Electrical Characteristics of the Insulation Paper Modified by Nano-TiO2 under Electrical-Thermal Aging

The research on the modified insulating paper by adding nano-materials has garnered increasing attentions. Scholars have investigated its performances under electrical aging or thermal aging, but there are few reports on its behaviors under electrical-thermal combined aging. In the laboratory, we prepared the modified insulating paper with different mass fractions of nano-TiO2, and subjected to 10-day accelerated electrical-thermal combined aging test. Samples were experimented at different aging stages to investigate the variations in AC breakdown field strength, relative permittivity and surface microstructure. The results demonstrate that the modified insulating paper samples exhibit better AC breakdown field strength, relative permittivity and other parameters compared to the unmodified samples. Among the modified samples, the one with 3% mass fraction of nano-TiO2 shows the best electrical properties and aging resistance under electrical-thermal combined aging. By adding an appropriate weight percentage of nano-TiO2, it was able to fill microscopic holes and gaps on the surface of the insulating paper, strengthen the connection between the cellulose fibers, and improve the electrical properties and aging resistance of the insulating paper.


Introduction
Oil-immersed power transformers are crucial for power transmission and ensuring their long-term safe and stable operation is vital for the power industry [1].However, the insulating paper within these transformers can be deteriorated due to various factors, ultimately affecting their performance [2].Therefore, study methods to enhance the properties of insulation paper for power systems is highly significant.
In recent years, there have been significant interest in improving the electrical properties of insulating paper by incorporating nano-materials.Researchers at Tianjin University, led by Kong Xiaoxiao, added cationic nano-crystalline primary fibrillated cellulose (CNFC) to insulating paper and observed the breakdown improvement performance and thermal aging resistance [3].Similarly, a team from Chongqing University, led by Liao Ruijin, studied the effects of nano-Al2O3 and nano-TiO2 doping on the electrical properties of insulating paper, finding improved electrical characteristics compared to unmodified paper [4].Chen Qingguo and colleagues modified insulating paperboard by incorporating nano-SiC, which resulted in more uniform electric field distribution within the oil paper insulation structure [5].These studies demonstrate the improved electrical properties achieved through the use of nano-materials.Currently, most research focuses on the single-factor impact of thermal aging or electrical aging on modified insulating paper with limited attention given to the combined effects of multiple factors.
In this paper, insulating paper was modified with nano-TiO2 and developed a specialized electricalthermal aging test setup.It also examined how the electrical properties of the modified paper changed during combined electrical and thermal aging.The study primarily investigated the electrical characteristics of nano-TiO2 modified insulating paper during electrical-thermal aging.

Sample Preparation
The testing used ordinary cellulose cardboard and 10 nm of nano-TiO2 particles.After vacuum drying the cellulose cardboard and nano-TiO2 at 130℃ for 24 hours, the cellulose cardboard was pulped for 15 minutes at 3000 r/min.Different mass fractions of nano-TiO2 were added to the cellulose pulp and stirred for 5 minutes at 800 r/min.cellulose paperboard samples were prepared with a thickness of 1 mm and mass fractions of 0%, 1%, 3%, 5%, and 7% using a paper leaf former.These samples were labeled as P0, P1, P2, P3, and P4, respectively.

Electrical-thermal Combined Aging Test
The accelerated electrical-thermal combined aging test setup for the testing as shown in figure 1.The samples were dried in a vacuum oven at 85°C for 48 hours to remove moisture.The 25# transformer oil was filtered and dried to eliminate impurities and moisture.To simulate the electrical aging environment in a transformer, a plate-plate structure was used with a diameter of 75 mm smooth surface brass pole plates for upper and lower electrodes.The accelerated electrical aging voltage was set to 5 kV (powerfrequency), and the entire setup was immersed in 25# transformer oil.The oil tank was placed inside a modified aging chamber at a temperature of 130°C for accelerated thermal aging.electrical-thermal combined aging tests were conducted on five groups of samples with different mass fractions.Each group underwent for 10-day accelerated electrical-thermal combined aging test.Samples were collected on the 0th, 1st, 3rd, 5th, and 10th days of the tests.

Performance Tests
In order to explore the effect of different mass fractions of TiO2 on modified insulating paper, a series of tests were carried out.In the electrical field, AC breakdown field strength tests and measurement of relative permittivity were conducted on the samples at different aging stages.In the physical field, Surface microstructure observation was proceeded.

