Electrical insulation characteristics of cold dielectric high temperature superconducting cable

For the optimization of electrical insulation design for high temperature superconducting (HTS) cable, evaluation of electrical insulation characteristics especially for butt gap of LN₂ impregnated cold dielectric (CD) which consists of the wrapped tape insulation impregnated with LN₂ plays an important role. This paper presents partial discharge (PD) inception and breakdown characteristics in LN₂ impregnated butt gap model which modeled a weak point of the wrapped tape insulation impregnated with LN₂ and cable model with short length with polypropylene laminated paper (PPLP^®), Nomex^® paper and cellulose paper. PD current pulse was found to have a steep rise time of ∼ ns and amplitude of ∼ tens μA at PD inception voltage region. Little dependency of breakdown stress on the insulating material is found. PD inception stress is almost independent of insulation thickness of 1 to 3 mm. The requirement insulation thickness for 66 kV class HTS cable is estimated to be ∼ 5 mm under PD-free condition from viewpoint of long-term reliability.