In order to realize the accurate assessment of the ageing state of transformer insulating paper, an ageing state assessing method of insulating paper based on combined assignment and improved TOPSIS was proposed. Firstly, in order to overcome the shortcomings of single feature quantity evaluation, polymerization degree, tensile strength, elongation at break, breakdown voltage, and dielectric loss factor were selected as ageing feature quantities to establish an evaluation system for the ageing state of insulating paper. Secondly, the combined weight value of feature quantities was calculated using the combined assignment model. Finally, an improved TOPSIS model was proposed to construct the Euclidean-gray correlation distance measure. The closeness degree was calculated using the improved TOPSIS model, and the closeness degree interval was set to grade and evaluate the ageing states of insulating paper. The results show that the ageing state assessing method of insulating paper based on combined assignment and improved TOPSIS not only considers the subjective experience weight of experts, but also uses the inherent characteristics of feature quantity data for weight correction, making the weight more scientific and reasonable. At the same time, it compensates for the distance criterion deficiency of the traditional TOPSIS model, making the calculated closeness degree more reasonable. Through the electricity-heat-force combined ageing test, the evaluation method has been verified to determine the ageing states of insulating paper accurately and effectively, and the grading evaluation of ageing states can be achieved by setting a closeness degree interval.

With the development and construction of offshore wind power, cross-linked polyethylene cables are gradually used in low-frequency power transmission. However, their insulation characteristics after ageing at low-frequency voltage are still unclear. So the insulation characteristics and ageing mechanism of cross-linked polyethylene after ageing at low frequency were studied in this paper. Firstly, cross-linked polyethylene samples were conducted accelerated ageing experiments at voltage frequencies of 20, 35, 50 Hz. Then, the physicochemical and electrical properties of the ageing samples were tested. Finally, the mechanism of voltage frequency on the electrical ageing of cross-linked polyethylene was investigated. The results show that with the decrease of the ageing voltage frequency, the crystallinity of cross-linked polyethylene decreases, the space charge accumulation and conductivity increase, and the AC electric strength decreases. The decrease of voltage frequency makes the ageing degree increase significantly. Combined with the infrared spectroscopy and crystallinity analysis, the decrease of voltage frequency makes the probability of charge into trap and forming hot electron be higher, the electroluminescence effect caused by electron-hole composite is stronger, which causes the increase of polymer molecular chain breakage and cleavage, and then its electric strength is reduced.

With the increase of grid capacity and voltage level, the insulating properties of epoxy composite insulation materials is crucial to the safe and stable operation of electrical equipment. Long-term high temperature and high humidity will lead to a serious decline in insulating properties of materials. Nano-SiO₂ was fluorinated (F-SiO₂) by the plasma technology of dielectric barrier discharge (DBD) in this paper, and fluorine-containing groups were successfully grafted on the surface of nano-SiO₂. The high-temperature and humid state of the material was simulated by the wet heat ageing experiment, and the surface flashover, charge dissipation characteristics and trap distribution characteristics of the samples at different ageing stages were tested. The effect of doping F-SiO₂ on the wet heat ageing resistance of epoxy composites was explored, and the inhibition mechanism of F-SiO₂ on the water intrusion was revealed from the molecular scale in combination with MD simulation. The experimental results show that doping F-SiO₂ reduces the saturated moisture absorption of epoxy composites by 16.16%, increases the surface flashover voltage by 13.06%, and can continue to maintain high insulating properties after wet heat ageing. The simulation results show that F-SiO₂ can reduce the free volume fraction of epoxy resin and the mean square displacement of water molecules, and enhance the barrier effect of epoxy composites on moisture. Plasma technology can graft fluorine elements onto the surface of nano-SiO₂, effectively inhibiting the intrusion of moisture and enhancing the surface insulating properties of epoxy composites in a humid state.

