Abstract
To study the effect of nano-SiO2 doping content on the properties of chopped fiber composite meta-nano-aramid insulating paper (IP) for transformer bushings, the laboratory-prepared chopped fiber composite meta-nano-aramid IP with different nano-SiO2 doping content is analyzed to assess its impact on the environment. The study has been carried out by considering crystallographic properties and morphology of chopped fiber composite meta-nano-aramid fiber, the influence-mechanism of nano-SiO2 doping content on the mechanical properties, the thermal stability and insulating properties of aramid paper. The experimental results show that the appropriate graft modification of nano-SiO2 enhances the thermal stability and mechanical properties of the meta-aramid (MA) insulating paper. It also shows lesser carbon emissions. When the results are compared with SiO2 without grafted silane coupling agent, SiO2 with a surface graft ratio of 6%, the former method (SiO2 without grafted silane coupling agent) shows a greater increase in the glass transition temperature of the MAIP. At the same time, it shows weakened molecular weight in terms of MSD. The proposed method shows more prominence in terms of the chain kinematic ability, the resistance to deformation, and shearing in terms of mechanical properties. When SiO2 with the graft ratio of 12% is observed, it does not show any improvement, and also the effects on thermal stability and mechanical properties of IP are not significant which is similar to grafting. After hot pressing at 245°C, 30 MPa for 5 min, the breakdown strength, and volume resistivity can reach as high as 37 kV/mm and 1.34 × 1015 Ω·cm respectively. The proposed method shows a significant environmental impact in terms of less emission of CO and CO2.
1. Introduction
1.1. Background Study
After long hours of operation at high-temperature, large-scale power transformers are prone to the problems such as material aging and insulation damage. This results in the deterioration of its health status, which seriously affects the safe and reliable operation of the entire power system. With the increasing complexity of the power grid system and the gradual improvement of the voltage level, the reliability requirements for power equipment such as transformers are also becoming higher. Transformers are the core equipment for the safe operation of power grids. Serious transformer failures will affect the reliability of the power supply of the power system. The economic cost of replacing transformers is high. It is very important to ensure the safe operation of the UHV power grid. The internal insulation of the transformer that is the composite insulation system is composed of insulating oil and IP. This is an important cause of transformer failure. The oil-paper insulation in the converter transformer is prone to the injection and accumulation of space charges under the action of the high-voltage DC electric field. The accumulation, migration, and transportation of space charges in the oil-paper insulation medium will cause the electric field inside the medium to be distorted [1–3]. When the electric field strength of the local distortion exceeds the strength of the insulating medium, it will cause damage to the insulating material. This process further triggers the phenomenon of partial discharge, thereby accelerating the aging of the material. In the case of a sudden change of voltage polarity of the converter transformer, it causes insulation damage leading to accidents. When transformers operate long term, their electrical and mechanical properties tend to deteriorate due to the action of moisture, high temperature, and electric field. The oil-paper insulating medium will age with the operation of the transformer, resulting in the decline of electrical and mechanical properties. The aged transformer oil can be removed by filtering or changing the oil to restore the insulation performance. The insulation paper cannot be replaced during the operation of the transformer, so the aging state of the insulation paper determines the aging life of the transformer. Reliable assessment of the insulation aging state in the transformer helps to discover potential insulation defects and fault risks in time and that in turn ensures the reliable operation of the transformer. MA fiber (poly-phenylene isophthalamide), abbreviated as MPIA, insulation paper has good electrical insulation, thermal stability, and corrosion resistance. It has become one of the main insulating materials for large transformers and has a wide range of applications in high-tech fields such as aerospace, transportation power, national defense and military, high-temperature insulating materials, and high-performance electronic equipment. Meta-aramid insulation paper (MAIP) is often used inside electronic devices, and its mechanical properties and insulation properties show direct impacts on the normal operation of the equipment. With the rapid development of electronic equipment in the direction of high power, integration, and miniaturization, the requirements for the insulation performance of MAIP are also getting higher. Therefore, improving the electrical insulating properties is of great significance to the development and application of MAIP.
The insulation material of the transformer shows the following properties:(i)Very high resistance or specific resistance.(ii)High dielectric strength.(iii)Low relative permittivity.(iv)Low dielectric loss.
