We have been studying a cryogenic electrical insulation system with PPLP (Polypropylene laminated paper)
® for its application to high-temperature superconducting cable. PPLP
®, which has the three-laminated structure of a kraft paper (KP), polypropylene (PP) film and a KP, has been reported to have an excellent insulation performance in liquid nitrogen (LN
2). However, questions still remain in dielectric breakdown mechanism of PPLP
® due to its laminated structure. In this paper, it is investigated the influence of prestressing on the impulse breakdown characteristic of KP and PP film to understand the basic breakdown characteristic of PPLP
® in LN
2 and we discussed the charge injection model into KP and PP film. As the result, the breakdown strength of KP and PP film under hetero polarity DC prestressing field application was not decreased compared with normal impulse breakdown strength, which was different from the general DC prestressing effects. Furthermore, it is particularly interesting that the breakdown strength of KP was increased under the positive DC prestressing field with negative impulse field in spite of hetero polarity DC prestressing field application. This result could be explained based on the charge injection model, in which negative charge may be easily injected inside KP through cellulose fiber interface.
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