A method for in-situ synthesis of nanoparticles based on free fibers in transformer oil

Abstract

The application relates to the technical field of transformer insulation materials, and discloses a method for in-situ synthesis of free-fiber-based nanoparticles in transformer oil, which comprises the following steps: extracting an aged transformer insulation oil sample containing free nanocellulose as a to-be-processed substrate; continuously and uniformly adding high-concentration modified zinc oxide transparent mother liquor into the aged transformer insulation oil sample, and controlling the volume ratio of the mother liquor and the insulation oil sample to be 1:88 to 1:92; and performing low-shear fluid mixing stirring at a temperature of 40 DEG C to 60 DEG C and maintaining for 3h to 4h. In the application, the intermolecular action of the hydroxyl groups on the surface of the free nanofiber in the aged insulation oil and the epoxy and ether bonds on the surface of the modified zinc oxide occurs to form a three-dimensional hydrogen bond network, the modified zinc oxide particles are spatially anchored on the fiber network nodes, the particle agglomeration and sedimentation path is blocked, the free fiber is in-situ wrapped, the polar aging components are consumed, and the electrical strength and the physical and chemical state of the transformer insulation oil are restored.

Description

Technical Field

[0001] This invention relates to the field of transformer insulation materials technology, specifically to a method for in-situ synthesis of free fiber-based nanoparticles from transformer oil. Background Technology

[0002] Transformer insulating oil is a liquid insulating medium that fills the inside of a power transformer. It mainly serves to provide insulation and cooling. During the transformer's operation, the solid insulating materials such as insulating paper inside the insulating oil are constantly exposed to high temperature and high voltage alternating electric field environments and continuously bear the effects of mechanical stress.

[0003] Under the combined influence of thermal and electrical stress, the cellulose macromolecular chains in solid insulating paper undergo pyrolysis and oxidative breakage, releasing a large amount of suspended nano-sized free cellulose into the surrounding insulating oil. Accompanying the thermal aging process, the macromolecular hydrocarbon structure of the insulating oil itself undergoes oxidative degradation, generating a large amount of low molecular weight organic acids. The above-mentioned chemical aging products accumulate continuously inside the insulating oil, causing the gradual deterioration of the oil's physical and chemical properties.

[0004] Current solutions for treating insulating oil in aging transformers mostly employ physical circulation filtration. However, physical interception methods cannot remove tiny nanoscale free cellulose and dissolved acidic components. Free cellulose suspended within the insulating oil tends to align and overlap under an electric field, forming conductive channels and causing a decrease in the oil's breakdown voltage. Some improvements attempt to add nanoparticles to the insulating oil to enhance its electrical insulation performance. However, nanoparticles without surface grafting treatment and lacking spatial anchoring structures have high surface energy, making them prone to collisions and aggregation after being added to the insulating oil system, failing to maintain the system's dispersion stability in the long term. Furthermore, simple physical filtration or conventional nanoparticle addition methods cannot consume the low-molecular-weight organic acids accumulated within the insulating oil, failing to address the issues of increased acid value and decreased oil-water interfacial tension, fundamentally hindering the recovery of the insulating oil's physicochemical state and electrical strength.

[0005] Therefore, the purpose of this invention is to provide an in-situ synthesis method for free fiber-based nanoparticles in transformer oil, so as to overcome the shortcomings of the prior art. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides an in-situ synthesis method for free fiber-based nanoparticles in transformer oil. This method solves the problems of free nanocellulose accumulating in aging transformer insulating oil easily forming conductive channels and low molecular weight organic acids causing deterioration of the oil's physicochemical properties. Existing physical filtration or simple addition of nanoparticles treatment methods are prone to particle agglomeration and sedimentation, failing to completely consume polar aging components and restore the dispersion stability, electrical strength, and physicochemical state of insulating oil in the long term.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for in-situ synthesis of free fiber-based nanoparticles from transformer oil, employing the following technical solution: A method for in-situ synthesis of free cellulose-based nanoparticles in transformer oil includes extracting an aged transformer insulating oil sample containing free nanocellulose as the matrix to be treated; continuously and uniformly adding a high-concentration modified zinc oxide transparent mother liquor to the aged transformer insulating oil sample, controlling the volume ratio of the high-concentration modified zinc oxide transparent mother liquor to the aged transformer insulating oil sample to be 1:88 to 1:92; and performing low-shear fluid mixing and stirring at a temperature of 40°C to 60°C for 3 to 4 hours.

[0008] By adopting the above technical solution, and by introducing a mother liquor containing modified zinc oxide nanoparticles into aged insulating oil containing free nanocellulose, and mixing it under temperature and low shear force field, the nanoparticles and the inherent free cellulose in the aged insulating oil undergo in-situ recombination, thus achieving the following technical effects: The in-situ recombination process involves specific spatial assembly mechanisms and chemical neutralization reactions, the specific reactions and assembly processes of which are as follows: First, the modified zinc oxide particles have an organic molecular layer grafted with epoxy groups and ether bonds on their surface, while the surface of the free nanocellulose molecular chains suspended in the aged insulating oil is rich in a large number of natural hydroxyl groups. Under low-shear mixing conditions, the epoxy groups and ether bonds on the surface of the modified zinc oxide act as hydrogen acceptor functional groups and interact with the hydroxyl groups on the surface of the free cellulose, which act as hydrogen donors, to generate intermolecular hydrogen bonds.

