A natural ester insulating oil and a method for producing the same

Abstract

The application relates to the technical field of electrical insulation materials, and discloses a natural ester insulating oil and a preparation method thereof, the preparation method of the natural ester insulating oil comprising the following steps: preparing a composite pour point depressant mother liquor containing polymethyl acrylate and an intermolecular spacer; atomizing and injecting the composite pour point depressant mother liquor into a base oil flow for online mixing, and controlling the momentum flux ratio to be 5-20; gradient quenching the mixed oil flow, and controlling the cooling rate to be 6-15 DEG C / min. According to the application, fluid mechanics control and thermodynamics locking are adopted, so that the polymethyl acrylate pour point depressant is in a non-equilibrium state solvated dispersion structure in the base oil. The obtained insulating oil has a filter blocking tendency value less than 1.05, the application has excellent low-temperature fluidity and micro cleanliness, solves the problems of uneven dispersion and easy agglomeration of the polymer pour point depressant in the natural ester, and is suitable for high-voltage power equipment.

Description

Technical Field

[0001] This invention relates to the field of electrical insulation materials technology, specifically to a natural ester insulating oil and its preparation method. Background Technology

[0002] Natural ester insulating oils are gradually replacing traditional mineral insulating oils in power transformers due to their high flash point, biodegradability, and good water absorption, among other environmentally friendly and safe properties. However, the main component of natural esters is triglycerides, and the saturated fatty acid ester components they contain are prone to crystallization at low temperatures, forming a three-dimensional network structure that causes the oil to solidify, severely limiting their application in cold climates.

[0003] Currently, adding polymeric pour point depressants such as polymethacrylate to natural esters is the main technical means to improve their low-temperature fluidity. However, due to the large molecular weight of polymeric pour point depressants and the differences in molecular structure and polarity between them and the triglyceride matrix, polymer molecular chains in natural ester-based oils tend to coil or entangle thermodynamically, forming microscale physical aggregates. Existing insulating oil preparation processes typically employ conventional heating and stirring blending methods. The shear force provided by this method is insufficient to overcome the strong interaction forces between polymer molecular chain segments, making it impossible to achieve uniform single-molecule dispersion of the polymer in the base oil.

[0004] This uneven dispersion at the microscopic level leads to two technical defects: First, the coiled or agglomerated polymer molecules cannot fully expose their alkyl side chains, resulting in a reduced efficiency in altering the growth habits of wax crystals. Often, an increased addition amount is required to achieve the expected pour point reduction effect, while excessively high polymer content will degrade the medium loss factor of the oil. Second, the presence of polymer micro-aggregates will significantly increase the filtration clogging tendency (FBT) of the oil. During the long-term circulating cooling or purification filtration process of transformers, it is very easy to cause filter element blockage or obstruction of the flow of fine oil channels, posing a potential hazard to the heat dissipation efficiency and operational safety of power equipment.

[0005] Therefore, this invention proposes a natural ester insulating oil and its preparation method to overcome the shortcomings of the prior art. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a natural ester insulating oil and its preparation method. This solves the problem that in the existing natural ester insulating oil preparation process, polymeric pour point depressants are difficult to achieve uniform single-molecule-level dispersion in the vegetable oil matrix, easily forming micro-agglomerates. This results in limited improvement of the low-temperature fluidity of the insulating oil and a high tendency to filter blockage, making it impossible to simultaneously ensure the stability of low-temperature performance and electrical insulation performance.

[0007] To address the above problems, the present invention provides the following technical solution: In a first aspect, the present invention provides a method for preparing a natural ester insulating oil, employing the following technical solution: A method for preparing a natural ester insulating oil includes the following steps: Step 1, Base oil pretreatment: After vacuum dehydration and degassing, the base oil is cooled to a predetermined temperature and transported in a laminar flow state; Step 2, Mother liquor preparation: Add molecular spacer and polymethyl methacrylate pour point depressant to the carrier solvent, heat to dissolve and shear homogenize to obtain a transparent and homogeneous composite pour point depressant mother liquor; Step 3, Atomized jet mixing: The high-temperature composite pour point depressurization mother liquor obtained in step 2 is pressurized and atomized, and injected into the base oil flow in step 1 for online mixing. The momentum flux ratio of the composite pour point depressurization mother liquor jet to the base oil flow is controlled between 5 and 20. Step 4, Gradient quenching: The mixed base oil stream is subjected to gradient quenching, and forced to cool to below 25°C within 3 minutes to lock in the micro-dispersion structure; Step 5, Post-processing: Precision filtration and filling are carried out under inert gas protection.

[0008] By employing the above technical solution and combining fluid dynamics control with thermodynamic locking, non-equilibrium solvation dispersion of polymethacrylate pour point depressant in natural ester-based oils was achieved. The specific mechanism and effects are described below: During the mother liquor preparation stage, the combined effects of high temperature and mechanical shear force overcome the van der Waals forces between polymethacrylate molecular chain segments, causing them to dissociate from a coiled or physically entangled state. With the solvation effect of the carrier solvent and the steric hindrance of the molecular spacer, the polymer molecular chains fully extend, reaching a critical disentanglement state with high free energy.

[0009] During the mixing stage, by controlling the momentum flux ratio between 5 and 20, sufficient kinetic energy is ensured for the composite pour point depressant jet to penetrate the laminar boundary layer of the base oil flow. A strong momentum exchange occurs between the jet and the base oil flow at the contact interface, initiating microscale turbulent mixing. This high-energy mixing method forces the stretched polymethyl methacrylate molecular chains to rapidly disperse and embed themselves into the intermolecules of the triglycerides in the base oil. The dispersion rate is significantly higher than the rate at which the molecular chains re-entangle, thus avoiding agglomeration caused by excessively high local concentrations.

[0010] During the gradient quenching stage, rapid cooling within 3 minutes utilizes the characteristic that the system viscosity increases sharply with decreasing temperature to create kinetic resistance. The increased viscosity of the base oil effectively restricts the thermal motion of the polymethyl methacrylate (PMMA) molecular chains, preventing them from reverting to a more thermodynamically stable coiled or aggregated state through conformational adjustment. This process fixes PMMA in a stretched, non-equilibrium state, allowing its alkyl side chains to more efficiently co-crystallize with wax crystals in the base oil at low temperatures, thus altering the wax crystal growth habit. Simultaneously, by reducing the formation of polymer agglomerates, the filter clogging tendency of the insulating oil is significantly reduced.

