U.S. patent application number 09/729379 was filed with the patent office on 2003-01-16 for high power superconducting cable.
Invention is credited to Metra, Piero, Nassi, Marco.
Application Number | 20030010527 09/729379 |
Document ID | / |
Family ID | 11372829 |
Filed Date | 2003-01-16 |
United States Patent
Application |
20030010527 |
Kind Code |
A1 |
Metra, Piero ; et
al. |
January 16, 2003 |
HIGH POWER SUPERCONDUCTING CABLE
Abstract
A superconducting cable (1) for high power with at least one
phase comprises a superconducting core (2) wherein a plurality of
elements (3) are housed, which are structurally independent and
magnetically uncoupled, each of which includes--for each phase--a
couple of phase and neutral coaxial conductors, each formed by at
least a layer of superconducting material, electrically insulated
from one another by interposition of a dielectric material (8).
Thanks to the distribution of the superconducting material into
several coaxial conductive elements (3), the cable (1) allows to
transmit high current amounts in conditions of superconductivity,
while using a high-temperature superconducting material sensitive
to the magnetic field.
Inventors: |
Metra, Piero; (Varese,
IT) ; Nassi, Marco; (Torino, IT) |
Correspondence
Address: |
L. P. Brooks, Esq.
Norris McLaughlin & Marcus, P.A.
P.O. Box 1018
Somerville
NJ
08876-1018
US
|
Family ID: |
11372829 |
Appl. No.: |
09/729379 |
Filed: |
December 4, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09729379 |
Dec 4, 2000 |
|
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|
08763909 |
Dec 11, 1996 |
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6255595 |
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Current U.S.
Class: |
174/125.1 |
Current CPC
Class: |
H01B 12/16 20130101;
Y02E 40/60 20130101; Y10S 505/886 20130101; H01B 12/14
20130101 |
Class at
Publication: |
174/125.1 |
International
Class: |
H01B 012/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1995 |
IT |
MI95A 002776 |
Claims
1. A high power superconducting cable having at least one phase
comprising: a superconducting core (2) comprising a phase conductor
(4) and a neutral conductor (5), external to the former and coaxial
to the same, each including at least a layer of superconducting
material, said coaxial conductors (4,5) being electrically
insulated from one another by interposition of a dielectric
material (8), means for cooling said core (2) at a temperature not
higher than the critical temperature of said superconducting
material; characterized in that it comprises, for each phase, a
plurality of magnetically uncoupled conductive elements (3), each
of said conductive elements (3) comprising a couple of phase (4)
and neutral (5) coaxial conductors.
2. Superconducting cable according to claim 1, characterized in
that each of said phase (4) and neutral (5) coaxial conductors
comprises a plurality of tapes of Superconducting material wound on
respective tubular cylindrical supports (6, 7).
3. Superconducting cable according to claim 1, characterized in
that said tapes of superconducting material are wound on said
supports (6, 7), with windup angles of from 10.degree. and
60.degree..
4. Superconducting cable according to claim 2, characterized in
that each of said phase (4) and neutral (5) coaxial conductors
comprises a plurality of layers of superconducting material placed
on said tubular cylindrical supports (6, 7).
5. Superconducting cable according to claim 1, characterized in
that the diameter of the phase conductor (4) of each of said
elements (3) is comprised between 25 and 40 mm.
6. Superconducting cable according to claim 1, characterized in
that said core (2) is cooled at a temperature of from 65.degree. to
90.degree. K.
7. Superconducting cable according to claim 1, characterized in
that said core (2) is cooled by means of liquid helium at a
temperature of about 40K.
8. Superconducting cable according to anyone of the Preceding
claims, characterized in that said superconducting material has the
following formula:
Bi.sub..alpha.PB.sub..beta.Sr.sub..gamma.Ca.sub..delta.Cu.sub..e-
psilon.O.sub.x (I) wherein .alpha. is a number of from 1.4 to 2. 0;
.beta. is a number of from 0 to 0.6; .gamma. is a number of from 0
to 2.5; .delta. is a number of from 0 to 2.5; .epsilon. is a number
of from 1.0 to 4.0; x is the stoichiometric value corresponding to
the different oxides present.
9. A method for transmitting a current quantity greater than a
prefixed value within a superconducting cable (1) having at least
one phase, characterized in that said current is split up, for each
phase, among a plurality of magnetically uncoupled conductive
elements (3) of a coaxial type, the number of said conductive
elements (3) being such that the current fraction transported in
each of them is lower than a value which determines a superficial
current density corresponding to a magnetic field capable of
generating a conductivity reduction of a superconducting material
used.
10. Method according to claim 9, characterized in that the current
is a multi-phase alternate current, and in that, for each phase,
said current is split up among said conductive elements (3).
11. Method according to claim 9, characterized in that the prefixed
value of said current quantity is at least equal to 5,000 A.
