WO2013029688A1

WO2013029688A1 - Current limiter

Info

Publication number: WO2013029688A1
Application number: PCT/EP2011/065200
Authority: WO
Inventors: Francis MUMFORD; Alison Baker
Original assignee: Alstom Technology Ltd
Priority date: 2011-09-02
Filing date: 2011-09-02
Publication date: 2013-03-07

Abstract

A current limiter (10) comprises at least one core element (12), the or each core element (12) include anisotropic magnetic material, and at least one electrically conductive wire (14) being wound to define a coil (16) enclosing a portion of the or each core element (12), the or each core element (12) being configured to align a direction of easy magnetization (20) of the anisotropic magnetic material to be non- parallel to the direction of a magnetic field that, in use, passes through the enclosed portion of the respective core element (12) and is generated by the respective coil (16).

Description

CURRENT LIMITER

This invention relates to a current limiter .

When operating any electrical apparatus, the electrical current flowing through the apparatus is typically maintained within a predetermined current rating of the electrical apparatus. However, fault or other abnormal operating conditions in the electrical apparatus may lead to the development of a high fault current exceeding the current rating of the electrical apparatus .

The development of high fault current may not only result in damage to the electrical apparatus components, but also result in the electrical apparatus being offline for a period of time. This results in increased cost of repair and maintenance of damaged electrical apparatus hardware, and inconvenience to end users relying on the working of the electrical apparatus .

The aforementioned adverse effects may be prevented by limiting the magnitude of the high fault current using a current limiter.

A known solution for a current limiter is the use of air-cored reactors. Air-cored reactors provides a large impedance to limit the peak magnitude of the current flowing through the electrical apparatus. However, the air-cored reactor presents the same impedance during normal and faulty operating conditions. This therefore leads to a constraint in the design of the electrical apparatus because it is necessary to take into account the large impedance of the air-cored reactor and its influence on current flow .

Another known solution for a current limiter is the use of I_s limiters. I_s limiters employ the use of a fuse element that melts upon detection of a high fault current so as to limit any further increases in current magnitude. The need to refurbish the I_s limiter or replace the fuse element after each fault instance leads to increased costs of repair and maintenance .

According to an aspect of the invention, there is provided a current limiter comprising at least one core element, the or each core element including anisotropic magnetic material, and at least one electrically conductive wire being wound to define a coil enclosing a portion of the or each core element, the or each core element being configured to align a direction of easy magnetization of the anisotropic magnetic material to be non-parallel to the direction of a magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil.

Configuring the or each core element to align the direction of easy magnetization to be non- parallel to the direction of the generated magnetic field has been found to reduce the magnetic permeability of the or each core element and therefore the impedance of the respective coil. This is because the or each core element exhibits maximum magnetic permeability when the direction of the generated magnetic field is the same as the direction of easy magnetization of the anisotropic magnetic material.

In the event of high fault current flowing through the wire, the resultant magnetic field is sufficiently strong to alter the magnetic domains of the anisotropic magnetic material and thereby align its magnetizing vectors in the direction of the magnetic field. This in turn causes the magnetic permeability of the or each core element and therefore the impedance of the respective coil to be increased. Subsequent removal of the high fault current causes the magnetizing vectors to be realigned in the direction of easy magnetization of the anisotropic magnetic material. This thereby causes the magnetic permeability of the or each core element and therefore the impedance of the respective coil to be restored to their original low values .

As such, the current limiter according to the invention presents low impedance when alternating current flowing through the or each wire is relatively low, and high impedance when alternating current flowing through the or each wire is relatively high. This allows the current limiter to have reduced influence on the flow of alternating current during normal, steady-state operating conditions, and limit the peak magnitudes of the fault current during abnormal operating conditions. The current limiter may, for example, be used to protect electrical transmission and distribution networks against fault currents that arise from short-circuits. The passive nature of the operation of the current limiter minimises the amount of detection and/or control equipment associated with the current limiter. This not only minimises hardware size, weight and costs, but also increases reliability of the current limiter by minimising the risk of breakdown of the associated detection and/or control equipment.

