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.

The main insulation of cross-linked polyethylene (XLPE) HVDC cable was sliced at radial inner, middle, and outer layers to as samples, and their equivalent thermal history was modeled by differential scanning calorimetry (DSC). The changes of chemical and electrical properties of each layer in cable insulation after type test were analyzed by infrared spectroscopy (FTIR), space charge, and DC electrical strength test. The results show that after type test, the equivalent heat treatment temperatures of each layer in cable main insulation are about 70℃, there is a small difference between the equivalent heat treatment time of the middle layer and the outer layer, while the equivalent heat treatment time of the inner layer is about 581 times bigger than that of the middle layer. There are little changes in DC electric strength and chemical properties of each layer in cable main insulation. Due to the equivalent heat treatment time of each layer is different, the impurities, polar small molecules, and other substances in inner insulation layer continue to diffuse from the inner layer to the middle layer, which results in the significant accumulation of heteropolar space charges.

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.

A monitoring method for the operating state characteristics of bushing in UHV converter valve hall was proposed from the perspective of intelligent image processing and three-dimensional modeling technology. This method mainly included using intelligent image processing technology to identify and classify the databases of infrared thermal imager and ultraviolet imager, using Kalman filtering technology to realize on-line measurement in real time for the insulation distance of typical metal fittings, and using three-dimensional modeling technology based on finite element method to build the electric field simulation model of typical main equipment bushing in valve hall, and the electric field distribution on the surface of key metal fittings was obtained. Combined with the image database information, insulation distance information, and electric field distribution information of typical main equipment bushing, its operation state parameters were obtained effectively, and its operation state was evaluated by the intelligent algorithm automatically. The results show that when the Kalman filtering technology is not used, the prediction deviation of the distance at key positions changes severely in 0‒160 s. After using the Kalman filtering technology, the jump amplitude of prediction deviation decreases and is basically controlled at the same error level. The neural network has good learning effect, the fitness function tends to be stable after 100 iterations, and the local weight of fuzzy neural network shows typical nonlinear characteristics. The research conclusion can explore potential hidden dangers and locate positive faults effectively, which provide effective data support and protection strategies for the operation and maintenance of main equipment bushing.

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.

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.

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.

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.

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.

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.