In this paper, the research progress on new eco-friendly insulating gases at home and abroad was reviewed, and the international mainstream promotion of perfluoroisobutyronitrile (C₄F₇N) gas and its application in electrical equipment were expounded emphatically. The gap and surface insulation characteristics of C₄F₇N gas mixture were summarized, and the design criteria of the related equipment insulation was proposed. The decomposition characteristics and gas-solid compatibility evaluation indices of C₄F₇N gas mixture under different operating conditions were analyzed. Additionally, the research progress on arc-quenching performance of C₄F₇N and its gas mixture and the development and application of a series of eco-friendly equipment were introduced, so as to provide a theoretical reference for the environmental protection upgrade of SF₆ electrical equipment in the current stage. Meanwhile, it was noted that the research and development efforts on new eco-friendly insulating gases with superior performance were still ongoing at home and abroad, which can provide technical support for the construction of a green and low-carbon power grid.

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.

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.

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 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.

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.

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.

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.