A research on several characteristics such as volume breakdown and surface discharge of insulators for a termination of power transmission class high temperature superconducting (HTS) cable was performed. In this paper, we investigated the surface discharge of glass fiber reinforced plastic (GFRP) in the air, nitrogen gas (GN 2 ) and cryogenic nitrogen gas (CGN 2 ) media. The breakdown characteristics of these media were also studied. Experimental results revealed that flashover voltage greatly depends on pressure, temperature, the kinds of insulating media and voltages. However, it is shallowly affected by shape and material of electrode. The breakdown voltage of liquid nitrogen (LN 2 ), GN 2 and CGN 2 deeply depends on the shape and dimension of electrode, kinds of voltages and pressure. Moreover, the breakdown voltage of CGN 2 and flashover voltage of GFRP in the CGN 2 is also influenced by temperature and vapor-mist density of the gas.Index Terms-Cryogenic insulation, HTS cable termination insulation, surface discharge, volume breakdown.

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A 154 kV class high-temperature superconducting (HTS) power cable system is developing in Korea. For insulation design of this cable, the grading method of insulating paper is proposed. The electrical insulation material has been used two kind of laminated polypropylene paper (LPP) that has different thickness. The use of graded insulation gives improved mechanical bending properties of the cable. Also, within a HTS cable technology the terminations are important components. A HTS cable termination is energized with the line-to-ground voltage between the coaxial center and outer surrounding conductors, in the axial direction there is a temperature difference from ambient to about 77 K. For insulation design of this termination, the insulation material of the termination body used glass fiber reinforced plastic (GFRP) and the capacitance-graded method is proposed. Therefore, in order to insulating design of a 154 kV class HTS power cable and termination, this paper will report on the experimental investigations in impulse breakdown and surface flashover characteristics of the insulation materials. Based on these experimental data, the electrical insulation design of a 154 kV class HTS power cable and termination were calculated.

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Recently, universities and companies are developing a power distribution and transmission class high temperature superconducting (HTS) transformer that is one of the 21st century superconducting frontier projects in Korea. For the development, it is necessary to establish the dielectric technology at cryogenic temperature such as insulating design, cooling system, manufacture, compact, and so on. Also, verification of insulating stability is one of important dielectric technology. Therefore, we prepared three models for a small simulated the HTS transformer, one is concentric arrangement, another is reciprocal arrangement, the other is Z continuous winding arrangement from Kapton insulated Cu tape and measured their insulation characteristics such as partial discharge (PD), AC (50 kV, 1 min) and impulse (154 kV, 1 2 50 s) withstand test. Before manufacture each model, we have been analyzed insulation composition and investigated electrical characteristics such as breakdown of LN 2 , polymer and surface flashover on fiberglass reinforced plastic (FRP) in LN 2 . We are going to compare with measured value and apply the value to most suitable insulating design of the HTS transformer.