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.

Oil-paper insulation is the main insulation material for oil-immersed power equipment, and its ageing under long-term complex working conditions will seriously affect the use safety of equipment. Therefore, the accurate evaluation of its insulation status is crucial. Multiple sets of oil-paper insulation models with different degrees of ageing were prepared in this paper, and wide temperature-wide frequency dielectric response tests at different test excitation amplitudes were conducted. Characteristic parameters characterized the ageing of oil-paper insulation was extracted on the basis of Disado-Hill relaxation model. The test results show that as the ageing of oil-paper insulation increases, the full frequency range loss factor curves show two characteristic frequency ranges, which are respectively related to the material conductivity characteristics, turning polarization and interface polarization process. According to the fitting calculation results, it is found that after ageing of the oil-paper insulation, the cellulose structure is damaged, and the number of impurity ions in the dielectric increases, which is manifested at the micro level as enhancing the inter cluster motion within the dielectric and weakening the intra cluster motion. Therefore, the characteristic frequency points in the model move towards higher frequencies. Meanwhile, to accurately obtain the ageing state of oil-paper insulation, a quantitative characterization relationship between model characteristic parameters and insulation ageing was constructed by eliminating the influence of test temperature and excitation amplitude. The research results provide theoretical support for the evaluation method of insulation ageing state of oil-immersed power equipment based on frequency domain dielectric response on-site detection technology.

Air gap defect is one of the common internal defects in GIS, which can cause electric field distortion. The introduction of dielectric functional graded materials can optimize the local electric field of basin insulators. However, under the combined effect of air gap defect and dielectric functional graded materials, its electric field characteristics are not yet known. Therefore, an electric field simulation model of GIS basin insulator with uniform medium and dielectric functional graded materials (ε-FGM) was built in this paper, and the influence of the existence of air gap and air gap position on the electric field distribution of insulators with uniform medium and ε-FGM was discussed. The results show that the introduced dielectric functional graded materials have a relatively ideal regulatory effect. When air gap defects are doped in a uniform medium, the field strength of air gap near the high-voltage conductor, insulator middle, and grounding shell will significantly increase, and the increase rate is 85.32%, 78.10%, and 68.69%, respectively. The influence trend of air gap defects on the electric field characteristics of insulators with ε-FGM is basically consistent with the influence trend on the uniform media, but from a specific numerical perspective, there is still a certain difference. Air gap defects have a greater impact on the electric field of insulators with ε-FGM. When designing insulators with ε-FGM, special attention should be paid to suppress the air gap defects in medium.

To investigate the impact of interface air gap and moisture defects on the multilayer composite dielectric insulation interface of ±320 kV DC cable intermediate joints on their electric field distribution mechanism, a simulation model of ±320 kV DC cable intermediate joint was built, and the electric field distribution of the cable joint with air gap and moisture defects on the multilayer composite dielectric interface region was calculated using the finite element method. The effect of these defects on the electric field distribution of the cable joint were analyzed at no load and load conditions, and their differences under AC/DC electric fields were discussed. The results show that there is significant electric field distortion at the composite dielectric interface of the DC cable joint due to air gap and moisture defects under the action of DC electric field. The multiple of electric field distortion of air gap defects is 9.6 times at no load and 1.7 times at load condition. The electric field strength in the water film defects decrease over 99.9% at both operating conditions. Interestingly, the electric field distortion induced by air gap defects in cable joints at no load is larger than that at load condition, and the temperature differentials across the joint play a role in field homogenization. However, the temperature differentials across the joint induced by moisture at load promote the accumulation of space charge in the joint, which leading to the multiple of electric field distortion at moisture defect bigger than than at no load. Notably, the multiple of electric field distortion resulting from defects in DC cable intermediate joint is bigger than that of AC cables.

High voltage cable is an important part of power supply system. Buffer layer ablation can lead to power cable failure. In recent years, several cable accidents caused by buffer layer ablation have occurred. In order to solve the above problems, a multi-parameter detection method for cable ablation defects was proposed based on buffer layer ablation characteristic gases, temperature and pressure variation. The CH₄, C₂H₆, and C₂H₄ content in the buffer layer was analyzed by Fourier transform infrared spectroscopy. The H₂ content in the buffer layer was measured by electrochemical sensor. The temperature and barometric pressure in the buffer layer were measured by temperature sensor and piezoelectric barometric pressure sensor. The performance of the system was tested by laboratory buffer layer ablation experiment and actual cable detection experiment. The results show that the functionality and accuracy of the system meet the requirements, and the four characteristic gases content, temperature, and pressure can be used as the parameters to characterize the buffer layer ablation defects. The detection period of the system is less than 30 s, and the detection error is within ±10%. The system has advantages in terms of safety, detection speed, and accuracy, which can guarantee the safe operation of power cables.

