DE19524579C2 - Transformer as a current limiter - Google Patents

Description

The invention relates to a transformer as a current limiter according to the Preamble of claim 1.

From European application EP 0 620 570 A1, according to which the preamble of claim 1 was formed is an arrangement known in which the primary coil, for example a soft iron core surrounded by a solid ceramic component consisting of egg ceramic superconductor cylinder, is surrounded concentrically. Of the superconducting ring is arranged in an annular container which with the coolant, e.g. B. liquid nitrogen is filled.

Problems associated with the need to cool down such electricity limiting units below the critical transition temperature of the Supra are connected, for example, in EP-PS 0 353 449 or by H. Kanbach, E. Bochenek, M. Trautmann, R. Fischer, H. Voigt, in: "Melt-processed YBCO for a Fault Current Limiter", 6th International Symposium on Superconductivity, ISS ′93, Hiroshima, Japan, October 26-29, 1993 described. This article is abbreviated: "Superconductors are conquering new applications" in etz. Vol. 115 (1994) Issue 3, P. 154 reproduced.

The current limiting effect in the event of an overcurrent is on the transition of the superconductor from the superconducting to the normal conducting state return. The ring experiences local, jerky heating and therefore mechanical tension. It also stretches the ring when passing through the acting electromagnetic Forces, and the brittle ceramic body builds the Zugbe exposure to mechanical damage, d. H. Cracking, from. The Ver The use of ceramic rings brings great problems with the Long-term stability of the current limiter arrangements with itself. It will Aging effects observed when switching the Strombe limiters to a massive degradation of the properties of the material can lead to the destruction of the component. The degradation effects  micro cracks are attributed to the thermal stress of the superconductor when switching.

From European application EP 0 620 630 A1 it is known that the zy to surround the Lindrian superconductor with a heat-insulating jacket, which can also increase the mechanical stability of the superconducting cylinder.

These effects become particularly problematic with a possible scale tion of such components for medium and high voltage networks where the use of superconducting current limiters instead of the usual one used explosion switch is expected. Cracking at Switching the current limiter is also added here for that spoken performance ranges components are necessary, the dimensions from Z. B. 1 m high and 50 cm in diameter.

Ceramic bodies of this size are difficult to handle and break easily. The previously known transformers for electricity limitation have the disadvantage that the secondary windings are very expensive dig are in production, the material costs of High temperature superconductors play a role.

The invention is therefore based on the object of a transformer of the type mentioned at the outset so that it is easy Way can be used as a current limiter in medium-voltage networks.

This object is achieved according to the invention by the in the license plate part of Features listed claim 1 solved.

Further developments of the invention are contained in the subclaims. The essence of the invention is that the secondary winding only consists of a thin layer. Through this is the manufacture of the Secondary winding very easy, and also cooling with liquid air is much easier. Because of the thermal stresses in the Transition from superconducting to normal conducting proposed special measures, in particular on the Sprö of the ceramic superconductor are matched.  

The invention is explained below with reference to the drawing.

It shows:

Fig. 1 is a representation of the principle of operation and

Fig. 2 shows a transformer in a practical design.

The transformer 1 consists of a core 6 , a primary winding 2 and the superconducting ring 3 , which is kept thermally insulated in a cooling vessel 4 (see FIG. 1). The advantage of the transformer according to the invention is that it can be used in the high current range of more than 1 kA alternating current in distribution networks as a generator transformer or device protection. At rated current, the secondary winding in the form of a short-circuit ring is flooded by a magnetic field below the critical field strength. This critical field strength can easily be set to a value by the number of primary turns, which corresponds to a maximum current I _max . At a multiple of this maximum current, the secondary winding changes from the superconducting to the normally conducting state and heats up very strongly in the process. For this reason, the ceramic ring from which the superconductor is made is formed as a thin layer 3 , which is applied to a carrier 5 , which prevents the superconducting ring from suddenly expanding too much and cracking.

