A method for preparing natural ester insulating oil with high insulation performance by low-temperature continuous coupling refining of vegetable oil

Abstract

This invention discloses a low-temperature, continuous, alkali-free refining method for preparing high-insulation-performance natural ester insulating oil from vegetable oil, belonging to the field of electrical insulation media and green oil refining technology. The method includes: sequentially subjecting vegetable oil to two-stage filtration, enzymatic degumming, vacuum membrane dehydration, deep adsorption with rice husk ash-based mesoporous silica, three-stage precision filtration, low-temperature vacuum deodorization, and cooling stabilization treatment. The entire process avoids alkali refining, producing no soap residue or alkaline wastewater, thus constructing a low-loss, high-efficiency, and zero-pollution green refining system. This invention employs a low-temperature, continuous processing flow, relying on rice husk ash-based mesoporous silica to simultaneously remove multiple impurities. The continuous production mode exhibits strong stability and wide raw material adaptability. The natural ester insulating oil prepared by this invention meets all indicators of the IEC 62770 industry standard, possesses excellent electrical insulation performance, and is suitable for power equipment such as distribution transformers, new energy transformers, and rail transit transformers.

Description

Technical Field

[0001] This invention belongs to the field of electrical insulation media and green oil refining technology, specifically relating to a method for preparing high-insulation-performance natural ester insulating oil by low-temperature, continuous, and alkali-free refining of vegetable oil. Background Technology

[0002] With the rapid development of new power systems, environmentally friendly, highly flame-retardant, and biodegradable natural ester insulating oils have become the core insulating medium for distribution transformers, wind power transformers, photovoltaic substations, and rail transit transformers. Natural ester insulating oils are made from vegetable oils such as soybean oil and rapeseed oil, and their insulation performance, thermal stability, and antioxidant capacity are highly dependent on the degree of refining. Phospholipids, free fatty acids, pigments, moisture, metal ions, and polar oxidation products contained in crude oil can significantly reduce breakdown voltage, increase dielectric loss, and accelerate thermal aging, threatening the long-term safe operation of transformers.

[0003] Currently, the industrial production of natural ester insulating oil still mainly relies on traditional intermittent alkali refining for acid removal and activated clay adsorption for decolorization. This process has a series of inherent defects: the alkali refining process uses a large amount of sodium hydroxide, generating high-COD alkaline wastewater and soap residue, resulting in high environmental treatment costs and failing to meet the development direction of green chemistry; activated clay has poor adsorption selectivity and low adsorption capacity, with an addition amount typically as high as 3% to 5%, and oil loss also reaches 3% to 5%, resulting in low raw material utilization. Traditional processes are mostly intermittent operations, resulting in poor batch stability, high dependence on manual labor, and low production efficiency, making it difficult to meet the needs of large-scale industrial continuous production. At the same time, high-temperature deacidification and high-temperature decolorization processes easily lead to oil oxidation, polymerization, and cracking, generating polar impurities, further increasing dielectric loss and decreasing breakdown voltage. Conventional clay materials have limited ability to remove trace metal ions and weakly polar oxidation products, making it difficult to meet the requirements for high-end electrical-grade insulating oils.

[0004] To overcome the aforementioned technical challenges, researchers and enterprises both domestically and internationally have undertaken a series of exploratory studies. For example, CN112725079A proposes a decolorization and refining process for kitchen waste oil-based natural ester insulating oil, employing activated clay for intermittent decolorization treatment; CN103073006A discloses a method for preparing mesoporous silica materials, which can be used for adsorption refining; CN116237008A has developed silica-based calcium and magnesium silicate adsorbents with mesoporous structures and applied them to the physical refining of edible oils such as palm oil and rice bran oil, as well as the refining of used fried oils. However, the aforementioned existing technologies still have significant shortcomings. In the process of CN112725079A, the amount of activated clay added is as high as 3.2% to 4.5%, and the processing temperature is higher than 80℃. This not only results in a high oil loss rate but also easily induces oxidation and deterioration of the oil under high-temperature conditions, failing to guarantee the long-term operational stability of the insulating oil. The mesoporous silica material described in CN103073006A requires the use of a template agent and high-temperature calcination, resulting in expensive raw materials and a complex preparation process. It is only suitable for small-scale laboratory purification and cannot be adapted to continuous industrial refining production of oils and fats. The adsorbent in CN116237008A is only optimized for basic indicators such as color and acid value of edible oils, without controlling key electrical parameters of insulating oils. It cannot remove trace metal ions and other impurities that seriously damage insulation performance, and therefore cannot meet the standards used in the power industry. Summary of the Invention

