CA2177169C - Current-limiting device - Google Patents
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- CA2177169C CA2177169C CA002177169A CA2177169A CA2177169C CA 2177169 C CA2177169 C CA 2177169C CA 002177169 A CA002177169 A CA 002177169A CA 2177169 A CA2177169 A CA 2177169A CA 2177169 C CA2177169 C CA 2177169C
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Abstract
In order to limit overload currents in electric lines, caused by a short circuit for instance, the invention calls for a current limiter to be inserted in the electric line to limit the overload current to a given multiple of the nominal current. The simple, compactly designed current limiter has, disposed on each side of a disc-shaped insulator (1), a composite conductor made up of a high-temperature superconductor (3, 3') and a normal conductor (4, 4') connected flat against each other to give good electrical contact. Between the composite conductors and the insulator (1) is a chemically insulating buffer layer (2) made of 2-µm-thick silver. Either the superconductor side or the normal-conductor side of the superconductor makes flat contact with the insulator (1). The electrical resistance of the shunt (4, 4) should not be greater than that of the superconductor (3, 3') connected to it. Each composite conductor is meander-shaped in design and electrically connected so that the current in adjacent strips of the composite conductor flows in opposite directions.
Description
CURRENT LIMITER DEVICE
BACKGROUND OF THE INVE:LVTION
s Field of the Invention The present invention relates to a current limiter device.
Discussion of Background to Referring to the prior art, JP 2-183915 A specifies a substrate with an oxide superconductor layer and a noble metal layer mounted on both surfaces of the substrate. Metals are considered suitable as substrate material.
US 4,961,066 specifies as a current limn er, for fast 15 current limitation in the event of short circuits, a rod shaped, a tubular and a planar layer structure, in each case consisting of a support insulator, a thin superconducting layer applied extensively thereon and a normal conductor resistive layer applied extensively on the latter. The last two layers can be 2o repeated one after the other. In this case, the resistance of the non-superconducting resistor is less than that of the superconductor in the normal conducting state. Disadvantages are large power losses in AC operation as well as relatively long conductors.
2s In the patent US 4,994,932 a superconducting current limiting apparatus is disclosed. The current limiting wires are mounted on surfaces of stack elements in parallel lines in which current flows in opposite directions in adjacent lines.
3o SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a further development of a current limiter device of the type mentioned at the outset in such a way that it is suitable for the resistive limitation of both direct and alternating currents. The intention is to limit an overcurrent, in the event of a short circuit, to a predeterminable multiple of the rated current.
An advantage of the invention consists in that the current limiter device is simple and compact. The super s conducting part of the current limn er is of modular design, that is to say that the superconductor is subdivided into units which, should the need arise, can be removed and replaced separately.
According to an advantageous configuration of the to invention, the compound conductors, consisting of super conductors and normal conductors, used in the current limn er can be arranged with low inductance. By virtue of a suitable conductor arrangement, AC losses which occur in the case of AC
applications can be greatly reduced.
15 The current limner device can also be used as an active switching element by introducing it into an external magnetic field. In this case, use is made of the fact that the critical current is very greatly reduced in magnetic fields. By switching on the external magnetic field, it is therefore zo possible to reduce the current in the superconductor to a fraction of the rated current.
BRIEF DESCRIPTION OF THE hRAWINGS
A more complete appreciation of the invention and many z5 of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Figure 1 shows, in cross section, a modular arrangement 30 of current limiters in a magnetic field coil, Figure 2 shows, in cross section, a current limiter according to Figure 1, Figure 3 shows, in a cross section, a superconductor of the current limiter according to Figure 2, with a meandering 35 conductor track, Figure 4 shows signal diagrams of AC losses with current limn ers, and Figures 5-8 show current limiters with different layer sequences.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, Figure 1 shows 4 current limiters or current to limiter modules (5) , which are arranged parallel to one another in a cryostat (7) filled with liquid nitrogen, are electrically connected in series and are connected to an electrical line (6).
During operation, a current (I) which, in the event of an overcurrent, for example as a result of a short circuit, is intended to be limited by the current limiter modules (5) to 3 times - 5 times a predeterminable rated current (IR) flows through the electrical line (6). The cryostat (7) is arranged within a magnetic field coil (8).
Figure 2 shows the layer structure of a current limn er 2o module (5) according to Figure 1. A thin silver buffer layer (2) whose thickness is in the range of 1 ~,m to 5 um is in each case applied onto a first main face (la) and onto a second main face (1b) opposite the former, of a plate-shaped ceramic sheet or of an insulator (1) having a thickness (dl). Suitable 2s materials for the insulator (1) are insulators, obtainable in sheet form, which have sufficient therma7_ stability and undergo, between room temperature and 77 K, a long-term thermal alteration which is comparable with that of the superconductor (3, 3'). Glass fiber reinforced casi~ resin sheets or Mg0 3o ceramic sheets are preferably used. They are most simply applied to the buffer layer (2) using a commercially available adhesive. This step is required for superconductor sheets (3, 3') which have been cast in a silver mold that is removed after casting, but is not required when using cast substrates on a 35 nickel based alloy or ceramic which need not be removed after production of the superconductor (3, 3') and can be used for mechanically stabilizing the latter.
Plate-shaped high temperature superconductors or superconductors (3, 3') having rectangular cross section and s meandering configuration according to Figure 3 and a respective thickness (dSC) are applied onto the two buffer layers (2), in extensive contact with the latter. In this case, a first main face (3) of each superconductor (3, 3') is, in order to electrically stabilize it, in good electrically conductive to contact with the respective buffer layer (2). The two superconductors (3, 3') have a mutual layer separation or conductor separation (D).
