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.

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.

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.

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.

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.

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.

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.

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.

Insulating materials are prone to flashover along the surface due to their inherent properties, contamination accumulation, rain and snow, and so on. In this paper, a multifunctional nanocomposite coating (SiO₂/MWCNTs/PDMS) was prepared through a "polymer+nanofiller" method by using polydimethylsiloxane (PDMS) elastomer as the matrix, SiO₂ nanoparticles and multi-walled carbon nanotubes (MWCNTs) as fillers. The trap distribution, DC surface flashover voltage, and hydrophobicity of the nanocomposite coating were tested and analyzed. The results show that the composite coating has a high surface roughness, which hinders the development of surface discharge. The surface flashover voltage of the composite coating increases from 17.7 kV to 24.3 kV, with a growth of 37.3%. With the increase of SiO₂ and MWCNTs content, the water contact angle of the composite coating has no obvious change the maximum water contact angle reaches 150.5°, and the composite coating has good self-cleaning ability. The research results can provide reference for the engineering application of composite insulation coating materials in the future.

Due to its advantages of high flash point, high ignition point, and biodegradability, natural ester insulating oil is gradually being promoted and applied in high voltage level power transformers. In order to provide basic data support for the insulation structure design of high voltage and large capacity natural ester transformer, we took mineral oil as a reference, and selected natural ester insulating oil as the test object to analyze its breakdown characteristics and gas generation law under lightning impulse voltage. The results show that the lightning impulse breakdown voltage of natural ester turn-to-turn insulation is above 200 kV, with the increase of turn-to-turn insulation thickness and insulation gap distance, it shows an upward trend and is slightly lower than the breakdown voltage of mineral oil under the same conditions. H₂ and C₂H₂ dissolved in oil are the main characteristic gases under lightning impulse voltage, and their relative percentage content increases with the increase of insulation gap. The Duval pentagon method is used for fault diagnosis of dissolved gases in oil, and the discharge at the fault points are basically diagnosed as high-energy discharge, which are consistent with the energy released by lightning impulse.