Bushing is an important component of oil-immersed transformers, and its internal insulation moisture, ageing, and development of some defects after long-term operation will cause distortion of local electric field, which seriously threatens the safe operation of bushing. In this paper, the variation patterns of dielectric parameters of the aged and damped oil-immersed paper under different test temperature environment were obtained, and a dynamic dielectric parameter calculation model was established. Meanwhile, a simulation model was established on the basis of actual structure of a 252 kV/1 250 A bushing. Electro-magnetic-thermal coupling multiphysics were used to conduct simulation analysis, and the transient electric field distribution and distortion severity inside the bushing under three typical defect conditions were obtained. The results indicate that when the end screen of bushing is unreliable grounding (resulting in floating potential), electric field distortion occurs at the contact side of bushing and end screen, and the voltage grading effectiveness of capacitive core is weakened. When there is wrinkle on the capacitance screen of bushing, the maximum electric field intensity at the wrinkle regions increases proportionally with the bending curvature. Notably, the field strength inversion phenomena are observed at the distortion zones when the capacitance screen reach specific bending curvature. The bubbles attached on the end screen surface have greater influence on the electric field distribution than bubbles dispersed in transformer oil. Such surface-attached bubbles are prone to produce partial discharge.

To develop polyimide (PI)-based composite films with low dielectric constant (D_k), low dielectric loss (D_f), and high temperature resistance at high frequency, a water-soluble polyamic salt (PAAS) was prepared by adding organic base into the polyamic acid, which is the precursor of fluoropolyimide (FPI), and composite films were prepared by compounding low dielectric polytetrafluoroethylene (PTFE) concentrated dispersion with the dried PAAS to form a water dispersion system. The effects of PTFE content on the dielectric, thermal, and mechanical properties of the composite films were studied. The experimental results show that with the increase of PTFE content, the dielectric constant of the composite films decreases continuously, but there will be some effects on the thermal stability and mechanical properties. The dielectric constant of 50%PTFE/FPI reaches the minimum value (D_k=1.5@8.5 GHz), and the dielectric loss of 10%PTFE/FPI in the frequency band of 9.25-10.25 GHz is less than 0.005. The glass transition temperature of the composite films is in the range of 289-297℃, and the thermal decomposition temperature at 5% is higher than 508℃. The thermal expansion coefficient of 40%PTFE/FPI is as low as 59.67×10^-6 K^-1. The tensile strength of 20%PTFE/FPI is 70 MPa, the tensile modulus is 1.59 GPa, and the elongation at break is 7.7%.

Polyimide (PI) is a class of high-performance polymer materials containing π-conjugated imide ring in the main chain, and its size of the band gap is one of the main factors directly affecting the thermal stability, optoelectronic properties, dielectric properties and other properties of the material. The electrically powered diamine group and the electrically absorbing dianhydride group in the conventional PI molecular structure determine that the value of band gap is nearby 3.0 eV, which directly affects its performance in the field of high-temperature energy storage, high-frequency communication, electrical insulation, etc. Because of the excellent structural designability of PIs, the band gap of PIs can be adjusted by modulating the monomer combination/chain segment structure/space structure, and the above properties of PIs can be optimized. In this paper, based on the main research progresses of PI bandgap modulation reported in recent years, the main strategies of PI bandgap modulation were elaborated from the perspectives of polymer structure and adjusting polymerization process, respectively, and the difficult problems faced in PI bandgap modulation were discussed with the example of its application in the field of dielectric energy storage. Finally, the future development direction of PI bandgap modulation was discussed on the basis of current research status of PI bandgap modulation.

