Fundamental Study on Composite Electrical Insulation Design for Superconducting Power Apparatuses Using Low and High Temperature Superconductors

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The purpose of this study is to investigate electrical and mechanical properties of composite insulating systems as well as their components in the cryogenic temperature region from liquid helium temperature to liquid nitrogen temperature. The main results obtained in these three years are as follows. They give inportant information on the insulation design of superconducting power apparatuses.1. Ethylene-propylene rubber (EPR), a kind of composites of base ethylene-propylene and additives, has excellent mechanical properties at cryogenic temperature because of small shrinkage and small stress caused during cooling. Furthermore, EPR has higher resistance to cracking than polyethylene (PE).2. Electric strength of EPR increases gradually as temperature decreases and is superior to that of PE at liquid helium temperature. Dissipation factor of EPR is larger than PE even at cryogenic temperature especially under high electric field The optimization of the components and amount of additives in EPR which are introduced to keep its enhanced mechanical properties is necessary.3. Extruded EPR insulated cable shows good electrical and mechanical properties at liquid helium temperature. It is confirmed that there is no problem in repetitive cooling of the cable4. Electric strength of polyvinylchloride (PVC), a typical polar polymer, shows peak value around 200K, which is quite different from that reported. This is attributed to morphology of PVC and space charge formation in PVC.5. Lifetime of polyethylene exposed to partial discharges in liquid nitrogen becomes shorter with the increase of magnetic field.