CN119639506A

CN119639506A - A method for preparing environmentally friendly plant-based transformer oil

Info

Publication number: CN119639506A
Application number: CN202411816116.2A
Authority: CN
Inventors: 高晓谋; 徐云帆; 杨桂芳
Original assignee: Anhui Zhong Tian Petrochemical Co ltd
Current assignee: Anhui Zhong Tian Petrochemical Co ltd
Priority date: 2024-12-11
Filing date: 2024-12-11
Publication date: 2025-03-18

Abstract

The invention discloses a preparation method of environment-friendly plant-based transformer oil, and belongs to the technical field of high-temperature transformer oil. The raw materials comprise, by mass, 1-8% of viscosity index improver, 2-5% of antioxidant, 0.05-0.2% of anti-emulsifying agent, 0.1-0.3% of acid scavenger, and the balance of modified castor oil base oil. The invention has the advantages that the viscosity index and the oil-water separation capacity of the transformer oil can be improved, the hydrolysis can be inhibited, the castor oil is used as the base oil, the degradability of the transformer oil is endowed, the castor oil base oil is improved, the flash point and the breakdown voltage of the transformer oil can be improved, the antioxidants can play a role in synergy, and the oxidation resistance of the transformer oil is obviously enhanced, so that the transformer oil prepared by the invention has excellent comprehensive performance, good flash point, breakdown voltage and oxidation resistance, is degradable, and has important application value in the technical field of high-temperature transformer oil.

Description

Preparation method of environment-friendly plant-based transformer oil

Technical Field

The invention belongs to the technical field of high-temperature transformer oil, and particularly relates to a preparation method of environment-friendly plant-based transformer oil.

Background

The transformer oil is used as an insulating medium of a transformer, plays a core role of insulation, cooling and protection, and the performance of the transformer oil is directly related to the safe and stable operation of the transformer. Under the continuous actions of high voltage, high current and strong magnetic field, heat accumulation is easy to generate in the transformer. As the running time increases, the transformer oil will undergo thermal and electrical ageing phenomena, leading to a gradual degradation of its performance and even to a transformer failure in severe cases. The temperature rise of the transformer not only affects the load capacity and the service life of the transformer, but also increases the energy consumption of the power system, and if the heat dissipation is poor, the normal operation of the equipment is threatened, and even faults are caused. Heat aging is a main cause of performance degradation of transformer oil, and transformer oil temperature is one of key indexes for measuring safe operation state of the transformer oil. According to national standards and industry specifications, the oil temperature of transformer oil is generally maintained within a range of 35-75 ℃, and the oil temperature can be allowed to rise to 85 ℃ under specific conditions. Therefore, the oxidation resistance and heat resistance of the transformer oil are particularly important.

At present, the transformer oil widely used at home and abroad is mainly mineral-based basic lubricating oil. Although mineral oils are widely used because of their low cost, many problems are exposed to be solved in practical applications. The primary problem is that mineral oil has a low ignition point, fire hazards exist in the using process, and each year, the fire accidents caused by the transformers not only damage adjacent electric facilities, but also can form a great threat to personnel safety. Secondly, oil leakage of the transformer is one of common faults, and once mineral oil leaks, the mineral oil can have serious influence on human health. In addition, since mineral oil is severely polluted and difficult to naturally degrade, one hundred tons of drinking water can be polluted per ton of leaked mineral oil, and a large number of microorganisms in the soil can die after the leakage into the soil. Obviously, with the advancement and development of society, the hazards presented by mineral oil as an insulating medium for transformers have clearly exceeded their own advantages. In view of this, the international and domestic research trends are gradually moving towards the development of new more environmentally friendly lubricating oils.

The natural ester type transformer oil prepared by synthesizing natural vegetable oil is used as a renewable energy source, and the vegetable oil content in the components is up to 98 percent. Compared with mineral oil, the natural ester has remarkable advantages in the aspects of oxidation resistance, aging resistance, sustainable development, environmental protection and the like. First, the natural ester oil has a fire point as high as 360 ℃ and little flammability, and belongs to high fire point insulating oil. Therefore, the transformer adopting the natural ester oil has higher fire-proof grade and the service life is obviously prolonged. And secondly, the vegetable oil serving as a raw material is a renewable resource, has obvious environmental protection advantage, and can be continuously supplied through agricultural production, so that the dependence on non-renewable resources is effectively reduced. Furthermore, vegetable oils exhibit excellent biodegradability, almost completely degrading in natural conditions, which greatly reduces their impact on the ecological environment. In contrast, mineral oil is difficult to degrade, and vegetable oil provides a more sustainable solution in terms of reducing environmental pollution, and becomes an ideal substitute for environment-friendly transformer oil.