Bisphenol A epoxy resin is widely used in the field of copper clad laminates. However, its flame retardancy and dielectric properties cannot meet the high-speed development of copper clad laminates. To simultaneously improve the flame retardancy and dielectric properties of epoxy resin, a reactive oligomeric flame retardant (PDDV) based on 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-HQ), 1,4-dichlorobenzene (DCX), and 4-vinylbenzyl chloride (VBC) was synthesized, and its chemical structure was determined by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) characterization. EP/PDDV composites were prepared by physical blending, and their thermal, flame retardancy, and dielectric properties were tested. The results show that when the mass fraction of PDDV is 30%, the char yield of EP/PDDV composites increases to 38.74%, the glass transition temperature (T_g) rises to 138.9℃, the limiting oxygen index (LOI) increases to 55%, UL-94 reaches V-0 grade, the dielectric constant (D_k) is 2.69, and the dielectric loss factor (D_f) is 0.007 91. The comprehensive performance of the composites is the best.

Polyimide (PI) is a class of insulating materials with great potential in the field of high-voltage and high-power device encapsulation, which has excellent insulating properties. However, its intrinsic thermal conductivity is low, which is easy to cause localized overheating and damage to the device. In this paper, the thermal conductivity of polyimide materials was improved by doping with thermal conductive fillers. PI/c-BN and PI/h-BN composite films were prepared by in situ polymerization with cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN) as filler, and the effects of different content of c-BN and h-BN on the thermal conductivity, thermal stability, and mechanical properties of the composite films were investigated. The results show that the thermal conductivity of the composite films increases with the increase of BN content. The thermal conductivity of PI/c-BN composite films is higher than that of PI/h-BN composite films within the studied content, and the thermal conductivity of PI/40c-BN composite films increases by 1 068% compared to pure PI. The thermal conductivity of PI matrix is improved by c-BN at lower content, and the tensile strength of PI/c-BN composite films still reaches 40 MPa when the filler mass fraction is 30%. In addition, the thermal stability of the composite films is improved.

The stator winding insulation of 10 kV three-phase asynchronous motor will subject to thermal, electrical, environmental, and mechanical stresses during operation, which will generate insulation defects and lead to partial discharge (PD). It is very difficult to obtain the reference voltage waveform in on-line monitoring condition, and the loss of reference phase makes it impossible to use PRPD pattern in PD recognition. To solve these problems, three kinds of PD models, e.g. internal discharge, slot discharge, and corona discharge, were made in the laboratory conditions, and the PD signals were collected. The polar coordinate method was used to convert PRPD pattern into circular polar coordinate pattern to solve the problem of reference phase loss. The results indicate that different types of PDs have different polar coordinate pattern characteristic distributions, the internal discharge has symmetrical distribution in the first and third quadrants. Slot discharge and corona discharge have unsymmetrical distribution, and the amplitude of the third quadrant is much larger than that of the first quadrant. The slot discharge has larger point cluster discharge concentration range and wider discharge shape than corona discharge. The characteristic parameters such as skewness, kurtosis, and discharge proportion are obviously different in different discharge types. With the increase of voltage level, the distribution and characteristic parameters of polar coordinate pattern will change obviously.

To address the challenges of epoxy resins in electrical insulation and flame-retardant applications, a phosphorus-containing bio-based flame-retardant resin—bis(methacryloyloxy-4-hydroxy-3-methoxyphenyl) phenyl phosphate (DGEBDB) was synthesized from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and renewable vanillin, and cured materials were prepared through blending it (0%, 25%, 50%, and 75%) with bisphenol A epoxy resin (DGEBA). Their flame retardancy, thermal properties, mechnical properties, and electrical properties were analyzed. The results show that when the mass fraction of DGEBDB is 75%, the limiting oxygen index (LOI) of epoxy bending system increases from 22.6% to 34.2%, and the flame retardant grade achieves V-0 in UL 94 tests. Cone calorimetry reveals that the DGEBDB can reduce the heat release rate (HRR) and total heat release (THR), showing excellent fire suppression. Electrical properties tests show that the electrical properties of the cured epoxy maintain good when the mass fraction of DGEBDB is as high as 75%, which ensures the reliability of DGEBDB in electrical application. Mechanical properties indicate that with the increase of DGEBDB content, the flexural strength and tensile strength increase, indicating an increase in structural integrity and load bearing capacity of the cured epoxy. Therefore, the addition of DGEBDB significantly enhances the flame retardantcy of composite, while maintaining the excellent thermal, mechanical, and electrical properties of epoxy resin, and when the mass fraction of DGEBDB is 25%, the overall performance is the best, which has a better prospect for practical applications.

