The traditional time-frequency domain reflection method (TFDR) is difficult to accurately obtain the time-frequency distribution of the acquired signal, which seriously reduces the location accuracy of cable defects. Therefore, a cable defect localization method based on Hilbert transform (HT) and TFDR was proposed in this paper. Firstly, the Gaussian frequency domain window function was introduced by Fourier digital filtering method to suppress the interference of the acquired signal. Then, the time-frequency distribution of incident traveling wave signal and reflect traveling wave signals were obtained by HT method. Finally, the instantaneous power spectrum of time-frequency distribution was used to characterize the time-frequency energy focusing characteristics of the defect reflected traveling wave signal, and the defect position in the cable was located through the peak value of the final curve obtained. The results show that, compared with the traditional Wigner distribution method and synchronous compression transformation method, the traveling wave signal obtained by this method has higher time-frequency resolution and no cross term interference, which can locate local defects in cables more accurately.

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.

Interfacial discharge between XLPE and SIR in cable joints is one of the main causes of cable failure. In order to improve this phenomenon, the XLPE sample surface was treated by plasma silicon deposition with different time, and the micro-morphology and interface discharge tests were carried out. The results show that the plasma silicon deposition technology can effectively improve the voltage resistance of XLPE/SIR interface. With the increase of plasma silicon deposition treatment time, the surface roughness of XLPE sample decreases at first and then increases, and its change trend is the same as that of the initial discharge voltage, breakdown voltage, and voltage increase amplitude of XLPE/SIR interface, and is opposite to that of the surface resistivity. The XLPE sample after 3 min of plasma silicon deposition has the smallest surface roughness (R _a=41.8nm) and the largest surface resistivity (857×10¹² Ω). Under this treatment time, the XLPE/SIR interface has the largest actual contact area, the fewest micro-pores, and the largest increase in breakdown voltage. Among them, the breakdown voltage increases by 66.7% compared with the untreated XLPE/SIR interface.

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 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.

High temperature liquid nuclear magnetic resonance (¹³CNMR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC) were used to study three kinds of polypropylene resins for DC capacitors at home and abroad with different temperature resistance grades, and analyze the effect of microstructure on electrical properties. The results show that the isotactic index, intermolecular chain defect distribution, and molecular weight characteristics jointly affect the crystallization characteristics of polypropylene, and thus affect the final high temperature dielectric properties of materials. The isotactic index of 5 units of imported polypropylene resin is greater than 97%, and the intermolecular chain stereodefect distribution is narrow (I=1.075), the weight average molecular weight is higher and the molecular weight distribution index is larger (PDI>5.6), showing excellent high temperature insulation properties (the electric strength α=533 kV/mm). While the molecular weight distribution of domestic polypropylene resin is narrow (PDI<5), the weight average molecular weight is lower, the isotactic index of 5 units is less than 97%, and the intermolecular chain stereodefect distribution is wider (I=1.106), which leads to its poor high temperature insulation performance and low electric strength (α=497 kV/mm).

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.

Surface flashover at the gas-solid interface of basin type insulator is the key problem that causes the ultra high voltage GIS/GIL failure, and it is the technical bottlenecks restricting the development of advanced electrical transmission system. In order to improve the DC flashover performance of the basin type insulator in SF₆/N₂ gas mixture, the zinc oxide (ZnO) nanoparticles with different contents were added to the epoxy resin to obtain epoxy/zinc oxide (EP/ZnO) coatings, and the EP/ZnO coatings were coated on the surface of epoxy material. Then the transport parameters and flashover performance were tested. The results show that EP/ZnO coating increase the shallow trap density and reduce the shallow trap energy level of epoxy composite, and increase the carrier mobility. The electrical conductivity of the epoxy materials coated by EP/ZnO coatings with 15% and 20% ZnO mass fraction show nonlinear characteristics, the existence of the coatings can reduce the electric field distortion and promote the dissipation of surface charge. The EP/ZnO coating can remarkably uniform electric field distribution and effectively inhibit the charge accumulation on the surface of epoxy materials, and improve the DC flashover performance. When the mass fraction of ZnO is 20%, the DC flashover voltage of the epoxy composite increases by 15.42%.

Polymer-based nanocomposites have received much attention for their application prospects in the development of capacitors with high energy storage density. In this paper, ultra-thin barium niobate (Ba₅Nb₄O₁₅, BNO) nanosheets were prepared by hydrothermal method, and were used as fillers to prepare nanocomposite films by combing with polyvinylidene fluoride trifluorochloroethylene (PVDF-CTFE) and polymethyl methacrylate (PMMA). The effects of ultra-thin BNO nanosheets on the delectric properties and energy storage properties of polymer composite films were studied. The results show that the dielectric constant and electric strength of polymer nanocomposite films increase significantly with a low BNO addition content, and thus the energy density increases. When the mass fraction of BNO is 0.5%, the maximum energy density of the nanocomposite film reaches 13.96 J/cm³, which is 2.6 times higher than that of pure P(VDF-CTFE)/PMMA polymer, and the energy storage efficiency reaches 67.4%.

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.