Researchers at the Karlsruhe Institute of Technology have developed a versatile superconducting cable, known as the HTS Cross Conductor (CroCo) with high current-carrying capacity. They have demonstrated that the CroCo can transmit very high DC currents while saving considerable space and weight. It is a continuously running, long length fabrication of soldered, round twisted-stacked-tape conductors that can be easily adapted to a reel-to-reel fabrication.
The CroCo would potentially be suitable for applications where space is limited, but with high power demand, such as on ships or in future electric airplanes. Other possible applications include power transmission for wind parks and up to a 300 kA power supply for electrolysis.
Cross Method Increases Geometrical Filling
The method developed at KIT arranges several REBCO tapes so that they form a cross. The shape was chosen to achieve high superconductor content and the associated high transport current in the round cross section. The conductor can be twisted to improve the magnetic properties and packed into a copper pipe.
“The cross was chosen to improve the filling factor of a twisted-stacked tape conductor of round outer shape,” commented KIT researcher Walter Fietz. “For a conventional stack of tapes of the same width, only about 64 % of the cross-section of a round strand can be used. By adding tapes of a second width, the best geometrical arrangement is a cross, and the maximum geometrical filling factor increases to about 79 %.
“Consequently, the critical current density increases as well. Indeed, it would be possible to increase the filling by adding tapes of a third, fourth, etc. … width, but the increase in filling would slow down significantly and require tapes in many individual widths, which is challenging for tape manufacturers, and not economical, unless you need hundreds of kilometers.”
CroCo Demonstrates High Current Carrying Capacity
An HTS CroCo produced from four and six mm wide REBCO tapes has a diameter of only 9.7 mm. At 70 K, a temperature that may be reached by cooling with liquid nitrogen, it is capable of transporting a direct current of more than 4 kA without any losses.
High-current cables may be produced by combining several CroCos. An optimized cable of 12 HTS CroCos may have a diameter of not more than 70 mm, including thermal and electrical insulation.
“We are using commercial tapes from several tape manufacturers and fabricate the HTS CroCo from these tapes,” Fietz said. “The single HTS CroCo consists typically of 30 to 40 REBCO and Cu tapes; the chosen numbers of REBCO and Cu tapes and the tape widths used depend on the application target. But we have built a 35 kA demonstrator from twelve HTS CroCo strands and tested it in a liquid nitrogen bath at 77 K. Details on these strands are available in IEEE Trans. Appl. Supercond. Early Access, 10.1109/TASC.2019.2909208.
“The current carrying capacity depends on the tapes that are used in the conductor and their individual performance. For example, for a HTS CroCo fabricated from REBCO tapes of two and three mm width, an Ic of ~ 1.5 kA was measured at 77 K, sf and nearly 5 kA at 4.2K, 12 T (B||c). The latter value corresponds to a current density of 482 A/mm², as reported in IEEE Trans. Appl. Supercond. 28 (4) 4802404 (2018).”
“With current improvement of the REBCO tapes, it is expected that the critical current density can increase even further. With new, optimized tapes, 6 kA at 4.2K, 12 T is possible.”
Technology Licensed to Vision Electric Super Conductors
The manufacturing process has been designed for large lengths and industrial production. Fabrication has reached a speed of one meter per minute.
Fietz noted that KIT has licensed the conductor and production technology to Vision Electric Super Conductors GmbH of Kaiserslautern, Germany: “The next step will be the successful implementation of the reel-to-reel fabrication and the demonstration of HTS CroCo in different use cases, such as a smaller 35 kA demonstrator with smaller diameter, tests of windings and so on. Together with Vision Electric Superconductors, we are looking forward also to seeing HTS CroCo in high direct current applications.