| Abstract: | 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 model cable specimens with different kinds of laminated tapes such as semisynthetic paper, polypropylene film, and kraft paper of insulation thickness less than 1 mm. First, ±dc, ac, and —Imp breakdown stresses of the semisynthetic papers were investigated. +dc breakdown stress was 21.7 kV/mm for a specimen of insulation thickness 1 mm and −dc breakdown stress was 41.5 kB/mm, so that the remarkable polar effect was observed. ac breakdown stress was 18 kV/mm. Also, the ratio of −Imp breakdown stress was about 3. Furthermore, ac breakdown stresses of other tapes were investigated, but remarkable differences among their stresses were not observed. Most of the specimens were broken from the “butt gap” between tapes. To understand this cause, measurements of the partial discharge were carried out. It was found that the inception stress of partial discharge of butt gap located just on the conductor was about 20 kV/mm, not dependent on the kind of tape. 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 the butt gap. © 1998 Scripta Technica, Inc. Electr Eng Jpn, 121(3): 20–26, 1997 |
