To resolve the issue that the traditional frequency domain reflection (FDR) method is easy to misjudge when diagnosing the moisture defects of cable joints, the influence law of thermal excitation on the characteristic impedance of moistened cable joints was firstly analyzed in this paper, and a diagnostic method for moisture in cable joints was proposed based on the frequency domain reflection characteristics under thermal excitation. Then a moistened joint was set up on a 10 kV XLPE cable with a length of 58 m, the temperature of the moistened joint was changed using a heating belt, and the moisture defects were located and diagnosed. The results show that when applying the thermal excitation, the unit capacitance and characteristic impedance of the cable body and normal joint hardly change with the temperature, the unit capacitance of the moistened cable joint decreases with the increase of temperature, and the characteristic impedance increases with the increase of temperature. The simulation results show that the amplitude of the frequency domain reflection spectrum at the moistened joint increases with the increase of temperature, while the amplitude of the frequency domain reflection spectrum at the normal joint is basically unchanged. Based on the above research results, the amplitude changes of the true cable reflection coefficient spectrum at the joint under different temperatures were obtained through thermal excitation, achieving the localization diagnosis of moisture defects.

The 110 kV epoxy resin dry type transformer has the phenomenon of flashover in the airway on the high voltage side due to its high electric field intensity. The high voltage side coil of transformer was modeled and simulated by finite element simulation software in this paper, and the standard lightning impulse was applied to verify the lightning impulse. According to the simulation results, the causes of flashover were analyzed and the field strength distribution at the airway was optimized. The results show that the electric field distribution is improved by adding a specific medium to the corresponding area inside the epoxy resin. At the same time, by changing the relative dielectric constant and position of the medium, the maximum field strength at the airway is reduced by 8.75% under the restriction of field strength inside the transformer, which meets the field strength requirement of the high-voltage side airway of the dry-type transformer to inhibit flashover.

To investigate the influence of the filler network constructed by alumina (Al₂O₃) on the thermal and dielectric properties of polypropylene (PP) composites, this study constructed a finite element model of randomly filled Al₂O₃ fillers, and systematically studied the effects of factors such as filler content, filler particle size, and size matching of binary fillers on the thermal conductivity and dielectric constant of Al₂O₃/PP composites. The results indicate that increasing the filler content can significantly bridge the Al₂O₃ filler, synergistically construct an interconnected thermal conductive network and electric displacement pathway, thereby significantly improving the thermal conductivity and dielectric constant of Al₂O₃/PP composites. For single Al₂O₃ filled PP composites, the thermal conductivity and dielectric constant of composites cannot be effectively enhanced by regulating the size of Al₂O₃ filler. After introducing the multiscale fillers, at the optimized binary filler ratio of 70∶30 (40 μm∶15 μm), the overall thermal conductivity and electrical displacement networks show the dominant skeleton of large-sized filler and the bridging branch of small-sized fillers features, which synergistically contributes to the Al₂O₃/PP composite reach the optimal thermal conductivity (0.55 W/(m·K)) and dielectric constant (5.6).

In order to study the change rule of microstructure and dielectric properties of polyethylene (PE) materials under the action of thermal stress, PE samples were conducted accelerated thermal ageing tests at 90℃, and the microstructure and charge transport properties of the thermal aged PE samples were characterized by Fourier transform infrared spectrometer (FTIR), differential scanning calorimetry (DSC), X-Ray diffraction (XRD), high field conductivity (HFC) test, and isothermal surface potential decay (ISPD) tests. The results show that the thermal ageing process of PE samples can be divided into recrystallization stage and thermo-oxidative degradation stage. In the recrystallization stage, the crystallinity of the samples increases, the crystal structure tends to be perfect, the deep-trap density and deep-trap energy level increase, the carrier migration is restricted, and the threshold field strength of space charge injection increase. In the thermo-oxidative degradation stage, the crystallinity of the samples decreases, the crystal structure appears deterioration, the deep-trap density and deep-trap energy level decrease, the carrier mobility increases, and the threshold field strength of space charge injection decreases. Through the characterization of conductivity properties under different temperatures and field strengths, the variation rules of three parameters A, B, and φ in Steven Boggs’ conductivity function were obtained. With the increase of ageing time, the φ and A increase at first and then decrease, and the B decrease at first and then remains unchanged, among which the material coefficient A is the most sensitive to thermal ageing.

