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.

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.

In this paper, the current research status of partial discharge (PD) measurement using non-sinusoidal voltage waveforms at home and abroad were summarized, the PD characteristics under non-sinusoidal voltage waveforms such as continuous pulse square-wave voltage, triangular-wave voltage, and trapezoidal-wave voltage were reviewed, especially the effects of voltage rise time, frequency, and duty cycle of square-wave voltage on the PD activity were discussed deeply. Finally, the great significance and existing problems of PD measurements using non-sinusoidal voltage waveforms were discussed. Future development of PD measurements using non-sinusoidal voltage waveforms was proposed, and how to use the non-sinusoidal voltage waveforms to enhance the understanding of discharge physical process and insulation ageing mechanism were prospected.

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.

Under the sinusoidal, bipolar square wave, and bipolar short impulsive voltage with 50 Hz of frequency, three typical corona-resistant turn-to-turn insulated twisted pairs for new energy vehicles were used to experiment, and the influences of three voltages on their partial discharge inception voltage (PDIV) characteristics were studied. The results show that within the effective bandwidth after frequency domain filtering, the discharge frequency domain energies under bipolar repetitive short pulse and square wave voltage distribute in 0.5‒0.9 GHz mainly. Keeping other test environments unchanged, the PDIVs under sinusoidal and bipolar repetitive square wave voltage are almost equal. The PDIVs under sinusoidal and square wave are about 20% lower than under bipolar repetitive short pulse voltage, and with the increase of pulse width, the PDIV decreases under bipolar repetitive short pulse voltage. According to the research results, when conducting PDIV test on turn-to-turn insulation, the sinusoidal and square wave voltage are more conservative test voltages for evaluating the PDIV of inverter-fed motor turn-to-turn insulation. When conducting PDIV test on capacitive sample of twisted pairs, the sinusoidal voltage can be considered to replace pulse voltage.

Internal water seepage is one of the common faults of cable. The accurate detection, identification, and location of internal water seepage defect in cable play a vital role in improving power safety and stability. On the basis of time-domain reflection technology (TDR) theory, the effects of injection pulse frequency and amplitude attenuation on the effective positioning distance of TDR were analyzed by simulation. The characteristic waveforms of different impedance points were obtained, the resolution method of water seepage defect waveforms was summarized, and the effect of complex reflection wave on the positioning accuracy was studied. The results show that the TDR technology can detect the water seepage defect in cable, and the effective detection distance is about 3.76 km. Different types of defects will return different characteristic waveforms, and when the degree of water seepage is different, the waveform amplitude and wave tail oscillation will also change.

A finite element simulating model of stator bar end was established by COMSOL software, and the influences of electrical properties parameters of insulating material on the electric field and potential distribution of bar end were analyzed. The results show that with the increase of relative permittivity of main insulation, the maximum electric field on stator bar end surface increases, therefore it is better to choose smaller relative permittivity of main insulating material. The resistivity of main insulation material has no significant effect on the maximum electric field on bar end surface. The relative permittivity of each anti corona layer has little effect on the maximum electric field on bar surface. It is better to choose larger nonlinear resistance coefficient of medium resistance anti corona layer. The nonlinear resistance coefficients of medium and high resistance anti corona layer have little effect on the maximum electric field on bar surface, and it is best to choose 1. The nonlinear resistance coefficient of high resistance anti corona layer has no effect on the maximum electric field on bar surface. The influence degree of initial resistivity of different anti corona layers on the maximum electric field of bar surface is different. Through partial correlation analysis, the influence degree of electrical properties parameters of insulating material on the maximum electric field at bar end is ranked, and the initial resistivity and nonlinear resistance coefficient of the medium resistance anti corona layer are the dominant factors.

From the perspective of the particle size of pollution particles, the particle size characteristics of the insulator surface pollution under two typical dust pollution sources of nickel plants (metal) and chemical plants were tested and analyzed, and the influence mechanism of the pollution particle size on the pollution accumulation was studied by establishing a pollution particle deposition model. The results show that the average particle size of pollution particles on insulator surface in chemical plant area is only 6.67 μm, while the average particle size of pollution particles on insulator surface in nickel plant area attains 24.5 μm. The deposition rate is the largest when the particle size of pollution particles is 20‒30 μm, therefore the surface of insulator in nickel plant area is polluted seriously. The simulation results based on the pollution particle deposition model can confirm with the on-site pollution accumulation phenomena, which explains the influence of pollution particle size on the insulator pollution accumulation.

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.