High voltage power cables demand stable and reliable connections with the power system through cable accessories, and silicone rubber (SIR) and ethylene-propylene-diene monomer (EPDM) are the main reinforced insulation materials for cable accessory due to their excellent dielectric properties and mechanical properties. However, with the continuous improvement of high voltage transmission level, the electrical properties of cable accessories also need further development. In this paper, the research status inside and out of China was summarized, then the factors affecting the electrical properties of SIR and EPDM for high voltage cable accessories were discussed. The research progress in improving method of electrical properties were analyzed, and the research progress in the interfacial electrical properties of double-layer dielectric was introduced. At last, the research difficulties of electrical properties modification of rubber reinforced insulation for high voltage cable accessories were summarized, and the research trends of the rubber insulating materials for high voltage cable accessories were prospected.

The fault type recognition and condition assessment play the crucial roles in fault diagnosis and maintenance. Different defects will produce partial discharge signals with difference, and the partial discharge signal also changes with the defect severity and the evolution of partial discharge. This situation can be regarded as pattern recognition of different severity levels and evolution stages, and the pattern recognition is a typical classification problem. In this paper, classification problems such as pattern recognition and state assessment were reviewed. Compared to classification results based on mathematical statistics, artificial intelligence has achieved nearly 100% of recognition accuracy. However, there are still some shortcomings in current research, this paper gives some solution strategies and prospects future research direction.

In order to obtain a silicone rubber composite with both good resistivity nonlinear characteristics and high electric breakdown characteristics, a copper calcium titanate (CCTO) ceramic powder was prepared, and the CCTO was mixed with two-component liquid silicone rubber to prepare CCTO/silicone rubber composite (the volume fraction of CCTO ceramic powder was 3%, 5%, and 10%, respectively). Their microstructure, dielectric spectrum characteristics, DC breakdown performance, and DC nonlinear resistivity were tested, and their application performance was compared by establishing a high-voltage DC cable terminal simulation model. The results show that the CCTO ceramic powder has obvious characteristic diffraction peaks, and the powder particle size is about 500 nm; with the increase of CCTO ceramic powder content, the dielectric constant of the CCTO/silicone rubber composite increases, the DC breakdown strength decreases, and the non-linear characteristics of resistivity are significantly enhanced. With the enhancement of the nonlinear characteristics of resistivity, the electric field strength in the reinforced insulation of cable terminal stress cone decreases significantly. When the volume fraction of CCTO ceramic powder in the composite is 5% or 10%, the composite can be used as a reinforced insulating material of stress cone for high voltage DC cable accessory.

A MWCNTs/T-ZnOw/PVB composite was prepared via solution blending method by using multi-walled carbon nanotubes (MWCNTs) and zinc oxide whisker (T-ZnOw) as fillers and polyvinyl butyral (PVB) as matrix. The thermal diffusivity of the composite was measured by transient electro-thermal technique (TET). The effects of different mass fraction ratio of fillers, heat treatment time and temperature, and ultraviolet accelerated ageing on the thermal conductivity of the composite were investigated. The results show that under the same mass fraction of fillers, the thermal conductivity of the composite increases at first and then decreases with the increase of MWCNTs content. When the heat treatment time is 1 h, with the increase of the heat treatment temperature from 80℃ to 140℃, the thermal conductivity of the composite decreases at first and then increases. When the heat treatment temperature is 140℃, the thermal conductivity increases after heating for 1 h, but it decreases with the extension of heating time. When the composite is accelerated aged by ultraviolet irradiation, the thermal conductivity of the composite decreases with the increase of irradiation time.

In order to research the influence of processing temperature of master-batch on the properties of HVDC cable insulation, three kinds of master-batch were prepared at different processing temperatures, and then the insulating properties of HVDC cable insulation manufactured by the master-batch were studied. Impurity detection was carried out at first, and then the chemical structure of crystal points detected by impurity detection was analyzed by attenuated total reflectance infrared spectroscopy (ATR-IR). At last, the electrical and mechanical properties of three HVDC cable insulation were compared. The results show that there are more crystal points detected in the HVDC cable insulation prepared by the master-batch under an excessive processing temperature, and part crystal points are light yellow. Compared with semitransparent crystal points, there is no strong absorption peaks in the infrared spectra of light yellow crystal points, but the excessive processing temperature has a greater negative effect on the electrical properties of HVDC cable insulation, while has a smaller effect on its mechanical properties.

A high temperature resistant insulating varnish for traction motors was prepared by modifying polyester-imide insulating varnish with meta-aramid resin. The effects of meta-aramid resin on the adhesion, electrical properties, flame retardancy, and heat resistance of the insulating coating were studied. The results show that when the content of meta-aramid resin is 15%, the electrical strength, the adhesion, the oxygen index of the insulating coating reaches 65 kV/mm, 12.9 MPa, 28.7%, respectively. After high temperature test at 220℃ for 500 h, the paint film shows slight discoloration without cracking and peeling off, which can meet the requirements of the traction motors.

