defects of mineral oils, but currently, synthetic ester insulating oils still have the following defects: unsaturated esters are easily oxidized, producing acidic substances that corrode equipment during long-term use; the formula is complex, the compatibility of additives is poor, and the cost is high. Summary of the Invention

[0004] The purpose of the present invention is to provide a synthetic ester insulating oil, which has the advantages of high oxidation stability, high thermal stability, low pour point, and high viscosity index.

[0005] Another purpose of the present invention is to provide a preparation method of a synthetic ester insulating oil. The preparation process is simple, and the insulating oil prepared by this method has the advantages of high oxidation stability, high thermal stability, low pour point, and high viscosity index.

[0006] The present invention solves its technical problems by adopting the following technical solutions.

[0007] On the one hand, an embodiment of the present invention provides a preparation method of a synthetic ester insulating oil, including the following steps: S1 Esterification reaction: Put C5-C10 saturated fatty polyols and C8-C18 saturated fatty acids into a reaction kettle, add a catalyst, displace the air in the reaction kettle with nitrogen, and gradually heat up to 200-210°C and react for 1-1.5 h;

[0008] S2 Refining: At 90-100°C, filter the materials in the reaction kettle, collect the filtrate, perform vacuum distillation, collect the middle fraction (the middle fraction with an acid value ≤ 0.1 mg KOH / g), decolorize with activated clay, filter, adsorb with molecular sieves, and filter to obtain the base oil;

[0009] S3 compounding: Heat the base oil to 50 - 60 °C, add additives, shear and stir, and filter to obtain the insulating oil. Among them, the rotation speed of shear stirring is 5000 rpm, the shear stirring time is 1 hour, and after shear stirring, the Hegman fineness of the insulating oil ≤ 10 μm.

[0010] In some embodiments of the present invention, the molar ratio of the above-mentioned C5 - C10 saturated fatty polyols to C8 - C18 saturated fatty acids is 1:(3 - 4).

[0011] In some embodiments of the present invention, the above-mentioned gradient temperature increase includes: the first stage: heat from 120 °C to 160 °C, the temperature increase time is 1 hour, and in this stage, the dehydration rate of the reaction system ≥ 30%;

[0012] The second stage: heat from 160 °C to 200 °C, the temperature increase time is 4 hours, and after the temperature increase in this stage ends, the acid value of the reaction system drops to ≤ 0.5 mg KOH / g;

[0013] The third stage: keep the temperature constant at 200 - 210 °C for 1 - 1.5 hours, and after the temperature increase in this stage ends, the acid value of the reaction system ≤ 0.2 mg KOH / g.

[0014] In some embodiments of the present invention, the above-mentioned C5 - C10 saturated fatty polyols are one or more of pentaerythritol, neopentyl glycol, 2,2 - bis(hydroxymethyl)butanol.

[0015] In some embodiments of the present invention, the above-mentioned C8 - C18 saturated fatty acids are one or more of lauric acid, stearic acid, palmitic acid.

[0016] In some embodiments of the present invention, in the above-mentioned step S2, a plate and frame filter press is used for filtration treatment, and the pore size of the filter membrane is 5 μm.

[0017] In some embodiments of the present invention, in the above-mentioned step S2, the vacuum degree of vacuum distillation is 1 KPa, and the temperature is 200 °C.

[0018] In some embodiments of the present invention, in the above-mentioned step S2, the activated clay decolorization includes adding activated clay to the middle distillate and stirring at 80 - 90 °C for 1 - 1.5 h, wherein the addition amount of activated clay is 3 - 5% of the mass of the middle distillate.

[0019] In some embodiments of the present invention, in the above-mentioned step S2, the molecular sieve adsorption includes adding molecular sieve to the middle distillate and standing still at room temperature for 10 - 12 h, wherein the addition amount of molecular sieve is 3 - 5% of the mass of the middle distillate.

[0020] In some embodiments of the present invention, the additives include antioxidants, corrosion inhibitors, and solvents. Among them, the antioxidant is Irganox 1076, and the addition amount is 0.5% of the mass of the base oil; the corrosion inhibitor is benzotriazole, and the addition amount is 0.2% of the mass of the base oil; the solvent is PMA (propylene glycol methyl ether acetate), and the addition amount is 0.3% of the mass of the base oil.

[0021] On the other hand, an embodiment of the present invention provides a synthetic ester insulating oil prepared by the above method.