12. Method according to claim 9, characterized in that the magnetic
field capable of generating a conductivity reduction of the
superconducting material is smaller than 200 mT.
13. Method according to claim 12, characterized in that the
magnetic field capable of generating a conductivity reduction of
the superconducting material is smaller than 20 mT.
Description
[0001] In a general aspect, the present invention relates to a
cable to be used to transmit current in condition of so-called
superconductivity, i.e., in conditions of almost null electric
resistance.
[0002] More particularly, the invention relates to a
superconducting cable for high power having at least one phase,
including a superconducting core comprising a phase conductor and a
neutral conductor, external to the former and coaxial to the same,
each including at least a layer of superconducting material, said
coaxial conductors being electrically insulated from one another by
interposition of a dielectric material, as well as means for
cooling said core at a temperature not higher than the critical
temperature of said superconducting material.
[0003] In the following description and the subsequent claims, the
term: cable for high power, indicates a cable to be used for
transmitting current quantities generally exceeding 5,000 A, such
that the induced magnetic field starts to reduce the value of the
maximum current density achievable in superconductivity
conditions.
[0004] 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 mercury, comprising a
superconducting phase having a substantially null resistively under
a given temperature, defined as critical temperature or Tc.
[0005] The term: superconducting conductor, or shortly, conductor,
indicates in the following any element capable of transmitting
electric current in superconductivity condition, such as for
instance a layer of superconducting material supported by a tubular
core, or tapes of superconducting material wound on supporting
core.
[0006] As is known, in the field of energy transmission, one of the
problems more difficult to solve is that of increasing as much as
possible both the current to be transmitted in superconductivity
conditions and the temperature at which the transmission takes
place.
[0007] Even though the so-called "high-temperature" superconducting
materials are available today, which can transmit currentings at
temperatures of the order of 70-77?K (about -203/-196BC), a
reduction in the current transmission capacity by said material is
noticed when the inducted magnetic field increases.
[0008] See on the matter, for instance, T. Nakahara "Review of
Japanese R&D on Superconductivity", Sumitomo Electric Technical
Review, Nr. 35, January 1993.
[0009] In superconductivity conditions, the sensitivity of
superconducting materials to the effects of the inducted magnetic
field is ever more marked the greater is the working temperature of
the superconducting core of the cable (i.e., the superconducting
materials with the highest critical temperature are more sensitive
to the effects of the magnetic field), so that in practice
high-temperature superconducting materials do not allow to
transmission current higher than some KA, on pain of an
unacceptable increase in the quantity of superconducting materials
to be used, and, along therewith, of the associated costs.
[0010] In the case of the so-called coaxial cables, whose
configuration is suitable to transmission high loads, the induced
magnetic field, the transmitted current and the diameter of the
conductor are tied by the following relation:
B=(.mu..sub.o.times.I)/(.pi..times.D)
[0011] wherein:
1 B = magnetic field on the surface of the conductor; I =
transmitted current; .mu..sub.0 = magnetic permeability; D =
diameter of the conductor.
[0012] (As is known, the values of B and I are to be understood as
direct current actual values, or as alternate current effective
values).
[0013] On the basis of this relation, it ensues that each increase
in the transmitted current brings about a proportional increase in
the induced magnetic field, which in turn limits, to a greater or
smaller extent, the maximum current density obtainable in
superconductivity conditions or technical critical current density,
"Je", defined as the ratio between the critical current and the
total cross section of the layer of superconducting material.
[0014] More particularly, it has been noticed that the critical
current density drastically decreases - sometimes up to two orders
of magnitude - starting from a threshold value of the magnetic
field, lower than the critical field above which the
superconductivity is substantially compromised; indicatively, such
value varies from 0.1 to 20 mT according to the superconducting
material used and to the working temperature; in this regard,
reference is made to, for instance, IEEE TRANSACTIONS ON APPLIED
SUPERCONDUCTIVITY, vol. 5, nr. 2, June 1995, pp. 949-952.
[0015] The attempts made to keep the critical current density at
acceptable values based on an increase in the conductor diameter,
have till now failed, due to both the practical difficulty of
making, transporting and installing a large diameter cable, and the
high costs necessary to cool the superconducting core, being the
thermal dissipations proportional to the diameter of the insulating
layer that surrounds the core of the superconductor.
[0016] Therefore, in view of these difficulties of technological
nature, in the field of coaxial cables the art has been
substantially restricted to either transmitting the desired high
current quantities by means of suitable metal or ceramic materials
at the temperature of 41K, at which the aforementioned phenomena
are less marked, or accepting an other than optimum exploitation of
the superconducting material at the maximum temperature (650-900K)
compatible with current transmission in superconductivity
conditions.
