ES2265749A1 - Current limiter for e.g. welding fragments of material includes superconducting elements in which weak zones are included - Google Patents

Abstract

The limiter may be inductive, resistive or hybrid type and includes superconducting elements comprised of a matrix of superconducting material. Weak zones are included in the superconducting elements.

Description

Current limiter based on elements superconductors with several weak zones created artificially

Technical sector

The present invention is a device current limiter based on superconducting elements, and is intended for use in improving quality and safety  of energy generation and distribution processes electric

State of the art

Superconducting materials have the interesting property of having a strongly non-linear resistance and dependent on several magnitudes: the temperature, the current that circulates through them and the magnetic field to which they are subjected. These materials are characterized by a zero resistance state that is reached below a certain critical temperature, T_ {c}, so they must be cooled by cryogenic liquids or electric refrigerators, and below a current, I_ {c} , and a field, B_ {c} (in aspects of characterization of the superconductor it is more proper to refer to the critical current density, J_ {c}, which is the I_ {c} per unit area crossed by the current, J_ { c} = I_ {c} / A). Above these critical values, a superconductor undergoes a transition to a dissipative state with non-linear resistance, which can lead to an ohmic resistance regime (such as that of a metal) if dissipation heats the material sufficiently, or circulating current exceeds a second critical value, called J * (see, for example, W. Klein et al ., J. Low Temp. Phys. 61 , 413 (1985); SG Doettinger et al ., Phys. Rev. Lett. 73 , 1691 (1994); ZL Xiao et al ., Phys. Rev. B 59 , 1481 (1999); and José María Viña Rebolledo, Contribution to the study of electrical transport in thin layers of superconducting cuprates: supercritical currents and paraconductivity , Thesis Doctoral, University of Santiago de Compostela (2003)).

This non-linear behavior of the resistance, function of the temperature, the current and the applied magnetic field, has suggested the use of superconductors in current limiting devices, aimed at reducing the pernicious effects of the high currents and voltages generated during a failure in an electrical distribution line, either in a production plant or a local network or branch (T. Verhaege and Y. Laumond 1998, Handbook of Applied Superconductivity , 2 , ed B. Seeber (Bristol: Institute of Physics Publishing) , p. 1691; WT Norris et al , Cryogenics, 37 , 657 (1997); W. Paul et al ., Physica C, 354 , 27 (2001); and P. Tixador, IEEE Trans. Appl. Supercond., 4 , 190 (1994)).

Depending on the way in which the superconductor in the circuit that you want to protect, you can distinguish two fundamental conceptions of limiter: resistive and inductive. The first can simply be an element superconductor (bar-shaped, thin film, coil, etc) connected in series with the circuit that you want to protect. He Limiter is designed so that when the line circulates rated current, do not exceed the critical value, I_ {c}, so that the resistance it offers is zero, and its presence is not perceive However, when a fault occurs and the current grows up to values greater than I_ {c}, the transition to dissipative (or even ohmic) state, and a resistance arises that effectively limits the current. Note that, in principle, a Once the line fault is eliminated, the superconductor returns to its state of zero dissipation. The limiter is, therefore, self-sufficient and does not need any additional elements that detect the start or disappearance of a fault (itself device is a detector). On the other hand, his reaction is almost instantaneous, and responds to a failure in times of the order of 1 ms, or less.

The inductive prototype has a type design transformer, in which the primary (usually metallic) is connected directly to the circuit that you want to protect, and the Secondary is a ring-shaped superconductor or hollow cylinder. While the nominal current circulates in the circuit, the flow magnetic that creates the primary in the magnetic core is canceled by which the superconductor generates, so that the inductance Effective transformer is null, and the only impedance present is the resistance of the primary coil (and components due to possible leakage inductances). When a fault occurs and it exceeds I_ {c}, the superconductor transition causes the cancellation of cessation flow, so an impedance arises from inductive and resistive character (depending on the design dominates one or the other component) that limits the current. In a way, this can be seen as an ideal transformer whose secondary happens to be shorted to be in open circuit.

