With the development of UHV DC transmission system engineering in China, the voltage level of converter transformers is getting higher and higher, and insulation problems are becoming more and more prominent. The study of partial discharge is of great significance to the insulation condition judgment and equipment life evaluation of converter transformer. In this paper, the insulation characteristics, the process of field tests, and the detection methods of partial discharges and their precautions of converter transformer were introduced, the research achievements of experts and scholars at home and abroad on the partial discharge of converter transformer were reviewed, and the research directions of partial discharge of converter transformer were prospected, which provided a reference for the operation, maintenance, and research of converter transformers.

Because the terahertz wave has strong penetrating ability, high imaging resolution, and strong anti-interference ability, the non-destructive testing technology based on terahertz reflection imaging has become a research hotspot in the field of insulator defect detection. In this paper, the propagation characteristics of terahertz wave in medium were firstly introduced, and a propagation model was constructed to analyze the terahertz wave refraction and reflection phenomena at the defect interface caused by the discontinuity of medium existing a defect. Then the finite difference time domain (FDTD) method of electromagnetic wave was elaborated, and geometric modeling and simulation of silicone rubber material was conducted by using a full wave 3D electromagnetic simulation software on the basis of the equation that electric field varies with time based on FDTD theory. It was found that the distribution difference of terahertz waves observed on the observation surface which was 2.5 mm away from the center of model, can reflect the size and location of internal defect, which provided a simulation method for theoretical studies. Finally, the academic cases of using terahertz imaging technology to detect internal defects in insulators were reviewed, and the problems that the detection distance affects the sensitivity, the background noise affects the imaging resolution, and the identification of defect types were analyzed and prospected.

Biaxially oriented polylactic acid (BOPLA) films were prepared by twin-screw extruder and static stretching machine, and they were buried in outdoor soil. The natural degradation process and mechanism of the BOPLA films were studied by analytical balance, Fourier transform infrared spectoscopy (FI-IR), scanning electron microscope (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The results show that the BOPLA film degrades slowly due to its dense surface at the initial degradation stage. After degrading for 10 months, the dense layer on the surface is eroded seriously, and the degradation goes deep into the film, the degradation is accelerated. The crystallinity, grain size, glass transition temperature, and other properties, which affected by the degradation of crystalline and amorphous regions, reach the peak value after degrading for 3 months, and then show a fluctuating downward trend.

In order to study the performance changes of silicone rubber and fluorinated silicone rubber after freezing at extremely cold temperature, the two materials were put into a low-temperature test box to freeze for 0, 150, 300, 450, 600, 750, 900, 1 050 h, and the temperature was set as -50℃. After freezing, the samples were taken out and stood for 2 hours at room temperature, and then the changes of breakdown voltage, maximum tensile force, tensile strength, hardness, and surface topography of the materials with freezing time were measured, and the principle were analyzed. The results show that with the increase of the low temperature freezing time, the breakdown voltage, maximum tensile force, and tensile strength of silicone rubber and fluorinated silicone rubber increase, while the hardness is almost constant. There are cracks on the sample surface after freezing for 300 h, there are obvious cracks and holes after freezing for 750 h, and the crack depth continues to deepen after freezing for 1 050 h. Throughout the freezing process in the extremely cold environment, the performance of fluorinated silicone rubber is better than that of silicone rubber, which indicates that the performance of fluorinated silicone rubber is more stable than that of silicone rubber in extremely cold environment.

The boron nitride nanosheets (BNNS) was surface modified by γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560), and then epoxy resin/functionalized BNNS (EP/BNNS-KH560) composites were prepared by doping BNNS-KH560 into epoxy resin matrix. The structure and morphology of the functionalized BNNS were measured, and the properties of EP/BNNS-KH560 composites were studied. The results show that the surface of BNNS is grafted with KH560 successfully, which exhibits a transparent thin layer structure under transmission electron microscope. The addition of BNNS-KH560 with a wide energy level gap can maintain the high insulating properties, excellent dielectric properties, and mechanical properties of the composites, and improve their thermal stability. Additionally, the BNNS-KH560 can form a good heat conduction channel in epoxy resin matrix, and improve the thermal conductivity of the composites effectively. When the mass fraction of BNNS-KH560 in composites is 20%, the thermal conductivity of the composites can reach 0.51 W/(m·K).