According to the special requirement of all glass fiber reinforced resin matrix composite (GFRP) transmission tower head on the fastening strength and insulation performance of connecting parts, GFRP insulated fasteners were developed. A medium-high temperature curing epoxy bulk molding compound (BMC) was designed and developed, and its initial viscosity decreases through blending modification of bisphenol A and bisphenol F epoxy resin. The mechanical properties and water absorption of samples with different ratio of Al(OH)₃, talcum powder, and SiO₂ fillers were tested to determine the material system that meets the requirements of molding process. Non-isothermal DSC measurement was conducted on the optimized material system, and T-β extrapolation and orthogonal tests were applied to evaluate, optimize, and determine the forming process parameters. The results show that the failure load, shear strength, and electrical properties of the product prepared by the optimal material system and forming process parameters meet the design requirements, which indicates this material system is suitable for the manufacture of GFRP fasteners with higher electrical performance.

In order to apply the frequency domain spectroscopy test technology to nondestructively test of ageing state for large electric machine stator bar insulation, we conducted accelerated ageing tests on the epoxy-mica insulation of stator bar under the combined action of multiple factors such as heat, electricity, and machinery, and tested the frequency domain spectroscopy of samples with different ageing degrees. The Davidson-Cole model parameters of frequency domain dielectric response were extracted as characteristic parameters by least square method, and the ageing degree of epoxy-mica insulation was analyzed according to the parameters. Finally, the relationship between characteristic parameters and ageing time was established by fitting. The results show that the dielectric loss factor (tanδ) of epoxy-mica insulation increases with the increase of ageing degree, especially at the frequency of 10^-3–10^-1 Hz; the characteristic parameters β and τ extracted by the Davidson-Cole model can be used to characterize the ageing degree of insulation. With the increase of ageing time, β decreases linearly, τ decreases exponentially, while Δε increases obviously. A formula for quantitatively characterizing the degree of insulation ageing is obtained through the fitting relationship between τ and ageing time, which can be considered as an empirical formula for evaluating the ageing degree.

In order to better use the repetitive surge oscillograph (RSO) method to quantitatively evaluate the turn-to-turn insulation defects of generator rotor winding, we conducted turn-to-turn short circuit simulation tests under different positions, different short circuit coils, and different short-circuit resistance on a real rotor when the generator was being repaired. The response waveform and characteristic waveform of the two poles were obtained by the RSO device. Three characteristic parameters that is voltage deviation rate, time deviation rate, and fault severity factor were proposed to study the characteristic waveform. The results show that RSO can be used to quantitatively evaluate the severity of turn-to-turn insulation defects of rotor, the fault type of rotor winding can be quickly determined according to the amplitude and deviation rate of voltage of characteristic waveform, and the fault location can be basically realized according to the time deviation rate. The voltage amplitude of the RSO characteristic waveform has a linear positive correlation with the number of short-circuit coils, and has an exponential relationship with the short-circuit resistance and the center time, respectively. The fault severity factor has a linear positive correlation with the number of short-circuit coils. According to the analysis results, two methods to quantitatively evaluate the severity of rotor turn to turn insulation defects are proposed, which can be used to accurately evaluate the turn-to-turn insulation status of rotors.

In order to study the influence of insulation ageing of cable joint on the electric field distribution at the typical defects of XLPE/silicone rubber interface, we built a simulation model of 10 kV cable joint by COMSOL Multiphysics simulation software. The electric field distribution in the composite interface with defect of conductive impurities, scratches, moisture, respectively was calculated before and after ageing of insulation. And the growth change of water tree in the cable insulation was analyzed combining with the simulation results. The results show that three kinds of defects can lead to obvious electric field distortion at the defect of composite interface, and the ageing of insulation can increase the electric field strength at the interface, and increase the effect of defects on the electric field distribution at the same time. The electric field distortion at the end of water tree is serious after initiation, with the growth and development of water branches, the electric field strength in the area close to each other decreases, but the local electric field concentration still exists.

In order to investigate the optical properties of epoxy-mica insulation in ageing process, we conducted multi-factor accelerated ageing treatment on epoxy-mica main insulation of generator stator bars. The blue whiteness of the samples at different voltage and temperature was measured, and the effect of ageing on the optical refractive index and absorption ratio of epoxy-mica insulation material was analyzed. The equation relationship between blue whiteness and ageing time was determined by regression analysis, and the validity of equation was proved by t test. Finally, the insulation life of epoxy- mica main insulation was fitted and analyzed by the variation characteristics of blue whiteness combining with Fallou model. The results show that the blue whiteness value of epoxy-mica insulation decreases constantly with the increase of ageing degree of insulation. Combining Weibull distribution probability with Fallou model, we can realize the life assessment of epoxy-mica insulation.

