Influence of prestressing on the breakdown of insulating paper-liquid nitrogen composite system

We have been studying a cryogenic electrical insulation system with polypropylene laminated paper (PPLP^®) for 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 excellent insulation performance in liquid nitrogen (LN₂). However, questions still remain in breakdown mechanism of PPLP^® due to its laminated structure. In this paper, it is investigated the influence of DC prestressing on the impulse breakdown characteristic of KP and PP film to understand basic breakdown characteristic of PPLP^® in LN₂. Furthermore, DC prestressing effects on impulse breakdown strength (Fb) of KP and PP film was investigated, and the charge injection model was discussed. As the result, the Fb of KP and PP film under hetero polarity with DC prestressing field application did not decreased compared with normal impulse Fb, which is different from the conventional DC prestressing effects. Furthermore, it is particularly interesting that the Fb of KP 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.