In this paper, the space charge formation mechanism and space charge effect were systematically described. Three methods of constructing trap energy levels, including filling inorganic nanoparticles, doping small molecule semiconductors, and molecular structure design, were emphatically reviewed. The design strategy to improve the energy storage performance of the dielectric material were deeply discussed, and the preparation methods of high performance polymer dielectric materials were prospected.

In this paper, combined with the application of insulation varnishes on the market in traction motors, the research status and development trend of insulation varnishes for traction motors at home and abroad were reviewed. The insulation varnish systems developed from 180-class (H) diphenyl ether system to the 200-class (N) epoxy anhydride system, and then to the 220-class (R) silicone insulation system, indicating that the insulation varnish system for traction motors were developing towards high heat resistance direction, and the future development and application trend was still high heat resistant insulation varnish systems.

In order to improve the adhesion of thermoplastic polyimide (TPI) between thermosetting polyimide (PI) film and copper foil in two-layer flexible copper clad plate (2L-FCCL), a series of TPI samples were prepared by one-step reaction using flexible aromatic dianhydride monomers and aromatic or alicyclic diisocyanate monomers. Then, the selected TPI solution was directly coated on the pretreated commercial thermosetting PI film to obtain the composite film, and the corresponding 2L-FCCL was obtained by hot pressing at high temperature. By observing the state of the TPI reaction solution and conducting DSC testing, TPI-8 is selected, which is soluble in organic solvent and has a T_g of 226.5℃. Then, TGA, TMA, and tensile tests show that the 2% thermal decomposition temperature (T_2%) of TPI-8 is 464℃, the thermal expansion coefficient (CTE) of 50-200℃ is 64.25×10^-6℃^-1, the tensile strength is 64.52 MPa, the elastic modulus is 1.75 GPa, and the elongation at break is 62%, and its chemical structure is confirmed by FTIR. Finally, the effect of alkaline pretreatment time of two kinds of thermosetting PI films on the bonding properties of resulting FCCLs was studied. It is found that the 90° peel strength (90°PS) of FCCLs are over 0.7 N/mm and the maximum value is 0.94 N/mm after SPI films treated with alkaline water at 60℃ for 1 min and 3 min. However, the 90°PS of FCCLs obtained by UPI films treated with the same hot alkaline water for 1-7 min are all less than 0.6 N/mm. Therefore, this method can be used to quickly and simply prepare TPI by one step, which is soluble and has good thermal, mechanical properties and dimensional stability. Meanwhile, it has good bonding properties for copper foil and SPI that the surface was pretreated with alkali water, and meets the requirements of relevant FCCL standards.

Polyvinylidene fluoride (PVDF)-based polymer has promising applications perspectives in the field of high energy storage density and high pulse energy storage. In this paper, barium titanate (BTO) nanoparticles were doped with polyvinylidene fluoride-chlorotrifluoroethylene (P(VDF-CTFE)) solution, and P(VDF-CTFE)/BTO composite films were prepared by solution casting method. The effects of different BTO doping contents on microstructure, dielectric properties, and energy storage characteristics of the composite films were investigated. Then the BTO was modified by dopamine (DA) to enhance the compatibility between BTO and polymer matrix and improve the dielectric constant and electric strength of composite films. At the same time, on the basis of the "breakdown blocking effect" between the films layers, the laminated composite films were prepared to enhance the energy storage properties. The results show that using the P(VDF-CTFE)/DA@BTO composite film with a DA modified BTO mass fraction of 10% as the interlayer, and the P(VDF-CTFE) as the outerlayer, the prepared PV-BT-PV laminated sandwich composite film has a dielectric constant of 10.44, a maximum electric strength of 362.25 kV/mm, and a charge discharge efficiency of 86.63% at an electric field strength of 500 kV/cm.

