Distributed, Autonomous, Shunt Protection of HTS Coils

Temperature Superconducting (HTS) coils will be used in motors, generators, magnets, transformers and inductors throughout space based and Terrestrial power systems. Unlike the conventional low temperature superconductors (LTS), the HTS conductors have, due to their materials of construction and higher operating temperature, higher heat capacity and lower thermal diffusivity. These facts combine to make HTS quenches difficult to detect and slow to propagate, necessitating the development of new protection devices and techniques. A low voltage, high energy cryogenic varistor (cryovaristor) proposed here in, is the ideal HTS coil protection device capable of shunting current around and away form any resistive (normal zone) location in the coil. The focus of the Phase I program is to demonstrate the cryovaristor tape fabrication technique. Voltage current (V-I) characteristic (measured at 40, 20, 77 and 293oK) and energy absorption will be correlated with microstructural analysis and HTS coil/cryovaristor application analysis in an integrated effort to fabricate optimal device. Prof. Iwasa of MIT will be our consultant. In Phase II cryovaristor prototypes will be fabricated in the form of a tape and tested in actual HTS coils. Incorporating the cryovaristor tape into the HTS conductor will be examined. If successful, it would lead to an integrated self protecting HTS conductor. The proposed cryovaristor tape represents an enabling technology in many areas of HTS applications. Distributed, cryo-varistor-based, turn-to-turn, and pancake-to-pancake shunt protection of HTS coils is important for their reliability and hence market penetration. The proposed fabrication technique also applies to low voltage, high energy room temperature varistors (which are currently not available) extending the benefits of this development into the conventional power systems.