Kelvin probe force microscope (KPFM) is an important tool to measure the surface potential of materials with nanometer resolution. Because of its sensitivity to the surface charge of materials, it has been widely used in the research of dielectric charge behavior in recent years. This paper introduced the principle of KPFM, summarized the latest research progress of KPFM applied to charge behavior in dielectrics, focused on the analysis of the diffusion and migration mechanism of surface and interface charges in dielectrics. And the application of KPFM in typical dielectrics such as inorganic materials, nanocomposites, and ferroelectric materials were also reviewed.

Defects will occur during production and operation of insulator, which will cause the accumulation of surface charge, and affect the distribution of electric field of insulators. In serious cases, surface flashover will occur, and then affect the operation safety of electrical equipment. Therefore, it is of great significance to study the nondestructive testing and evaluation methods of defects in insulators. Firstly, this paper summarized the influence of insulator defect types such as bubble, crack, and metallic foreign matter on insulator insulation performance, and analyzed the generation and development mechanism of different defects. Then, the testing principle and research status of new nondestructive testing methods based on acoustic, optical, and thermal characteristics were summarized and introduced. Finally, the detection methods were comprehensively evaluated from the perspectives of speed and accuracy, the detection effects of different detection methods on insulator defects were evaluated, and the research direction of insulator defect detection in the future were prospected.

In order to improve the ageing resistance of insulating paper in oil-immersed transformer, three kinds of common silane coupling agents were selected to modify the insulating papers by immersing. The obtained samples were subjected to the combined thermal ageing test of oil paper at 130℃ for 35 days, and the tensile strength, permittivity, and breakdown voltage of the insulating papers were measured periodically. The results show that all the three kinds of silane coupling agents can improve the ageing resistance of insulating paper, the insulating paper modified by 3-aminopropyltriethoxysilane almost maintains the original three-dimensional network structure of cellulose during the thermal ageing process, and it exhibits the optimal mechanical strength, dielectric properties, and breakdown resistance.

In order to obtain epoxy adhesive materials with high thermal stability for metal foil resistors, combining with molecular simulation and experiment, we studied the effects of molecular structures of methyl hexahydrophthalic anhydride (MeHHPA), methyl tetrahydrophthalic anhydride (MeTHPA), and hexahydrophthalic anhydride (HHPA) on the thermodynamic properties of epoxy adhesive materials for metal foil resistors. Firstly, the cross-linking models of three kinds of acid anhydride molecules and fluoren-based epoxy resin (DGEBF) were established, respectively, and the macroscopic thermodynamic parameters of the cross-linking models were analyzed through molecular simulation calculation. Secondly, the influence mechanism of the molecular structure of acid anhydride on the thermodynamic properties of the cross-linking system was explained according to microscopic parameters. Finally, the simulation results were verified by experiments. The results show that hexahydrophthalic anhydride system has the smallest free volume (FFV) and mean square displacement (MSD), and the free volume proportion is only 15.15%. In the experiment, the change trend of thermodynamic properties of the three kinds of curing anhydride system is consistent with the simulation results. The hexahydrophthalic anhydride system has the best thermodynamic performance, and its glass transition temperature reaches 435 K, bending strength reaches 84.46 MPa, and volumetric thermal expansion coefficient is only 1.65×10^-4 K^-1.

In this research, cyanate ester-bismaleimide-hydrocarbon (BT-CH) composite resin system and BT-CH copper clad laminates (CCL) were prepared by introducing hydrocarbon (CH) resin into ester-bismaleimide (BT) resin system. The curing kinetic parameters of BT-CH composite resin system were studied by differential scanning calorimetry (DSC), Fourier infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The dielectric loss (D_f) and thermal oxygen ageing properties of BT-CH composite resin were investigated after curing. The results show that the reaction order, activation energy, and frequency factors of BT-CH composite resin system are smaller than those of BT system, CH can promote the curing reaction of BT. According to the FTIR spectra of BT-CH CCL, the characteristic peaks of cyanate group, imide group, and vinyl group disappear or weaken, while that of triazine ring appear, the reaction of resin system is sufficient. SEM shows there is no hole or other microdefect in the BT-CH substrates. The D_f of BT-CH substrate is 25% higher than that of BT substrate, and the D_f of BT-CH substrate increases by 6% after thermal oxygen ageing at 153℃ for 4 weeks, which has excellent thermal oxygen ageing resistance.

