1 ATM Subcooled Liquid Nitrogen Cryogenic System for Oxide Superconducting Power Transformer

A subcooled liquid nitrogen cryogenic system was developed for an oxide superconducting power transformer. The performance of the transformer was improved in capacity and in reliability. The transformer is cooled by 1 atm subcooled liquid nitrogen supplied by a pump. The cryogenic system has the refrigeration power of 100 W at 70 K and the capacity of the transformer was improved from 500 kVA at 77 K to 800 kVA at 66 K. Because of the steep temperature gradient at the vicinity of the liquid surface, there is a saturated liquid nitrogen at the liquid surface and an ordinary open-dewar can be used as a cryostat for the transformer. Below the steep temperature gradient region, uniformly subcooled bulk liquid is found and there are no bubbles in the cryostat during operations. This feature makes a reliability of the superconducting power transformer increased.