Current transformer, and protection device and electrical circuit breaker including such a transformer

Abstract

A current transformer (1) comprises a magnetic circuit (2) made of magnetic material for placement around a primary conductor (3), and a secondary winding (4) wound around a portion (5) of the magnetic circuit (2) for delivering a secondary current to a processing circuit (32). In the current transformer, the magnetic circuit (2) comprises at least one device (6, 6A, 6B) for varying the magnetization of a portion of the magnetic circuit as a function of temperature, so as to limit or reduce the magnetic flux in the magnetic circuit when the temperature of the magnetic circuit increases. A protection device and a circuit breaker comprising such a transformer.

Description

Technical Field

[0001] The present invention relates to a current transformer, comprising:

[0002] - a magnetic circuit made of magnetic material, adapted to be placed around the primary conductor; and

[0003] - a secondary winding wound onto a portion of the magnetic circuit in order to deliver a secondary current to a processing circuit.

[0004] The invention also relates to a protection device and a circuit breaker comprising such a current transformer. Background Art

[0005] Figure 1 A known current transformer 1 is shown in FIG. It comprises a magnetic circuit 2 intended to be placed around a primary conductor 3, and a secondary winding 4 wound on a portion 5 of the magnetic circuit. The magnetic circuit is usually manufactured as a stack of sheets. These sheets are mainly made of iron and are designed to operate at the frequency of the grid, in particular at a frequency of 50 Hz or 60 Hz.

[0006] When the fundamental frequency of the power grid increases and exceeds 100 Hz, or when high-frequency currents or current harmonics are superimposed on the load current circulating in the primary conductor 3, energy losses are generated in the iron of the magnetic circuit. These iron losses lead to heating of the magnetic circuit and of the components adjacent to or in contact with the magnetic circuit. These temperature increases are particularly significant when the current transformer is integrated in an insulating housing with other electrical or electronic components. The use of an insulating housing ensures that electrical insulation is provided between the primary conductor 3 and the rest of the electrical equipment associated with the current transformer. Summary of the invention

[0007] The object of the present invention is a current transformer capable of avoiding or reducing temperature rise in the magnetic circuit, and a protection device and a circuit breaker comprising such a current transformer.

[0008] In a current transformer according to the present invention, the current transformer comprises a magnetic circuit made of magnetic material for placement around a primary conductor, and a secondary winding wound on a portion of the magnetic circuit for delivering a secondary current to a processing circuit, the magnetic circuit comprising at least one device for varying the magnetization of a portion of the magnetic circuit in dependence on temperature so as to limit or reduce the magnetic flux in the magnetic circuit when the temperature of the magnetic circuit increases.

[0009] Preferably, the means for varying the magnetization restrict the circulation of the magnetic flux to a value less than 50% of the value at 20°C when the temperature is above 150°C.

[0010] Advantageously, the means for varying the magnetization create an additional full or partial gap in the magnetic circuit so as to restrict the circulation of the magnetic flux when the temperature exceeds a predetermined threshold. Preferably, the predetermined threshold is in the range of 120°C to 180°C.

[0011] According to a first preferred embodiment of the invention, the means for varying the magnetization comprise mechanical means housed in a recess of the magnetic circuit in order to create the gap or increase the gap by opening the magnetic circuit when the temperature reaches said predetermined threshold.

[0012] Advantageously, the mechanical means comprise bimetallic strips mounted end to end and housed in the recesses in order to achieve separation of the magnetic circuits, thereby creating the gap.

[0013] Preferably, the mechanical arrangement comprises 6 to 12 bimetallic strips.

[0014] Advantageously, said gap is greater than 0.5 mm when the temperature is above 120° C. or the magnetic circuit experiences a temperature increase of 120 K.

[0015] According to a second preferred embodiment of the invention, the means for varying the magnetization comprise a static device housed in a section of said magnetic circuit, said static device being made of a magnetic material having a Curie point or Curie temperature lower than that of the magnetic material of the magnetic circuit.

[0016] Preferably, the material of the static device has a Curie temperature in the range of 80°C to 200°C.

[0017] Advantageously, the material of the static device is mainly made of iron-nickel or ferrite.

[0018] Preferably, the material of the static device comprises cutouts for avoiding current losses via eddy currents.

[0019] The electronic protection device according to the invention comprises an electronic processing unit connected to at least one current transformer and delivering an electronic signal in response to a protection function, the electronic protection device comprising a current transformer as defined above, having a secondary winding connected to said processing unit to provide a secondary measurement or supply current.

