Investigation on Flow Electrification of Polyimide Films by Spinning Disk System in the Liquid Nitrogen

During the operation of high temperature superconducting (HTS) transformer, electrostatic charges will be accumulated gradually on the surface of solid dielectric due to the friction between the liquid nitrogen (LN) and the polyimide (PI) film. A large amount of accumulating charge will distort the electrical field to result in the discharge along the solid insulating surface, which causes damage to the insulation of the transformer. Eventually, it will lead to the breakdown of the composite insulation system. It is very necessary to carry out prophylactic tests for the electrostatic charging tendency (ECT) of LN to retain the safety of HTS transformer. In this paper, the streaming electrification of LN is produced and measured by the spinning disk system. The mechanism of LN flow electrification is analyzed, and the influence factors on electrification, such as contact area, flow time and flow velocity, are discussed. Based on the results, it is found that the diameter and rotation velocity of the disks are the two main factors to affect the streaming current. In addition, the flowing current and the volume charge density which describe the ECT can be calculated by the mathematical model. In the end, the methods to suppress LN flow electrification are proposed based on the experimental results.