[0005] To address the problems of high pollution from alkali refining, high oil loss from clay refining, low efficiency from intermittent production, easy oxidation at high temperatures, insufficient refining depth, high oil loss, and unstable electrical properties of finished products in existing natural ester insulating oil refining processes, this invention provides a method for preparing high-insulation-performance natural ester insulating oil by low-temperature continuous alkali-free refining of vegetable oil. This method achieves efficient, green, low-temperature, continuous, and alkali-free refining of vegetable oil and produces high-insulation-performance natural ester insulating oil that meets the IEC 62770 standard.

[0006] The present invention provides a method for preparing high-insulation-performance natural ester insulating oil from vegetable oil through low-temperature continuous alkali-free refining coupled with the following steps: the vegetable oil is subjected to two-stage filtration, enzymatic degumming, vacuum membrane dehydration, deep adsorption of rice husk ash-based mesoporous silica, three-stage precision filtration, low-temperature vacuum deodorization, and cooling stabilization treatment in sequence; the method uses continuous equipment and does not use alkali refining process throughout the entire process.

[0007] Furthermore, the feed flow rate of vegetable oil is 15–25 L / h.

[0008] Furthermore, the two-stage filtration includes first coarse filtration through a 30-100 mesh filter screen, and then fine filtration through a 1-10 μm precision filter element.

[0009] Furthermore, the enzymatic degumming temperature is 40–55°C, the enzyme is phospholipase A1, the amount added is 0.03%–0.08% of the vegetable oil mass, and the reaction time is 20–30 minutes.

[0010] Furthermore, the vacuum membrane dehydration temperature is 40–60°C, the vacuum degree is 0.08–0.1 MPa, and the water content of the dehydrated oil is ≤50 ppm.

[0011] Furthermore, the amount of rice husk ash-based mesoporous silica added is 0.8% to 2.0% of the mass of the vegetable oil, the adsorption temperature is 40 to 55°C, and the adsorption residence time is 25 to 50 minutes.

[0012] Furthermore, the three-stage precision filtration adopts a three-stage series security filter with progressively decreasing pore size, and its filtration accuracy is 5-10μm, 1-5μm and 0.2-1μm respectively.

[0013] Furthermore, the low-temperature vacuum deodorization temperature is 40–75°C, the vacuum degree is 0.09–0.1 MPa, and the deodorization time is 30–50 minutes.

[0014] Furthermore, the cooling stabilization treatment includes: cooling the deodorized oil to below 25°C and allowing it to stand for aging for 6 to 12 hours.

[0015] The present invention also provides a natural ester insulating oil with high insulating properties prepared by the above method.

[0016] Furthermore, the natural ester insulating oil has an acid value ≤0.20mgKOH / g, a breakdown voltage ≥70kV, a water content ≤50ppm, and a dielectric loss factor ≤0.025 at 90℃.

[0017] This invention first removes solid impurities from vegetable oil through graded filtration, then degummes it using specific bio-enzymes in a low-temperature, neutral environment, avoiding the pollution problems associated with traditional acid-base refining. Next, a vacuum thin-film evaporator rapidly removes water from the oil through instantaneous evaporation, reducing irreversible damage to insulation performance caused by moisture. Combined with rice husk ash-based mesoporous silica, which, with its high specific surface area, ordered mesoporous channels, and surface polar functional groups, simultaneously adsorbs and removes various harmful impurities. Multi-stage precision filtration thoroughly separates the solid adsorbent, preventing fine particle residue from causing partial discharge. Finally, volatile impurities are removed in a low-temperature vacuum environment, and after cooling and stabilization, a qualified natural ester insulating oil is obtained. Compared with existing technologies, this invention has the following advantages:

[0018] 1. This invention abandons the traditional alkali refining process and adopts enzymatic degumming under neutral conditions at 40-55℃. The entire process does not use strong alkaline reagents such as sodium hydroxide, generates no soap residue, and discharges no alkaline wastewater. It solves the industry pain point of serious pollution from traditional processes and belongs to green and environmentally friendly refining process.

