To solve the problem of poor thermal conductivity of polyimide (PI), PI was doped with hexagonal boron nitride (h-BN) modified by a surface wetting agent (F068) through in-situ polymerization, and then a series of h-BN/PI high thermal conductivity composites were prepared. The structure of h-BN/PI composites was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. The thermal conductivity mechanism of h-BN/PI composites was explored by introducing the Y.Agari model, and the influence of modified h-BN filler doping content on the thermal conductivity, mechanical, heat resistance, and insulating properties of the composites was analyzed. The results show that with the increase of modified h-BN doping content, the thermal conductivity and thermal stability of h-BN/PI composites increase, while their tensile strength, elongation at break, and electric strength exhibit a significant downward trend. When the doping amount of modified h-BN is less than 30%, the influence of polymer crystal size factor on the thermal conductivity of the composites is dominant. When the doping amount of modified h-BN is higher than 30%, the influence of the free factor of the thermal chain formed by thermal conductive particles on the thermal conductivity of the composites is dominant. When the doping amount of modified h-BN is 50%, the thermal conductivity of h-BN/PI composite reaches 0.83 W/(m·K), the electric strength is 198.6 kV/mm, the 5% thermal decomposition temperature is 595℃, the tensile strength is 64.8 MPa, and the elongation at break is 10.4%.

High sensitivity defect detection technology is urgently needed to evaluate the health status of gas insulated metal enclosed switchgear (GIS) insulator. In this paper, a defect detection method for GIS basin insulator based on ultrasonic scanning frequency signal injection was proposed. The propagation law of scanning frequency ultrasonic signals on clean/defective basin insulators were investigated by simulation and experiment, and three defects including internal bubbles, surface cracks, and surface metal particle attachment were considered. In the simulation and experiment, the ultrasonic emission and reception probes were located opposite the flange of the basin insulator. The ultrasonic scanning signal was a frequency conversion sine wave with an amplitude of 1 V and a frequency range of 0.5-1 MHz, which was energized by the transmitting probe and injected into the basin insulator through a flange, and the ultrasonic signal was received by the receiving probe on the opposite side. The results show that the ultrasonic signal directly propagates through the basin insulator to the receiving piezoelectric probe, and the amplitude of the time-domain ultrasonic signal received on the defective insulator is higher than that on the clean insulator, but it is still difficult to be used as a basis for evaluating the health status of the insulator. The main frequency of ultrasonic signals propagated through detective insulators is lower than that propagated through clean insulators. The main frequency signals propagated through clean, crack containing, metal particle defective, and bubble defective insulators are 24.00, 14.83, 10.51, and 12.13 kHz, respectively, which can be used as the basis for defect detection of GIS basin insulators.

The salt alkali resistance modification of polyurea protective coating of the composite pole can improve the operation reliability of the composite pole in heavily polluted area. In this paper, the surface treatment of montmorillonite was carried out with KH550 coupling agent, and the salt spray protection effect of montmorillonite modified polyurea resin with different contents on pole composite materials was analyzed. The results show that after 28 days of salt spray ageing, the properties stability of montmorillonite modified polyurea resin (sample PFRP-4) with mass fraction of 1.5% is better. The bending and tensile properties were reduced by 12.77% and 25.70%, respectively, and the retention rates of electric strength and flashover electric field strength strength reach 77.60% and 83.83%, respectively. After salt spray ageing, the surface damage degree of PFRP-4 is small, the thermal weight loss is low, and the residual rate is 67.15%. Montmorillonite can effectively fill the free volume and cracks in the polyurea resin, adsorb water molecules and corrosive ions, and reduce the overall water absorption of the coating. The sample PFRP-4 protective coating can effectively improve the hydrophobicity and thermal stability of the composite materials, and has good bonding properties, which can provide a strong guarantee for the operation of the composite pole in the harsh environment such as high salt spray and high humidity.

