Impulse and ac partial discharge inception and breakdown characteristics of LN2/polypropylene laminated paper composite insulation system

Partial discharge (PD) inception and breakdown (BD) characteristics of liquid nitrogen (LN₂)/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables are presented. In this paper, we measured PD inception characteristics under lightning impulse and AC voltage applications by electrical, optical and acoustic methods. The impulse PD inception characteristics were compared with those under AC voltage application, and the PD generation mechanisms are discussed. Experimental results reveal that PD detection sensitivity under lightning impulse voltage application is as small as 6 pC. PD inception at the lower stress was verified to be generated not only in the butt gaps but also in micro gaps between PP laminated paper layers under both lightning impulse and AC voltage applications. After PD inception, PD activity could be developed in the butt gaps at the higher stress, finally leading to BD