A second main face (3b) of the respective superconductor (3, 3') is, in order to electrically and 15 thermally stabilize it, in extensive good electrically conductive contact with a first main fare (4a) of a respective non-superconducting resistor or normal conductor (4, 4!) having a thickness (dN~), wherein dN~ is the thickness of the normal conductor. A second main face of the normal conductor (4, 4'), 20 opposite the first main face (4a) , is de:~ignated (4b) . Suitable metals for the normal conductors (4, 4') are, in particular, those which have a resistivity at room to=_mperature of > 10 x,52 cm and are still ductile at -200°C. Preferable ones are: tin, zinc " bismuth and alloys thereof as wel=L as non-magnetic metals 2s based on steel or nickel. The normal conductors (4, 4') can be applied onto the superconductor (3, 3') by electrolytic methods, flame spraying, plasma spraying, bonding using a conductive adhesive, soldering or sintering a cold sprayed metal powder.
The thickness of the normal conductor layer (4, 4') should be 3o such that the electrical resistance of this layer is approximately equal to that of the adjoining superconductor layer (3, 3') in the non-superconduct:ing state, for example 50 ~,m when the thickness (ds~) of the superconductor (3, 3' ) is 1 mm, wherein dSC is the thickness of the superconductor. The contact resistivity, expressed in term: of area, between the second main face (3b) of the superconductors (3, 3') and the first main face (4a) of the normal conductors (4, 4' ) should be < 1 mS2 cmz, preferably 5 10 x,52 cm2.
Figure 3 shows a cross section through the superconductor (3) according to Figure ~:, in a section that is perpendicular to the layer representation in Figure 2. A band shaped conductor is produced using cuts or meander gaps (9) in a sheet with a rectangular, preferably square contour. Silver contacts (10, 11) are applied to this conductor at the end, for electrically connecting it. Neighboring meander gaps (9) have a io perpendicular separation (b) corresponding to a meander path width. The meanders are most simply produced by reciprocal cuts by milling or sawing or laser or water-jEa cutting, and this can be done before applying the normal conductor (4, 4') or else before applying the mechanical stabilization, that is to say the insulator ( 1 ) .
The meandering superconductors (3; 3') are arranged, in the current limiter modules (5), on both sides of the insulator (1) in such a way that the current (I) flows in the opposite direction in respectively opposite meander paths, so that the 2o self-field components perpendicular to the band plane compensate for each other. The effect of this is that the current limner modules (5) are low-inductance and low-loss.
Fig. 4 shows, in AC loss curves (12) and (13) for different values of the ratio of the meander path width (b) to the thickness (dsC) of the superconductor (3, 3' ) , the AC losses which occur in the abovementioned current return line.
In this case the conductor separation (D) in mm is plotted on the abscissa and the ratio of the electrical AC power loss (P) to a length (1) of the superconductor (3, 3' ) in mW/m is plotted on the ordinate. The AC loss curve (12) shows that, for b/ds~ - 2, the AC losses decrease with g - 94/123 US
increasing conductor separation (0), while with b/dgC =
15 they increase with increasing conductor separation (0), cf. AC loss curve (13). The current return arrangement in a current limiter module (5) only leads to a reduction in the losses with increasing conductor separation (D) when the. meander track of the superconductor (3, 3') is sufficiently flat, that is to say when the ratio b/dSC is sufficiently large, up to a critical value of 3.5. For bands with b < 3.5 ~ dSC, the current return increases the AC power loss (P) instead of reducing it.
Figs. 5-8 show a simplified representation of various layer sequences which can be used instead. of the layer structure in Fig. 2. In this case (NC) designates normal conductors (4, 4', 14, 15) and (SC) designates superconductors (3, 3').
According to Fig. 5, a compound conductor or laminate consisting of a normal conductor (4, 4') and a superconductor (3, 3') can be extensively connected to the insulator (1) or the respective buffer layer (2) in such a way that the normal conductor (4, 4') is in each case arranged on the insulator side.
According to Fig. 6, a compound conductor consisting of a normal conductor (4, 4') and a superconductor (3, 3') can be extensively connected to the insulator (1) or the respective buffer layer (2) in such a way that the insulator (1) is in superficial contact, via one of its main faces, with the superconductor (3), according to the arrangement of Fig. 2, and via its other main face with a normal conductor (4'), according to the arrangement of Fig. 5.
It is also possible to provide a further insulator (1') which is extensively connected to the superconductor (3' ) via a buffer layer (2) .
Fig. 7 shows a layer structure according to Fig. 5, in which the outer faces of the superconductors (3, 3') are in extensive, highly conductive electrical contact with additional normal conductors (14) and (15) .
. ~17'~169 Fig. 8 shows a current limiter module (5) which has a layer structure according to Fig. 7 on one side of the insulator (1) and a layer structure according to Fig. 2 on the other side.
In the current limiter modules (5) according to Fig. 1, the superconductors (3, 3') are resistively coupled into an electrical circuit. Below a certain critical current strength (jc) the superconductor (3, 3') is in the superconducting state and therefore has virtually no electrical resistance. If the critical current strength is exceeded, for example because of a short circuit, then the superconductor (3, 3') undergoes transition into its normal conducting state.
The resistance which results therefrom limits the current to a value which is much smaller than the short circuit current.