In this paper, tetracyano-benzoquinone dimethane organic small molecules with narrow band gap were introduced, and polyethylene/tetracyano-benzoquinone dimethane composites were prepared by solution blending. The effects of doping content of tetracyano-benzoquinone dimethane on DC breakdown properties, high field conductance and trap distribution properties of the composite were investigated. The results show that the electric strength of the composite increases first and then decreases with the increase of the doping content of tetracyano-benzoquinone dimethane, the addition of tetracyano-benzoquinone dimethane increases the electric strength of polyethylene by a maximum of about 54%, and effectively decreases its conductivity. Trap distribution measurement and quantum chemical simulation confirm that tetracyano-benzoquinone dimethane introduces charge deep traps, enhances the trapping effect of charge carriers, and improves the DC electric strength of polyethylene.

In order to improve the cracking resistance of the encapsulating insulating material for dry-type air-core reactor, it is necessary to improve the toughness of the epoxy resin composite while ensuring the good electrical properties. In this paper, an epoxy glass fiber composite with good mechanical and electrical properties was obtained by adding different content of epoxy-terminated liquid nitrile butadiene rubber (ETBN), different content and partical size of Al₂O₃ powders to the epoxy glass fiber (EP/GF) composite, and its structure and morphology, mechanical and electrical properties were tested. The results show that when the mass fraction of ETBN, 5 μm Al₂O₃, and 100 nm Al₂O₃ is 10%, 30%, and 3%, respectively, the bending strength of the EP/GF composite reaches the highest of 291.0 MPa, which increases by 33.5% than that of the unmodified composite, and the electric strength is 24.3 kV/mm, the comprehensive properties are the best.

Room temperature vulcanized silicone rubber (PRTV) material is the most commonly used anti-pollution flashover coating in power system at present. In order to further improve the insulating performance of PRTV material, the boron nitride (BN) nanoparticles was modified by silane coupling agent KH570, and a BN/PRTV composite was prepared. The effects of BN doping content on the electrical properties and hydrophobicity of the composite material were studied. The results show that the surface of the BN nanoparticle is successfully grafted with coupling agent. With the increase of BN nanoparticle content, the section interface roughness of the composite increases and the mastoid structure appears. The introduction of BN nanoparticles reduces the dielectric constant of PRTV material, and introduces more deep traps inside PRTV, which inhibitis the charge detrapping ability and increases the flashover voltage of PRTV material. When the mass fraction of BN nanoparticles is 5%, the hydrophobicity of the composite is the best and the and flashover voltage is the highest.

The molecular chain structure has great influence on the electrical insulation properties of silicone rubber. In this paper, the regularity and mechanism of the blending of different ratios and types of blended silicone rubber on the electrical properties of composite silicone rubber were explored through a combination of experiments and molecular simulations. The results show that the electric strength of phenyl silicone rubber/vinyl blended silicone rubber (PMVQ/MVQ) improves significantly, when the mass fraction of PMVQ is 15%, the volume resistivity of PMVQ/MVQ composite is 1.04×10¹⁶ Ω·cm, and the electric strength is 106.87 kV/mm, which increase by 1.77 times and 13.22% than that of the vinyl silicone rubber. The dielectric properties of trifluoropropyl silicone rubber/vinyl blended silicone rubber (FMVQ/MVQ) improves, when the mass fraction of trifluoropropyl silicone rubber is 15%, the dielectric constant of FMVQ/MVQ composite is 3.74, which is 40.07% higher than that of vinyl silicone rubber. Molecular simulation results show that the free volume of PMVQ/MVQ decreases with the increase of phenyl silicone rubber content, and the free volume of FMVQ/MVQ increases with the increase of trifluoropropyl silicone rubber content, which is consistent with the breakdown phenomenon of composite silicone rubber.

In order to meet the stress grading requirements of the 18 kV hydro-generator in a large hydropower station with an altitude of 2 500 m, the air breakdown field strength and its influencing factors at the target altitude were analyzed theoretically, and the overhang electric field characteristics was evaluated by establishing a three-dimensional simulation model of stator winding overhang, the influences of the clearance distance and binding structure on the electric field at overhang hypotenuse were analyzed, the scheme of controlling the minimum clearance distance at the overhang hypotenuse of the stator winding to 15 mm was determined. Finally, the stress grading performance of 18 kV hydro-generator stator coil and winding was tested and verified in a large high-altitude environmental simulation laboratory. The results show that the stress grading performance of the 18 kV hydro-generator is good and can meet the stress grading requirements at an altitude of 2 500 m.