These properties tend to degrade when the transformer ages.
1.2. Literature Review
To identify the solution to the above problem, we have carried out an extensive literature review.
The literature we have referred to is as follows:
In [4], the authors propose a composite model of surface-modified SiO2 doped PMIA and study the effect of SiO2 on PMIA with surface grafting rates of 6% and 12%, respectively. They also study the effects of the glass transition temperature, thermal stability, and mechanical properties. In [5], the authors propose a method of applying nano-SiO2 particle-modified aramid insulation paper to DC power transmission and transformation equipment to improve the insulation life of the oil-paper insulation system. The space charge characteristics of thermally stimulated depolarization current, DC breakdown strength, and volume resistivity of aramid IP samples with different nano-SiO2 contents are tested under pressure and decompression short-circuit conditions. In [6], the authors enlist the insulating constituents used in the transformer such as paper, and transformer board. These are made from cellulose. The major problem in using dry paper is that it is very hygroscopic. This disturbs the voltage distribution in heterogeneous dielectric systems and results in a dielectric loss. In [7], the authors state that the degradation of paper in the transformers results in the emission of the products such as CO and CO2. They mention that the drawback with this approach is that it requires a shutdown of the transformer. Considering the scenario, the authors study the insulation used in the power transformer. In [8], the authors carry out the test in the laboratory, using oil-saturated insulating system models. They assessed the changes in features of insulating cellulose materials. The authors find a good correlation between the quantity of gas generated from IPs in insulating oil and the retaining of tensile strength and the degree of polymerization. They infer that the degree of degradation of IPs in transformers can be gauged by the quantity of gas. In [9], the authors analyze the internal mechanism of nano-modified meta-aramid insulation paper (MAIP). The outcome shows that the k-polyphenylsilsesquioxane modified MAIP was superior to b-PPSQ modified MAIP when the thermal stability, mechanical, and electrical properties are considered. In [10], the authors study the models of the crystalline and amorphous regions of MA fibers. The model of the crystalline area was compared with the experimental data. They analyzed the variation of temperature, density, energy, and size of the cell related to the dynamic time. The experiment reveals that crystalline regions show a higher modulus value than those in the amorphous regions. The incompressibility, plasticity, and toughness of the crystalline regions are higher than that of amorphous regions, whereas the toughness of the amorphous regions is more.
Although there are many proposed methods for the insulation of the transformers, in this research, we focus on the properties of chopped fiber composite meta-nano-aramid IP and analyze the performance of chopped fiber composite meta-nano-aramid IP for transformer bushings when the mass content of nano-SiO2 is 1wt%, 6%, and 12wt% respectively. Our work proves to emit less CO and CO2 as compared to existing methods.
1.3. Contribution of the Paper
(i)Explanation of raw materials and preparation method of chopped fiber composite meta-nano-aramid IP.(ii)Describing the production method.(iii)Analyzing the effect of chopped fiber composite meta-nano-aramid IP nanoparticles.(iv)Studying the Characterization of chopped fiber composite meta-nano-aramid
1.4. Organization of the Paper
The flow of the research is as follows:
Section 1: the background details of the research study, review of the related work, and the contribution of our research.
Section 2: explanation of raw materials and preparation method of chopped fiber composite meta-nano-aramid IP, describing the production method.
Section 3: the experimental results and the corresponding analysis have been elaborated.
Section 4: the conclusion and scope for the future work of the paper are discussed.
2. Raw Materials and Preparation Method of Chopped Fiber Composite meta-Nano-aramid IP
2.1. Raw Materials
The MA chopped fibers and MA pulp selected in the experiment are all poly-meta-phenylene isophthalamide fibers (meta-structure), which are provided by a chemical fiber company in China. Among them, the average length of the MA staple fiber is 6 mm, the average linear density is 1.75 × 10−4 g/m, the average length of the MA pulp is 0.5∼3 mm, and the beating degree is 40°SR.
2.2. Test Equipment
We used NicoletiS50 Fourier transform infrared spectroscopy (FTIR) instrument, Flex1000 scanning electron microscope (SEM), GeminiSEM500 field emission scanning electron microscope, SDTQ600 thermogravimetric analysis (THG) instrument, AT-XW standard fiber dissociator, IMT-CP0916 paper forming machine Tablet machine, GJJ-1/25 homogenizer, PCH-600DG flat plate hot press, TL5502 B AC withstand voltage tester, TH2683 A/B insulation resistance tester, and D8DISCOVER X-ray diffraction (XRD) instrument.