The 110 kV epoxy resin dry type transformer has the phenomenon of flashover in the airway on the high voltage side due to its high electric field intensity. The high voltage side coil of transformer was modeled and simulated by finite element simulation software in this paper, and the standard lightning impulse was applied to verify the lightning impulse. According to the simulation results, the causes of flashover were analyzed and the field strength distribution at the airway was optimized. The results show that the electric field distribution is improved by adding a specific medium to the corresponding area inside the epoxy resin. At the same time, by changing the relative dielectric constant and position of the medium, the maximum field strength at the airway is reduced by 8.75% under the restriction of field strength inside the transformer, which meets the field strength requirement of the high-voltage side airway of the dry-type transformer to inhibit flashover.

After the occurrence of line tree barriers, it is difficult to successfully reclose the line, which can easily cause line shutdown. There are multiple major power outages caused by tree barriers both at home and abroad, but the electric field distribution characteristics and discharge mechanism of insulated conductors with different types of tree barrier are still unclear. Therefore, electrostatic field models of insulated conductors with different types of tree barrier were established on the basis of finite element theory in this paper, and the erosion mechanism of tree barrier discharge was revealed according to the obtained electric field distribution and gas-solid discharge theory. The results show that the tree barrier types of insulated conductor are mainly divided into four categories: single side wedge-shaped penetration defects, flat cut defects, double wedge-shaped penetration defects, and critical contact between wood throns and defects tree barrier-wire types. The electric field undergoes distortion at the defects of tree barrier, and the wedge-shaped penetration defects can cause gas breakdown and discharge at the site of tree branch-insulation wire, which poses the greatest damage to the surface of insulation wire and the erosion of tree branches. The field strength at the defect site is affected by the degree of defect and air gap, and the maximum electric field strength of the wood thorn penetrating into the insulation layer is 2.64 times higher than that of the wood thorn penetrating only into the shielding layer. The wind disturbance causes the critical contact between the wood thorn and defect to generate air gap, and its electric field is 2.54 times higher than that without air gap (5.65 kV/mm). The research results provide a theoretical basis for different types of tree barrier defense methods, avoiding the drawbacks of cutting tree indiscriminately.

Insulating oil plays a critical role as a dielectric medium in reactors, and the breakdown voltage is a key indicator evaluating its insulating properties, which is closely related to the quality of insulating oil. In this paper, 155 reactor insulating oil samples were selected for experiments, which included the measurement of breakdown voltage and collection of multi-frequency ultrasound signals after propagation in the oil samples. The relationship between the breakdown voltage and the amplitude-frequency and phase-frequency responses of ultrasonic acoustic parameters was analyzed. A breakdown voltage prediction method was then proposed by combining multi-frequency ultrasound technology with a grey wolf optimizer (GWO) optimized random forest (RF) algorithm. The results show that the GWO-RF model achieves 4.04% of mean relative error and 95.96% of accuracy on the test set, and there is 20.25% of improvement in prediction accuracy compared to the unoptimized RF model. The proposed prediction model, which integrates multi-frequency ultrasound detection and GWO-RF optimization, demonstrates significant feasibility for predicting the breakdown voltage of insulating oil in reactor.

To reduce the partial discharge (PD) of the basin-type insulator in gas-insulated switchgear (GIS), reduce the occurrence of insulation faults, and improve the safety and stability of GIS operation, we took the 252 kV basin-type insulator as the simulation analysis object. The basin-type insulators with and without grounding shield ring were conducted simulation calculation by Comsol software, and basin-type insulators with different sizes of grounding shield ring were conducted simulation comparison analysis. The insulating and mechanical properties of the basin-type insulator were further verified through experiments. The results show that adding a ground shield ring in the basin-type insulator can effectively improve the electric field in the wedge air gap region and significantly reduce the partial discharge value of the basin-type insulator.