The broadband dielectric response of oil-paper insulation was studied, and broadband dielectric response tests were carried out on laboratory samples of oil-paper insulation with different moisture contents. According to the relaxation polarization part of dielectric spectrum, the spectral parameters characterizing the moisture degree were extracted, and their relationship with the moisture content in the oil-paper insulation was analyzed. Further moisture tests for the oil-immersed bushing scaled-down model were designed, and the application of the spectral parameters in the moisture diagnosis of bushing was discussed. The results show that the low-frequency polarization process in the relaxation polarization spectrum is more sensitive to the moisture content, and its loss peak value can be used as a characteristic parameter to characterize the moisture content; under different moisture types of the bushing, the change patterns of the loss peaks and characteristic frequencies of the low-frequency and high-frequency relaxation polarization are different, which can be used as a preliminary criterion for the moisture type of the bushing.

Three kinds of soluble polyimide (PI) resins were prepared from 4,4′-hexafluoroisopropylidene)bisphthalic anhydride (6FDA) and three kinds of romatic diamine monomer containing rigid-rod amide bonds in the molecular chains by one-step high temperature solution polycondensation reaction, respectively. And then three kinds of transparent PI films were prepared by the N,N-dimethylacetamide (DMAc) solution of the above resins. The films were tested by attenuated total reflection Fourier transform infrared spectroscopy, thermogravimetric analysis, dynamic mechanical analysis, thermomechanical analysis, and UV-visible spectrum, and their yellowness index was tested. The results show that the PI films exhibit good optical transparency, their ultraviolet cutoff wavelength is below 380 nm and the optical transmittance is over 80% at the wavelength of 500 nm. In addition, the PI films exhibit good thermal stability with the glass transition temperatures (T_g) higher than 320℃ and the 5% weight loss temperatures (T_5%) over 520℃. The linear thermal expansion coefficients (CTE) of the PI films are all lower than 50×10^-6/K, indicating that the introducing of rigid amide bonds can efficiently enhance the high-temperature dimensional stability of the solution-processable fluoro-containing PI films.

In order to explore the influence of distributed optical fiber on the ageing life of transformer insulation system, we carried out accelerated thermal ageing tests on the transformer oil sample containing distributed optical fiber at 110, 120, 130℃. The breakdown voltage of the samples was measured regularly during thermal ageing process. On the basis of the breakdown voltage cumulative loss rate, the relationship between the activation energy of the transformer oil containing optical fiber and pure oil was analyzed, a method for calculating activation energy of transformer oil containing optical fiber was proposed, and a thermal ageing life model of transformer oil containing optical fiber was constructed. The results show that the optical fiber in the transformer oil can affect the strength of the transformer insulation system, and the ethylene tetrafluoroethylene (ETFE) optical fiber has the least effect on the ageing of transformer oil. The calculated results of three life evaluation models of transformer oil containing ETFE, poly(tetrafluoroethylene) (PTFE), and thermoplastic polyester-ether elastomer (TPEE) fiber established based on calculated activation energy and breakdown voltage cumulative loss kinetic equation, are in good agreement with the evaluation results of 10℃ life criterion in engineering experience, which verifies the effectiveness of the model.

In order to study the failure mechanism of transformer sealing ring in low temperature environment, we collected some failed rubber sealing rings of transformers in extremely cold area, and studied their microstructure and mechanical properties. The changes of micro morphology caused by low temperature and mechanical stress were characterized by scanning electron microscope, the changes of molecular chain structure and thermal motion of rubber samples were characterized by Fourier transform infrared spectroscopy and differential scanning calorimetry, and the changes of micro indentation hardness and reduced Young's modulus of rubber samples were characterized by non-destructive micron indentation test. The results show that due to the long-term joint action of low temperature, insulating oil, and mechanical stress, both the structure and arrangement of the molecular chain in nitrile butadiene rubber (NBR) have changed, resulting in the obvious decrease of micro indentation hardness and reduced Young's modulus of the rubber, and the sealing effect loses.

The effects of doping micro-SiO₂ on the electrical tree resistance, thermal stability, thermal conductivity, and trap characteristics of epoxy resin under repetitive frequency bipolar square wave voltage were investigated. The results show that the electrical tree breakdown time of epoxy resin is prolonged by doping micro-SiO₂, and the prolong effect is the most obvious at 10 kHz, the electrical tree breakdown time is prolonged from 6 min to 44 min. Doping micro-SiO₂ can also increase the thermal conductivity of the epoxy resin and reduce the degradation of epoxy molecular chain at high temperature. The micro-SiO₂ filler can introduce more deep level traps, hinder the development of electrical tree channels, extend and even block the development path of electrical tree, which decreases the damage degree of molecular chain. Therefore, doping micro-SiO₂ can effectively improve the electrical tree resistance of epoxy resin under repetitive frequency bipolar square wave voltage.