2.3. Production Method
The production flow chart of MAIP is shown in Figure 1.
The fiber dissociator used in the laboratory is the ZQS4 type fiber dissociator. The pattern maker is the ZQJ1-B-II pattern extractor (sheet former), and the high-temperature calendering equipment is the NYG300 high-temperature three-roll calendar. First, the MA chopped fibers and precipitating fibers are pretreated and then put into a fiber dissociator, An appropriate amount of dispersant is added for high-speed stirring so that the two fibers are fully dispersed and mixed evenly stirring speed is 3000 r/min. The dissociated pulp is put into a paper maker, It is shaped on a circular screen under reduced pressure. It is then pressed and dried to obtain a handmade sheet of IP. Finally, the IP handsheets are placed on a high-temperature calendar to be hot pressed. The hot-pressing parameters are: temperature 220°C and pressure 200 N/mm. The hot pressing is carried out twice [6–8].
The mixed slurry was shaped with a paper sampler and hot pressed to obtain a dry sheet. Finally, the dry sheet was calendered with a calendar to obtain an insulating sheet having a diameter of 20 cm and a thickness of 0.075 mm to analyze the impact of nanoparticle content on the properties of MAIP. The mass content of nano-SiO2 in the experimental MAIP was 1wt%, 6%, and 12wt%, respectively.
3. The Effect of Chopped Fiber Composite meta-Nano-aramid IP Nanoparticles
Nanoparticles (ultrafine particles) are particles having a radius between 1 nm and 100 nm which are atomic or molecular groups consisting of a very small number of atoms/molecules. Nanoparticles are distinguished from macroscopic objects with the fact that their area makes a large proportion, while surface atoms are amorphous layers with neither long-range nor short-order. High Gilibs pressure is generated because of the large surface area and small radius. This may lead to the deformation of the internal structure.
This structural feature of nanoparticles makes it have five effects. These effects are the small size, surface, quantum size, macroscopic quantum tunneling effect, and dielectric confinement effect [9–11]. The nano-effect of nanoparticles makes the properties of the material itself different from general materials in terms of optics, heat, mechanics, and magnetism, so it plays an important role in the fields of industry, medicine, electronics, and environmental protection. In terms of modified polymers, nanoparticles have also been deeply researched and widely used, and have achieved excellent results [12].
The actual strength of polymers is only 1% to 1/1000th of the theoretical strength. How to strengthen and toughen polymers and tap the potential of mechanical properties of polymers has always been a hot topic of research by polymer scholars all over the world. Filling modification is one of the significant methods. Nanoparticles have a large specific surface area, many surface-active atoms, and strong interaction with polymers. Filling nanoparticles into polymers can significantly improve the rigidity, toughness, strength, wear resistance, and other mechanical properties of polymer composites. It can be seen that it is feasible to add nanoparticles to polymer materials to prepare composite materials. The addition of nanoparticles will not reduce the performance of polymer materials but can increase the performance of polymers in some aspects. Among them, nano-SiO2, as a commonly used modified nanoparticle, plays a vital role in the research of polymer modification and has shown great application prospects in high-voltage and UHV power transmission and transformation equipment and lines [13–15].
4. Results and Discussion
4.1. Characterization of Chopped Fiber Composite meta-Nano-aramid
To characterize the crystallinity and thermal stability of the raw materials for papermaking, X-ray diffraction (XRD) and thermogravimetric (THG) tests were performed on the chopped fiber composite meta-nano-aramid. In order to characterize the crystalline state of the chopped fiber composite meta-nano-aramid fibers, the samples were tested by XRD.
PANalytical Empyrean X-ray diffractometer is used as the measuring instrument. The test parameters are Cu target, Kα ray, wavelength 1.5406 × 10−10m, tube voltage 40 kV, current 40 mA, scanning-speed 4°/min, step size 0.02°, and diffraction angle range 5°∼50°. In order to observe the fiber dispersion and bonding on the surface and cross section of the aramid IP, a part of the sample was made brittle in liquid nitrogen. The cross section and the surface of the non-brittle sample were sprayed with gold. A field emission scanning electron microscope (SEM, model FEInova400nanoSEM) was used to observe the microscopic morphology of the surface and sample’s cross section.