[0019] 2. The entire process of this invention operates at a temperature ≤75℃, significantly lower than the over 80℃ of traditional alkali refining processes and the 90-110℃ of activated clay decolorization. This mild, low-temperature environment effectively prevents the thermal cracking and oxidative polymerization of triglycerides in the oil, reducing the generation of polar oxidizing impurities at the source and ensuring the long-term stability of the oil's electrical properties. Accelerated aging tests have verified that the oil prepared by this invention, after aging at 120℃ for 96 hours, shows a dielectric loss factor increase of no more than 8%, demonstrating significantly better oxidation stability than traditional processes, making it suitable for the long-term enclosed operation environment of transformers.

[0020] 3. This invention uses rice husk ash-based mesoporous silica adsorbent, which has a well-developed pore structure and strong adsorption selectivity. The amount added is only 0.8% to 2.0% (compared to 3% to 5% for traditional activated clay). The overall oil loss is controlled within 1.2%, and as low as 0.95% under optimal working conditions (compared to 3% to 5% oil loss in traditional clay processes). This significantly improves the utilization rate of raw materials, reduces production costs, and has significant economic benefits.

[0021] 4. This invention employs a continuous coupled system (continuous filtration, continuous enzymatic hydrolysis, continuous thin-film evaporation, continuous adsorption, continuous filtration, and continuous deodorization), enabling 24-hour uninterrupted operation with a stable flow rate controlled at 15–25 L / h. Compared to traditional intermittent processes, production efficiency is increased by 3–5 times (intermittent processes require 8–12 hours per batch, while this invention produces 20 L per hour with continuous operation), batch performance deviation is less than 1.5% (traditional processes often have batch-to-batch deviations greater than 5%), manual intervention is reduced by more than 70%, and the degree of automation is high.

[0022] 5. This invention relies on the high specific surface area (550-750 μm²) of rice husk ash-based mesoporous silica. 2 With its ordered mesoporous channels (3-8 nm) and surface polar functional groups (silanol groups), it can simultaneously adsorb and remove phospholipids (removal rate >92%), free fatty acids (acid value reduced from 0.86 mg KOH / g to 0.18 mg KOH / g), pigments (oil color changed from dark yellow to light yellow and transparent), moisture (reduced from 765 ppm to 42 ppm), and metal ions (not detected by ICP-MS). It has a wide refining coverage and high purification depth.

[0023] 6. The natural ester insulating oil prepared by this invention has a breakdown voltage ≥70kV, a dielectric loss factor ≤0.025 at 90℃, an acid value ≤0.20mgKOH / g, and a water content ≤50ppm. All indicators are superior to those of traditional alkali refining processes and commercial SBA-15 mesoporous materials, and fully meet the requirements of IEC 62760 industry standard and GB / T 46134-2025 national standard.

[0024] 7. The process of this invention has good refining effects on various vegetable oil raw materials such as soybean crude oil and rapeseed crude oil. Especially for inferior raw materials with high acid value and high impurities, by increasing the amount of adsorbent added to 2.0%, raising the adsorption temperature to 55℃, and extending the residence time to 50 minutes, high-quality insulating oil with a breakdown voltage of 74.1kV and a dielectric loss of 0.018 can still be prepared, demonstrating excellent process tolerance and versatility.

[0025] 8. The rice husk ash-based mesoporous silica adsorbent used in this invention is made from agricultural solid waste rice husk ash through simple acid washing and calcination. It does not require template agents or high-temperature calcination, resulting in lower raw material costs and realizing the resource utilization of solid waste, which meets the requirements of the circular economy.

