Characterization of Partial Discharge With Polyimide Film in $\hbox{LN}_{2}$ Considering High Temperature Superconducting Cable Insulation

High temperature superconducting (HTS) cable has now reached the stage where their application in power systems is in the process of being implemented. Liquid Nitrogen (LN2) and polyimide film have been used as a coolant and insulation in HTS cable systems. Partial discharge (PD) events may lead to material degradation and total breakdown. PD testing is an important quality check for the insulation of HTS cable. Thus it is necessary to consider PD for the insulation testing in LN2 which is generated in void defects of the insulation layers. The PD characterizations of polyimide film in different defects were investigated under 50 Hz AC voltages in LN2 and at room temperature (298 K). The results show that the number of discharges and the discharge quantity increase with the increasing of the applied voltage and the defect size. The PD inception voltage decreases when the void defect diameter enlarged and it is higher in LN2 than that at room temperature. The maximum field strength for HTS tape insulation increases with the defect close to the tape boundary and the addition of contaminant relative permittivity. Obtained results at cryogenic temperature provide necessary information for insulation design and pre-shipment inspections of HTS cable system.