Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
2001-10-05
2004-05-04
Pert, Evan (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C029S599000, C505S230000, C505S885000
Reexamination Certificate
active
06730851
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
In a general aspect, the present invention relates to a superconducting cable comprising a cryogenic fluid, a superconducting conductor and a cryostat.
More particularly, the invention relates to a superconducting cable comprising a cryogenic fluid, a superconducting conductor and a cryostat wherein the cryogenic fluid cannot reach a direct contact with the superconducting material.
2. Description of the Related Art
The term “superconducting cable” encompasses any cable to be used to transmit current in conditions of so-called superconductivity, i.e. in conditions of almost null electric resistance. See, for example, Engelhardt J. S. et al., Application Consideration for HTSC Power Transmission Cable, 5
th
Annual Conference on Superconductivity and Application, Buffalo, N.Y., Sep. 24-26, 1991.
The term “superconducting conductor” indicates in the following any element capable of transmitting electric current in superconductivity conditions. An example would include a layer of superconducting material supported by a tubular core. Another example would include tapes of superconducting material partially or totally surrounded by a noble metal pipe, which are wound on a supporting core.
The term “superconducting material” or “superconductor” indicates a material such as, for example, special ceramics based on mixed oxides of copper, barium, and yttrium (usually called YBCO); of bismuth, lead, strontium, calcium, and copper (usually called BSCCO); or of thallium or mercury and barium, calcium, and copper, comprising a superconducting phase having a substantially-null resistivity under a given temperature, defined as the critical temperature or T
c
. For example, for the above-mentioned materials, the T
c
ranges from about 80 K (−193° C. to about 150 K (−123° C).
Usually, the superconducting material, particularly the BSCCO material, is produced and used in the form of mono- or multi-superconductor element tapes. The material is surrounded by a metal, generally silver, optionally with aluminum or magnesium added.
The operative temperature of a superconductive cable is lower than the T
c
of the superconductive material present therein.
In view of this the superconducting cables are provided with at least one channel for the flow of the cryogen. The cryogen is typically liquid helium, liquid nitrogen, liquid hydrogen and/or liquid argon, operating at temperature and pressure specific for the application.
The term “operative temperature” indicates in the following the temperature at which the superconducting cable transmit electric current in superconductivity conditions. Specifically such temperature is lower than the T
c
.
For maintaining the superconducting material at the operative temperature a close contact between the superconducting material and the cryogen is generally recommended. See, for example, EP-A-0 786 783 (in the Applicant's name) wherein the cryogen is said to flow both in the inside of the conductive elements and in the interstices between such elements and the tubular shell. U.S. Pat. No. 4,966,886 (in the name of Junkosha Co., Ltd) discloses a cable wherein the liquid nitrogen penetrates into the crystalline arrangement of the superconducting ceramic and is effectively absorbed to yield a stabilised superconducting cable. Analogously, EP-A-0 412 442 (in the name of Sumitomo Electric Industries, Ltd.) discloses a pipe supporting the superconducting tapes and defining the cooling space, said pipe being provided with holes in order to increase the efficiency of the cooling.
EP-B-0 297 061 (in the name of Saes Getters S.p.A.) discloses a vacuum insulated superconducting electrical conductor employing a getter device. More specifically, this document claims an electrical conductor wherein a thermally insulating evacuated space surrounds the superconducting elements and the liquid nitrogen. Said evacuated space takes the place of the cryostat which is absent in the cable described in the above patent.
WO 98/09004 (in the name of American Superconductor Corporation) discloses that the infiltration of cryogenic liquid into the porous ceramic structure of the superconducting material is detrimental for the integrity of the conductor. In fact, when the article is rapidly heated, the cryogenic liquid entrapped in the interstices of the ceramic material quickly expands, thus creating “balloons” in the matrix and damaging the intragrain bonds thereof. This causes a decrease of the mechanical strength and current carrying capacity of the article. According to said patent application, it is known to “pot” certain superconducting articles with thick layers of epoxy resin for minimising the likelihood of contact between the liquid and the superconducting tape. Alternatively, when the article cannot be protected in such a way (the use of a heavy epoxy coating is considered not feasible because of a number of reasons such as packing factor and flexibility requirements) other cooling means, such as conductive cooling are used. Nevertheless, it is said that the cooling by conduction is not deemed adequate for applications such as transmitting cables. This patent application proposes to solve the “balloon” problem by a superconducting conductor wherein the superconducting ceramic tape has at least one surface, which is vulnerable to cryogenic infiltration, sealed to a non-porous metal laminate impervious to said infiltration. In particular, the metal is stainless steel, copper, copper alloy, or superalloys.
SUMMARY OF THE INVENTION
The Applicant has found that the “balloon” phenomenon does not only occur due to cryogenic fluid leaking from the flowing channel into the superconducting tape area. Actually any kind of fluid directly in contact with the superconductor may liquefy at a temperature equal or higher than the operative temperature of the cable and penetrate into the superconductor. When the temperature of the article rapidly increases, for example when the cable is brought to room temperature for maintenance operations, such a liquefied fluid will abruptly turn into gaseous status, thus expanding its volume and consequently damaging the superconductor according to the “balloon” effect discussed above.
Moreover, in the Applicant's view, the prior art technique of individually protecting each tape to prevent the “balloon” formation, which implies the production of superconducting cables provided with this specific kind of tapes, is economically inconvenient as further material (stainless steel, copper, copper alloy, or superalloys) and further processing steps (lamination and sealing) are necessary.
It has been found that the “balloon” effect damaging the superconducting material can be effectively eliminated by providing a layer of material impervious to the cryogenic fluid. The layer of material can be added between the superconducting conductor and the fluid flow. This addition causes the superconducting material to operate in a space free from fluids that liquefy at a temperature equal to or higher than the operative temperature of the superconducting material.
Therefore, the present invention relates to a superconducting cable that comprises a cryogenic fluid, a superconducting conductor, and a cryostat. Further, a layer impervious to the cryogenic fluid is provided between the superconducting conductor and the cryogenic fluid. This permits the superconducting conductor to operate in a space substantially free from fluids that liquefy at a temperature equal to or higher than the operative temperature of the superconducting cable.
The superconducting cable of the present invention may be a warm dielectric (WD) or cold dielectric (CD) cable. See, for example, Engelhardt J. S. et al. supra,
FIG. 5
for a WD cable, and
FIG. 6
for a CD cable.
A WD cable generally comprises superconducting tapes wound on a support, typically tubular, defining the cryogen fluid flow channel. Externally to the superconducting tapes a cryostat and an electric insulation are provided.
A CD cable generally comprises, in
Caracino Paola
Ladie' Pierluigi
Nassi Marco
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Patel I B
Pert Evan
Pirelli Cavi e Sistemi S.p.A.
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