The invention relates to an electrical device for conditioning
of electric current.
An electrical device for current conditioning is known
. Examples of current conditioning devices are also sometimes referred
to as current limiters or fault current limiters. In
US 5, 379,020
, the current conditioning is provided by a electrical device consisting
of a primary spool, a secondary spool which contains a quenchable superconductor
exhibiting a transition from a low resistive state, commonly referred to as the
superconducting state, to a high resistive state when a critical value of the electric
current is exceeded. The secondary spool is coupled through a common part of magnetic
flux with the primary spool. The secondary spool further comprises a metallic member
which forms a closed loop circuit. The secondary spool is positioned in a cryostat
that provides a cooling of the secondary spool. Magnetic coupling of two spools
is provided though a ferromagnetic core.
A fault current limiting device is also known from the
. The device comprises a primary spool comprising a metal or alloy and
a secondary spool which includes quenchable superconductor which exhibits a transition
from the low resistive state to the high resistive state when a critical value of
the electric current is exceeded. Both of these spools are coupled through a common
part of magnetic flux provided in a ferromagnetic core. The secondary spool is based
on a number of thin and flat discs substrates coated with a layer of the quenchable
superconductor, namely, a high temperature superconductor. The discs are provided
with central openings which allow to install the discs in a cryostat and to position
them around a magnetic core. The secondary spool may also comprise a cylindrical
substrate coated with quenchable, high temperature superconductor.
Both of the above-referenced devices aim to condition electrical
current in an external circuit which is connected in series to the primary spool.
They may provide the function of a current limiter which provides limitation of
the "primary" overcurrents. However, the prior art devices display a reaction time
which is to be too slow for some applications and may be too slow for efficient
control of electric power.
It is desirable to further improve the performance of current
conditioning devices and to provide a more efficient control of electrical power
where much shorter reaction times are required in order to not only provide a quick
circuit protection but also to provide an desired fast dynamics for such protection.
It is an object of the present invention to provide a device
for conditioning of the electrical current with a short reaction time. A further
object of the present invention is to provide a device with a controllable reaction
time, so that the reaction time may be predetermined and preinstalled. A further
object of the invention is to provide an electrical device which may be produced
These objects are provided by the subject matter of the
independent claims. Further advantageous embodiments are the subject matter of the
According to the present invention, an electrical device
comprises a primary spool, a secondary spool which comprises a quenchable superconductor.
A quenchable superconductor exhibits a transition from a low resistive state to
a high resistive state when a critical value of an electric current is exceeded.
The secondary spool is coupled through a common part of magnetic flux with the primary
spool. The secondary spool further comprises a metallic member which forms a closed
loop circuit. The electrical device further comprises a cryostat which provides
a cooling of the secondary spool.
The secondary spool comprises at least one element or portion
comprising a substantial fraction of a non-quenchable conductor and a fraction of
the quenchable superconductor. The non-quenchable conductor exhibits a minor dependence
of its resistance on current and magnetic field. The non-quenchable conductor may
exhibit metallic properties. In this context a substantial fraction is defined as
more than 50% by volume. The fraction f of non-quenchable conductor of the element
of the secondary spool is in the range of 50% ≤ f ≤ 95%, preferably
60% ≤ f ≤ 95%, more preferably 75% ≤ f ≤ 95%, even more
preferably 80% ≤ f s 90% by volume. The volume of the element is the sum
of the fraction of the non-quenchable conductor and the fraction of the quenchable
The secondary spool comprises at least one turn which comprises
the non-quenchable conductor and the quenchable superconductor electrically jointed
in series to provide a closed loop circuit.
The construction or arrangement of the secondary spool
according to the invention results in more homogeneous quenching of the quenchable
superconductor and leads to a reduction of the reaction time of the whole device.
The electric device may also further comprise an iron core.
The non-quenchable conductor in this electrical device
may be based on a highly conductive metal as Cu, Ag, Au, Al, In or on a superconductor.
In an embodiment, the quenchable superconductor is a so called low temperature superconductor,
for example Nb3Sn or NbTi when the device is aimed for low temperature
operation, i.e. at the temperatures of 2-10K.
