The mechanical properties of insulation pressboard are key factors in modeling the structure of transformer windings. The operating transformer winding is subjected to periodic electromagnetic forces, and the static stress-strain test results cannot characterize the dynamic mechanical properties of insulation pressboard. In this paper, an asymmetric hysteresis model was proposed to study the dynamic mechanical properties of the insulation pressboard under periodic electromagnetic forces, and the model includes not only linear damping and stiffness coefficients, but also nonlinear dynamic stiffness. In the experiment, a pneumatic vibration exciter was used to simulate the amplitude and frequency of the electromagnetic force, and the stress-strain data of the insulation pressboards under different clamping forces was obtained. A multi-objective optimization algorithm based on NSGA-II algorithm was used to train the model parameters. The results show that the stiffness and damping of the insulation pressboard are closely related to the clamping force, and the oil-immersed pressboard has obvious damping characteristics. As the clamping force increases, the dynamic stiffness of the insulation pressboard decreases during the loading and unloading processes. Compared with the amplitude of periodic electromagnetic force, the frequency of periodic electromagnetic force has more significant effect on the mechanical characteristics of insulation pressboard.

Transformer oil is prone to microbial contamination during storage, utilization, and transportation, which affects the quality and insulating performance of transformer oil. In this study, the effects of temperature and humidity on the microorganism growth in transformer oil were mainly investigated, and the quality changes of transformer oil before and after treatment were analyzed. The functional group structure and organic species of transformer oil were identified, and some effective treatment methods for microbial pollution in transformer oil were proposed. The results show that both the transformer running oil and faulty oil contain bacillus subtilis, bacillus using sugar to produce acid, bacillus using sugar to produce acid and gas, and bacillus producing acid and gas without sugar. The number of microorganisms in faulty oil is much greater than that in running oil, and the number of bacillus subtilis is the largest, while there is no microorganism cultured in new transformer oil. The high temperature could restrict the growth of microorganism in transformer oil. When the temperature is 60℃, the microorganisms will not be completely inactivated, and once the temperature returns to 37℃, they will start to grow and reproduce again. When the mass ratio of oil and water is 99∶1, 49∶1, 19∶1, and 9∶1, four types of bacillus all exist at 37℃, and the number of bacillus increases with the increase of oil-water mass ratio. The regeneration treatment method of high-pressure sterilization+molecular sieve coarse filtration+ultrafiltration can make the contaminated transformer oil clear and transparent, and the microorganisms disappear. The regenerated oil mainly contains O-H, C-H, C=O, and unsaturated bonds. After regeneration treatment, the breakdown voltage and volume resistivity of transformer oil increase from 25.83 kV and 5.70×10¹⁰ Ω·m to 49.50 kV and 4.68×10¹¹ Ω·m, respectively, and the dielectric loss factor decreases from 6.052% to 0.215%, which meet the requirement of DL/T 1419—2015.

An interturn insulation test platform was constructed, and the voltage-withstand tests were conducted on vegetable insulating oil-paper insulation and mineral insulating oil-paper insulation under lightning impulse voltage, switching impulse voltage, and power frequency voltage. The results show that under power frequency voltage, the breakdown voltage of vegetable oil-paper insulation is higher than that of mineral oil-paper insulation. Under switching impulse voltage, the breakdown voltage of vegetable oil-paper insulation is slightly higher than that of mineral oil-paper insulation. Under negative lightning impulse voltage, the breakdown voltage of vegetable oil-paper insulation is significantly lower than that of mineral oil-paper insulation. Considering the differences in breakdown voltage of interturn insulation with different specification of wires, on the basis of test results, the field strength of mineral oil-paper interturn insulation and vegetable oil-paper interturn insulation were simulated and calculated by finite element method so as to guide the design of interturn insulation for vegetable insulating oil transformer. The simulation results show that the maximum field strength of mineral oil-paper interturn insulation is higher than that of vegetable oil-paper interturn insulation under the breakdown voltage.

The partial discharge ultrasonic signal attenuates severely on power transformer equipment shell, resulting in larger differences in the partial discharge signals received by different positions of sensors, which greatly affects its measurement accuracy and detection efficiency in field application. In this paper, a partition model of oil-immersed power transformer was built using finite element simulation software, the partitioning rule of partial discharge ultrasonic signal on the main surface of oil tank was obtained, and the transformer spatial sound pressure and partition sound pressure distribution maps were formed. On the basis of monitoring point optimization method of Pearson correlation coefficient and K-means clustering, the monitoring points with the best sensitivity of spatial sound pressure and partition sound pressure were preferably selected and compared. The results show that the average detection efficiency of the A-level monitoring points is 86.0%, which realizes the efficient detection of transformer partial discharge and provides a new method for selecting the placement points of ultrasonic sensors.

In this paper, the modeling methods of three-capacitance model, conductance model, and plasma model for the numerical simulation of partial discharges under DC voltage and their advantages and disadvantages were introduced, and the recent research achievements of experts and scholars at home and abroad using these models were reviewed. The researches on partial discharge testing under DC voltage in the recent years was summarized, and the impacts of temperature, insulation material, voltage harmonics, atmospheric pressure, and defect on the partial discharge charactristic under DC voltage were summarized. Finally, the limitations of current numerical simulation studies were discussed, and the potential areas for further research were prospected.