With the rapid development of the power industry, large-capacity, high-parameter and high-voltage transformers are increasingly used. In recent years, the successful operation of 420kV power transformers using natural ester insulating oil at the first stage worldwide marks an important step for replacing mineral oil with natural ester oil. In the continuous testing and application process, the environment-friendly transformer based on the natural ester oil has the outstanding advantage, and is particularly suitable for power systems of 35kV and below. The application of these ultra-high voltage transformers puts higher demands on the performance of the natural ester transformer oil.

Currently, natural ester insulating oils for transformers are mainly derived from three vegetable oils, palm oil, soybean oil and rapeseed oil. Palm oil is known for its high saturated fatty acid content, which imparts to it far superior oxidation stability to soybean oil and rapeseed oil, and even better than mineral insulating oils, however, palm oil is prone to solidification at normal temperature and its ignition point fails to meet the class K fire protection standard. The transformer natural ester insulating oil prepared from the soybean oil has the antioxidant capacity close to that of pure mineral oil by supplementing additives, but the special design and production process are needed for the soybean oil natural ester transformer oil due to the characteristics of higher viscosity, pour point and rapid oxidation in air. The high oleic rapeseed oil contains higher unsaturated fatty acid, and the overall performance of the high oleic rapeseed oil is similar to that of soybean oil. Aiming at the problems of the three vegetable oils, the invention is needed to provide the transformer oil with high oxidation resistance, low pour point, excellent insulating property and high biodegradability so as to meet the higher demands of the current market.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a preparation method of environment-friendly plant-based transformer oil.

The aim of the invention can be achieved by the following technical scheme:

the preparation method of the environment-friendly plant-based transformer oil comprises the following steps:

Heating the modified castor oil base oil to 90-100 ℃ in a reaction kettle, stirring for 30min, adding a viscosity index improver, an antioxidant, an anti-emulsifying agent and an acid catching agent, controlling the temperature to 60-70 ℃, and stirring for 1-3h to obtain the environment-friendly plant-based transformer oil.

Further, the raw materials comprise, by mass, 1-8% of viscosity index improver, 2-5% of antioxidant, 0.05-0.2% of anti-emulsifying agent, 0.1-0.3% of acid scavenger, and the balance of modified castor oil base oil.

Further, the viscosity index improver is one or two of a derivative of polymethacrylate, an ethylene-propylene copolymer and polyisobutylene.

Further, the anti-emulsifying agent is one of a cationic anti-emulsifying agent and an anionic anti-emulsifying agent.

Further, the acid scavenger is one of an epoxy compound and a carbodiimide.

The viscosity index improver is added to improve the viscosity index of the transformer oil, the natural ester base oil contains a large amount of ester groups in the molecular structure, the lubrication and protection effects are improved, meanwhile, poor oil-water separation capacity is brought, the anti-emulsifying agent is added to improve the oil-water separation capacity, the damage to the transformer caused by the mixing of water is reduced, and finally, the acid value of the acid catcher is reduced, and the hydrolysis of the ester groups in the transformer oil is inhibited.

Further, the improved castor oil base oil is prepared by the steps of:

A1, adding castor oil, long-chain fatty alcohol and a catalyst (the mass is 0.1% of the total mass of reactants) into a three-neck flask with a condenser tube, heating to 50-150 ℃ under the protection of nitrogen, stirring and reacting for 1-10h, and then washing reaction liquid to be neutral, and removing solvent to obtain long-chain fatty alcohol ester castor oil, wherein the molar ratio of the castor oil to the long-chain fatty alcohol is 1:1-5;

under the action of a catalyst, castor oil and long-chain fatty alcohol undergo transesterification reaction to obtain hexanol ester castor oil, wherein the specific process is as follows:

A2, adding long-chain fatty alcohol ester castor oil, long-chain fatty acid and a catalyst (the mass is 1% of the total reactant mass) into a three-neck flask with a water separator, adding toluene as a water carrying agent, heating and reacting for 5-15h at 100-200 ℃ until no more excessive water is generated in the water separator, washing to be neutral after the reaction is finished, and distilling under reduced pressure to remove the solvent to obtain an esterified product, wherein the molar ratio of the long-chain fatty alcohol ester castor oil to the long-chain fatty acid is 1:1-5;