[0009] Furthermore, under the influence of intermolecular hydrogen bonds, a three-dimensional hydrogen bond network is formed through self-assembly within the fluid. The modified zinc oxide nanoparticles are spatially anchored at the nodes of the free cellulose three-dimensional network structure. The interplay of steric hindrance and intermolecular hydrogen bonds restricts the free collisions of the modified zinc oxide particles within the insulating oil system, blocking the aggregation and sedimentation paths of the nanoparticles and maintaining the long-term dispersion stability of the in-situ composite insulating oil system. The free nanocellulose molecular chains are in-situ encapsulated and fixed by the modified zinc oxide, eliminating the physical conditions for the cellulose molecules to oriented and overlap in an electric field to form conductive channels.

[0010] Furthermore, the reactivity of the modified zinc oxide particle surface and the weakly alkaline properties of the zinc oxide core enable acid-base neutralization and chemisorption reactions with low-molecular-weight organic acids in insulating oil that cause an increase in acid value and a decrease in interfacial tension. The in-situ composite construction of free nanocellulose and nanoparticles consumes polar aging components and restores the electrical insulation strength and physicochemical properties of aged insulating oil.

[0011] Preferably, the high-concentration modified zinc oxide transparent mother liquor contains surface-modified zinc oxide nanoparticles dispersed in fresh insulating oil; the surface-modified zinc oxide nanoparticles are prepared by covalently grafting γ-glycidoxypropyltrimethoxysilane onto the surface of nano-zinc oxide powder after micro-aqueous induced hydrolysis. The mass concentration of modified zinc oxide in the high-concentration modified zinc oxide transparent mother liquor is 10%; the mass ratio of γ-glycidoxypropyltrimethoxysilane to nano-zinc oxide powder is 5% to 10%; and the particle size distribution of the nano-zinc oxide powder is 5 nm to 10 nm.

[0012] The preparation method of surface-modified zinc oxide nanoparticles includes: dispersing nano-zinc oxide powder in anhydrous ethanol, and sequentially performing magnetic stirring and ultrasonic dispersion to obtain a homogeneous zinc oxide dispersion; adding deionized water to γ-glycidoxypropyltrimethoxysilane, and adding an acid regulator to adjust the pH of the mixture to 5 to 6, so as to promote the complete hydrolysis of γ-glycidoxypropyltrimethoxysilane to generate silanol groups, and obtaining a slightly acidic aqueous silane mixture; uniformly adding the slightly acidic aqueous silane mixture to the homogeneous zinc oxide dispersion, and stirring under water bath heating and reflux conditions, so that the silanol groups generated by silane hydrolysis and the natural hydroxyl groups on the surface of nano-zinc oxide undergo a dehydration condensation reaction to complete the covalent grafting.

[0013] The acid regulator is glacial acetic acid or propionic acid; the dropwise flow rate of the slightly acidic aqueous silane mixture is 0.1 parts by volume / min; the water bath heating temperature is 80℃, and the stirring time under reflux conditions is 2 to 3 hours. After the covalent grafting reaction, the resulting suspension is centrifuged at 8000 r / min and the supernatant is removed. The precipitate is washed with anhydrous ethanol to remove unreacted free silane, and then dried in a vacuum drying oven at 70℃ to 80℃ for 5 to 6 hours to obtain surface-modified zinc oxide nanoparticles. The surface-modified zinc oxide nanoparticles are dispersed in new insulating oil and mechanically stirred at 500 to 600 r / min for 15 to 20 minutes, and then continuously dispersed under ultrasonic conditions for 1 to 1.5 hours until a high-concentration transparent mother liquor of modified zinc oxide without obvious particle suspension is formed.

[0014] By employing the above technical solution, the micro-water-induced hydrolysis step achieves covalent grafting on the surface of nanoparticles. The reaction mechanism is as follows: the introduction of deionized water and a suitable slightly acidic environment promote the hydrolysis of the methoxy groups of γ-glycidoxypropyltrimethoxysilane molecules to generate silanol groups. The silanol groups generated by hydrolysis further dehydrate and condense with the natural hydroxyl groups on the surface of the nano-zinc oxide particles, forming zinc-oxygen-silicon covalent bonds and silicon-oxygen-silicon crosslinking networks.

[0015] The coupling agent molecules chemically bond and adhere to the surface of the nanoparticles, reducing the surface energy of the zinc oxide nanoparticles. The hydrolysis and heating reaction conditions controlled the epoxy groups at the end of the coupling agent to prevent ring-opening reactions, thus preserving the hydrogen acceptor functional groups that would later form a hydrogen bond network with free cellulose in the insulating oil.

[0016] Centrifugal washing and vacuum drying processes remove unreacted free components and moisture, while mechanical stirring and ultrasonic dispersion treatment promote the formation of a high-concentration, transparent mother liquor without suspended particles in the new insulating oil matrix from modified zinc oxide nanoparticles.

[0017] Preferably, the flow rate of the high-concentration modified zinc oxide transparent mother liquor added to the insulating oil sample of the aged transformer is controlled at a constant 10 parts by volume / min. The specific implementation of low-shear fluid mixing and stirring is as follows: When the equipment is powered off and offline, a mechanical stirrer is used at a speed of 30 to 40 r / min to simulate the natural flow state of the insulating oil inside the transformer; or, without stopping the transformer's energized operation, the natural heat convection generated by the inherent oil temperature during normal transformer operation is utilized, combined with the fluid power of an external forced oil circulation pump to replace mechanical stirring, continuously injecting the high-concentration modified zinc oxide transparent mother liquor into the transformer's sealed oil tank system through an external forced oil circulation bypass system for in-situ compounding.