The cable joint is the weakest link in the cable line, and the contact resistance between the conductor and the connecting pipe at the aluminum alloy conductor cable joint is the key factor affecting the temperature distribution and failure rate of the cable joint. In order to study the influence of different crimping processes on the temperature distribution characteristics of cable joints, a 35 kV aluminum alloy conductor was used as the research object, and the contact resistance between conductor samples with different crimping percentages and connecting tubes were measured through the contact resistance measurement circuit. The quantitative relationshipes between contact resistance and crimping percentage under two crimping processes of pit pressure and confining pressure were obtained. The electro-thermal coupling model of a cable joint with conductor cross-sectional area of 400 mm² was established by COMSOL finite element simulation software, and the corresponding relationship between contact resistance and temperature distribution characteristics of cable joint under different crimping processes was studied. The results show that the contact resistance increases with the increase of the crimping percentage. Under the premise of ensuring that the maximum temperature of the long-term operation of the cable does not exceed 90℃ and meeting the mechanical performance requirements, the crimping percentage under the pit pressing process should be controlled at 27.86%-35.44%, and the crimping percentage under the confining pressure process should be controlled at 16.01%-22.21%. The temperature rise curves of the two crimping processes have the same change trend. The highest temperature of the cable core appears at the position of the connecting pipe, and gradually decreases along the axial direction. The temperature of the outer surface of the joint shows a downward trend as a whole, but the temperature of the connecting pipe increases in a small range due to the existence of contact resistance.

Converter transformer is the key equipment in high-voltage direct current (HVDC) transmission system, and its valve-side windings are subjected to complex composite voltage waveforms during operation. The existing researches primarily focus on partial discharge characteristics under single voltage form or AC-DC composite voltages. However, due to the influence of power electronic devices such as converter valves, converter transformers also endure pulsating voltages. In this paper, the impact of positive polarity DC pulsating voltage on partial discharge of oil-paper insulation under actual operating conditions was studied, and the partial discharge characteristics of two typical oil-paper insulation defects, needle tips discharge in oil and surface discharges on oil-paper boards, were obtained under DC pulsating voltage. The results show that the DC component only changes the frequency and amplitude of partial discharge, and has same effect on the two types of defects. As the amplitude of the pulsating component increases, the discharge pulses of the two types of defects increase gradually, forming pulse clusters and exhibiting significant polarity effects. The pulse clusters for needle tips discharge in oil are concentrated on the rising edge, while those for surface discharges on oil-paper boards are concentrated on the falling edge.

As a key device in ultra-high voltage and extra-high-voltage transmission projects, the deflagrate accidents of bushings seriously threaten the safe and stable operation of the power system. Adhesive impregnated paper bushing is an alternative to oil-impregnated paper bushings with deflagration risks, while there is relatively little research on the thermodynamic behavior of its core insulating material of epoxy resin impregnated paper during arc faults and the risk of bushing deflagration. In this paper, the thermogravimetric experiments and differential scanning calorimetry analysis at different heating rates were conducted on epoxy resin impregnated paper, and its thermal decomposition process was analyzed. Flynn-Wall-Ozawa (FWO) method and Coast-Redfern (CR) method were used to obtain the pyrolysis kinetic models of epoxy resin impregnated paper at high temperature in an oxygen-free environment. The results show that the thermal decomposition process of epoxy resin impregnated paper in adhesive impregnated paper bushing can be divided into three stages: epoxy melting, endothermic reactions, and decomposition gasification, and the first two stages conform to three-dimensional diffusion model, random nucleation and subsequent growth model, respectively. After an arc breakdown fault occurs in bushing cores, the thermal decomposition mass loss of epoxy resin impregnated paper is as high as 90% under the effect of high temperature. The high temperatures and gas expansion produce high amplitude internal stress inside the core, which leads to core cracking and even explosion.

Aiming at the problem that the energy storage characteristics of polypropylene (PP) capacitor films reduce greatly under high temperature and high electric field, the polypropylene was modified by melt grafting with maleic anhydride (MAH) at the molecular level, and the effects of grafting monomer content, initiator mass fraction, and reaction temperature on the high-temperature energy storage characteristics of the films were investigated. Then a maleic anhydride grafted polypropylene (PP-g-MAH) biaxially oriented capacitor film with superior high-temperature energy storage characteristics was prepared, and its structure and performances were characterized and tested. The results show that with the increase of monomer content, initiator mass fraction, and reaction temperature, the charge and discharge efficiency of the grafted sample increases at first and then decreases. The graft modification promotes the increase of crystal nuclei and the reduce of spherulite size in PP, which increases the trap depth and trap density of the film effectively, reduces the carrier mobility, and enhances the electric strength and energy storage characteristics at high temperature.