Under the action of a catalyst, the long-chain fatty alcohol ester castor oil and the long-chain fatty acid are subjected to esterification reaction to obtain an esterification product, wherein the specific process is as follows:

A3, adding an esterification product, unsaturated olefin and an initiator (the mass is 0.2% of the total reactant mass) into a three-neck flask with a condenser pipe under the protection of nitrogen at room temperature, then initiating a polymerization reaction by a heating or ultraviolet irradiation method for 3-6 hours, purifying methanol to remove impurities, and removing a solvent by rotary evaporation to obtain an improved castor oil base oil, wherein the mol ratio of the esterification product to the unsaturated olefin is 1:1-10;

Under the action of an initiator, unsaturated carbon double bonds in the esterified product molecules undergo free radical polymerization reaction with unsaturated olefin to obtain the improved castor oil base oil, wherein the specific process is as follows:

Further, in the step A1, the long-chain fatty alcohol is all isomers in alcohols with 5-18 total carbon numbers.

Further, the long chain fatty acid in step A2 is all isomers of monocarboxylic acids having a total carbon number of 8 to 16.

Further, the catalyst in the steps A1 and A2 is one of Lewis acid catalyst, organic base catalyst and inorganic base catalyst.

Further, the catalyst in the steps A1 and A2 accounts for 0.01% -1% of the total reactant mass.

Further, the initiator in step A3 may be one of a photoinitiator and a thermal initiator according to the polymerization reaction.

Castor oil is a vegetable oil which is degradable, and is a mixture of various fatty acid triglycerides, wherein the castor acid triglycerides account for about 99%, and the castor oil has relatively good low-temperature performance, oxidation and decay resistance, hydrolysis resistance and excellent electrical insulation, and the flash point and breakdown voltage of the castor oil can be improved by modifying the castor oil.

Further, the antioxidant is prepared by the steps of:

S1, adding a drying pipe into a three-neck flask provided with a thermometer, an electromagnetic stirrer and a spherical condenser pipe, and connecting the drying pipe with a tail gas absorbing device to absorb generated hydrogen chloride gas, mixing pentaerythritol and benzene, slowly dropwise adding phosphorus trichloride under stirring, controlling the temperature to be not more than 30 ℃, controlling the reaction temperature to be 75 ℃ after dropwise adding, carrying out reflux reaction for 4 hours, and distilling to remove solvent benzene and excessive phosphorus trichloride to obtain an intermediate 1, wherein the dosage ratio of the pentaerythritol, the benzene and the phosphorus trichloride is 13.6g to 100mL to 28.1g;

The nucleophilic substitution of pentaerythritol and phosphorus trichloride reacts to obtain an intermediate 1, and the specific reaction is as follows:

S2, uniformly mixing and stirring the intermediate 1, 6-hydroxycaproic acid, potassium carbonate (acid binding agent) and N, N-Dimethylformamide (DMF) in a three-neck flask with a stirring device, controlling the reaction temperature to be 70 ℃, carrying out heat preservation reaction for 6 hours, filtering, removing part of solvent by rotary evaporation, purifying by column chromatography (eluent adopts a mixed solvent of benzene and ethyl acetate in a volume ratio of 1:3), and removing the eluent by rotary evaporation to obtain an intermediate 2, wherein the dosage ratio of the intermediate 1, 6-hydroxycaproic acid, potassium carbonate and N, N-dimethylformamide is 26.3g:27.4g:30mL:150mL;

Under the catalysis of potassium carbonate, the intermediate 1 and 6-hydroxycaproic acid react, and two chlorine groups on the intermediate 1 participate in the reaction by controlling the molar ratio of the intermediate 1 and the 6-hydroxycaproic acid to be close to 1:2 and slightly excessive, so as to obtain an intermediate 2, wherein the specific reaction process is as follows:

S3, uniformly stirring and mixing the intermediate 2, the para-hydroxy diphenylamine and the N, N-dimethylformamide in a three-neck flask with a stirring device, adding dibutyltin oxide (catalyst) and dicyclohexylcarbodiimide (DCC, dehydrating agent), controlling the reaction temperature to be 80 ℃, keeping the temperature for 6 hours, continuously stirring during the reaction, filtering, removing most of the solvent by rotary evaporation firstly, purifying by column chromatography (eluting solution adopts a mixed solvent of ethyl acetate/benzene, the volume ratio of the ethyl acetate/benzene is 5:2), and removing the eluting solution by rotary evaporation to obtain an antioxidant, wherein the dosage ratio of the intermediate 2, the para-hydroxy diphenylamine, the N, N-dimethylformamide, dibutyltin oxide and the DCC is 45.6g:38.6g:200mL:0.4g:41.2g;