[0018] By adopting the above technical solutions, the system adapts to different operating conditions of transformers and provides corresponding in-situ synthesis methods. A low shear field can maintain the stability of the three-dimensional physical cross-linked structure formed by the self-assembly within the composite insulating oil. Excessive shear stress will cause physical damage to the hydrogen bond cross-linked structure, leading to a decrease in internal frictional resistance and the ionization of nanoparticles.

[0019] Mechanical stirring at a controlled speed or the use of natural thermal convection in conjunction with a circulating pump to provide fluid power promotes macroscopic mixing of the mother liquor and base oil, preventing the shear field from damaging the established hydrogen bond network. The forced oil circulation bypass injection method under energized operation allows for online repair of the insulating oil's physicochemical state without transformer shutdown or power outage, maintaining the transformer's continuous operating capability.

[0020] Preferably, the composite insulating oil obtained after in-situ synthesis meets the following characteristics: the free cellulose content is reduced to undetectable levels, the breakdown voltage is increased to 62.4kV to 65.1kV, the acid value is reduced to 0.032mgKOH / g to 0.038mgKOH / g, and the oil-water interfacial tension is restored to 41.2mN / m to 42.5mN / m.

[0021] By adopting the above technical solution, the in-situ encapsulation of free fiber chains by modified zinc oxide particles removes free cellulose in aged insulating oil. The consumption of polar aging components reduces the acid value of the oil and increases the interfacial tension, thereby achieving the direct restoration and improvement of the dielectric properties and physicochemical parameters of the insulating oil of waste aged transformers.

[0022] This invention provides a method for in-situ synthesis of free fiber-based nanoparticles from transformer oil. It has the following beneficial effects: 1. This invention involves adding a high-concentration, transparent mother liquor of modified zinc oxide to an aged transformer insulating oil sample for in-situ composite processing. The natural hydroxyl groups enriched on the surface of the free nanofibers in the insulating oil sample interact with the epoxy and ether bonds on the surface of the modified zinc oxide through intermolecular interactions, forming a three-dimensional hydrogen bond network within the system. This anchors the modified zinc oxide particles in the network structure nodes of the free fibers, restricting free collisions between particles, blocking the aggregation and sedimentation path of the nanoparticles, and maintaining the long-term dispersion stability of the insulating oil system.

[0023] 2. By setting low shear force fluid mixing and stirring conditions within a temperature range, this invention promotes macroscopic mixing of the mother liquor and the base oil while avoiding the destruction of the established hydrogen bond network by high shear stress. This causes the suspended free nanocellulose to be in situ encapsulated and fixed by modified zinc oxide particles, eliminating the conditions for free fibers to oriented and overlap to form conductive channels under the action of an electric field. This removes free cellulose from the insulating oil and increases the breakdown voltage of the insulating oil.

[0024] 3. This invention sets up an assembly and composite process of free nanofibers and modified zinc oxide within the insulating oil system. By utilizing the reactivity of the modified zinc oxide surface and the weak alkalinity of its core, it neutralizes and chemically adsorbs the low molecular weight organic acids in the aged insulating oil that cause physicochemical degradation. This directly consumes the polar aging components accumulated within the system, thereby reducing the acid value of the aged transformer insulating oil and restoring key physicochemical indicators such as oil-water interfacial tension. Attached Figure Description

[0025] Figure 1 The Fourier transform infrared spectra of the test samples of this invention show the wavenumber distribution of the main characteristic absorption peaks. Figure 2 The graph shows the apparent viscosity of each test oil sample system of the present invention as a function of shear rate. Figure 3 This is a comparison chart of the key electrical and physicochemical indicators of the various test oil samples of this invention; Figure 4 This is a comparison chart of the particle size and centrifugal absorbance retention rates of various test oil samples before and after aging, according to the present invention. Detailed Implementation

[0026] 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.

[0027] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a high-concentration modified zinc oxide transparent mother liquor, including the following steps: (1) Preparation of dispersion: 12 parts by weight of nano zinc oxide powder with a particle size distribution of 5 nm were dispersed in 500 parts by volume of anhydrous ethanol, and magnetically stirred for 45 min at room temperature. Then, the mixture was ultrasonically dispersed in an ultrasonic cleaner for 15 min to obtain a homogeneous zinc oxide dispersion. (2) Micro-aqueous induced silane hydrolysis: Weigh 0.6 parts by weight of γ-glycidoxypropyltrimethoxysilane as a coupling agent (the mass ratio of coupling agent to zinc oxide is 5%), add 0.14 parts by weight of deionized water to the coupling agent, and add an appropriate amount of glacial acetic acid to adjust the pH of the mixture to 5, so as to promote the silane coupling agent to fully hydrolyze and generate silanol groups; then add the above slightly acidic aqueous silane mixture to the homogeneous zinc oxide dispersion in step (1) at a flow rate of 0.1 parts by volume per minute. (3) Covalent grafting reaction: The mixture is placed in an 80°C water bath equipped with a reflux condenser and stirred for 2 hours to allow the silanol produced by silane hydrolysis to undergo a dehydration condensation reaction with the natural hydroxyl groups on the surface of nano zinc oxide, thus completing the covalent grafting of the coupling agent on the zinc oxide surface. (4) Washing and purification: After the reaction, the suspension was centrifuged at 8000 r / min for 10 min, the supernatant was discarded, and the precipitate was washed twice with anhydrous ethanol to remove unreacted free silane. Then it was dried in a vacuum drying oven at 70℃ for 5 h to obtain surface-modified zinc oxide nanoparticles. (5) Preparation of mother liquor: The modified zinc oxide nanoparticles obtained above are dispersed in 130 volumes of new insulating oil (so that the mass concentration of modified zinc oxide nanoparticles is about 10%). The mixture is mechanically stirred at 500 r / min for 15 min. Then the dispersion system is continuously dispersed under ultrasonic conditions for 1 h until a high-concentration transparent mother liquor of modified zinc oxide without obvious particle suspension is formed.