[0017] In the first case, one has to face the high costs associated
with the need of cooling the superconducting core at a very low
temperature, while in the second case it is necessary to use a very
high quantity of superconducting material.
[0018] According to the invention, it has now been found that the
problem of transmitting within a coaxial cable having at least one
phase high current quantities at the maximum working temperature of
the superconducting materials available today (650-900K, determined
by the usable materials and cooling fluids) can be solved by
splitting up for each phase the superconducting material within the
cable into a plurality of "n" elements, structurally independent
and magnetically uncoupled, each of which comprises a couple of
phase and neutral coaxial conductors, insulated from one another,
and transmits a fraction "I/n" of the total current.
[0019] According to the invention, in fact, it has been found that
with such distribution of the superconducting material it is
possible to:
[0020] a) reduce the size of the cable, with the same use
conditions of the superconducting material, with the ensuing
easiness of construction, transport and installation of the
cable,
[0021] b) use, with the same quantity of superconducting material,
the same quantity of electric insulating material of conventional
cables;
[0022] c) limit, with the same quantity of superconducting
material, the size of the thermal insulation layers (cryostat)
which surround the superconducting core of the cable, with an
advantageous reduction in thermal losses;
[0023] d) have superconducting elements which, in case of need, can
independently supply different loads.
[0024] Preferably, the phase and neutral coaxial conductors of each
of said elements comprise a plurality of superimposed tapes of
superconducting material, wound on a tubular cylindrical support,
for instance made of metal or insulating material in order to
reduce as much as possible the possible mechanical stresses in
their inside, the tapes of superconducting material are wound on
said support according to windup angles--either constant or
variable from tape to tape and within each individual tape--of from
10B and 60B.
[0025] Alternatively, the phase and neutral coaxial conductors of
each of said elements may comprise a plurality of layers of
superconducting material, superimposed and laid on the tubular
cylindrical support.
[0026] According to the invention, the maximum number of coaxial
conductive elements is determined by the minimum diameter of such
elements compatible with the winding deformations of the tapes made
of superconducting material, or anyhow compatible with the critical
tensile deformation of the superconducting material chosen.
[0027] Preferably, the diameter of the phase conductor of each of
said elements varies from 25 to 40 mm.
[0028] According to the invention, the superconducting core of the
cable is cooled at temperatures not higher than 650-900K,
advantageously using so-called high-temperature superconducting
materials and liquid nitrogen as cooling fluid.
[0029] Among these high-temperature superconducting materials, use
may advantageously be made of those known in the art by the
initials BSCCO having the formula:
Bi.sub..alpha.Pb.sub..beta.Sr.sub..gamma.Ca.sub..delta.Cu.sub..epsilon.O.s-
ub.x
[0030] wherein:
[0031] .alpha. is a number of from 1.4 to 2.0; .beta. is a number
of from 0 to 0.6; .gamma. is a number of from 0 to 2.5; .delta. is
a number of from 0 to 2.5; .epsilon. is a number of from 1.0 to
4.0; x is the stoichiometric value corresponding to the different
oxides present.
[0032] According to the invention, particularly preferred are mixed
oxides of the following ideal general formula:
(BiPb).sub.2Sr.sub.2Ca.sub.n-1Cu.sub.nO.sub.x
[0033] wherein n is a whole number of from 1 to 3 and x is the
stoichiometric value corresponding to the different oxides
present.
[0034] Among them, particularly advantageous results have been
obtained with the mixed oxide known as BSCCO-2223 (i.e., in which
n--3), or with suitable mixtures of mixed oxides of the
aforementioned metals, in such ratios as to obtain a mean
stoichiometry of the mixture corresponding to that of the
BSCCO-2223 oxide.
[0035] In another aspect, the present invention relates to a method
for transmitting a current quantity higher than a prefixed value
within a superconducting cable having at least one phase, which
method is characterized in that said current is split up, for each
phase, -among a plurality of magnetically uncoupled conductive
elements of a coaxial type, the number of such conductive elements
being such that the current fraction carried in each of them is
lower than a value which determines a superficial current density
corresponding to a magnetic field capable of generating a
conductivity reduction of a superconducting material used. In a
particular embodiment, such current is a multiphase alternate
current, and said conductive elements among which the current is
split up, carry a single phase of said current.
[0036] In a preferred embodiment of the method, said prefixed
quantity of current is at least equal to 5,000 A. In the method
according to the invention, and if liquid nitrogen is used as
cooling fluid, the magnetic field capable of generating a
conductivity reduction of the superconducting material used is
lower than 200 mT, preferably lower than 100 mT and more preferably
lower than 20 mT.
[0037] Further characteristics and advantages will appear more
clearly from the following description of some examples of
superconducting cables according to the invention, made--by way of
non limitative illustration--with reference to the attached
drawings.