[0022] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects:

[0023] The synthetic ester insulating oil provided by the present invention uses saturated polyols and saturated fatty acids as raw materials, and synthesizes saturated fatty esters through an esterification reaction. The fatty acid chains and alcohol components in the ester molecules are both saturated structures (without double bonds). High oxidation stability: Avoid the increase in acid value and the generation of by-products caused by the oxidation of double bonds; high thermal stability: The decomposition temperature is high (>250 °C), suitable for high-temperature insulating oils. The alcohol component uses branched polyols (such as saturated fatty polyols like neopentyl glycol, etc.), and the ester molecules have a three-dimensional spatial structure. Low pour point: The branched chain destroys the regular arrangement of molecules, reducing the freezing point (below -40 °C); high viscosity index: The viscosity changes little with temperature, broadening the applicable temperature range. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 It is a schematic flow chart of the method for preparing the insulating oil in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0027] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to specific embodiments.

[0028] On the one hand, an embodiment of the present invention provides a method for preparing synthetic ester insulating oil, comprising the following steps:

[0029] S1 Esterification reaction: Put C5-C10 saturated fatty polyols and C8-C18 saturated fatty acids into a reaction kettle, add a catalyst, displace the air in the reaction kettle with nitrogen, and gradually heat up to 200-210 °C and react for 1-1.5 h; under the action of the catalyst, the alcohol and the acid undergo an esterification reaction to generate an ester. And before the reaction, by displacing the air with nitrogen, the air and oxygen in the reaction kettle are removed, so that an inert atmosphere is maintained in the reaction kettle to avoid the oxidation of reactants and products. The molar ratio of C5-C10 saturated fatty polyols to C8-C18 saturated fatty acids is 1:(3-4). By adjusting the molar ratio of the alcohol and the acid, the esterification degree of the product is adjusted, that is, the performance of the insulating oil is adjusted. For example, when there is less acid, mainly triester is generated, and some hydroxyl groups do not participate in the esterification reaction, which can improve the polarity of the insulating oil but slightly reduce the thermal stability. When the hydroxyl group in the alcohol and the carboxyl group in the acid are 1:1, complete esterification occurs, there is no free hydroxyl group, and the obtained insulating oil has the best thermal temperature performance and the highest viscosity.

[0030] It should be noted that regarding the selection of fatty acids, C8 fatty acids have low viscosity, ultra-low pour point, but relatively low dielectric strength, and are suitable for extremely low temperature environments;

[0031] C12 fatty acids have balanced comprehensive properties such as viscosity, pour point, and dielectric strength, and need to be compounded with additives during use, and can be used as general-purpose insulating oils;

[0032] C18 fatty acids have high dielectric strength, high thermal stability, and high viscosity (assisted by a pour point depressant), and are suitable for high-temperature and high-pressure equipment.

[0033] Regarding the selection of catalysts, solid acid catalysts: suitable for long-chain fatty acids (C16-C18), with stable activity at high temperatures; biocatalyst enzymes: dedicated to short-chain fatty acids (C8-C10), and the water content needs to be controlled ≤ 0.1%; composite metal oxide catalysts: compatible with mixed acid systems and can introduce flame retardant elements (such as phosphorus).

[0034] Among them, the gradient heating is as follows: the first stage: heat up from 120 °C to 160 °C, and the heating time is 1 hour. In this stage, the dehydration rate of the reaction system ≥ 30%;

[0035] The second stage: heat up from 160 °C to 200 °C, and the heating time is 4 hours. After the heating in this stage ends, the acid value of the reaction system drops to ≤ 0.5 mg KOH / g;

[0036] The third stage: keep the temperature constant at 200-210 °C for 1-1.5 hours. After the heating in this stage ends, the acid value of the reaction system ≤ 0.2 mg KOH / g.

[0037] S2 Refining: At 90 - 100 °C, filter the materials in the reaction kettle to recover the catalyst, collect the filtrate, perform vacuum distillation, and collect the middle fraction (the middle fraction with an acid value ≤ 0.1 mg KOH / g).

[0038] Decolorize with activated clay, filter, adsorb with molecular sieve, filter to obtain the base oil;

[0039] S3 Blending: Heat the base oil to 50 - 60 °C to reduce its viscosity, then add additives (antioxidants, anti-corrosion agents, solvents, etc.), perform shear stirring, and filter to obtain the insulating oil. Among them, the rotation speed of shear stirring is 5000 rpm, the shear stirring time is 1 hour, and after shear stirring, the Hegman fineness of the insulating oil ≤ 10 μm.

[0040] The above C5 - C10 saturated fatty polyols are one or more of pentaerythritol, neopentyl glycol, 2,2-bis(hydroxymethyl)butanol. More preferably, pentaerythritol is selected as the raw material, which has many branches and better heat resistance stability after esterification.