[0028] Preparation Example 2: This preparation example provides a method for preparing a high-concentration modified zinc oxide transparent mother liquor, including the following steps: (1) Preparation of dispersion: 16 parts by weight of nano zinc oxide powder with a particle size distribution of 7 nm were dispersed in 500 parts by volume of anhydrous ethanol, and magnetically stirred for 45 min at room temperature. Then, the mixture was ultrasonically dispersed in an ultrasonic cleaner for 18 min to obtain a homogeneous zinc oxide dispersion. (2) Micro-water-induced silane hydrolysis: Weigh 1.28 parts by weight of γ-glycidoxypropyltrimethoxysilane as a coupling agent (the mass ratio of coupling agent to zinc oxide is 8%), add 0.35 parts by weight of deionized water to the coupling agent, and add an appropriate amount of propionic acid to adjust the pH of the mixture to 5.5, so as to promote the silane coupling agent to fully hydrolyze and generate silanol groups; then add the slightly acidic aqueous silane mixture to the homogeneous zinc oxide dispersion in step (1) at a flow rate of 0.1 parts by volume per minute. (3) Covalent grafting reaction: The mixture is placed in an 80°C water bath equipped with a reflux condenser and stirred for 2.5 h to allow the silanol produced by silane hydrolysis to undergo a dehydration condensation reaction with the natural hydroxyl groups on the surface of nano zinc oxide, thus completing the covalent grafting of the coupling agent on the zinc oxide surface. (4) Washing and purification: After the reaction, the suspension was centrifuged at 8000 r / min for 12 min, the supernatant was discarded, and the precipitate was washed three times with anhydrous ethanol to remove unreacted free silane. Then it was dried in a vacuum drying oven at 75℃ for 5 h to obtain surface-modified zinc oxide nanoparticles. (5) Preparation of mother liquor: The modified zinc oxide nanoparticles obtained above are dispersed in 170 volumes of new insulating oil (so that the mass concentration of modified zinc oxide nanoparticles is about 10%). The mixture is mechanically stirred at 500 r / min for 20 min. Then the dispersion system is continuously dispersed under ultrasonic conditions for 1.5 h until a high-concentration transparent mother liquor of modified zinc oxide without obvious particle suspension is formed.

[0029] Preparation Example 3: This preparation example provides a method for preparing a high-concentration modified zinc oxide transparent mother liquor, including the following steps: (1) Preparation of dispersion: 20 parts by weight of nano zinc oxide powder with a particle size distribution of 10 nm were dispersed in 500 parts by volume of anhydrous ethanol, and magnetically stirred for 45 min at room temperature. Then, the powder was ultrasonically dispersed in an ultrasonic cleaner for 20 min to obtain a homogeneous zinc oxide dispersion. (2) Micro-water-induced silane hydrolysis: Weigh 2.0 parts by weight of γ-glycidoxypropyltrimethoxysilane as a coupling agent (the mass ratio of coupling agent to zinc oxide is 10%), add 0.6 parts by weight of deionized water to the coupling agent, and add an appropriate amount of glacial acetic acid to adjust the pH of the mixture to 6, so as to promote the silane coupling agent to fully hydrolyze and generate silanol groups; then add the slightly acidic aqueous silane mixture to the homogeneous zinc oxide dispersion in step (1) at a flow rate of 0.1 parts by volume per minute. (3) Covalent grafting reaction: The mixture is placed in an 80°C water bath equipped with a reflux condenser and stirred for 3 hours to allow the silanol produced by silane hydrolysis to undergo a dehydration condensation reaction with the natural hydroxyl groups on the surface of nano zinc oxide, thus completing the covalent grafting of the coupling agent on the zinc oxide surface. (4) Washing and purification: After the reaction, the suspension was centrifuged at 8000 r / min for 15 min, the supernatant was discarded, and the precipitate was washed 4 times with anhydrous ethanol to remove unreacted free silane. Then it was dried in a vacuum drying oven at 80℃ for 6 h to obtain surface-modified zinc oxide nanoparticles. (5) Preparation of mother liquor: The modified zinc oxide nanoparticles obtained above are dispersed in 220 volumes of new insulating oil (so that the mass concentration of modified zinc oxide nanoparticles is about 10%). The mixture is mechanically stirred at 600 r / min for 20 min. Then the dispersion system is continuously dispersed under ultrasonic conditions for 1.5 h until a high-concentration transparent mother liquor of modified zinc oxide without obvious particle suspension is formed.

[0030] Examples 1-3: Example 1: This embodiment provides a method for in-situ synthesis of free fiber-based nanoparticles from transformer oil, including the following steps: 12,000 volume portions of aged transformer insulating oil were extracted from the interior of a decommissioned 10kV transformer and used as the substrate for treatment. 130 volume parts of the high-concentration modified zinc oxide transparent mother liquor obtained in Preparation Example 1 were added to the above-mentioned aged transformer insulating oil sample at a flow rate of 10 volume parts per minute, and the volume ratio of the mother liquor to the aged transformer insulating oil sample was controlled to be 1:92. At a temperature of 40℃, the oil was mechanically stirred at a speed of 30r / min for 3 hours with low shear force to simulate the natural flow state of the insulating oil inside the transformer, thus completing the in-situ composite of free nanofibers and modified zinc oxide in the insulating oil sample of the aged transformer.