[0011] Preferably, the polymethacrylate pour point depressant is prepared by: using pentaerythritol tetra-2-ethylhexanoate as the polymerization solvent, adding a monomer mixture of dodecyl methacrylate and octadecyl methacrylate under an inert atmosphere, and heating to 75-80°C; adding the initiator azobisisobutyronitrile, and reacting at a constant temperature for 5-6 hours; after the reaction is completed, heating to 110-120°C and maintaining under vacuum for 0.5-1.5 hours to remove residual monomers and volatiles.

[0012] By adopting the above technical solution and selecting a synthetic ester with a chemical structure similar to the base oil as the polymerization solvent, the compatibility of the pour point depressant system with the base oil is ensured, and solvent replacement is unnecessary. Specific polymerization temperature and initiator addition method ensure that the copolymer has a suitable sequence structure distribution. The vacuum post-treatment step effectively removes small molecule volatiles, preventing them from affecting the flash point and dielectric properties of the insulating oil.

[0013] Preferably, the preparation temperature of the composite pour point depressant mother liquor is controlled at 100-105℃, the shearing and homogenization speed is 3000-5000 rpm, and the time is 5-10s, to ensure that the molecular chains of the polymethyl methacrylate pour point depressant are below the critical detangling concentration.

[0014] By employing the above technical solution, a specific combination of temperature and shear rate constitutes a process window for detangling polymer molecular chains. If the temperature is below 100℃ or the rotation speed is insufficient, the physical entanglement nodes between molecular chains cannot be effectively destroyed; if the temperature is too high or the shearing time is too long, it will lead to thermal oxidative degradation or mechanical chain breakage of the polymer. Under these conditions, the polymer molecular chains can maintain a stretched and structurally intact state.

[0015] Preferably, the injection pressure of the composite pour point depressant mother liquor is 2.0 to 3.5 MPa, forming a solid conical jet through a nozzle with an orifice diameter of 0.8 to 1.2 mm; the momentum flux ratio is defined as the ratio of the momentum flux of the composite pour point depressant mother liquor jet to the momentum flux of the base oil flow, that is, the product of the density of the composite pour point depressant mother liquor and the square of its velocity divided by the product of the density of the base oil and the square of its velocity.

[0016] By employing the above technical solution, a pressure of 2.0–3.5 MPa, combined with a solid conical nozzle, atomizes the mother liquor into micron-sized droplets, significantly increasing the contact area between the two phases. The momentum-flux ratio, as a core control parameter, quantifies the jet's penetration and mixing capabilities within the mainstream field. Within this defined ratio range, the mother liquor droplets rapidly break up and complete mass transfer in the base oil, achieving homogeneous mixing.

[0017] Preferably, the pretreatment temperature of the base oil is controlled at 45-50°C, and the flow rate of the base oil is controlled at 14.0-20.0 m / s; the moisture content of the base oil is treated to below 30 ppm.

[0018] By adopting the above technical solution, the temperature of the base oil is controlled at 45-50℃, which not only imparts a suitable Reynolds number to the base oil to facilitate jet mixing, but also reduces the heat load of the subsequent quenching process. The specific flow rate range maintains a stable flow field environment inside the pipe, preventing uneven mixing caused by flow rate fluctuations.

[0019] Preferably, the gradient quenching employs a plate heat exchanger, uses a refrigerant at 5-10°C, controls the cooling rate at 6-15°C / min, and maintains nitrogen protection with a dew point below -40°C throughout the process.

[0020] By employing the above technical solution, a cooling rate of 6–15 °C / min is the key thermodynamic parameter for the formation of a non-equilibrium structure. An excessively slow cooling rate leads to polymer chain relaxation and rearrangement, resulting in structural degradation; an excessively fast cooling rate may induce localized supercooling and crystallization. The plate heat exchanger, combined with low-dew-point nitrogen protection, ensures efficient heat exchange while preventing condensate contamination, thus guaranteeing that the moisture content of the insulating oil meets standards.

[0021] Secondly, the present invention provides a natural ester insulating oil, which adopts the following technical solution: A natural ester insulating oil is prepared by the preparation method described in the first aspect and is composed of the following components in parts by weight: base oil: 90.0-95.0 parts; composite pour point depressant mother liquor: 5.0-10.0 parts; wherein the base oil is a vegetable oil with an oleic acid content greater than 80% by weight; the composite pour point depressant mother liquor comprises: a carrier solvent, a polymethyl methacrylate pour point depressant, a molecular spacer, and an antioxidant; wherein the polymethyl methacrylate pour point depressant has a non-equilibrium solvated dispersion structure in the natural ester insulating oil, and the filter clogging tendency value of the natural ester insulating oil is less than 1.05.

[0022] By employing the above technical solution, this natural ester insulating oil exhibits characteristics distinct from traditional blended oils at the microscopic level. The polymethyl methacrylate pour point depressant is distributed in the continuous phase of the base oil in a non-equilibrium solvated extension conformation, rather than in a thermodynamically stable aggregate form. This dispersion increases the contact surface area between the alkyl side chains on the pour point depressant molecule and the base oil, thereby enhancing its adsorption probability of trace amounts of saturated fatty acid ester crystals (wax crystals) precipitated in the base oil.

[0023] This non-equilibrium structure enhances oil performance: on the one hand, the extended pour point depressant molecular chains can more efficiently alter the growth habit of wax crystals through co-crystallization, inhibiting the formation of a three-dimensional network crystal structure, thereby improving low-temperature fluidity at low addition levels; on the other hand, by eliminating large-sized polymer aggregates, the microscopic uniformity of the oil is improved, keeping the filter clogging tendency (FBT) below 1.05, reducing the risk of filter element clogging in transformer cooling systems. Simultaneously, the high-oleic vegetable oil base ensures the oil's oxidative stability and good viscosity-temperature characteristics.

[0024] Preferably, the composite pour point depressant mother liquor is made from raw materials comprising the following parts by weight: carrier solvent: 75.0-80.0 parts; polymethyl methacrylate pour point depressant: 15.0-20.0 parts; molecular spacer: 3.0-5.0 parts; composite antioxidant: 1.0-2.0 parts; wherein the carrier solvent is pentaerythritol tetra-2-ethylhexanoate; and the molecular spacer is epoxidized soybean oil.

[0025] By adopting the above technical solution, pentaerythritol tetra-2-ethylhexanoate exhibits excellent thermal stability and low volatility, and its solubility parameters are similar to those of polymethyl methacrylate and vegetable oil base materials, effectively promoting the compatibility between components. Epoxidized soybean oil, acting as a molecular spacer, utilizes the interaction between the polar epoxy groups on its molecular chain and the polymethyl methacrylate ester groups to insert between polymer molecular chain segments, providing steric hindrance, preventing the re-aggregation of polymer molecular chains during the standing process, and stabilizing the dispersion structure; in addition, the epoxy groups can also absorb trace acidic substances produced by oil aging, improving chemical stability.