[0020] The circuit breaker according to the present invention comprises at least one main electrical contact for interrupting the current in the circuit, a mechanism for controlling the disconnection of the electrical contact and a protection device for transmitting a control signal to the control mechanism, and the circuit breaker comprises at least one current transformer as described above connected to a processing unit of the protection device.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Other advantages and features will become more apparent from the following description of particular embodiments of the invention given for non-limiting illustrative purposes only and shown in the accompanying drawings, in which:

[0023] Figure 1 A current transformer of the prior art is shown;

[0024] Figure 2 A current transformer according to a first embodiment of the present invention is shown;

[0025] Figure 3 A current transformer according to a second embodiment of the present invention is shown;

[0026] Figure 4 A current transformer according to a third embodiment of the present invention is shown;

[0027] Figures 5 to 8 shows the arrangement of a first mechanical embodiment of the device for varying the magnetization;

[0028] Figures 9 to 11 shows details of a strip forming the device for varying the magnetization according to a first embodiment;

[0029] Fig.12 A second static embodiment of the device for varying the magnetization is shown;

[0030] Fig.13 shows a magnetization curve for a device that varies magnetization as a function of temperature;

[0031] Fig.14 and 15 shows an arrangement of a second embodiment of the device for varying the static magnetization; and

[0032] Fig.16 A block diagram of a circuit breaker and a protection device including a mutual inductor according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0033] Figure 1 A current transformer 1 is shown comprising a magnetic circuit 2 made of magnetic material for placement around a primary conductor 3, and a secondary winding 4 wound on a portion 5 of the magnetic circuit 2 for delivering a secondary current to a processing circuit. The magnetic circuit comprises sheets mainly made of iron designed to operate at the frequency of the grid, in particular at a frequency of 50 Hz or 60 Hz.

[0034] To avoid heating of the magnetic circuit when energy losses in its iron increase, the magnetic circuit comprises at least one device 6 for modifying the magnetization of a portion 7 of said magnetic circuit so as to limit or reduce the magnetic flux in the magnetic circuit when its temperature increases. Figure 2A current transformer according to an embodiment of the invention is shown, wherein the device 6 is inserted in the portion 7 of the magnetic circuit.

[0035] Energy losses in the iron of the circuit increase particularly when the fundamental frequency of the grid increases and exceeds 100 Hz, or when high-frequency currents or current harmonics are superimposed on the load current circulating in the primary conductor 3 .

[0036] For example, when the temperature is above 150° C., the means for changing the magnetization limit the circulation of the magnetic flux to a value less than 50% of the value at 20° C. In a preferred embodiment, the means for changing the magnetization create an additional full or partial gap in the magnetic circuit in order to limit the circulation of the magnetic flux when the temperature exceeds a predetermined threshold. The means 6 are thus means for limiting the magnetic flux as a function of the temperature. Such a means has the advantage that no additional gap is created at low current levels, for which a good energy transfer to the secondary winding is required. Furthermore, at low current levels, heating is kept low.

[0037] Figure 3 A current transformer according to another embodiment of the invention is shown. The magnetic circuit has a partially circular shape and comprises a magnetic shunt 8 with a gap 9. When the magnetization of the magnetic circuit exceeds a predetermined level, the shunt 8 diverts the magnetic flux to reduce the magnetic flux in the portion 5 of the magnetic circuit receiving the secondary winding 4. The purpose of the shunt 8 is to limit the secondary current of the winding 4 when the current in the primary conductor increases. Figure 3 In the embodiment of the invention, the shunt 8 is made of the same sheet material as the magnetic circuit 2. However, other embodiments are also possible. For example, the shunt can be added to the magnetic circuit and made of a different material, in particular iron powder, to better regulate the current in the secondary winding.

[0038] Figure 4 Another embodiment of a current transformer according to the invention is shown. The means 6A for changing the magnetization of the portion 7A of the magnetic circuit are held by means 10 of the magnetic circuit. If the means 6A are mechanical, the holding means 10 also serve to ensure a return to the normal position. The means 6B for changing the magnetization of the portion 7B of the magnetic circuit can also be arranged in the gap 9 of the shunt 8. In this case, if the means 6B are mechanical means and if an opening is provided on the portion 7B of the magnetic circuit remote from the means 6B, a larger opening on the portion 7B is feasible to reduce the magnetization of the magnetic circuit 2.

[0039] according to Figures 5 to 8In the first embodiment of the device for limiting the magnetic flux as a function of temperature shown, the means 6 for varying the magnetization comprise mechanical means 11 housed in a recess 12 of the magnetic circuit 2 so as to increase the gap or create a gap 13 by opening the magnetic circuit 2 when the temperature reaches said predetermined threshold. Preferably, said predetermined threshold is within the range of 120°C to 180°C.

[0040] The mechanism 11 generates an additional gap that will increase starting from a threshold temperature. To this end, the mechanism 11 comprises a bimetallic strip 14 mounted head to tail and housed in a recess 12 to achieve separation of the magnetic circuit, thus generating a gap 13. The recess 12 allows the double strip to remain in place so as to generate a thrust perpendicular to its deformation under the effect of temperature. The means 10 for maintaining the magnetic circuit comprise a return spring that serves to ensure the correct closure of the magnetic circuit or its reclosing and to avoid noise.