Additionally, the structure of the current limiter is straightforward to manufacture and readily adapted to fit into any apparatus requiring one or more current limiters.

Preferably the or each core element is configured to align a direction of hard magnetization of the anisotropic magnetic material to be substantially parallel to the direction of a magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil.

Configuring the or each core element in this manner minimises coil impedance when the alternating current flowing through the or each wire is relatively low. This is because the or each core element exhibits minimum magnetic permeability when the direction of the generated magnetic field is the same as the direction of hard magnetization of the anisotropic magnetic material. This allows the current limiter to have minimal influence on the flow of alternating current during normal, steady-state operating conditions.

The anisotropic magnetic material is preferably a grain-oriented magnetic alloy, such as, for example, cold rolled grain-oriented silicon iron having a cube-on-edge configuration, in which the direction of easy magnetization of the cold rolled grain-oriented silicon iron is the [100] crystalline direction.

In embodiments employing the use of grain- oriented magnetic alloy material, the or each core element may be configured to align a direction of the anisotropic magnetic material at an angle between 55 and 90 degrees to the direction of the magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil, the preferred angle in a further embodiment being 55 degrees.

It was found that a core element based on cold rolled grain-oriented silicon iron exhibits minimum magnetic permeability when the direction of the generated magnetic field is at an angle of 55 degrees to the [100] crystalline direction, i.e. in the direction of the [111] crystalline direction of cold rolled grain-oriented silicon iron, and intermediate magnetic permeability when the direction of the generated magnetic field is at an angle of 90 degrees to the [100] crystalline direction, i.e. in the direction of the [110] crystalline direction of cold rolled grain-oriented silicon iron. This results in a lower magnetic permeability of the or each core element based on cold rolled grain-oriented silicon iron in comparison to a similar core element configured to align the [100] crystalline direction to be parallel to the direction of the generated magnetic field. In embodiments of the invention, the or each core element may include a plurality of first layers of anisotropic magnetic material arranged in a laminated structure.

The provision of a plurality of first layers in the or each core element helps to provide a current limiter core in which the power losses resulting from the creation of eddy currents are reduced. The magnitude of any eddy currents induced in the or each core element when a changing flux flows through the or each core element is greatly reduced by the relatively small cross-section of each first layer of the or each core element, which restricts the circulation of the eddy currents.

In such embodiments, the or each core element may further include a plurality of second layers of electrically insulating material, the first and second layers being arranged in a laminated structure of alternating first and second layers.

The inclusion of insulating material not only provides electrical insulation between neighbouring first layers of anisotropic magnetic material, but also provides a supporting structure to hold neighbouring first layers in place.

In other such embodiments, at least one core element may be separated from one or more other core elements by an air gap.

The low permeability of air improves the isolation between neighbouring core elements and thereby minimises the risk of magnetic flux passing from one core element to another. The structure of the current limiter may vary depending on the requirements of the application associated with the current limiter. Preferably the or each core element is in the form of, for example, a rod, bar or toroid, and/or the cross-section of the or each core element is circular, oval or polyhedral in shape. In addition, the or each coil is preferably in the form of, for example, a solenoid or a toroid.

Preferably the current limiter further includes one or more additional magnetic field sources, the or each additional magnetic field source being configured to generate, in use, a magnetic field in the direction of easy magnetization of the anisotropic magnetic material of the or each core element. Such an additional magnetic field source may be, for example, in the form of an electromagnet or a permanent magnet.

The use of one or more additional magnetic field sources in the current limiter helps maintains the arrangement of the crystalline axes of the or each core element against the rotational forces exerted by the generated magnetic field when alternating current flows through the or each wire. This ensures that the magnetic permeability of the or each core element and therefore the impedance of the respective coil is kept at their respective low values during normal, steady- state operating conditions.

In other embodiments of the invention, the or each electrically conductive wire may be operably connected, in use, to one or more electrical circuits. In such embodiments, the or each electrically conductive wire may present an impedance to minimise a fault current created by a fault, in use, in an electrical circuit.

The current limiter may be used to minimise fault current in one or more associated electrical circuits during fault conditions or other abnormal operating conditions so as to prevent damage to the or each associated electrical circuit.

Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which:

Figure 1 shows a current limiter comprising a coil wound around a portion of a core element.

A current limiter 10 according to an embodiment of the invention is shown in Figure 1.

The current limiter 10 comprises a core element 12 including anisotropic magnetic material, and an electrically conductive wire 14.

The core element 12 has an annular, square cross-section, and the anisotropic magnetic material is cold rolled grain-oriented silicon iron having a cube- on-edge configuration. In other embodiments, the shape and size of the core element may vary depending on the requirements of the current limiter. For example, the core element may be in the form of a rod, bar or toroid, and/or the cross-section of the core element may be circular, oval or polyhedral in shape.

The provision of a core element 12 including anisotropic magnetic material results in a higher concentration of the magnetic field lines, and thereby a higher magnetic flux density, within the core element 12 when compared to an air-gapped coil. This is because the anisotropic magnetic material has a higher permeability than that of air.

Preferably the core element 12 includes a plurality of first layers of anisotropic magnetic material and a plurality of second layers of electrically insulating material, the first and second layers being arranged in a laminated structure of alternating first and second layers.

The provision of a plurality of first layers in the core element 12 helps to provide a magnetic fault current limiter core in which the power losses resulting from the creation of eddy currents are reduced. The magnitude of any eddy currents induced in the core element 12 when a changing flux flows through the core element 12 is greatly reduced by the relatively small cross-section of each first layer of the core element, which restricts the circulation of the eddy currents.

The inclusion of insulating material improves electrical isolation between the first layers of anisotropic magnetic material and provides a supporting structure to hold neighbouring first layers in place.

The electrically conductive wire 14 is wound around one side of the core element 12 to define a coil 16 in the form of a solenoid. The shape of the coil 16 results in an axial magnetic field that has a direction parallel to the axis of the solenoid.

It is envisaged that in embodiments of the invention, the current limiter may include a plurality of core elements. In such embodiments, a single electrically conductive wire may be wound around a portion or the whole of each of the plurality of core elements. In other such embodiments, at least one core element may be separated from one or more other core elements by an air gap.

It is also envisaged that in other embodiments, the current limiter may include a plurality of electrically conductive wires. In such embodiments, each wire may be wound around a portion or the whole of one or more core elements, and/or a plurality of wires may be wound around a portion or the whole of a core element.

The core element 12 is configured to align the [111] crystalline direction 18 of the cold rolled grain-oriented silicon iron to be parallel with the direction of the magnetic field that, in use, passes through the enclosed side of the core element 12 and is generated by the coil 16, as shown in Figure 1.

The core element 12 is also configured to align the [100] crystalline direction 20 of the cold rolled grain-oriented silicon iron to be at an angle of 55 degrees to the direction of the magnetic field that, in use, passes through the enclosed side of the core element 12 and is generated by the coil 16, as shown in Figure 1.

It was found that the core element 12 exhibits minimum magnetic permeability when the direction of the generated magnetic field is the same as the direction of the [111] crystalline direction 18 of the cold rolled grain-oriented silicon iron, and maximum magnetic permeability when the direction of the generated magnetic field is the same as the direction of the [100] crystalline direction 20 of the cold rolled grain-oriented silicon iron. As such, the [111] and [100] crystalline directions 18,20 respectively represent the direction of hard magnetization and the direction of easy magnetization of the anisotropic magnetic material of the core element 12.

In use, the electrically conductive wire 14 carries an alternating current, which may take the form of a sinusoidal waveform or other types of waveforms. As such, the current limiter 10 may be operably associated with one or more electrical circuits carrying alternating current such as power converters and electric motors.

The operation of the current limiter 10 is carried out as follows:

Initially the electrical circuit associated with the current limiter 10 is in an off state such that there is no current flowing through the coil and the core element is unmagnetized .

The switching of the associated electrical circuit to an on state results in the flow of alternating current through the electrical circuit and the coil 16 of the current limiter 10. The flow of alternating current in the coil 16 results in the generation of a magnetic field about the coil 16. The direction of the magnetic field at any one time is dependent on the direction of the alternating current.