[0041] The above C8 - C18 saturated fatty acids are one or more of lauric acid, stearic acid, palmitic acid. More preferably, lauric acid and stearic acid are selected as the raw materials.

[0042] In the above step S2, a plate and frame filter press is used for filtration treatment, and the pore size of the filter membrane is 5 μm. The vacuum degree of vacuum distillation is 1 KPa, and the temperature is 200 °C. The decolorization with activated clay includes adding activated clay to the middle fraction and stirring at 80 - 90 °C for 1 - 1.5 h, where the addition amount of activated clay is 3 - 5% of the mass of the middle fraction. The adsorption with molecular sieve includes adding molecular sieve to the middle fraction and standing still at room temperature for 10 - 12 h, where the addition amount of molecular sieve is 3 - 5% of the mass of the middle fraction.

[0043] The additives include antioxidants, anti-corrosion agents, solvents. Among them, the antioxidant is Irganox 1076, and the addition amount is 0.5% of the mass of the base oil; the anti-corrosion agent is benzotriazole, and the addition amount is 0.2% of the mass of the base oil; the solvent is PMA (propylene glycol methyl ether acetate), and the addition amount is 0.3% of the mass of the base oil.

[0044] The features and properties of the present invention will be further described in detail below in conjunction with examples.

[0045] Example 1

[0046] Prepare the insulating oil of this example according to the following method:

[0047] Step 1: Esterification reaction

[0048] Charging and premixing: Pentaerythritol (1 mol), lauric acid (2.4 mol), and stearic acid (1.6 mol) are charged into the reaction kettle.

[0049] Add a catalyst (sulfonated carbon-based solid acid, 2% of the total raw material mass), and displace with nitrogen three times (oxygen content in the reaction kettle ≤ 50 ppm).

[0050] Subsequently, carry out a gradient temperature rise reaction:

[0051] Stage 1: 120°C → 160°C (1 hour, dehydration rate in the reaction kettle ≥ 30%);

[0052] Stage 2: 160°C → 200°C (4 hours, acid value drops to ≤ 0.5 mg KOH / g);

[0053] Stage 3: Keep the temperature constant at 200°C (1 hour, acid value ≤ 0.2 mg KOH / g).

[0054] On-line monitoring:

[0055] During the temperature rise reaction process, it is necessary to detect the acid value in real time (automatic titrator), and the end point determination condition: acid value ≤ 0.2 mg KOH / g; the reaction water in the reaction kettle is discharged through the water separator (dehydration amount ≥ 95% of the theoretical value).

[0056] Step 2: Post-treatment and refining

[0057] Catalyst separation: Thermal filtration (90°C, plate and frame filter press, pore size 5 μm), catalyst recovery rate ≥ 95%;

[0058] Catalyst regeneration: Wash with ethanol, dry at 120°C, and recycle.

[0059] Vacuum distillation: Vacuum degree 1 kPa, temperature 200°C, to remove unreacted fatty acids and low-boiling substances; collect the intermediate fraction with an acid value ≤ 0.1 mg KOH / g.

[0060] Adsorption purification: Add activated clay to the collected intermediate fraction, the addition amount is 4% of the intermediate fraction mass, stir at 80°C for 1 hour, and then filter;

[0061] Then add 3A molecular sieve, the addition amount is 3% of the intermediate fraction mass, adsorb at room temperature for 12 hours, and then filter to obtain the base oil.

[0062] Step 3: Compound modification

[0063] Base oil preheating: Heat the base oil to 60°C and keep it constant to reduce the viscosity;

[0064] Additive mixing: Add Irganox 1076 (0.5%), benzotriazole (0.2%), and PMA (0.3%) in sequence; perform shear stirring at 5000 rpm for 1 hour, with Hegman fineness ≤ 10 μm.

[0065] Final filtration: Filter through a 5-μm filter element to remove undispersed particles, and thus obtain insulating oil.

[0066] Example 2

[0067] Prepare the insulating oil of this example according to the following method. Select long-chain fatty acids. This insulating oil is applicable to UHV transformers and DC transmission equipment.

[0068] Step 1: Esterification reaction

[0069] Charging and premixing: Charge pentaerythritol (1 mol) and stearic acid (4 mol) into the reaction kettle;

[0070] Add a catalyst (sulfonated carbon-based solid acid, 2% of the total raw material mass), and displace with nitrogen 3 times (oxygen content in the reaction kettle ≤ 50 ppm).

[0071] Subsequently, perform gradient temperature rise reaction:

[0072] Stage 1: 120°C → 160°C (1 hour, dehydration rate in the reaction kettle ≥ 30%);

[0073] Stage 2: 160°C → 200°C (4 hours, acid value drops to ≤ 0.5 mg KOH / g);

[0074] Stage 3: Keep the temperature at 200°C (1 hour, acid value ≤ 0.3 mg KOH / g).