An important factor is the dimensioning of. the superconductor (3, 3'), its electrical, thermal and mechanical stabilization, the AC losses which result during operation and the connections between the current limiter modules (5).
The electrical and thermal stabilization is achieved using at least one normal conductor (4, 4', 14, 15) as a parallel conductor, which must locally be in good electrical and thermal contact with the superconductor (3, 3'). This bypass resistor (4, 4', 14, 15) can, should need be, locally take a part of the current from the superconductor (3, 3') and thereby protect the latter from excessive heating and destruction. In order for it to be possible to relieve the load on the superconductor (3, 3') effectively, the bypass resistance (4, 4', 14, 15) should not be greater than the normal resistance of the superconductor (3, 3'). The thickness (dNC) of the bypass resistor (4, 4', 3 5 14 , 15 ) must consequently be z dSC PNC'PSC ~ PNC and pSC
being the resistivities of the bypass resistor (4, 4', 14, 15) and of the superconductor (3, 3'), respectively. Since the intention is for the bypass resistor (4, 4', 14, 15) to receive as much heat as _ g _ possible, a high thermal inertia and consequently a high resistivity pNC are beneficial.
In the operating state, the superconductor (3, 3' ) must be able to carry the rated current (IR) which gives a lower limit for its cross-sectional area F, according to:
F ? 1.414 - IR/jc.
In the limiter state the currE.nt (I) is intended to rise to at most n times the rated current (IR), values of between 3 and 5 being required ir~ practice. This requirement gives the to minimum conductor length (1) of the superconductor (3,3'), according to:
1 >_ b ' (dsc/Psc + dNC/PNC) ° 1. 414 ~ UR/ (n ' IR) , UR being the rated voltage of a current source (not represented) and b being the strip width of a compound conductor consisting Zs of a bypass resistor (4, 4', 14, 15) and a superconductor (3, 3' ) .
The AC power loss (P) of a superconductor (3, 3') through which current flows depends greatly on the local magnetic field (self-field and possible external fields). In 2o the case of band-shaped superconductors (3, 3'), as they are used according to Fig. 1, above all the field components which are perpendicular to the band plane h<~ve a very unfavorable effect on the AC power loss (P). The conductor geometry must therefore be realized in such a way that the field in the 2s superconductor (3, 3') is oriented predominantly parallel to the band plane. In a single thin current carrying band, the magnetic field in the conductor is for the most part perpendicular to the band plane, in which case the AC power loss (P) would not be acceptable for the application. An efficient reduction in the 30 perpendicular field components can be achieved with a conductor geometry which consists of pairs of closely adjacent conductor segments, perpendicular to the band plane, with antiparallel current (I). For each pair of such conductors the magnetic field in the conductor is for the most part parallel to 35 the band plane, which results in a substantially smaller AC
~1~7~16~
a ' power loss (P). The AC power loss (P) per unit conductor length (1) is given by:
P/1 = 4 ' jc ' C-A(xec) ' F + (~) A(x) dfl .
A(x) being the vector potential at maximum current, xec being the so-called electrical center of the superconducting band, at which the electric field is always = 0, and F being the cross-sectional area of the band. The integral extends over the entire conductor cross section F. It can be seen from the above formula that the current return concept is effective if the conductor separation (0) of the superconductors (3, 3') perpendicular to the .band plane is substantially less than the meander path width (b). For O » b, the superconductors (3, 3') behave as 2 individual conductors with high AC power loss (P). For b. 2 mm and dSC - 0.5 mm, with current return a reduction in the AC power loss (P) by a factor of 2 can be achieved.
The current return design can be realized with bands arranged in meander or spiral form, cf. Fig. 3. In this case the conductor separation (0) is preferably selected to be < 10 mm.
Example 1 with layer structure according to Fig. 2:
Rated power PR 20 kW, Rated voltage UR 200 V, Rated current IR 100 A, Maximum current Imax 300 A, Critical current density jc 1 kA/cm2, Conductor width b 1.4 cm, Meander gap width (9) 1 mm, Conductor length 1 per module (5) 126 cm, Total conductor length . 8.8 m, Number of modules (5) 7.
AC power loss P at 77 K 0.62 W.
A modularly designed high temperature superconductor (3, 3') based on Bi:Sr:Ca:Cu - 2:2:1:2 was applied to a thickness (dSC) of 1 mm onto both sides of a ceramic sheet (1) with an area of 10 cm 10.4 cm and a thickness (dl) of 1 mm. There was a 21'7"169 silver layer (2) with a thickness of 2 ~m between the ceramic sheet (1) and the superconductor (3, 3'). The silver acts both as an electrical stabilizer (bypass resistor) and as a chemical insulator between the superconductor (3, 3') and the ceramic substrate (1). A
lead layer (4, 4') with a thickness (dNC) of 10 ~.m, which likewise contributes to the electrical stabilization, was applied:to the other side of the superconductor (3, 3').
Meander gaps (9) according to Fig. 3 were cut from the superconducting layer on both sides of the sheet. The two conductor tracks (3, 3') on both sides of the ceramic sheet (1) are electrically connected to each other in such a way that the current (I) in directly opposite band parts flows in antiparallei directions. The current return effect for reducing the AC power loss (P) is thereby achieved.
Example 2: Switch function according to Fig. 1 By enclosing the current limiter modules (5) according to Example 1 with a magnetic field coil (8) , as represented in Fig. 1, the device according to the invention can be used as an active switching element.
