At present, the high-voltage transmission system is developing in the direction of larger capacity and higher voltage, and the safe and stable operation of the high-voltage transmission system has become particularly important. Power transformer is one of the key equipment of transmission system, and the insulation performance of transformer is also facing unprecedented challenges. At present, most power transformers are oil-immersed transformers, and their insulation systems are composed of insulating oil and cellulose insulating paper. Among them, cellulose insulating paper is prone to partial discharge, which makes it easy to break down and difficult to replace, making it a weak link in oil-paper insulation. Therefore, it is of great practical significance to improve the insulation performance of insulating paper.

At present, the main method to improve the insulation performance of insulating paper is to add inorganic nanoparticles to insulating paper, such as SiO2, TiO2, Al2O3 and so on. Due to its nano-scale quantum effect and large specific surface area, nanoparticles can adsorb carriers in insulating materials to reduce their energy, thereby improving the insulation performance of insulating materials. Therefore, it is widely used in the modification of insulating materials. However, nanoparticles have high specific surface area and surface activity, and are prone to agglomeration in the polymer matrix. Nanoparticles at the agglomeration are prone to accumulate charge and cause local breakdown. At present, researchers mainly improve the dispersion of nanoparticles on the surface of matrix materials by surface modification, thus inhibiting the agglomeration of nanoparticles. Silane coupling agent is the most commonly used surface modifier. One end of the silane coupling agent can be combined with the hydroxyl group on the surface of the nanoparticles by condensation reaction, and the amino group on the other end can be combined with the hydroxyl group on the surface of the insulating paper by hydrogen bonding force to improve the dispersion of the nanoparticles on the surface of the insulating paper. However, the surface hydroxyl content of inorganic nanoparticles is low, and the modification effect of silane coupling agent is not ideal, and the activity of silane coupling agent will decrease with time. Therefore, it is necessary to hydroxylate the inorganic nanoparticles to improve the binding rate with the silane coupling agent and solve the agglomeration problem of the nanoparticles.

Fig. 1 The action principle of silane coupling agent

Plasma treatment

Low temperature plasma treatment has been proved to produce etching, grafting, crosslinking and other effects on the surface of the material, thus endowing the material with new properties. Among them, the grafting of plasma can graft the corresponding functional groups on the surface of the material according to the application requirements, so it is widely used in the field of material surface modification.

The effect of plasma treatment on the surface element composition of nano-SiO2 particles was analyzed

XPS test can characterize the elemental composition and proportion of the compound. The results of XPS spectrum analysis of nano-SiO2 particles before and after plasma modification are shown in table 1. The surface of nano-SiO2 particles mainly contains four elements : Si, C, N and O. The content of O element on the surface of nano-SiO2 particles increases after plasma modification. The C1s peak fitting curve is shown in Fig.2. The proportion of C − C bonds and O − C = O bonds on the surface of nano-SiO2 particles decreased, and the proportion of C − O bonds ( i.e., C − OH bonds ) increased, especially in the humid air environment. Plasma modification can further increase the hydroxyl content on the surface of nano-SiO2 particles. This is due to the impact of a large number of high-energy particles produced by the plasma, which breaks the C − C bond and C = O to produce free radicals, and then combines with the hydroxyl radicals ( · OH ) produced by the decomposition of water molecules to form C − OH bonds.

Fig.1 C1s peak fitting curve of plasma modified nano-SiO2 particles

Analysis of the effect of plasma treatment on the surface chemical composition of nano-SiO2

Fourier transform infrared spectroscopy was used to analyze the functional groups on the surface of nano-SiO2 particles before and after plasma modification. The results are shown in Fig.3. For pure nano-SiO2 particles, there is a corresponding 1 ∶ 2 relationship between Si and O, while the corresponding relationship between Si and O on the surface of SiO2 particles cannot be fully satisfied. During the preparation process, nano-SiO2 will adsorb water in the environment to form a certain number of hydroxyl groups, so the unmodified nano-SiO2 particles will also have a weak hydroxyl characteristic peak. Compared with the unmodified nano-SiO2 particles, the hydroxyl characteristic peak at 3440cm − 1 of the plasma-modified nano-SiO2 particles in dry air is slightly enhanced. This may be due to the inevitable existence of a small amount of water vapor in the reactor, resulting in the decomposition of water molecules under the action of plasma to produce hydroxyl radicals and grafted on the surface of nano-SiO2 particles, so that the intensity of the hydroxyl characteristic peak is slightly enhanced. Compared with the plasma modification in dry air, the hydroxyl characteristic peak intensity of nano-SiO2 particles after plasma modification in wet air is further enhanced, which is consistent with the characterization results of XPS. This is mainly due to the collision of high-energy particles produced by plasma with water molecules to produce hydroxyl radicals, which are grafted onto the surface of nano-SiO2 particles. The increase of hydroxyl content on the surface of nano-SiO2 particles is beneficial to improve the grafting rate with silane coupling agent, so as to improve the dispersion of nano-SiO2 particles on the surface of the matrix material.

Fig.3 Fourier transform infrared spectroscopy of nano-SiO2 particles before and after plasma modification in different atmospheres