[0026] 9. This invention achieves comprehensive progress in environmental protection, energy consumption, raw material utilization, production efficiency, and product quality. Its continuous production mode exhibits strong stability, a wide range of compatible raw materials, simple processes, low production costs, and low difficulty in industrialization. It also boasts excellent electrical insulation performance, making it suitable for use in power equipment such as distribution transformers, new energy transformers, and rail transit transformers, with broad market application prospects. The promotion and implementation of this invention will strongly drive the green upgrading of the natural ester insulating oil industry, providing high-performance, low-cost, and environmentally friendly insulating media for the construction of my country's new power system. Detailed Implementation

[0027] The technical solution of the present invention will be further described in detail and completely below with reference to specific embodiments. However, the following embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0028] Unless otherwise specified, all raw materials and reagents used in this invention are commercially available industrial-grade raw materials. Phospholipase A1 uses commercially available food-grade enzyme preparation. The rice husk ash-based mesoporous silica used is prepared by the following method: rice husk ash is washed with water to remove surface dust and impurities, calcined at 550℃ for 2 hours to remove residual carbon and organic matter; after calcination, the rice husk ash is mixed with a 2 mol / L sodium hydroxide aqueous solution at a solid-liquid ratio of 1 g: 6 mL, stirred at 90℃ for 2 hours, filtered to remove insoluble residue, and sodium silicate leachate is obtained; CTAB template agent is added to the sodium silicate leachate at a mass ratio of sodium silicate to template agent of 4:1, and stirred at room temperature for 30 minutes to mix evenly; the pH of the mixture is adjusted to 10.0 with dilute hydrochloric acid, transferred to a hydrothermal reactor, and crystallized at 125℃ for 14 hours; the crystallized product is filtered, washed with deionized water until neutral, dried, and calcined at 500℃ for 4 hours to remove the template agent, yielding rice husk ash-based mesoporous silica with a specific surface area of ​​635 m². 2 / g, pore size 5.2nm, pore volume 0.62m³ 3 / g, surface silanol content 2.8mmol / g, residual carbon mass fraction 1.8%.

[0029] Example 1

[0030] Soybean crude oil was selected as the processing feedstock. The initial physicochemical properties of the feedstock were: acid value 0.86 mg KOH / g, breakdown voltage 34.2 kV, moisture content 765 ppm, and dielectric loss factor at 90℃ 0.085. Refining was carried out according to the following steps:

[0031] (1) Two-stage filtration: Soybean crude oil is fed into a continuous filtration device at a constant flow rate of 20L / h. It passes through a 50-mesh stainless steel coarse filter and a 5μm polypropylene pleated fine filter in sequence to remove suspended particles, mechanical impurities and colloidal aggregates. The filtered oil is clear and transparent, which can effectively avoid the problems of pore blockage and adsorption poisoning of the subsequent adsorbent.

[0032] (2) Enzymatic degumming: The filtered oil is sent to a constant-temperature enzymatic hydrolysis reactor, with the temperature controlled at 45℃. Phospholipase A1 is continuously added online at a ratio of 0.05% of the vegetable oil mass. The stirring speed is 300 r / min, and the reaction is carried out at a constant temperature for 25 min to promote the full enzymatic hydrolysis of large phospholipid molecules in the oil into water-soluble small molecules. After the reaction, a tubular centrifuge is used to separate the oil at a high speed of 8000 r / min to remove the gum residue and insoluble impurities. After this process, the phospholipid removal rate of the oil reaches 92%, which greatly avoids the problem of insulation performance degradation caused by phospholipid impurities.

[0033] (3) Vacuum film dehydration: The degummed oil is fed into a scraped film evaporator and dehydrated under a vacuum of 0.085 MPa, a heating temperature of 55°C, and a scraping speed of 300 r / min. The oil film thickness is controlled at 0.2 mm, and the material stays in the evaporation chamber for 30 seconds. The principle of instantaneous film evaporation is used to quickly remove free water from the oil, avoiding the deterioration of the oil caused by prolonged high-temperature boiling. After dehydration, the water content of the oil is reduced to 42 ppm, which meets the pre-treatment water content control standard for natural ester insulating oil.

