The high frequency repetitive impulse generated by the inverter can lead to uneven overvoltage with high amplitude along the stator windings, which will accelerate the ageing of insulation system and even cause faults. On the basis of the transient analysis software PSCAD/EMTDC, the equivalent circuit model for three-phase stator windings of 2.5 MW permanent magnet direct-drive generator was established, and the parameters of corresponding equivalent circuit were calculated by finite element method. Moreover, the experimental platform was built for verifying the validity of the calculation results. The results show that under the effect of three-phase pulse width modulation (PWM) pulse voltage, the amplitudes of the overvoltage to ground, coil to coil overvoltage, and turn to turn overvoltage of the stator winding for permanent magnet direct-drive wind generator show a decreasing trend from the connecting terminal of motor to the neutral point of winding. The experimental results further verify the accuracy of the equivalent circuit model of three-phase stator winding.

Aiming at the problerm of seleting energy in the intrinsic conductivity measurement by charge decay method,the intrinsic conductivity of polyimide was measured by charge decay method under the electron irradiation energy range from 5 keV to 20 keV. The influence of irradiated electron energy on the test results was studied, and the fitted method of surface potential decay data was improved by adding a polarization decay time constant. The results show that a higher irradiated electron energy causes a higher charged balance potential on the surface of insulating material, resulting in more serious polarization inside the dielectric, which affects the test results of the conductivity in charge decay method. The surface potential decay data can be fitted and interpreted well by the improved method, which provides an basis for the selection of irradiated electron energy.

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.