Doping nanoparticles in insulating paper can improve the insulating properties of insulating paper by limiting their charge carriers. However, the high specific surface area and surface activity of nanoparticles are prone to "agglomeration", resulting in partial discharge and breakdown at the accumulation of nanoparticles. In order to effectively inhibit the agglomeration of nanoparticles and further improve the insulation characteristics of insulating paper, Jiang Nan, Li Zhiyang, Peng Bangfa, Li Jie, and Wu Yan from the School of Electrical Engineering of Dalian University of Technology published a report in the 24th issue of Transactions of China Electrotechnical Society in 2023. In the above paper, the dielectric barrier discharge (DBD) plasma was used to hydroxylation the nano-SiO2 particles, the hydroxyl number on the surface of the nano-particles was increased, and the coupling effect with the silane coupling agent molecules through hydrogen bonding was carried out to bridge the insulating paper matrix, which improved the compatibility between the nanoparticles and the insulating paper, thereby improving their agglomeration, and the cellulose insulating paper doped with nano-SiO2 particles was prepared by in-situ polymerization.
Background:
The high-voltage transmission system is developing in the direction of large capacity and high voltage, and higher requirements are put forward for the insulation performance of the transformer. The internal insulation of the transformer is composed of insulating oil and cellulose insulating paper, in which the insulating paper is prone to thermal aging and electrical aging, and is difficult to replace, which has become a weak link in the insulation system. Improving the performance of insulating paper is the key to the continuous and safe operation of the transmission system.
At present, the main method to improve the dielectric properties of insulating paper is to add inorganic nanoparticles to the insulating paper, and the nanoparticles are prone to agglomeration in the polymer matrix and cause local breakdown. To this end, our team proposed to use atmospheric dielectric barrier discharge (DBD) plasma to hydroxylation nano SiO2 particles to improve their binding rate with silane coupling agents, solve the problem of agglomeration of nanoparticles, and apply it to improve the pressure resistance of insulating paper.
Outline of Research
In this paper, a dual-dielectric barrier discharge reactor driven by high-frequency AC power supply at atmospheric pressure is established, and the nano-SiO2 particle plasma hydroxylation modification is modified, and then modified with silane coupling agent KH550, and the experimental setup is shown in Figure 1. On this basis, FTIR, XPS, SEM and other methods were used to characterize and analyze the nano-SiO2 particles before and after plasma modification.
Fig.1 Experimental setup
Scanning electron microscopy (SEM) was used to characterize the microscopic morphology of the surface of the insulating paper doped with SiO2 particles before and after plasma modification. It was found that the modified nano-SiO2 particles in the humid air environment had the smallest particle size and the best dispersion (Fig. 2).
Fig.2. SEM image of nano-SiO2 particles on the surface of insulating paper
Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), specific surface and porosity analyzer (BET) combined with titration were used to analyze the changes of surface chemical composition and hydroxyl number of nano-SiO2 particles before and after modification, and it was found that the relative humidity was 75% Under air conditions, a large number of hydroxyl groups were grafted on the surface of nano-SiO2 particles after plasma modification, and the active sites available for binding of the coupling agent increased, which strengthened the effect of the coupling agent, and the dispersion of nano-SiO2 particles on the surface of the insulating paper was significantly improved (Fig. 3 and Fig. 4).
Figure 3 (a) XPS characterization of unmodified (b) humidified air plasma-modified nano-SiO2
Fig. 4(a) Fourier transform infrared spectroscopy of nano-SiO2 before and after modification and (b) Effect of plasma treatment time on the hydroxyl number on the surface of SiO2
The influence of hydroxylated nano-SiO2 doping on the insulation properties and mechanical properties of insulating paper was explored. The results show that the breakdown field strength, volume resistivity and partial discharge initial voltage of the insulating paper increase first and then decrease with the increase of the doping content of nano-SiO2 particles, reaching the maximum value when the mass fraction of SiO2 particles is 3%, which is increased by 52.3%, 106.2% and 24.1% respectively compared with that of undoped nano-SiO2 particles.
This is due to the fact that the nano-SiO2 modified by plasmonic hydroxylation is more evenly dispersed on the surface of the insulating paper, and the charged particles in the insulating paper preferentially hit the nano-SiO2, which weakens its own energy, thereby limiting the migration of charged particles and improving the insulation performance of the insulating paper.
However, when the doping amount of nano-SiO2 is too high, the SiO2 particles are easy to agglomerate and form a conductive path due to the high surface activity, and the agglomeration of nano-SiO2 particles leads to the shallowness of the interface trap of the insulating paper, and the charged particles are more likely to fall off and sink on the surface of the insulating paper, and the limiting effect of the nano-SiO2 particles on the charged particles is weakened, resulting in the deterioration of insulation performance.
The tensile strength of insulating paper also increased first and then decreased with the increase of nano-SiO2 particle content. When the mass fraction of nano-SiO2 was 3%, the tensile strength of the insulating paper reached the maximum value (2.34 kN/m), which was 103.5% and 35.4% higher than that of the undoped and unmodified nano-SiO2 insulating paper, respectively. When the doping amount of nano-SiO2 increases, the number of hydrogen bonds between nano-SiO2 and the insulating paper increases, the restraining effect of nano-SiO2 on the deformation of the insulating paper is enhanced, and the mechanical properties of the insulating paper are improved.
However, when the doping amount of nano-SiO2 is too high, the nano-SiO2 particles are easy to agglomerate, which causes uneven stress on the insulating paper on the one hand, and a large number of defects in the interface area between the nano-SiO2 and the insulating paper on the other hand, thereby reducing the mechanical strength of the insulating paper.
Fig.5 Effect of nano-SiO2 doping amount on the electrical and mechanical properties of insulating paper under different modified atmospheres
conclusion
In this paper, an atmospheric pressure dual-dielectric barrier discharge reactor was established to characterize and analyze the surface functional groups, elemental component proportions, and dispersion properties of SiO2 nanoparticles before and after plasma modification. The results show that when the mass fraction of nano-SiO2 is 3% and the relative humidity of the air is 75%, the breakdown field strength and volume resistivity of nano-SiO2 are increased by 52.3% and 106.2%, respectively, when the nano-SiO2 is treated with plasma and then silane coupling agent, respectively.
The results of this work were published in the 24th issue of Transactions of China Electrotechnical Society in 2023, with the title of "Effect of Plasma Hydroxylation Modified Nano-SiO2 Particles on the Insulation Characteristics of Insulating Paper". This project is supported by the National Natural Science Foundation of China.