A novel film capacitor was prepared by using a mass-produced surface modified high dielectric polypropylene film, and the power density, energy storage properties, capacity retention ratio, and the cycle life under large current of the capacitors were studied. The results show that the maximum energy storage density of the new capacitors with the surface modification polypropylene film can reach to 3.2 J/cm³, and the capacitor shows excellent comprehensive performances.

The relationship between the withstand voltage and the thickness of the sheath was studied when the air gap distance is 0, 5, 10, 15 cm, respectively. The influence of the length of sheath on the development of creeping discharge was studied when the sheath surface is dry, wet, and dirty, respectively. The results show that with the increase of the length and thickness of gap, the breakdown voltage of the insulating sheath-air combined gap increases nonlinearly, and the increase rate of the breakdown voltage also increases. The surface flashover electric field intensity of the insulating sheath decreases obviously in dirty state, the average flashover electric field intensity is 1.2 kV. Based on the above research, the configuration of anti bird droppings flashover insulating sheath was obtained. The thickness of the sheath is determined according to the line voltage level, and then the covering length is determined by the maximum surface flashover distance. It is recommended that the thickness and covering length of sheath in 110 kV transmission line is 6 mm and 60 cm, respectively; while the thickness and covering length of sheath in 220 kV transmission line is 8 mm and 110 cm, respectively.

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 this paper, a multi-physics finite element model of actual 10 kV T-type cable terminal joint was established. The electric field and temperature field distribution characteristics of the T-type cable terminal joint with air gap defect at insulation interface were studied, and the change laws of electric field and temperature field amplitudes were analyzed, then the characteristic quantities reflecting the air gap defect of T-type cable terminal joints effectively and the monitored position were proposed. The results show that different degrees of air gap defects in T-type cable terminal joints have a certain impact on the electric field distribution characteristics at the interface, but the electric field and temperature field amplitudes at other typical positions do not change significantly. The air gap defect position would near breakdown under air gap damping condition, at this time, the increase of electric field amplitude on flange base and upper surface of box can reach 20%, and the temperature rise can reach 15℃. In actual engineering, the potential critical hazard of partial breakdown discharge in T-type cable terminal joint can be reflected by monitoring the increase of electric field amplitude and temperature rise of the flange base and the upper surface of box.

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.

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.

The surface of meta-aramid paper was coated with poly(m-phthaloyl-m-phenylenediamine) (PMIA) slurry with different solid contents, and the coated meta-aramid paper was dried. The morphology, mechanical properties, dielectric properties, and electric strength of the modified meta-aramid paper were characterized by cold field emission scanning electron microscope, universal testing machine, broadband dielectric impedance spectrometer, and electric strength testing instrument. The results show that the PMIA slurry can change the structure of aramid paper and decrease the internal void of aramid paper. When the solid content of PMIA slurry is 10%, the transverse and longitudinal tensile strength of meta-aramid paper increases to 51.3 MPa and 94.4 MPa, respectively, the dielectric constant of meta-aramid paper is 2.07, and the electric strength of meta-aramid paper increases to 29.83 kV/mm. The mechanical properties and electric strength of the meta-aramid paper coated with PMIA slurry are better than those of original aramid paper. After treated at 180℃, the mechanical properties and electric strength of the modified meta-aramid paper are still better than those of original meta aramid paper, which can meet the applications of aramid paper in higher electric field environment and further broaden the applications field of aramid paper.

A medium-density aramid pressboard was prepared by using meta-aramid chopped fiber and precipitated fiber as raw materials, the effects of chopped fiber length, grammage of single pulp layer, and moisture content of paper billet on the properties of the aramid pressboard were studied. The conventional properties, compatibility, and processing performance of the aramid pressboard were studied. The results show that when the chopped fiber length is 7 mm, the grammage of single pulp layer is 64 g/m², and the moisture content of paper billet is 63%, the aramid pressboard has excellent electrical and mechanical properties, good compatibility with synthetic ester transformer oil and silicone transformer oil, and good processing performance, which meets the application requirements of traction transformer and offshore wind power oil-immersed transformer.

The oil-paper insulation systems, composed of insulating paper and monoester insulating oil or mineral insulating oil, were conducted thermal ageing tests, the ageing temperature was 130℃ and 140℃, and the ageing atmosphere was nitrogen and air, respectively. The thermal ageing characteristics of two insulation systems were compared. The results show that after the ageing of oil-paper insulation, the acid value and water content of monoester insulating oil are higher than that of the mineral insulating oil. Under the nitrogen atmosphere, the monoester insulating oil can prolong the ageing life of insulating paper significantly compared with mineral insulating oil, but at 130℃, the monoester insulating oil only prolong the ageing life of insulating paper slightly compared with mineral insulating oil. There is little difference between the thermal ageing life of insulating papers in the two insulating oils under air atmosphere.

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.