Mastering the theoretical calculation model of gas relative insulation strength can greatly accelerate the research of SF₆ substitute gas. On the basis of molecular topological index theory, a molecular topological index of insulating gas was proposed, and the electrical topological state indexes of 22 types of atomic for 47 insulating gas molecules were calculated. At the same time, the electrical topological state indexes of carbonyl and cyano groups were introduced. A multivariate linear prediction model of gas insulation strength was constructed by stepwise multiple regression method, and the groups which have significant indigenous influence on the insulation strength were obtained by screening principle. The results show that >CH-, =C=, -F, -C≡N, -Cl, and >C=O have significant effects on the gas insulation strength. Among them, -F, -C≡N, -Cl, and >C=O have higher electronegativity, and their electrical topological state index (ETSI) value is high and has a positive contribution to the insulation strength. These groups can be preferentially considered when designing and screening SF6 substitute gases.

In order to study the thermal conductivity of high thermal conductive (HTC) mica tape, the micro structure of different high thermal conductive VPI dry mica tapes were analyzed, and a thermal conductivity test method of HTC dry mica tape was proposed. A comprehensive evaluation method of thermal conductivity was discussed. The results show that under the existing VPI system, the thermal conductivity of the main insulation with high thermal conductive mica tape increase by up to 45% compared with the main insulation with conventional mica tape. Using the comprehensive thermal conductivity λ_s to measure the contribution of high thermal conductive mica tape is more objective and comprehensive, and the comprehensive thermal conductivity of the main insulation with high thermal conductive mica tape increase by up to 33% compared with the main insulation with conventional mica tape.

A series of polypropylene/ethylene propylene copolymer (PP/EPC) cable materials were prepared by blending polypropylene (PP) with two kinds of ethylene propylene copolymer (EPC) with different ethylene content. The effects of EPC types and contents on the mechanical, thermal, and electrical properties of PP were investigated. The results show that the addition of EPC improves the toughness of PP significantly on the basis of maintaining the original excellent thermal and electrical properties of PP, and the toughening effect is more obvious by increasing the amount of EPC and the ethylene content in EPC.

A transformer oil circulating flow device was established, the flow transformer oil containing metal particles was conducted partial discharge (PD) experiment, and the PD characteristic parameters were extracted. By combining with the movement behavior of metal particles in flow transformer oil, the influence mechanism of electrode covering paper on the PD of metal particle in flow transformer oil was discussed. The results show that when the electrode is covered with paper, the partial discharge initial voltage (PDIV) increases, and the discharge amplitude and frequency decrease. When the double electrodes are bare or only the lower electrode is covered with paper, the discharge concentrates in two phase sections of 0°-90° and 300°-360°. When the two electrodes are covered with paper, the discharge phases concentrate near 45° and 160° mainly. The metal particles will be settled on the paper because of the existence of insulating paper and lead to the increase of horizontal distance between two adjacent collision points. In addition, after the electrode is covered with paper, the electric field in the oil will decrease, which makes the vertical velocity of the particles decrease and the collision frequency between particles and electrodes decrease, resulting in the decrease of PD strength and change of the discharge phase distribution.

Micro-water is one of the main factors leading to the deterioration of transformer oil-paper insulation, so it is important to discuss the affecting mechanism of micro-water in the pyrolysis process of oil-paper insulation. In this paper, a compound molecular model of oil-paper insulation was established, and on the basis of reactive molecular dynamics, the pyrolysis process of oil-paper insulation containing micro-water was simulated. The affecting mechanism of water molecules in different pyrolysis stages of oil-paper insulation was investigated, and the roles of hydrogen ions and hydrogen radicals in the pyrolysis process were analyzed. The results show that in the oil-paper insulation composite model containing micro-water, the initial formation time of small molecules such as water and hydrogen molecules is earlier, and their formation rate and quantity are higher than that of the oil-paper insulation composite model without micro-water. By analyzing the reaction path, it is found that at the initial stage, water molecules firstly destroy the hydrogen bond network of insulating paper, and with the increase of high temperature time, the water molecule act as a catalyst to promote the pyrolysis of insulating paper. As the reaction proceeds, water molecules as carriers carrying hydrogen carboxylate ions diffuse into the insulating oil, which promotes the pyrolysis of insulating oil. During the reaction process, the hydrogen carboxylate ion has the effect of accelerating the insulation pyrolysis, and which can accelerate the pyrolysis of oil-paper insulation synergistically with water molecules, while the hydrogen radical does not have the accelerating effect.