Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
2001-06-22
2003-10-14
Norris, Jeremy (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C505S110000, C505S704000, C505S887000
Reexamination Certificate
active
06633003
ABSTRACT:
FIELD OF INVENTION
In a general aspect thereof, the present invention relates to a cable to be used to transmit electric current in conditions of so-called superconductivity, i.e. in conditions of almost null electric resistance.
More particularly, the invention relates to a superconducting cable comprising:
a) a layer of tapes comprising superconducting material,
b) a tubular element for supporting said layer of tapes comprising superconducting material
c) a cooling circuit adapted to cool the superconducting material to a working temperature non higher than its critical temperature.
In the following description and the subsequent claims, the term: superconducting material, indicates a material, such as for instance special niobium-titanium alloys or ceramics based on mixed oxides of copper, barium and yttrium, or of bismuth, lead, strontium, calcium, copper, thallium and mercury, comprising a superconducting phase having a substantially null resistivity under a given temperature, defined as critical temperature (in the following also shortly referred to as Tc).
The term: cable for high power, indicates a cable to be used for transmitting current quantities generally exceeding 3,000 A, such that the induced magnetic field starts to reduce the value of the maximum current density achievable in superconductivity conditions.
The term: superconductor cable indicates in the following any element capable of transmitting electric current in superconductivity conditions, such as for example tapes of superconducting material wound onto a supporting core.
The superconducting cables comprise a structural element consisting of the supporting tubular element of the superconducting material.
Patent application EP 97202433.5 in the name of the Applicant discloses a supporting tubular element entirely consisting of a tube made of polymeric material, typically polytetrafluoroethylene or polyamide.
The same patent application EP 97202433.5 also discloses a supporting tubular element made of metallic material such as steel, copper or aluminum.
The superconducting cable is installed at room temperature, as well as the electrical (to the terminals) and hydraulic connections (attached to the cooling circuits of the cable).
After the installation the cable is brought to its working temperature by means of the cooling liquid. During such cooling each component of the cable is submitted to mechanical stresses of thermal nature, according to the thermal coefficients of the constituting materials.
In particular mechanical stresses are generated in the layers of superconducting materials and at the terminals connected to the ends of the cable.
The Applicant has noticed that the supporting element must not only offer a satisfactory mechanical support to the layer or layers of superconducting material, but also at the same time perform a number of additional functions not less important for the good operation of the cable.
More particularly, the supporting element should:
i) ensure that during cooling of the cable no internal stresses are generated within the superconducting material nor at the ends of the cable;
ii) ensure the mechanical stability of the cable, that is to say, the cable can be bent according to bending radiuses compatible with the diameters of the reels onto which the cable is wound for its transport;
iii) contribute to the mechanical resistance of the cable during the installation; and
iv) substantially contribute to the cryostability of the cable in case of short circuit, this term indicating both keeping the superconducting material below its critical temperature and keeping the cooling fluid in liquid state.
The Applicant has found that the use of a substantially composite supporting tubular element allows to reduce the stresses imparted to the superconducting material both in radial direction and along a longitudinal direction, while ensuring at the same time a sufficient amount of metallic material for ensuring the cryostability of the cable.
According to a first aspect the invention relates to a superconducting, cable of the above indicated type, which is characterized in that said tubular element is composite and comprises a predetermined amount of a first material having a first thermal expansion coefficient and a second material having a thermal expansion coefficient higher than that of said first material, said thermal expansion coefficients and said amounts of said first.and second material being predetermined in such a way that said tubular element has an overall thermal shrinkage between the room temperature and said working temperature of the cable such as to cause a deformation of said tapes comprising superconducting material lower than the critical deformation of the same tapes.
In a second aspect thereof, the invention relates to a superconducting element characterized in that said tubular element is substantially composite and comprises a predetermined amount of a first metallic material in electrical contact with the layer of superconducting material and at least one second polymeric material associated to said first material.
According to a third aspect of the invention, a method for limiting the tensile stresses along a longitudinal direction imparted to opposite fixing terminals of a superconducting cable of the type with clamped heads as a consequence of cooling is provided, the cable comprising at least one layer of superconducting material, which is characterized by providing in the cable a composite tubular element for supporting the layer of superconducting material.
In the following description and in the subsequent claims, the term: superconducting cable of the type with clamped heads, indicates a cable whose opposite ends are mechanically constrained to respective fixing terminals in such a way that no substantial relative sliding in axial direction between tapes and supports and with respect to the terminal themselves takes place.
Advantageously, the aforesaid composite supporting tubular element is able not only to adequately support the superconducting material, but also to limit the stresses induced along a longitudinal direction in the layer of superconducting material and in the terminals connected to the ends of the cable and to provide at the same time an amount of metal in electrical connection with the superconducting material, capable to substantially contribute to the cryostability of the cable during the short circuit transient.
In particular, it has been found that such composite supporting tubular element, thanks to the presence of the above indicated second material having a higher thermal expansion coefficient, has an overall thermal expansion coefficient equal to or higher than that of the superconducting material, and therefore during the cooling step of the cable is able to shrink in radial direction to a greater extent with respect to entirely metallic supports, or, anyway, to an extent such as not to cause unacceptable deformations in the tapes.
In this way, the composite support according to the invention allows a greater shrinkage thereof along a longitudinal direction and, hence, allows to reduce the stresses along a longitudinal direction within the superconducting material due to the so-called constrained shrinking by the clamped heads.
Additionally, the use of a composite supporting tubular element advantageously allows to reduce in a substantial way also the stresses exerted along a longitudinal direction by the ends of the superconducting cable on the terminals with respect to the tubular elements entirely made of metal whenever the second material of the composite supporting tubular element also has a Young's modulus (E) lower than that of the first metallic material.
The longitudinal stresses to which the supporting element of the cable is submitted in operation, in fact, are proportional to the product of the thermal expansion coefficient and the respective Young's modulus (E) of the material which constitutes the supporting tubular element.
In contrast to the tubular element entirely made of polymeric material,
Ladie′ Pierluigi
Nassi Marco
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Norris Jeremy
Pirelli Cavi e Sistemi S.p.A.
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