Polyvinylidene fluoride (PVDF) is widely used in the fields of energy storage, sensors, and energy collection due to its excellent dielectric, piezoelectric, and ferroelectric properties. PVDF has a variety of crystalline phases, which correspond to different molecular conformations and physical properties. At present, there are still controversies about which PVDF crystal phases dominate under certain conditions, the corresponding morphological characteristics of each crystal phase, and the influence on the physical properties of PVDF. This article reviewed the structural characteristics of the three main crystal forms (α, β and γ) of PVDF and the current mainstream characterization methods, and analyzed the difficulties and potential ways to accurately establish the "structure-activity" relationship of PVDF polymorphs. The factors affecting the formation of PVDF polymorphs and the transformation rules were also reviewed.

Polylactic acid (PLA) is a kind of the most widely studied green biodegradable polymer material and is currently recognized as the most promising industrial application due to its excellent biodegradability, biocompatibility, processability, high mechanical strength, non-toxicity, and no stimulation. However, the disadvantages of slow crystallization rate, low crystallinity, and poor heat resistance of PLA severely limit its application in the fields with higher temperature requirements. This paper reviewed the current research on the heat resistance of PLA inside and outside China, mainly from the crystal type modification, blending modification, and cross-linking modification, and then prospected the future development of high heat resistance PLA.

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.

In order to evaluate the application of epoxy potting sealant in China on the packaging of insulated gate bipolar transistor (IGBT) power modules, we selected two domestic epoxy pouring sealant to compare their properties, including the viscosity, density, and gel time before curing, and the basic properties, thermal properties, and insulating properties after curing. The difference between the two epoxy potting sealants were analyzed, and IGBT power modules were encapsulated with them, respectively. Environmental tests such as high temperature storage, low temperature storage, and temperature cycle were conducted on the IGBT modules. The results show that the two epoxy potting adhesives have different toughening mechanism, mixing ratio, curing temperature, mechanical strength, and T_g value, which have some effect on the package. CTE value is the most important parameter that affects the application of epoxy potting adhesives in IGBT module package.

The cross-linking rheological properties of the cross-linked polyethylene (XLPE) insulation material can reflect the premature cross-linking phenomenon and processing performance during the extrusion process. The hindered phenol antioxidant in the insulation material can not only inhibit the thermal oxidation and degradation during extrusion process, but also effectively capture free radicals to inhibit the cross-linking reaction, thereby improving the processing performance of the polyethylene melt. In this paper, insulation materials with different types and contents of hindered phenol antioxidants were prepared, the changes in the torque and temperature of the insulation material under shear at 140℃ were tested, the cross-linking rheological properties of insulating materials were studied, and the cross-linking degree and cross-linking by-products content of the insulating materials were characterized. The results show that the worse the ability of the insulating material to inhibit premature crosslinking, the more gel is produced, and the greater the equilibrium torque. The hindered phenol antioxidant can inhibit the premature cross-linking reaction, compared with the molecular weight and steric hindrance of hindered phenol antioxidant, the number of hydroxyl groups per unit mass is more closely related to the ability to inhibit premature crosslinking.

The insulation master batch was prepared by twin-screw extruder and internal mixer, and then it was used to prepare HVDC cable insulation. The dispersing property of nanoparticles in insulation was quantitatively analyzed by transmission electron microscope (TEM) and Image J software, and the electrical properties of insulation were tested. The results show that it is difficult to determine the dispersion and distribution degree of nanoparticles in nano-composite DC materials by qualitative analysis through TEM, while the characteristic data of nanoparticles in TEM images can be extracted by Image J, so as to quantitatively analyze the dispersion properties of nanoparticles. Compared with DC material using the master batch prepared by internal mixer, the DC material using the master batch by twin-screw extruder have better electrical properties, and nanoparticles are more evenly distributed and better dispersed in it.

