Partial discharge characteristics in composite insulation systems with PPLP for HTS cable

The electrical insulation system of high-temperature superconducting (HTS) cable consists of liquid nitrogen (N 2 (l)) and polypropylene laminated paper (PPLP). Partial discharge (PD) may occur in butt gaps of the insulation layers and its characteristics imply the insulation performance of HTS cables. N 2 (l) cooling system is installed in the power system and N 2 (l) will flow through the cables during the system operation. Filling the HTS cable with N 2 (l) in order to perform pre-shipment inspection is timeconsuming and costly for cable manufacturers. Therefore, the authors are trying to find a cost effective method for pre-shipment inspections. One alternative is to use high pressure gaseous nitrogen (N 2 (g)) instead of N 2 (l). This article investigates PD characteristics such as PD inception electric field (PDIE) and PD extinction electric field (PDEE) in butt gaps of HTS cables in 0.1-MPa to 0.3-MPa N 2 (l) and 0.1-MPa to 1.0-MPa N 2 (g) environments. For assessing the surface/volume effects, PD characteristics are measured with changing the size of butt gaps. It turns out that PDIE and PDEE in N 2 (g) are linearly correlated with those in N 2 (l) at any gas pressure in our testing, and PDIE in 1.0-MPa N 2 (g) is almost 30% of that in 0.2-MPa N 2 (l). It suggests that PD characteristics in N2(l) can be extrapolated from those in N2(g).