In a further embodiment, the quenchable superconductor
comprises a ReBa2Cu3O7-x -based composition or
a fine mixture thereof, where Re is one of more rare earth elements, preferably
one or more elements from the group consisting of Y, Ho, Nd, La, Pr, Gd, Tb, Dy
The quenchable superconductor in the electrical device
is may be formed as a single layer or may comprise a plurality of layers forming
a multilayer structure.
A ReBa2Cu3O7-x based layer
or multilayered structure are preferably provided in a form of a coated tape which
is based on metallic substrate tape. Either one side or two opposing sides of the
substrate tape may be coated by a single ReBa2Cu3O7-x-based
layer or by a multilayer structure including a number of single ReBa2Cu3O7-x
based layers. The substrate tape may comprise a stainless steel or a Hastelloy or
a Ni-based alloy or a NiCr-based alloy tape which may exhibit a resistively of more
than 80 µ&OHgr;cm (microohm cm.) The substrate tape may also further comprise
one or more additional buffer layers disposed between the surface of the metallic
substrate tape and the superconductor layer. The buffer layer may prevent undesired
chemical reactions between the metallic substrate tape and the superconductor.
Electrical jointing of the quenchable conductor to the
non-quenchable conductor is achieved employing a layer of normal conductor such
as In, Cu,or Pb or a superconductor such as BiSCCO (i.e. Bi2Sr2CaCu2Ox,
or (BiPb)2Sr2Ca2Cu3Ox), Eu(Bi)CCO,
or their mixtures which have a different composition from the compositions used
in either quenchable superconductor or non quenchable conductor.
In an embodiment, the secondary spool of the electrical
device comprises a number of turns, each turn comprising a non-quenchable conductor
and a quenchable superconductor according to one of the embodiments of the invention.
In a further embodiment, the non-quenchable conductor and
the quenchable conductor forming a turn are capable of guiding the current in a
perpendicular direction to the main axis of the magnetic flux. The magnetic flux
may be provided by an iron core around which the primary spool and secondary spool
In an embodiment, at least one of the non-quenchable conductor
and the quenchable conductor comprises at least one portion which is capable of
guiding the current along the main axis of magnetic flux. This allows a re-distribution
of the current between different turns, and thus to form a desired reaction time
of the entire device.
In an embodiment, a ratio of geometrical dimensions of
different portions of the non-quenchable conductor or/and different portions of
the quenchable superconductor varies for different turns of the non-quenchable conductor
and the quenchable superconductor. This embodiment provides an efficient opportunity
to achieve a pre-determined quench reaction.
In an embodiment, the ratios of geometrical dimensions
of different portions of the non-quenchable conductor or/and different portions
of the quenchable superconductor follow a numerical sequence or form a smooth distribution
function. This advantageously enables jumps to be avoided during the quenching of
the entire device.
In the latter embodiment, the width of the distribution
function determines the time performance, such as the reaction time of the device.
In the case that the secondary spool comprises a number
of the turns, the quenchable superconductor may advantageously comprise a plurality
of ReBa2Cu3O7-x coated tapes.
If the secondary spool comprises a plurality of ReBa2Cu3O7-x
coated tapes, the secondary spool may comprise at least two tapes exhibiting different
thresholds of electrical current that causes quench. The at least two tape may exhibit
different superconducting properties such as Jc (critical current density)
or Tc (critcal temperature). This provides an additional freedom in controlling
the response function of the device and, thus, in providing a desired reaction time.
In an embodiment, the cryostat comprises at least one metallic
wall forming a closed loop circuit which comprises at least a part of the common
The metallic member and the metallic wall of the cryostat
may be the same element of the electrical device.
The reaction time may be controlled by replacing or partially
replacing the metallic member of the secondary spool with an "external" element
of the cryostat apparatus.
The electrical device according to the invention may be
more cost efficiently produced as the amount of quenchable superconductor provided
in the device is substantially, by 50 to 90 % lower compared to the known technical
solutions. Superconducting tapes and in particular coated superconducting tapes
which are fabricated by a number vacuum deposition steps are relatively expensive
to produce. Since the secondary spool of the current conditioning device of the
invention comprises a substantial fraction of non-quenchable conductor, the fraction
of quenchable superconductor is reduced. Additionally, the production costs for
the cryostat are reduced as according to the present invention, parts of the cryostat
may be based on conventional metallic elements.
