Dielectric properties of mica-alumina combined insulation at high temperatures

Development of sodium immersed type Electro-Magnetic Pumps (EMP) is expected to improve reliability, safety, maintainability, and construction cost of FBR compared with the conventional mechanical pumps. In order to realize the EMP, establishment of a high-temperature-resistant electrical insulating system is indispensable. We selected a mica-alumina combination for the insulating system at temperatures higher than 500degC. Using this insulating system, an ageing pre-test was performed. The ageing condition was to apply 1.5 kV AC voltage in the nitrogen atmosphere at 550degC for 4000 hours. The dielectric properties before and after the ageing were measured to understand the tendency of temporal change in the ageing. In this result, the dielectric performance after the ageing was improved compared with the initial performance. The curious phenomenon was clarified to be caused by a decrease of organic component in the insulation, which remained owing to incomplete burning. Also the possibility of the thermal runaway was considered to be higher as a dielectric breakdown mechanism. (author)