[0026] Preferably, the polymethacrylate pour point depressant has the following characteristics: a weight-average molecular weight of 70,000 to 100,000 g / mol and a polydispersity index of 1.5 to 1.8; the polymethacrylate pour point depressant is copolymerized from dodecyl methacrylate and octadecyl methacrylate, and the mass ratio of dodecyl methacrylate to octadecyl methacrylate is 60 to 75: 25 to 40.

[0027] By adopting the above technical solution, the weight-average molecular weight is controlled at 70,000–100,000 g / mol, balancing shear stability and pour point depressant efficiency: too low a molecular weight leads to insufficient co-crystallization ability, while too high a molecular weight easily causes mechanical shear degradation and increases oil viscosity. A narrow polydispersity index of 1.5–1.8 ensures the uniformity of molecular chain size. In the monomer composition, the long side chain of octadecyl methacrylate is used for co-crystallization with wax crystals, while the short side chain of dodecyl methacrylate provides oil solubility. The specific mass ratio of the two ensures that the pour point depressant can effectively precipitate at low temperatures, while avoiding premature separation that would lead to oil turbidity.

[0028] Preferably, the composite antioxidant is selected from a composition of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 1-(N,N-bis(2-ethylhexyl)aminomethyl)-4-methylbenzotriazole, and the mass ratio of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate to 1-(N,N-bis(2-ethylhexyl)aminomethyl)-4-methylbenzotriazole is 4:1 to 6:1.

[0029] By employing the above technical solution, a hindered phenolic primary antioxidant is combined with a metal deactivator. Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate interrupts the oxidation chain reaction by capturing free radicals; 1-(N,N-bis(2-ethylhexyl)aminomethyl)-4-methylbenzotriazole forms a film on the surface of metals such as copper through coordination bonds, inhibiting the catalytic activity of metal ions. The combination of these two in a specific ratio produces a synergistic antioxidant effect, and this combination exhibits good solubility in natural esters and does not precipitate at low temperatures, ensuring the long-term dielectric properties and anti-aging stability of the insulating oil.

[0030] This invention provides a natural ester insulating oil and its preparation method. It has the following beneficial effects: 1. This invention achieves non-equilibrium solvation locking of polymethacrylate pour point depressants in base oils through atomized jet mixing and gradient quenching processes with a specific momentum flux ratio. This structure forces the pour point depressant molecules to remain in a highly extended state, increasing the contact area between their side chains and wax crystals in the base oil. This allows for more effective modification of wax crystal growth habits through co-crystallization, inhibiting the formation of a three-dimensional crystalline network structure. This enables the resulting natural ester insulating oil to achieve excellent low-temperature fluidity even at relatively low addition levels.

[0031] 2. Benefiting from the synergistic effect of shear homogenization of the mother liquor, the steric hindrance effect of the molecular spacer, and online jet dispersion, this invention effectively eliminates the large-sized physical aggregates or micelles commonly found in polymeric pour point depressants in vegetable oil matrices, keeping the product's filtration clogging tendency (FBT) at an extremely low level. This highly uniform microstructure avoids filter clogging or obstruction of fine oil passages that may occur in transformer circulating cooling systems, effectively ensuring the heat dissipation efficiency and operational safety of power equipment.

[0032] 3. This invention constructs a synergistic formulation system comprising a specific carrier solvent, a molecular spacer, and a composite antioxidant. The excellent compatibility between the carrier solvent and the base oil, combined with the stabilizing effect of the molecular spacer, ensures that the pour point depressant does not undergo phase separation or precipitation under long-term static or low-temperature conditions. Simultaneously, the specific combination of primary and secondary antioxidants improves the oxidative stability of the natural ester insulating oil without affecting its dielectric properties, thus extending the performance of the insulating oil throughout the transformer's entire lifespan. Attached Figure Description

[0033] Figure 1 This is a comparison chart of the filter clogging tendency test results of each embodiment, comparative example, and reference oil sample in Test Example 1 of the present invention; Figure 2 The figure shows the test results of conformational stability and low-temperature rheological dynamics in Test Example 2 of the present invention; wherein, (a) shows a comparison of the kinematic viscosity index of different samples, and (b) shows the evolution curve of the apparent viscosity of the sample with the standing time under constant temperature of -20℃. Figure 3 This is a comparison diagram of the pour point changes of each sample in Test Example 3 of the present invention before and after undergoing thermal cycling aging; Figure 4 The following are the test results of the electrical insulation performance of each sample in Test Example 4 of the present invention; wherein, (a) shows the average value and standard deviation of the breakdown voltage, and (b) shows the comparison of the dielectric loss factor at 90°C. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0036] High-oleic soybean oil (CAS No. 8001-22-7), a refined-grade insulating oil base material, with an oleic acid content greater than 85% by mass, a saturated fatty acid content less than 8% by mass, an acid value less than 0.03 mg KOH / g, and a moisture content less than 50 ppm; pentaerythritol tetra(2-ethylhexanoic acid) ester (CAS No. 7299-99-2), with a purity greater than 98%, is used as a carrier solvent; epoxidized soybean oil (CAS No. 8013-07-8), with an epoxy value greater than 6.2%, is used as a molecular spacer; octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (CAS No. 2082-79-3) and 1-(N,N-bis(2-ethylhexyl)aminomethyl)-4-methylbenzotriazole (CAS No. 80584-90-3) are used as components of a composite antioxidant. Dodecyl methacrylate (CAS No. 142-90-5) and octadecyl methacrylate (CAS No. 32360-05-7) are used as monomers for synthesizing pour point depressants; azobisisobutyronitrile (CAS No. 78-67-1) is used as a polymerization initiator.

[0037] Preparation Example 1: This preparation example provides a method for preparing a polymethacrylate pour point depressant (PMA-1) for natural ester insulating oil, comprising the following steps: (1) Add 100g of pentaerythritol tetra(2-ethylhexanoic acid) ester as a polymerization solvent to a 500mL four-necked flask equipped with a mechanical stirrer, reflux condenser, thermometer and nitrogen inlet tube, and purge with high-purity nitrogen for 30 minutes to replace the air in the system. (2) Add the monomer mixture to the flask. The monomer mixture consists of 65g dodecyl methacrylate and 35g octadecyl methacrylate. Turn on the stirring and heat to 80°C. (3) Dissolve 0.4g of initiator azobisisobutyronitrile (AIBN) in 10g of the above polymerization solvent, and slowly add it dropwise into the flask. The dropwise addition time is controlled within 15 minutes. After the dropwise addition is completed, keep the reaction at 80-85℃ for 5 hours. (4) The reaction system is heated to 115°C and kept under a vacuum of -0.08MPa to -0.09MPa for 1 hour to completely decompose the residual initiator and remove trace amounts of unreacted monomers or volatile impurities, so as to obtain a viscous and transparent liquid pour point depressant PMA-1.

