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.

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.

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 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.

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.

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.

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.

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.

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.

Gas chromatography is a common way to determine the dissolved gas content in insulating oils, but there is many types of natural insulating oil, and the Ostwald coefficient is not clear. In order to be able to accurately calculate the dissolved gas content in natural insulating oils, the Ostwald coefficient of seven typical dissolved gases in four natural ester insulating oils was measured using double equilibrium method and compared with the Ostwald coefficient in mineral oil. The results show that there is little difference on Ostwald coefficient of the seven dissolved gases in the three natural esters, while the Ostwald coefficient of dissolved gas in the modified esters is slightly greater than that in the natural esters. The Ostwald coefficient of dissolved gas in FR3 has little relative error compared with the available data, which proves that the data obtained by the method has good credibility.