Impulse and ac PD inception characteristics of LN2/polypropylene laminated paper composite insulation system

We discussed partial discharge (PD) inception characteristics of liquid nitrogen (LN₂)/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables. In this paper, we measured PD inception characteristics under lightning impulse voltage application by both electrical and optical methods. The impulse PD inception characteristics were compared with those under ac voltage application, and the PD generation mechanisms were discussed. Experimental results revealed that PD detection sensitivity under lightning impulse voltage application reached as small as 6 pC. PD was verified to be generated not only inside the butt gap but also outside the butt gap under both lightning impulse and ac voltage applications. AE sensors could also be applied to PD measurement under ac voltage application.