In order to solve the problem of poor interaction between black shale (BS) and coupling agent in the application process of polymer filler, and the strong odor of sulfur-containing elements during the processing, BS was processed by calcination, the effect of calcination on the composition and morphology of BS was analyzed by XPS, FTIR, and SEM. Then calcinated BS/high density polyethylene (HDPE) composites were prepared, the thermodynamic, mechanical, and electrical properties of the composite materials were tested. The results show that after calcination, the organic matter and pyrite in BS are removed, and the molar ratio of carbon atom in BS decreases from 35.95% to 6.98%, while that of sulfur atom is no longer detectable. The shale maintains a layered structure and has lamellar fragmentation, and the spacing between shale strata is slightly reduced. The impact strength of calcined BS/HDPE composites are significantly higher than that of natural BS/HDPE, because there is a large amount of interfacial bonding between calcined shale and HDPE, while natural shale and HDPE are completely separated. The volume resistivity of the calcined BS/HDPE composites maintains at 4.79×10¹⁶ Ω·cm or higher than that, it is showed the addition of calcined BS has little effect on the dielectric properties of the composites, so that they have good electrical insulation properties.

Glass fiber reinforced resin polymer (GFRP), which is widely used in high voltage electrical equipment, is easily to be eroded by hygrothermal environment under long-term operation conditions, resulting in insulation deterioration, which affects the safe and stable operation of power system. In this paper, glass fiber was modified by nano SiO₂, and GFRP composites were synthesized by infiltrating epoxy resin. Accelerated hygrothermal ageing treatment was carried out on the composites, and the influence of different concentrations of nano SiO₂ on water invasion and ageing resistance of GFRP was analyzed by experimental test and simulation. The results show that when the mass fraction of SiO₂ is 9.4%, GFRP has the best inhibition effect on water intrusion. At the same time, the addition of SiO₂ can make the GFRP composites maintain high surface insulation properties before and after ageing. In addition, combined with the simulation results, the inhibition effect of SiO₂ on water intrusion of GFRP composites and the influence mechanism of hygrothermal ageing resistance of GFRP are revealed from the molecular scale.

In order to study the ageing characteristics of stator bar insulation for pumped storage generator set, VPI stator bar for 18 kV pumped storage generator stator was taken as research object, thermal ageing test at different temperatures and different periods was carried out, its physical and chemical properties, thermal properties, and mechanical properties were analyzed, and the change rule of material properties before and after thermal ageing was studied. The results show that during the thermal ageing process, the composition change of epoxy resin in the insulation is the main reason for the change of its microstructure, thermal properties, and mechanical properties. There is a high coincidence between the measured results of physical and chemical properties, thermal properties, and mechanical properties.

In order to explore the internal mechanism of the difference in space charge characteristics of epoxy based materials used for insulation support in ultra-high voltage power equipment, the dynamic characteristics of space charge of epoxy resin and micro-alumina/epoxy composites under different electrical and thermal environments were studied, and the trap distribution characteristics were discussed. The results show that the space charge distribution of epoxy resin and its composites exhibit obvious temperature characteristics and field strength characteristics. Under high temperature and high electric field strength, space charge is more easily injected from electrodes and migrates in the samples, and the amount of trapped charges increases. The high thermal conductivity of alumina fillers at high temperatures makes charge dissipation faster,and the charge accumulation of alumina/epoxy composite material is improved. At high field conditions, alumina/epoxy composites are more likely to accumulate homopolar charges than epoxy materials, the trap density of alumina material is smaller, resulting in a greater degree of charge accumulation and field strength distortion inside the sample under the same conditions.

Under high voltage DC electric field, the particles and defects on the surface of epoxy resin will distort the local electric field and form a non-uniform electric field, and that will induce surface flashover. Most of the existing epoxy resin surface modification schemes focus on the uninformity of treatment, which has limited improvement on the voltage resistance under non-uniform electric field. In this paper, a simulation study was carried out on non-uniform electric field, and the gradient modification scheme was designed according to the characteristics of the electric field. The plasma fluorination modification technology was combined with the concept of gradient insulation, and the plasma step gradient fluorination modification of epoxy resin was realized on the basis of traditional fluorination modification. The results show that the surface morphology, chemical composition, and electrical properties of epoxy resin exhibit a step gradient distribution after plasma step gradient fluorination. Moreover, plasma step gradient fluorination can not only reduce the maximum surface field strength of epoxy resin, but also regulate the dynamic behavior of interface charge, and greatly improve the surface flashover performance of epoxy resin, the improvement effect is better than that of plasma uniform fluorination.