A first embodiment of the invention is shown in FIG. 1. The composite body 7 consists of a superconducting ring 3 made of YBaCuO and a carrier 5 which surrounds the ring 3 . In this case, the carrier 5 is made of glass fiber reinforced plastic (GRP). The composite is heated in the oven at 120 ° C, which significantly increases the final strength of the GRP, whereby values of up to 500 Mpa can be achieved.

If the composite body is then used as a current limiter, then even after switching a hundred times, none of the usual Degra dations observed.

In another exemplary embodiment, a is used instead of the GRP sleeve Steel cylinder used.  

It is particularly advantageous to use a carrier cylinder with a suitable one Coating processes such as electrophoresis, screen printing or plasma spraying to coat on the inside or outside. Suitable Layer thicknesses are 10 µm to 2 mm. It can go beyond that Interlayers are used to cover the growing conditions or adhering to the base of the Optimize superconductor layers.

Such coating processes are especially for components of large di suitable dimensions and offer in addition to the better handling of the Component additional cost savings through lower material costs.

Another advantage of the invention results when the carrier 5 is used as the coupling medium for the cooling. For example, the carrier can carry cooling coils or be double-walled, the cooling medium being guided in the cavities. Another possibility is that the carrier 5 itself represents a boundary wall of a bathroom cryostat or is immersed as a whole composite body in a cooling medium. For this reason, it is advantageous to use a metallic support. The carrier can also serve to accommodate fasteners that are necessary to anchor the composite body in a cryostat.

In a further advantageous embodiment of the invention, the carrier 5 consists of several layers of a band of glass or carbon fibers which are glued together.

In order to make crack formation more difficult, the surface of the superconducting ring densely sintered and polished. The polishing happens especially by water jet grinding.

In an advantageous embodiment of the invention, a clamp or cuff 9 made of a high-strength alloy is increased in diameter by heating so that it can be pushed over the superconducting ring 3 together with the carrier cylinder 5 . The alloy has a high specific resistance in order to limit the current resulting from induction and the resulting losses to a value that is as low as possible. When cooling, a Druckvorspan voltage is generated, which holds the superconducting ceramic and the support cylinder under such a bias that they can not burst when the ceramic suddenly heats up. The carrier 5 is preferably made of a ceramic material on which the superconducting layer 3 adheres well. Otherwise an adaptation layer 8 (see FIG. 2) is applied to the carrier.

The transformer is dimensioned so that it is in the event of a short circuit or Overvoltages that the power that occurs can be transmitted. First at the maximum size of the primary ampere turns the core come close to saturation to get a good shutdown keep guarantee.

The cooling of the superconducting ring 3 takes place in a heat-insulated vessel 4 , into which liquid nitrogen 12 is introduced at the inlet nozzle 11 , which leaves the cooling vessel 4 at the outlet 10 and is pumped through a refrigerator or supplied to a storage vessel. The cooling vessel 4 is preferably double-walled.

It is considered favorable if the primary winding 2 is applied as an inner winding on the transformer core 6 and the cooling vessel 4 with the superconducting ring 3, which takes up a little more space, is mounted concentrically thereon as a secondary winding. It is only important in principle, however, that the primary and secondary fluxes are chained together in such a way that the secondary impedance is transformed to the primary side and the inductive resistance is practically zero there as long as the ring 2 is superconducting. If the primary circuit is overloaded, the critical magnetic field strength in the ring is exceeded, and the ring behaves like a winding with high resistance, ie almost like an insulator. As a result, the normal inductance of the choke with closed iron core in the network is effective and limits the current to a value that can be set by the size of the inductance of the choke.

In a particularly preferred embodiment of the invention a carrier cylinder with a thin layer of a ceramic supra coated conductor. All high temperatures come as coating materials rature superconductors with a high transition temperature and high  Current carrying capacity in question. Suitable high-temperature superconductors fin in the group Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Bi-Pb-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O, Ba-K-Bi-O. These layers can be through thick layers procedures such as sol-gel procedures, plasma spraying, electrophoresis, sieve pressure or electrochemical deposition.