[0041] Thus, the deflection of the bimetallic strip separates the edges of the gap starting from the set temperature. In this case, the operation is reversible, since the mechanical elasticity of the magnetic circuit is used. A separation or gap of 0.5 mm is sufficient to significantly reduce the iron losses by 50%, thereby reducing or limiting the heating of the magnetic circuit. This separation of the magnetic circuit has almost no effect on the secondary current when the primary current is at the nominal operating current. Advantageously, the gap is greater than 0.5 mm when the temperature is above 120°C.

[0042] Preferably, the mechanical device comprises 6 to 12 bimetallic strips. Figures 5 to 8 In the embodiment of the mechanical restraint device, there are eight bimetallic material strips mounted end to end.

[0043] As shown, the recesses are preferably separated by cutout sections in the magnetic circuit. However, the recesses can also be arranged on the outside or edge of the magnetic circuit or on a support causing a gap separation.

[0044] Figures 9 to 11 Detail of a strip forming the means for changing magnetization according to a first embodiment is shown. An example of a bimetallic strip is Fig. 9 As shown, the first side 16 of the belt is made of a first material having a first expansion coefficient, and the second side 17 is made of a second material having a second expansion coefficient lower than the first expansion coefficient. Fig. 9 Two belts 15 are shown mounted end-to-end or face-to-face at low temperatures, such as 20°C. Fig.11The same strip is shown at an elevated temperature of about 130°C. The deformation of the strip is intended to push against the wall of a section of the magnetic circuit in order to create a gap at elevated temperatures. In one embodiment, the dimensions of each strip are 8.5 mm long, 3.00 mm wide and 0.6 mm thick. Thus, when the temperature is above 120°C or the magnetic circuit experiences a temperature increase of 120K, the movement or length is greater than 0.5 mm. For a set of 8 strips, the movement is 0.6 mm. The movement of 10 strips will be about 0.74 mm. The bimetallic strip material is preferably selected to have a linear deformation from -20°C to 200°C. This movement is a theoretical value which does not take into account the pressure of the return spring holding the device 10. In practice, the movement may be slightly shorter depending on the spring.

[0045] In order to avoid mechanical movement of the magnetic circuit, in a second embodiment of the device for limiting the magnetic flux as a function of temperature, the means for varying the magnetization comprise a static device 18 housed in a section of said magnetic circuit. According to a second embodiment of the invention, the static device 18 is made of a magnetic material having a Curie point or a Curie temperature lower than that of the material of the magnetic circuit. The material of the static device 18 preferably has a Curie temperature in the range of 80° C. to 200° C., for example about 130° C. Advantageously, the material of the static device is mainly made of iron nickel (FeNi) or ferrite. Another example of such a material is "Phytherm" by Aperam.

[0046] Therefore, in order to limit the temperature of the magnetic circuit, the magnetic circuit includes a cutout that can create a gap. An element made of a magnetic material with a low Curie point (Tc) is inserted into this cutout or opening. The magnetization of this material decreases with temperature until it cancels out for the temperature Tc. Within a temperature range that does not pose any risk to the sensor (for example, below 120°C), the material has magnetic properties that allow normal operation of the sensor while allowing the magnetic flux to pass freely through the opening. In this case, the power delivered by the sensor at low current levels is hardly affected. When approaching the defined limit temperature, the material loses its magnetization sufficiently to behave like air. As in the first solution with a bimetallic strip, iron losses and heating are then reduced.

[0047] Fig.12 An embodiment of a static device 18 for changing the magnetization is shown, comprising cutouts 19 for avoiding eddy current losses. When the material is ferrite, no cutouts are needed, since this material is not a good electrical conductor or insulator.

[0048] Fig.13 The magnetization curve 20 of a device for changing the magnetization as a function of temperature is shown. At around 20° C., the magnetization is still at a maximum. Then, as the temperature increases, the magnetization of the material decreases until it becomes almost zero beyond the Curie point 21 .

[0049] Fig.14 and 15 An arrangement according to a second embodiment of the device 18 for varying the static magnetization to limit the magnetic flux as a function of temperature is shown. The magnetic circuit comprises a notch or opening for creating a gap at high temperatures. Fig.14 In the embodiment, the gap will be complete since the opening 22 occupies the entire cross section. In this case, the retaining device 10 prevents any movement of the magnetic circuit. Fig.15 In the embodiment, the gap is local, wherein the notch 23 occupies only a part of the cross section of the magnetic circuit. Therefore, the element 18 made of a magnetic material with a low Curie point (Tc) is inserted into the notch or opening 22 or 23. Preferably, the distance of the gap of the opening 22 or 23 is in the range of 0.5 mm to 1 mm.