[0034] (4) Deep adsorption of rice husk ash-based mesoporous silica: The dehydrated oil is transported to a continuous countercurrent adsorption tower. Rice husk ash-based mesoporous silica is continuously added online as an adsorbent at a ratio of 1.2% of the vegetable oil mass. The temperature inside the adsorption tower is 45℃, the stirring speed is 250r / min, and the material stays in the adsorption tower for 35min. Relying on the ordered porous structure and surface active silanol groups of the mesoporous silica, the free fatty acids, pigments, trace metal ions and polar oxidation products inside the oil are removed simultaneously through the dual action of physical adsorption and chemical chelation, thus completing the deep purification and refining.

[0035] (5) Three-stage precision filtration: The oil after adsorption treatment flows through three-stage precision security filters of 5μm, 1μm and 0.5μm in series, which successively intercept and remove fine adsorbent powder and solid particles, and completely eliminate the hidden danger of partial discharge caused by solid particles.

[0036] (6) Low-temperature vacuum deodorization: The filtered oil is sent into a vacuum deodorization tower, the vacuum degree inside the tower is controlled at 0.095MPa, the heating temperature is kept constant at 75℃, the oil spreads on the packing surface to form a film and stays for 40 minutes, removing volatile organic compounds, odor substances and heat-sensitive oxidation by-products from the oil, and further improving the storage stability of the oil.

[0037] (7) Cooling and stabilization treatment: The deodorized oil is cooled to 25°C by a plate heat exchanger and then transferred to a closed stabilization tank for 12 hours of standing and aging to promote uniform molecular distribution and natural sedimentation of weak suspended impurities. Finally, it is filtered through a 0.2μm terminal precision filter and then sealed and bottled to obtain the finished natural ester insulating oil.

[0038] Example 2

[0039] Soybean crude oil of the same quality as in Example 1 was selected as the processing raw material. The entire process flow and equipment used were consistent with those in Example 1, except that the process parameters for the deep adsorption stage of rice husk ash-based mesoporous silica were adjusted as follows: the amount of rice husk ash-based mesoporous silica added was adjusted to 0.8% of the mass of the vegetable oil, the adsorption temperature was reduced to 40°C, the stirring speed was maintained at 250 r / min, and the material adsorption residence time was shortened to 25 min. The process parameters for the remaining steps were the same as in Example 1.

[0040] Example 3

[0041] Low-quality soybean crude oil was selected as the processing raw material. The initial raw material acid value was 1.32 mg KOH / g, breakdown voltage was 26.3 kV, moisture content was 822 ppm, and dielectric loss factor at 90℃ was 0.398. The entire process flow and equipment used were consistent with Example 1, except that the process parameters for the deep adsorption stage of rice husk ash-based mesoporous silica were adjusted as follows: the amount of rice husk ash-based mesoporous silica added was increased to 2.0% of the vegetable oil mass, the adsorption temperature was increased to 55℃, the stirring speed was adjusted to 300 r / min, and the material adsorption residence time was extended to 50 min to enhance the adsorption and purification capacity. The process parameters for the remaining steps were the same as in Example 1.

[0042] Example 4

[0043] Rapeseed crude oil was selected as the processing raw material, with an initial acid value of 0.93 mg KOH / g, a breakdown voltage of 31.8 kV, a water content of 642 ppm, and a dielectric loss factor of 0.117 at 90°C. The entire process flow, process parameters, and equipment used were consistent with those in Example 1.

[0044] Comparative Example 1

[0045] Conventional soybean crude oil is selected, and a traditional industrial refining process combining alkali refining and activated clay is employed. The soybean crude oil is heated to 85°C, and a 20% sodium hydroxide aqueous solution is added at a ratio of 1.2% of the vegetable oil mass. The mixture is stirred at a constant temperature for 20 minutes for alkali refining and deacidification. Subsequently, the temperature is raised to 90°C for centrifugal desap removal, followed by two washes with 90°C hot water. This process generates alkaline wastewater and soap residue. After washing, the oil is vacuum dried until the moisture content is below 100 ppm. Then, 3.5% activated clay is added to the oil, and the mixture is intermittently stirred and adsorbed at 80°C for 30 minutes. Finally, the oil is filtered through a plate and frame filter press to obtain the finished natural ester insulating oil.

[0046] Comparative Example 2

[0047] Commercial SBA-15 mesoporous silica was used to replace the rice husk ash-based mesoporous silica in Example 1. The entire process flow and equipment used were consistent with those in Example 1, and the process parameters for each step were also the same as those in Example 1.