An electrical device for current conditioning in accordance
with the present invention will now be described, by way of example, with reference
to the following accompanying figures:
Fig. 1 A schematic view of a first embodiment of the electrical device according
to the present invention;
Fig. 2 A schematic view of a single turn of secondary spool based on electrically
jointed a non-quenchable conductor and a quenchable superconductor according to
the present invention;
Fig. 3 A schematic view of a circular variant of a single turn of secondary
spool based on electrically jointed a non-quenchable conductor and a quenchable
superconductor according to the present invention;
Fig. 4a A cross sectional schematic view of a second embodiment of the electrical
device according to the present invention;
Fig. 4b An alternative schematic view of a second embodiment of the electrical
device according to the present invention;
Fig. 5 A schematic view of a variant of arrangement of a number of turns
in the secondary spool;
Fig. 6 A schematic view a further variant of arrangement of a number of turns
in the secondary spool according to the present invention;
Fig. 7 A schematic view of an "inhomogeneous" arrangement of a number of
turns of secondary spool according to the present invention.
Fig. 1 reveals a schematic view of the first embodiment of the electrical
device according to the present invention. The device comprises a primary spool
1 which comprises a normal metallic conductor such as Cu or Al, and a secondary
spool which comprises a number of turns. Each turn 2 3 4 comprises a non-quenchable
conductor 3, which in this case has the from of a Cu wire or tape, and a
quenchable superconductor 2, which in this case is provided by a YBCO coated
tape. The YBCO tape superconductor may be a biaxially textured YBCO film or coating
which has been deposited on one side or two opposing sides of a flexible metal or
alloy tape substrate. One or more buffer layers which also have a biaxial texture
may be positioned between the substrate tape and the YBCO superconductor. The non-quenchable
conductor 3 and the quenchable superconductor 2 are electrically jointed
in series in such a way that they form a closed loop circuit. Jointing of these
conductors is provided within jointing areas 4. The jointing areas
4 are positioned towards the ends of the superconductor tape 2 and the ends
of the copper wire 3. In this embodiment the superconductor tape 2 is longer than
the gape between the two ends of the copper wire so that the ends of the superconductor
tape overlap the ends of the copper wire. The jointing area is therefore formed
by these overlapping regions. The joint between the non-quenchable conductor
3 and the quenchable superconductor 2 is achieved by employing a layer
of normal In conductor which was employed as a soldering material forming a thin
interlayer. In this embodiment, one side of the metallic substrate is coated by
YBCO superconductor. The joint between the superconductor tape 3 and the
copper wire 2 is formed between the uncoated metallic substrate of the superconductor
tape 3 and the copper wire 2.
Experiments have demonstrated that a sufficient quality
or electrical jointing may be provided employing pressed or soldered contacts based
on layers of Cu, Pb or high temperature superconductors as BiSCCO or, Eu(Bi)CCO.
The plurality of turns 2-4 are capable of guiding the current in a perpendicular
direction to the main axis of the magnetic flux (marked with a point-dash line in
Fig. 1). The turns 2-4 are placed in a cryostat 5. The device
is equipped with a Cu member 6 that forms closed loop. The secondary spool
of the device also comprises metallic spacers 7 which are electrically connected
to the non-quenchable conductor 3 between adjacent turns of the secondary
spool in order to allow to guide the current along the main axis of magnetic flux
(dash-point line of Fig. 1) provided in an iron core 8 that create
an efficient coupling of the primary spool with the secondary spool based on elements
2-4 and 6. In practice, the primary spool 1 may be located not only
in a way shown in Fig. 1 but at any other position at the iron core 8. This spool
may be located as well inside or outside the secondary spool at the right arm of
the iron core 8.
Further elements of the apparatus of the electrical device
of the invention are an insulating cylinder core 9 serving as holder of the
primary spool 1 and current leads 10 delivering current to the primary
In operation, the current leads 10 are connected
in series to an ac power circuit not shown in the figures. The cryostat
5 is filled with liquid nitrogen or other condensed gas. While the current
that flows through the primary coil does not reach a threshold value, the electrical
device introduces a very small drop of voltage which results in a small power loss.