Under the catalysis of dibutyl tin oxide and DCC, carboxyl on the intermediate 2 and hydroxyl on the parahydroxydiphenylamine are subjected to esterification reaction, and the two carboxyl groups on the parahydroxydiphenylamine are reacted by controlling the molar ratio of the carboxyl to the hydroxyl to be close to 1:2 and slightly excessive, so that the antioxidant is obtained, wherein the structure of the antioxidant is as follows:

The prepared antioxidant takes phosphite ester as a center and is connected with ester group and diphenylamine, the phosphite ester can be used as an auxiliary antioxidant, and hydroperoxide generated by oxidation and aging in a polymer can be decomposed to be inactive substances, so that the aim of stopping or delaying the oxidative degradation of the polymer is fulfilled, the oxidation resistance of transformer oil can be enhanced, the introduced ester group can improve the compatibility of the antioxidant and transformer oil, and finally, the introduced diphenylamine belongs to arylamine antioxidants, has excellent oxidation resistance, can cooperate with the phosphite ester, and greatly enhances the oxidation resistance of the transformer oil.

The invention has the beneficial effects that:

1. The transformer oil prepared by the invention can improve the viscosity index and the oil-water separation capacity of the transformer oil and inhibit hydrolysis by adding various additives;

2. the castor oil is used as the base oil, so that the degradability of the transformer oil is endowed, the castor oil is modified, and the flash point and the breakdown voltage of the transformer oil are improved;

3. the prepared antioxidant has synergistic effect of each group in the molecule, so that the oxidation resistance of the transformer oil is obviously enhanced;

therefore, the transformer oil prepared by the method has excellent comprehensive performance, good flash point, breakdown voltage and oxidation resistance, is environment-friendly and degradable, and has important application value in the technical field of high-temperature transformer oil.

Detailed Description

The technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Example 1

Preparation of an improved castor oil base oil:

A1, adding 100g of castor oil, 110g of hexanol and 0.21g of sodium hydroxide catalyst into a three-neck flask with a condenser tube, heating to 70 ℃ under the protection of nitrogen, stirring and reacting for 3 hours, washing the reaction liquid to be neutral, and removing the solvent to obtain long-chain fatty alcohol ester castor oil;

A2, adding 100g of long-chain fatty alcohol ester castor oil, 150g of stearic acid and 2.5g of acid ion resin catalyst into a three-neck flask with a water separator, adding 100mL of toluene serving as a water carrying agent, heating and reacting for 5 hours at 110 ℃ until no more excessive water is generated in the water separator, washing to be neutral after the reaction is finished, and distilling under reduced pressure to remove the solvent to obtain an esterified product;

A3, adding 100g of esterified product, 20g of heptene and 0.24g of azodiisobutyronitrile initiator into a three-neck flask with a condenser tube at room temperature under the protection of nitrogen, heating to 80 ℃ to initiate polymerization, continuing for 3 hours, finishing the reaction, purifying methanol to remove impurities, and removing the solvent by rotary evaporation to obtain the improved castor oil base oil.

Example two

Preparation of an improved castor oil base oil:

A1, adding 100g of castor oil, 150g of hexanol and 0.25g of sodium hydroxide catalyst into a three-neck flask with a condenser tube, heating to 80 ℃ under the protection of nitrogen, stirring and reacting for 3 hours, washing the reaction liquid to be neutral, and removing the solvent to obtain long-chain fatty alcohol ester castor oil;

A2, adding 100g of long-chain fatty alcohol ester castor oil, 120g of lauric acid and 2.2g of acid ion resin catalyst into a three-neck flask with a water separator, adding 100mL of toluene serving as a water carrying agent, heating and reacting for 5 hours at 100 ℃ until no more excessive water is generated in the water separator, washing to be neutral after the reaction is finished, and distilling under reduced pressure to remove the solvent to obtain an esterified product;

A3, adding 100g of esterified product, 20g of decene and 0.24g of azodiisobutyronitrile initiator into a three-neck flask with a condenser tube at room temperature under the protection of nitrogen, heating to 80 ℃ to initiate polymerization, continuing for 3 hours, finishing the reaction, purifying methanol to remove impurities, and removing the solvent by rotary evaporation to obtain the improved castor oil base oil.