The transformer according to the invention can advantageously in one energy distribution network can be used to limit short-circuit currents. For electrical engineering reasons, it makes sense to close the geometry in this way choose that the total height of the secondary turn in relation to its outer diameter is selected so that it is like n: 1 (with less than or equal to n less than or equal to 10). This can be done either an appropriately shaped carrier cylinder or by a stack of coated rings.

example 1

At a nominal current of 2.5 kA, for example, the current intensity should be limited in the event of a short circuit if the current in the primary coil 2 of the transformer 1 rises above 14 kA. The diameter of the iron core 6 is 40 cm, the core itself is 2 m high and has an O-shape, as shown in Fig. 2. The primary coil 2 is designed in copper cross-section for a nominal current density of 2.5 A / mm². It is cooled by a convection cooler. The secondary turn 3 is part of the composite body 7 , which was produced as a sintered ceramic. The carrier cylinder 5 consists of a 1 m high Al₂O₃ cylinder, on which a 60 µm thick and 90 cm high layer of melt-processed YBa₂Cu₃O₇ was applied by internal coating. Between the Al₂O₃ cylinder and the superconductor layer is a buffer layer 8 , here applied cerium oxide with a thickness of 0.5 µm, which prevents unwanted interdiffusion between the carrier 5 and the superconductor layer 3 , which is known to significantly impair the superconductor properties. Other materials, such as e.g. B. yttrium-stabilized zirconium oxide, MgO, SrTiO₃ and other systems with adapted lattice parameters are suitable. The carrier cylinder has a wall thickness of 3 cm. This composite body is held in a double-walled cryostat in a bath made of liquid nitrogen.

The critical field strength can be set at around 14 kA by the number of turns n of the primary coil. In the superconducting material used, the manufacture and properties of which are described in the patent DE 42 26 942 C1, the magnetic flux in which the superconducting core changes to the normal conducting state is approximately 100 T. The compound cylinder 7 is mechanically stable and shows no cracks, even after multiple short-circuit tests. The superconducting layer also shows no signs of degradation. This is due to the fact that the ceramic cylinder, to which this layer is applied, absorbs the high pressure loads and, in the event of a short circuit, initially behaves like a very strong sleeve, preventing any expansion of the superconducting layer. The large surface area of the superconducting layer also ensures that the heat loss that accompanies the transition from superconducting to normal conducting is dissipated very quickly to the liquid nitrogen.

The carrier cylinder 5 can of course also be made from other ceramic materials. Quartz is a good choice for electrical reasons, but the thermal expansion coefficients of carrier and superconducting layer material and the lattice adaptation when applying the possible superconducting layers to the substrate must be taken into account.

Example 2

In a further embodiment, the compressive stress is not applied by the carrier cylinder 5 itself, but by a bandage applied externally after the coating.

This allows a further degree of freedom in the choice of the carrier cylinder dermaterials, since here the coordination of the thermal expansion coefficient efficient less weight.

A metallic carrier 5 , e.g. B. made of nickel-based alloys, which is however as thin as possible to avoid excessive losses due to inevitably induced eddy currents in the carrier. The retrofitted Ban dage then puts the laminate under pressure and mechanically stabilizes the support.

In addition to GRP or carbon fiber reinforced plastic bandages the prestressing also from the outside with half shells attached from one high-strength alloy, which are clamped together, who applied the.

Example 3

In a further embodiment, the carrier cylinder 5 is coated on the outside with the superconductor. As in the previous example, the compressive stress, which protects the superconductor against tensile loads in the limited case, is protected by a bandage 9 which is subsequently laminated on.

