Magnetothermal current limiting device

Abstract

A current limiting device including a transformer including an element made from a magnetothermal material, a primary conductor, and a secondary winding. Heat is generated by the current flowing through the primary conductor and when the current exceeds a certain threshold it modifies the coupling coefficient of the transformer, which makes it possible to limit the current in the primary conductor.

Claims

1. A current limiting device comprising: a primary configured to be connected to an electrical line; a secondary comprising a limiting resistance; and a magnetic circuit that couples the primary and the secondary, wherein the magnetic circuit comprises a magnetocaloric material, magnetization of which increases with temperature above a first temperature greater than or equal to 330 K and reaches a maximum at a second temperature of less than 420 K, the magnetocaloric material being of NiCoMnX type, wherein X{Al, In, Sb, Sn}.

2. The device as claimed in claim 1, wherein the magnetization maximum is greater than 40 emu/g.

3. The device as claimed in claim 2, wherein the magnetocaloric material has a magnetization maximum that increases with a magnetic field applied to the material.

4. The device as claimed in claim 1, wherein the magnetic circuit forms a torus with first portions made of magnetocaloric material and second portions made of material for which the magnetization is not modified above the first temperature.

5. The device as claimed in claim 4, wherein the first and second portions alternate to form sectors of the torus.

6. The device as claimed in claim 1, wherein the secondary comprises a winding of a conducting wire around the magnetic circuit.

7. The device as claimed in claim 1, wherein limiting resistance of the secondary referred to the primary is equal to a voltage of a network divided by a desired value for the limited current.

8. The device as claimed in claim 1, wherein the primary comprises a conductor passing through the magnetic circuit and including two connection terminals at ends of the conductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and features will become clearer from the following description of particular embodiments of the invention, which are given by way of illustration and are not at all limiting, and which are represented in the appended drawings.

(2) FIG. 1, already described, illustrates an inductive limiter with a superconductor.

(3) FIG. 2 shows a device according to the invention, with reference to the inductive limiter of FIG. 1.

(4) FIG. 3 shows the characteristics of the material which can be used in the magnetic circuit of a limiter according to the invention.

(5) FIGS. 4A and 4B represent limiting devices according to two embodiments of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(6) The device according to the invention takes up the principle of the superconducting inductive limiter described previously with reference to FIG. 1. However, the value of the resistance of the secondary circuit is not modified by the current. When a current exceeds a threshold, this modifies the magnetic coupling by changing the magnetization of the magnetic circuit, as indicated in FIG. 2.

(7) In particular, the material of the magnetic circuit is chosen for its magnetothermal, or magnetocaloric, properties. More specifically, as illustrated in FIG. 3, the material is such that its degree of magnetization M exhibits a peak dependent on temperature. Notably, at low temperature, the material is not very, or even not at all, magnetic. When the temperature increases, above a first temperature T.sub.0, the magnetization M of the material increases rapidly, to reach a maximum at a second temperature T.sub.1, above which the magnetization decreases until it is canceled at the Curie temperature Tc of the material. These various temperatures T.sub.0, T.sub.1, Tc are themselves dependent on the magnetic field H applied (see the variations obtained for a field of 0.2 T and a field of 7 T in FIG. 3).

(8) For a use according to the invention, the first temperature T.sub.0 is chosen to be greater than 330 K, preferably close to 350 K. This choice is made possible through the use of materials of the NiCoMnX family, where X{Al, In, Sb, Sn}, preferably aluminum or tin. For these materials, the transition is very marked with a temperature T.sub.1 close to T.sub.0 (difference of 10 to 30 K) and a high magnetization, in the order of 70 emu/g. Notably, for Ni.sub.40Co.sub.10Mn.sub.33Al.sub.17: T.sub.0=347 K, Mmax=90 emu/g.

(9) In particular, according to a preferred embodiment illustrated in FIG. 4A, the limiting device 10 according to the invention comprises a primary conductor 12 equipped at its ends with means 14 of coupling to the electrical line 5 to be protected, notably connection terminals. The primary 12 is magnetically coupled to a secondary 16, which comprises a limiting resistance 18, the characteristics of which are configured according to the result, with notably the limiting resistance 18 of the secondary 16 referred to the primary 12 equal to the voltage of the network divided by the desired value for the limited current. The coupling is produced by means of a magnetic torus 20 through which the two conductors, primary and secondary 12, 16, pass. In the embodiment illustrated, the primary 12 is in the form of a wire conductor which passes through the torus only once, while the secondary 16 is coiled several times around the torus 20.

