This article is a review of the electrical properties of the insulating media at cryogenic temperatures especially at liquid helium temperature. In general, the dielectric strength increases and the losses owing to the conduction current or the dielectric loss decrease as the temperature decreases. Although the vacuum withstands fairly high voltages, prebreakdown currents produce losses and the support spacers are necessary for the mechanical support of the conductors. Cryogenic liquids or gases have low dielectric strength in comparison with the vacuum or the solids, and the support spacers are also necessary. The dielectric losses, however, are low except under high electric fields. Solids at cryogenic temperature have good dielectric strength, but the dielectric losses produce the heat loads to the refrigeration system. The matching of the thermal contraction with the metals is also required.
Development of superconducting generator requires us to solve various technical problems. The article describes the basic concepts on the design of a 6MVA machine and suggests the problems in construction of main parts of superconducting generators. The three year study of 6MVA superconducting generator is being executed in collaboration of Mitsubishi Electric and Fuji Electric and supported by Ministry of International Trade and Industry; one of government agencies of Japan. The time schedule of the study is described. Mainly the first step is to develop and test the main parts of a superconducting machine which include technical problems to be solved. The 2nd step is to construct a 6MVA machine and the last step is to test the machine. The test results of main parts made in the first step are described also. Superconducting field coils were ascertained to have satisfactory mechanical strength by rotation tests at room temperature and liquid helium temperature. The behaviors in cooling and energizing processes were satisfactory also. As for transfer coupling for liquid helium supply into the rotating cryostat, ferrofluidic sealing was applied. These test results show applicability of our sealing structure while the heat loss in the transfer coupling could not be determined separately from bulk losses in the experimental transfer system. From the tests of the model of armature coil, the temperature rise is tolerable under normal operation condition carrying the rated current. On the basis of these preliminary test results the program is progressing into the 2nd step.