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 in the form of a solid-state electronic current limiter. These current limiters employ the use of semiconductor devices to provide a rapid response to the development of high fault current. However, this leads to high power consumption and decreased reliability due to the need to employ additional switching, detection and control devices when operating the current limiter.

A further 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.

Superconducting fault current limiters may be employed to limit the fault current flowing through the electrical apparatus. These superconducting fault current limiters however have limited availability and are relatively expensive, and may therefore be too costly for small-scale electrical operations.

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 a hard magnetic material, and at least one electrically conductive wire being wound around a portion of the respective core element to define a coil, wherein in use the electrically conductive wire carries an alternating electrical current.

The provision of hard magnetic material in the or each core element results in a passive current limiter which presents low impedance when the alternating current is relatively low, and high impedance when the alternating current is relatively high. This is because the level of magnetic hysteresis and eddy currents losses in the or each core element and therefore the effective impedance of the current limiter increases with the peak magnitudes of the alternating current. This allows the current limiter to have minimal influence on the flow of alternating current during normal 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 provision of hard magnetic material in the or each core element improves the efficiency of the current limiter when it comes to limiting the fault current, because of higher coil impedance resulting from the higher levels of magnetic hysteresis in hard magnetic material at high levels of excitation in comparison to soft magnetic materials. Otherwise the use of soft magnetic material in place of the hard magnetic material leads to low coil impedance which may be insufficient to limit the fault current. 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 easily adapted to fit into any apparatus requiring one or more current limiters.

The or each electrically conductive wire is preferably wound around a portion of the hard magnetic material of the respective core element.

The winding of the or each coil around the hard magnetic material of the respective core element allows the magnetic hysteresis in the hard magnetic material to have a greater effect on the coil impedance, and thereby improve the efficiency of the current limiter.

Preferably the or each coil is operably connected in use to one or more electrical circuits. In such embodiments, the or each coil presents 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. In embodiments of the invention, the or each core element may be a rod, bar or a toroid.

In other embodiments, the cross-section of the or each core element is circular, oval or polyhedral in shape.

In further embodiments, the or each electrically conductive wire is in the form of a solenoid or a toroid.

The structure of the current limiter may vary depending on the requirements of the current limiter .

Preferably the or each core element includes a plurality of first layers of hard 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 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 or each core element when a changing flux flows through the 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 core elements, but also provides a supporting structure to hold neighbouring core elements in place.

In other such embodiments, the or each first layer may be separated from each neighbouring first layer by an air gap.

The low permeability of air improves the isolation between neighbouring first layers and thereby minimises the risk of magnetic flux passing from one first layer to another.

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

Figures 1 to 3 show current limiters according to first, second and third embodiments of the invention ;

Figure 4 is a cross-sectional view of a current limiter including a core element in the form of a laminated structure of alternating layers of hard magnetic material and electrically insulating material;

Figure 5 illustrates the formation of eddy currents in the core element of the current limiter of Figure 4 ;

Figure 6 illustrates the magnetic behaviour of the core element of the current limiter of Figure 4 at low levels of alternating current flowing through the coil; and

Figure 7 illustrates the magnetic behaviour of the core element of the current limiter of Figure 4 at low and high levels of alternating current flowing through the coil. A current limiter 10 according to a first embodiment of the invention is shown in Figure 1.

The current limiter 10 comprises a core element 12 including a hard magnetic material, and an electrically conductive wire wound around at least a portion of the core element 12 to define a coil 14.

In the embodiment shown in Figure 1, the core element 12 is provided in the form of a rod. In other embodiments, the core element 12 may be provided in the form of a bar (Figure 2) or in the form of a toroid (Figure 3) .

It is envisaged that the cross-section of the core element 12 may be circular, oval or polyhedral in shape. The shape and size of the core element 12 may vary depending on the requirements of the current limiter 10.

Preferably the core element 12 includes a plurality of first layers 16 of hard magnetic material arranged in a laminated structure. The core element 12 preferably further includes a plurality of second layers 18 of electrically insulating material, the first and second layers 16, 18 being arranged in a laminated structure of alternating first and second layers 16,18, as shown in Figure 4.

The provision of a plurality of first layers 16 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 20 (Figure 5) are reduced. The magnitude of any eddy currents 20 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 16 of the core element 12, which restricts the circulation of the eddy currents 18.

The inclusion of insulating material improves electrical isolation between first layers 16 of hard magnetic material and provides a supporting structure to hold neighbouring first layers 16 in place .

In other embodiments, the or each first layer 16 may be separated from each neighbouring first layer 16 by an air gap.

The electrically conductive wire may be wound around a portion or the whole of the core element 12 to form the coil 14, which may be in the form of a solenoid or a toroid.

It is envisaged that in embodiments of the invention, the current limiter 10 may include a plurality of core elements 12. 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 12 to form a coil 14.

It is also envisaged that in other embodiments, the current limiter 10 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 12, and/or a plurality of wires may be wound around a portion or the whole of a core element 12.

The or each electrically conductive wire may be wound around a portion of the hard magnetic material of the respective core element 12. This allows the magnetic hysteresis in the hard magnetic material to have a greater effect on the coil impedance, and thereby improve the efficiency of the current limiter 10.

In use, the electrically conductive wire 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 associated electrical circuit is in an off state such that there is no current flowing through the coil 14 and the core element 12 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 14 of the current limiter 10. The flow of alternating current in the coil 14 results in the generation of a magnetic field about the coil 14. The direction of the magnetic field at any one time is dependent on the direction of the alternating current.

The provision of a core element 12 including a hard 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 hard magnetic material has a higher permeability than that of air.

Figures 6 and 7 illustrate the magnetic behaviour of the core element 12 during the flow of alternating current through the coil 14 of the current limiter 10.

