Aging characteristics of cryogenic insulator for development of HTS transformer

In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper V−t characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter.