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

近年来，超导发电机、超导电缆、超导变压器等超导电力设备的开发在世界范围内得到了广泛的重视。这些都增强了低温电绝缘技术的重要性。在这种情况下，有必要对聚合物材料在低温下的电强度和击穿机理进行深入的研究。此外，它们通常具有更高的机械强度，并且它们的损耗角正切在低温温度区域中变为可允许的水平。这些事实导致极性聚合物可能是低温电绝缘材料的良好候选者的想法。然而，它们的击穿机理，特别是极性聚合物薄膜的Fb在低温区呈现负温度依赖性的原因，至今还不清楚。用这种试样测得的PVC薄膜的直流电强度在-80 ° C附近有一个峰值，在-120 ° C以下几乎不变，这与已报道的凹进试样的直流电强度有很大不同。由于由相同PVC材料制成的凹进样品给出的电强度的温度依赖性与其他人报道的几乎相同，上述结果是Jilm固有的，并且可能是由于材料形态的差异。电场强度与外加电场上升速率和预加应力的关系表明，极性聚合物在低温下的电击穿明显受到空间电荷效应的影响。这些结果表明，极性聚合物在低温区的电击穿可以用电子雪崩击穿和空间电荷效应来解释。

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.