[0031] Example 2: This embodiment provides a method for in-situ synthesis of free fiber-based nanoparticles from transformer oil, including the following steps: 15,000 volume portions of aged transformer insulating oil were extracted from the interior of a 110kV main transformer in the remaining life assessment stage as the substrate to be treated. 170 volume parts of the high-concentration modified zinc oxide transparent mother liquor obtained in Preparation Example 2 were added to the above-mentioned aged transformer insulating oil sample at a flow rate of 10 volume parts per minute, and the volume ratio of the mother liquor to the aged transformer insulating oil sample was controlled to be 1:88. At a temperature of 50℃, the oil was mechanically stirred at a speed of 40 r / min for 4 hours with low shear force to simulate the natural flow state of the insulating oil inside the transformer, thus completing the in-situ composite of free nanofibers and modified zinc oxide in the insulating oil sample of the aged transformer.

[0032] Example 3: This embodiment provides a method for in-situ synthesis of free fiber-based nanoparticles from transformer oil, including the following steps: The closed oil tank system containing 20,000 volume fractions of aged transformer insulating oil from an actual 220kV UHV transformer was used as the substrate to be treated. Without stopping the transformer from operating under power, 220 volume parts of the high-concentration modified zinc oxide transparent mother liquor obtained in Preparation Example 3 were continuously injected into the transformer through a forced oil circulation bypass system outside the transformer at a constant flow rate of 10 volume parts per minute, and the total volume ratio of the mother liquor to the aging transformer insulating oil sample was controlled to be 1:91. By utilizing the natural heat convection generated by the inherent oil temperature of 60℃ during normal operation of the transformer, and in conjunction with the fluid power of an external forced oil circulation pump to replace mechanical stirring, the system was maintained in operation for 4 hours, and the assembly and composite of free nanofibers and modified zinc oxide were completed in situ within the transformer insulating oil system.

[0033] Comparative Examples 1-5: Comparative Example 1: Compared with Example 2, the difference is that no high-concentration modified zinc oxide transparent mother liquor was added for in-situ composite treatment, and the original aged transformer insulating oil sample was directly used. All other aspects are the same.

[0034] Comparative Example 2: Compared with Example 2, the difference is that a high-concentration modified zinc oxide transparent mother liquor was not added. Instead, an insulating filter membrane with a pore size of 0.5 micrometers was used to perform conventional physical circulation filtration on the aging transformer insulating oil sample. All other aspects are the same.

[0035] Comparative Example 3: Compared with Example 2, the difference is that γ-glycidoxypropyltrimethoxysilane was not used for hydrolysis grafting modification when preparing the mother liquor. Instead, equal weights of pure nano zinc oxide powder without any treatment were directly ultrasonically dispersed in the new insulating oil to prepare a mixture. All other aspects are the same.

[0036] Comparative Example 4: Compared with Example 2, the difference is that deionized water was not introduced when adding γ-glycidoxypropyltrimethoxysilane coupling agent during the preparation of the mother liquor, and an absolutely anhydrous environment was maintained throughout the process; all other aspects are the same.

[0037] Comparative Example 5: Compared with Example 2, the difference is that the substrate to be treated was replaced with a brand new transformer insulating oil sample that does not contain any free nanocellulose; otherwise, they are the same.

[0038] Test Examples 1-5: Test Example 1: Verification of the Chemical Bonding Mechanism of Surface Modification (1) Take unmodified nano zinc oxide powder, pure γ-glycidyl etheroxypropyltrimethoxysilane and the dried modified zinc oxide nanoparticles obtained in Preparation Examples 1 to 3 as test samples.

[0039] (2) For solid samples, the potassium bromide tableting method is used to prepare the sample. The solid test sample is mixed with dry potassium bromide powder at a mass ratio of 1:100. After being thoroughly ground in an agate mortar, it is pressed into a transparent sheet in a tablet press.

[0040] (3) For pure liquid silane coupling agent samples, the liquid film method is used to prepare the sample, and the liquid is evenly spread between two blank potassium bromide salt windows.

[0041] (4) Turn on the Fourier transform infrared spectrometer and preheat it, setting the scanning range to 4000 to 400 cm⁻¹. -1 The resolution is 4cm. -1 The number of scans was 32.

[0042] (5) First, use a blank potassium bromide thin film or a blank salt window to perform background scanning to subtract background absorption. Then, place each processed test sample in the optical path for scanning and testing, record the corresponding infrared transmission spectrum curves, and extract the wavenumber distribution data of the characteristic absorption peaks.

[0043] Table 1. Wavenumber distribution of main characteristic absorption peaks in Fourier transform infrared spectra of each sample. Conclusion: Combining Table 1 and Figure 1 Spectral data show that unmodified nano-zinc oxide has a wavelength of 435.2 cm⁻¹. -1 A Zn-O lattice stretching vibration peak was detected at 3442.8 cm⁻¹. -1 A surface hydroxyl stretching vibration peak is present at this location; the characteristic absorption peak of the pure coupling agent is mainly observed at 908.6 cm⁻¹. -1 The epoxy group peak at 1085.4 cm⁻¹ and the peak at 1085.4 cm� -1 The peak of the methoxy-related group at that location.

[0044] After grafting treatment, the modified zinc oxide samples from Preparation Examples 1, 2, and 3 all exhibited structural shifts and new absorption peaks in their spectra. The peaks for all three samples were at 910.8 cm⁻¹. -1 Up to 912.4cm -1The presence of characteristic peaks for epoxy groups within the specified range indicates that the coupling agent molecules have adhered to the surface of the nanoparticles, and that the hydrogen acceptor functional groups have not undergone ring-opening reactions. The prepared sample showed a peak at 1042.7 cm⁻¹. -1 Up to 1048.1cm -1 A new absorption band was added to the interval. This absorption band belongs to the vibrational region of the Zn-O-Si covalent bond and Si-O-Si crosslinking network formed by the dehydration condensation of hydrolyzed silanol groups and zinc oxide surface hydroxyl groups, relative to the pure product's 1085.4 cm⁻¹. -1 The newly added peak shows a red shift, indicating that silane molecules form chemical bonds with the zinc oxide surface.