The frequent faults caused by buffer layer defects in high-voltage cross-linked polyethylene cables have seriously threatened the safe operation of power system. In this paper, the basic structure and function of buffer layers was introduced at first, and the relevant research on buffer layer failure at home and abroad were summarized. Secondly, the main reason of buffer layer defects was analyzed through electric field simulation from the material characteristic and internal structure of buffer layers, and the insulating properties and physicochemical characteristics of the white powder in buffer layer were summarized to propose its formation mechanism. Finally, the detection methods of buffer layer defects were summarized, it is proposed to use computer tomography technology to detect cable buffer layer defects to make up for the shortcomings of existing detection methods, and it is recommended to optimize the materials or structures of aluminum sheaths and buffer layers to prevent the generation of buffer layer defects.

Currently, the dielectric of plastic film capacitors is usually linear dielectric polymers with a low dielectric constant, leading to a relatively low energy storage density of capacitors. High-energy-density dielectric polymers under research often suffer from excessive dielectric loss, limiting their practical utilization. Poly(methyl methacrylate) (PMMA) has been commonly employed to enhance the mechanical and breakdown properties of high-energy-density fluoropolymer beceause of their good compatibility between them. However, the commercially available PMMA also exhibits too high intrinsic dielectric loss. In order to decrease the dielectric loss of PMMA, a copolymer named MS was synthesized from methyl methacrylate (MMA) and styrene (St). Subsequently, a series of composites were fabricated by incorporating low content of MS into bulk polymerized PMMA, and their dielectric properties, energy storage characteristics, and insulating peoperties were investigated. The results show that the composites can significantly reduce the dielectric loss of PMMA, making it more suitable as the modified material of high-energy-density polymer compared to PMMA. Under 5 500 kV/cm of electric field, the discharge energy density of the dielectric film composed of 10% MS achieves 5 J/cm³, and its charge-discharge efficiency can attain 83%.

To meet the growing demand for high thermal conductive and electrically insulating composites, a silver nanoparticles (AgNPs) modified nonwoven fabric (AgNPs@NWF) was prepared by polydopamine (PDA) modification and in situ reduction process using PET nonwoven fabric (NWF) as a template. The continuous boron nitride nanosheets (BNNS) thermal conductive network (BNNS@NWF) and AgNPs/BNNS synergistic thermal conductive network (AgNPs/BNNS@NWF) were constructed by adsorbing BNNS on the surfaces of NWF and AgNPs@NWF through the dispersion and interfacial bonding of nanocellulose through a cyclic impregnation adsorption and layer-by-layer assembly process. BNNS-AgNPs/BNNS-BNNS sandwich-structured thermal conductive composite films were prepared by hot pressing process with BNNS@NWF as the surface layer and AgNPs/BNNS@NWF as the intermediate layer, and their microstructure, thermal conductivity, insulating properties, mechanical properties, and actual thermal management performance is characterized and tested. The results show that a synergistic three-dimensional thermal conductive network of AgNPs/BNNS was constructed in the composite films, at the same time the insulating properties are ensured. When the mass fraction of BNNS and AgNPs is 34.8% and 3.3%, the in-plane thermal conductivity of the composite films reaches 7.56 W/(m·K), the volume resistivity reaches 3.54×10¹³ Ω·cm, and the mechanical properties is good. The actual application test show that the composite films have good thermal management performance.

High-temperature-vulcanized (HTV) silicone rubber has poor resistance to ultraviolet (UV) ageing, and its hydrophobicity would gradually lost, tensile strength decrease, and hardness significantly increase after ultraviolet irradiation. In order to improve the hydrophobicity and UV resistance of HTV silicone rubber, the permanent-room-temperature-vulcanized (PRTV) silicone rubber and four needle-like zinc oxide/nano-zinc oxide (tZnO/nZnO) particles with different structures were introduced onto the surface of HTV silicone rubber by embedding-curing method to prepare the hydrophobic and anti-UV composite coatings with different morphologic morphology. The effects of the composition of composite coating on the microstructure and anti-ageing properties of HTV silicone rubber were studied by the ageing experiment and water contact angle, surface hardness, and mechanical properties tests. The results show that the surface holes of HTV silicone rubber are covered, its hydrophobicity increases, and water content and water absorption decrease after the introduction of PRTV silicone rubber on the surface of HTV silicone rubber. The further introduction of ZnO particles greatly improves the hydrophobicity, UV resistance, and thermal stability of HTV silicone rubber materials. The PRTV silicone rubber-tZnO/nZnO composite coating introduced by stepwise embedding-curing method can effectively construct a micro-nano mastoid structure and UV shielding layer on the HTV silicone rubber surface, and its enhancing effect on the hydrophobicity and UV ageing resistance are more obvious than that of single embedding of tZnO or nZnO particles.

Compared with mineral insulating oil, natural ester has the advantages of higher ignition point, better degradability, and can delay the thermal ageing rate of insulating paper, so natural esters are receving extensive attention as liquid insulating medium for extending the life of transformers or improving short term overload capacity. In this paper, we used rapeseed oil, which accounted for a large proportion of cooking oil in China, to prepare rapeseed oil based natural esters, and the accelerated thermal ageing characteristics of insulating paper with different water contents in rapeseed oil based natural ester and mineral oil were investigated. The effect of oxygen on the accelerated thermal ageing characteristics of insulating paper in the two different insulating liquids was studied. The experimental results show that at the same temperature, water content is the main factor affecting the thermal ageing rate of insulating paper, and oxygen will reduce the inhibitory effect of rapeseed oil based natural esters on the thermal ageing rate of insulating paper with low water content. Combined with the measured data of specific strength and degree of polymerization of insulating paper during thermal ageing in insulating liquid, a kinetic model of cellulose decomposition reaction rate of insulating paper considering the influence of water content and oxygen was established. The model can be used to predict the thermal ageing process of insulating paper in insulating liquids.