[0038] In the drawings:
[0039] FIG. 1 shows a schematic view, in perspective and partial
section, of a triphase superconducting cable, according to an
embodiment of this invention;
[0040] FIG. 2 shows a schematic view, in perspective and partial
section, of a single phase superconducting cable, according to a
further embodiment of this invention;
[0041] FIG. 3 shows a further embodiment of a cable according to
this invention, using low-temperature superconductors;
[0042] FIG. 4 shows an electric connection scheme of a single phase
cable according to the invention with two independent loads;
[0043] FIG. 5 shows a qualitative graph of magnetic field values
within coaxial conductors.
[0044] With reference to FIG. 1, a triphase superconducting cable 1
according to this invention comprises a superconducting core
globally indicated by 2, comprising a plurality of conductive
elements 3, indicated by 3a, 3b, 3c for each phase,
housed--preferably loosely--within a tubular containing shell 9,
made e.g. of metal, such as steel, aluminum and the like.
[0045] Each of the conductive elements 3 comprises in turn a couple
of coaxial conductors, respectively phase and neutral conductors 4,
5, each including at least one layer of superconducting
material.
[0046] In the examples shown in the drawings, the superconducting
material is incorporated in a plurality of superimposed tapes,
wound on respective tubular supporting elements 6 and (possibly) 7,
made of a suitable material, for instance formed with a
spiral-wound metal tape, or with a tube made of plastics or the
like.
[0047] The coaxial phase conductors 4 and neutral conductors 5 are
electrically insulated from one another by interposing a layer 8 of
dielectric material.
[0048] Cable 1 also comprises suitable means to cool the
superconducting core 2 to a temperature adequately lower than the
critical temperature of the chosen superconducting material, which
in the cable of FIG. 1 is of the so called "high-temperature"
type.
[0049] The aforementioned means comprises suitable pumping means,
known per se and therefore not shown, supplying a suitable cooling
fluid, for instance liquid nitrogen at a temperature typically of
from 65.degree. to 90.degree. K, both in the inside of each of the
conductive elements 3 and in the interstices between such elements
and the tubular shell 9.
[0050] In order to reduce as much as possible, the thermal
dissipations towards the external environment, the superconducting
core 2 is enclosed in a containing structure or cryostat 10,
comprising a thermal insulation, formed for instance by a plurality
of superimposed layers, and at least a protection sheath. A
cryostat known in the art is described, for instance, in an article
of IEEE TRANSACTIONS ON POWER DELIVERY, Vol. 7, nr. 4, October
1992, pp. 1745-1753.
[0051] More particularly, in the example shown, the cryostat 10
comprises a layer 11 of insulating material, formed, for instance,
by several surface-metallized tapes (some tens) made of plastics
(for instance, a polyester resin), known in the art as "thermal
superinsulator", loosely wound, with the possible help of
interposed spacers 13. Such tapes are housed in an annular hollow
space 12, delimited by a tubular element 14, in which a vacuum in
the order of 10.sup.-2 N/m.sup.2 is maintained by means of known
apparatuses. The tubular element 14 made of metal is capable of
providing the annular hollow space 12 with the desired fluid-tight
characteristics, and is covered by an external sheath 15, for
instance made of polyethylene.
[0052] Preferably, the tubular metal element 14 is formed by a tape
bent in tubular form and welded longitudinally, made of steel,
copper, aluminum or the like, or by an extruded tube or the like.
If the flexibility requirements of the cable so suggest, 15 element
14 may be corrugated.
[0053] In addition to the described elements, cable traction
elements may also be present, axially or peripherally located
according to the construction and use requirements of the same, to
ensure the limitation of the mechanical stresses applied to the
superconducting elements 3; such traction elements, not shown, may
be formed, according to techniques well known in the art, by
peripherally arranged metal reinforcements, for instance by roped
steel wires, or by one or more axial metal ropes, or by
reinforcements made of dielectric material, for instance aramidic
fibers.
[0054] According to the invention, several superconducting elements
are present for each phase, in particular, as shown by way of
example in FIG. 1, each phase (a, b, c) comprises two
superconducting elements, respectively indicated by the subscripts
1, 2 for each of the three illustrated superconducting elements 3a,
3b, 3c, so that the current of each phase is split up among several
conductors (two in the example shown).
[0055] FIGS. 2 and 3 schematically show two different embodiments
of this invention, both of them relating to a monophase cable.
[0056] In the following description and in the figures, the
components of the cable structurally or functionally equivalent to
those previously described with reference to FIG. 1 will be
indicated by the same reference numbers and will be no longer
discussed.
[0057] In the embodiment of FIG. 2 four superconducting elements
3.sup.I, 3.sup.II, 3.sup.III, 3.sup.IV, structurally independent
and magnetically uncoupled, are enclosed in the tubular containing
shell 9.