In this paper, low density polyethylene (LDPE) was used as the matrix material, boron nitride nanosheets (BNNs) was selected as the first doping filler, magnesium hydroxide (Mg(OH)₂) was selected as the second filler, and an insulating composites with multi filler structure was prepared by melt blending method. The effects of two filler contents on the thermal conductivity of the composites with different thickness were studied under normal temperature and pressure. The results show that the Mg(OH)₂ can change the orientation and connection of BNNs in matrix, so as to affect the thermal conductivity of composites. High amount of Mg(OH)₂ can enhance the axial thermal conductivity of the composites with three thicknesses. The addition of Mg(OH)₂ is not conducive to improve the radial thermal conductivity of the composite with thin thickness. Appropriate amount of Mg(OH)₂ can enhance the radial thermal conductivity of the composite with thicker thickness. When the mass fraction of BNNs is 20%, the mass fraction of Mg(OH)₂ is 40%, the thermal conductivity of the composite can reach 12 times bigger than that of pure LDPE.

Aiming to the copolymerized polypropylene (PP) material with low transparency, an electrical tree initiation voltage test system based on high frequency current sensor (HFCT) was designed. It was confirmed that the test system could detect partial discharge signal as low as 5 pC, and the test sensitivity was higher. Through the identification test of partial discharge signal, it was found that the main spectra of discharge caused by electrical tree initiation located at 12.9 MHz, which were different from the other disruptive partial discharge signals probably generated in test system. It was proved that the obtained partial discharge signal with a specific frequency characteristic was caused by the electrical tree initiation of sample. The transparent XLPE sample was conducted verification experiment, it was found that the electrical tree initiation voltage measured by the test system was almost the same as that measured by microscopic observation system, which confirmed the practicability of the electrical tree initiation voltage test system based on HFCT. Then, the effects of three voltage stabilizers of AOHBP, RBBT, and RQCT on the AC electrical tree initiation voltage of copolymerized PP material were investigated, respectively. The results show that three kinds of voltage stabilizers can all increase the initiation voltage and quenching voltage of partial discharge in the process of electrical tree initiation of PP with appropriate addition amounts. The optimal components are PP+0.6 phr AOHBP, PP+0.4 phr RBBT, and PP+0.6 phr RQCT, respectively. Among them, PP+0.6 phr AOHBP has the best inhibiting effect of electrical tree initiation, making the initial voltage of electrical tree increase by 33.0%.

In order to evaluate the impregnating resin for driving motor of new energy vehicles, epoxy resin and polyester imide resin were selected as research objects. The properties of the two impregnating resins were compared by thermal weight loss and bonding strength tests, and their long-term ageing characteristics were compared emphatically. The results show that the VOC content and long-term thermal resistance of the two impregnating resins meet the requirements. The bonding strength of epoxy resin is higher than that of polyester imide at 25℃, but decreases rapidly at 150℃ and 180℃. Ageing at 200℃ for 2 000 h, high and low temperature impact for 600 cycles, high and low temperature for 8 cycles under ATF oil immersion, high and low temperature for 8 cycles under ATF oil + 0.5% water closed immersion, and ageing at 150℃ for 2 000 h under ATF oil immersion experiments have similar effects on the bonding strength of the two impregnating resins. However, under the condition of ATF oil + 0.5% water closed immersion at 150℃, the bonding strength of epoxy resin is only 16.41%, 13.64%, 4.29%, and 4.29% of the initial value after aged for 500, 1 000, 1 500, and 2 000 h, while the bonding strength of polyester imide is higher than the initial value under the same condition. The structural and elemental changes of epoxy resin before and after its bonding strength decreasing significantly were compared by Fourier transform infrared spectroscopy and X-ray electron spectroscopy, it is inferred that the hydrolysis at high temperature is the main reason for the decline of bonding strength.

