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 (N₂(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. N2(l) cooling system is installed in the power system and N₂(l) will flow through the cables during the system operation. Filling the HTS cable with N₂(l) in order to perform pre-shipment inspection is time-consuming 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 (N₂(g)) instead of N₂(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 N₂(l) and 0.1-MPa to 1.0-MPa N₂(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 N₂(g) are linearly correlated with those in N₂(l) at any gas pressure in our testing, and PDIE in 1.0-MPa N₂(g) is almost 30% of that in 0.2-MPa N₂(l). It suggests that PD characteristics in N₂(l) can be extrapolated from those in N₂(g).