Natural ester is an environmentally friendly liquid dielectric with high fire point, which is mainly used in power equipment such as transformer. Dissolved gas analysis (DGA) is one of the most effective ways for transformer fault diagnosis. At present, there are some research results on the monitoring of dissolved gas in natural ester insulating oil. In this paper, the research progresses of the dissolved gas in natural ester insulating oil were introduced generally, and the key problems that should be paid attention to in the future application of natural ester insulating oil were pointed out.

In order to improve the mechanical strength and weather resistance in special environment of direct-drive wind generator insulation system, the partial model of direct-drive wind generator stator was impregnated by epoxy modified unsaturated polyesterimide impregnating resin. At first, the performance of the resin and the initial performance of the partial model were tested. Then the partial model was subjected to 12 cycles of water immersion and cold-thermal shock tests, and the performance changes before and after the test were analyzed. The results show that the resin has low odor, low curing volatile content, and excellent mechanical properties, insulating properties, and storage stability. The initial performance of the partial model is excellent. The insulation resistance of the model coil at 150℃ and after water immersion increases at first and then decreases during test process, while the dielectric loss factor at 150℃ increases gradually. After test, the insulating properties of the model coil can meet the requirements of direct-drive wind generator.

ZnO varistors doped by Al₂O₃ were prepared under different sintering temperatures, and their microstructure and electrical characteristics were studied by scanning electron microscope, X-ray diffraction, current-voltage test, and capacitance-voltage test. The results show that with the increase of sintering temperature, the leakage current of the ZnO varistor doped by Al₂O₃ was suppressed significantly, this is due to the continuous increase of donor degree and interface state density increases the barrier height of grain boundary. With the increase of sintering temperature, Al³⁺ will continuously dissolve into the ZnO grains, the grain resistivity would reduce, and the residual voltage ratio of the ZnO varistor when a large current is passed decreases. When the sintering temperature is 1150℃, the electrical properties of the ZnO varistor are the best, the voltage gradient is 418.7 V/mm, the leakage current is 0.74, the residual voltage ratio is 1.68, and the nonlinear coefficient is 67.5, which is contribute to improve the protective performance of ZnO arrester and limit the overvoltage of the power system, especially the UHV system deeply.

The correlation between methanol content and polymerization degree of insulating paper was studied through the detection and analysis of ageing characteristic product methanol of insulating paper, and the characteristic parameter of methanol content in oil, which can characterize the ageing state of insulating paper, was put forward. The results show that there is a good linear correlation between the polymerization degree logarithm of insulating paper and the methanol content, and the linear correlations are highly similar under different experimental conditions, which can be expressed as lnDP=aC+b. The characteristic values of methanol content in oil proposed for transformers with different voltage levels can judge the ageing state of insulating paper effectively, and thus the operation status of transformer can be diagnosed and evaluated.

In the application scenarios of natural ester insulating oil, the technology of using natural ester insulating oil to replace mineral insulating oil to prolong the service life of in-service old transformers has attracted extensive attention at home and abroad. During the oil replacing process, the residual mineral insulating oil in transformer will mix with the newly added natural ester insulating oil inevitably. In this study, the natural ester was mixed with mineral insulating oil, and its volume fraction was 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, respectively. The properties of the mixed oils were studied by accelerated thermal ageing tests at 100℃ for 168 h. The results show that with the increase of natural ester content, the degradation range of breakdown voltage of the aged oil decreases, and the effect of water content on the breakdown voltage of the aged oil decreases, while the acid value increases. When the volume fraction of the natural ester exceeds 30%, the dielectric loss factor of the aged oil is lower than that of the oil before ageing. When the volume fraction of mineral oil is 10%, the ignition point of the mixed oil is only 68.5% of that of natural ester.

The electric multiple unit roof insulators with different operating years were selected as research objects, and the hydrophobic analysis (contact angle method), Fourier infrared analysis, and scanning electron microscope analysis were conducted on them, respectively. The results show that with the increase of service time, the hydrophobicity of the insulator decreases rapidly, while the decreasing trend slows down year by year, and the hydrophobicity of the top umbrella skirt is generally lower than that of the middle and bottom umbrella skirts, which is consistent with the change trend of Si-CH₃ content in insulator analyzed by FTIR. With the increase of service time, the surface morphology of the insulator changes significantly. The micromorphology of the electric multiple unit roof insulator serviced for 6 years is similar to that of the insulator serviced in 110 kV transmission grid for 15‒19 years, which is one of the important reasons that the hydrophobic ageing speed of electric multiple unit insulator is faster than that of transmission grid insulators.

In this paper, the withstand voltage characteristics of cross-linked polyethylene (XLPE) and its nanocomposites at different temperatures was systematically measured by step voltage rising method. A new method for calculating the key parameters of the Crine model was proposed, and the activation energy and charge acceleration distance of XLPE and its nanocomposites at different temperatures were calculated on the basis of the experimental results of step voltage rising method. The results show that the characteristic breakdown time of both the materials decreases with the increase of temperature. At the same temperature and time step, the characteristic breakdown voltage and breakdown time of XLPE nanocomposites are higher than that of XLPE, and the electrical ageing life of XLPE nanocomposites is longer than that of XLPE under high voltage. With the increase of temperature, the ageing activation energy and charge acceleration distance of the two materials increase. Under the same temperature, both the ageing activation energy and charge acceleration distance of XLPE nanocomposite are smaller than that of XLPE. It is more intuitive to reflect the ageing resistance of insulating materials by activation energy and charge acceleration distance on basis of Crine model than the ageing life index on basis of the inverse power model or exponential model.

A high resistance insulating layer for edge protection of carbon-ceramic linear resistors was prepared using SiO₂-Al₂O₃-B₂O₃ ternary glass as matrix and mullite as filler. The adaptability of expansion coefficient between high resistance insulating layer and resistance ceramic was studied, and the voltage shock resistance was tested using impulse voltage of 1.2/50 μs. The results show that the glass content affects the expansion coefficient, porosity, and electric strength of the high resistance insulating layer. When the glass content in the high resistance insulating layer is more than 80%, the electric strength of the resistance shows a significant downward trend. With the increase of the coating thickness of the high resistance insulating layer, the electric strength of the carbon-ceramic linear resistance increases obviously. When the thickness of the high resistance insulating layer is controlled at about 0.35 mm, the electric strength of the resistance reaches the maximum value. Using SF₆ gas in place of air can increase the resistance significantly. In the SF gas environment of 0.2 MPa, under the protection of high resistance insulating layer, the withstand flashover voltage of the resistance reaches 22.5 kV, which is 80 % higher than that without protection.

Insulating paper is an important insulating material that produced by papermaking technology and widely used in electrical equipment such as transformers, capacitors, motors, wires and cables. According to its composition, insulating paper can be divided into cellulose paper and non-cellulose paper. With the sustainable development of China’s power industry, motors and other electric equipment are gradually getting more efficient and smaller. Therefore, higher property requirements are put forward to insulating paper, especially high-temperature resistant performance. Although cellulose papers have a long history, their heat resistance is not high enough to meet the operating requirements of electrical insulation systems, which need the thermal class of F or above. Non-cellulose papers with higher heat resistance have sprung up. This article briefly introduces the development history, performance characteristics, main uses, production status, and development trends of various high-temperature resistant non-cellulose insulating papers in China.

In order to accelerate the dissipation of the surface charge and improve the flashover voltage of insulator, we propose the plasma fluorination modification technology. Changing the surface modification time, the surface physical, chemical, and dielectric properties of epoxy resin sample with the same formula as insulator was test before and after modification. The results show that as a method of both physical and chemical surface modification, plasma modification can introduce hydrophilic groups to the surface of the sample and change its wettability. With the increase of modification time, the surface roughness of the sample increases at first and then decreases. At the same time, the plasma modification can introduce fluorine to the surface of the material, shallow the surface traps, improve the surface conductivity, and reduce the accumulation of surface charge. Under the selected parameters, the surface flashover voltage increases to the maximum after modification for 9 minutes, and the Weibull distribution calculation shows that the increase rate is about 37.17%. After plasma surface modification for too long, the material structure is damaged, the surface traps become deeper, the surface conductivity and surface flashover voltage decrease.