Investigation of electrical insulating property of slush nitrogen for next generation superconducting cable

2019 Fiscal Year Final Research Report

• Project/Area Number: 18K13743
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 21010:Power engineering-related
• Research Institution: Toyohashi University of Technology
• Principal Investigator: Kawashima Tomohiro 豊橋技術科学大学, 工学(系)研究科(研究院), 助教 (70713824)
• Project Period (FY): 2018-04-01 – 2020-03-31
• Keywords: 極低温電気絶縁 / スラッシュ窒素 / 液体窒素 / 部分放電 / 放電波形 / 特徴量

Outline of Final Research Achievements

In immersion cooling superconducting equipment, the refrigerant plays the double role of cooling and electrical insulation, but electric field concentration on the refrigerant causes partial discharge (PD), which can be a weak point in electrical insulation. Although slush nitrogen, which is a mixed refrigerant of liquid nitrogen and solid nitrogen particles, has high cooling performance; however, the effects of the introduction of solid nitrogen particles on the insulation performance has not been clarified. From the analysis of PD waveform characteristics, it was shown that the solid nitrogen particles have a certain barrier effect against discharge, but basically act as a weak point in electrical insulation. On the other hand, the breakdown voltage of slush nitrogen with high solid phase ratio is about 80% of that of liquid nitrogen. It was suggesting that the high cooling performance and sufficient insulation performance may be compatible depending on the insulation design.

Free Research Field

電気絶縁工学

Academic Significance and Societal Importance of the Research Achievements

スラッシュ窒素の冷却・運搬特性は，液体窒素よりも優れることが多数報告される中で，電気絶縁性能は，液体窒素との優劣の観点においても知見が少なく，固体窒素の導入による絶縁性能の変化がメカニズムに則って議論された報告はなかった。本研究によって，PD波形の特徴量解析より，固体窒素は放電に対して一定のバリア効果を有するが，基本的には絶縁上の弱点として作用し，液体窒素に比べて短時間の絶縁破壊特性は低下すことを明らかにした。一方で，固相率の選択によっては，絶縁破壊電圧の低下を抑制できることも示した。これらは，超電導機器の絶縁設計をスラッシュ窒素の放電メカニズムに則って行う上で，重要な知見になるものと考える。