Example III

Preparation of an antioxidant:

the antioxidant is prepared by the following steps:

S1, adding a drying pipe into a three-neck flask provided with a thermometer, an electromagnetic stirrer and a spherical condenser pipe, connecting the drying pipe with a tail gas absorbing device to absorb generated hydrogen chloride gas, mixing 13.6g of pentaerythritol with 100mL of benzene, slowly dropwise adding 28.1g of phosphorus trichloride under stirring, controlling the temperature to be not more than 30 ℃, controlling the reaction temperature to be 75 ℃ after dropwise adding, refluxing for 4 hours, finishing the reaction, and distilling to remove solvent benzene and excessive phosphorus trichloride to obtain an intermediate 1;

S2, uniformly mixing 26.3g of intermediate 1, 27.4g of 6-hydroxycaproic acid, 30mL of potassium carbonate and 150mLN, N-dimethylformamide in a three-neck flask with a stirring device, controlling the reaction temperature to be 70 ℃, carrying out heat preservation reaction for 6 hours, filtering, removing part of solvent by rotary evaporation, purifying by column chromatography (eluent adopts a mixed solvent of benzene/ethyl acetate with the volume ratio of 1:3), and removing the eluent by rotary evaporation to obtain an intermediate 2;

S3, stirring and mixing 45.6g of intermediate 2, 38.6g of parahydroxydiphenylamine and 200mLN, N-dimethylformamide uniformly in a three-neck flask with a stirring device, adding 0.4g of dibutyltin oxide and 41.2g of dicyclohexylcarbodiimide, controlling the reaction temperature to be 80 ℃, keeping the temperature for 6 hours, continuously stirring during the reaction, finishing the reaction, filtering, removing most of the solvent by rotary evaporation, purifying by column chromatography (the eluent adopts a mixed solvent of ethyl acetate and benzene, the volume ratio of the ethyl acetate to the benzene is 5:2), and removing the eluent by rotary evaporation to obtain the antioxidant.

Example IV

Heating the modified castor oil base oil prepared in the first embodiment to 90 ℃ in a reaction kettle according to the mass percentage of each raw material in the table 1, stirring for 30min, adding an ethylene-propylene copolymer, an antioxidant prepared in the third embodiment, nonylphenol polyoxyethylene ether and 2-oxabicyclo [4.1.0] heptane-7-carboxylic acid, controlling the temperature at 60 ℃, and stirring for 1h to obtain the environment-friendly plant-based transformer oil;

TABLE 1

Formulation of	Content (mass percent content)
		Improved castor oil base oil	90%
Ethylene-propylene copolymers	5%
		Antioxidant prepared in example III	2.5%
Polyoxyethylene nonylphenol ether	0.2%
		2-Oxabicyclo [4.1.0] heptane-7-carboxylic acid	0.3%

Performance measurements were performed for example four, and the test results are shown in table 2:

TABLE 2

Example five

Heating the modified castor oil base oil prepared in the second example to 90 ℃ in a reaction kettle according to the mass percentage of each raw material in the table 3, stirring for 30min, adding polyisobutylene, the antioxidant prepared in the third example, nonylphenol polyoxyethylene ether and 2-oxabicyclo [4.1.0] heptane-7-carboxylic acid, controlling the temperature at 60 ℃, and stirring for 1h to obtain the environment-friendly plant-based transformer oil;

TABLE 3 Table 3

Formulation of	Content (mass percent content)
		Improved castor oil base oil	85%
Polyisobutene (S)	8%
		Antioxidant prepared in example III	2.5%
N-phenyl-alpha-anilines	2%
		Polyoxyethylene nonylphenol ether	0.2%
2-Oxabicyclo [4.1.0] heptane-7-carboxylic acid	0.3%

Performance measurements were performed for example five, and the test results are shown in table 4:

TABLE 4 Table 4

Comparative example one

The modified castor oil in the fifth embodiment is replaced by the unmodified ordinary castor oil with the same dosage, and the rest steps are the same as those in the fifth embodiment to prepare the transformer oil.

Comparative example two

The same quality phosphite antioxidant is used for replacing the antioxidant in the fifth embodiment, and the rest steps are the same as those in the fifth embodiment, so that transformer oil is prepared.

The performance of comparative examples one and two was measured and the test results are shown in table 5:

TABLE 5

As can be seen from the tables, the transformer oil prepared by the embodiment of the invention has excellent comprehensive performance, is environment-friendly and degradable, has higher flash point, breakdown voltage and oxidation resistance than those of the comparative example, and has important application value in the technical field of high-temperature transformer oil.