Oil-paper insulation is the main insulation material for oil-immersed power equipment, and its ageing under long-term complex working conditions will seriously affect the use safety of equipment. Therefore, the accurate evaluation of its insulation status is crucial. Multiple sets of oil-paper insulation models with different degrees of ageing were prepared in this paper, and wide temperature-wide frequency dielectric response tests at different test excitation amplitudes were conducted. Characteristic parameters characterized the ageing of oil-paper insulation was extracted on the basis of Disado-Hill relaxation model. The test results show that as the ageing of oil-paper insulation increases, the full frequency range loss factor curves show two characteristic frequency ranges, which are respectively related to the material conductivity characteristics, turning polarization and interface polarization process. According to the fitting calculation results, it is found that after ageing of the oil-paper insulation, the cellulose structure is damaged, and the number of impurity ions in the dielectric increases, which is manifested at the micro level as enhancing the inter cluster motion within the dielectric and weakening the intra cluster motion. Therefore, the characteristic frequency points in the model move towards higher frequencies. Meanwhile, to accurately obtain the ageing state of oil-paper insulation, a quantitative characterization relationship between model characteristic parameters and insulation ageing was constructed by eliminating the influence of test temperature and excitation amplitude. The research results provide theoretical support for the evaluation method of insulation ageing state of oil-immersed power equipment based on frequency domain dielectric response on-site detection technology.

To explore the thermal mechanical stress distribution of motor insulation structure at high temperatures, we take the insulation structure of organic silicone system as the research object, test its thermal expansion coefficient and elastic modulus at different temperatures, and analyze the influence of thermal stress parameters at different temperatures on the thermal mechanical stress of organic silicone insulation system. The results show that within -50-200℃, the thermal expansion coefficient of insulation structure show a change trend of increases, decreases, and increases again with the increase of temperature, and its value is 0.8×10^-5-1.7×10^-5 K^-1. The elastic modulus increases at first and then decrease with the increase of temperature, and its value is 0.2×10³-3×10³ MPa. The thermal mechanical stress borne by insulation structures increases linearly with the increase of thermal expansion coefficient and elastic modulus. The thermal mechanical stress of insulation structures firstly increases and then decreases with the increase of temperature, depending on the size of thermal expansion coefficient and elastic modulus at different temperatures. The feasibility of testing parameters and simulation analysis was verified through stress testing of the wire rod and motor, and the deviation between simulation value and testing value is within 15%. The simulation analysis results show that at 120℃, the maximum thermal mechanical stress point of motor insulation is located at the slot insulation, and the maximum stress value is 11.37 MPa. After sealing, the maximum thermal stress value of the slot insulation can be reduced by about 45%.

High voltage cable is an important part of power supply system. Buffer layer ablation can lead to power cable failure. In recent years, several cable accidents caused by buffer layer ablation have occurred. In order to solve the above problems, a multi-parameter detection method for cable ablation defects was proposed based on buffer layer ablation characteristic gases, temperature and pressure variation. The CH₄, C₂H₆, and C₂H₄ content in the buffer layer was analyzed by Fourier transform infrared spectroscopy. The H₂ content in the buffer layer was measured by electrochemical sensor. The temperature and barometric pressure in the buffer layer were measured by temperature sensor and piezoelectric barometric pressure sensor. The performance of the system was tested by laboratory buffer layer ablation experiment and actual cable detection experiment. The results show that the functionality and accuracy of the system meet the requirements, and the four characteristic gases content, temperature, and pressure can be used as the parameters to characterize the buffer layer ablation defects. The detection period of the system is less than 30 s, and the detection error is within ±10%. The system has advantages in terms of safety, detection speed, and accuracy, which can guarantee the safe operation of power cables.

