Design and Operating Performance of Cryocooled Helium Thermosiphon Loops for HTS Transformers

Since the large coils in a cryocooled HTS utility transformer can be located more than a meter away from the cryocooler cold heads, conductive cooling would require very large, heavy thermal links to keep temperature differentials small. Under a U.S. Department of Energy (USDOE) Superconductivity Partnership with Industry (SPI) agreement with Waukesha Electric Systems (WES), SuperPower Inc. (SP), Energy East (EE), and Oak Ridge National Laboratory (ORNL), we have developed a closed‐loop helium system that couples the coils to the cold heads using natural convection cooling. The loop operates on a single‐phase thermosiphon principle at temperatures and pressures from 20 to 45 K and 10 to 20 bars, and needs no external compressor to maintain helium gas circulation. Design procedures for the heat exchangers on both the cold heads and coils and for the interconnecting piping will be summarized. A 30‐W cooling loop was successfully operated on a 1‐MVA prototype transformer. A 350‐W system for a 5/10‐MVA protot...