[0075] On-line monitoring:

[0076] During the temperature rise reaction process, it is necessary to perform real-time detection of the acid value (automatic titrator). The end-point determination condition: acid value ≤ 0.3 mg KOH / g; the water separator discharges the reaction water in the reaction kettle (dehydration amount ≥ 95% of the theoretical value).

[0077] Step 2: Post-treatment and refining

[0078] Catalyst separation: Perform hot filtration (90°C, plate and frame filter press, pore size 5 μm), with catalyst recovery rate ≥ 95%;

[0079] Catalyst regeneration: Wash with ethanol, dry at 120°C, and recycle.

[0080] Vacuum distillation: Vacuum degree 1 kPa, temperature 220°C, remove unreacted fatty acids and low-boiling substances; collect the middle fraction with acid value ≤ 0.1 mg KOH / g.

[0081] Adsorption purification: Activated clay is added to the collected middle fraction, with the addition amount being 4% of the mass of the middle fraction, and stirred at 80 °C for 1 hour, followed by filtration;

[0082] Then, 3A molecular sieve is added, with the addition amount being 3% of the mass of the middle fraction, and adsorbed at room temperature for 12 hours, followed by filtration to obtain the base oil.

[0083] Step 3: Compound modification

[0084] Base oil preheating: Heat the base oil to 60 °C and keep it constant to reduce the viscosity;

[0085] Additive mixing: Irganox 1076 (0.5%), benzotriazole (0.2%), and PMA (0.3%) are added in sequence; shear and stir at 5000 rpm for 1 hour, and the Hegman fineness ≤ 10 μm.

[0086] Final filtration: Filter with a 5 μm filter element to remove undispersed particles, and the insulating oil is obtained.

[0087] Example 3

[0088] The insulating oil of this example is prepared by the following method. Short-chain fatty acids are selected, and this insulating oil is applicable to transformers in cold regions and charging piles for new energy vehicles:

[0089] Step 1: Esterification reaction

[0090] Feeding and premixing: Pentaerythritol (1 mol) and octanoic acid (3.2 mol) are put into the reaction kettle;

[0091] Add a catalyst (lipase, Novozyme 435, 1.5% of the total raw material mass), and replace with nitrogen 3 times (the oxygen content in the reaction kettle ≤ 50 ppm).

[0092] Subsequently, carry out gradient temperature rise reaction:

[0093] Stage 1: 50 °C → 60 °C (0.5 hour, oxygen content ≤ 50 ppm);

[0094] Stage 2: Keep at 60 °C (24 hours, acid value drops to ≤ 0.5 mg KOH / g);

[0095] Stage 3: 60 °C → 70 °C (6 hours, acid value ≤ 0.2 mg KOH / g).

[0096] Gradient temperature rise from 50 °C → 60 °C → 70 °C, total duration 32 hours (acid value ≤ 0.2 mg KOH / g)

[0097] Online monitoring:

[0098] During the heating reaction process, the acid value needs to be detected in real time (using an automatic titrator), and the end-point determination condition is: acid value ≤ 0.2 mg KOH / g; the reaction water in the reaction kettle is discharged through a water separator (the dehydration amount ≥ 95% of the theoretical value).

[0099] Step 2: Post-treatment and refining

[0100] Recovery of enzyme catalyst: Pressurized filtration through a 5-μm nylon filter membrane at 60 °C, and the recovery rate ≥ 90%;

[0101] Regeneration: Wash with pH 7.0 phosphate buffer solution and store at 4 °C (the activity retention ≥ 80%, and it can be recycled 3 times)

[0102] Solvent removal:

[0103] Vacuum distillation at 50 °C (vacuum degree 10 kPa), and the recovery rate of n-hexane ≥ 95%;

[0104] The solvent is dehydrated by molecular sieve (3A type, 4% mass ratio) and then reused.

[0105] Product refining:

[0106] Decolorization with activated carbon (1% addition amount, stirring at 60 °C for 1 hour);

[0107] Dehydration with molecular sieve (adsorbing at room temperature for 12 hours, water content ≤ 100 ppm);

[0108] Sealed storage by filling nitrogen through a 0.45-μm filter membrane.

[0109] Step 3: Compound modification

[0110] Preheating of base oil: Heat the base oil to 60 °C - 70 °C for constant temperature to reduce the viscosity;

[0111] Additive mixing: Add Irganox 1076 (0.5%), benzotriazole (0.2%), PMA (0.3%), Span 80 (0.1%) in sequence; Shear and stir at 800 rpm for 2 hours, and the Hegman fineness ≤ 10 μm.