A small power loss of 0.03% was observed for a 6 kW testing prototype.
When the current in the primary spool exceeds the threshold
value, the quenchable superconductor of the secondary spool enters a highly resistive
state and the electrical device starts to introduce additional impedance to the
external electrical circuit to which it is coupled. The current flowing in the external
electrical circuit is conditioned. The device acts partly as an inductive load and
partly as a resistive load which conditions the current in the primary circuit regarding
its phase and amplitude. One of the known tasks that may be solved by such current
conditioning is a limitation of the current.
In the case of an electrical device comprising 8 turns
based on YBa2Cu3O7-x coated tapes as a quenchable
superconductor, the current was limited to 1600 A (rms) in the secondary spool and
respectively to 14 A (rms) in the primary 380V-circuit. Each coated tapes was 1
cm wide and 10mm thick. Thickness of the YBCO layer was about 2µm. Critical
current of single tape was approximately of 280A (at 77K and self field); current
density in YBa2Cu3O7-x was of 1.4MA/cm2.
Interface of the YBa2Cu3O7-x was coated with a
0.5µm thick silver or gold protective layer that was electrically jointed with
the metallic substrate. The onset current in the secondary spool is additionally
determined by the metallic member 6 which under full power load functions
as an effective shunt that protects the quenchable superconductor against too high
overcurrent. The reaction time of the device corresponds to 45 microseconds. The
device exhibited an extra short, recovery time, i.e. the time needed to return to
the initial state of the electrical device: the recovery time was less than 50ms
at full power loads (to be compared with a typical value of 0.5 to 20 seconds for
Further variants of the electrical device shown in the
Fig. 1 may be based on a different location of the primary spool
1 relative to the iron core 8 and the secondary spool 2-4, 6.
The primary spool may be located at any position including its coaxial positioning
relative to the secondary spool 2-4, 6. In the latter case, the primary spool
1 may either located outside the outer surface of the cryostat
5 or between the cryostat 5 and the iron core 8. A further
alternative is to provide the iron core 8 in a different form; not as a rectangular
shaped core as it is shown in Fig. 1 but for example as a toroid.
Two variants of the construction of a single turn are depicted
in Fig. 2 and Fig. 3. Fig. 2 reveals a rectangular shaped turn and
Fig. 3 relates to a circular shaped turn. Each turn has a portion of non-quenchable
conductor 3 and a portion of a quenchable superconductor 2 mechanically and
electrically connected to form a closed loop. In both figures, the same denotations
as in Fig. 1 are used.
A second embodiment of the electrical device according
to the invention is schematically depicted in Fig. 4a and Fig. 4b.
The non-quenchable Cu conductor 3 has the form of, in this case, a toroid
with a slit in the external wall. To the edges 14 of the slit, an YBCO coated
tape is jointed in such a way that it forms a semi-closed ring 12. A metallic
member 16 is formed as Cu bridge soldered to the same edges 14. The
secondary spool comprising parts 3, 12, 16 is placed in a cryostat
5. The primary spool as well as a complete view of the iron core
8 is not shown in Fig. 4a and Fig. 4b.
In operation, the device was found to exhibit a very similar
performance compared to the first embodiment of Fig. 1 with a difference
in the reaction time which in the actual case becomes even shorter: it is less than
Fig. 5 represents a variant of arrangement of a number of turns in the secondary
spool. In this variant no interconnection of turns in the direction which is parallel
to the direction of magnetic flux is provided. All turns are substantially equal
and comprise a quenchable superconductor 22 and a non-quenchable conductor
23 electrically jointed in areas 24. The non-quenchable conductor
23 has a U-shape with a superconductor tape 22 disposed between the
two ends of the U-shape to form a closed loop. In this embodiment the U-shaped non-quenchable
conductor 23 has a wall thickness approximately that of the width of the
superconductor tape 22 and the superconductor tape 22 extends between
the top surface of each of the arms of the U-shaped non-quenchable conductor
23. An electrical device having such an arrangement exhibits a rather short
reaction time of about 50 microseconds.
