抄録
A composite insulation system of laminated tape and high vacuum is newly proposed for a superconducting bus-line cable operated at relatively low voltage. Its breakdown characteristics were investigated using the model cable specimens having different kinds of laminated tapes such as semi-synthetic paper, polypropylene film and kraft paper of the insulation thickness of less than 1mm. First, ±DC, AC and -Imp breakdown stresses of the semi-synthetic papers were investigated. +DC breakdown stress was 21.7kV/mm for specimen of insulation thickness of 1mm and -DC breakdown stress was 41.5kV/mm, so that the remarkable polar effect was observed. AC breakdown stress was 18kV/mm. Also, the ratio of-Imp breakdown stress was about 3. Furthermore, AC breakdown stresses of other tapes were investigated, however the remarkable differences among their stresses were not observed. The most of specimens were broken from "butt gap" between tapes. To make this cause clear, the measurements of the partial discharge were carried out. As a result, the inception stress of partial discharge of butt gap located just on the conductor was about 20kV/mm not depending on the kind of tapes. This value is much lower than the breakdown stress of the same length of vacuum gap. Therefore it is considered that the trigger of the breakdown of this insulation system is the surface discharge along the edge of butt gap.
