We have discussed a cross-linked polyethylene (XLPE) nanocomposite insulating material that is able to DC voltage applications. Nanocomposites, which are composed in polymer matrix mixed with nano-fillers, have received considerable attention because of their potential benefits as dielectrics. The nano-sized alumina oxide (Al 2 O 3 )/XLPE nanocomposite was prepared, and three kinds of test, such as DC breakdown, DC polarity reversal breakdown, and volume resistivity were performed. By the addition of nano-sized Al 2 O 3 filler, both the DC breakdown strength and the volume resistivity of XLPE were increased. A little homogeneous space charge was observed in Al 2 O 3 /XLPE nanocomposite material in the vicinity of electrode through the polarity reversal breakdown test. From these results, it is thought that the addition of Al 2 O 3 nano-filler is effective for the improvement of DC electrical insulating properties of XLPE.

-Sulfur hexafluoride (SF6) gas is used widely in electric power equipment such as Gas Insulated Switchgear (GIS), Gas Insulation transmission Line (GIL), and Gas Circuit Breaker (GCB). But applications of SF6 should be restricted because SF6 gas is one of the greenhouse effect gases. To reduce use of SF6 gas, a study on eco-friendly alternative insulation medium is needed. In this paper, we investigated lightning impulse (LI) breakdown of dry-air which is attracting attention as an ecofriendly alternative gas and the LI breakdown of hybrid insulation combined with dry-air and solid insulation (Room-Temperature Vulcanizing Silicone Rubber-RTV SIR) and dry-air in inhomogeneous fields according to gap distance and pressure. The experiment results showed that the LI breakdown strength of hybrid insulation system was higher than that of dry-air insulation system. It was verified that the development of technology related to eco-friendly power apparatus compact such GIS, GCB and GIL can be used as basic research data.

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-In this study, AC breakdown properties of cenosphere/epoxy composite are investigated.The thermal properties of the composite are also discussed. The cenosphere/epoxy composites were prepared by epoxy resin filled with 0~40 wt% of hollow fly ash particles (cenospheres). The AC breakdown strength of the composites was measured at 25~125℃. The permittivity, glass transition temperature, and coefficient of thermal linear expansion of the composites were also measured. The experimental results of the cenosphere/epoxy composite were compared with those of the silica/epoxy composite which are widely used as insulating materials in power apparatus. The cenosphere/epoxy composite showed the similar or better performance than silica/epoxy composite in terms of breakdown strength, glass transition temperature, and coefficient of thermal expansion.