Influence of Combined AC-DC Voltages on Breakdown Characteristics of Epoxy Resin in LN2

In a dc superconducting cable system, the ripple waves contained lots of ac components that can superimpose dc voltage and current and are widely applied in rectifiers and other power electronic devices, which threaten the reliability and safety of a high-temperature superconducting (HTS) insulation system. In this paper, the partial discharge (PD) characteristics of the epoxy resin are studied with different ac/dc proportions in liquid nitrogen and room temperature. The results show that the PD inception electric field and breakdown strength of the epoxy resin increase as the dc component increases in both liquid nitrogen and room temperature. According to the phase-resolved PD distribution obtained in the inception stage, PDs show different behaviors at a higher dc component. The mean and total PD amplitude decrease as the dc component increases in both the inception stage and prebreakdown stage; they depend on the electric field modified by space charges. Compared with the PD characteristics in room temperature, they are weaker in liquid nitrogen, indicating the PD detection of insulation materials should be enhanced to eliminate the latent destruction in the HTS system. The results establish the foundation for diagnosis and assessment of insulation materials in liquid nitrogen with combined ac-dc voltages.