The following figure shows the chopped fiber composite meta-nano-aramid fiber prepared by the solution jet method. During the spinning process, the solvent volatilized well, the fiber surface was smooth and flat, the thickness was uniform, and the average fiber diameter was 226nm. The chopped fiber composite meta-nano-aramid fiber prepared by the solution jet method is shown in Figure 2.
The XRD pattern of the chopped fiber composite meta-nano-aramid fiber is shown in Figure 3.
Using the XRD test results, the crystallinity of the chopped fiber composite meta-nano-aramid fiber was obtained by the Jade software using the peak splitting method, as shown in Table 1.
The structure of chopped fiber composite meta-nano-aramid contains an amorphous region and crystalline region structure. The crystallinity itself is very low. From Figure 3, it is visible that the XRD pattern of the chopped fiber composite meta-nano-aramid fiber has sharper characteristic peaks at 2θ of 17.1°, 22.1°, 26.1°, etc., and the maximum intensity of the diffraction peak appears at 26.1°.
From Figure 3 and Table 1, it can be seen that the chopped fiber composite meta-nano-aramid is ordered in some segments, and the crystallinity is 14.15. Crystallization makes the polymer chains regularly arranged and tightly packed, thus enhancing the force between the molecular chains. The lower the fiber crystallinity, the lower the modulus of the fiber, the softer the fiber appears, and the weaker the fiber itself is. The softer the chopped fiber composite meta-nano-aramid, the greater the possibility of fiber flocculation. Therefore, the higher the crystallinity of the chopped fiber composite meta-nano-aramid fiber, the better the stiffness, which is more conducive to improving the dispersibility of the fiber in water and improving the evenness of the aramid IP.
4.2. Characterization of the Surface of Chopped Fiber Composite meta-Nano-aramid IP
The SEM images of the chopped fiber composite meta-nano-aramid IP with a fiber ratio of 1:0 and 1:2 are shown in Figures 4, 4(a) and 4(b) respectively.
(a)
(b)
Pulp is in a finely fibrous state, curled, branched, film-like, or ribbon-like, randomly entangled together. In the aramid paper made from pulp alone, the pulp is closely connected, the holes are fewer, and the surface of the aramid paper is smoother. In the aramid paper with chopped fibers added, the fibers are ribbon shaped with a diameter of about 10 μm, and are randomly distributed in the pulp, some fibers have flocculation, and the connection between the fibers and the pulp is not tight enough solid, resulting in more holes around the fibers.
4.3. Properties of Chopped Fiber Composite meta-Nano-aramid IP
4.3.1. Mechanical Properties
Hooke's law can be used to express the relationship between strain and stress in solid materials, as shown in (1).
In (1), is the 6×6 elastic stiffness coefficient matrix. In theory, all mechanical properties of materials can be derived from . The study found that the elastic strain of the vast majority of materials is symmetric, that is, in the matrix. For isotropic materials within the error range, the elastic stiffness matrix can be simplified as shown in (2).
In (2), and are the Lamé constants, which can be calculated from (3) and (4) as follows:
The Elastic Modulus E (will be rereferred as EM) and shear modulus G (will be referred to as SM) of the material mechanical parameters can be obtained from (5) and (6) respectively. The EM (E) is also known as Young's modulus. The larger the value, the stronger the deformation resistance of the material; the SM (G), also known as the stiffness modulus or, is the ratio of shear stress to strain.
The EM and SM data of four different nanometer proportions of chopped fiber composite meta-nano-aramid IP at the transformer working temperature of 343 K are shown in Table 2.
It can be seen from Table 2 that, the EM and SM of chopped fiber composite meta-nano-aramid fiber are increased by 30% and 45%, respectively, as compared to pure MA fibers; 6% SiO2-chopped fiber. The EM and SM of the composite meta-nano-aramid fiber are increased by about 66% and 61%, respectively. The EM and SM of the 12% SiO2-chopped fiber composite meta-nano-aramid fiber increased by 36% or higher. This is close to the improvement effect of chopped fiber composite m-aramid fiber. The improvement effect of SM is not as good as that of pure m-aramid fiber doped with nano-SiO2. It can be seen that the doping of nano-SiO2 can enhance the rigidity of pure MA fiber material. Appropriate graft modification of the SiO2 surface can make this enhancement effect more prominent, but excessive graft modification of the SiO2 surface will worsen the situation.
