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 time-consuming and costly for cable manufacturers. Therefore, they 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 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 N-2(l) can be extrapolated from those in N-2(g).