A cryogenic sensing element for measurement current transformers

The design of a cryogenic sensing element for superconducting DC current transformers working up to 100 kA at 4.2 K is proposed. Limitations of μ-metal magnetic characteristics at cryogenic conditions are overcome by a soft Ni 81-Mo 5-Fe magnetic alloy (referred to as cryogenic permalloy). An optimized geometrical design allows cores saturation due to mechanical constraints to be avoided. The effectiveness of the design is demonstrated through electromagnetic simulations exploiting measurement results from the characterization of cryogenic permalloy samples. Moreover, a preliminary lab-scale cryogenic DCCT prototype for a nominal current of 20 kA is implemented and characterized. A typical precision of the transducer below ±0.05% is achieved along with nonlinearity better than ±0.05%.