In either direction of the alternating current, the direction of the magnetic field passing through the enclosed side of the core element 12 is coaxially aligned with the [111] crystalline direction 18 of the anisotropic magnetic material, i.e. the direction of hard magnetization.

The impedance of the coil 16 is a function of coil resistance and reactance.

Additional energy is expended as a consequence of the presence of magnetic hysteresis and eddy currents in the core element 12. The expenditure of this additional energy contributes to an increase in coil resistance, and coil resistance increases with the level of magnetic hysteresis and eddy current losses in the core element 12.

Coil reactance is a function of coil inductance, which in turn is proportional to the core element's magnetic permeability.

The impedance of the coil 16 is therefore dependent on the level of magnetic hysteresis and eddy current losses in the core element 12 and the magnetic permeability of the core element 12.

During normal, steady-state operating conditions, the magnitude of the alternating current in the associated electrical circuit is relatively low. This results in the generation of a magnetic field about the coil 16 with relatively low strength that is insufficient to affect the orientation of magnetic domains within the crystal structure of the anisotropic magnetic material of the core element 12. This thereby results in low magnetization and therefore low magnetic permeability of the core element 12, while the low magnitude of current in the coil 16 results in low levels of magnetic hysteresis and eddy current losses in the core element 12. Consequently the electrically conductive wire presents a low coil impedance to the associated electrical circuit during normal, steady- state operating conditions.

A fault or other abnormal operating condition in the associated electrical circuit may lead to high fault current and thereby higher peak values of the alternating current flowing in the associated electrical circuit.

In the event of high fault current flowing through the coil 16, the resultant magnetic field is sufficiently strong to alter the magnetic domains of the anisotropic magnetic material and thereby align its magnetizing vectors in the direction of the magnetic field. This in turn causes the magnetic permeability of the core element 12 to be increased. The magnetic permeability of the core element 12 increases with the magnitude of the fault current flowing through the coil 16.

The increase in magnetic permeability of the core element 12 and the increased levels of magnetic hysteresis and eddy current losses in the core element 12 therefore contribute to an increase in coil impedance .

The provision of high coil impedance limits the magnitude of the fault current through the current limiter 10 and the associated electrical circuit. Preferably the high impedance presented to the associated electrical circuit is such that the peak value of the fault current is kept within the current rating of the associated electrical circuit to protect the various components of the associated electrical circuit .

When the magnetizing vectors of the anisotropic material of the core element 12 are fully aligned in the direction of the generated magnetic field, maximum permeability of the core element 12 and therefore maximum coil impedance are established. Further increases in coil current will tend to saturate the core element 12, leading to decreases in magnetic permeability and coil impedance.

Subsequent removal of the high fault current causes the magnetizing vectors to be realigned in the direction of easy magnetization of the anisotropic magnetic material of the core element 12. This thereby causes the magnetic permeability of the core element 12 and therefore the impedance of the coil 16 to be restored to their original low values. As such, the current limiter 10 is restored to its original state for use in normal, steady-state operating conditions of the associated electrical circuit .

Configuring the core element 12 to align the [111] and [100] crystalline directions 18,20 in the manner set out above is advantageous in that the current limiter 10 provides an associated electrical circuit with a fault protection mechanism, which has minimal influence on the associated electrical current during normal, steady-state operating conditions and limits the peak magnitude of the fault current in the event of a fault or other abnormal operating condition in the associated electrical circuit. The passive nature of operation of the current limiter 10 means that it may be possible to minimise or eliminate the use of detection and/or control equipment normally employed to monitor and control the current within the associated electrical circuit. Moreover the structure of the current limiter 10 is straightforward to manufacture and readily adapted to fit into any apparatus requiring one or more current limiters 10.

In other embodiments, it is envisaged that the core element is configured to align the [100] crystalline direction of the cold rolled grain-oriented silicon iron to be at an angle between 55 and 90 degrees to the direction of the magnetic field that, in use, passes through the enclosed portion of the core element and is generated by the coil. This can be achieved by, for example, configuring the core element to align the [110] crystalline direction of the cold rolled grain-oriented silicon iron in the direction of the magnetic field that, in use, passes through the enclosed portion of the core element and is generated by the coil.