[0048] The performance of the natural ester insulating oil products obtained in Examples 1 to 4 and Comparative Example 1 were tested respectively, and the results are shown in Table 1.

[0049] Table 1

[0050]

[0051] According to the test data in Table 1, the natural ester insulating oils prepared in Examples 1-4 of this invention are significantly superior to the traditional alkali refining process of Comparative Example 1 in terms of key indicators such as breakdown voltage, dielectric loss factor, water content, acid value, and oil loss rate. Furthermore, their overall performance is slightly better than the refined oil product of Comparative Example 2 using commercial SBA-15 mesoporous silica adsorbent. Specifically, Example 1 (optimal process for soybean oil) has a breakdown voltage of 72.5 kV, a dielectric loss factor of 0.021, and an oil loss rate of 0.95%; Example 3 (high acid value, inferior raw material) has a breakdown voltage of 74.1 kV and a dielectric loss factor as low as 0.018, demonstrating the strong adaptability of the process to high-impurity raw materials; Example 4 (rapeseed oil) has a breakdown voltage of 71.6 kV and a dielectric loss factor of 0.022, indicating that the process has good versatility for different vegetable oil raw materials. All finished products from these examples fully meet the IEC62770 industry standard for natural ester insulating oils. The oil processed by this invention exhibits high breakdown voltage, low dielectric loss, and minimal residual harmful impurities, with the oil loss rate strictly controlled below 1.2%, far superior to traditional processing methods. This green and pollution-free low-temperature continuous processing flow aligns with the green and low-carbon development requirements of the modern power industry. The process boasts strong fault tolerance and versatility, possessing extremely high value for industrial application and promotion.

Claims

1. A method for preparing high-insulation-performance natural ester insulating oil from vegetable oil through low-temperature continuous alkali-free refining coupled with refining, characterized in that, The method includes the following steps: sequentially subjecting vegetable oil to two-stage filtration, enzymatic degumming, vacuum membrane dehydration, deep adsorption of rice husk ash-based mesoporous silica, three-stage precision filtration, low-temperature vacuum deodorization, and cooling stabilization treatment; the method uses continuous equipment and does not use alkali refining process throughout the entire process.

2. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that, The feed flow rate of the vegetable oil is 15-25 L / h.

3. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that, The enzymatic degumming temperature is 40–55°C, the enzyme is phospholipase A1, the amount added is 0.03%–0.08% of the vegetable oil mass, and the reaction time is 20–30 minutes.

4. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that: The vacuum membrane dehydration temperature is 40–60°C, the vacuum degree is 0.08–0.1 MPa, and the water content of the dehydrated oil is ≤ 50 ppm.

5. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that: The amount of rice husk ash-based mesoporous silica added is 0.8% to 2.0% of the mass of the vegetable oil, the adsorption temperature is 40 to 55°C, and the adsorption residence time is 25 to 50 minutes.

6. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that: The low-temperature vacuum deodorization process involves a temperature of 40–75°C, a vacuum degree of 0.09–0.1 MPa, and a deodorization time of 30–50 minutes.

7. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that, The two-stage filtration includes: first, coarse filtration through a 30-100 mesh filter, and then fine filtration through a 1-10μm precision filter element; the three-stage precision filtration adopts a three-stage series security filter with progressively decreasing pore size, and their filtration accuracies are 5-10μm, 1-5μm and 0.2-1μm respectively.

8. The method for preparing high-insulation-performance natural ester insulating oil from vegetable oil using low-temperature continuous alkali-free refining coupled with refining according to claim 1, characterized in that, The cooling and stabilization treatment includes: cooling the deodorized oil to below 25°C and allowing it to stand for aging for 6 to 12 hours.

9. A natural ester insulating oil with high insulating properties, characterized in that, It is prepared by the method described in any one of claims 1 to 8.

10. The high-insulation-performance natural ester insulating oil according to claim 9, characterized in that: The natural ester insulating oil has an acid value ≤0.20mgKOH / g, a breakdown voltage ≥70kV, a water content ≤50ppm, and a dielectric loss factor ≤0.025 at 90℃.

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