[0112] Final filtration: Filter through a 1-μm filter element to remove undispersed particles, and store in a nitrogen-filled sealed manner to obtain insulating oil.

[0113] The equipment and process parameters used in the preparation method of the above examples are shown in Table 1:

[0114] Table 1 Equipment and parameters

[0115]

[0116] Experimental example

[0117] Taking the insulating oils of Examples 1-3 as the detection objects, the properties of each insulating oil were tested, and the results are shown in Table 2.

[0118] Table 2 Properties of Each Insulating Oil

[0119]

[0120] Among them, the mineral insulating oil of the comparative example was directly purchased from the market.

[0121] It can be concluded from Table 2 that the insulating oil prepared in the examples has the advantages of high oxidation stability, high thermal stability, low pour point, and high viscosity index.

[0122] In summary, the synthetic ester insulating oil provided by the present invention uses saturated polyols and saturated fatty acids as raw materials, and synthesizes saturated fatty esters through an esterification reaction. The fatty acid chains and alcohol components in the ester molecules are both saturated structures (without double bonds). High oxidation stability: Avoid the increase in acid value and the generation of by-products caused by the oxidation of double bonds; High thermal stability: The decomposition temperature is high (>250 °C), suitable for high-temperature insulating oils. The alcohol component uses branched polyols (such as saturated fatty polyols like neopentyl glycol), and the ester molecules have a three-dimensional spatial structure. Low pour point: The branches disrupt the regular arrangement of molecules and lower the freezing point (such as below -40 °C); High viscosity index: The viscosity changes little with temperature, broadening the applicable temperature range.

[0123] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

Claims

1. A preparation method of synthetic ester insulating oil, characterized in that, It includes the following steps: S1 Esterification reaction: Put C5-C10 saturated fatty polyols and C8-C18 saturated fatty acids into a reaction kettle, add a catalyst, displace the air in the reaction kettle with nitrogen, gradually heat up to 200-210 °C, and react for 1-1.5 h; S2 Refining: At 90-100 °C, filter the materials in the reaction kettle, collect the filtrate, conduct vacuum distillation, collect the middle fraction, decolorize with activated clay, filter, adsorb with molecular sieve, and filter to obtain the base oil; S3 Compound: Heat the base oil to 50-60 °C, add additives, shear and stir, and filter to obtain the insulating oil.

2. The preparation method of the synthetic ester insulating oil according to claim 1, characterized in that, The molar ratio of the C5-C10 saturated fatty polyols to the C8-C18 saturated fatty acids is 1:(3-4).

3. The preparation method of the synthetic ester insulating oil according to claim 1, wherein, The gradual heating includes: the first stage: heat up from 120 °C to 160 °C, the heating time is 1 hour, and the dehydration rate ≥ 30%; The second stage: heat up from 160 °C to 200 °C, the heating time is 4 hours, and the acid value of the reaction system drops to ≤ 0.5 mgKOH / g; The third stage: keep the temperature constant at 200-210 °C for 1-1.5 hours, and the acid value of the reaction system ≤ 0.2 mg KOH / g.

4. The preparation method of the synthetic ester insulating oil according to claim 1, characterized in that, The C5-C10 saturated fatty polyols are one or more of pentaerythritol, neopentyl glycol, 2,2-bis(hydroxymethyl)butanol, etc.

5. The preparation method of the synthetic ester insulating oil according to claim 1, characterized in that, The C8-C18 saturated fatty acids are one or more of lauric acid, stearic acid, palmitic acid, etc.

6. The preparation method of the synthetic ester insulating oil according to claim 1, wherein, In the step S2, a plate and frame filter press is used for filtration treatment, and the pore size of the filter membrane is 5 μm.

7. The preparation method of the synthetic ester insulating oil according to claim 1, characterized in that, In the step S2, the vacuum degree of the vacuum distillation is 1 KPa, and the temperature is 200 °C.

8. The preparation method of the synthetic ester insulating oil according to claim 1, characterized in that, In the step S2, the decolorization with activated clay includes adding activated clay to the middle fraction and stirring at 80-90 °C for 1-1.5 h. Among them, the addition amount of activated clay is 3-5% of the mass of the middle fraction.

9. The preparation method of the synthetic ester insulating oil according to claim 1, wherein, In the step S2, the adsorption with molecular sieve includes adding molecular sieve to the middle fraction and standing still at room temperature for 10-12 h. Among them, the addition amount of molecular sieve is 3-5% of the mass of the middle fraction.

10. A synthetic ester insulating oil, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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