Investigation of partial discharges on insulating foils in liquid nitrogen

A current objective in the field of grid stabilization is the implementation of resistive superconductive fault current limiters (R-SFCLs). After successful development of R-SFCLs at medium voltage level, the technology shall be applied at high voltage level exceeding 100 kV. In the majority of cases high temperature superconductive tapes of the second generation are winded as bifilar coils surrounded by liquid nitrogen (LN) which serves as coolant and isolation medium. The tapes themselves are additionally insulated against each other using nonconductive film materials. To assess the long-term operation ability of this arrangement, three different polymer foils made of PTFE, PI and PET were tested on their inception of partial discharges (PDs) in LN2. The investigation took place at a special electrode configuration simulating the arrangement of a bifllar coil and involved foil thicknesses from 25 μm up to 340 μm, depending on the particular material. For PTFE as well as PI an empiric formula was found approximately predicting the PD inception voltage of partial surface discharges.