Quench protection and stagnant normal zones in a large cryostable SMES

The quench protection system for a 30 MW SMES built by Babcock and Wilcox is discussed. The magnet uses a cryostable conductor with an aluminum stabilizer cooled by pool boiling helium. A general approach to the protection of a cryostable magnet is presented. The phenomenon of stagnant zones is identified as a catastrophic process which can raise the conductor temperature in a localized area to hundreds of K in several minutes and has been selected as the target for quench protection systems. A stagnant zone can appear in an area covered by vapor and is characterized by uncontrollably growing temperature in the center, while its fronts slowly advance in cryostable regions. Stagnant zones are difficult to detect because of the cryostable nature of the conductor and the existence of other types of normal zones analyzed in the paper, such as steady zones or traveling normal zones, each with its unique behavior. The complexity of quench detection for cryostable magnet and the authors´ approach to the problem are discussed.