In order to investigate the impact of thermal ageing on the development of DC partial discharge of surface scratched cross-linked polyethylene (XLPE) cables, a surface scratched defect model weas made using a physical 10 kV cable, and subjected to the accelerated thermal ageing treatments with different times. The partial discharge characteristics of defective cables under DC voltage were studied using the pulse current method. On basis of the variation of partial discharge with voltage applying time, the statistical charts of the maximum discharge capacity, average discharges capcity, and discharge repetition rate in different discharge intervals were extracted. Then based on the charge transport theory, partial discharge and charge transport models were proposed to explain the partial discharge results of defective cable. The results show that the initial partial discharge voltage of the defective cable increases with the increase of thermal ageing time, the discharge quantity-time spectrum shows "comb" discharge characteristics. As the thermal ageing time increases, the average discharge capacity and discharge repetition rate of the defective cable in the interval greater than 100 pC decrease gradually, and the “comb" discharge characteristics disappear completely after 21 days of ageing, demonstrating the self-healing ability of partial discharge. During thermal ageing, the surface oxidation of XLPE scratched defects produces charge traps, resulting in a uniform and dense space charge layer, which reduces the internal electric field of the defects and weakens partial discharge in defective cables. The research results provide data support for the study of DC partial discharge of XLPE cable defects.

DC cable mainly uses oil-paper and crosslinked polyethylene (XLPE) as insulation, and the conductivity characteristics of the two materials are different, which has a significant effect on the electric field distributions in cable. In this paper, a finite element model was established based on the 500 kV typical DC cable structure, and the thermal-electrical coupled simulation analyses were performed on the oil-paper and XLPE insulated cables. The electric field distribution characteristics in the two insulation under different temperatures and temperature drops were studied, and the influences of the conductivity temperature and electric field coefficient on the field strength distribution were analyzed. The conductivity variations of oil-paper and XLPE insulations among the same temperature and field ranges were compared. The results show that the electric field distribution in oil-paper insulation is affected by the insulation temperature difference in the electric field strength range of 10-25 kV/mm and temperature range of 20-90℃, and hardly changes with the insulation temperature under definite temperature difference, while the electric field distribution in XLPE insulation is affected by both insulation temperature and temperature difference. In addition, the overall conductivity of oil-paper insulation is higher than that of XLPE insulation, and its change amplitude with temperature and field strength is larger, which may be the reason why the electric field inversion in oil-paper cable insulation occurs at a lower temperature difference. For the insulating materials used in HVDC cables, the conductivity temperature coefficient should to be controlled small so as to reduce the temperature sensitivity.

In the process of injection molding of the epoxy insulator on EMU roof, there will be two closing joints on the surface of the umbrella skirt along the vertical direction, the convex edges will distort the electric field near it. In this paper, the influence of the closing joint on the flashover path of the epoxy insulator on EMU roof under wet conditions was studied. Through the spray flashover test, the flashover probability between the closing joint and other parts of the umbrella skirt was compared, the motion characteristics of water droplets on the surface of the epoxy resin umbrella skirt were observed, and the effects of the closing joint on the formation of water band, electric field intensity, and current density distribution were analyzed by simulation. The results show that the flashover channel is easily form at the closing joint, and under the action of electric field, the water band near the closing joint will stretch or even bridge the adjacent skirt. The current density of water band at the closing joint of high voltage end umbrella skirt is more than one time higher than that at other parts of the umbrella skirt. The water band bridging skirt will distort the local electric field, decrease the creepage distance and increase the voltage value borne by the unit creepage distance.

Metallized polypropylene film capacitor has the characteristics of high reliability, self-healing, and low dielectric loss. However, it needs to withstand the combined action of strong direct current electric field and high temperature thermal field for a long time during the working process, which makes the space charge easy to accumulate inside the polypropylene dielectric film, resulting in the degradation of its electrical insulating properties. A self-healing model of metallized polypropylene film was established in this paper. The changes of self-healing area at the self-healing point under different self-healing development time and applied voltage were studied through the evaporation model of metal layer, and the changes of space charge density in the discharge channel under different self-healing development time, applied voltage, and temperature were studied by using the bipolar carrier transport model, so as to explore the microscopic mechanism of self-healing of metallized polypropylene film. The results show that there is a linear relationship between the evaporation area of metallized film and the self-healing development time. With the increase of the applied DC voltage and temperature, the charge migration process in discharge channel strengthens, and the electrons and holes in discharge channel are more likely to migrate and recombine with the charges of opposite polarity. With the self-healing process progresses, the charge amount injected into the discharge channel increases gradually, and the migration of charge to both ends of the discharge channel intensifies, resulting in uneven potential distribution in discharge channel and the electric field will distort in discharge channel.

