Correlation between DC electric field intensity and electrical breakdown of butt gap in LN2/PPLP composite insulation system

In the design of DC superconducting cables, the electrical insulation design is a critical factor in the cable´s performance and reliability. To evaluate the insulation design of DC superconducting cables, DC electric field analysis and experimental verifications should be performed. Wrapped polypropylene laminated paper (PPLP) tape has generally been used to insulate superconducting cable. During the wrapping process, butt gaps are inevitably introduced, and these are filled with liquid nitrogen (LN₂) during normal operating conditions. Therefore, the insulation characteristics of the resulting combination of PPLP and liquid insulation should be carefully verified. The objective of this work was to determine the DC electric field transitions when a butt gap is present in a LN₂/PPLP composite insulation system. DC electric field distribution and transition were simulated by using COMSOL Multiphysics^® software. Also, to verify the characteristics of DC electric field transitions, two kinds of breakdown tests were performed: a ramp voltage breakdown test and a step voltage breakdown test. In both the experimental and analytical works, it was observed that the electric field distributions were totally different while the DC field transition. And, due to the different distributions of electric field, the breakdown characteristics of LN₂/PPLP composite insulation systems could be altered.