When the magnetic field is turned on, the critical current strength (jc) in the superconductor (3, 3') is reduced, so that the superconductor (3, 3') undergoes transition into the resistive state. This causes the current (I) to be reduced to a fraction of the rated current (IR). Because of the texturing of the superconductor (3, 3'), the reduction in the critical current strength (jc) is a maximum if the applied magnetic field is perpendicular to the plane of the superconducting band, as in Fig. 1.
Production of a superconducting sheet (1):
Superconductor powder of composition BiaSnbCacCudOe with a, b, d = 1.8-2.2, c = 0.8-1.2, a =
7.5-8.5 is introduced; in the dry state or as a suspension with a liquid, into a suitable flat mold. In a preferred procedure, silver powder and/or Bi203 powder is mixed in a concentration range of 0.5%-5%
with this superconductor powder, which has a positive influence on the melting and compacting of the melt.
Any material which does not react with the powder during the subsequent melting of the powder and is structurally stable at temperatures of around 900°C is suitable as a casting mold. Molds made of sheet silver, of nickel alloys with a silver protective layer and ceramic sheets made of magnesium oxide and stabilized zirconium oxide were used. The metal molds were easily able to be provided, for example by deep drawing or folding, with a rim having a height of approximately 10 mm. Conductive silver was used as the buffer layer (2) or adhesive. The filling level was selected in such a way that, when the powder was compacted to 100% by the melting, a thickness (dgC) of 0.3 mm-3 mm results.
The highest possible so-called green density of the powder, which was achieved by uniaxial subsequent pressing of the loose powder bed, is advantageous for the achievable current density and homogeneity. A
pressure of 10 MPa is sufficient for this. The casting method is described in DE-Al-4,234,311.
Application of the electrical stabilization:
Superconductor sheets (1), which were produced in silver or ceramic molds, were provided with a metallization (4, 4', 14, 15) which serves as electrical stabilization. For this purpose the silver must be removed from the superconductor sheet (1), which can be done before the metallization or after a mechanical stabilization has been applied.
When casting molds of silvered nickel-based alloy are used, separate electrical stabilization is not necessary if the resistance of the silver/nickel-based alloy combination already corresponds to that of the superconductor (3, 3').
Example 3:
Square casting molds with dimensions of 100 mm 100 mm, having rims with a height of 6 mm, were folded by hand from a 100 ~m thick silver sheet. These casting molds were in each case filled with a slurry of 60 g Bi2Sr2Ca1Cu2Og+8 powder in ethanol, 0 s 8 s 0.3. After the liquid had dried, the powder bed was compressed by uniaxial pressing with a pressure of 2 GPa. The specimens were then subjected to a heat treatment, in an oxygen atmosphere, which consisted of a melting step at 900°C for a period of time in the range from 20 h -80 h. Homogeneous compact superconductor sheets (3, 3') with a thickness of approximately 1 mm, from which the silver could easily be shaved off, were obtained as a result. A 50 ~m thick tin layer was deposited onto these superconductor sheets (3, 3') by flame spraying for electrical stabilization. These superconductor sheets (3, 3') were then adhesively bonded onto .an aluminum sheet and made into a meandering form by water-jet cutting, conductors with a cross section of 14 mm ~ 1 mm and a length of approximately 70 cm being produced as a result. After the aluminum sheet had been removed, 2 superconductor sheets (3, 3') were in each case aligned relative to each other in such a way 'that their tracks on the front and rear side of the ceramic sheet (1) run parallel and the ends with the silver contacts (10, 11) are on top of each other. The silver contacts (10, 11) are bonded on with a silver-filled epoxy resin adhesive, and the ends can be connected in series with low impedance using them. The contact resistivity of the adhesive bond with silver-filled epoxy resin is 0.05 x.52 cm2. At a current (I) of 1 kA
the current limiter module (5) formed a resistance of 5 S2.
It is expedient to select the ratio of the conductor separation (O) to the meander path width (b) of a current limiter module (5) at < 0.5, preferably < 0.1.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
BACKGROUND OF THE INVE:LVTION
s Field of the Invention The present invention relates to a current limiter device.
Discussion of Background to Referring to the prior art, JP 2-183915 A specifies a substrate with an oxide superconductor layer and a noble metal layer mounted on both surfaces of the substrate. Metals are considered suitable as substrate material.
US 4,961,066 specifies as a current limn er, for fast 15 current limitation in the event of short circuits, a rod shaped, a tubular and a planar layer structure, in each case consisting of a support insulator, a thin superconducting layer applied extensively thereon and a normal conductor resistive layer applied extensively on the latter. The last two layers can be 2o repeated one after the other. In this case, the resistance of the non-superconducting resistor is less than that of the superconductor in the normal conducting state. Disadvantages are large power losses in AC operation as well as relatively long conductors.
2s In the patent US 4,994,932 a superconducting current limiting apparatus is disclosed. The current limiting wires are mounted on surfaces of stack elements in parallel lines in which current flows in opposite directions in adjacent lines.
3o SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a further development of a current limiter device of the type mentioned at the outset in such a way that it is suitable for the resistive limitation of both direct and alternating currents. The intention is to limit an overcurrent, in the event of a short circuit, to a predeterminable multiple of the rated current.
An advantage of the invention consists in that the current limiter device is simple and compact. The super s conducting part of the current limn er is of modular design, that is to say that the superconductor is subdivided into units which, should the need arise, can be removed and replaced separately.