(10) In the presence of a nominal current in the primary 12, the limiting device 10 operates in the usual manner, with standard magnetic coupling defined by the cross-section of the conductors 12, 16, the value of the resistance 18 and the predefined characteristics of the magnetic circuit 20, the magnetization of which is constant at ambient temperature of use, notably between 30 C. et +50 C.

(11) When the current in the line 5 increases, notably when it exceeds a fault value, the primary 12 is heated by the Joule effect, and the thermal effect thus produced leads to an increase in temperature within the magnetic circuit 20 which exceeds its phase change temperature T.sub.0. The material is magnetized, and this non-magnetic/magnetic transition leads to a modification in the coupling coefficient of the transformer 10. Due to this change of coupling, the secondary circuit resistance 18, referred to the primary, increases and therefore makes the current in the primary circuit 12 fall.

(12) Advantageously, a direct thermal contact is provided between the magnetic circuit 20 and the conductor 12. In fact, the magnetocaloric material sees its magnetic state dependent mainly on temperature. It is to be noted however that the magnetic circuit 20 sees its magnetic state dependent also on the magnetic field to which it is subjected, i.e. notably on the field induced by the primary 12: an increase in the primary current leads to a rise in the field H, and a modification of the magnetization curve with notably an increase in magnetization (see FIG. 3). Thus, the higher the primary current, the more marked the non-magnetic/magnetic transition, and the limiting effect thereof is strengthened, due to the strengthening of the primary/secondary coupling.

(13) The dimensioning of the system 10 thus provides for setting the corresponding value of overload current in the primary 12 in order to locate, as a function of the induced field, the temperature generated at the torus 20 in the range [T.sub.0, T.sub.1] of non-magnetic/magnetic phase transition of the material. In particular, it is possible to act on the cross-section and the number of turns of the primary and secondary electrical circuits 12, 16, on the value of the resistance 18 at the secondary, and on the length and cross-section of the magnetic circuit 20.

(14) Furthermore, as illustrated also in FIG. 4A, it is possible to modify the proportion of active material in the magnetic circuit 20 so as to optimize the coupling coefficient and to refine the operating parameters of the limiter 10. In particular, according to the embodiment illustrated, the magnetic circuit 20 comprises an alternating arrangement of sectors 22 of magnetothermal material, and of sectors 24 of material that is inert with regard to temperature. Preferably, the magneto-stable sectors are formed by a material that is ferromagnetic regardless of the temperature, to ensure good coupling and therefore optimal limiting when the temperature increases.

(15) Alternatively, the magnetic circuit 20 can be formed by first portions 22 of magnetocaloric material dispersed within a matrix 24 of ferromagnetic material. Any other alternative is possible for distributing first magnetocaloric portions 22 and second thermally stable magnetization portions. In particular, it is possible to use, for the magnetic coupling circuit, a lamination stack with an alternating arrangement of ferromagnetic sheets (FeSi, FeNi, FeCo) and of magnetocaloric sheets (NiMnCoX). The properties are then adjusted either by the relative thickness of the sheets of the two kinds, or by the number of sheets of each kind. The primary and secondary circuits can, for their part, be produced from any conducting material, notably copper or aluminum.

(16) The device 10 according to the invention thus provides for a limiting of the current by a change of state of the magnetic circuit 20, obtained directly by the rapid variation of the properties of the material during a phase transition triggered by the effects of the fault current, which thus occurs at ambient temperature, therefore without requiring a cryogenic device.

(17) These state change properties can also be used in the direct insertion of an inductance in the line to be protected in order to replace a superconductor by a magnetothermal material. Said inductance also changes from the non-magnetic state to the magnetic state according to the current flowing in the line itself, to in return modify the value of said current. This embodiment hence has the advantage of also limiting a DC current, but it requires a coiled primary thereby increasing dissipation in normal operation.

(18) Although the invention has been described with reference to a limiting device 10 with a toric magnetic circuit, it is not limited thereto. In particular, as illustrated in FIG. 4B, a limiter 10 according to the invention can comprise two aligned terminals 14. The primary 12 passes through the magnetic circuit 20 more than once, which magnetic circuit 20 can for example be in a figure-of-eight shape in order to be inserted around the branches of a U-shape forming a primary circuit 12. In particular, the magnetic circuit 20 can be a magnetic parallelepiped 24 with two holes 26, the walls on one side at these holes 26 being formed by magnetocaloric material 22. The secondary is preferably coiled the same number of times at each of the holes 26. Any other alternative embodiment is possible.