The insulation resistance of the outer sheath of high-voltage cables is an important parameter for measuring the insulation state of high-voltage cables. In this paper, an online monitoring method based on metal sheath circulation current was proposed to evaluate the insulation resistance of the outer sheath of high-voltage cables, so as to realize the real-time monitoring of the insulation state of high-voltage cables. The formation of mental sheath circulation current and its relationship with the insulation resistance of outer sheath were analyzed, and a calculation model was derived. Then the online monitoring of high-voltage cable with single-end grounding and cross-linked grounding have been carried out. The results show that the error between the online monitoring method and the offline measurement method is less than 20%, and the measurement effect of the method is significantly better in the cross-linked grounding segments than in the two-sided cable segments, which is of great significance for the condition monitoring and fault location of the metal sheath of high-voltage cables.

The research on the pyrolysis gas generation law of cable can provide a basis for cable fire early warning. In this paper, a high temperature pyrolysis experiment platform was set up for common cables. The pyrolysis gas generation laws of PVC outer sheath of cables at 75, 100, 120, and 150℃, as well as the pyrolysis gas generation laws of cable internal materials at 185℃ and 225℃ were studied, respectively. The results show that PVC outer sheath of cables has been pyrolyzed at 75℃, and the pyrolysis gases are CO and CO₂. When the temperature is below 120℃, the pyrolysis gas components are only CO and CO₂, so CO and CO₂ are suitable as early characteristic gas components for commonly used cable fires. When the temperature is 150℃, gas components such as CH₄, C₂H₄, C₃H₆, and CH₃Cl appear in the pyrolysis gas, so the CH₄, C₂H₄, C₃H₆, and CH₃Cl can be used as the characteristic gas components of cable overheating faults.

In order to improve the accuracy of live detection of line porcelain insulator degradation, the influences of layout methods and deterioration degree on the voltage distribution characteristics of high-voltage AC insulator strings were studied in this paper. Firstly, a finite element simulation calculation model with single voltage distribution of AC 220 kV porcelain insulator strings under different layout methods was established, and the voltage distribution characteristics of the insulator strings were simulated and analyzed. Then, combined with the actual measured voltage distribution of 220 kV AC transmission line insulator string in Shandong province, the consistency between simulation calculation and actual measurement was verified. The results show that the tower type of transmission circuit, the number and layout method of the insulator string, and the position and deterioration degree of the degraded insulator have great effects on the voltage distribution characteristics and deterioration discrimination of the insulator strings. Among them, the maximum distributed voltage difference of different tower types can reach 35%, and the maximum distributed voltage difference of different layout methods can reach 18%. The deterioration situation of insulators at different positions can be judged by the distribution voltage drop ratio.

COMSOL finite element simulation software was used to study the effect of bird droppings with different morphological characteristics on sulator flashover. By studying the influences of the length and viscosity of bird droppings on the flashover and the process of insulator flashover caused by the bird droppings falling, the calculation method of the combined air gap breakdown voltage composed of multiple sections bird droppings with different viscosities was obtained. The results show that the higher the viscosity of bird droppings, the better the continuity of bird droppings, the lower the breakdown voltage of the air gap, which is easy to cause the composite insulator flashover. Therefore, in order to improve the protection ability of anti-bird shield to high viscosity long bird droppings. the anti-bird shield must have the ability to reduce the length of bird droppings. In this study, a deflector groove was designed on the surface of anti-bird shield, and the high viscosity long bird droppings can be divided into several sections of short bird droppings through the deflector groove. By reducing the length of bird droppings, the air gap length increases, so as to increase the breakdown voltage and decrease the probability of bird droppings flashover.