With the increase of grid capacity and voltage level, the insulating properties of epoxy composite insulation materials is crucial to the safe and stable operation of electrical equipment. Long-term high temperature and high humidity will lead to a serious decline in insulating properties of materials. Nano-SiO₂ was fluorinated (F-SiO₂) by the plasma technology of dielectric barrier discharge (DBD) in this paper, and fluorine-containing groups were successfully grafted on the surface of nano-SiO₂. The high-temperature and humid state of the material was simulated by the wet heat ageing experiment, and the surface flashover, charge dissipation characteristics and trap distribution characteristics of the samples at different ageing stages were tested. The effect of doping F-SiO₂ on the wet heat ageing resistance of epoxy composites was explored, and the inhibition mechanism of F-SiO₂ on the water intrusion was revealed from the molecular scale in combination with MD simulation. The experimental results show that doping F-SiO₂ reduces the saturated moisture absorption of epoxy composites by 16.16%, increases the surface flashover voltage by 13.06%, and can continue to maintain high insulating properties after wet heat ageing. The simulation results show that F-SiO₂ can reduce the free volume fraction of epoxy resin and the mean square displacement of water molecules, and enhance the barrier effect of epoxy composites on moisture. Plasma technology can graft fluorine elements onto the surface of nano-SiO₂, effectively inhibiting the intrusion of moisture and enhancing the surface insulating properties of epoxy composites in a humid state.

After the occurrence of line tree barriers, it is difficult to successfully reclose the line, which can easily cause line shutdown. There are multiple major power outages caused by tree barriers both at home and abroad, but the electric field distribution characteristics and discharge mechanism of insulated conductors with different types of tree barrier are still unclear. Therefore, electrostatic field models of insulated conductors with different types of tree barrier were established on the basis of finite element theory in this paper, and the erosion mechanism of tree barrier discharge was revealed according to the obtained electric field distribution and gas-solid discharge theory. The results show that the tree barrier types of insulated conductor are mainly divided into four categories: single side wedge-shaped penetration defects, flat cut defects, double wedge-shaped penetration defects, and critical contact between wood throns and defects tree barrier-wire types. The electric field undergoes distortion at the defects of tree barrier, and the wedge-shaped penetration defects can cause gas breakdown and discharge at the site of tree branch-insulation wire, which poses the greatest damage to the surface of insulation wire and the erosion of tree branches. The field strength at the defect site is affected by the degree of defect and air gap, and the maximum electric field strength of the wood thorn penetrating into the insulation layer is 2.64 times higher than that of the wood thorn penetrating only into the shielding layer. The wind disturbance causes the critical contact between the wood thorn and defect to generate air gap, and its electric field is 2.54 times higher than that without air gap (5.65 kV/mm). The research results provide a theoretical basis for different types of tree barrier defense methods, avoiding the drawbacks of cutting tree indiscriminately.

To investigate the impact of interface air gap and moisture defects on the multilayer composite dielectric insulation interface of ±320 kV DC cable intermediate joints on their electric field distribution mechanism, a simulation model of ±320 kV DC cable intermediate joint was built, and the electric field distribution of the cable joint with air gap and moisture defects on the multilayer composite dielectric interface region was calculated using the finite element method. The effect of these defects on the electric field distribution of the cable joint were analyzed at no load and load conditions, and their differences under AC/DC electric fields were discussed. The results show that there is significant electric field distortion at the composite dielectric interface of the DC cable joint due to air gap and moisture defects under the action of DC electric field. The multiple of electric field distortion of air gap defects is 9.6 times at no load and 1.7 times at load condition. The electric field strength in the water film defects decrease over 99.9% at both operating conditions. Interestingly, the electric field distortion induced by air gap defects in cable joints at no load is larger than that at load condition, and the temperature differentials across the joint play a role in field homogenization. However, the temperature differentials across the joint induced by moisture at load promote the accumulation of space charge in the joint, which leading to the multiple of electric field distortion at moisture defect bigger than than at no load. Notably, the multiple of electric field distortion resulting from defects in DC cable intermediate joint is bigger than that of AC cables.