The mechanical properties of modified IP can be improved by the nanoparticles. On the one hand, the nanoparticles can act as a lubricant to improve the tensile properties and flexibility when the aramid fiber is exposed to mechanical tension. On the other hand, when the modified IP is exposed to shear stress, partial force is borne or transmitted by the nanoparticles, sharing the force of the aramid fiber. At the same time, because of the Van Der Waals force and hydrogen bond between the nanoparticles and the aramid fiber’s tensile property shows improvement. Therefore, while improving the tensile properties and flexibility of IP, nanoparticles can still show an improvement in the mechanical properties of materials such as rigidity and incompressibility.
4.3.2. Thermal Stability
(1) Determination of glass transition temperature. The chopped fiber composite meta-nano-aramid IP is in a glass state when the temperature is lower than its glass transition temperature (TG) and is in a high elastic state when the temperature is more than TG. In the high elastic state, the inter-molecular chain hinders become smaller, so the overall movement of the molecular chain is more likely to occur. The violent movement of the molecular chain worsens the thermal stability of the IP. The most commonly used method to determine the TG is the volume-temperature curve method. Dynamic simulation: extract the density data from the trajectory file and take the average value, the reciprocal of which is the specific volume; then take the temperature from 350 to 750 K as the abscissa to make a scatter plot of temperature and specific volume, and then determine the TG according to the experimental value. The approximate range is used to fit the data points. The abscissa of the intersection of the fitting line between the low-temperature section and the high-temperature section is TG, as shown in Figures 5, 5(a), 5(b), 5(c), and 5(d).
(a)
(b)
(c)
(d)
Figure 5(a) shows that the simulated TG value of pure MA fibers in this study is 549 K, which is about 3 K lower than the experimental value. The results of the simulation study show good reliability. The value is closer to the experimental value and more closely matches the real material. Figure 5 shows that the TG of chopped fiber composite meta-nano-aramid is 19 K higher than that of pure MA fiber, and the TG of 6% SiO2-chopped fiber composite meta-nano-aramid is higher than that of chopped fiber. The composite meta-nano-aramid fiber increased by 11 K, indicating that nano-SiO2 doping and certain modification and grafting of SiO2 can improve the thermal properties of the composite material. However, the TG of 12% SiO2-chopped fiber composite meta-nano-aramid did not show further improvement but was 10 K lower than that of 6% SiO2-chopped fiber composite meta-nano-aramid.
(2) Mean square displacement (MSD). The MSD is one of the important indicators to describe the overall motion of the molecular chain. It can directly reflect the thermal motion of the molecular chain. The more intense the motion of the molecular chain, the higher the MSD value, and the thermal stability of the matrix Sex is also worse. The calculation expression of the MSD is shown in (7):
In (7): represents the position of molecules or atoms at time ; represents the position of molecules or atoms at time 0; the angle brackets represent the average value.
The MSD of the MA molecular chain is shown in Figure 6, 6(a), 6(b), 6(c), 6(d), and 6(e).
