Dielectric breakdown mechanism of polypropylene laminated paper in liquid nitrogen

Breakdown characteristics of PPLP (Polypropylene laminated paper) impregnated with LN₂ for its application to high-temperature superconducting cable are presented. In this paper, we investigated the breakdown characteristics of PPLP, which was the three-laminated structure of kraft paper (KP), polypropylene film (PP film) and KP, in LN₂ by comparison with those of the papers with different laminated structure, e.g., single-layer of KP, single-layer of PP film and the 2 layer laminated paper of KP and PP. The breakdown characteristics under the application of impulse voltage, ac voltage and dc voltage were obtained, and the breakdown mechanisms were discussed. Experimental results revealed that the breakdown strength of PPLP under the application of impulse voltage and dc voltage was higher than that of the single-layer material. This could be attributed to the dispersion effect of the streamer by KP layer of PPLP. In order to obtain the unified explanation for the breakdown mechanism with the different voltage polarity and the different laminated structure, we suggested the injection model of the negative charge into the KP layer of PPLP and the less-injection model of the charge into the PP.