DC Surface Flashover Characteristics of Insulation Film in LN2 and Bubbles Composite

A combination of resistive type superconducting fault current limiters (R-SFCLs) and mechanical-type DC circuit breaker is a reliable way to protect the medium voltage and high voltage DC systems. Using the insulation barrier plays an important role in enhancing the insulation strength of R-SFCL. It promotes the development of R-SFCLs to a higher voltage level. However, the bubbles effect on the DC flashover characteristics of the insulation barrier is unknown. The objective of this paper is to obtain the DC flashover characteristics of insulation film with and without bubbles in liquid nitrogen (LN2). In the experiments, a triangle-plane electrode was put horizontal and immersed in LN2. A 14 W heater was placed right below the gap of two electrodes. Negative flashover characteristics of PTFE and Kapton were studied with different gap distances. The results show that the negative flashover voltage of PTFE film increases with the increase of gap distance both in conditions with and without bubbles. The shorter the gap distance between the triangle and plane electrodes is, the stronger the influence of bubbles on the negative flashover voltage is. The flashover voltage of PTFE is 1.15 times of Kapton both in the conditions with and without bubbles. The results could be used to design the insulation structure of high voltage R-SFCLs.