[0038] The weight-average molecular weight (Mw) of the obtained polymer was approximately 75,000 g / mol and the polydispersity index (PDI) was 1.65, as determined by gel permeation chromatography (GPC) with polystyrene as a standard.

[0039] Preparation Example 2: This preparation example provides a method for preparing a high molecular weight polymethacrylate pour point depressant (PMA-2) for natural ester insulating oil, comprising the following steps: (1) Add 100g of pentaerythritol tetra(2-ethylhexanoic acid) ester to a four-necked flask equipped with a mechanical stirrer, reflux condenser, thermometer and nitrogen inlet tube, and purge oxygen with nitrogen for 30 minutes. (2) Add the monomer mixture consisting of 70g dodecyl methacrylate and 30g octadecyl methacrylate, stir and heat to 75°C; (3) Dissolve 0.25g of initiator azobisisobutyronitrile in 10g of polymerization solvent and drop it into a flask, and react at a constant temperature of 75-80℃ for 6 hours; (4) Heat to 115°C and keep warm under vacuum for 1 hour for post-treatment to obtain pour point depressant PMA-2.

[0040] The weight-average molecular weight (Mw) of the obtained polymer was determined to be approximately 98,000 g / mol, and the polydispersity index (PDI) was 1.72.

[0041] Explanation of the calculation of momentum flux ratio (J): In all embodiments and comparative examples of the present invention, the momentum flux ratio (J) is used as a key parameter for measuring the mixing intensity of the two phases, and its calculation is based on the following formula: In the formula: The momentum flux ratio (dimensionless); The density of the composite pour point depressant mother liquor is 0.95 g / cm³, and to ensure consistency in the calculated data, this instruction manual uses a uniform value of 0.95 g / cm³. 3 (This value is an approximation based on measured values ​​at the process temperature.) The jet velocity of the composite pour point depressant mother liquor at the nozzle outlet is expressed in m / s. The density of the base oil is 0.92 g / cm³. To ensure consistency in calculation data, this instruction manual uses a uniform value of 0.92 g / cm³. 3 ; The velocity of the base oil in the mixing zone (the throat of the venturi tube) is expressed in m / s.

[0042] Example 1: This example provides a natural ester insulating oil and its preparation method, using the pour point depressant PMA-1 synthesized in Example 1, specifically including the following steps: (1) Base oil pretreatment: High oleic soybean oil is pumped into a vacuum degassing tower and treated at a temperature of 110°C and an absolute pressure of 300Pa until the moisture content drops to 25ppm; then it is cooled to 48°C through a plate heat exchanger and pumped into the main channel of the Venturi mixer, and the base oil flow rate at the venturi throat is controlled to be 15.5m / s.

[0043] (2) Mother liquor preparation: Add the formulated amounts of the carrier solvent pentaerythritol tetra(2-ethylhexanoate) (76.5 parts by mass) and the molecular spacer epoxidized soybean oil (4.0 parts by mass) to a nitrogen-filled and protected pre-dissolution kettle, and heat to 70°C; start stirring (80 rpm), and slowly add the pour point depressant PMA-1 obtained in Preparation Example 1 (18.0 parts by mass) and the compound antioxidant (1.5 parts by mass, where the mass ratio of the main antioxidant to the metal deactivator is 5:1), and stir at a constant temperature for 15 minutes until the particles are completely wetted; then quickly raise the temperature to 100°C, and perform online homogenization for 5 seconds through a pipeline high-shear emulsifier (rotation speed 4000 rpm) to obtain a transparent homogeneous mother liquor.

[0044] (3) Atomized jet mixing: Pressurize the above-mentioned mother liquor at 100°C to 2.8 MPa through a medium-pressure piston pump, and spray it out through a solid cone nozzle (aperture 1.0 mm) located at the center of the Venturi throat to form an atomized jet with an outlet velocity of 53.9 m / s; adjust the flow rate of the base oil and the injection pressure of the mother liquor, and control the momentum flux ratio (J) to be 12.5 (meeting the preferred range of 5 < J < 20); under these conditions, the mother liquor and the base oil are mixed online at a mass ratio of 8:92.

[0045] (4) Gradient quenching: The mixed oil flow immediately enters a two-stage plate cooler, uses 5°C chilled water as the refrigerant, and quickly reduces the oil temperature from the mixing temperature to 25°C within 3 minutes; maintain a slightly positive pressure protection of dry nitrogen with a dew point of -45°C from the cooler outlet to the subsequent pipeline.

[0046] (5) Filtration and filling: Under the conditions of 25°C and nitrogen sealing, the oil product passes through a 5μm bag filter and a 1μm pleated filter element in sequence, and then is vacuum-filled to obtain the finished natural ester insulating oil.

[0047] Example 2: This example provides a natural ester insulating oil and its preparation method, which specifically includes the following steps: (1) Base oil pretreatment: After vacuum dehydrating and degassing the high-oleic acid soybean oil, cool it to 50°C, pump it into the Venturi mixer, and control the base oil flow rate at the throat to be 19.3 m / s.

[0048] (2) Mother liquor preparation: Prepare the mother liquor according to the same ratio as in Example 1, the difference is that PMA-1 of Preparation Example 1 is used, and the final heating temperature of the mother liquor is 102°C, and it is immediately injected after online homogenization.

[0049] (3) Atomized jet mixing: Pressurize the mother liquor to 2.1 MPa, spray it out through a nozzle with an aperture of 1.2 mm to form an atomized jet with an outlet velocity of 48.0 m / s; at the same time, adjust the base oil flow rate to control the momentum flux ratio (J) at 6.2; the mixing mass ratio of the mother liquor to the base oil is 9:91.

[0050] (4) Gradient quenching: The mixed oil flow is cooled to 26°C within 4 minutes, and dry nitrogen protection is implemented throughout the process.

[0051] (5) Filtration and filling: After double-stage filtration of 5μm and 1μm, the product is filled to obtain the finished product.

[0052] Example 3: This example provides a natural ester insulating oil and its preparation method, specifically including the following steps: (1) Base oil pretreatment: After vacuum dehydration and degassing of high oleic soybean oil, it is cooled to 45°C and pumped into a Venturi mixer, and the flow rate of base oil in the throat is controlled at 14.4 m / s.

[0053] (2) Preparation of mother liquor: The mother liquor was prepared according to the same ratio as in Example 1, and the final heating temperature of the mother liquor was 105℃.

