SUPERCONDUCTIVITY GETS PRACTICAL
Superconductors deliver high current density - if you have manufacturing know-how.
- Published in Southwire's Power Cable Update newsletter in March 1998
- Reprint permission granted
Superconductive power cables are simple enough in principle. Wrap a bismuth-based ceramic material in a silver tape. Submerge it in liquid nitrogen at -321'F. Then run current through it. But how do you build this stuff for commercial installations? Southwire is finding answers.
"Southwire brings advanced cable construction expertise to the superconducting party," say R.L. Hughey, project manager. "We're developing practical manufacturing designs. For example, one requirement is that the assembled superconductive cable must be flexible for coiling on reels and for ease of installation."
Flexible Pipes Cool and Insulate
In the center of Southwire's superconducting cable is a flexible, porous-wall pipe. Circulating liquid nitrogen runs down the porous pipe and floods the conductor, dielectric and shields.
A flexible outer pipe surrounds the cable core and provides a return path for the nitrogen. A chiller brings the returning nitrogen back down to design temperature and re-circulates it through the inner core. The outer pipe has double walls with a vacuum between them for thermal insulation.
Southwire has designed and built a machine that wraps the superconducting tape around the inner pipe. Four tape layers are wound in a reverse-lay construction. One wraps to the left, the other wraps to the right.
The tape is delicate-about 0.008 inches thick. Because of the fragility of the tape, the wrapping machine rotates only a few times per minute, compared with several hundred times per minute for conventional cable stranders.
Medium-Voltage Operation Requires Full Shielding
In Southwire's pilot program, the current-carrying superconductor runs at 12.4kV above ground. The superconductive cable uses a fully shielded design similar to NW-90 power cable architecture.
On top of the current-carrying conductor there is a conductor shield layer, then a dielectric layer, then an insulation shield. On top of the insulation shield is a ground-potential superconductive shield, cooled by the returning liquid nitrogen.
The dielectric is formed of multiple layers of polymeric tape. One of the requirements for the tape is that it must retain flexibility at liquid nitrogen temperatures, to resist cracking from vibration and thermal expansion.
"Terminations also are a complex design challenge," says Hughey. "We've got to bring 12.4kV at 1,250A out of 150 psi and -321'F without letting the heat from the external bushing boll the liquid nitrogen, and without letting the bushing coat itself with ice. The prototype terminations are big: eight feet long and 24 inches in diameter."
Hughey concludes, "It has taken a lot of careful engineering to get from laboratory samples to our manufacturing pilot program. Southwire Customers will get the benefit of leading-edge technology that really works in the field." |