(a)
(b)
(c)
(d)
(e)
It can be seen from Figure 6(a) that the molecular chain motion strength of pure MA fibers at the working temperature is the largest, followed by 12% SiO2-chopped fiber composite meta-nano-aramid and chopped fiber composite meta-nano-aramid. The molecular chain of 6% SiO2-chopped fiber composite meta-nano-aramid has the weakest molecular chain movement strength. Figure 6(b)–6(e) shows that with the increase in temperature, the trend of molecular chain motion is enhanced, but it does not show a strong temperature dependence; this is because of the temperature simulated in this study. The range does not yet have a large impact on its physicochemical properties. From 6(b)–6(e), it can also be found that the distance between MSD-temperature curves at adjacent temperatures in the low-temperature region is not large, but with the increase in temperature, there will be different degrees of jumps, which is due to the temperature. The increase of α helps to overcome the interaction force between molecular chains so that the degree of freedom of molecular chain movement is increased, and the overall movement is intensified. The temperature of the first transition of MSD of pure MA fiber is 363 K, while the temperature of the first transition of MSD of chopped fiber composite meta-nano-aramid and 6% SiO2-chopped fiber composite meta-nano-aramid is 383 K and 403 K respectively. The MSD corresponding to 12%SiO2-chopped fiber composite meta-nano-aramid at 343 K has a rapid rise during the simulation process. This abnormal phenomenon may have occurred because the PMIA molecular chain interacts with the surrounding environment during the dynamic simulation process. The molecular chain does not form a strong interaction, making it easier to overcome this energy barrier when it enters the energy peak, but it can be seen from Figure 6(e) that the MSD of 12% SiO2-chopped fiber composite meta-nano-aramid one jump temperature is 363 K or 383 K. The above phenomenon shows that the doping of nano-SiO2 can inhibit the phenomenon of molecular chain movement jump caused by temperature rise to a certain extent, improve the thermal stability of MAIP, and properly graft the surface of SiO2. The effect of improving thermal stability is better. If the surface grafting rate is too high, the effect of improving thermal stability is not significant.
4.3.3. Insulation
The breakdown strength of aramid nanofiber IPs with different hot-pressing pressures and different ratios was measured. The breakdown strength of chopped fiber composite meta-nano-aramid IP is shown in Figure 7.
The volume specific resistance of chopped fiber composite meta-nano-aramid IP is shown in Figure 8.
Figure 7 shows that, with the increase of the content of para-aramid nanofibers, the breakdown strength of the aramid nanofiber IP shows an increasing trend first and then decreasing. Among them, when the hot-pressing pressure is 30 MPa and the mass fraction of para-aramid nanofibers reaches 50%, the breakdown strength of the aramid nanofiber IP can be as high as 37 kV/mm. At the same time, we also measured the volume resistivity of aramid nanofiber IPs with different hot-pressing pressures and different ratios.
Figure 8 shows that as the content of para-aramid nanofibers increases, the volume resistivity of the aramid nanofiber IP also shows a trend of increase first and then decrease. Among them, when the hot-pressing pressure is 30 MPa and the mass fraction of para-aramid nanofibers reaches 50%; the volume-specific resistance can be as high as 1.38×1015Ω·cm. Compared with the MA fiber prepared by spinning, the crystalline properties of para-aramid nanofibers are better, and their insulating properties are also better. Therefore, with the increase of the content of para-aramid nanofibers, the insulating properties of IP are improved. However, further increasing the content of para-aramid fiber and reducing the MA fiber that plays a role in bonding are not conducive to enhancing the compactness of the IP through the subsequent hot-pressing process. The defects and physical properties of the composite film can be improved by the hot-pressing process, the structure of the composite film is more uniform, and the structure of the whole system is more compact, thereby reducing the free volume. This effectively weakens the movement of the molecular chain and improves the composite film.
5. Conclusion
The TG MSD, EM, SM, and insulation properties of chopped fiber composite meta-nano-aramid IP for transformer bushings are analyzed. After the comparative analysis, the following conclusions can be drawn: (1) SiO2 nano-doping can improve the thermal stability and mechanical properties of MAIP, and proper grafting of silane coupling agent on the surface of SiO2 can further enhance the improvement effect. (2) In terms of improving the thermal properties of aramid IP, the improvement effect when the graft ratio of SiO2 surface is 12% is not more significant than that when the graft ratio is 6%, but it is close to the improvement effect when no grafting is carried out. Even the shear resistance is weaker than the lifting effect without grafting. (3) Appropriate grafting on the surface of SiO2 can further reduce the free volume of the system and make the molecules more tightly combined, but when the grafting rate is higher, the free volume of the system increases, and the molecules are relatively loose. The space available for molecular chain movement is larger, which is an important reason why it is inferior to the former in terms of thermal stability and mechanical properties. (4) After hot pressing at 245°C, 30 MPa, and 5 min, the breakdown strength and volume resistivity can be as high as 37 kV/mm and 1.34×1015Ω·cm, respectively. [16–20].
6. Future Scope
The above work has been carried out in a simulated laboratory environment. In the future, our proposed work will be tested in a long-term real environment with the actual transformer.
Data Availability
The data can be obtained from the corresponding author on request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.