In this paper, BNNS@SiO₂/epoxy composite dielectrics with different filler content were prepared, the chemical characteristics and microscopic morphology of the composite dielectrics interface area were studied, and the trap characteristics and breakdown properties of the composite dielectrics were studied by thermal stimulation depolarization current method and breakdown test. The results show that BNNS@SiO₂ has a core-shell structure and the thickness of the coated SiO₂ is at the nanoscale. There is a clear chemical bonding effect on the surface of BNNS@SiO₂, which can form a strong interface zone with the epoxy matrix, so as to improve its compatibility with the epoxy matrix. A small amount of BNNS@SiO₂ can effectively improve the electric strength of the composite dielectric, and when the mass fraction of BNNS@SiO₂ is 1%, the electric strength of the composite dielectric increases by 52.3%. When the content of BNNS@SiO₂ nanoparticles is small, the deep traps in the epoxy composite dielectric increases. When content of BNNS@SiO₂ nanoparticles is large, the deep traps decrease and the shallow traps increase. In addition, the thermal conductivity of the core-shell structure BNNS@SiO₂/epoxy composite dielectric increases significantly, which is conducive to the heat dissipation of insulation under high electric field. Through the comprehensive effect of deep trap effect and thermal conductivity improvement, the breakdown performance of epoxy composite dielectric is improved significantly.

Natural ester and synthetic ester insulating oils are getting more and more application in transformer industry because of their eco-friendly and excellent fire resistance characteristics. At present, there is no precedent in China for the application of environment friendly ester insulating oil in 220 kV power transformers for offshore substation. According to the characteristics and requirements of offshore substation, the selection of transformer ester oil types and insulation system types were studied in this paper. Taking SZ-150000/220 type transformer as the object, an improved model of the transformer body structure was established for simulation analysis, and the simulation results show that the ester insulating oil meet the requirements of insulation and temperature rise of 220 kV power transformer. The feasibility of ester insulating oil applied in 220 kV power transformer for offshore substation was discussed through comparative analysis and simulation calculation, and an optimized design scheme was proposed to make the equipment adapt the requirements of offshore operating environment, which provides a reference for the application of ester oil transformer in offshore substation.

Thermosetting epoxy resin is widely used in electrical equipment field due to its excellent thermal conductivity and insulating properties. Due to the stable three-dimensional network structure and insolubility of the thermosetting epoxy resin, its recycling and reuse pose challenges. In this paper, we added different mass fractions of boron nitride (BN) into the epoxy (EP)/4-methylhexahydrophthalic anhydride degradation system to prepare high thermal conductivity, high insulating, and degradable BN/EP composites, and studied their thermal conductivity, dielectric properties, and degradability. The results show that taking 2,4,6-tris(dimethylaminomethyl) phenol as the catalyst, the EP/BN composites can achieve atmospheric pressure degradation at 200℃ under the action of ethylene glycol. The thermal conductivity of BN/EP composite with 15% mass fraction of BN is 0.335 W/(m·K), which is 34% higher than that of pure EP resin; the AC electric strength is 101.7 kV/mm, which is 13% higher than that of pure EP resin. BN/EP composites can be degraded through transesterification to obtain EP degradation products (EDP). After the BN/EP composites are compounded with EDP, the thermal conductivity and electric strength of the BN/EP composites is basically unchanged.

Vegetable insulating oil has gained wide attention due to its green and renewable characteristics as well as excellent fire protection properties, while nanoparticles can effectively improve the electrical properties of vegetable insulating oil. In this paper, the effects of nanoparticles on the electrical properties of vegetable insulating oil were summarized from dielectric loss factor and volume resistivity, and the effects of nanoparticle type, concentration, and surface modification on the breakdown properties of vegetable insulating oil were also discussed. Combining with the differences in the effects of nano-modification on the electrical properties of mineral insulating oil and vegetable insulating oil, we summarized the current research difficulties of nano modified vegetable insulating oil, and prospected the research direction and prospects of nano modified vegetable insulating oil.

In view of three common defects of 10 kV cables, three-dimensional cable defect models were established and conducted electromagnetic-thermal coupled temperature field analysis to study the transient temperature models of three defective cables. The effects of different operating currents and laying conditions on the temperature of three defective cables were analyzed. The results show that the harm of defects on the cable ranked from high to low is metal tip defect, air gap defect, and scratch defect. Under the same current carrying capacity, the temperature at the metal tip defect is higher than that at the other two defects. It is determined that the heat dissipation effect of tunnel laying is the best when the cable has defects. The internal temperature of the cable without defects decreases gradually from the core to outer sheath in the radial direction, and the internal temperature field of the cable is distorted under the action of defects. The fitting coefficients between core temperature and outer sheath temperature of the defective cable, and the fitting coefficients between temperature at the defect and the core temperature of cable are both close to 1. The simulation fitting results provide theoretical support for the judgment and identification of cable defects.