[0058] In the cable of FIG. 3, phase and neutral coaxial conductors
40, 50 of four elements 3.sup.I, 3.sup.II, 3.sup.III, 3.sup.IV,
comprise a superconducting material made of niobiumtitanium alloy,
for which the superconductivity conditions are reached by cooling
the superconducting core 2 to about 4.degree. K my means of liquid
helium. In this further embodiment, the cryostat comprises, besides
a first layer of tapes 11, a hollow space 16 in which liquid
nitrogen circulates at 65.degree.-90.degree. K, and a second layer
of tapes 17, having a structure similar to the preceding ones.
[0059] FIG. 4 schematically shows an example of connection of the
four elements, wherein a monophase generator G is connected to the
respective phase and neutral superconductors 4 and 5 of elements
3.sup.I, 3.sup.II, 3.sup.III, 3.sup.IV, on their turn, the elements
3.sup.I, 3.sup.II, 3.sup.III, are connected to a first load
C.sub.1, and element 3.sup.IV is independently connected to a
second load C.sub.2.
[0060] With reference to what has been described hereinabove, some
examples of superconducting cables according to the invention will
be described hereunder by way of non-limitative illustration.
EXAMPLES 1-3
Invention
[0061] According to the invention, three high power superconducting
cables of the monophase type were designed, incorporating
respectively 37,19 and 7 conductive elements 3 within the
superconducting core 2.
[0062] All the cables were designed to be used in d. c. at a
voltage of 250 kV (high voltage), using a thickness of the
dielectric layer equal to 10 mm.
[0063] In all the cables the superconducting material used was the
mixed oxide known as BSCCO-2223. As the cryogenic, fluid used in
this case is constituted by liquid nitrogen at a temperature of
from 65.degree. to 90.degree. K, the cables possess the structure
schematically illustrated in FIG. 2, using a cryostat 10 having an
overall thickness equal to about 10 mm.
[0064] The, design current was equal to 50 kA.
[0065] The design characteristics in d.c. of the cables were:
[0066] working magnetic field at the decay threshold of the
critical current density, at the temperature of the cryogenic fluid
(about 77.degree. K)=20 mT;
[0067] working magnetic field to which corresponds a critical
current density equal to 50% of that with a field .ltoreq.20 mT, at
the temperature of the cryogenic fluid (about 77.sup.0K)=100
mT.
[0068] As concerns d.c. losses, it has been assumed by way of
approximation that:
[0069] the losses of the conductor were negligible compared with
the other losses;
[0070] the losses in the dielectric were negligible compared with
the other losses;
[0071] the thermal dissipation losses from the
cryostat--proportional to the surface thereof--were expressed by a
ratio between the entering thermal power and the cryostat surface,
equal to 3.5 W/m.sup.2;
[0072] the efficiency of the cooling plant were expressed by a
ratio between the installed power W.sub.i and the extracted thermal
power W.sub.e equal to 10 W/W.
[0073] Therefore, as a first approximation, it is necessary to
install for the cables considered a cooling plant having a power
W.sub.i equal to 35 W/m.sup.2.
[0074] Then for all cables the mean exploitation efficiency of the
superconductor was evaluated based on the following working
hypotheses:
[0075] that the magnetic field generated within the superconducting
material had to increase linearly from a 0 (zero) value on the
internal surface of each of the phase coaxial conductors 4 (radius
R1) and respectively on the external surface of the neutral ones 5
(radius R4), up to maximum values respectively on the external
surface of the phase conductors 4 (radius R2) and on the internal
surface of the neutral ones 5 (radius R3), as is schematically
shown in FIG. 5, while in the hollow space between the phase and
neutral conductors (between radiuses R2 and R3), the field changes
according to the already mentioned law 1 B = 0 I 2 r R2 r ,
[0076] wherein r is the radius of the element and I is the
[0077] current transmitted by conductors 4 and 5;
[0078] that the exploitation efficiency of the superconducting
material had a decreasing linear trend through the thickness, with
threshold values equal to look on the surface having zero field and
up to the threshold level of the field, and equal to the level
corresponding to the decay produced by the maximum working field on
the surface having maximum field, for each of the phase and neutral
conductors (in particular 100% was assumed between 0 and 20 mT and
50% at 100 mT).
[0079] The structural and functional characteristics of the
resulting cables are summarized in the following table I.
EXAMPLE 4
Comparison
[0080] In order to compare the cables of the invention with those
of the prior art, a cable was designed comprising within the core 2
a single coaxial element incorporating superconducting material
BSCCO-2223 cooled in liquid nitrogen.
[0081] The design conditions were the same of preceding examples
1-3, with the additional working limitation constituted by the fact
of keeping a mean exploitation efficiency of the superconducting
material equal to 100%.
[0082] The structural and functional characteristics of the
resulting cables are summarized in the following table I.
