Medium factors of electrical insulation systems in high temperature superconducting power apparatus with coil structure for equivalent ac withstand voltage test at room temperature

Abstract

Equivalent insulation test voltage at room temperature of high temperature superconducting (HTS) power apparatus with coil structure for power frequency withstand voltage tests is discussed based on medium factors of insulation weak parts in electrical insulation elements of the HTS power apparatus. Uniform and non-uniform field gaps, triple junction and solid insulator surface are selected as the insulation weak parts, and their medium factors are theoretically and experimentally calculated as the ratio of partial discharge (PD) inception or direct-breakdown voltages in liquid nitrogen or cryogenic nitrogen gas to those in atmospheric air at room temperature. The results show that the type of insulation weak part affects the relationships between the medium factor and the gap length between electrodes as well as the state of cryogenic surrounding medium. These medium factors are compared with the medium factor of a coil-to-coil insulation element of HTS pancake coils that contains some of the insulation weak parts, and it is verified that the medium factor of the coil-to-coil insulation element can be obtained by using that of triple junction or solid insulator surface. Finally, a selection method of a significant unique medium factor is discussed to determine the equivalent insulation test voltage at room temperature for the confirmation of the insulation reliability of all the parts of the HTS apparatus.