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 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.

In order to obtain the influence of orientation angle of two-dimensional microsheet on the growth characteristics of electrical tree in composite dielectrics, on the basis of WZ breakdown model of fractal media, we studied the growth and fractal dimension characteristics of electrical tree under different microsheet orientation angles, growth probability index, and discharge threshold voltage. The change of electric field caused by the change of two-dimensional microsheet orientation angle was analyzed, and the influence mechanism of microsheet on the growth of electrical tree was explained. The results show that when the microsheet is oriented perpendicular to the electric field, the electrical tree has the largest fractal dimension and is most difficult to breakdown. With the decrease of the angle formed by microsheet and electric field direction, both the fractal dimension and breakdown time of electrical tree decrease. With the increase of the growth probability index or threshold voltage, the fractal dimension of electrical tree decreases, and the influence on the electrical tree fractal dimension in microcomposite dielectrics with different orientation angles increases. When the orientation angle of microsheet is perpendicular to the electric field, the electric field distribution is relatively uniform. With the increase of microsheet orientation angle, the electric field concentrates at the both ends of microsheet, and the closer the adjacent microsheet, the more concentration the electric field.

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.

Compared with mineral insulating oil, natural ester insulating oil has the advantages of high biodegradability, non-toxicity, and strong fire resistance, which has the potential to replace mineral insulating oil in power transformer. A set of lightning impulse discharge characteristics test system including high-speed image acquisition system, test power supply, and electrical signal measurement system was established. The positive and negative lightning impulse discharge characteristics of palm insulating oil and mineral insulating oil were tested, and the stop length, development velocity, and gap breakdown characteristics of the streamer in two oils under positive and negative lightning impulse were compared and analyzed. The results show that under the same applied voltage and polarity, the streamer stop length in palm insulating oil is longer and the streamer development velocity is faster compared with the mineral insulating oil. The contents of vitamin A and vitamin E in palm insulating oil have no effect on the streamer stop length, development velocity, and breakdown voltage. The differences of streamer development characteristics and breakdown characteristics under positive and negative voltages mainly comes from the influence of space charge in insulating oil on the streamer development process.

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.

On the basis of a new type of three-element mixed insulating oil with excellent physichemical and electrical properties, and taking the mineral insulating oil-paper composite system as a reference, the creeping discharge characteristics of the three-element mixed insulating oil-paper composite system were analyzed under needle-plate electrode structure. The electric field distribution of two insulating oil-paper composite systems were simulated under needle-plate electrode structure. On the basis of density functional theory, the energy band structures of natural ester molecule and mineral oil molecule were studied comparatively. The results show that compared with the mineral insulating oil-paper composite system, the three-element mixed insulating oil-paper composite system has smaller partial discharge inception voltage, flashover voltage and larger maximum discharge magnitude, discharge pulse number under creeping discharge fault. There is small difference on electric field distribution between two insulating oil-paper composite systems under needle-plate electrode structure. The energy gap of palm oil molecule and soybean oil molecule in mixed oil are smaller than that of the mineral oil molecule, therefore, the mixed oil molecule is more likely to ionize under strong electric field.

This paper mainly studied the feasibility of retrofilling traditional mineral oil transformer with ester insulating oil, including natural ester and mono-ester insulating oil. At first, two kinds of ester insulating oil were mixed with mineral oil, respectively, and their electrical and physicochemical characteristics were studied. On this basis, two 10 kV mineral oil transformers were retrofilled with two kinds of ester insulating oil, respectively, and the overall characteristics of the transformers before and after retrofilling were tested. The results show that after using the mono-ester insulating oil to retrofill the mineral oil transformer, the temperature rise of the transformer decreases, the capacitance to ground increases, and the insulation resistance decreases. After using the ester insulating oil to retrofill the mineral oil transformer, the temperature rise of the transformer increases, the capacitance to ground increases, and the insulation resistance decreases. It is feasible to retrofill mineral oil transformer with two kings of ester insulating oils, and using the mono-ester insulating oil to retrofill the old mineral oil transformer can improve its heat dissipation capacity. However, it should pay attention to the reduction of transformer insulation resistance after retrofilling the mono-ester insulating oil.

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.