The polyimide (PI) film is widely used in power apparatus due to its excellent electrical properties, but space charge will accumulate in PI film to result in the degradation of its insulating properties. In order to obtain the PI film with excellent insulating properties, it is necessary to further study the dynamic accumulation and migration process of space charge inside PI film and optimize its internal structure. On the basis of bipolar charge transport model, the space charge distribution of PI film under DC voltage was analyzed, and the effects of electric field intensity and temperature on the space charge distribution characteristics of PI film were discussed. The results show that the space charge accumulated in PI film increases with the extension of applied voltage time. After applying the voltage for 180 min, the anode and cathode accumulate 0.635 C/m³ and -0.712 C/m³ of charge, respectively. The existence of space charge causes serious distortion of potential inside the PI film. The increase of electric field strength and temperature make the charge obtain more energy to overcome the interface barrier and inject into the film, so that the film accumulates more space charge, and the electric field distortion is more serious. The distortion rate of electric field inside the film reaches 0.968% at 308 K. Because the migration rate of charges becomes faster with the increase of temperature and electric field strength, the distribution of space charge in the PI film is more complicated under strong electric field and high temperature field.

The operating environment of EMU is complex, under the influences of multiple factors such as electricity, heat, and high-speed airflow, the performance of insulating material for high-voltage cable terminal on the roof is an important factor affecting the safety and stability of power supply system for EMU. In this study, the thermal conductivity, glass transition temperature, and dielectric properties of typical insulating materials for cable terminal on the roof were tested and analyzed by combining experimental analysis with simulation calculation. The electric field distribution at room temperature and the temperature distribution at different ambient temperatures of the cable terminal were studied by establishing an electric-thermal coupling simulation model of the roof cable terminal. The results show that the thermal conductivity of both ethylene-propylene-diene monomer (EPDM) and epoxy resin (EP) increases with the increase of temperature, and decreases with the increase of temperature when the temperature is higher than 80℃. At room temperature, the thermal conductivity of EPDM and EP is 0.251 W/(m·K) and 0.431 W/(m·K), respectively. The dielectric constant of EPDM decreases gradually with the increase of temperature, decreasing from 3.7 at -60℃ to 3.28 at 100℃. The dielectric constant of EP increases gradually with the increase of temperature, increasing from 5.02 at -60℃ to 5.29 at 100℃, which is due to the difference of their glass transition temperature. The simulation results show that the change of electric field at the terminal of AC cable caused by temperature is small, the maximum distortion point occurs at the root of stress cone, and the electric field is 3.12 kV/mm. The secondary electric field concentrates in the main insulation of cable, and the electric field is 2.9 kV/mm.

Electrical tree is a common type of insulation deterioration in power cables. The growth of electrical trees will change the performance of insulating materials, thereby threatening the safe operation of cables. However, traditional detection methods are often unable to realize effective detection of the tiny electrical trees deteriorated area in cable. A 10 kV single-core cross-linked polyethylene (XLPE) was selected, and electrical tree defects were introduced into its insulation. The insulation conductivity and equivalent capacitance between the inner and outer semiconducting layers at different growth stages of electrical trees were measured, and the electrical trees deteriorated area was tested and located by broadband impedance spectroscopy and inverse Fourier transform method. The results show that the growth of electrical trees increases the conduction current of the cable insulation significantly. The increase of electrical tree cumulative damage makes the capacitance value between the inner and outer semiconducting layers increase. Due to the change of conductivity and capacitance, the wave impedance of the electrical tree deteriorated area distorts. Using the broadband impedance spectroscopy and inverse Fourier transform methods can realize the accurate location of the electrical tree deteriorated area. At the same time, the different growth stages of electrical tree will significantly affect the amplitude of distortion points in the impedance spectroscopy and inverse Fourier transform spectrum.

The dry and cold environment of the Qinghai-Tibet Plateau is easy to make the insulators of railway catenary ice-coated, causing serious ice flash accidents. To study the operating status of ice-coated insulators in catenary, a two-dimensional ice-coated model of cantilever insulators was established according to the winter environmental characteristics of Golmud station on the Qinghai-Tibet railway, and the porcelain insulator XP-160 and composite insulator FXBW-110/100 were used as contrast. The spatial electric field distribution and surface temperature distribution characteristics of different types of ice-coated insulators were analyzed by electrostatic field and Joule heat field simulation. Finally, the shed structure of the insulator was optimized. The results show that as the thickness of ice layer and the length of ice crystal increase, the field intensity of each insulator increases gradually, and the surface temperature continues to rise. When the insulator is bridged by the ice crystal, the field intensity decreases, which is slightly lower than the field intensity of clean insulator. The increase of arrangement angle for the cantilever insulator will lead to the increase of electric field, but it has little effect on the temperature. Therefore, the arrangement angle of cantilever insulator should decrease during installation. The insulators with one large and two small shed structure can effectively reduce the increase of field intensity and suppress the temperature rise.