[0054] (3) Atomized jet mixing: The mother liquor is pressurized to 3.4 MPa and sprayed out through a nozzle with an orifice diameter of 0.8 mm to form an atomized jet with an outlet velocity of 61.0 m / s; at the same time, the base oil flow rate is adjusted so that the momentum flux ratio (J) is controlled at 18.5; the mixing mass ratio of mother liquor to base oil is 7.5:92.5.

[0055] (4) Gradient quenching: The mixed oil flow is forcibly cooled to 24°C within 2.5 minutes to prevent heat accumulation, and nitrogen protection is provided throughout the process.

[0056] (5) Filtration and filling: After double-stage filtration of 5μm and 1μm, the product is filled to obtain the finished product.

[0057] Example 4: This example provides a natural ester insulating oil and its preparation method, using the high molecular weight pour point depressant PMA-2 synthesized in Preparation Example 2, specifically including the following steps: (1) Base oil pretreatment: The process conditions are the same as in Example 1. The base oil is cooled to 48°C and the flow rate of the base oil is controlled at 15.0 m / s.

[0058] (2) Mother liquor preparation: Given the high molecular weight of PMA-2, its concentration needs to be adjusted to below the critical overlap concentration to avoid molecular chain entanglement. The mother liquor composition is: 79.5 parts by mass of carrier solvent, 4.0 parts by mass of epoxidized soybean oil, 15.0 parts by mass of PMA-2 (high molecular weight), and 1.5 parts by mass of composite antioxidant. In the preparation process, the wetting temperature is 75℃, the final dissolution temperature is 105℃, and the online homogenization time is extended to 8 seconds.

[0059] (3) Atomized jet mixing: The mother liquor injection pressure is 3.0 MPa, the nozzle orifice diameter is 1.0 mm, and an atomized jet with an outlet velocity of 56.1 m / s is formed. At this time, the momentum flux ratio (J) is controlled to be 14.0. The mother liquor and base oil are mixed at a mass ratio of 8:92.

[0060] (4) Subsequent steps: Quenching, filtering and filling are the same as in Example 1, and the final cooling temperature is controlled at 25°C.

[0061] Example 5: This example provides a natural ester insulating oil and its preparation method, adjusting the amount of molecular spacer ESO, specifically including the following steps: (1) Base oil pretreatment: The process conditions are the same as in Example 1, and the base oil flow rate is controlled at 15.5 m / s.

[0062] (2) Preparation of mother liquor: The formula of the mother liquor was adjusted as follows: 77.5 parts by weight of carrier solvent, 3.0 parts by weight of epoxidized soybean oil, 18.0 parts by weight of PMA-1, and 1.5 parts by weight of composite antioxidant. The preparation process is the same as in Example 1.

[0063] (3) Atomized jet mixing: The mother liquor is sprayed at a pressure of 2.8 MPa and sprayed out through a nozzle with an orifice diameter of 1.0 mm to form an atomized jet with an outlet velocity of 53.7 m / s. At this time, the momentum flux ratio (J) is controlled to be 12.0; the mother liquor and base oil are mixed at a mass ratio of 8:92.

[0064] (4) Subsequent steps: Same as in Example 1, to obtain the finished product.

[0065] Comparative Example 1: This comparative example uses a traditional batch mixing process. The difference from Example 1 is that a Venturi mixer and atomizing jet device are not used; the base oil is heated to 60°C and then added to a mixing vessel with a frame stirrer. The mother liquor (with the same formula as in Example 1) is added directly, and the mixture is stirred at 300 rpm for 2 hours, then allowed to cool naturally to room temperature. All other raw material specifications are the same.

[0066] Comparative Example 2: This comparative example aims to verify the importance of the lower limit of momentum flux ratio (J). Compared with Example 1, the difference is that the injection pressure of the mother liquor was significantly reduced to 0.5 MPa, which reduced the exit velocity of the atomized jet, and the calculated momentum flux ratio (J) was 2.5; the rest of the steps and parameters were the same.

[0067] Comparative Example 3: This comparative example aims to verify the importance of the upper limit of the momentum flux ratio (J). The difference from Example 1 is that the mother liquor injection pressure was increased to an excessively high level, and the calculated momentum flux ratio (J) was 35.0; all other steps and parameters were the same.

[0068] Comparative Example 4: This comparative example aims to verify the importance of "kinetic stabilization," i.e., gradient quenching. Compared with Example 1, the difference is that the forced quenching process of the plate cooler in step (4) is cancelled; the mixed oil enters the buffer tank and is naturally cooled to room temperature (cooling time is about 4 hours); the remaining steps and parameters are the same.

[0069] Comparative Example 5: This comparative example aims to verify the importance of temperature in the "de-entanglement pretreatment at critical concentration". The difference from Example 1 is that in step (2) of the mother liquor preparation process, the final heating temperature of the mother liquor is only 60°C. At this time, although the polymethyl methacrylate (PMA-1) has dissolved, it is still in a coiled and agglomerated state, and then jet mixing is carried out directly; the other steps and parameters are the same.

[0070] Comparative Example 6: This comparative example aims to verify the role of the "molecular spacer layer". The difference from Example 1 is that the mother liquor formulation does not contain the molecular spacer epoxidized soybean oil (ESO), and the missing 4.0 parts by mass are made up by the carrier solvent; all other steps and parameters are the same.

[0071] Test Example 1: Filter Blocking Propensity (FBT) and Microscopic Dispersion State Assessment Experimental description: This test case, based on ASTM-D2068 standard method B, evaluates the dispersion uniformity of polymethyl methacrylate (PPD) in modified insulating oil and the presence of large micelles by measuring the pressure difference change across the filter membrane at a constant flow rate.

[0072] Take 500 mL of fresh oil samples prepared in Examples 1-5 and Comparative Examples 1-3, and place them in a constant temperature environment of 25±0.5℃ for 24 hours to eliminate the influence of thermal history.

[0073] Check the sealing of the test instrument circuit, install a glass fiber filter membrane with a nominal pore size of 1.6 μm, and set the peristaltic pump flow rate to 20 mL / min.

[0074] Start the pumping program and continuously record the gauge pressure (P) upstream of the filter membrane at a frequency of 1 Hz; the test termination condition is set to a filtration volume of 300 mL or a pressure of 105 kPa.

[0075] Read the maximum pressure value (P) at the end of the test, according to the formula. Calculate the filter blocking tendency value.

[0076] Each sample was tested independently three times, and the arithmetic mean was taken.