On the basis of filling magnesium hydroxide sulfate hydrates（MHSH）whisker and aluminium trihydrate (ATH) blends, EVA/LDPE composites were prepared by adding borax, zinc borate (ZnB) and microencapsulated red phosphorus (MRP), respectively. The effects of multiple synergistic flame retardant containing MHSH whisker on the flame retardancy, mechanical and electrical properties of EVA/LDPE composites were analyzed by thermogravimetric analyzer, cone calorimeter, limiting oxygen index (LOI) meter, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), universal tensile machine, AC medium strength tester, and high resistance meter. The results show that the initial decomposition temperature T_d, the peak temperature of weight loss rate T_p2 and T_p3, and the residual rate at 800℃ of EVA/LDPE composites are significantly increased by adding MRP. Compared with the samples adding borax and zinc borate, the samples adding MRP show the best flame retardancy, the peak value of heat release rate curve and total heat release value decreases to 219.86 kW/m² and 19.38 MJ/m², respectively; and the time to ignition (TTI) and LOI increases to 59 s and 28.4%, respectively; during the combustion process, the carbon layer of the sample is continuous and compact, without melting and dropping. The mechanism of MRP cooperating with MHSH whisker and ATH to improve the flame retardancy of EVA/LDPE composite is due to the synergistic effect of gas phase flame retarding and condensed phase flame retarding. The addition of MRP reduces the elongation at break of the composites, but they still have good electrical insulation properties.

In order to clarify the interaction of electrical stress, thermal stress and mechanical stress on the insulating properties of ethylene propylene diene monomer (EPDM), we prepared EPDM samples and measured the polarization current of the sample at temperatures of 30‒120℃, pressures of 0‒1.0 MPa, and electric field strengths of 1‒25 kV/mm, respectively. For further evaluating the insulation performance of EPDM, the absorption ratio (K) was introduced. Based on the steady-state electrical conduction currents in the polarization currents, the quasi-steady-state electrical conduction current densities and conductivities of the sample were calculated. The results show that the insulation performance of EPDM is negatively correlated with temperature and electric field strength. In the low-field-strength area, the electrical conductivity of the sample conforms to the ohmic conductivity characteristics; in the high-field-strength area, conductivity conforms to the space charge limited current mechanism. By calculating the carrier mobility of EPDM, it is found that the carrier mobility decreases at first and then increases with the increase of pressure, which leads to the change of the insulation performance of EPDM.

In order to improve the bonding strength of SiO₂ and methyl vinyl silicone rubber (MVSR) matrix, we selected poly(methyl methacrylate) (PMMA) as the graft chain of SiO₂. Molecular simulation was used to calculate the change of binding energy, molecular overlap length, and mean orientation shift. The change law of interface bonding strength between SiO₂ and MVSR matrix under different graft densities and lengths of PMMA short chains and the effects of electric field and temperature on the interface bonding strength were studied, and compared with SiO₂ ungrafted and KH570 grafted. The results show that as a graft chain, PMMA has a better improve effect on the interface bonding strength of SiO₂ and MVSR than the common silicone coupling agent KH570. When the graft density is 13% or 16%, and the graft chain length is 5, PMMA shows the best grafting effect. Under negative electric field, the interfacial bonding strength increases with the increase of the field intensity, while it decreases with the increase of the field intensity under the positive electric field. With the increase of temperature, the bonding strength between SiO₂ and MVSR interface decreases continuously.

Vegetable insulating oil has a higher ignition point and better environmental protection, but its breakdown voltage is lower than mineral insulating oil. Using nanoparticles to modify vegetable insulating oil can improve the breakdown voltage of vegetable insulating oil. In this study, the lightning impulse breakdown voltage test was conducted on vegetable insulating oil modified by different volume fractions of nano-TiO₂, at the same time, pictures of the production, development, and termination of the streamer during the breakdown process were taken. The streamer development morphology of the original vegetable oil and the nano-modified vegetable insulating oil were observed, and the effect law of TiO₂ nanoparticles on the lightning impulse breakdown voltage of vegetable insulating oil and the development process of streamers was analyzed. The results show that adding TiO₂ nanoparticles can improve the positive polarity lightning impulse performance of vegetable insulating oil, and the development speed of streamer is relatively slow, while the development speed of streamer is faster under negative polarity lightning impulse voltage.

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 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.