[0045] Meanwhile, the Zn-O lattice vibration peak in each preparation example shifted to the right to 441.5 cm⁻¹. -1 Up to 445.6cm -1 The wavenumber of the surface hydroxyl stretching vibration peak shifts to a lower frequency to 3425.9 cm⁻¹. -1 up to 3431.2cm -1 The synchronous shift reflects the large consumption of surface hydroxyl groups and the change in the chemical environment of the nanoparticle surface caused by the formation of new covalent bonds. The above data confirm that the coupling agent molecules have completed covalent grafting modification on the nano zinc oxide surface, retaining active groups that can be used to form hydrogen bonds with free cellulose in the insulating oil of aging transformers.

[0046] Test Example 2: Verification of In-situ Hydrogen Bond Framework Assembly Mechanism (1) Take the in-situ composite insulating oil sample prepared in Example 2, the original aged transformer insulating oil sample of Comparative Example 1, and the non-free fiber transformer insulating composite oil sample of Comparative Example 5 respectively as the liquid matrix for rheological property testing.

[0047] (2) Start the rotary rheometer and install the coaxial cylindrical test fixture. Before testing, perform the instrument zero-point calibration and inertia calibration procedures.

[0048] (3) Inject each liquid matrix into the test barrel of the rheometer, set the ambient temperature of the test system to 50°C, and after the system reaches the preset temperature, let the liquid matrix stand in the barrel for 5 minutes to eliminate the flow shear history applied during the sampling and injection process.

[0049] (4) Establish a steady-state shear test program in the operating software, and set the scanning range of the shear rate to 0.1s. -1 up to 100s -1 Data points were collected using a logarithmic incremental mode. The instrument automatically recorded the apparent viscosity values ​​of the liquid matrix at each shear rate. Each group of samples was tested in three parallel tests and the average value was extracted.

[0050] Table 2. Apparent viscosity of each test oil sample system at different shear rates Conclusion: Combining Table 2 and... Figure 2 The test data shows that in 0.1s -1 up to 100s -1 Within the shear rate range, the apparent viscosity of Comparative Example 1 remained between 7.15 mPa·s and 7.26 mPa·s, while that of Comparative Example 5 remained between 8.55 mPa·s and 8.67 mPa·s. The apparent viscosity of both samples remained essentially constant under different shear fields, exhibiting typical Newtonian fluid characteristics. The overall viscosity of Comparative Example 5 was higher than that of Comparative Example 1 because rigid modified nanoparticles were dispersed within the system of Comparative Example 5, leading to an increase in the viscosity of the basic fluid. However, the hydrodynamic characteristics remained unchanged, indicating that the modified nanoparticles alone were discretely distributed in the fiber-free insulating oil and failed to construct an internal spatial structure.

[0051] Example 2 at 0.1s -1 The apparent viscosity under low shear rate conditions is 34.82 mPa·s, and increases with shear rate up to 100 s⁻¹. -1 In Example 2, the apparent viscosity continuously decreased to 8.94 mPa·s, exhibiting the characteristics of a pseudoplastic non-Newtonian fluid with shear thinning. The aged insulating oil matrix used in Example 2 contained free nanocellulose, and the surface hydroxyl groups interacted with the epoxy and ether bonds on the modified zinc oxide surface, forming a three-dimensional hydrogen bond network within the fluid. Under low shear conditions, the relative slippage of the base oil molecules was restricted, resulting in a higher macroscopic viscosity. Under high shear conditions, the physically cross-linked structure underwent stress failure, the internal frictional resistance of the system decreased, and the apparent viscosity returned to the characteristic value of the base fluid, confirming that spatial anchoring assembly was completed between the free cellulose and modified zinc oxide particles in the aged oil.

[0052] Test Example 3: Recovery Test of Key Electrical and Physicochemical Indicators (1) Take 500 mL of each of the in-situ composite insulating oil samples prepared in Examples 1 to 3, the original aged transformer insulating oil sample of Comparative Example 1, and the insulating oil sample of Comparative Example 2 after physical filtration. Place all oil samples in a constant temperature sealed glass container at 25°C and let stand for 24 hours to remove tiny air bubbles mixed in during the sampling process.

[0053] (2) The breakdown voltage of the oil sample was determined by a fully automatic insulating oil dielectric strength tester. The electrode spacing was set to 2.5 mm and the voltage rise rate was 2 kV / s. Each oil sample was subjected to 6 breakdown tests in a row, with a 5-minute interval between each test. The test results were recorded and the arithmetic mean was calculated.

[0054] (3) The acid value of the oil sample was determined by using an automatic acid value analyzer combined with neutralization titration. A specified mass of oil sample was taken, and a mixed extraction solvent of isopropanol and toluene was added. The sample was heated and shaken to fully extract the acidic components in the oil into the solvent phase. The sample was then titrated with a 0.02 mol / L potassium hydroxide standard solution. The volume of potassium hydroxide consumed was recorded and converted into the acid value.

[0055] (4) The interfacial tension of oil and water was determined by the ring method using a fully automatic interfacial tensiometer. Under constant temperature of 25℃, the platinum ring was pulled horizontally out of the oil-water interface, and the maximum tensile force at the moment of liquid film rupture was recorded.