According to an advantageous configuration of the to invention, the compound conductors, consisting of super conductors and normal conductors, used in the current limn er can be arranged with low inductance. By virtue of a suitable conductor arrangement, AC losses which occur in the case of AC
applications can be greatly reduced.
15 The current limner device can also be used as an active switching element by introducing it into an external magnetic field. In this case, use is made of the fact that the critical current is very greatly reduced in magnetic fields. By switching on the external magnetic field, it is therefore zo possible to reduce the current in the superconductor to a fraction of the rated current.
BRIEF DESCRIPTION OF THE hRAWINGS
A more complete appreciation of the invention and many z5 of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Figure 1 shows, in cross section, a modular arrangement 30 of current limiters in a magnetic field coil, Figure 2 shows, in cross section, a current limiter according to Figure 1, Figure 3 shows, in a cross section, a superconductor of the current limiter according to Figure 2, with a meandering 35 conductor track, Figure 4 shows signal diagrams of AC losses with current limn ers, and Figures 5-8 show current limiters with different layer sequences.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, Figure 1 shows 4 current limiters or current to limiter modules (5) , which are arranged parallel to one another in a cryostat (7) filled with liquid nitrogen, are electrically connected in series and are connected to an electrical line (6).
During operation, a current (I) which, in the event of an overcurrent, for example as a result of a short circuit, is intended to be limited by the current limiter modules (5) to 3 times - 5 times a predeterminable rated current (IR) flows through the electrical line (6). The cryostat (7) is arranged within a magnetic field coil (8).
Figure 2 shows the layer structure of a current limn er 2o module (5) according to Figure 1. A thin silver buffer layer (2) whose thickness is in the range of 1 ~,m to 5 um is in each case applied onto a first main face (la) and onto a second main face (1b) opposite the former, of a plate-shaped ceramic sheet or of an insulator (1) having a thickness (dl). Suitable 2s materials for the insulator (1) are insulators, obtainable in sheet form, which have sufficient therma7_ stability and undergo, between room temperature and 77 K, a long-term thermal alteration which is comparable with that of the superconductor (3, 3'). Glass fiber reinforced casi~ resin sheets or Mg0 3o ceramic sheets are preferably used. They are most simply applied to the buffer layer (2) using a commercially available adhesive. This step is required for superconductor sheets (3, 3') which have been cast in a silver mold that is removed after casting, but is not required when using cast substrates on a 35 nickel based alloy or ceramic which need not be removed after production of the superconductor (3, 3') and can be used for mechanically stabilizing the latter.
Plate-shaped high temperature superconductors or superconductors (3, 3') having rectangular cross section and s meandering configuration according to Figure 3 and a respective thickness (dSC) are applied onto the two buffer layers (2), in extensive contact with the latter. In this case, a first main face (3) of each superconductor (3, 3') is, in order to electrically stabilize it, in good electrically conductive to contact with the respective buffer layer (2). The two superconductors (3, 3') have a mutual layer separation or conductor separation (D).
A second main face (3b) of the respective superconductor (3, 3') is, in order to electrically and 15 thermally stabilize it, in extensive good electrically conductive contact with a first main fare (4a) of a respective non-superconducting resistor or normal conductor (4, 4!) having a thickness (dN~), wherein dN~ is the thickness of the normal conductor. A second main face of the normal conductor (4, 4'), 20 opposite the first main face (4a) , is de:~ignated (4b) . Suitable metals for the normal conductors (4, 4') are, in particular, those which have a resistivity at room to=_mperature of > 10 x,52 cm and are still ductile at -200°C. Preferable ones are: tin, zinc " bismuth and alloys thereof as wel=L as non-magnetic metals 2s based on steel or nickel. The normal conductors (4, 4') can be applied onto the superconductor (3, 3') by electrolytic methods, flame spraying, plasma spraying, bonding using a conductive adhesive, soldering or sintering a cold sprayed metal powder.
The thickness of the normal conductor layer (4, 4') should be 3o such that the electrical resistance of this layer is approximately equal to that of the adjoining superconductor layer (3, 3') in the non-superconduct:ing state, for example 50 ~,m when the thickness (ds~) of the superconductor (3, 3' ) is 1 mm, wherein dSC is the thickness of the superconductor. The contact resistivity, expressed in term: of area, between the second main face (3b) of the superconductors (3, 3') and the first main face (4a) of the normal conductors (4, 4' ) should be < 1 mS2 cmz, preferably 5 10 x,52 cm2.
Figure 3 shows a cross section through the superconductor (3) according to Figure ~:, in a section that is perpendicular to the layer representation in Figure 2. A band shaped conductor is produced using cuts or meander gaps (9) in a sheet with a rectangular, preferably square contour. Silver contacts (10, 11) are applied to this conductor at the end, for electrically connecting it. Neighboring meander gaps (9) have a io perpendicular separation (b) corresponding to a meander path width. The meanders are most simply produced by reciprocal cuts by milling or sawing or laser or water-jEa cutting, and this can be done before applying the normal conductor (4, 4') or else before applying the mechanical stabilization, that is to say the insulator ( 1 ) .
The meandering superconductors (3; 3') are arranged, in the current limiter modules (5), on both sides of the insulator (1) in such a way that the current (I) flows in the opposite direction in respectively opposite meander paths, so that the 2o self-field components perpendicular to the band plane compensate for each other. The effect of this is that the current limner modules (5) are low-inductance and low-loss.
Fig. 4 shows, in AC loss curves (12) and (13) for different values of the ratio of the meander path width (b) to the thickness (dsC) of the superconductor (3, 3' ) , the AC losses which occur in the abovementioned current return line.
