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.

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.

The dielectric constant and dielectric loss of composite insulating paper can be reduced by doping different content of aramid 1313 (PMTA) in the natural cellulose pulp, but the mechanical properties of composite insulating paper are greatly reduced due to the poor combining ability of two fibers. In order to solve this problem, we studied the improvement effect of low-temperature plasma treatment on the binding capacity of two kinds of cellulose emphatically, and tested and analyzed the properties of the prepared composite insulating paper. The results show that the low-temperature plasma treatment can effectively improve the binding force between the two kinds of fibers, so as to make up for the loss of mechanical properties, and it has little influence on the dielectric and breakdown characteristics of the composite insulating paper. The results of SEM and XPS show that the surface of PMTA fiber after low temperature plasma treatment become rough, the polarity functional group increases, the binding force with cellulose matrix increases, and the voids of composite insulating paper sample decreases significantly. The power frequency relative permittivity of the composite insulating paper prepared by doping low temperature plasma treated PMTA in natural cellulose pulp is reduced, while the AC/DC electric strength and volume resistivity remain unchanged, and the tensile strength and elongation at break are compensated to the maximum.

A derived diamine monomer of 1,3-bis(4-aminobenzoyoloxymethyl)-1,2,2-trimethylcyclopentane (BABMT) was synthesized from a biomass small molecule natural-(D)-camphor. It was polymerized with aromatic structure dianhydrides (BPDA and ODPA) to prepare semi-alicyclic structure polyimide (PI) films, respectively, and their thermal properties, mechanical properties, optical properties, and solubility were compared with that of the wholly aromatic structure PI films prepared by alicyclic structure diamine (DMB) polymerizing with alicyclic structure dianhydrides (BPDA and ODPA), respectively. The results show that the semi-alicyclic structure PI films have good thermal properties, the glass transition temperature (T_g) is 281℃ and 236℃, respectively, and the 5% weight loss temperatures (T_d5) is not lower than 390℃. At the same time, the semi-alicyclic structure PI films retain good mechanical properties, the tensile modulus is 2.13 GPa and 2.41 GPa, respectively, the tensile strength is 109 MPa and 97.9 MPa, respectively, and the elongation at break is 7.7% and 11%, respectively. Compared with the wholly aromatic structure PI films, the semi-alicyclic structure PI films show better transparency with the transmittance at 500 nm (T₅₀₀) over 78%, and better solubility in common organic solvents. Wide-angle X-ray diffraction results show that all the semi-alicyclic structure PI films present amorphous structure, and their interchain spacing (d) are larger than that of the wholly aromatic PI films.

The aromatic ketone polymer 4-allyloxy-2-hydroxybenzophenone (AOHBP) was selected as voltage stabilizer. In order to inhibit the migration and precipitation of voltage stabilizer in polymer, dicumyl peroxide (DCP) was used as initiator to graft AOHBP on PP, and an anti-degradation additive styrene derivative (St) was added to inhibit the material degradation caused by the grafting process of voltage stabilizer. The physical and chemical properties of the modified material were characterized, and the electrical treeing initiation characteristics and DC breakdown characteristics of the modified material were studied. The results show that the AOHBP can graft on the PP matrix by the free radical reaction initiated by DCP, and the St and AOHBP can inhibit the macromolecular degradation of PP during the grafting reaction process synergistically. The grafting modified PP insulating material can inhibit the initiation of electrical tree and improve the DC electric strength significantly.

In order to study the high temperature resistance of hydrophobic properties of PRTV coating and the difference of thermal ageing characteristics between epoxy resin coated with PRTV coating (EPTL) and epoxy resin uncoated with PRTV coating (EP). The EPTL and EP were conducted thermal ageing tests at 80, 100, 120℃, and their dielectric properties, internal microstructure, thermal stability, and hydrophobic properties were compared and analyzed. The results show that after the thermal ageing tests under different temperature, the dielectric properties and thermal stability of EPTL are better than that of EP, and the internal microstructure is more complete. Although EPTL still shows strong hydrophobic properties after ageing, the contact angle decreases rapidly, which indicates that the hydrophobicity of PRTV coating is affected by high temperature greatly.

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.

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.

Considering the equivalence between sequential stress test and constant stress test, we proposed a mixed stress test method that the sample was applied voltage in the order of sequential advance stress-constant stress-sequential advance stress, and then the voltage withstand index n of XLPE flake sample was evaluated by this method combined with Weibull statistical analysis. The results show that the voltage tolerance index n calculated by the mixed stress test is 16.11, the deviation between it and the n evaluated by the constant stress test is 5.36%, and its total evaluation time is shortened by 98.04% than that of the constant stress test, which proves that this method can evaluate the voltage withstand index of XLPE insulation effectively.

In order to study the design and selection of the composite insulated cross arm for 35 kV transmission line, the feasibility analysis was conducted from material technology, electrical scheme, and structural scheme, the finite element stress analysis was conducted on the tension and compression rods of composite insulated cross arm, and the practical application of the composite insulated cross arm was verified. The results show that the composite insulated cross arm of 35 kV transmission line, which adopts the winding forming E-glass fiber/epoxy resin composite and the single strut and single cable-stayed structure, can meet the requirements of mechanical properties. The composite insulated cross arm has been in good operation since it is applied in the pilot project.

In order to study the ageing characteristics of cross-linked polyethylene (XLPE) cable and explore the microscopic factors affecting the dielectric properties of materials, the XLPE cables with different ageing states were conducted isothermal relaxation current (IRC) and partial discharge (PD) tests. A pure polyethylene molecular chain P₁, an aged polyethylene molecular chain P₂ containing hydroxyl groups, and a further thermal-oxidative aged polyethylene molecular chain P₃ containing carboxyl groups were established, and polyethylene molecules with different ageing states were carried out quantum chemical calculation. The results show that with the increase of ageing time, the time constant of depolarization current and ageing factor A increase, and the trap number and energy level depth in insulating medium increase. Besides, there are more partial discharges with higher magnitude occurring in the aged cable. The carbon-oxygen group generated by thermal oxygen ageing in polyethylene molecular chain is an important reason for the increase of trap number and energy level depth, and the type of functional group will affect the distribution of trap energy level directly. Under the effect of electric field, the stability of the aged polyethylene molecular chain becomes worse, the dipole moment is larger, and the charge transfer is more obvious, which make the charges in the aged cable insulation more likely to be trapped and accumulate, resulting in electric field concentration and increase of partial discharge level.

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.

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.

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.

On the basis of a new equivalent topology of oil-paper insulation—hybrid model, a variety of model characteristic parameters were extracted to realize the quantitative evaluation of water content in oil-paper insulation. At first, the polarization branch value range of the hybrid model was determined on the basis of parameter coincidence degree by using the measured frequency domain spectrum of oil-paper insulation equipment. And then, the difference between the hybrid model and extended Debye model in reflecting the damp information of oil-paper insulation was compared combined the experimental data and measured data. At last, the effects of water content in oil-paper insulation on the different branch parameters were studied on the basis of hybrid model, and the quantitative relation expression between the parameters and the damp state of oil-paper insulation were established. The results show that when the total number of polarization branch is 5-7, the identification effect of model parameter is better. The hybrid model can express the dielectric response information of oil-paper insulation system more accurately. The extracted model characteristic parameters can realize the quantitative evaluation of water content in insulating paper preliminarily.