[0050] Fig.16 A block diagram of a circuit breaker 30 and a protection device 31 including a transformer 1 according to an embodiment of the present invention is shown.

[0051] The electronic protection device 31 comprises an electronic processing unit 32 connected to at least one current transformer and delivering an electronic signal D in response to a protection function. The protection device comprises a current transformer 1 for changing magnetization according to an embodiment of the invention. The transformer 1 has a secondary winding 4 connected to the processing unit 32 for delivering a secondary measurement or supply current Is. Fig.16 In the particular case of , the protection device comprises a current transformer 1 having means for varying magnetization as a function of temperature to supply power to a processing unit 32 , and a current sensor 33 for delivering a current measurement signal Im to the processing unit.

[0052] The circuit breaker 30 comprises at least one main contact 34 for interrupting the current lp in the circuit 35, a mechanism 36 for controlling the opening of said electrical contact 34, and a protection device 31 which delivers a control signal D to said control mechanism 36. The circuit breaker comprises at least one current transformer 1 having means for changing the magnetization as a function of the temperature for powering a processing unit 32 of the protection device 31. A primary conductor 3 forming part of the circuit 35 passes inside the magnetic circuit 2 of the current transformer 1.

Claims

1. A current transformer (1), comprising: - a magnetic circuit (2) made of magnetic material, intended to be placed around the primary conductor (3); and - a secondary winding (4) wound onto a portion of the magnetic circuit (2) in order to deliver a secondary current to a processing circuit (32), Characterized in that the magnetic circuit (2) comprises at least one device (6, 6A, 6B) for varying the magnetization of a portion (7) of the magnetic circuit as a function of the temperature, so as to limit or reduce the magnetic flux in the magnetic circuit when the temperature of the magnetic circuit increases, wherein the means (6, 6A, 6B) for changing the magnetization comprises mechanical means (11, 14) housed in a recess (12) of the magnetic circuit (2) so as to create a gap (13) or increase the gap (13) by disconnecting the magnetic circuit (2) when the temperature reaches a predetermined threshold, And wherein the mechanical device (11, 14) includes a bimetallic strip (15), which is installed end to end and accommodated in the recess (12) to achieve separation of the magnetic circuit, thereby generating a gap.

2. The current transformer according to claim 1, characterized in that: When the temperature is above 150°C, the means (6, 6A, 6B) for varying the magnetization restrict the circulation of the magnetic flux to a value less than 50% of the value at 20°C.

3. The current transformer according to claim 1 or 2, characterized in that: The means (6, 6A, 6B) for changing the magnetization create additional full or partial gaps in the magnetic circuit in order to restrict the flow of magnetic flux when the temperature exceeds a predetermined threshold.

4. The current transformer according to claim 3, characterized in that: The predetermined threshold is in the range of 120°C to 180°C.

5. The current transformer according to claim 1, characterized in that: The mechanical device (11, 14) comprises 6 to 12 bimetallic strips (15) having bimetallic parts (16, 17).

6. The current transformer according to claim 1 or 5, characterized in that: When the temperature is above 120°C or the magnetic circuit experiences a temperature increase of 120 K, the gap (13) is greater than 0.5 mm.

7. The current transformer according to any one of claims 1 to 4, characterized in that: The device (6, 6A, 6B) for changing the magnetization comprises a static device (18) housed in a section of the magnetic circuit (2), the static device being made of a magnetic material having a Curie point or Curie temperature lower than that of the magnetic material of the magnetic circuit (2).

8. The current transformer according to claim 7, characterized in that: The material of the static device (18) has a Curie temperature in the range of 80°C to 200°C.

9. The current transformer according to claim 7 or 8, characterized in that: The material of the static device (18) is mainly made of iron nickel (FeNi) or ferrite.

10. The current transformer according to any one of claims 7 to 9, characterized in that: The material of the static device (18) comprises cutouts (19) for avoiding current losses due to eddy currents.

11. An electronic protection device (13) comprising an electronic processing unit (32) connected to at least one current transformer (1) and delivering an electronic signal in response to a protection function, the electronic protection device being characterized in that it comprises a current transformer (1) according to any one of claims 1 to 10, the current transformer having a secondary winding (4), the secondary winding (4) being connected to the processing unit (32) to provide a secondary measurement or supply current (ls).

12. A circuit breaker (30), comprising: At least one main electrical contact (34) for interrupting the current (lp) in the circuit (35); a control mechanism (36) for controlling the disconnection of the electrical contact (34); and a protection device (31) for transmitting a control signal to the control mechanism (36), characterized in that the circuit breaker comprises at least one current transformer (1) according to any one of claims 1 to 10, which is connected to a processing unit (32) of the protection device (31).

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