The thermal ageing behavior of insulating paper and the evolution of refractive index of cellulose fiber during thermal ageing process were explored. Accelerated thermal ageing experiments were carried out on the oil-paper samples at 120℃ and 130℃, respectively. The degree of polymerization, furfural content, and carbon and oxygen gas content were measured at different ageing periods, and their changing laws were explored. According to the change trend of the crystallinity of insulating paper in the test, a reasonable inference was made about the change of its refractive index, and further analysis was carried out by the Abbe refractometer. On the basis of dispersion staining color method, the evolution rule of refractive index of the oil-paper cellulose fiber during thermal ageing was explored. The results show that as the ageing progresses, the degree of polymerization decreases, while the furfural content and the carbon oxygen gas content increase, the refractive index and crystallinity of the insulating paper increases, and the refractive index of the oil-paper cellulose fiber increases also. The refractive index of cellulose can be considered as a new type of ageing characteristic of insulating paper.

A detection platform of insulation partial discharge (PD) under pulse voltage was constructed by using electromagnetic detection method with the help of a high-voltage pulse power source with adjustable voltage parameters of repetitive pulse. The influence law of frequency variation of repetitive square wave voltage on the PD statistical characteristics of single point and multipoint interturn insulation discharge model for inverter-fed motor was studied. The results show that with the increase of frequency, the amplitude of both insulation single point and multipoint discharge decrease significantly. This may be caused by composite factors such as the change of residual charge distribution on insulation surface, the increase of the excitation probability of initial electron, and the increase of surface temperature after discharge. According to the research results, the influence of square wave pulse voltage frequency on the PD pulse should be carefully considered in the PD detection of motor insulation, and the appropriate voltage frequency should be selected to improve the sensitivity and test accuracy.

The traditional insulation reliability evaluation method based on failure life data for solid dielectric need long test time, and the failure data is little and difficult to obtain. In view of this issue, a reliability assessment method based on the test data of accelerated performance degradation was studied in this paper. Firstly, the average charge density was used as the characteristic parameter of performance degradation, and accelerated performance degradation tests were conducted to obtain performance degradation data under 140 kV/mm, 160 kV/mm, and 180 kV/mm of field strength, respectively. Secondly, the pseudo-failure life of XLPE insulation material under different field strengths was calculated, and its statistical distribution was determined. Finally, the pseudo-failure life data was expanded in combination with the virtual augmentation theory, and the expanded data was analyzed by statistical analysis methods. The results show that the performance reliability indicators such as sample reliability, failure distribution density function, and average life can be obtained through using accelerated performance degradation test method to assess the reliability of XLPE insulation. In combination with the accelerated life model theory, the life expectancy of the XLPE insulating material is 15.3 years under 20 kV/mm of stable field strength, and the reliability when reaching the expected life is 0.445 1.

According to the structure of cable buffer layer, a partial discharge test platform with controllable humidity environment for buffer layer was established. Three typical defect models were designed and conducted partial discharge tests by pulse current method. The results show that the variation of relative humidity has little effect on the geometric shape of partial discharge pattern, while has large effect on the characteristic parameters of partial discharge. With the increase of humidity, the discharge amplitude of three models decreases at first and then increases. Under constant humidity and voltage, the partial discharge activity in the needle-plate system is higher than that in the other models. Under 180 V of voltage and 90% of humidity, the maximum pulse amplitude reaches 770 mV.

In areas with large temperature difference between day and night, negative pressure is easy to form in the expander of oil-immersed inverted current transformers, which will affect the distribution of water in the oil-paper insulation, and then affect the insulating properties of the oil-paper insulation system. In this paper, the saturation solubility of water in insulating oil under different pressure and temperature was measured by indirect method. On the basis of the test results, the relationship between the saturation solubility of water in insulating oil and temperature and voltage in Arrhenius equation was fitted by recursive least square method, and the effect law was studied. The results show that with the increase of negative pressure, the water saturation solubility in insulating oil decreases constantly, and this trend becomes more obvious with the increase of temperature. Under the effect of negative pressure, water in the oil separates out and turns into suspended water in insulating oil or transfers to insulating paper. The translation factor n related to pressure was introduced in this paper, and a mathematical expression involving pressure, temperature, and water saturation solubility in oil was established. The acid value, temperature, and pressure were taken as the comprehensive factors to evaluate the water saturation solubility in insulating oil, and the multi factor influence law was established, in which the influence of acid value and pressure can cancel with each other to some extent. The water distribution and insulation state of the oil-paper insulation system for oil-immersed inverted current transformers can be evaluate more accurately by the above research results.

The influence of sampling period, voltage rising speed, and overcurrent breaking time on the breakdown voltage of insulating materials in the process of breakdown voltage test was analyzed. Proper improving technology and method was select to eliminate or reduce the error. The results show that the dispersion of test results is reduced by digital filtering, weighted average, and other methods, and the accuracy of dynamic measurement is improved at the same time.