In both cases, due to the high energy that dissipates in the form of heat in the current limitation process, it is desirable that the limiter does not act more than a few network cycles, which is the time required by current power switches. circuit to detect the fault and open the line (they must wait until a zero of the current is reached). While increasing the temperature of the superconductor can be beneficial in terms of increasing the impedance of the device (which should happen in a similar time to the reaction, i.e. \1 ms), it can also cause a very slow recovery of the device. once the fault was eliminated, since the superconductor would continue to dissipate and offer impedance on the line, somewhat undesirable (MR Osorio et al ., Applied Superconductivity 1999 (EUCAS'01), Inst. Phys. Conf. Ser. No 167, 1 , ed X .; Obradors et al . (Bristol: Institute of Physics Publishing, JW Arrowsmith Ltd.) p. 1013 (2000); and MR Osorio et al ., Physica C, 372-376 , 1635 (2002)).

On the other hand, overheating can cause degradation of the superconducting element (including its melting), especially due to the presence of hot spots or weak areas , which are regions of the material with, for example, lower critical temperature, T_ {c}, or lower critical current density, J_ {c}. The so-called low critical temperature superconductors (SBT), whose T_ {c} is lower than that of boiling liquid nitrogen (77 K), are usually metallic, and the homogeneity is quite good. In addition, they have a high thermal conductivity (~ 100 W / mK), so that the heat generated in the transition can be easily evacuated to the cooling medium. These properties make them little sensitive to the problem of hot spots. However, critical high temperature superconductors (SAT) are ceramic and much more heterogeneous. On the one hand, with regard to stoichiometry, a dominant phase may coexist with non-negligible proportions of precursors or intermediate products. On the other hand, they have a less uniform structure than a metal. Thus, granular SATs are formed by a multitude of small grains linked together, so that the transition to the dissipative state first affects the intergranular borders and then the interior of the grain. In addition, its thermal conductivity is much reduced
(sim 1 W / mK), so if there is a weak zone and it overheats, its expansion would be slow and local damage could occur to the material that would worsen the performance of the limiter.

The main problem of weak areas is which, in general, are not capable of giving rise to Good limitation of the fault current. Therefore, they assume a excessive dissipation and may eventually cause degradation of any superconducting element and a limiter malfunction of current. For the fault to be controlled in a way effective, even if there are weak areas, its extension and distribution spatial as well as its physical properties (current density and critical temperature, resistivity) should be adequate, which is virtually impossible to achieve by chance. This is, weak areas naturally present in the superconductor They would never give rise to this situation. They could be too much small, or the values of J_ {c} very different from each other, or the resistivity too low to give rise to resistance what large enough to avoid excessive increase in temperature, etc.

Description of the invention

This invention consists of a device, a limiter, capable of limiting a fault current in a power distribution line using superconducting elements. The limiter can be of the resistive, inductive or hybrid type, and is based on one or more superconducting elements formed by fragments characterized by a specific critical current and temperature density, between which, fragments of material with values are suitably inserted. lower than the aforementioned properties, or changes in dimensions are made (cross section). These fragments can be welded together following one of the procedures already described in the literature to manufacture large superconductors from small fragments that join with a similar material (A. Leenders et al ., IEEE Trans. Appl. Supercond., 11 , 3728 (2001); K. Iida et al ., Physica C, 370 , 53 (2001); and S. Haseyama et al ., Physica C, 354 , 437 (2001)). This way of manufacturing the superconducting elements causes that, during the operation of the limiter, most of the dissipation is assumed by the altered zones (weak zones). Therefore, they heat up much more than the rest of the material. Once the fault has disappeared, thermal recovery is carried out not only by convection with the cooling medium (liquid or gas at cryogenic temperatures), but also by the conduction of heat from the hot to the cold parts. This mechanism would not take place if the superconducting element were homogeneously constructed, since in that case the entire heat flow would be evacuated exclusively by convection as there were no parts colder than the others.