In such configurations, the core element exhibits intermediate magnetic permeability when a magnetic field is generated by the coil, the intermediate magnetic permeability having a value between the aforementioned minimum and maximum magnetic permeabilities .

The cold rolled grain-oriented silicon iron of the core element 12 may be replaced by other types of anisotropic magnetic material or grain-oriented magnetic alloys. Such a core element may be configured to align a direction of easy magnetization of the anisotropic magnetic material to be non-parallel to the direction of a magnetic field, and/or a direction of hard magnetization of the anisotropic magnetic material to be substantially parallel to the direction of the magnetic field, whereby the magnetic field, in use, passes through the enclosed side of the core element and is generated by the coil.

Preferably the current limiter 10 further includes one or more additional magnetic field sources, the or each additional magnetic field source being configured to generate, in use, a magnetic field in the direction of easy magnetization of the anisotropic magnetic material of the core element 12. Such an additional magnetic field source may be, for example, in the form of an electromagnet or a permanent magnet.

The use of one or more additional magnetic field sources in the current limiter 10 helps maintains the arrangement of the crystalline axes of the anisotropic magnetic material of the core element 12 against the rotational forces exerted by the generated magnetic field when alternating current flows through the coil 16. This ensures that the magnetic permeability of the core element 12 and therefore the impedance of the coil 16 is kept at their respective low values during normal, steady-state operating conditions .

Claims

1. A current limiter comprising at least one core element, the or each core element including anisotropic magnetic material, and at least one electrically conductive wire being wound to define a coil enclosing a portion of the or each core element, the or each core element being configured to align a direction of easy magnetization of the anisotropic magnetic material to be non-parallel to the direction of a magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil.

2. A current limiter according to Claim 1 wherein the or each core element is configured to align a direction of hard magnetization of the anisotropic magnetic material to be substantially parallel to the direction of a magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil.

3. A current limiter according to Claim 1 or Claim 2 wherein the anisotropic magnetic material is a grain- oriented magnetic alloy.

4. A current limiter according to Claim 3 wherein the grain-oriented magnetic alloy is cold rolled grain- oriented silicon iron.

5. A current limiter according to Claim 4 wherein the direction of easy magnetization of the anisotropic magnetic material is the [100] crystalline direction.

6. A current limiter according to any of Claims 3 to 5 wherein the or each core element is configured to align a direction of the anisotropic magnetic material at an angle between 55 and 90 degrees to the direction of the magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil.

7. A current limiter according to Claim 6 wherein the or each core element is configured to align a direction of the anisotropic magnetic material at an angle of 55 degrees to the direction of the magnetic field that, in use, passes through the enclosed portion of the respective core element and is generated by the respective coil.

8. A current limiter according to any preceding claim wherein the or each core element includes a plurality of first layers of anisotropic magnetic material arranged in a laminated structure.

9. A current limiter according to Claim 8 wherein the or each core element further includes a plurality of second layers of electrically insulating material, the first and second layers being arranged in a laminated structure of alternating first and second layers.

10. A current limiter according to Claim 8 wherein at least one core element is separated from one or more other core elements by an air gap.

11. A current limiter according to any preceding claim wherein the or each core element is in the form of a rod, bar or toroid.

12. A current limiter according to any preceding claim wherein the cross-section of the or each core element is circular, oval or polyhedral in shape.

13. A current limiter according to any preceding claim wherein the or each coil is in the form of a solenoid or a toroid.

14. A current limiter according to any preceding claim further including one or more additional magnetic field sources, the or each additional magnetic field source being configured to generate, in use, a magnetic field in the direction of easy magnetization of the anisotropic magnetic material of the or each core element .

15. A current limiter according to Claim 14 wherein the or each additional magnetic field source is in the form of an electromagnet or a permanent magnet.

16. A current limiter according to any preceding claim wherein the or each electrically conductive wire is operably connected, in use, to one or more electrical circuits .

17. A current limiter according to Claim 16 wherein the or each electrically conductive wire presents an impedance to minimise a fault current created by a fault, in use, in an electrical circuit.