Insulation Design of 60 kV Class Bushing at the Cryogenic Temperature

The technologies to be required to commercialization a HTS transformer are of a reduction in AC loss, an improvement of conductor characteristics, cooling technology, high voltage technology, bushing technology and so on. Amongst the technologies, the development of bushing is the important technology which should be accomplished to apply high voltages into the winding of the HTS transformer. However, the studies and developments have done on bushing for HTS transformer are not enough. The bushing is energized with the line-to-ground voltage between the coaxial center and outer surrounding conductors, in the axial direction there is a temperature difference between room temperature and 77 K. So it has to endure for electrical insulation as well as the thermal contraction as well. Therefore, in order to insulating design of a 60 kV class bushing for HTS transformer this paper will report on the experimental investigations in impulse breakdown and surface flashover characteristics of the insulation materials. Insulation design was realized in a way of calculating breakdown voltage of 0.1% through Weibull statistical treatment of the obtained experimental results and calculating insulation thickness and separation distance meeting BIL 325 kV, a target voltage.