[0077] The experimental data are shown in Table 1: Table 1: Record of Filtration Pressure Response and FBT Characteristics for Each Oil Sample

[0078] (Note: The base pressure differential of the reference oil is mainly derived from the bulk viscosity of the natural ester at 25°C.) Conclusion: Based on Table 1 and the appendix Figure 1 The data show that the rheological resistance of each group of samples when passing through the microporous medium is significantly different, which directly reflects the microscopic existence form of PPD in the base oil.

[0079] The average filtration endpoint pressures in Examples 1, 4, and 5 remained in the range of 21.9–23.5 kPa, with converted FBT values ​​(1.021–1.025) extremely close to those of pure reference oil (1.015). Considering the high viscosity of natural esters, this small pressure differential increment indicates the near absence of particles or micelles with an equivalent diameter greater than 1.6 μm within the system. This result confirms that PPD molecules, after detangling pretreatment and locking with a specific momentum flux (J=12–14), exist in the oil phase as highly extended monomolecular or oligomolecular solvation units. This “non-equilibrium locked” structure has an extremely small hydrodynamic radius, allowing it to pass smoothly through the filter membrane pores without stagnation or blockage.

[0080] In contrast, the average pressure of Comparative Example 1 (blended in a batch) soared to 96.3 kPa, with an FBT value as high as 1.353, approaching the upper limit of the instrument's measurement range. This indicates that in the absence of strong shear dispersion in the thermodynamic equilibrium process, the PPD molecular chains underwent severe self-entanglement, forming large-sized aggregates or soft micelles, leading to rapid blockage of the filter membrane pores.

[0081] The boundary effect of momentum flux ratio (J) was verified in Comparative Examples 2 and 3. The FBT value of Comparative Example 2 (J=2.5) was 1.198, significantly higher than that of Example Group, indicating that when the jet energy is insufficient to penetrate the laminar boundary layer, inefficient mixing at the interface leads to excessively high local concentrations and the formation of agglomerates. Although Comparative Example 3 (J=35.0) was better than Comparative Example 2, its FBT value (1.077) was still higher than that of Example 1, and the data dispersion was increased, suggesting that excessive shearing may have disrupted the stability of the molecular spacer layer, triggering secondary collisional agglomeration due to intensified Brownian motion.

[0082] In summary, the data show that the process described in this invention successfully constructed the expected "non-equilibrium solvation-locked" structure, which exhibits excellent filtration permeability on a macroscopic level and corresponds to the extreme dispersion and extension of PPD molecular chains on a microscopic level.

[0083] Test Example 2: Conformational Stability and Low-Temperature Rheological Dynamics Test Experimental description: This test case aims to verify the conformational extension and long-term storage stability of polymethyl methacrylate (PPD) molecular chains in the base oil by examining the viscosity-temperature characteristics and low-temperature shear history of the insulating oil under different thermodynamic states. The test includes kinematic viscosity index (VI) calculation and long-cycle low-temperature Brookfield scanning viscosity testing.

[0084] Take 300 mL of each of the oil samples prepared in Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6. Use an Ubbelohde viscometer to determine the kinematic viscosity of each sample at 40°C and 100°C according to ASTM-D445 standard, and calculate the viscosity index (VI) according to ASTM-D2270.

[0085] Take 100 mL of each of the above samples and place them in a standard glass test tube, then place the tube in a programmable low-temperature thermostat. First, heat the samples to 60°C and hold for 90 minutes to eliminate temporary thermal history, then cool them to room temperature at a rate of 1°C / min.

[0086] The constant temperature bath was set to -20±0.1℃, and the samples were kept in a constant temperature static curing environment at this temperature.

[0087] The apparent viscosity of the samples was measured using a Brookfield-LV viscometer equipped with a slender rotor at three time points: 12 hours, 24 hours, and 48 hours of curing. To capture subtle gel structures, all tests were conducted at a low shear rate (rotation speed set to 0.3 rpm), and the viscosity values ​​were recorded after the readings stabilized.

[0088] The experimental data are shown in Table 2: Table 2: Viscosity-temperature characteristics and viscosity evolution data at -20℃ for each oil sample

[0089] (Note: The reference oil loses its fluidity after 24 hours at -20℃, exceeding the instrument's measuring range.) Conclusion: Based on Table 2 and the appendix Figure 2 The data, by comparing the viscosity index and low-temperature viscosity evolution of each group of samples over time, confirmed the decisive influence of different processes on the conformation and stability of PPD molecular chains.

[0090] Example 1 exhibited the highest viscosity index (VI=218) and an extremely low 48-hour viscosity growth rate (2.5%). The high VI value indicates that PPD molecules maintained a large hydrodynamic volume in both the high-temperature (100°C) and low-temperature (40°C) ranges. Combined with the low-temperature Brookfield viscosity data, during a 48-hour settling period at -20°C, the viscosity value of Example 1 only fluctuated slightly (from 2485 to 2548 mPa·s), indicating that the internal structure of the system reached a kinetically frozen state. This confirms that the "gradient quenching" process successfully locked the detangled PPD molecular chains in a metastable extended conformation. PPD effectively suppressed the growth and interlocking of wax crystals in the base oil, and did not undergo significant relaxation or re-agglomeration.

[0091] In Comparative Example 4 (without quenching), the VI value decreased to 198, and the low-temperature viscosity increased significantly over time (by 46.2%). This indicates that during the slow, natural cooling process, the PPD molecular chains, which were originally stretched in the high-temperature jet, had sufficient time for thermal motion, resulting in conformational retraction and a return from the extended state to a partially coiled state. The coiled state of PPD reduces its modification efficiency on wax crystals, and simultaneously, physical entanglement gradually forms between the molecular chains, leading to a continuous increase in macroscopic viscosity over time.

[0092] Comparative Example 5 (low-temperature mother liquor) showed the worst performance, with the lowest VI value (182) and an initial low-temperature viscosity as high as 4215 mPa·s, which rapidly deteriorated to 7340 mPa·s over time. This confirms that if the mother liquor preparation temperature does not reach above the critical solvation temperature and the critical deentanglement condition, PPD is injected into the base oil in the form of tightly aggregated agglomerates. This type of structure has almost no pour point depressing ability, and the agglomerates themselves become the thickening core within the system.

[0093] Comparative Example 6 (without ESO), despite undergoing jet dispersion and quenching, still showed a viscosity increase of 45.2% after 48 hours. This indicates that, lacking steric protection from the intermolecular spacer, the PPD molecular chains, driven by Brownian motion, gradually displace the solvation layer and aggregate over time. This result conversely demonstrates the necessity of the polar intermolecular spacer layer in maintaining the long-term stability of the PPD microstructure within the "non-equilibrium solvation lock" mechanism.

[0094] Test Example 3: Depletion Efficiency and Thermal Cycling Aging Recovery Characteristics Test Experimental description: This test case aims to evaluate the low-temperature fluidity retention capability of modified natural ester insulating oil after undergoing simulated transformer operation thermal cycling. By comparing the difference between the initial pour point and the pour point after aging (i.e., the "recovery rate"), the microstructural stability and thermal relaxation resistance of PPD in the base oil are verified.

[0095] Take 200 mL of each sample from Examples 1-5 and Comparative Examples 1-4 and 6, and take high-oleic soybean oil without additives as a blank reference.

[0096] According to the ASTM-D97 standard method, the initial pour point (IPP) of all fresh samples was determined using an automatic pour point tester. Each sample was measured twice, and the smaller value was recorded.

[0097] After determining the initial pour point, the samples were placed in a sealed container and subjected to accelerated aging thermal cycling in a programmable high and low temperature test chamber. The cycle program was set as follows: first, the temperature was increased to 60°C at a rate of 2°C / min and held at that temperature for 24 hours (simulating the operating oil temperature of a transformer under load); then, the temperature was decreased to -10°C at a slow rate of 0.5°C / min and held at that temperature for 48 hours (simulating low-temperature settling after shutdown); finally, the temperature was allowed to rise naturally to room temperature. This process was repeated for 3 cycles, with a total time of approximately 9 days.

[0098] After the thermal cycling is completed, the sample is removed and left to stand at room temperature for 2 hours, and the final pour point (FPP) is determined again according to the ASTM-D97 standard.

[0099] Calculate the pour point recovery value (Reversion = FPP - IPP), which reflects the degree of failure of the pour point depressant conformation or the degree of recovery of the wax crystal growth habit in the system.

[0100] The experimental data are shown in Table 3: Table 3: Initial pour point and pour point data after thermal cycling aging for each oil sample

[0101] (Note: The ASTM-D97 standard typically uses 3°C intervals for manual testing. This experiment uses high-precision automated instruments, and data is recorded to the nearest whole number of 1°C.) Conclusion: Based on Table 3 and the appendix Figure 3 The data showed that the low-temperature performance evolution of each sample after undergoing a hot-cold cycle exhibited obvious regular differences, profoundly revealing the influence of process parameters on the microstructure stability of PPD.

[0102] Examples 1, 3, 4, and 5 exhibited excellent pour point depressing effects and extremely high stability, with a pour point rebound of only 1-2°C. This confirms the effectiveness of the "non-equilibrium solvation lock-in" mechanism: PPD molecules are "frozen" in a metastable extended conformation, and even under long-term heat treatment at 60°C, the molecular chains are unable to overcome steric hindrance and curl up, continuously inhibiting wax crystal growth.

[0103] The rationality of the upper and lower limits of the momentum flux ratio (J) was fully verified in this test: Comparative Example 2 (J = 2.5): Although it is slightly better than the kettle blending (Comparative Example 1), the initial pour point is only -24°C, and it rises significantly to -16°C (a rise of 8°C) after aging. This shows that too low jet energy cannot achieve uniform dispersion at the molecular level, and the locally formed aggregates are extremely likely to further coalesce during thermal cycling, resulting in the failure of the pour point depressing function.

[0104] Comparative Example 3 (J = 35.0): Its initial pour point (-30°C) is acceptable, but it rises to -24°C (a rise of 6°C) after aging, and flocculates appear. This indicates that too high turbulence intensity may cause the dispersed droplets to collide and coalesce, or too strong shear may cause some molecular chains to break, increasing the thermodynamic instability of the system, and it is not as stable as the samples within the preferred range (5 < J < 20).

[0105] The rise value of Comparative Example 4 (without forced quenching) is as high as 13°C, confirming that without rapid quenching and freezing, the conformational rearrangement of PPD molecular chains will occur. The rise value of Comparative Example 6 (without ESO) is 9°C, confirming the necessity of the molecular spacer layer for preventing the microscopic aggregation of PPD.

[0106] In summary, only the products under the specific process window of the present invention can simultaneously meet the requirements of deep pour point depression at low temperatures and long-term stability.

[0107] Test Example 4: Test on Dielectric Loss and Breakdown Voltage Characteristics Experimental Description: Based on the IEC-60156 and IEC-60247 standards, this test example measures the insulation strength and dielectric loss factor of the modified natural ester insulating oil. This test is used to evaluate the degree of interference of the polymethacrylate (PPD) additive on the insulation performance of the base oil in different dispersion states, and focuses on whether aggregates form charge traps or cause interfacial polarization under a strong electric field.

[0108] Select the oil samples prepared in Example 1, Example 5, Comparative Example 1, and Comparative Example 6, and at the same time take untreated high-oleic soybean oil as a reference. All samples are filtered through a micron filter and treated in a vacuum drying oven (60°C, 100 Pa) for 1 hour before testing to exclude the interference of trace moisture and air bubbles on the electrical properties, ensuring that the test results only reflect the influence of the additive dispersion state.

[0109] The breakdown voltage test is carried out according to the IEC-60156 standard. Use an oil cup equipped with standard hemispherical electrodes (gap distance 2.5 mm), slowly inject the oil sample and let it stand for 10 minutes to eliminate the agitated air bubbles. Apply an AC voltage with a frequency of 50 Hz, and the voltage rise rate is set at 2.0 kV / s until breakdown occurs. Each sample is continuously subjected to 6 breakdown tests, record the breakdown voltage value each time, and calculate the average value and standard deviation.

[0110] The dielectric loss factor (tanδ) was tested according to the IEC-60247 standard. The three-electrode test cell was cleaned and dried, and after injecting the oil sample, it was placed in a temperature-controlled chamber and heated to 90°C. After the temperature stabilized, a 50Hz, 1000V AC voltage was applied, and the dielectric loss factor was measured and recorded using a precision bridge. Each sample was tested independently three times.

[0111] The experimental data are shown in Table 4: Table 4: Breakdown Voltage (BDV) and Dielectric Loss Factor Test Data of Each Insulating Oil Sample

[0112] Conclusion: Based on Table 4 and the appendix Figure 4 The data showed that modified insulating oils prepared by different processes exhibited significant statistical differences in electrical properties, confirming the structure-property relationship between micro-dispersion structure and macro-insulation properties.

[0113] The average breakdown voltage of Example 1 reached 72.0 kV, which is on par with that of pure reference oil (74.5 kV), with a standard deviation of only 2.4 kV. This indicates that the PPD introduced through the process of this invention does not become an "electrical impurity." Under the "non-equilibrium solvation lock" mechanism, the PPD molecular chains are uniformly distributed with a very small hydrodynamic volume, and the surrounding ESO solvation layer effectively shields the local polarity of the PPD side chain ester groups. This highly homogeneous system avoids electric field distortion, making it difficult for electron avalanche to be initiated at the additive interface, thereby maintaining the inherent high pressure resistance of the base oil.

