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.

In order to analyze the technological factors affecting the dielectric loss factor of low-voltage forming winding motor stator, the influence rules of four factors including uneven core, oil contamination, insulation moisture, and insulation paint loss on the dielectric loss factor of stator and the corresponding disposal measures were studied. The results show that uneven core, oil contamination, and insulation paint loss lead to the increase of dielectric loss factor of the stator insulation, and the dielectric loss factor of the motor insulation increases significantly when the insulation gets damp. The rise of dielectric loss factor caused by insulation paint loss can be decreased by secondary impregnation.

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 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 order to study the gas production characteristics of ester insulating oils under transformer fault conditions, three ester insulating oils including single ester, natural ester, and synthetic ester insulating oils were selected to analyze their gas production characteristics under simulated low energy discharge conditions. The results show that the hydrogen production ratio of ester insulating oil is higher under low energy discharge, and the production of acetylene is much lower than that of mineral insulating oil, especially the acetylene production of synthetic esters is the lowest. The low energy discharge of mono ester and natural ester insulating oils can be determined by three-ratio, Duval triangle, and Duval pentagon methods. The low energy discharge of the synthetic ester can be determined by the Duval triangle method.

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.

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.

In this paper, the thermal conductivity mechanism of filled epoxy resins was firstly introduced, and then the research status of the modified epoxy resins doped with different dimensional inorganic thermal conductive fillers was summarized mainly. On the basis of design idea for building the thermal conductive paths, the effect of different dimensions of filler size, distribution orientation, mixed filling, surface functionalization and other factors on improving the thermal conductivity of epoxy resin composites were described emphatically, and a comparative analysis was given. Finally, a brief outlook on the future development of the filled epoxy resin research field was put forward.

In the extrusion process of XLPE cable materials, when the local extrusion temperature is higher than the decomposition temperature of crosslinking agent, the crosslinking agent will be thermally decomposed in advance, resulting in a low degree of pre-crosslinking in materials. To study the effect of pre-crosslinking on the space charge characteristics of XLPE DC cable materials, the low-degree pre-crosslinking treatment at different temperatures (140℃, 150℃, and 160℃) was carried out before high-temperature crosslinking of XLPE DC cable materials. The aggregated structure was characterized by X-ray diffraction, differential scanning calorimetry, and crosslinking test, and the space charge characteristics at different temperatures were studied. The results show that pre-crosslinking will hinder the normal crosslinking process of XLPE at high temperature, which leads to the decrease of crosslinking degree and crystallinity. Incomplete crosslinking structure makes the pre-crosslinked XLPE samples accumulate a lot of the same polarity of charges injected by electrodes. At the same time, more impurity molecules remaining in the sample will dissociate with the increase of temperature to produce large number of the different polarity of charges, which leads to serious electric field distortion. The decrease of crystallinity also leads to the increase of trap density and depth at the interface between the crystal region and the amorphous region, which limits the charge migration and detrapping process.

Thermoplastic polypropylene material has excellent electrical and thermal properties. It does not require cross-linking and degassing in the production process of power cable insulation, has low energy consumption and good environmental friendliness, and can be recycled compared with cross-linked polyethylene. In this paper, medium-voltage polypropylene cable and cross-linked polyethylene cable insulation were used as the research object. The directly synthesized polypropylene and cross-linked polyethylene were analyzed by differential scanning calorimetry, Fourier infrared spectroscopy, and X-ray diffraction analysis, and their mechanical tensile properties, dielectric constant and loss, breakdown strength, and other macroscopic properties were tested. The results show that the structure of the directly synthesized polypropylene is interspersed with ethylene segments, which leads to the melting temperature of polypropylene material decrease, but its melting temperature is still much higher than that of the cross-linked polyethylene. The ethylene segment will participate in the crystallization process of polypropylene, but will not crystallize alone. The elongation at break of polypropylene is 712%, which is higher than that of the cross-linked polyethylene (566%), which is more conducive to the transportation and installation of cable. The breakdown strength of polypropylene at 90℃ is 91.5 kV/mm, which is 123% higher than that of the cross-linked polyethylene at the same temperature, and the breakdown strength of polypropylene has better stability with respect to temperature. In terms of comprehensive mechanical properties and electrical properties, the performance of directly synthesized polypropylene is not inferior to cross-linked polyethylene, which can be used as an insulating material for medium voltage cables.