Fig. 6 depicts a further variant of arrangement of a number of turns in the
secondary spool. As before, all turns are substantially equal. Each turn comprises
a portion of a quenchable superconductor 32 electrically jointed to a non-quenchable
conductor 33 in areas 34. The non-quenchable conductor 33 possesses
a common part 35 that interconnects the different turns in the direction
of the magnetic flux. Some parts of the non-quenchable conductor 33 are not
interconnected because of a set of slits 36 provided in the conductor
33. The non-quenchable conductor 33 can be thought as being a rectangular
block with a channel disposed on one face of the block to from two protruding arms.
The non-quenchable conductor 33 has a cross-sectional U-shape. A series of
slits, each having substantially the same dimensions, are positioned in the outer
surface of each of the two arms to provide a series of turns of substantially the
same width and height which are mechanically and electrically joined by the base
of the block. The two series of slits are therefore substantially aligned with one
Thus, the non-quenchable conductor is capable in this case
of guiding the current along the main axis of magnetic flux. This results in a re-distribution
of the current between different turns, and in an improved time stability of the
device while deviation of local parameters of quenchable coated tapes
32 provide less influence to the performance of the entire electrical device.
A similar effect may be achieved using a partial interconnection
through the quenchable superconductor. The reaction time is equally short as in
the case considered in Fig. 5.
A schematic view of an "inhomogeneous" arrangement of a
number of turns of secondary spool according to the present invention is shown in
Fig. 7. In this case different turns 42-44 are provided with a different
and different length h of the turn portions 45 .These portions
do not provide electrical interconnection to other turns. Such interconnection is
provided in a "common" part 46 of the non-quenchable conductor. In the present
example, geometrical parameters
are determined by a position and depth of slits 47. Similarly to
Fig.6, the non-quenchable conductor 43 can be thought as being a rectangular
block with a channel disposed on one face of the block to from two protruding arms.
The non-quenchable conductor 43 has a cross-sectional U-shape. A series of
slits are positioned in the outer surface of each of the two arms to provide a series
of turns. In this embodiment the slits are provided with different dimensions. In
Fig. 7 it can be seen that the depth of the slit decreases from the front
to the back of the block shown in the orientation of Fig. 7. The two series
of slits positioned in each arm are substantially aligned with one another forming
a series of pairs of slits. Each pair has substantially the same size to provide
a series of protrusions each forming a portion of a turn. The height and width of
the protruding portions, therefore, varies and can be thought of an inhomogeneous.
It is shown experimentally that the reaction time as well
as a time dependence of the impedance increase during current conditioning is strongly
dependent on the distribution of these geometrical parameters for a given number
of turns. At constant parameters of the strips of the quenchable superconductor
42, a variation of w1 between 10mm and 17 mm with a 1 mm step or interval was provided
for secondary coil based on 8 turns. An increase of reaction time of the entire
device from 40ms to 110 ms was observed. The response of the electrical device during
current conditioning may, therefore, be controlled by providing the secondary coil
with an arrangement of the quenchable superconductor and non-quenchable conductor
which produces the desired response time.
The smoothness of the distribution function of such variations
results in smooth time-variation of impedance during current conditioning. This
may be understood taking into account geometrical parameters of channels in the
not-quenchable conductor determine distribution of current between different turns,
and, therefore, a sequence of their quenching at current overloads. Thus, a width
of the distribution function of geometrical parameters (which in turn defines amplitude
of variations of these parameters) is determining the time-width of electrical response
of entire device, i.e. the entire reaction time. Furthermore, the "smoothness" of
the distribution function enables current jumps to be avoided during operation of
the entire device.
Similar results may be provided by varying the electrical
parameters of the pieces of the quenchable superconductor employed in different
turns. In practice, both possibilities, i.e. the variation of parameters of the
not-quenchable conductor and of the quenchable superconductor, may be used together
with an additional advantage that the entire reaction time may be not only increased
but also be shortened in a controllable way due to a compensation of the intrinsic
inhomogeneity of critical current in the quenchable superconductor by properly chosen
ratio of geometrical parameters of the not-quenchable conductor within each turn.
Employment of a multitude of short quenchable superconductors
in the examples demonstrated in Fig. 1 and Fig. 5 - Fig. 7 results
in an improvement of cost efficiency of production and maintenance of the electrical
device as mounting and replacement procedures become less time- and material consuming.