Claims (19)

1. Transformer as a current limiter, in which a current to be limited in the event of a fault flows through the primary coil ( 2 ) and a superconducting ring ( 3 ) is provided on a carrier cylinder ( 5 ) as a secondary coil, which in a cooling vessel ( 4 ) below the transition temperature of the High-temperature superconductor is cooled, characterized in that the superconductor is applied as a thin hollow cylindrical layer ( 3 ) on the carrier cylinder ( 5 ) so that it forms a composite body ( 7 ) with this, and that the superconducting layer is surrounded by a cylinder is that puts the superconducting layer under compressive stress in the switching case, in such a way that in the switching case the cracking in the superconductor is prevented. 2. Transformer according to claim 1, characterized in that the layer ( 3 ) on the inside of the carrier cylinder ( 5 ) is applied. 3. Transformer according to claim 1, characterized in that the layer ( 3 ) on the outside of the carrier cylinder ( 5 ) is applied. 4. Transformer according to one of claims 1 to 3, characterized in that the layer ( 3 ) is 10 µm-2 mm thick. 5. Transformer according to one of claims 1 to 4, characterized in that the layer ( 3 ) is 50-500 µm thick. 6. Transformer according to one of claims 1 to 5, characterized in that the carrier cylinder ( 5 ) has a wall thickness less than 1/10 of the outer diameter. 7. Transformer according to one of claims 1 to 6, characterized in that the superconducting layer ( 3 ) in the composite body ( 7 ) by prestressing the carrier cylinder ( 5 ) is placed under compressive stress. 8. Transformer according to one of claims 1 to 7, characterized in that the carrier cylinder ( 5 ) is formed from a metal, in particular from a nickel-based alloy. 9. Transformer according to one of claims 1 to 8, characterized in that the electrical resistance of the carrier cylinder ( 5 ) is so large that in the limited case in the carrier cylinder at most one tenth of the current flows compared to the current in the superconducting layer ( 3 ). 10. Transformer according to one of claims 1 to 9, characterized in that the carrier cylinder ( 5 ) is formed from an insulator, in particular from a ceramic and / or from a glass fiber reinforced plastic and / or from a carbon fiber reinforced plastic. 11. Transformer according to one of claims 1 to 10, characterized in that the superconducting layer ( 3 ) is produced by a ceramic process as a solid material. 12. Transformer according to one of claims 1 to 11, characterized in that the superconducting layer ( 3 ) is produced by a coating process, in particular by a thick-film process such as the sol-gel process, plasma spraying, electrophoresis, screen printing or electrochemical deposition. 13. Transformer according to one of claims 1 to 12, characterized in that the primary coil ( 2 ) concentrically surrounds the superconducting layer ( 3 ). 14. Transformer according to one of claims 1 to 13, characterized in that the superconducting layer ( 3 ) concentrically surrounds the primary coil ( 2 ). 15. Transformer according to one of claims 1 to 14, characterized in that the carrier cylinder ( 5 ) is designed as a boundary wall of the cooling device. 16. Transformer according to one of claims 1 to 15, characterized in that the superconducting layer ( 3 ) made of at least one ceramic superconductor from the group Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Bi-Pb -Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O, Ba-K-Bi-O. 17. Transformer according to one of claims 1 to 16, characterized in that the layer ( 3 ) as a ceramic superconductor is a compound of the general formula Y _{1 + 2x + y} Ba _{2 + x} Cu _{3 + x} O _z with 0 x 0.5 ; 0 y 0.5 and 6 z 7. 18. Transformer according to one of claims 1 to 17, characterized in that the height of the superconducting cylinder ( 3 ) to its outer diameter behaves like n: 1, where 1 n is 10. 19. Transformer according to one of claims 1 to 18, characterized in that between the layer ( 3 ) and the carrier cylinder, an adaptation layer ( 8 ) to improve the adhesiveness of the superconducting layer ( 3 ) and to avoid damage from diffusion from adjacent layers is provided.