Cross-linked polyethylene (XLPE) is subject to various environmental or human factors during production, laying, and operation, which can generate structural defects at different scales and affect the electrical performance of XLPE. In this paper, the molecular defects, aggregated structural defects, and macroscopic air gap defects in XLPE power cables were reconstructed and characterized by X-ray Diffraction (XRD) and transmission and reflection modules of terahertz time-domain spectroscopy (THz-TDS). The results show that the amplitude and phase information of terahertz time-domain spectroscopy exhibit sensitive to the structural changes of polar molecules and aggregates, respectively. The location and size of hidden air gap defects can be accurately identified and calculated by the combined technique of transmission and reflection modules of terahertz time-domain spectroscopy, realizing the nondestructive detection of defects at different scales.

Currently, the dielectric of plastic film capacitors is usually linear dielectric polymers with a low dielectric constant, leading to a relatively low energy storage density of capacitors. High-energy-density dielectric polymers under research often suffer from excessive dielectric loss, limiting their practical utilization. Poly(methyl methacrylate) (PMMA) has been commonly employed to enhance the mechanical and breakdown properties of high-energy-density fluoropolymer beceause of their good compatibility between them. However, the commercially available PMMA also exhibits too high intrinsic dielectric loss. In order to decrease the dielectric loss of PMMA, a copolymer named MS was synthesized from methyl methacrylate (MMA) and styrene (St). Subsequently, a series of composites were fabricated by incorporating low content of MS into bulk polymerized PMMA, and their dielectric properties, energy storage characteristics, and insulating peoperties were investigated. The results show that the composites can significantly reduce the dielectric loss of PMMA, making it more suitable as the modified material of high-energy-density polymer compared to PMMA. Under 5 500 kV/cm of electric field, the discharge energy density of the dielectric film composed of 10% MS achieves 5 J/cm³, and its charge-discharge efficiency can attain 83%.

In hot and humid environment, composite insulators with interface defects are more prone to occur serious accidents such as decay-like breakage and interface breakdown. However, there is a lack of studies on the interface ageing mechanisms of composite insulators with core-sheath interface defects under hygrothermal environment. In this paper, composite insulator short samples with different interface defects were subjected to hygrothermal ageing. The temperature rise and discharge results of each sample were compared under conditions of moisture absorption and dry condition. The influence of hygrothermal environment on the performance of composite insulators with interface defects was studied through dissection observation, microscopic morphology observation, and surface elements and functional groups analysis. The results show that the insulator interface defects will expand into interface failures under the action of heat and humidity. Partial discharge and polarization loss of moisture at the interface will trigger abnormal heating faults, and the temperature rise caused by metal defects is significantly higher than that of other defects. As the interface defects gradually expand, epoxy resin undergoes oxidation and decomposition from interface to interior, and the glass fibers exposes and deteriorates.

To meet the growing demand for high thermal conductive and electrically insulating composites, a silver nanoparticles (AgNPs) modified nonwoven fabric (AgNPs@NWF) was prepared by polydopamine (PDA) modification and in situ reduction process using PET nonwoven fabric (NWF) as a template. The continuous boron nitride nanosheets (BNNS) thermal conductive network (BNNS@NWF) and AgNPs/BNNS synergistic thermal conductive network (AgNPs/BNNS@NWF) were constructed by adsorbing BNNS on the surfaces of NWF and AgNPs@NWF through the dispersion and interfacial bonding of nanocellulose through a cyclic impregnation adsorption and layer-by-layer assembly process. BNNS-AgNPs/BNNS-BNNS sandwich-structured thermal conductive composite films were prepared by hot pressing process with BNNS@NWF as the surface layer and AgNPs/BNNS@NWF as the intermediate layer, and their microstructure, thermal conductivity, insulating properties, mechanical properties, and actual thermal management performance is characterized and tested. The results show that a synergistic three-dimensional thermal conductive network of AgNPs/BNNS was constructed in the composite films, at the same time the insulating properties are ensured. When the mass fraction of BNNS and AgNPs is 34.8% and 3.3%, the in-plane thermal conductivity of the composite films reaches 7.56 W/(m·K), the volume resistivity reaches 3.54×10¹³ Ω·cm, and the mechanical properties is good. The actual application test show that the composite films have good thermal management performance.

Low-frequency cable is a key equipment in the flexible low-frequency AC transmission system, and the characteristics of insulation medium under low-frequency voltage are of great significance to the design and operation of cable. In order to study the growth and partial discharge characteristics of electrical tree in cross-linked polyethylene under low-frequency voltage, a real-time observation system for electrical tree growth at different frequencies and a synchronous measurement system for partial discharge were designed and constructed. The initiation, growth, and partial discharge characteristics of electrical tree in XLPE samples were investigated at four frequencies of 20, 30, 40, and 50 Hz. The results show that the influence of voltage frequency on the growth and partial discharge characteristics of electrical tree in XLPE has obvious rules. In the range of 20-50 Hz, with the decrease of voltage frequency, the tree starting voltage of XLPE increases slightly, but the growth rate of electrical tree is accelerated, the damage area increases, the amount and number of partial discharge increase, and the partial discharge phase is basically unchanged.