EXAMPLE 5
Comparison
[0083] Again to compare the cables of the invention with those of
the prior art, a cable was designed comprising within the core 2 a
single coaxial element incorporating superconducting material
BSCCO-2223 cooled in liquid nitrogen.
[0084] The design conditions were the same of the preceding example
4, with additional working limitation constituted by the fact of
fixing the working magnetic field to 100 mT.
[0085] As a consequence, the mean exploitation efficiency of the
superconducting material was equal to about 70%
[0086] The structural and functional characteristics of the
resulting cable are summarized in the following table I.
EXAMPLE 5bis
Comparison
[0087] Again to compare the cables of the invention with those of
the prior art, a cable was designed comprising within the core 2 a
single coaxial element incorporating superconducting material
BSCCO-2223 cooled in liquid conditions were the same of the
preceding with the additional working limitation the fact of fixing
the working magnetic nitrogen.
[0088] The design conditions were the same of the preceding example
4, with additional working limitation constituted by the f act of
fixing the diameter of the cryostat at a value equal to that of the
preceding example 3 (0.195 m).
[0089] As a consequence, the mean exploitation efficiency of the
superconducting material decreased to a value of about 60%.
Therefore, compared with the cable of the invention, it is
necessary to introduce--with the same diameter--a greater quantity
of superconducting material with a remarkable increase both of the
costs and of the technological manufacturing difficulties of the
same cable.
[0090] The structural and functional characteristics of the
resulting cable are summarized in the following table I.
EXAMPLES 6-8
Comparison
[0091] In order to compare the cables of the invention with those
of the prior art, three cables were designed comprising within the
core 2 a single coaxial element and incorporating respectively a
superconducting material BSCCO-2223 (Example 6) and a
niobium-titanium alloy (Examples 7 and 8).
[0092] Since the cryogenic fluid used was liquid helium at 40K, the
cables have the structure schematically shown in FIG. 3, using a
cryostat 10 having an overall thickness equal to about 70 mm.
[0093] In these cases, it has been assumed as design data a minimum
diameter of the single conductive element equal to 0.025 m, to
respect the construction sizes that maintain the mechanical
stresses within acceptable values.
[0094] The d.c. design characteristics were, consequently, a
working magnetic field at the temperature of the cryogenic fluid
(4.degree. K) of 800 mT, to which corresponds a current density
equal to 100% and 25% of the critical one, for the Examples 6 and 8
respectively, and a working magnetic field of 260 mT at the
temperature of the cryogenic fluid (4.degree. K) in Example 7.
[0095] As concerns d.c. losses, it has been assumed, by way of
approximation, that:
[0096] the losses of the conductor are negligible compared with the
other losses;
[0097] the losses in the-dielectric are negligible compared with
the other losses;
[0098] the thermal dissipation losses from the cryostat
proportional to the surface thereof--are expressed by a ratio
between the entering thermal power and the cryostat surface, equal
to 0.5 W/m.sup.2;
[0099] the efficiency of the cooling plant is expressed by a ratio
between the installed power W.sub.i and the extracted thermal power
W.sub.e equal to 300 W/W.
[0100] Therefore, as a first approximation, it is necessary to
install for the cables considered a cooling plant having a power
W.sub.i equal to 185 W.
[0101] Then for all cables the mean exploitation efficiency of the
superconductor was evaluated based on the criteria illustrated in
the preceding Examples 1-5.
[0102] The structural and functional characteristics of the
resulting cables are summarized in the following table I.
EXAMPLES 9-11
Invention
[0103] According to the invention, three high power superconducting
cables were designed, incorporating respectively 37, 19 and 7
conductive elements inside the superconducting core 2.
[0104] The design data were the same as for the preceding Examples
10 1-3, except for the d.c. use voltage, equal in this case to 1 kV
(low voltage).
[0105] Therefore, a thickness of the dielectric material layer 8
equal to 1 mm was used.
[0106] In all cables, the superconducting material used was the
mixed oxide known as BSCCO-2223.
[0107] Since the cryogenic fluid used in this case is liquid
nitrogen at a temperature of 77.degree. K, the cables possess the
structure schematically illustrated in FIG. 1, using a cryostat 10
having an overall thickness equal to about 10 mm.
[0108] Also in this case, the design current was equal to 50 kA.
The structural and functional characteristics of the resulting
cables are summarized in the following table II.
EXAMPLE 12
Comparison
[0109] In order to compare the cables of the invention with those
of the prior art, a cable was designed comprising within the core 2
a single coaxial element incorporating the superconducting material
BSCCO-2223 cooled in liquid nitrogen.
[0110] The design conditions were the same of preceding Examples
9-11, with the additional working limitation constituted by the
fact of keeping a mean exploitation efficiency of the
superconductor equal to 100%. The structural and functional
characteristics of the resulting cables are summarized in the
following table II.