On the other hand, when traveling to the state dissipative only some small parts of the element superconductor, its total resistance could be less than the generated by a homogeneous element with uniform heating. This problem can be solved by designing weak areas so that they are very resistive compared to the rest of the material:

one.

When the weak zone is a section change, making this alteration more accentuated, since the resistance of a conductive path is equal to R = \ frac {\ rho} {A} \ ell, being \ rho the resistivity of the material, A the section of the road and its length. A smaller section, greater resistance.

2.

When the weak zone is a fragment of different material, it must have a resistivity greater than the of the rest of the sample, so that it compensates for the shortest length of way, \ ell, which occupies.

Example 1

An example of this conception is the limiter inductive superconductor that we have built and that we present in this invention. It is formed by a core of silicon-iron (3%) with three arms, with a primary consisting of two windings connected in series and located on the side arms and a superconducting cylinder of Bi-2223 (Bi_ {1. 8} Pb_ {0. 26} Sr 2 Ca 2 Cu 3 O 10 + X) in the exchange. The scheme of the device shown in Figure 1. a., and a photograph of the same in Figure 1. b, and in them you can see the nucleus (N), the Two coil primary (P) and superconducting cylinder (S).

The core consists of sheets of 0.3 mm thick material, rolled over each other, but electrically isolated from each other, to minimize losses produced by the currents that are induced in them (the insulator represents a loss of magnetic material of 5%). The length of magnetic path is 12.5 cm, and the section of each of the 1.6 cm2 arms. The magnetic saturation field, B_ {sat}, is about 1.7 T, and the relative permeability, µ 5000. The primary is constructed with 0.5 mm copper wire of diameter and has 300 turns. The superconducting cylinder is 4 cm height, 2.1 cm internal diameter and 0.25 cm wall thickness. Its critical temperature is T_ {c} = 108 K, and its density of critical current at the boiling temperature of nitrogen Liquid (77 K) is J_c \ approx 1000 A / cm2. In this cylinder there is a weak zone that occupies its entire height and approximately one tenth of its circumference, as show the characterization experiments performed. This zone weak has its origin in structural defects of the element superconductor or some local variation of stoichiometry of the material. As a consequence of its existence, the element superconductor does not behave homogeneously throughout its extension.

To study the performance of the limiter, we connected to an electrical circuit in which a fault was simulated Remove a resistance. The impedance of the limiter before failure is approximately 1 \ Omega, and after it, of \ approx 2.2 \ Omega. The current that must circulate through the primary to cause the transition from secondary to state dissipative is approximately 4 A. The applied voltage is 9.3V effective.

Figure 2 shows the temperature variation measured with a thermocouple in the weak zone, naturally present in the analyzed superconducting sample. At the start of the fault (I) the limiter reaction occurs and the temperature of the superconducting element increases. This increase is more prominent in the weak zone, since its properties are different than those of the rest of the cylinder material and, therefore, when the transition to the non-superconducting state occurs, the dissipation is greater and warms up more. At the end of the fault (F), the circuit opens and the Joule dissipation ceases, so the temperature begins to drop. This process is called thermal recovery . The lines represent the cooling predictions obtained with a purely convective model, and when the transmission of heat by conduction between the weak (hot) and the cold zone is also taken into account. As can be seen, only when the combination of convection and conduction is taken into account, the experimental results are predicted. This indicates that the heat exchange by conduction contributes fundamentally to the cooling of the weak zone.

Legend of Figure 2:

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- Experimental data

- - - - -

- Alone convection

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- Convection and driving.

Example 2

Another example of this conception would be the high power inductive superconducting limiter that we have numerically simulated using realistic parameters for the Superconducting element and magnetic core. The simulation was made with the Matlab calculation program (The Mathworks, Inc, USES).

The characteristics of the limiter would be the following:

- For the magnetic core: cross section: 50 cm2 and magnetic path length of 90 cm. The rest of properties (material, saturation field and permeability, as in Example 1).