In order to ensure the safe and stable operation of capacitor voltage transformer (CVT), improve its insulation reliability, the insulation state of CVT was evaluated based on improved criteria importance through intercriteria correlation (CRITIC) method. Firstly, the CVT was conducted the artificially accelerated thermal ageing, and its capacitance and dielectric loss factor, polarization and depolarization current, voltage and current were tested under different ageing cycles. The results show that after ageing, the total capacitance of CVT increases from 10.106 nF to 10.155 nF, the time constant of the third branch increases from 62.35 s to 82.80 s, the current and voltage phase difference increases from 96.3° to 97.8°, and only the primary side apparent impedance value decreases among the insulation characteristic parameters. Subsequently, the CRITIC method was improved by residual expectation coefficient and activation entropy, and the improved CRITIC method was used to weight and score the CVT insulation characteristic parameters. In the empowerment results, the weight of 0.1 Hz low frequency dielectric loss factor is the largest, which is 0.323 3, and the weight of C1 dielectric loss factor is the smallest, which is 0.091 2, and the improved CRITIC method solves the problem of information incompleteness. Finally, the insulation score of CVT was correlated with the operating time under actual working condition, and its insulation status was evaluated. When the insulation score is 0 points, the corresponding operating time under working condition is 29.88 years, which is consistent with the actual project. According to CVT insulation score, the internal insulation state can be judged.

In order to investigate the effect of combined heat-force on the performance of ethylene propylene diene monomer self-fusing insulation, ageing experiments with different temperatures and tensile rates were conducted on the ethylene propylene diene monomer self-fusing insulation, and the tensile and dielectric properties of ageing samples were characterized respectively. The results show that during the ageing process, the tensile strength and elongation at break of the sample decrease due to the gradual destruction of molecular structure and swelling of adhesive molecules, and the strong tensile stress will exacerbate the degree of ageing, make the defects gradually develop and expand, and further deteriorate the mechanical properties of the sample. With the increase of ageing time, the polar groups inside the sample increases, the steering polarisation enhances, and the polarisation loss increases. When the stretching rate gradually increases, the increasing rates of relative dielectric constant and dielectric loss factor show the trend of “slow first and fast later”. At the same time, the breakage of molecular chains and the generation of microscopic defects increase the concentration of carrier, and the larger tensile rate weakens the intermolecular forces, reduces the barrier energy level, and makes the carriers easier to migrate, resulting in a continues increase of its volume conductivity. In the later stages of ageing, the molecular chain cleavage under tensile stress is serious, and the thermal expansion of the matrix increases the free volume, leading to a decrease in electric strength. A larger tensile rate will further reduce the trap energy level inside the sample, resgreater decrease in electric strength.

The ablation of the buffer layer of high-voltage cables is accompanied by the release of characteristic gas, so the characteristic gas can be used as an indicator for the detection and evaluation of ablation faults. However, the correlation between the type and concentration of released gas and the degree of ablation is still unclear. In this paper, a buffer layer ablation simulation experiment platform was established. After comprehensive consideration, C₂H₆, C₂H₄, C₂H₂, and CO were selected as the characteristic gases for buffer layer ablation evaluation, and the change law of their stoichiometric characteristics with buffer layer ablation degree and moisture content were analyzed. The results show that the average molar mass of characteristic gas decreases with the increase of moisture content in buffer layer, and the ratio of C to H atoms in characteristic gases under humid conditions is higher than that under dry conditions. The concentrations of C and H atoms continuously increase with the burning time, and with the increase of burning time, the concentrations of C and H atoms first decrease, then increase and then decreases, and finally monotonically increase with the increase of moisture degree. Analysis suggests that the differences of damage to the buffer layer structure under different conditions are the reasons for the change in the stoichiometric characteristics of the released gas.