[0114] The electrical performance of Comparative Example 1 (blended in a kettle) deteriorated significantly, with the average BDV dropping to 49.9 kV and exhibiting extremely high data dispersion (standard deviation 8.2 kV). The dielectric loss factor at 90°C surged to 2.85%. This was due to the formation of micron-sized inhomogeneous phases in the oil by PPD agglomerates resulting from low-shear mixing. According to the Maxwell-Wagner interfacial polarization theory, these agglomerates exhibit significant differences in dielectric constant and conductivity compared to the base oil. Under the influence of an alternating electric field, charge easily accumulates at the phase interface, leading to a significant increase in dielectric loss. Simultaneously, the agglomerates act as "weak points" or charge traps in the electric field, inducing partial discharge under high voltage, resulting in a sharp drop in breakdown voltage and drastic numerical fluctuations.

[0115] The data for Comparative Example 6 (without ESO) falls between those of Example 1 and Comparative Example 1. Its BDV (60.3 kV) is acceptable, but its dielectric loss factor (1.62%) is significantly higher than that of Example 1 (1.14%). This confirms that the role of the molecular spacer ESO is not only in physical dispersion but also in electrical shielding. Without the directional adsorption of ESO, the polar groups on the PPD molecular chains are directly exposed to the electric field, increasing energy dissipation due to orientation polarization. Furthermore, aging tests (combined with the aforementioned Test Example 3) show that the ESO-free system is prone to slow aggregation, indicating a risk of long-term degradation in its electrical insulation performance.

[0116] In summary, the data demonstrates that this invention, through specific hydrodynamic control and molecular spacer design, solves the industry pain point that polymer additives typically reduce the electrical performance of insulating oils, achieving a synergy between excellent low-temperature fluidity and high insulation strength.

[0117] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a natural ester insulating oil, characterized in that, Includes the following steps: Step 1, Base oil pretreatment: After vacuum dehydration and degassing, the base oil is cooled to a predetermined temperature and transported in a laminar flow state; Step 2, Mother liquor preparation: Add a molecular spacer and polymethyl methacrylate pour point depressant to the carrier solvent, heat to dissolve and shear homogenize to obtain a transparent and homogeneous composite pour point depressant mother liquor; the molecular spacer is epoxidized soybean oil; the preparation temperature of the composite pour point depressant mother liquor is controlled at 100-105℃, the shear homogenization speed is 3000-5000 rpm, and the time is 5-10s; Step 3, Atomized Jet Mixing: The obtained high-temperature composite pour point depressurizing mother liquor is pressurized and atomized, with the injection pressure controlled at 2.0–3.5 MPa. A solid conical jet is formed through a nozzle with an orifice diameter of 0.8–1.2 mm and injected into the base oil flow for online mixing. The momentum flux ratio of the composite pour point depressurizing mother liquor jet to the base oil flow is controlled between 5 and 20. The momentum flux ratio is defined as the ratio of the momentum flux of the composite pour point depressurizing mother liquor jet to the momentum flux of the base oil flow, that is, the product of the density of the composite pour point depressurizing mother liquor and the square of its velocity divided by the product of the density of the base oil and the square of its velocity. Step 4, Gradient quenching: The mixed base oil stream is subjected to gradient quenching, and forced to cool to below 25°C within 3 minutes to lock in the micro-dispersion structure; Step 5, Post-processing: Precision filtration and filling are carried out under inert gas protection.

2. The method for preparing natural ester insulating oil according to claim 1, characterized in that, The preparation method of the polymethacrylate pour point depressant is as follows: Using pentaerythritol tetra-2-ethylhexanoate as the polymerization solvent, a monomer mixture of dodecyl methacrylate and octadecyl methacrylate was added under an inert atmosphere, and the temperature was raised to 75-80°C. Add the initiator azobisisobutyronitrile (AIBN) dropwise and react at a constant temperature for 5–6 hours. After the reaction is complete, the temperature is raised to 110–120°C and maintained under vacuum for 0.5–1.5 h to remove residual monomers and volatiles.

3. The method for preparing the natural ester insulating oil according to claim 1, characterized in that, The temperature of the base oil pretreatment is controlled at 45-50℃, and the flow rate of the base oil is controlled at 14.0-20.0m / s; the moisture content of the base oil is treated to below 30ppm.

4. The method for preparing the natural ester insulating oil according to claim 1, characterized in that, The gradient quenching employs a plate heat exchanger, uses a refrigerant at 5–10°C, controls the cooling rate at 6–15°C / min, and maintains nitrogen protection with a dew point below -40°C throughout the process.

5. A natural ester insulating oil, prepared by the method according to any one of claims 1-4, characterized in that, It is made from the following components in parts by weight: Base oil: 90.0–95.0 parts; Composite pour point depressant mother liquor: 5.0–10.0 parts; The base oil is a vegetable oil with an oleic acid content greater than 80% by mass; The composite pour point depressant mother liquor comprises: a carrier solvent, a polymethyl methacrylate pour point depressant, a molecular spacer, and an antioxidant; The polymethacrylate pour point depressant exhibits a non-equilibrium solvated dispersion structure in the natural ester insulating oil, and the filter clogging tendency value of the natural ester insulating oil is less than 1.

05.

6. The natural ester insulating oil according to claim 5, characterized in that, The composite pour point depressant mother liquor is made from raw materials comprising the following parts by weight: Carrier solvent: 75.0–80.0 parts; Polymethyl methacrylate pour point depressant: 15.0–20.0 parts; Molecular spacer: 3.0–5.0 parts; Compound antioxidant: 1.0–2.0 parts; The carrier solvent is pentaerythritol tetra-2-ethylhexanoate; the molecular spacer is epoxidized soybean oil.

7. The natural ester insulating oil according to claim 6, characterized in that, The polymethacrylate pour point depressant has the following characteristics: The weight-average molecular weight is 70,000–100,000 g / mol, and the polydispersity index is 1.5–1.

8. The polymethacrylate pour point depressant is copolymerized from dodecyl methacrylate and octadecyl methacrylate, with a mass ratio of dodecyl methacrylate to octadecyl methacrylate of 60-75:25-40.

8. The natural ester insulating oil according to claim 6, characterized in that, The composite antioxidant is selected from a combination of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 1-(N,N-bis(2-ethylhexyl)aminomethyl)-4-methylbenzotriazole, and the mass ratio of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate to 1-(N,N-bis(2-ethylhexyl)aminomethyl)-4-methylbenzotriazole is 4:1 to 6:1.

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