- For the superconducting element: density of critical current at 77 K, J_ {c} \ approx 20 KA / cm2, T_ {c} = 108 K, height 4 cm, wall thickness of 0.24 cm and external diameter of approx 10 cm. Weak areas are supposed to have a resistivity in the dissipative state slightly greater than the rest of the material (1.01 times, so that they heat up more), and that their Normal state resistivity is double.

Figure 3.a. show an outline of how it would be a superconducting cylinder, through which a current (I) circulates, with a weak zone (D) of a length (L) equal to 20% of the circumference (darkest part). In Figure 3.b., this area weak has been fragmented into several smaller ones (specifically 6). In this way, the cylinder can be seen as an association in series of resistances due to the homogeneous matrix (R h) and a weak areas (R d).

In Figure 4.a., you can see the comparison between thermal recovery for a homogeneous cylinder (no zones weak) and those described in Figure 3. Note that, despite being higher temperature of weak areas at the time of opening the circuit (F), these are recovered before the homogeneous cylinder, that has been heated uniformly, as long as its length be small Figure 4.b. shows that the impedance for the cylinder with weak zones is greater in the first bars of the failure (after I), and finally reaches a value of around 60% of the impedance of the homogeneous cylinder. This relationship can be modified by altering the resistivity of weak areas in the normal state, since the resistance offered by the superconductor It is proportional to her. Thus, for a higher resistivity, the impedance curves could equalize for longer times of 0.125 s.

Figure 5 shows how this fast effect Thermal recovery depends on the extent of weak areas. For a total length of weak zone, \ ell, the time of recovery (normalized to the value obtained for a cylinder homogeneous) decreases as it is divided into "n" smaller parts This occurs up to a certain limit (in the figure, up to n \ approx 10), above which there is no improvement substantial.

Legend of Figure 4:

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- Homogeneous cylinder.

\hskip0.3cm

\cdot\cdot\cdot\cdot - Cilindro con una zona débil de longitud un 20% de la circunferencia.

 \ hskip0.3cm 

\ cdot \ cdot \ cdot \ cdot - Cylinder with a weak area of length 20% of the circumference.

- - - -

- Cylinder with the weak zone divided into 10 small fragments.

       \ vskip1.000000 \ baselineskip
    

Legend of Figure 5:

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- Points calculated

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- Line for the eyes.

Example 3

Another possible way to create weak areas artificially it is, instead of welding superconducting fragments with different properties, starting from a superconducting element homogeneous and practice changes in its cross section. This form, the critical current and resistance are altered locally without altering the basic characteristics of the material (that is, critical current density and resistivity). In the Figure 6 you can see examples of section changes made locally: wall narrowing (S) and height reduction (H). This method can be applied in several areas, although Simplicity, the drawings only show one. The results would be equivalent to those obtained in Example 2.

Claims (3)

1. Current limiter based on superconducting elements with several artificially created weak zones, and which can be of inductive, resistive or hybrid type, characterized by being constituted by one or more superconducting elements formed by a matrix of homogeneous properties in which they are inserted alternately several weak areas of length much shorter than that of the entire element; these are characterized by having lower values of the critical current and / or temperature than the matrix, and they can be fragments of different and more resistive superconducting material that are welded to it or regions where the cross-section has been mechanically reduced; so that the lower current and / or critical temperature of the weak zones make them offer a non-zero resistance even when the rest of the material is still in a superconducting state. 2. Superconducting current limiter according to claim 1, characterized in that the weak zones are more resistive fragments than the matrix, either because their dimensions have been reduced, for example the cross section, or because their resistivity has been increased by adding defects in the material, such as impurities; In this way, limit impedance values comparable to the case in which the superconducting elements are homogeneous can be achieved. 3. Superconducting current limiter, according to the preceding claims, characterized in that it is used in the processes of production and distribution of electrical energy to reduce the high voltages and currents that arise when a fault occurs in any of the elements that make up the network electric; This fault may be due to a short circuit, a lightning strike, a voltage peak, etc.