The main insulation of large motor will produce many kinds of insulation defects during its operation process, and only used the phase resolved partial discharge (PRPD) pattern cannot reflect the characteristics of the defects fully. In this paper, four typical insulation defects such as internal air gap, coil wear in slot, overlapping part fault of end semiconductor coating and anti corona layer, and surface contamination were simulated in laboratory, their partial discharge (PD) information were collected in the electromagnetic shielding chamber, and the PRPD patterns and PSA patterns were drawn respectively. Combined with the characteristics of PRPD and PSA patterns under different voltage levels, the discharge characteristics of the typical insulation defects were analyzed. The results show that considering the symmetry, discharge amplitude, spectrum envelope shape of PRPD pattern and the discharge cluster number, discharge cluster distribution position, discharge cluster shape of PSA pattern, the four typical insulation defects can be identified accurately.

A glass fiber winding reinforced epoxy resin composite with high thermal conductivity was prepared by filling the bisphenol A type epoxy resin system with a mixture of alumina and silica powder fillers. The effects of filler content on the mechanical properties, electrical properties, and thermal conductivity of composite were studied. The results show that the addition of mixed fillers is beneficial for improving the thermal stability and thermal conductivity of the composite. When the mass fraction of the fillers is 62% (of which the mass fraction of silicon powder is 48% and the mass fraction of alumina is 14%), the tensile strength (0° and 90° directions), bending strength, and NOL ring tensile strength of the composite are 160.25, 24.50, 245.40, and 134.30 MPa, respectively. The dielectric constant is 1.889, the dielectric loss factor is 0.673×10^-2, the volume resistivity is 3.36×10¹⁶ Ω·cm, and the thermal conductivity is 1.257 W/(m·K), which is 155% higher than the thermal conductivity before addition.

This paper introduced the research and application progress on green solvents in the preparation of polyimide from the views of the classification, manufacturing procedures, solvent composition of polyimide films, and the application status of green solvents. Emphatically, the current research status of biobased solvents, including γ-valerolactone (GVL), dimethyl isosorbide (DMI), and dihydrolevoglucosenone (Cyrene^TM) and their applications on the preparation of PI films were presented. The future developing trends of the applications of green solvents in the manufacturing of PI films were prospected.

In order to investigate the improving mechanism of polysiloxane with different substituents on atomic oxygen (AO) erosion resistance of polyimide in space environments, reactive molecular dynamics (MD) simulations was used to analyze the AO erosion resistance of PI composited with polyhedral oligomeric silsesquioxane (POSS) with trifluoromethyl (-CF₃) group and methyl (-CH₃) group. The results show that both the composites exhibit strong AO erosion resistance by forming an SiO₂ layer that blocks AO propagation into the polymer matrix and and heat transfer. The PI/CF₃-POSS shows the best performance, and its normalized mass is 0.83 after 35 ps of AO exposure, while the nomalized mass of the PI/CH₃-POSS composite is 0.78.

In order to improve the solution processability of traditional thermoplastic polyimide (TPI) and enhance its adhesion and heat resistance, several soluble thermoplastic polyimide (TPI) resins were prepared from different commercial dianhydride monomers and self-made diamine monomers with pyridine and diphenyl ether structures and active phenol side groups or benzene side group in the main chain through the two-step method. And then the corresponding TPI films and flexible copper clad laminates (FCCLs) were prepared. The properties of TPI resins were analyzed by solubility test and gel permeation chromatography (GPC). The structure, water absorption, mechanical properties, and dielectric properties of TPI films were tested, and the related properties of the FCCLs were also tested. The results show that all the TPI resins can dissolve in strong polar organic solvents such as NMP, and the glass transition temperature (T_g) and 5% thermal weight loss temperature (T_5%) of the corresponding films are in the range of 236.8-325.6℃ and 508.7-553.7℃, respectively, and the residue rate at 800℃ (R₈₀₀) is higher than 64%. The coefficient of thermal expansion (CTE), tensile strength, elongation at break, and water absorption is in the range of 56.36×10^-6-78.30×10^-6℃^-1, 64.93-109.18 MPa, 9.09%-24.60%, and 0.74%-3.75%, respectively. The dielectric constant and dielectric loss of TPI-4 with better comprehensive properties are also lower than that of other samples, and the peeling strength of the corresponding FCCL reaches 0.95 N/mm, but it can only pass the floating welding test at 288℃ for 10 seconds. In addition, the polar phenol side group in TPI-4 not only ensures its organic solubility, but also significantly enhances its heat resistance and mechanical strength, and also reduce its CTE. The existence of reactive phenol side groups provides great convenience for the subsequent chemical modification of TPI. By adding a small amount of cyanate ester resin (CE01) to modify the TPI, the peel strength and the floating welding resistance of the responding FCCLs can be obviously improved.

