In oil-immersed systems, the rapid development of discharge faults has the characteristic of high gas production and fast gas generation rates. As a result, the generated characteristic gases may not have enough time to dissolve in oil. The majority of these gases escape to the oil surface and enter the gas relay, leading to that a large amount of effective gas for transformer diagnosis and early warning is unable to reach dissolution equilibrium in time, which makes the commonly used dissolved gas analysis method in power industry unable to accurately diagnose faults. Based on this, an experimental platform for studying the gas production law on the oil surface under fast-developing discharge faults of oil-immersed insulation system was constructed. The characteristic gas information on the oil surface of the oil-immersed insulation system during fast-developing discharge faults was obtained. The results show that the concentration of characteristic gases in the liquid phase does not increase significantly in the short period after fault occurred, while there is a large amount of characteristic gases in the gas phase at this time. When there is high-energy discharge in the system, CO, CO₂, CH₄, and H₂ will accumulate on the oil surface, and these four gases can be used as characterization basis for high-energy discharge faults. On this basis, C₂H₆, C₂H₄, and C₂H₂ will also accumulate on the oil surface when there is spark discharge, and these three gases can be used as diagnostic basis for spark discharge.

The use of silicon carbide (SiC) inverters for power supply can reduce the size, weight, and temperature rise of traction inverter motors and improve the motor efficiency. However, the high frequency and short rise time of the inverter output voltage would bring challenges to the insulation of traction inverter motors. The turn-to-turn insulation will be subjected to higher voltage loads, which will lead to premature insulation failure. In this paper, the voltage waveforms of turn-to-turn insulation of high-voltage inverter traction motor were firstly simulated when the SiC inverter is at the maximum switching frequency and the actual cable length. The partial discharge inception voltage (PDIV) test and endurance life test were conducted on the electromagnetic wire of high-voltage inverter motor under different voltage parameters (frequency, voltage level, rise time), different temperatures, and different thicknesses of turn-to-turn insulation (0.23 mm, 0.30 mm). Then according to the actual operating conditions of traction motor and the electrical load failure mechanism of insulation system, the change mechanism of PDIV, partial discharge characteristics, and life was analyzed. The PDIV test results show that the change of frequency and rise time has no influence on the PDIV. The PDIV derceases with the increase of temperature. However, due to the fabrication process, the PDIV increases at some temperatures. The increase in thickness has a greater increase in PDIV. Under the premise of controllable cost and efficiency, increasing the thickness can effectively improve the turn-to-turn insulation performance of inverter motors. Endurance test results show that the changes in temperature and rise time would cause large changes in insulation life. The effect of frequency and voltage level on insulation life is non-linear, and it can be expressed by the inverse power model. In industrial applications, the effect of frequency on the insulation life can be directly converted through a proportional relationship.

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%.

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.

In order to solve the problem of turn-to-turn insulation for oxygen-free copper coils in radio frequency plasma of antenna built-in type, a process capable of sintering a glass insulating layer on the surface of oxygen-free copper was studied, and the effects of factors such as different surface treatment method, binder, and sintering temperature of the process on the performance of glass insulating layer were investigated in detail. The results show that the oxygen-free copper surface after acid washing pretreatment can significantly improve the wettability of low-temperature glaze on the copper surface compare to alkali washing, and reducing the "porosity" on the insulating layer surface. At the same time, an appropriate adding amount of Na₂SiO₃-5H₂O binder can improve the interface combination effect between the glass insulating layer and the copper surface, and improve the hardness of glass insulating layer surface. In addition, the surface hardness of the insulating layer increases with the increase of the sintering temperature, and the number of "porosity" on the insulating layer surface is gradually reduced. An oxygen-free copper coil antenna with a glass insulating layer on the surface was finally prepared according to this process, which achieved a long-time discharge maintenance and effectively solved the turn-to-turn insulation problem during discharge process.

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.

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.

Compared with mineral insulating oil, natural ester has the advantages of higher ignition point, better degradability, and can delay the thermal ageing rate of insulating paper, so natural esters are receving extensive attention as liquid insulating medium for extending the life of transformers or improving short term overload capacity. In this paper, we used rapeseed oil, which accounted for a large proportion of cooking oil in China, to prepare rapeseed oil based natural esters, and the accelerated thermal ageing characteristics of insulating paper with different water contents in rapeseed oil based natural ester and mineral oil were investigated. The effect of oxygen on the accelerated thermal ageing characteristics of insulating paper in the two different insulating liquids was studied. The experimental results show that at the same temperature, water content is the main factor affecting the thermal ageing rate of insulating paper, and oxygen will reduce the inhibitory effect of rapeseed oil based natural esters on the thermal ageing rate of insulating paper with low water content. Combined with the measured data of specific strength and degree of polymerization of insulating paper during thermal ageing in insulating liquid, a kinetic model of cellulose decomposition reaction rate of insulating paper considering the influence of water content and oxygen was established. The model can be used to predict the thermal ageing process of insulating paper in insulating liquids.

The stable operation of dry-type reactors affects the transmission reliability of new power system. The encapsulating material of dry-type reactor is made of glass fiber filament impregnated epoxy resin cured at high temperature. In this paper, a multiphysics coupled finite element method was used to consider the influence of thermal conductivity of the encapsulating material for dry-type reactor on its hot spot temperature rise, and a COMSOL microscopic simulation model of epoxy composites and an electro-magnetic and flow-thermal coupling calculation model of dry-type reactor under the constraints of external circuits were established. The temperature field and flow field distribution were calculated by using the loss under electromagnetic field as the heat source, and the influence of conventional/high thermal conductive epoxy composites on the hot spot temperature rise of the dry-type reactor at 25℃ of ambient temperature was studied. The results show that the high thermal conductive epoxy resin has a significant improving effect on the thermal conductivity of composites. The maximum hot spot temperature rise in the temperature field area of the encapsulating material body and the surrounding air is 103.75℃, which appears at the upper end of the fourth layer of encapsulating material. The epoxy resin composite with different thermal conductivity has obvious difference on decreasing the hot spot temperature of dry-type reactor, and the hot spot temperature of the dry-type reactor with high thermal conductive epoxy resin composite is reduced by 7.55℃.