Influence of DC prestress on impulse breakdown strength of insulating paper-liquid nitrogen composite system

Breakdown characteristics of PPLP (Polypropylene laminated paper)® impregnated in liquid nitrogen (LN₂) are presented. PPLP has three-laminated structure of kraft paper (KP), polypropylene film (PP film) and KP and has been used as an electrical insulating material in high-temperature superconducting cables. In this report, we investigated the influence of DC prestress on impulse breakdown strength of KP, PP film and PPLP in LN₂. We performed DC prestress test in which DC voltage of 10 kV was applied as prestress and then impulse breakdown voltage was superimposed for the breakdown test. By the application of DC prestress of homo polarity with impulse voltage, the impulse breakdown strength of KP and PP film was found to be increased. On the other hand, by the application of DC prestress of hetero polarity, the breakdown strength of KP was found to be increased, which was unique to paper material. We proposed the negative charge injection model into KP to explain this result. Furthermore, the breakdown strength of PPLP was almost the same under the application of DC prestress of 10 kV. The influence of DC prestress on impulse breakdown strength appeared to be different depending on the specimen, and the possible model at this moment was discussed.