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The flashover at the solid-liquid interface is the main dielectric failure in the cryogenic insulation system, which has great relation with the carrier mobility. In this paper, epoxy$/\mathrm {B}\mathrm {N}$(boron nitride)/Graphene composites with different contents were prepared and the surface potential decay (SPD) method was employed to analyze the transit time and the carrier mobility. The experimental results indicate that the behaviors of surface potential and carrier mobility are affected by specimen temperature. Obtained results show that BN and Graphene particles can affect the carrier mobility and modulate the discharge speed, which may further influence the flashover voltage at solid-liquid interface at cryogenic temperature.

Cable accessories made of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. The charges are likely to accumulate at the interface between insulation materials with different conductivity and permittivity, which may induce the occurrence of partial discharge and breakdown. Nanoparticles can be applied to adjust the interface charge behaviors. In this paper, carbon black (CB) nanoparticles were dispersed into EPDM with 0, 0.5, 1, 3 and 5 wt% respectively. The effects of nanoparticle doping on the interface charge distribution of LDPE and EPDM composite insulation were measured under −15 kV/mm. Furthermore, dielectric properties and DC conduction were introduced to discuss the suppression mechanism of carbon black nanoparticle doping. Obtained results the interface charge density can be suppressed to 0.5 C/m3 with 1 wt% carbon black nanoparticle doping, much less than the undoped and other doped groups. The mechanism of interface charge suppression is attributed to the decrease of conduction and permittivity mismatch by carbon black nanoparticle doping, which was proved by the dielectric spectroscopy and conduction current tests. The interface charge buildup depends on the time constant controlled by the doping proportion.

Moisture in oil-immersed transformer will become small bubbles suspending in insulation oil at high temperature. In order to investigate the influence of suspending bubbles on tracking failure of oil-impregnated pressboard in oil-paper insulation, a model based on needle-plate electrodes was established. This model was used to study effect of the number of suspending bubbles generated by dropped distilled water on the discharge voltage, how the discharge signals varied during the process of tracking failure on the oil-impregnated pressboard surface with the effect of suspending bubbles, and the surface carbonization process of oil-impregnated pressboard during tracking failure. The results show that when the number of suspending bubbles increases with the increase of dropped distilled water, the partial discharge inception voltage in oilpaper insulation descends slowly while the surface flashover inception voltage descends sharply. The discharge process can be divided into three periods orderly according to the discharge signals during tracking failure. The discharge signals in each period have some relationship with each other and the recurrence plot (RP) method is used to analyze characteristics of the discharge signals. The surface carbonization process can be also divided into three periods the same as the discharge process. The carbonization on the oil-pressboard surface is slow under partial discharge while it's fast under surface flashover. Because suspending bubbles make surface flashover much easier, so they make the tracking failure of the oil-impregnated pressboard much easier too, although the carbonization in the insulation oil is more difficult than that in the air. This paper is helpful to understand the process of tracking failure of oil-impregnated pressboard in insulation oil with the effect of suspending bubbles, which will be beneficial for the insulation protection in oil-immersed transformer.

With the advantages of clean, renewable and wide distribution, wind power generation becomes an important part of the smart grid. Wind generator is the key apparatus in application of wind energy, which has been paid great attention to its operation reliability. Partial discharge in the insulation system is a serious threat to its safe operation. Polyimide (PI) has been widely applied as one of the most important organic materials owing to its excellent physical and chemical properties. And nano particals are incorporated in polyimide to enhance the properties, such as breakdown characteristics. In this paper, polyimide/Al 2 O 3 (PI/Al 2 O 3 ) nanocomposite films were prepared using in-situ dispersion polymerization process. The mechanical and electrical characteristics of the PI/Al 2 O 3 nanocomposite films with different alumina content comparing to the unfilled polyimide film, including the tensile strength and elongation, dielectric spectroscopy, dielectric breakdown strength and the corona resistance property as well, were studied.