Basic Research for High-Performance Cryogenic Electrical Insulation System with Polar Polymeric Materials

点击翻译按钮获取中文摘要

Recently, developments of superconducting power apparatuses such as generators, cables and transformers, are promoted in many organizations in the world. These enhance the importance of the electrical insulation technology at cryogenic temperature. Under these situations, it is necessary to obtain much Knowledge of electric strengths and breakdown mechanisms of polymeric materials at cryogenic temperature.At cryogenic temperature, electric strengths (Fb) of polar polymers are higher than of non-polar polymers. Further, they have generally higher mechanical strengths and their tan delta becomes allowable level in the cryogenic temperature region. These facts lead to the idea that polar polymers may be a good candidate for cryogenic electrical insulating materials. However, their breakdown mechanism, especially why Fb of polar polymers show negative temperature dependence in low-temperature region, has been scarcely clarified.In this study, a new specimen is developed to measure intrinsic electric strengths of polymeric films at cryogenic temperature. The DC electric strengths of PVC film measured with this specimen have a peak around -80 ﾟC and are almost constant below -120 ﾟC, which is quite different from those already reported with recessed specimens. Since the recessed specimens made of the same PVC material give almost same temperature dependence of electric strength as that reported by the others, the above results are inherent to the jilm and are probably due to a difference in morphology of the material. The dependence of the electric strengths on both the rise rate of applied field and the prestressing showed that the electrical breakdown of polar polymers are apparently affected by a space charge effect at cryogenic temperature. These results show that the electrical breakdown of polar polymers in the low-temperature region can be interpreted by an electron avalanche breakdown and the space charge effect.