In this case the conductor separation (D) in mm is plotted on the abscissa and the ratio of the electrical AC power loss (P) to a length (1) of the superconductor (3, 3' ) in mW/m is plotted on the ordinate. The AC loss curve (12) shows that, for b/ds~ - 2, the AC losses decrease with g - 94/123 US
increasing conductor separation (0), while with b/dgC =
15 they increase with increasing conductor separation (0), cf. AC loss curve (13). The current return arrangement in a current limiter module (5) only leads to a reduction in the losses with increasing conductor separation (D) when the. meander track of the superconductor (3, 3') is sufficiently flat, that is to say when the ratio b/dSC is sufficiently large, up to a critical value of 3.5. For bands with b < 3.5 ~ dSC, the current return increases the AC power loss (P) instead of reducing it.
Figs. 5-8 show a simplified representation of various layer sequences which can be used instead. of the layer structure in Fig. 2. In this case (NC) designates normal conductors (4, 4', 14, 15) and (SC) designates superconductors (3, 3').
According to Fig. 5, a compound conductor or laminate consisting of a normal conductor (4, 4') and a superconductor (3, 3') can be extensively connected to the insulator (1) or the respective buffer layer (2) in such a way that the normal conductor (4, 4') is in each case arranged on the insulator side.
According to Fig. 6, a compound conductor consisting of a normal conductor (4, 4') and a superconductor (3, 3') can be extensively connected to the insulator (1) or the respective buffer layer (2) in such a way that the insulator (1) is in superficial contact, via one of its main faces, with the superconductor (3), according to the arrangement of Fig. 2, and via its other main face with a normal conductor (4'), according to the arrangement of Fig. 5.
It is also possible to provide a further insulator (1') which is extensively connected to the superconductor (3' ) via a buffer layer (2) .
Fig. 7 shows a layer structure according to Fig. 5, in which the outer faces of the superconductors (3, 3') are in extensive, highly conductive electrical contact with additional normal conductors (14) and (15) .
. ~17'~169 Fig. 8 shows a current limiter module (5) which has a layer structure according to Fig. 7 on one side of the insulator (1) and a layer structure according to Fig. 2 on the other side.
In the current limiter modules (5) according to Fig. 1, the superconductors (3, 3') are resistively coupled into an electrical circuit. Below a certain critical current strength (jc) the superconductor (3, 3') is in the superconducting state and therefore has virtually no electrical resistance. If the critical current strength is exceeded, for example because of a short circuit, then the superconductor (3, 3') undergoes transition into its normal conducting state.
The resistance which results therefrom limits the current to a value which is much smaller than the short circuit current.
An important factor is the dimensioning of. the superconductor (3, 3'), its electrical, thermal and mechanical stabilization, the AC losses which result during operation and the connections between the current limiter modules (5).
The electrical and thermal stabilization is achieved using at least one normal conductor (4, 4', 14, 15) as a parallel conductor, which must locally be in good electrical and thermal contact with the superconductor (3, 3'). This bypass resistor (4, 4', 14, 15) can, should need be, locally take a part of the current from the superconductor (3, 3') and thereby protect the latter from excessive heating and destruction. In order for it to be possible to relieve the load on the superconductor (3, 3') effectively, the bypass resistance (4, 4', 14, 15) should not be greater than the normal resistance of the superconductor (3, 3'). The thickness (dNC) of the bypass resistor (4, 4', 3 5 14 , 15 ) must consequently be z dSC PNC'PSC ~ PNC and pSC
being the resistivities of the bypass resistor (4, 4', 14, 15) and of the superconductor (3, 3'), respectively. Since the intention is for the bypass resistor (4, 4', 14, 15) to receive as much heat as _ g _ possible, a high thermal inertia and consequently a high resistivity pNC are beneficial.
In the operating state, the superconductor (3, 3' ) must be able to carry the rated current (IR) which gives a lower limit for its cross-sectional area F, according to:
F ? 1.414 - IR/jc.
In the limiter state the currE.nt (I) is intended to rise to at most n times the rated current (IR), values of between 3 and 5 being required ir~ practice. This requirement gives the to minimum conductor length (1) of the superconductor (3,3'), according to:
1 >_ b ' (dsc/Psc + dNC/PNC) ° 1. 414 ~ UR/ (n ' IR) , UR being the rated voltage of a current source (not represented) and b being the strip width of a compound conductor consisting Zs of a bypass resistor (4, 4', 14, 15) and a superconductor (3, 3' ) .
The AC power loss (P) of a superconductor (3, 3') through which current flows depends greatly on the local magnetic field (self-field and possible external fields). In 2o the case of band-shaped superconductors (3, 3'), as they are used according to Fig. 1, above all the field components which are perpendicular to the band plane h<~ve a very unfavorable effect on the AC power loss (P). The conductor geometry must therefore be realized in such a way that the field in the 2s superconductor (3, 3') is oriented predominantly parallel to the band plane. In a single thin current carrying band, the magnetic field in the conductor is for the most part perpendicular to the band plane, in which case the AC power loss (P) would not be acceptable for the application. An efficient reduction in the 30 perpendicular field components can be achieved with a conductor geometry which consists of pairs of closely adjacent conductor segments, perpendicular to the band plane, with antiparallel current (I). For each pair of such conductors the magnetic field in the conductor is for the most part parallel to 35 the band plane, which results in a substantially smaller AC
~1~7~16~
a ' power loss (P). The AC power loss (P) per unit conductor length (1) is given by:
P/1 = 4 ' jc ' C-A(xec) ' F + (~) A(x) dfl .