In recent years, ablation failure of high-voltage cable buffer layer occurs frequently. In order to solve the problem, this paper established a simulation model for XLPE cables and studied the electric field distribution characteristics of buffer layers under different resistivity. Based on the fault mechanism, a buffer layer repair scheme and a full set of on-site repair process were proposed. Repair tests were conducted on 220 kV high-voltage cross-linked polyethylene fault cables with a length of 1.2 m and 6 m, respectively. The repair effect was evaluated from the perspectives of contact resistance and capacitance current. The results show that with the increase of volume resistivity of the buffer layer, the electric field distortion between the buffer layer and the aluminum sheath is serious, which is easy to cause partial discharge and lead to cable failure. With the decrease of volume resistivity of the buffer layer, the electrical connection between the buffer layer and the aluminum sheath gradually recovers, and the electric field distribution tends to be uniform. After the injection of conductive repair medium, the resistance between the buffer layer and the aluminum sheath decreases by 41.67%, indicating that the electrical connection performance between the buffer layer and the aluminum sheath has been restored.

In order to study the effects of pressure and temperature on the conductivity of ethylene propylene rubber (EPDM) insulation for cables, the polarization current of EPDM under different electric field, temperature, and pressure were measured. The polarization index and conductivity were extracted, and the change law of the performance of EPDM was analyzed. The effect of pressure and temperature on the conduction mechanism of EPDM on pressure and temperature was studied based on the relationship between quasi-steady-state conductance current density and electric field intensity. The results show that the polarization index and conductivity are linearly related to temperature, but nonlinear related to pressure. This is because the conduction mechanism of EPDM changes under the influence of pressure and temperature, and the increase of temperature can promote the increase of the number of carriers in EPDM. The lower pressure limits the intermolecular movement of EPDM, while the higher pressure can destroy the molecular chain, resulting in an increase in electrical conductivity.

Reasonable prediction of the ageing state of cross-linked polyethylene (XLPE) cables and timely replacement of severely aged cables are very important for the stable operation of power grid. In this paper, XLPE insulated cable samples were subjected to accelerated thermal ageing test at 145℃.The microphysical and chemical properties of these samples were analyzed, and the correlation between dielectric parameters and microstructure of terahertz frequency range was established. The results show that with the increase of aging time, the crystallinity and thermal stability of XLPE decrease, while the carbonyl index increases, indicating that the ageing degree of XLPE is gradually intensified. In the terahertz frequency range, the real part of the dielectric constant is affected by the combination of multiple structural factors such as crystallinity and polar group content, resulting in an unclear relationship with ageing time. However, the change of the imaginary part of dielectric constant is mainly caused by the dipole high elastic polarization of polar groups, showing a trend of gradually increasing with the ageing time. After fitting, it is found that the imaginary part of dielectric constant is positively correlated with the carbonyl index. Therefore, the imaginary part of dielectric constant in the terahertz frequency range can well reflect the ageing degree of XLPE.

Based on the mechanical properties of the accessory silicone rubber material, a mainstream accessory on the market at present were conducted accelerated thermal ageing test, and then the tensile properties and thermal weight loss characteristics of the accessory silicone rubber were tested. The life model of silicone rubber was built based on its breaking elongation retention rate by using Arrhenius formula. The results show that the mechanical properties of silicone rubber decrease after thermal ageing, which is manifested by the increase of elastic modulus and the decrease of tensile strength and elongation at break. When only consider the thermal ageing of silicone rubber material, the theoretical operating life of cable accessories is 13.0–44.5 years.

In order to study the influence of the relative dielectric constant of motor insulation structure on the electric field distribution at different frequencies, this article took the relative dielectric constant of a main insulation structure at different frequencies as a variable to calculate its influence on the electric field distribution of the main insulation sandwich structure in the slot and the notch. At the same time, the optimization measures for the electric field of slot were analyzed. The results show that in the sandwich structure of main insulaton, because the film has a lower dielectric constant, it bears a larger electric field than mica tape. When the relative dielectric constant of the air at the notch is constant, the electric field strength of the air at the notch decreases slightly with the decrease of the relative dielectric constant of the main insulation. The insulating paint in the small gap at the chamfer of the notch iron core can reduce the electric field strength of air; without insulatin paint, the small gap generated by chamfer can produce a larger electric field strength of air. Vacuum potting of the notch can effectively reduce the electric field strength of air at the notch.