EXAMPLE 13
Comparison
[0111] Again in order to compare the cables of the invention with
those of the prior art, a cable was designed comprising within the
core 2 a single coaxial element incorporating the superconducting
material BSCCO-2223 cooled in liquid nitrogen.
[0112] The design conditions were the same of preceding Examples
9-11, with the additional working limitation constituted by the
fact of fixing the working magnetic field at 100 mT.
[0113] As a consequence, the mean exploitation efficiency of the
superconducting material was equal to 70%. The structural and
functional characteristics of the resulting cables are summarized
in the following table II.
EXAMPLE 13bis
Comparison
[0114] Again in order to compare the cables of the invention with
those of the prior art, a cable was designed comprising within the
core 2 a single coaxial element incorporating the superconducting
material BSCCO-2223 cooled in liquid nitrogen.
[0115] The design conditions were the same of preceding Examples
9-11, with the additional working limitation constituted by the
fact of fixing the diameter of the cryostat at a value equal to the
preceding Example 11 (0.142 m).
[0116] As a consequence, the mean exploitation efficiency of the
superconducting material dropped to a value of about 50%.
[0117] Therefore, compared with the cable of the invention, it is
necessary to introduce--with the same diameter--a greater quantity
of superconducting material with a remarkable increase both of the
costs and of the technological manufacturing difficulties of the
same cable.
[0118] The structural and functional characteristics of the
resulting cables are summarized in the following table II.
EXAMPLES 14-16
Comparison
[0119] In order to compare the cables of the invention with those
of the prior art, three cables were designed comprising within the
core 2 a single coaxial element and incorporating respectively a
superconducting material BSCCO-2223 (Example 14) and a
niobium-titanium alloy (Examples 15 and 16).
[0120] As the cryogenic fluid used was liquid helium at 4.degree.
K, the cables have the structure schematically shown in FIG. 3,
using a cryostat 10 having an overall thickness equal to about 70
mm.
[0121] The design characteristics and the d.c. losses of the cables
were determined in the same way as that illustrated in Examples
6-9.
[0122] The mean exploitation efficiency of the superconducting
material was evaluated based on the criteria illustrated in
preceding Examples 1-5.
[0123] The structural and functional characteristics of the
resulting cables are summarized in the following table II.
[0124] In the following tables I and II, the cooling costs have
been indicated with reference, respectively, to the cables of
Examples 3 and 11, for which the size and the costs for cooling the
superconducting core 2 resulted to have a minimum value, at the
loss of a non optimum use of the superconducting material, with the
ensuing need of using a greater quantity of the same and with a
higher level of electric losses.
[0125] With regard to the data reported in tables I and. II, it
should also be noted that the material B SCCO-2223 works with a
100% efficiency with a magnetic field equal to 800 mT (Examples 6
and 14), and that the NbTi alloy has, on the contrary, a 100%
efficiency up to a magnetic field of about 260 mT (Examples 7 and
15), and equal to 25% at 800 mT (Examples 8 and 16).
[0126] From what has been described and illustrated hereinabove, it
is immediately evident that the invention allows to couple a
transmission of high current quantities with an optimum
exploitation of high- temperature superconducting materials.
[0127] All this is achieved by keeping the size of the cables and
the cooling costs at values fully acceptable from a technological
point of view.
[0128] If the problems and costs associated to a non optimum use of
the high-temperature superconductor should not be determinant for
the purposes of the specific application, the invention allows all
the same to reduce to a minimum the size of the cable--as shown by
Examples 3 and 11--facilitating the construction, transport and
installation operations, up to values quite comparable with
helium-cooled cables of the known art, which have much higher
manufacturing and operational costs.
[0129] In particular, it has to be observed that, while a cable
according to the invention--with the same transmitted current--has
an overall diameter (cryostat included) lower than 0.3 m, such as
to allow, for instance, its winding on a reel, a cable of the known
art, using a single coaxial conductive element, would have a
diameter greater than 1 meter, if the superconducting material were
used at a look efficiency (magnetic field lower than 20 mT).
[0130] In the same way, if a 70% efficiency of the superconducting
material is accepted (magnetic field up to 100 mT), a cable
according to this invention may have a diameter of 0.14 m, while a
cable according to the known art would have a diameter of no less
than 0.23 m, with the associated drawbacks, such as for instance a
60%; increase of the cooling costs.
[0131] It must be noted that the subdivision into several
superconducting elements does not involve an increase in the
overall surface of the same conductors, and therefore it does not
cause any actual increase in the volume of the insulation used.