A(x) being the vector potential at maximum current, xec being the so-called electrical center of the superconducting band, at which the electric field is always = 0, and F being the cross-sectional area of the band. The integral extends over the entire conductor cross section F. It can be seen from the above formula that the current return concept is effective if the conductor separation (0) of the superconductors (3, 3') perpendicular to the .band plane is substantially less than the meander path width (b). For O » b, the superconductors (3, 3') behave as 2 individual conductors with high AC power loss (P). For b. 2 mm and dSC - 0.5 mm, with current return a reduction in the AC power loss (P) by a factor of 2 can be achieved.
The current return design can be realized with bands arranged in meander or spiral form, cf. Fig. 3. In this case the conductor separation (0) is preferably selected to be < 10 mm.
Example 1 with layer structure according to Fig. 2:
Rated power PR 20 kW, Rated voltage UR 200 V, Rated current IR 100 A, Maximum current Imax 300 A, Critical current density jc 1 kA/cm2, Conductor width b 1.4 cm, Meander gap width (9) 1 mm, Conductor length 1 per module (5) 126 cm, Total conductor length . 8.8 m, Number of modules (5) 7.
AC power loss P at 77 K 0.62 W.
A modularly designed high temperature superconductor (3, 3') based on Bi:Sr:Ca:Cu - 2:2:1:2 was applied to a thickness (dSC) of 1 mm onto both sides of a ceramic sheet (1) with an area of 10 cm 10.4 cm and a thickness (dl) of 1 mm. There was a 21'7"169 silver layer (2) with a thickness of 2 ~m between the ceramic sheet (1) and the superconductor (3, 3'). The silver acts both as an electrical stabilizer (bypass resistor) and as a chemical insulator between the superconductor (3, 3') and the ceramic substrate (1). A
lead layer (4, 4') with a thickness (dNC) of 10 ~.m, which likewise contributes to the electrical stabilization, was applied:to the other side of the superconductor (3, 3').
Meander gaps (9) according to Fig. 3 were cut from the superconducting layer on both sides of the sheet. The two conductor tracks (3, 3') on both sides of the ceramic sheet (1) are electrically connected to each other in such a way that the current (I) in directly opposite band parts flows in antiparallei directions. The current return effect for reducing the AC power loss (P) is thereby achieved.
Example 2: Switch function according to Fig. 1 By enclosing the current limiter modules (5) according to Example 1 with a magnetic field coil (8) , as represented in Fig. 1, the device according to the invention can be used as an active switching element.
When the magnetic field is turned on, the critical current strength (jc) in the superconductor (3, 3') is reduced, so that the superconductor (3, 3') undergoes transition into the resistive state. This causes the current (I) to be reduced to a fraction of the rated current (IR). Because of the texturing of the superconductor (3, 3'), the reduction in the critical current strength (jc) is a maximum if the applied magnetic field is perpendicular to the plane of the superconducting band, as in Fig. 1.
Production of a superconducting sheet (1):
Superconductor powder of composition BiaSnbCacCudOe with a, b, d = 1.8-2.2, c = 0.8-1.2, a =
7.5-8.5 is introduced; in the dry state or as a suspension with a liquid, into a suitable flat mold. In a preferred procedure, silver powder and/or Bi203 powder is mixed in a concentration range of 0.5%-5%
with this superconductor powder, which has a positive influence on the melting and compacting of the melt.
Any material which does not react with the powder during the subsequent melting of the powder and is structurally stable at temperatures of around 900°C is suitable as a casting mold. Molds made of sheet silver, of nickel alloys with a silver protective layer and ceramic sheets made of magnesium oxide and stabilized zirconium oxide were used. The metal molds were easily able to be provided, for example by deep drawing or folding, with a rim having a height of approximately 10 mm. Conductive silver was used as the buffer layer (2) or adhesive. The filling level was selected in such a way that, when the powder was compacted to 100% by the melting, a thickness (dgC) of 0.3 mm-3 mm results.
The highest possible so-called green density of the powder, which was achieved by uniaxial subsequent pressing of the loose powder bed, is advantageous for the achievable current density and homogeneity. A
pressure of 10 MPa is sufficient for this. The casting method is described in DE-Al-4,234,311.
Application of the electrical stabilization:
Superconductor sheets (1), which were produced in silver or ceramic molds, were provided with a metallization (4, 4', 14, 15) which serves as electrical stabilization. For this purpose the silver must be removed from the superconductor sheet (1), which can be done before the metallization or after a mechanical stabilization has been applied.
When casting molds of silvered nickel-based alloy are used, separate electrical stabilization is not necessary if the resistance of the silver/nickel-based alloy combination already corresponds to that of the superconductor (3, 3').