The Effect of Different Mass Fractions on the AC Breakdown Field Strength
The power-frequency AC breakdown field strengths of TiO2 nano-modified insulating paper samples with different mass fractions at different electro-thermal combined aging stages are shown in figure 2. In figure 2, the breakdown field strength of the five samples decreases gradually from the 1st day to the 3rd day of aging, with an average of 1.2879 kV/mm.However, from the 5th day to the 10th day of aging, the breakdown field strengths of the samples decrease significantly, with an average of 10.0375 kV/mm.This indicates that intermolecular fracture between the fiber molecules in the insulating paper occurs slowly in the first three days.As the electrical-thermal combined aging time increases, temperature factors intensify the thermal degradation of cellulose, resulting in main chain breakage and removal of side groups from the main chain [6].Electric field factors also cause surface charges on the cellulose to aggregate, leading to the destruction of the cellulose structure in the insulating paper [7].The combination of these factors intensifies the cracking process during the later stage of combined electrical-thermal aging, resulting in a steep decline in the AC breakdown field strength.
The power-frequency AC breakdown field strengths of insulating papers modified with different mass fractions of nano-TiO2 are consistently higher than that of the unmodified paper P0 throughout the aging cycle.Among the modified samples, P2 exhibits the highest AC breakdown field strength, with an increase of 13.42% compared to P0.This increase is attributed to fill the pores by nano-TiO2 particles, which improves the trap density on the paper surface and ensures a more uniform electric field distribution.These factors collectively contribute to the improvement of the AC breakdown field strength [8].
Increasing the mass fraction of nano-TiO2 from 1% to 3% leads to an increase in the AC breakdown field strength of the modified insulating paper.However, when the mass fraction exceeds 3%, the AC breakdown field strength gradually decreases.At lower nano-TiO2 content, the nanoparticles are evenly dispersed throughout the insulating paper.But as the nano-TiO2 content exceeds 3%, the nanoparticles tend to agglomerate, forming larger micron-sized particles.This aggregation affects the hydrogen bonds between the paper fibers and results in a decrease in the AC breakdown field strength.

The Effect of Different Mass Fractions on Relative Dielectric Constant
Figure 3 shows the relative permittivity of modified insulating paper samples with the different mass fractions of nano-TiO2 at different electrical-thermal combined aging stages.In figure 3, the relative permittivity gradually increases during aging.However, the unmodified insulating paper has high relative permittivity than the modified paper with different mass fractions of nano-TiO2.This suggests that adding nano-TiO2 can reduce the relative permittivity of the insulating paper.Among the modified samples, P2 show a slower increase in relative permittivity and better dielectric properties compared to other samples.This improvement is due to the strong interaction between nano-TiO2 and cellulose, which restricts cellulose chain mobility, suppresses the polarization of cellulose hydroxyl groups, and lowers the relative permittivity of the modified insulating paper [9].

The Effect of Different Mass Fractions on Surface Microstructure
Since the results of the electrical performance test of the modified insulating paper, P2 were better than the other samples, the surface microstructures of the samples of P0, P2 without aging and after electricalthermal combined aging for 10 days were compared respectively.Figure 4 shows that the surface of unmodified insulating paper P0 is relatively rough, whereas the surface of modified paper P2 is comparatively flat.Figure 5 displays the surface microstructures of P0 and P2 for 10 days of electro-thermal aging.P0 exhibits significant surface fiber fracture, resulting in numerous holes and severe damage to the cellulose structure.In contrast, P2 maintains a relatively flat surface and better overall fiber structure retention.By figure 4 and figure 5 indicating that the addition of nano-TiO2 fills in surface holes and gaps has improved the overall flatness, improves the aging resistance of the insulating paper In nano-TiO2 modified insulating paper, the surface of nano-TiO2 can undergo hydrolysis with water molecules, generating hydroxyl groups.The cellulose surface also contains hydroxyl groups.The interaction between the hydroxyl groups of nano-TiO2 and cellulose forms hydrogen bonds (as shown in figure 6), which helps the preservation of the fiber structure during aging.This improves the aging resistance of the modified insulating paper.

Conclusions
In this paper, the electrical characteristics and the aging resistance of the nano-TiO2 modified insulating paper were analyzed.The main conclusions are as follows: (1) The addition of nano-TiO2 in appropriate mass fraction improves the electrical properties of insulating paper, and the electrical properties and aging resistance of nano-TiO2 modified insulating paper with 3% mass fraction are the best.
(2) Adding nano-TiO2 to insulating paper improves its electrical properties and aging resistance by enhancing surface flatness and creating strong bonding between TiO2 hydroxyl groups (formed through hydrolysis) and cellulose, resulting in a more robust cellulose structure.
(3) When the mass fraction of nano-TiO2 exceeded 3%, the electrical properties of the modified insulating paper started to gradually decline.This is because the nanoparticles tend to agglomerate at higher concentrations, disrupting the hydrogen bonds between the fibers of the insulating paper and resulting in the deterioration of its electrical properties.

Figure 2 .
Figure 2. Power-frequency AC breakdown field strength at different aging stages.

Figure 3 .
Figure 3. Relative dielectric constant for different aging stages.

Figure 6 .
Figure 6.The process of hydrogen bonds.