[0056] (5) The free cellulose content was quantitatively determined by the microporous membrane filtration method. 100 mL of oil sample was measured and vacuum filtered through a pre-weighed microporous membrane with a pore size of 0.22 μm. The membrane was washed multiple times with petroleum ether to remove residual oil. The membrane was then dried in a vacuum drying oven at 80 °C until constant weight. The mass concentration of suspended free cellulose in the oil sample was calculated by the differential method.

[0057] Table 3. Test data of key electrical and physicochemical properties of each oil sample Conclusion: Combining Table 3 and... Figure 3 The data shows that the breakdown voltage of the original aged transformer insulating oil sample in Comparative Example 1 was 28.3 kV, the acid value was 0.125 mg KOH / g, the interfacial tension was 18.4 mN / m, and the free cellulose content was 14.8 mg / L. The test values ​​indicate that polar aging products accumulated inside the aged insulating oil. Nanoscale cellulose is prone to forming conductive channels under the action of an electric field, leading to insulation failure. Comparative Example 2 was filtered using a filter membrane with a pore size of 0.5 micrometers. After treatment, the free cellulose content of the insulating oil sample decreased to 3.5 mg / L, and the breakdown voltage rose back to 41.2 kV. The physical interception method removed some large-scale fiber particles, but it could not filter out dissolved acidic components and nanoscale cellulose with a size smaller than the pore size of the filter membrane. Therefore, the acid value (0.118 mg KOH / g) and interfacial tension (21.6 mN / m) of the insulating oil sample in Comparative Example 2 did not improve.

[0058] After treatment with modified zinc oxide transparent mother liquor in Examples 1 to 3, the free cellulose content in the insulating oil samples all dropped to zero, the breakdown voltage of the insulating oil samples increased to between 62.4kV and 65.1kV, the acid value decreased to between 0.032mgKOH / g and 0.038mgKOH / g, and the interfacial tension recovered to above 41mN / m. The disappearance of the free cellulose test value proves that the free cellulose molecular chains in the aged insulating oil are in situ wrapped by the coupling agent groups on the surface of modified zinc oxide through a hydrogen bond network, eliminating the hidden danger of cellulose overlapping in the electric field.

[0059] The high reactivity of the modified zinc oxide particles' surface and the weak alkalinity of its core neutralize and adsorb low molecular weight organic acids in the insulating oil, consuming the polar components that cause the oil's acid value to increase and the interfacial tension to decrease. The changes in test data indicators prove that the in-situ composite construction of waste fiber skeleton and modified zinc oxide particles has achieved the restoration of the electrical strength and physicochemical state of the insulating oil.

[0060] Test Example 4: Long-term anti-agglomeration and dispersion stability test of insulating oil system (1) Take 100 mL of each of the insulating oil samples from Example 2, Comparative Example 3, Comparative Example 4 and Comparative Example 5 and inject them into a clean glass reagent bottle for later use.

[0061] (2) The initial Z-average hydrodynamic particle size of the insulating oil samples was determined using a dynamic light scattering instrument. The test temperature was set to 25℃, and each group of insulating oil samples was measured three times consecutively and the arithmetic mean was recorded.

[0062] (3) The insulating oil sample that has completed the initial test is placed in a constant temperature aging test chamber at 90°C and subjected to a thermal aging cycle of 168h. After the aging cycle is completed, the insulating oil sample is taken out and cooled to 25°C. The Z-average average hydrodynamic particle size of the aged insulating oil sample is measured again using a dynamic light scattering instrument.

[0063] (4) Take 20 mL of each heat-aged insulating oil sample and transfer them into centrifuge tubes. Place the centrifuge tubes into a benchtop high-speed centrifuge, set the speed to 4000 r / min, and the centrifugation time to 30 min.

[0064] (5) After the centrifugation process is completed, use a pipette to extract the supernatant of the centrifuge tube. Use a UV-Vis spectrophotometer to test the absorbance of the supernatant at a wavelength of 600 nm. Compare the test value with the absorbance value of the aged insulating oil sample before centrifugation and calculate the absorbance retention rate of the supernatant of each insulating oil sample.

[0065] Table 4. Test data on particle size and centrifugal absorbance retention rate of each oil sample before and after aging. Conclusion: Combining Table 4 and... Figure 4The data shows that in Comparative Example 3, the initial average hydrodynamic particle size of unmodified pure nano-zinc oxide dispersed in insulating oil reached 145.2 nm. After thermal aging cycles, the particle size increased to 684.5 nm, and the absorbance retention rate of the supernatant decreased to 12.4%. The test values ​​indicate that the untreated pure zinc oxide particles have high surface energy and are prone to agglomeration and sedimentation in the insulating oil matrix. In Comparative Example 4, no deionized water was added during the preparation process, and the silane coupling agent failed to undergo hydrolysis and grafting. After thermal aging, the particle size of the insulating oil sample increased to 412.3 nm, and the absorbance retention rate was 35.7%. The comparative data proves that the micro-water-induced hydrolysis step is a necessary prerequisite for achieving covalent grafting of nanoparticles onto the surface.

[0066] Comparative Example 5 used brand-new transformer insulating oil without free fibers as the liquid matrix. The initial average hydrodynamic particle size of the insulating oil sample was 81.5 nm. After thermal aging, the particle size increased to 215.6 nm, and the absorbance retention rate decreased to 64.1%. The test results show that the surface grafting modification process reduced the surface energy of the nanoparticles. However, in the insulating oil system lacking free nanocellulose as a spatial anchoring skeleton, the nanoparticles will still collide and agglomerate under thermal stress.

