Electrical insulation characteristics in the simulated electrode system of HTS double pancake coil

For the experiment four types of spacer were distinguished by an arrangement that could improve the dielectric strength by making the path of flashover longer. One of the spacers could be applied to an insulator between windings of a high temperature superconducting (HTS) transformer. The flashover characteristic of each type was investigated and the flashover phenomena were observed to understand breakdown mechanism in liquid nitrogen (LN₂). In the first, the flashover characteristic in LN₂ was compared with that in air using the simulated electrode made from five turns of HTS tape. The dielectric strength of spacers was improved with an arrangement in air but not in LN₂. Through the observation of flashover phenomena, a micro gap between the spacer and the coil electrode generating bubbles was regarded as the main cause. In the second, the effect of the micro gap on flashover was investigated with a plane-plane electrode system. The micro gap decreased the flashover voltage by 70 percent. Finally, the dielectric strength of the spacer in LN₂ also could be improved by filling of a small amount of epoxy in the gap between the coil and the spacer.