The corona accelerated ageing was conducted on silicone rubber, the insulation characteristic parameters of silicone rubber under different ageing time were measured, and the ageing mechanism of silicone rubber was deeply discussed according to the parameters. The results show that corona ageing causes serious damage to the surface structure of silicone rubber, and obvious holes and cracks are formed on its surface, resulting in serious pulverization. Corona ageing makes the long molecular chain of silicone rubber break into small molecular chain, reduces the flexibility of silicone rubber molecular chain, forms various defects, which leads to the decrease of the insulation performance of silicone rubber. With the increase of corona ageing time, the energy level centers of electron and hole traps decrease slowly at first and then decrease fast. Corona ageing leads to the gradual development of deep traps into shallow traps, reduces the binding effect on electrons, and finally leads to the occurrence of surface flashover of silicone rubber.

In order to explore the impact of the performance of silicone dry mica tape on the reliability of insulation system of traction motor, we conducted the material basic performance, laboratory simulation of wire rod, and insulation system reliability evaluation on two imported mica tapes and two domestic mica tapes. The results show that the mica tape with less resin and higher air permeability shows a better compatibility with the silicone impregnating varnish. The formed insulation system has a lower thermal dielectric loss factor and a higher curing rate, which improves the breakdown voltage and improves the reliability of the insulation system.

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.

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.

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.

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 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.

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.

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.

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.