[0067] In Example 2, the initial average hydrodynamic particle size of the insulating oil sample was 85.4 nm. After the same thermal aging cycle, the particle size only changed to 88.7 nm. The absorbance retention rate of the supernatant under high-speed centrifugation conditions reached 96.2%. The hydroxyl groups enriched on the surface of the free nanofibers inside the insulating oil sample formed intermolecular hydrogen bonds with the epoxy and ether bonds on the surface of the modified zinc oxide, anchoring the modified zinc oxide nanoparticles to the nodes of the cellulose three-dimensional network structure. The steric hindrance and intermolecular hydrogen bonds worked together to restrict the free collision of the modified zinc oxide particles in the insulating oil system. The difference in test data indicators confirmed the synergistic anchoring effect of the surface covalent grafting process and the original aged fiber skeleton inside the insulating oil, which blocked the aggregation and sedimentation path of the nanoparticles and maintained the long-term dispersion stability of the in-situ composite insulating oil system.

Claims

1. A method for in-situ synthesis of free fiber-based nanoparticles from transformer oil, characterized in that, Includes the following steps: An aging transformer insulating oil sample was extracted from inside the transformer and used as the substrate for treatment. The high-concentration modified zinc oxide transparent mother liquor was continuously and uniformly added to the aging transformer insulating oil sample, and the volume ratio of the high-concentration modified zinc oxide transparent mother liquor to the aging transformer insulating oil sample was controlled to be 1:88 to 1:

92. The in-situ composite of free nanofibers and modified zinc oxide in the aging transformer insulating oil sample was completed by mixing and stirring the fluid under low shear force at a temperature of 40℃ to 60℃ for 3 to 4 hours.

2. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 1, characterized in that, The high-concentration modified zinc oxide transparent mother liquor contains surface-modified zinc oxide nanoparticles dispersed in the new insulating oil. The surface-modified zinc oxide nanoparticles are prepared by covalently grafting γ-glycidoxypropyltrimethoxysilane onto the surface of nano zinc oxide powder after micro-water-induced hydrolysis.

3. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 2, characterized in that, The high-concentration modified zinc oxide transparent mother liquor contains 10% modified zinc oxide by mass. The mass ratio of the γ-glycidoxypropyltrimethoxysilane to the nano zinc oxide powder is 5% to 10%; the particle size distribution of the nano zinc oxide powder is 5 nm to 10 nm.

4. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 2, characterized in that, The preparation method of the surface-modified zinc oxide nanoparticles includes: The nano zinc oxide powder was dispersed in anhydrous ethanol and then subjected to magnetic stirring and ultrasonic dispersion in sequence to obtain a homogeneous zinc oxide dispersion. Deionized water was added to the γ-glycidoxypropyltrimethoxysilane, and an acid regulator was added dropwise to adjust the pH of the mixture to 5 to 6, so as to promote the complete hydrolysis of the γ-glycidoxypropyltrimethoxysilane to generate silanol groups, thereby obtaining a slightly acidic aqueous silane mixture. The slightly acidic aqueous silane mixture is added dropwise at a uniform rate to the homogeneous zinc oxide dispersion. The mixture is stirred under water bath heating and reflux conditions to allow the silanol groups generated by silane hydrolysis to undergo a dehydration condensation reaction with the natural hydroxyl groups on the surface of the nano zinc oxide, thus completing the covalent grafting.

5. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 4, characterized in that, The acid regulator is glacial acetic acid or propionic acid; The dripping rate of the slightly acidic aqueous silane mixture is 0.1 parts by volume / min; The water bath heating temperature is 80℃, and the stirring time under the condensation reflux condition is 2h to 3h.

6. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 4, characterized in that, After the covalent grafting reaction was completed, the resulting suspension was centrifuged at 8000 r / min and the supernatant was removed. The precipitate was washed with anhydrous ethanol to remove unreacted free silanes, and then dried in a vacuum drying oven at 70℃ to 80℃ for 5 to 6 hours to obtain the surface-modified zinc oxide nanoparticles. The surface-modified zinc oxide nanoparticles are dispersed in the new insulating oil and mechanically stirred at a speed of 500 r / min to 600 r / min for 15 min to 20 min, and then continuously dispersed under ultrasonic conditions for 1 h to 1.5 h, until a high-concentration transparent mother liquor of modified zinc oxide without obvious particle suspension is formed.

7. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 1, characterized in that, The flow rate of the high-concentration modified zinc oxide transparent mother liquor added to the aging transformer insulating oil sample was controlled at a constant 10 parts by volume / min.

8. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 1, characterized in that, The specific implementation method of the low shear force fluid mixing and stirring is as follows: when the equipment is powered off and offline, a mechanical stirrer is used to mechanically stir at a speed of 30r / min to 40r / min to simulate the natural flow state of the insulating oil inside the transformer.

9. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 1, characterized in that, The specific implementation method of the low shear force fluid mixing and stirring is as follows: without stopping the transformer from running under power, the natural heat convection generated by the inherent oil temperature during normal operation of the transformer is used, and the fluid power of the external forced oil circulation pump is used to replace mechanical stirring. The high concentration modified zinc oxide transparent mother liquor is continuously injected into the transformer's closed oil tank system through the forced oil circulation bypass system outside the transformer for in-situ compounding.

10. The method for in-situ synthesis of free fiber-based nanoparticles in transformer oil according to claim 6, characterized in that, The step of washing the precipitate with anhydrous ethanol specifically involves washing the precipitate with anhydrous ethanol 2 to 4 times to remove unreacted free silane.

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