Surface Flashover Characteristics of the Interface Between PPLP and Epoxy According to Tensile Stress Under Cryogenic Environments

Stop joint boxes (SJB) serve to maintain constant cooling efficiencies by separating the cooling sections on superconducting DC cable systems. SJBs are composed of polypropylene laminated paper (PPLP) and an epoxy spacer. PPLP is wound against a superconductor and epoxy spacer to improve surface flashover performance. The interface between the insulating paper and epoxy is vulnerable to surface flashover due to the extreme distortion of electric fields at the interface. In addition, PPLP is subject to constant tensile stress that affects the dielectric characteristics of insulating paper. The breakdown characteristics of insulating paper with diverse tensile stress in cryogenic environments has been previously reported. However, research with regard to the surface flashover characteristics of composite insulation structures under cryogenic environments depending on tensile stress has not yet been reported. Therefore, in this paper the DC surface flashover characteristics of the interface between PPLP and epoxy was investigated in liquid nitrogen in accordance with the tensile stress of PPLP. TM systems which are able to convert rotational motion into linear motion by torque were devised for changing and subjecting the tensile stress to PPLP. From the experiments, it was confirmed that the DC surface flashover strength of PPLP decreased over 40% due to tensile stress. In the case of the interface, tensile stress had no effect on the DC surface flashover strength. Consequently, tensile stress was an important design factor in a single insulation structure with PPLP but was not essentially considered in the composite insulation structure with PPLP and epoxy.