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.

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.

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.

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.

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.

To improve the UV resistance of room temperature vulcanized silicone rubber (RTV), we used titanate coupling agent (PN) to modify nano zinc oxide (ZnO), and then nano ZnO-RTV composites were prepared by doping different mass fraction of modified nano ZnO into RTV. The ultraviolet accelerated ageing experiments were conducted on pure RTV and nano ZnO-RTV composites, and the effect of nano ZnO on the ultraviolet resistance of RTV was compared and analyzed. The results show that nano ZnO can effectively improve the UV shielding performance of RTV, and the UV shielding rate of nano ZnO-RTV composite with 1.0% of ZnO can increase by 72.63%; after adding nano ZnO, the electrical strength of RTV is slightly improved. After UV ageing for 2 500 hours, the decreasing amplitude of static con-tact angle and the ageing degree which reflected by surface micro-morphology of pure RTV are far greater than that of nano ZnO-RTV composite, it is verified by the changes of Si-(CH₃)₂, Si-CH₃, and C=O of RTV molecular chains before and after ageing. At the same time, after UV ageing for 2500 hours, the thermal stability of nano ZnO-RTV composite is significantly better than that of pure RTV.

The paper proposes a evaluation method for the micro properties of impurity content for cable insulation, which can effectively avoid the effect of subjective factors of manual microscopic observation in the purity evaluation process. On the basis of broadband dielectric impedance spectroscopy method, the change law of dielectric loss factor of 35 kV and 110 kV XLPE cable insulation samples with the test frequency during different thermocycling processes were investigated, and the stress characteristics at the interface between XLPE insulation and impurity during the thermocycling process were simulated. The correlation between the macrophysical parameter broadband dielectric loss and the microscopic property cleanliness for XLPE cable insulation were analyzed. The results show that after 10 cycles of thermocycling between -196℃ and 50℃, the low-frequency dielectric loss factor of 35 kV XLPE cable insulation at 0.1-1.0 Hz is more than doubled, while the dielectric loss factor of 110 kV XLPE sample does not change obviously. The simulation analysis shows that during the thermocycling process, there is interface stress between the polyethylene insulation and the micro-impurities, the stress fatigue enlarges the microscopic defects in the interface area and then accelerates the increase of low-frequency dielectric loss factor.

Polypropylene laminated paper (PPLP) used in DC superconducting cable is in a wrap-around structure. Due to the existence of rectifier, PPLP withstands AC-DC composite voltage, and it is easy to generate partial discharge at the butt gap. Under long-term working voltage, the insulating property of PPLP changes and affects partial discharge. In this paper, the DC electrical ageing experiment was conducted on PPLP in liquid nitrogen, and the butt gap was simulated by internal discharge model. The electric field distribution of internal discharge model was obtained by simulation calculation, and the electrical ageing mechanism and partial discharge results were discussed and analyzed. The results show that with the increase of ageing time, the electrical conductivity and relative permittivity of PPLP show an increasing overall trend; the DC electric strength of PPLP decreases gradually, but still maintained good insulation properties; the intensity of partial discharge increases the partial discharge phase mainly concentrates in the rising stage of composite voltage, and the ratio between AC and DC of composite voltage is the main factor affecting partial discharge.

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.