Effects of copper former on the DC and AC transmission losses for conductor on round core cables

Conductor on round core (CORC) cables have become one of the most promising superconducting cables for power applications. AC loss is a crucial parameter in determining cryogenic maintenance costs. In this work, CORC cable models were built, and the effects of copper formers with different wall thicknesses on the critical current measurements and AC transition loss were studied. It was found that the voltage on the copper former emerged when the total applied currents were below the critical current, Ic, of the cable sample, causing a DC loss as high as 3.3 mW/cm within the former for the original cable samples. An improved approach by secondary soldering is proposed to eliminate voltage in the copper former for the reason that the secondary-soldering reduced contact resistance, thereby facilitating the current transfer from the former to CCs. AC loss of copper formers shows highly frequency dependent behavior with increased wall thickness because of the skin effect. Experimental results demonstrate that the AC losses of CORC cable samples with copper formers of different wall thicknesses are all frequency dependent. The AC loss of the sample with a copper bar is larger than that with a copper tube of the same diameter at low frequencies, while the contrary behavior is observed at high frequencies, suggesting copper tubes as former cores are good for CORC cables to reduce the AC loss at low frequencies.