To investigate the influence of the filler network constructed by alumina (Al₂O₃) on the thermal and dielectric properties of polypropylene (PP) composites, this study constructed a finite element model of randomly filled Al₂O₃ fillers, and systematically studied the effects of factors such as filler content, filler particle size, and size matching of binary fillers on the thermal conductivity and dielectric constant of Al₂O₃/PP composites. The results indicate that increasing the filler content can significantly bridge the Al₂O₃ filler, synergistically construct an interconnected thermal conductive network and electric displacement pathway, thereby significantly improving the thermal conductivity and dielectric constant of Al₂O₃/PP composites. For single Al₂O₃ filled PP composites, the thermal conductivity and dielectric constant of composites cannot be effectively enhanced by regulating the size of Al₂O₃ filler. After introducing the multiscale fillers, at the optimized binary filler ratio of 70∶30 (40 μm∶15 μm), the overall thermal conductivity and electrical displacement networks show the dominant skeleton of large-sized filler and the bridging branch of small-sized fillers features, which synergistically contributes to the Al₂O₃/PP composite reach the optimal thermal conductivity (0.55 W/(m·K)) and dielectric constant (5.6).

Crepe paper is an essential insulating material for transformers and high-voltage cables. Currently, all high-performance crepe paper is imported. In order to realize the localization of crepe paper for high-end power equipment and improve the long-term reliability of crepe paper, it is urgent to carry out research on the physicochemical properties and performance improvement technology of crepe paper. The physicochemical and macroscopic properties of crepe papers from different manufacturers were tested and analyzed in this paper, and the key physicochemical parameters that affect the performance of crepe papers were extracted. Three functional groups were introduced into the cellulose systems through grafting modification, the properties of the cellulose systems before and after grafting modification were analyzed using Materials Studio software, and the effects of different functional groups on the various properties of the cellulose systems were investigated. The results show that the benzene ring has the most obvious improvement on the thermal properties of the cellulose system, the methylene long chain can improve the insulating properties of the cellulose system, and the benzene ring can improve the mechanical properties of the cellulose system.

In order to study the change rule of microstructure and dielectric properties of polyethylene (PE) materials under the action of thermal stress, PE samples were conducted accelerated thermal ageing tests at 90℃, and the microstructure and charge transport properties of the thermal aged PE samples were characterized by Fourier transform infrared spectrometer (FTIR), differential scanning calorimetry (DSC), X-Ray diffraction (XRD), high field conductivity (HFC) test, and isothermal surface potential decay (ISPD) tests. The results show that the thermal ageing process of PE samples can be divided into recrystallization stage and thermo-oxidative degradation stage. In the recrystallization stage, the crystallinity of the samples increases, the crystal structure tends to be perfect, the deep-trap density and deep-trap energy level increase, the carrier migration is restricted, and the threshold field strength of space charge injection increase. In the thermo-oxidative degradation stage, the crystallinity of the samples decreases, the crystal structure appears deterioration, the deep-trap density and deep-trap energy level decrease, the carrier mobility increases, and the threshold field strength of space charge injection decreases. Through the characterization of conductivity properties under different temperatures and field strengths, the variation rules of three parameters A, B, and φ in Steven Boggs’ conductivity function were obtained. With the increase of ageing time, the φ and A increase at first and then decrease, and the B decrease at first and then remains unchanged, among which the material coefficient A is the most sensitive to thermal ageing.

A 0.6/1 kV cable core with double-layer insulation structure was prepared through the double-layer co-extrusion process, and the effect of extrusion temperatures on its surface quality, eccentricity, microstructure, and mechanical properties was investigated. The prepared cable core with double-layer insulation structure was conducted multiple properties tests and heat-resistance life evaluation. The results show that when the extrusion temperature increases from temperature I to temperature II, the prepared double-layer insulation core has flat surface, good brightness, and lower eccentricity between the inner and outer insulation layers (≤15%), and the elongation at break of the insulation core increases from 109% to 221%. When the extrusion temperature increases from temperature II to temperature III, the surface quality of insulation core is deteriorated with distortion and shrinkage on it. Meanwhile, many different sizes of pores can be observed in the longitudinal section, and the tensile strength and elongation at break decrease to 10.7 MPa and 140%, respectively. Compared with the values of JG/T 442—2014 standard, the insulation resistance constant at 20℃, tensile strength and elongation at break before and after ageing, and thermal extensibility of insulation core after electron irradiation cross-linking are significantly improved, indicating that the mechanical properties, ageing properties, and thermal elongation properties of the insulation core with double-layer structure are relatively excellent. Furthermore, after ageing at 165℃/168 h, the elongation at break of insulation core with double-layer strucure has 98% of retention rate, indicating that the insulation core with double-layer structure possesses the outstanding heat-resistance life.