Composite insulators have been widely used in power transmission lines. In recent years, there are many grid accidents caused by insulator core rod deterioration. It is urgent to find out the influence factors and rules of composite insulator core rod deterioration. In order to study the influences of different voltage applied time and environmental humidity on the core rod deterioration of composite insulator under uniform electric field, the electric field distribution generated by the dielectric barrier discharge method was simulated and analyzed. An electrical ageing experiment platform was designed and built, and the core rod samples were conducted 200 h, 400 h, and 800 h of electrical ageing tests at power frequency under dry and humid environments. The physical and chemical properties of new sample and the aged samples were tested, and the corresponding influence laws were analyzed. The results show that after electrical ageing, the epoxy resin content in the core rod decreases. When the ageing time is less than 400 h, the epoxy resin content under humid and dry environments has little difference. When the ageing time increases to 800 h, the epoxy resin content under humid environment is significantly lower than that under dry environment.

In this paper, the positive and negative DC breakdown voltage of ester insulating oils and mineral insulating oil in extremely uneven field under small and medium gap were studied, and then compared with their lightning breakdown voltage. The results show that the DC breakdown voltage of ester liquids is lower than that of mineral insulating oil. The negative DC breakdown voltage is lower than the lightning breakdown voltage under small gaps, but it is higher than the lightning breakdown voltage under medium gaps.

In order to clarify the electric strength test conditions of arc-extinguishing nozzle materials, an electric strength test model of nozzle materials was established by using COMSOL finite element analysis software, and the electrostatic field distribution of the test model were calculated under different electrode shapes, environmental media, and sample thicknesses. The results show that the spherical electrodes and equal-diameter electrodes are better than non-equal-diameter electrodes, the best environmental media is transformer oil, and the sample thickness of 1 mm is better. By comparing the electric strength test results of the nozzle material samples with 1 mm of thickness in transformer oil measured with spherical electrodes and equal-diameter electrodes, it is found that the electric strength of the samples measured with spherical electrodes is higher, and the dispersion is smaller, the standard deviation is only 1.172 kV/mm. The research results provide a theoretical basis for the selection of electric strength test conditions for nozzle materials.

To realize the development of fluorescent optical fiber partial discharge detection system, according to the light radiation characteristics of insulator creeping discharge, the key performance and matching efficiency of the fluorescence photoelectric measurement system were compared and analyzed. The main components, characteristic spectral lines, and spectral statistical changing with discharge intensity of the insulator creeping discharge spectrum were obtained through experiments. Combined with the emission spectrum of insulator creeping discharge, excitation spectrum of fluorescent fiber, and capture efficiency of the fiber, the fluorescence excitation efficiency of two fluorescent fibers for partial discharge was compared and analyzed, and the excitation intensity and excitation efficiency of two typical PS fluorescent fibers were obtained. The results show that the fiber with 299-477 nm of excitation spectrum is better than the fiber with 294-410 nm of excitation spectrum. The R3896 PMT and fluorescent fiber system has better matching efficiency, and the SNR of PMT reaches a peak value of 12.45 dB under the optimal driving bias. Under the optimal bias voltage, the self-temperature drift effect of natural cooling system is approximately 7%, and the cooling recovery time is approximately 40 min.

In order to accurately evaluate the maximum impulse voltage which corona resistance enameled wire endured, the corona resistance time of enameled wire under different impulse voltage was tested respectively, and the optimal fitting relationship curve equation between them was analyzed. The results show that the corona resistance time decreases exponentially with the increase of impulse voltage. Because the corona resistance time in the fitting equation has practical physical significance, no matter how the corona resistance time changes, it is greater than zero, which can indirectly solve the maximum impulse voltage that the corona resistance enameled wire can endure.