Polyimide films with high porosity were prepared using 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-diamino-2,2′-dimethyl-biphenyl (mTB) as monomers. During the preparation process, pore-forming agent was added into the system, and the high-temperature thermal imidization of polyamide acid was controlled by the principle of thermo-induced phase separation. The micropore morphology, mechanical properties, and dielectric properties of the films were analyzed. The results show that the porosity of porous films reach 50%-60%. Under the frequency of 10 GHz, the dielectric constant and the dielectric loss factor of the sample PI-1-250-4 h is 2.14 and 0.002 8, respectively, while maintaining good mechanical properties and heat resistance. Due to the introduction of rigid biphenyl building blocks, the porous film still maintains a low coefficient of thermal expansion. The adhesive-free double-sided copper clad laminate, which prepared by combining the porous film with thermoplastic polyimide (TPI) and then pressing copper foil at high temperature, has good interface adhesion and solder resistance, and can apply to high-frequency and high-speed flexible copper clad laminates.

Metal particle pollutants inside gas-insulated metal enclosed switchgear (GIS) equipment can cause severe electric field distortion, leading to surface discharge of insulators. This paper proposed a method of dielectrically functionally gradient insulator for suppressing metal particles. The electric field distribution around the basin insulator and the motion characteristics of metal particles were simulated. The results show that metal particles released from the shell are affected by the axial electric field force and tend to move towards the insulator. The permittivity distribution of the laminated functionally gradient (ε_L-FGM) insulator decreases in the radial direction, which can homogenize the electric field distribution along the insulator surface and drive away the metal particles by reversing the electric force with the axial electric field. However, the ε_L-FGM insulator enhances the electric field on the surface of the shell, and the jump height of metal particle is 2.7 times higher than those around the uniform insulator. Compared with ε_L-FGM insulator, the ε_T-FGM insulator with a spatial permittivity gradient, which designed based on the topology optimization, reduces the jump height of metal particles and performs better effect in regulating the electric field and expelling the particles.

In order to study the influence of different interface pressure on the electric field distortion of the composite interface of crosslinked polyethylene/silicon rubber (XLPE/SR) insulation in the cold shrinkable cable intermediate joint, a simulation model of 35 kV cold shrinkable cable intermediate joint was established by the finite element simulation software. The electric-thermal-force field of the composite interface without defects and with metal impurities, circumferential scratches, and longitudinal scratches was simulated and calculated. The results show that the interface pressure has different effects on the joint deformation and electric field strength distortion under different conditions. The joint deformation with longitudinal scratch is more significant, and the field strength distribution at the defect is more obviously affected by the interface pressure. The field strength of the shielding tube is inversely proportional to the interface pressure, and the field strength of the stress cone in the joint with longitudinal scratches is directly proportional to the interface pressure. The overall field strength of the insulation composite interface except for the defects decreases with the increase of the interface pressure.

In order to investigate the arc resistance of coating modified polyimide fabric, a waterborne polyurethane was used as matrix to prepare a barrier thermal insulating coating with SiO₂ aerogel and a reflective thermal insulating coating with TiO₂ filler, and then two coatings were applied on the polyimide fabric, respectively. The effect of filler addition amount and the matrix on the thermal protection performance and rupture performance of polyimide fabric under the effect of fault arc was studied. The results show that the thermal protection performance of the modified polyimide fabric increases with the increase of filler addition, the thermal protection performance of polyimide modified by thermal insulating coating with 6% of SiO₂ and 6% of TiO₂ increases by 34.79% and 21.78%, respectively. Waterborne polyurethane matrix has no effect on the thermal protection performance of polyimide fabrics, but it can make their rupture performances increase by 20%.

Four kinds of copolymerized polyimide (PI) membranes were prepared through thermal imidization method after solution polymerization between two kinds of self-made diamine monomers 9,9′-bis[4-(4-amino-3-hydroxyphenoxy) phenyl] fluorene (BAHPPF) and 4-(4′-tertbutyl)phenyl-2,6-di(4′-aminophenyl)pyridine (TPAPP) and 4,4′-(hexafluoroisopropene)phthalic anhydride (6FDA), and the molar ratio of BAHPPF and TPAPP is 9∶1, 8∶2, 7∶3, and 6∶4, respectively. Then the corresponding copolymerized thermal rearrangement (TR) membranes were obtained after heat treatment at 450℃. The thermal, mechanical, and gas separation properties of the membrane materials were measured. The results show that the glass transition temperature of the copolymerized PI membranes is around 350℃. As the molar ratio of TPAPP increases, the tensile strength of the copolymerized PI membranes decreases. After thermal rearrangement, the mechanical properties of the copolymerized TR membranes decrease significantly, while the d values increase. When the molar ratio of BAHPPF and TPAPP is 8∶2, the copolymerized TR membrane exhibits the best gas permeability, and the permeation coefficients for H₂, CO₂, O₂, and N₂, is 346.4, 304.5, 72.43, and 13.34 Barrer respectively. Meanwhile, the O₂/N₂ separation performance of the four TR membranes is close to the Robeson upper limit in 2008.