In order to deeply study the variation rule of cable joint temperature and stress under micro-gap discharge and explore its influence on the cracking of the composite interface of cable joints, we took 110 kV cable intermediate joint as the prototype, established a three-dimensional simulation model combining with the most common air gap position in actual cable joint. Temperature, stress, and interface crack were taken as indexes to measure the damage degree of cable joint, the initial grip force between cable joint and ontology as well as the air gap pressure were taken into consideration, the charge law of temperature and stress of the joint with the change of gap discharge energy, air gap pressure, air gap thickness, and air gap position was calculated by finite element method. The effect of the change of gap discharge energy and air gap pressure on cracking situation of the composite interface of the joint was studied in detail. The results show that with the increase of the gap discharge power, the local temperature and stress near the gap increase rapidly, and the interface cracks appear at a distance from the original air gap and new gap generates. With the increase of the air gap pressure, the gap pressure decreases at first and then increases. The negative air gap pressure can increase the local stress, while the positive air gap pressure can reduce the local stress in a certain range, which leads to the interface cracking on both sides of the gap, results in the continuous expansion of the original gap.

In order to investigate the effect of hydrolysable antioxidants on long-term properties of XLPE cable rejuvenation, we selected two kinds of rejuvenation fluid containing different antioxidants to inject water tree ageing cable, and conducted secondary thermal ageing, then tested and analyzed the electrical performance changes of cable samples before and after rejuvenate and thermal ageing. The results show that the two rejuvenation fluid can significantly reduce the dielectric loss factor, leakage current, and conductivity of water tree ageing cable, and increase the breakdown voltage, the rejuvenate samples with hydrolysable antioxidant show a higher breakdown voltage. After secondary thermal ageing, the dielectric loss factor, leakage current, and conductivity of the two groups of rejuvenate samples increase again, while the breakdown voltage decreases. However, the parameters of the rejuvenate samples with hydrolyzable antioxidant are generally superior to those with non-hydrolyzable antioxidant. It is proved that oligomers are generated by hydrolyzation-condensation reaction of hydrolysable antioxidant siloxane, which can elongate the molecular chain segment and improve the ability of migration resistance, and can be retained in the cable insulation for a long time to inhibit the thermal oxygen ageing process of polymer molecules.

In order to solve the problem of small adhesion on the surface of aluminum wire during insulating for conductor in transmission and transformation equipment, the adhesion mechanism of conductor insulation coating was described in detail, and the experimental theory and method of laser cleaning were analyzed. The effect of laser cleaning times on the roughness was studied by experiments, and then pull method and shear method for measuring adhesion were compared and analyzed. The results show that the surface roughness R_a decreases significantly after cleaning for many times, and the first cleaning efficiency is proportional to the coverage energy per pulse area. However, when R_a reaches to 2.5 μm, the cleaning effect begins to slow down. The reduction of roughness can effectively improve the adhesion of the insulating coating surface, the adhesion of epoxy resin and silicone rubber on aluminum plate begins to decrease again when the surface roughness R_a reaches to 4.5 μm and 4.0 μm, respectively.

In recent years, failures of water-blocking buffer layer in high voltage cables occur frequently, which has become a seriously threat to the security of the power transmission system. This paper summarized the latest research status of buffer layer failure, and put forward the possible preventive measures. Firstly, the accidents caused by the failure of water-blocking layer reported in recent years were enumerated, and the common characteristics of the failure were carefully analyzed. Then, on the basis of the introduction of structure and function of the water blocking buffer layer, combining with the literature and the experimental results of scanning electron microscope (SEM), energy disperse spectrometer (EDS), and X-ray diffraction (XRD), the phenomenon of white spot in buffer layer was explained, and it was proposed that the neutral water-blocking powder could effectively inhibit the generation of white spot. According to the research of buffer layer ablation, the ablation mechanism of buffer layer was divided into partial discharge ablation and current thermal effect ablation, and the problems of simulation modeling in the research were discussed. Finally, the existing researches on the ablation of the buffer layer was summarized and prospected, and it is suggested that reducing the gap between aluminum sheath and preventing the buffer layer from getting damp can effectively slow down the buffer layer ablation, but it needs further research to achieve.

The micro-water content in transformer oil is an important factor to measure whether the transformer can operate stably for a long time. Based on multi-frequency ultrasonic detection combined with artificial neural network algorithm, a method for predicting micro-water content in transformer oil was proposed in this study. Firstly, the micro-water content in 210 groups of oils was determined by Carl Fischer titration. Secondly, 210 groups of oil samples were detected by multi-frequency ultrasound to analyze the relationship between micro-water content in oil samples and amplitude and phase signals in multi-frequency ultrasonic data. Finally, the original 242-dimensional multi-frequency ultrasonic data was reduced to 23-dimensional by PCA. Two prediction models for micro-water content in transformer oil based on PCA-GA-BPNN and PCA-PSO-GRNN were established by combining with BPNN and GRNN artificial neural networks as well as GA and PSO optimization algorithms. The prediction results were compared with the actual results. The results show that the forecast accuracy of both models is higher than 90%, which indicates that the method proposed in this study can effectively detect the moisture content in transformer oil.