To explore the thermal mechanical stress distribution of motor insulation structure at high temperatures, we take the insulation structure of organic silicone system as the research object, test its thermal expansion coefficient and elastic modulus at different temperatures, and analyze the influence of thermal stress parameters at different temperatures on the thermal mechanical stress of organic silicone insulation system. The results show that within -50-200℃, the thermal expansion coefficient of insulation structure show a change trend of increases, decreases, and increases again with the increase of temperature, and its value is 0.8×10^-5-1.7×10^-5 K^-1. The elastic modulus increases at first and then decrease with the increase of temperature, and its value is 0.2×10³-3×10³ MPa. The thermal mechanical stress borne by insulation structures increases linearly with the increase of thermal expansion coefficient and elastic modulus. The thermal mechanical stress of insulation structures firstly increases and then decreases with the increase of temperature, depending on the size of thermal expansion coefficient and elastic modulus at different temperatures. The feasibility of testing parameters and simulation analysis was verified through stress testing of the wire rod and motor, and the deviation between simulation value and testing value is within 15%. The simulation analysis results show that at 120℃, the maximum thermal mechanical stress point of motor insulation is located at the slot insulation, and the maximum stress value is 11.37 MPa. After sealing, the maximum thermal stress value of the slot insulation can be reduced by about 45%.

Composite insulators are widely used in transmission lines, but their ageing and fracture problems cannot be ignored. In order to study the cause of fracture of composite insulators and the mechanism of mechanical-electro-thermal ageing, a mechanical-electro-thermal ageing experiment platform was designed to carry out multi-field coupling ageing tests on short rod samples of composite insulators. The results show that the ageing insulator rod sample has fracture phenomenon. During the process of rod fracture, the macroscopic characteristics are the expansion of crack and fracture surface, and there are discharge marks near the high-voltage electrode. The microscopic characteristics are epoxy decomposition, glass fiber microcrack and fracture, and fiber-epoxy interface failure. At the same time, a multi-field fracture failure evaluation model of mechanical-electro-thermal coupling is established based on the unit break rate, and the inverse relationship between the unit break rate and the energy barrier is revealed. Under the given electrical, thermal, and mechanical stress conditions, the model can evaluate the ageing degree of composite insulator rod according to the ratio of fracture area, and can provide a reference for the ageing failure analysis of composite insulator rod under the synergistic action of multiple stresses.

The main insulation of hydro generator stator bar ends is susceptible to thermal ageing due to temperature rise and bombardment of high-energy particles from partial discharges during long-term operation, which would deteriorate the insulation performance and intensify the partial discharges. In this paper, three kinds of typical end defect models were made according to the insulation structure of F-class stator bar for hydropower station, and accelerated thermal ageing tests were carried out at 170℃. The morphological structure and molecular structure of the main insulation samples under different ageing cycles were analyzed by scanning electron microscope and Raman spectrometer to reveal the thermal ageing mechanism of main insulating material, which is epoxy glass mica tape. A partial discharge test platform was set up, the changes of discharge amount, discharge number, and ozone concentration before and after ageing were measured by the pulse current method and ozone detection method, and their influence law on the partial discharge characteristics of end defects were explored. The results show that the accelerated thermal ageing cycle is proportional to the insulating structure change of the main insulating material, and the longer the ageing time, the rougher the surface of the samples. Thermal ageing destroys the molecular chain structure of the epoxy glass mica tape, and the characteristic peak changes of C-C bond and C=C bond on the benzene ring and other aromatic and carbon rings have correlation with the insulation resistance. Thermal ageing has a small effect on the discharge amount and discharge number of the internal air gap, and has a large effect on the discharge number of low discharge amount of the anti-corona layer shedding defects. The saturation concentration and saturation time of ozone in different insulation defect models have large differences, and the changes of ozone concentration of different discharge types before ageing have stage growth characteristics, while the ozone concentration after ageing has no obvious stage growth characteristics.