In the description of the present specification, the descriptions of the terms "one embodiment," "example," "specific example," and the like, mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

The foregoing is merely illustrative and explanatory of the invention, as various modifications and additions may be made to the particular embodiments described, or in a similar manner, by those skilled in the art, without departing from the scope of the invention or exceeding the scope of the invention as defined in the claims.

Claims

1. A method for preparing an environmentally friendly plant-based transformer oil, characterized in that it comprises the following steps:

In a reaction kettle, the improved castor oil base oil is heated and stirred, and then a viscosity index improver, an antioxidant, an anti-emulsifier and an acid scavenger are added, the temperature is controlled at 60-70° C., and the mixture is stirred for 1-3 hours to obtain an environmentally friendly plant-based transformer oil;

Wherein, the improved castor oil base oil is prepared by the following steps:

A1. Heat castor oil, long-chain fatty alcohol and catalyst to 50-150° C. under nitrogen protection, stir and react for 1-10 hours, wash and remove the solvent to obtain long-chain fatty alcohol ester castor oil;

A2. Add long-chain fatty alcohol ester castor oil, long-chain fatty acid and catalyst into a three-necked flask with a water separator, add toluene, heat and react at 100-200° C. for 5-15 hours, wait until no more excess water is generated in the water separator, wash to neutrality, and distill under reduced pressure to obtain an esterification product;

A3. At room temperature and under nitrogen protection, add the esterification product, unsaturated olefin and initiator into a three-necked flask, and then initiate the polymerization reaction by heating or ultraviolet light irradiation for 3-6 hours. After the reaction is completed, purify with methanol and then rotary evaporate to obtain improved castor oil base oil.

2. The method for preparing an environmentally friendly plant-based transformer oil according to claim 1, characterized in that the molar ratio of castor oil to long-chain fatty alcohol in step A1 is 1:1-5.

3. The method for preparing an environmentally friendly plant-based transformer oil according to claim 1, characterized in that in step A1, the long-chain fatty alcohol is all isomers of alcohols with a total carbon number of 5-18.

4. The method for preparing an environmentally friendly plant-based transformer oil according to claim 1, wherein the long-chain fatty acid in step A2 is all isomers in a monocarboxylic acid having a total carbon number of 8-16.

5. The method for preparing an environmentally friendly plant-based transformer oil according to claim 1, characterized in that the molar ratio of the long-chain fatty alcohol ester castor oil to the long-chain fatty acid in step A2 is 1:1-5.

6. The method for preparing an environmentally friendly plant-based transformer oil according to claim 1, characterized in that the molar ratio of the esterification product to the unsaturated olefin in step A3 is 1:1-10.

7. The method for preparing an environmentally friendly plant-based transformer oil according to claim 1, wherein the antioxidant is prepared by the following steps:

S1. After pentaerythritol and benzene are mixed, phosphorus trichloride is added dropwise under stirring, and the temperature is controlled not to exceed 30° C. After the addition is completed, the mixture is reacted at 75° C. for 4 hours. After the reaction is completed, the mixture is distilled to obtain intermediate 1;

S2, mixing intermediate 1, 6-hydroxyhexanoic acid, potassium carbonate and N,N-dimethylformamide (DMF) and stirring evenly, reacting at 70° C. for 6 h, filtering, rotary evaporating, column chromatography purification, and rotary evaporating to obtain intermediate 2;

S3. Stir and mix the intermediate 2, p-hydroxydiphenylamine and N,N-dimethylformamide evenly, add dibutyltin oxide and dicyclohexylcarbodiimide, and react at 80°C for 6 hours with continuous stirring during the reaction. After the reaction is completed, filter, rotary evaporate, purify by column chromatography, and rotary evaporate to obtain an antioxidant.

8. The method for preparing an environment-friendly plant-based transformer oil according to claim 7, wherein the ratio of the amounts of pentaerythritol, benzene and phosphorus trichloride in step S1 is 13.6 g:100 mL:28.1 g.

9. A method for preparing an environment-friendly plant-based transformer oil according to claim 7, characterized in that, in step S2, the ratio of the amount of intermediate 1, 6-hydroxycaproic acid, potassium carbonate, and N,N-dimethylformamide is 26.3g:27.4g:30mL:150mL.

10. The method for preparing an environmentally friendly plant-based transformer oil according to claim 7, characterized in that the ratio of the amount of intermediate 2, p-hydroxydiphenylamine, N,N-dimethylformamide, dibutyltin oxide, and DCC in step S3 is 45.6 g:38.6 g:200 mL:0.4 g:41.2 g.