In order to study the influence of the cosine square wave voltage frequency on the breakdown time of typical cable insulation defects, we prepared the needle plate electrode flaws and water tree defect models. Breakdown tests were carried out with the cosine square wave voltages of 0.1 Hz, 0.5 Hz, and 1.0 Hz, respectively. The distribution of breakdown time and the moment of breakdown point were counted, the microscopic morphology of the breakdown channel was observed, and the mechanism of the influence of the cosine square wave voltage frequency on the breakdown time of the typical insulation defect of the cable was discussed. The results show that with the increase of the frequency of the cosine square wave voltage, the breakdown time of the typical insulation defect of the cable is significantly reduced, and the dispersion is reduced. The vast majority of breakdowns occur during the voltage polarity change phase, which is the rising or falling edge of the cosine square wave. The increase of the synthetic field strength during the polarity conversion process caused by space charge injection and the increase in the number of polarity conversions per unit time may be the main reasons for the above phenomenon.

Sheath-core interface is an important part of the internal insulation for composite insulators, and it is closely related to the abnormal running defects and failures of composite insulator. In this paper, an aged 500 kV composite insulator with abnormal heating was systematically investigated. The ageing characteristics of materials in sheath-core interface were studied by a variety of characterization techniques. The results show that the ageing of the sheath at the sheath-core interface is significantly affected by the electric field strength of its location. The silicone rubber sheath with abnormal temperature rise or discharge adheres well to the epoxy glass fiber core, but there are particle defects at the sheath-core interface. The peak strength of Si-O bond of silicone rubber sheath increases gradually from the inside to the outside, and the peak strength of -NO₂ produced at 1 650 cm^-1 also shows the same trend. These results indicate that the ageing of silicone rubber sheath develops gradually from the inside to the outside, which is contrary to the normal ageing order. The core surface at the interface is already aged and turns to white, and the relative content of epoxy resin in the core material decreases from 17.7% to 11.4%. The X-ray diffraction analysis of the interface particles shows that its main component is aluminum hydroxide (ATH), and it is deduced that the interface particles could be formed due to the aggregation of ATH. This kind of interface particle defect is generated in the manufacturing process of composite insulator, and it is hard to find from appearance.

The flash point measured value of transformer oil is significantly affected by atmospheric pressure, therefore the standard flash point value of transformer oil at 101.3 kPa need to be obtained through pressure correction on the measured value. The open-cup and closed-cup flash point value of new 25^# transformer oil and used transformer oil under different air pressure was measured by flash point tester. The results indicate that there is a certain linear relationship between the reciprocal value of flash point (1/T_F-exp) and the logarithm of experimental pressure (lnP). The linear expression of 1/T_F-exp-lnP for each experimental oil sample is determined, and the range of 1/T_F-exp-lnP slope value is (-2.31--1.22)×10^-4 (for the open-cup flash point) and (-6.68--2.32)×10^-5 (for the closed-cup flash point) for different oil samples. The flash point value of the experimental oil sample determined under different pressure (60.0-102.0 kPa) is corrected to the standard flash point value at 101.3 kPa, and it is found that the correction deviation is far less than the deviation caused by using the current standard method on basis of the T_F-exp-P linear relationship. The correction method based on the linear relationship of 1/T_F-exp-lnP has a better correction accuracy and a wider pressure range, which has good practical significance to obtain more accurate corrected flash point value of transformer insulating oil.

With the continuous advancement of electronic technology, the oriented polypropylene (BOPP), which is currently the most widely used in commercial applications, cannot meet the requirements of electronic products due to its small energy storage density. Among many dielectric film materials, polyvinylidene fluoride (PVDF)-based nanocomposites have received widespread attention due to their high dielectric constant and good processing properties. In this paper, the theoretical model of dielectric composites was introduced, the main preparation strategies of PVDF-based nanocomposites: filler modification, multi-phase blending, and construction of novel structures, were reviewed systematically, and the development of PVDF-based composite energy storage materials was prospected.

A low viscosity, fast-curing single component thermally conductive structural adhesive base glue was prepared by using bisphenol A epoxy resin E124 as the main resin, polythiol as the curing agent, and imidazole as the accelerator, its curing process was discussed by surface drying time and gel time test and DSC test. A thermal conductive structural adhesive was prepared by adding thermal conductive fillers, and the particle size, shape and amount of fillers on the properties were studied. The results show that when the mass ratio of the base glue to two different sizes of alumina, which is m(base glue): m(40 μm Al₂O₃):m(2 μm Al₂O₃)=15:7:3, m(base glue): m(40 μm Al₂O₃):m(5 μm Al₂O₃)=15:7:3, m(base glue):m(40 μm Al₂O₃):m(10 μm Al₂O₃)=15:8:2, the three high thermal conductive structural adhesives could cure at 90℃ in 15 min. Their thermal conductivity is 2.46, 2.59, and 2.42 W/(m·K), adhesive strength is 5.92, 6.46, and 6.49 MPa, viscosity is 63 800, 74 100, and 87 000 mPa·s, respectively, which meets the basic performance requirements of screen printing process and chip package heat dissipation materials.