[0132] According to the invention, furthermore, it is
advantageously possible to:
[0133] reduce the size of the cable--with the same exploitation of
superconducting material--with ensuing easiness of construction,
transport and installation of the cable (compare Example 2 with
Example 4, and Example 3 with Example 5);
[0134] use--compared with the cables of the known art--the same
quantity of electric insulation with the same quantity of
superconducting material;
[0135] limit the size of the thermal insulation layers (cryostat)
which surround the superconducting core of the cable, with an
advantageous reduction in thermal losses (compare Examples 1 and 2
with Example 4, and Example 3 with Example 5);
[0136] have magnetically uncoupled conductive elements capable of
supplying different loads;
[0137] make flexible, high-efficiency superconducting bus bars;
[0138] use in the best way and therefore reduce the quantity of
superconducting material present in the various phase and neutral
conductors, with the same cable diameter and therefore also with
the same cooling costs.
[0139] It should be noted that, should one wish to make a high
voltage cable (250 KV) with a diameter of 0.14 m according to the
known art, i.e. with a single element of the coaxial type, a
magnetic field of 175 mT would be reached to which corresponds an
exploitation efficiency of the superconducting material equal to
50%, compared with the 70% obtainable according to the invention
(see on the matter Examples 3 and 5bis).
[0140] In the same way, should one wish to make a low voltage cable
(1 KV) with a diameter of 0.2m according to the known art, i.e.
with a single element of the coaxial type, a magnetic field of 130
mT would be reached to which corresponds an exploitation efficiency
of the superconducting material equal to 60%, compared with the 70%
obtainable according to the invention (see on the matter Examples
11 and 13bis). What has been illustrated with reference to cables
of the monophase type, applies also to cables of the triphase type
or, more generally, multi-phase, of the type shown in FIG. 1, in
which a remarkable advantage is reached by splitting up the
conductive elements of each phase into several elements, each of
which carries a fraction of the global current of the phase.
[0141] For instance, a triphase cable for supplying 1700 MVA at 20
KV, manufactured with a single conductive element for each phase
would require a diameter on the cryostat of 0.52m; according to the
present invention, by splitting up each phase into 7 phase
conductors, the cable would have a diameter on the cryostat of 0.43
m, with the same use of the superconducting material.
[0142] In the same way, a triphase cable for supplying 35 MVA at
400 V, manufactured with a single conductive element for each
phase, would require a diameter on the cryostat of 0.48m; according
to the present invention, by splitting up each phase into 7 phase
conductors, the cable would have a diameter on the cryostat of
0.32m, with the same use of the superconducting material.
[0143] With regard to the method of the invention, it has also been
observed that current quantities higher than a prefixed value,
generally equal to at least 5,000 A, may be carried--with the
aforementioned advantages--by splitting up the total current into a
number of magnetically independent conductors such that the current
fraction carried within each of them is smaller than a threshold
value inducing a magnetic field capable of limiting the
conductivity of the superconductive material used.
[0144] Obviously, those skilled in the art may introduce variants
and modifications to the above described invention, in order to
satisfy specific and contingent requirements, variants and
modifications which fall anyhow within the scope of protection as
is defined in the following claims.
2TABLE I Material BSCCO NbTi Example 1 2 3 4 5 5bis 6 7 8 Nr.of 37
19 7 1 1 1 1 1 1 elements per phase Critical 1350 2630 7140 50000
50000 50000 50000 50000 50000 current for cond. [A] Working 77 77
77 77 77 77 4 4 4 [.degree. K.] Working 20 20 100 20 100 130 800
260 800 magnetic field [mT] Mean 100 100 70 100 70 60 100 100 90
exploitation efficiency of the SC material [%] (Approx.) single
0.027 0.053 0.0285 1 0.2 0.15 0.025 0.077 0.025 phase conductor [m]
single 0.057 0.083 0.0585 1.03 0.23 0.18 0.055 0.107 0.055 element
[m] cryostat 0.419 0.435 0.195 1.05 0.25 0.195 0.195 0.247 0.195
[m] Cooling 2.1 2.2 1 5.4 1.3 1 5.3 6.7 5.3 costs
[0145]
3TABLE II Material BSCCO NbTi Example 9 10 11 12 13 13bis 14 15 16
Nr.of 37 19 7 1 1 1 1 1 1 elements per phase Critical 1350 2630
7140 50000 50000 50000 50000 50000 50000 current for cond. [A]
Working 77 77 77 77 77 77 4 4 4 temp. [K] Working 20 20 100 20 100
175 800 260 800 magnetic field [mT] Mean 100 100 70 100 70 50 100
100 90 exploitation efficiency of the SC material [%] (Approx.) .O
slashed. single 0.027 0.053 0.0285 1 0.2 0.11 0.025 0.077 0.025
phase conductor [m] single 0.039 0.065 0.0405 1.012 0.212 0.122
0.037 0.089 0.037 element [m] cryostat 0.293 0.343 0.142 1.032
0.232 0.149 0.177 0.229 0.177 [m] Cooling 2.1 2.4 1 7.3 1.6 1 6.6
8.6 6.6 costs
* * * * *