The fluidity of granular dry unsaturated polyester molding compound was tested at different pressure and temperature by using capillaries of different specifications, and the fluidity and curing behavior during the molding process of the granular dry unsaturated polyester molding compound were studied. The results show that the best conditions for testing the fluidity of dry unsaturated polyester molding compounds by Rasig method are as follows: the test temperature is 160℃, the load pressure is 50 kg, the capillary template pore size is 1.0 mm, and the remaining ingot height is greater than 1.5 mm.

A 500 kV decay-like composite insulator was conducted infrared temperature rise tests under dry conditions, 55%, 75%, 95% of ambient humidity and 3, 5, 7 m of observation distances to study the temperature rise characteristics of the decay-like composite insulator under different humidity and observation distance. The results show that the temperature rise of the decay-like composite insulator increases with the increase of ambient humidity, its temperature rise in 95% of humidity environment increase by 22%-40% compared with that under dry environment. When the humidity is constant, the temperature rise decreases with the increase of observation distance. The deteriorating section length of the core rod can be judged by the temperature rise distribution curve characteristics on the axis of decay-like composite insulator, which can realize the non-destructive detection of the deteriorating section length of decay-like insulator.

With the development of power capacitors towards lightweight and miniaturization, dielectric materials with high energy storage density have broad application prospects in the field of capacitors, among them, nanocomposite dielectrics with core-shell structure have attracted extensive attention due to their excellent energy storage properties. In this paper, the latest progress of nanocomposite dielectrics with core-shell structure were introduced, the preparation methods of nanofillers with core-shell structure were introduced, including organic surface modification and inorganic shell covering. Then the effect mechanism of nanofillers with core-shell structure on the electric strength, dielectric constant, and dielectric loss of nanocomposite dielectrics were discussed, and the construction strategies of nanocomposite dielectrics with core-shell structure were provided. At last, the existing problems in the design of high energy storage density nanocomposite dielectrics with core-shell structure were summarized, and their future development direction was prospected.

The partial discharge inception voltage (PDIV) of the turn-to-turn insulation for low-voltage random-wound inverter-fed motor changes with the rise time of pulse voltage, resulting in that it is difficult to find the vulnerable spot of the insulation system when the rise time changes. Therefore, considering the rise time of pulse width modulation, we proposed a three-curve method to determine the insulation vulnerable spot of inverter-fed motors by combining the PDIV test technology under repetitive pulse and sinusoidal voltages. At first, the minimum PDIV distributions of main insulation and phase-to-phase insulation were obtained by testing their PDIV under the sinusoidal voltage. Then, the turn-to-turn insulation of inverter-fed motor was conducted PDIV tests using the repetitive pulse voltage with 50-200 ns of rising time, and the fitting curves of PDIV with rising time were obtained. At last, according to the three-curve diagrams, considering the rise time of motor, the vulnerable spot of insulation system was determined, and the feasibility of three-curve method was verified by breakdown experiment. The results show that the insulation vulnerable spot of low-voltage random-wound inverter-fed motor can be judged by the three-curve method, and the feasibility of the three-curve method is verified by breakdown test.

In order to improve the capacitance and energy density of polypropylene film, the mixed slurry of high dielectric constant nanoparticles and acrylate binder was used to modify the surface of polypropylene film. By comparing with the properties of polypropylene film before modification, the effects of base film stretching technology, surface modification temperature, and high dielectric nanoparticles size on the properties of the modified polypropylene film were discussed. The results show that after modification, the dielectric constant of polypropylene film increases from 2.20 to 2.89 at 1 kHz, and the energy storage density increases from 2.56 J/cm³ to 3.29 J/cm³ under 500 kV/mm of electric field, which increases by 28.5%. At the same time, the modified polypropylene film maintains stable dielectric properties in the range of 20-120℃, and the variation range of dielectric constant is less than 6.9% under 1 kHz. The solution method used to modify polypropylene film in this study is simple and easy, and is feasible for large-scale preparation, which has a certain reference value for the large-scale preparation of dielectric materials with high energy storage density.

In order to study the influence of winding heat release during actual transformer operation process at extremely low temperature on the breakdown characteristics of transformer oil, the 45^# transformer oil was selected as the research object, and the saturated moisture content of the transformer oil with different temperature under extremely low temperature were tested. A breakdown test platform under low temperature environment was built, and the electric strength of the transformer oil was measured under uniform temperature and gradient temperature conditions. The results show that the relationship between saturated moisture content of the transformer and temperature satisfies Arrhenius equation under low temperature. After fully standing at low temperature, the electric strength of uniform temperature transformer oil increases to a certain extent in low temperature range, and the electric strength at -10℃ increases most obvious where the electric strength is the minimum, which is 51.5% higher than that before standing. The electric strength change curve of gradient temperature transformer oil with temperature is similar to that of the uniform temperature transformer oil, showing a “U” shape distribution as a whole. The minimum electric strength of gradient temperature transformer oil does not decrease compared with that of uniform temperature transformer oil, but slightly increases, and the temperature reaching the electric strength minimum value increases.

Firstly, the actual flashover test of large fittings-composite insulator-metal grounding body was carried out in the laboratory environment, the quantitative relationship curve between flashover voltage and space net distance was obtained. Further more, the three-dimensional finite element models of two typical structure of high voltage bushing composite insulators for capacitive core converter transformer and double-layer metal shield through wall were established, and their distribution characteristics of typical voltage and electric field under operating state were obtained, the relationships between local high field strength and corona, flashover phenomena of composite insulators for bushing were analyzed. On the basis of the above theoretical analysis and test data, the size altitude correction and umbrella skirt contour topology optimization of composite insulator for UHV AC/DC bushing were carried out. Finally, all the electric field strength of key parts can meet the control requirements, which proves that the rationality and effectiveness of the design of composite insulator for bushing at UHV level.

Nuclear grade medium voltage cables were conducted thermal-water tree and irradiation-water tree ageing tests. The nuclear grade medium voltage cables with different ageing time were tested by PDC method to obtain the conductivity under different voltages, and the nonlinear coefficients under different ageing conditions were obtained through the conductivity. The results show that with the increase of ageing time, the nonlinear coefficients of the cable samples under two ageing conditions increase gradually. Under the condition of heat-water tree combined ageing, with the increase of ageing time, the absorption peak intensity of O-H bond increases, indicating that the cable samples are water tree aged more seriously. In the irradiation-water tree ageing test, the absorption peak intensity of O-H bond increases slightly, indicating that the ageing degree of water tree increases. At the same time, although the characteristic peak of O-H bond appears near 1 640 cm^-1 in the infrared spectra of water tree aged cable sample, the peak value changes little with the increase of irradiation dose. Therefore, the results of O-H bond absorption peak in infrared spectrum test can not well reflect the radiation ageing state of cable, but can only reflect the ageing state of water tree.

In this paper, a series of preparation and modification researches of polyphenylene oxide resin in the field of super high-frequency and low dielectric were reviewed, including the preparation of low molecular weight polyphenylene oxide, chemical structure modification, composite with other resins, organic/inorganic hybrid. Then the existing problems of low dielectric polyphenylene oxide resin were summarized, and its future development trend was prospected.

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.