In order to realize the synergistic enhancement of electrical-mechanical properties of epoxy resin for power electronics encapsulation, CaCO₃ oligomer inorganic nanoparticle filler was prepared by solution method, epoxy resin was carried out organic matrix-inorganic filler modification together with organic silicone, and single inorganic modified, single organic modified, and organo-inorganic jointly modified epoxy resin composites were prepared. Taking the pure epoxy resin as a reference, the AC electric strength, DC volume conductivity, surface potential attenuation characteristics of the four epoxy resin composites were investigated, and the internal trap energy levels were calculated from them, while their bending strength and impact strength characteristics were also investigated. The results show that the electrical properties of single inorganic modified, single organic modified, and organic-inorganic co-modified epoxy resin composites are significantly improved, and their electric strength increases by 6.55%, 12.66% and 9.61%, respectively. Their DC volume resistivity is 5 times bigger than that of pure epoxy resin, the surface potential decay rate decreases, the trap density and trap energy increase, but the improvement of electrical properties by the organic-inorganic co-modification do not superimpose compared with single modification. The flexural and impact strengths of the modified epoxy resin composites significantly increase, with exponential growth compared to the pure epoxy resin. The research results provide a reference for the performance improvement of modified epoxy resin composites in the field of power electronics insulation and encapsulation.

Meta-aramid insulation paper is widely used in transformers and high-voltage bushings due to its excellent dielectric and mechanical properties, but its low thermal conductivity limits its application in high thermal load environments. To enhance the thermal conductivity of nano-filler/aramid insulation paper, this study employed simulation-guided experimental design to calculate the thermal conductivity of aramid simulation models. Three nano-fillers with excellent thermal conductivity properties, namely nano SiO₂ (KH570), nano TiO₂, and nano C₃N₄, were screened out. The mechanical and insulation performance of laboratory composite aramid paper hand sheets were compared, and the optimal filling amount was determined. Further, the influence of nano-fillers on the thermal conductivity of composite aramid paper was explored, and their inherent thermal conduction mechanism was analyzed. The results show that the optimal addition amounts of three nano-fillers are: nano SiO₂ (KH570) at 10%, nano TiO₂ at 4%, nano C₃N₄ at 4%. Through thermal conductivity testing, nano C₃N₄ is determined to be the optimal nano-filler, and the composite aramid paper with 4% of nano C₃N₄ showing an increase in elastic modulus by 135%, Young's modulus by 198%, electric strength by 60.24%, volume resistivity by 3 713%, and thermal conductivity by 304.31% compared with pure aramid paper.

With the development of economy and technology, the demand of safer and more stable packaging materials for electronic devices has increased. Traditional polyimide (PI) base materials are extensively used due to their excellent mechanical and heat-resistant properties. In this study, a novel fluorinated polyimide (FPI) porous composite film was developed by introducing fluorine groups and a porous structure into polyimide, along with adding the flame retardant 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Experimental results demonstrate that under 80%RH of ambient humidity and 5% of DOPO mass fraction, the new FPI porous composite film exhibits uniform and regularly distributed pore size, excellent mechanical properties, and lower water absorption. The incorporation of flame retardant DOPO forms a protective phosphorus-containing layer on the film surface, which significantly improves the flame retardancy and hydrophobic properties of film. This research provides a new material with enhanced safety for the packaging of electronic devices, which is expected to play a crucial role in improving the stability and safety of electronic equipment.