Example 3:
Square casting molds with dimensions of 100 mm 100 mm, having rims with a height of 6 mm, were folded by hand from a 100 ~m thick silver sheet. These casting molds were in each case filled with a slurry of 60 g Bi2Sr2Ca1Cu2Og+8 powder in ethanol, 0 s 8 s 0.3. After the liquid had dried, the powder bed was compressed by uniaxial pressing with a pressure of 2 GPa. The specimens were then subjected to a heat treatment, in an oxygen atmosphere, which consisted of a melting step at 900°C for a period of time in the range from 20 h -80 h. Homogeneous compact superconductor sheets (3, 3') with a thickness of approximately 1 mm, from which the silver could easily be shaved off, were obtained as a result. A 50 ~m thick tin layer was deposited onto these superconductor sheets (3, 3') by flame spraying for electrical stabilization. These superconductor sheets (3, 3') were then adhesively bonded onto .an aluminum sheet and made into a meandering form by water-jet cutting, conductors with a cross section of 14 mm ~ 1 mm and a length of approximately 70 cm being produced as a result. After the aluminum sheet had been removed, 2 superconductor sheets (3, 3') were in each case aligned relative to each other in such a way 'that their tracks on the front and rear side of the ceramic sheet (1) run parallel and the ends with the silver contacts (10, 11) are on top of each other. The silver contacts (10, 11) are bonded on with a silver-filled epoxy resin adhesive, and the ends can be connected in series with low impedance using them. The contact resistivity of the adhesive bond with silver-filled epoxy resin is 0.05 x.52 cm2. At a current (I) of 1 kA
the current limiter module (5) formed a resistance of 5 S2.
It is expedient to select the ratio of the conductor separation (O) to the meander path width (b) of a current limiter module (5) at < 0.5, preferably < 0.1.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
Claims (7)
1. A current limiter device a) having at least one superconductor (3, 3') of thickness dSC and b) having at least one non-superconducting shunt resistor (4, 4', 14, 15) in parallel with the superconductor (3, 3'), c) the superconductor (3, 3') having, via at least a first main face (3b), extensive contact with a main face (4a) of a shunt resistor (4, 4', 14, 15) and forming a compound conductor with the latter, d) a first main face (la) and e) a second main face (1b) of an insulator (1, 1') being each in extensive contact with a compound conductor consisting of a superconductor (3, 3') and a shunt resistor (4, 4' , 14, 15), wherein f) each compound conductor is designed in the form of a band of width b, g) the compound conductors arranged on both main faces (la, lb) of the insulator (1) are electrically connected to one another in such a way that a current (I) flows in antiparallel directions in directly opposite bands of the compound conductors, and h) the band width (b) is > 3.5 × the thickness (dSC) of the superconductor (3, 3'), wherein dSC is the thick-ness of the superconductor.
2. The device as claimed in claim 1, wherein a ratio of a conductor separation (a) between two superconductors (3, 3') in directly opposite bands of the compound conductors to the band width (b) of the superconductors (3, 3') is < 0.5.
3. The device as claimed in claim 2, wherein each compound conductor is designed in the form of a meander.
4. The device as claimed in claim 2, wherein at least one main face (la, 1b) of an insulator c;1, 1' ) is in extensive contact, via a main face (4b) of a shunt resistor (4, 4', 14, 15), with a compound conductor consisting of a superconductor (3, 3') and a shunt resistor (4, 4', 14, 15).
5. The device as claimed in claim 2, wherein the superconductor (3') is in extensive contact, via one of its two main faces, with a main face of a further insulator (1').
6. The device as claimed in claim 2, wherein at least one main face (la, 1b) of an insulator (l, 1' ) is in extensive contact with this compound conductor via an electrically conducting and chemically insulating buffer layer (2).
7. The device as claimed in claim 2 wherein a) the electrical resistance of a shunt resistor (4, 4', 14, 15) is not greater than the electrical resistance, in its non-superconducting state, of the superconductor (3, 3') connected to it, and b) the following inequality is satisfied for a minimum conductor length 1 of the superconductor (3, 3'):
1 >= b .cndot. (dSC/pSC + dNC/pNC) .cndot. 1.414 .cndot. UR/(n .cndot.
IR), dSC and dNC being the thicknesses of the super-conductor (3, 3' ) and the shunt. resistor (4, 4' , 14, 15) respectively, pSC and pNC being the resistivities of the superconductor (3, 3') and of the shunt resistor (4, 4', 14, 15) respectively, IR being the rated current, n being the .ratio of the maximum permissible current to IR, UR being the rated voltage of a current source and b being the band width of a compound conductor.
1 >= b .cndot. (dSC/pSC + dNC/pNC) .cndot. 1.414 .cndot. UR/(n .cndot.
IR), dSC and dNC being the thicknesses of the super-conductor (3, 3' ) and the shunt. resistor (4, 4' , 14, 15) respectively, pSC and pNC being the resistivities of the superconductor (3, 3') and of the shunt resistor (4, 4', 14, 15) respectively, IR being the rated current, n being the .ratio of the maximum permissible current to IR, UR being the rated voltage of a current source and b being the band width of a compound conductor.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP4434819.3 | 1994-09-29 | ||
| DE4434819A DE4434819C5 (en) | 1994-09-29 | 1994-09-29 | Current limiting device |
| PCT/CH1995/000215 WO1996010269A1 (en) | 1994-09-29 | 1995-09-25 | Current-limiting device |
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| Publication Number | Publication Date |
|---|---|
| CA2177169A1 CA2177169A1 (en) | 1996-04-04 |
| CA2177169C true CA2177169C (en) | 2006-04-11 |
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|---|---|---|---|
| CA002177169A Expired - Fee Related CA2177169C (en) | 1994-09-29 | 1995-09-25 | Current-limiting device |
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| US11101059B2 (en) | 2017-03-31 | 2021-08-24 | Tokamak Energy Ltd | Quench detection in superconducting magnets |
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| US11101059B2 (en) | 2017-03-31 | 2021-08-24 | Tokamak Energy Ltd | Quench detection in superconducting magnets |
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