For the purposes of this specification, positive and negative terms refer to the direction of the magnetizing force and the magnetic flux density.

In order to generate a first magnetic field about the coil 14, the alternating current flows through the coil 14 in a forward direction. The increase in magnitude of the alternating current in the forward direction leads to a corresponding increase in magnetizing force in a positive direction, which in turn leads to a development of a positive magnetic flux density within the core element 12. The positive magnetic flux density reaches its maximum value 22 at the peak value of the alternating current in the forward direction.

After arriving at its peak value in the forward direction, the alternating current begins to decrease to zero current, which leads to a corresponding decrease in magnetizing force in the positive direction. At zero current, the magnetizing force has a zero value.

The decrease in magnetizing force in the positive direction leads to a decrease in the positive magnetic flux density within the core element 12. The core element 12 however retains a positive magnetic flux density at zero alternating current and magnetizing force. This is because application of a magnetizing force to the hard magnetic material of the core element 12 leads to magnetization of the hard magnetic material, which tends to remain magnetized even after the magnetizing force is completely removed. The retention of magnetism in the core element 12 after removal of the magnetizing force is known as permanent magnetism.

In order to generate a second magnetic field about the coil 14, the alternating current flows through the coil 14 in a reverse direction. The increase in magnitude of the alternating current in the reverse direction leads to a corresponding increase in magnetizing force in the negative direction. The magnetic hysteresis in the core element 12 however means that the magnetic flux density in the core element 12 will remain positive until the magnetizing force in the negative direction is sufficiently large to demagnetize the core element 12.

After the core element 12 is demagnetized, further increases in magnitude of the alternating current in the reverse direction and thereby the magnetizing force in the negative direction leads to a development of a negative magnetic flux density within the core element 12. The negative magnetic flux density within the core element 12 reaches its maximum negative value 24 at the peak value of alternating current in the reverse direction.

During the change of flow of the alternating current from the reverse direction to the forward direction, the core element 12 retains a negative magnetic flux density until the magnetizing force in the positive direction is sufficiently large to demagnetize the core element 12. After demagnetization of the core element 12, further increases in magnitude of the alternating current in the forward direction, and thereby the magnetizing force in the positive direction, leads to a development of a positive magnetic flux density within the core element 12 up to its maximum value at the peak value of the alternating current in the forward direction.

The above described behaviour of the core element 12 during the flow of alternating current through the current limiter 10 forms a cycle of magnetisation and demagnetisation. The cycle continuously repeats until the associated electrical circuit reverts to an off state so that there is no flow of alternating current through the current limiter 10.

This cyclic behaviour of the core element 12 defines a closed hysteresis loop 26a, 26b (Figures 6 and 7) which describes a relationship between the magnetizing force and the magnetic flux density. The size of a hysteresis loop for a core element 12 is dependent on the peak values of the alternating current in the forward and reverse directions. The hysteresis loop of the core element 12 tends to widen with increasing peak values of alternating current because the core element 12 exhibits a higher degree of magnetization, and thereby higher magnetic hysteresis, at higher values of current. The widening of the hysteresis loop leads to steeper rates of change of magnetic flux density with magnetizing force as the alternating current approaches its peak positive or negative value.

The shape and size of the magnetic hysteresis loop affects the effective impedance presented by the coil to the associated electrical circuit .

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

Additional energy is required to overcome the magnetic hysteresis of the core element 12. The expenditure of this additional energy contributes to an increase in coil resistance. Coil resistance therefore increases with the level of magnetic hysteresis 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 magnetic permeability of the core element 12 is equal to the rate of change of magnetic flux density with magnetizing force, which is dependent on the shape and size of the magnetic hysteresis loop. As outlined above, the widening of the hysteresis loop leads to steeper rates of change of magnetic flux density with magnetizing force, and thereby higher values of magnetic permeability, as the alternating current approaches its peak positive or negative value. These higher values of magnetic permeability lead to increases in coil inductance and thereby coil reactance. The impedance of the coil is therefore dependent on the level of magnetic hysteresis in the core element 12.

During normal operation of the associated electrical circuit, the peak values of the alternating current remain relatively low and within the current rating of the associated electrical circuit. These relatively low peak values of the alternating current leads to a hysteresis loop 26a having a narrow magnetic hysteresis loop area and gradual rates of change of magnetic flux density with applied magnetic field strength, as the alternating current approaches its positive or negative peak value. This results in low coil resistance and reactance, as outlined above. As such, the coil 14 presents low impedance to the associated electrical circuit during normal operating conditions, the low impedance being advantageous due to its limited influence on the flow of alternating current .

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. These higher peak values of the alternating current mean that the core element 12 will experience a higher degree of magnetization, which leads to a hysteresis loop 26b having a wider magnetic hysteresis loop area and steeper rates of change of magnetic flux of magnetic flux density with magnetizing force, as the high fault current approaches its positive or negative peak value. This leads to an increase in coil resistance and reactance as outlined above. As such, the coil 14 presents high impedance to the associated electrical circuit to limit the peak values of the fault current during fault or other abnormal operating conditions.

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 .

The use of hard magnetic material in the core element 12 is advantageous in that hard magnetic material typically exhibits a greater increase in magnetic hysteresis than soft magnetic material, as the peak values of the alternating current increase. This allows the coil 14 to present low impedance at low values of alternating current and high impedance at high values of alternating current. The use of soft magnetic material in place of the hard magnetic material would decrease the efficiency of the current limiter 10 when it comes to limiting the fault current, because the low levels of magnetic hysteresis in soft magnetic materials lead to low coil impedance which may be insufficient to limit the fault current.

The current limiter 10 according to the invention therefore provides an associated electrical circuit with a fault protection mechanism, which has minimal influence on the associated electrical current during normal 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 .

In embodiments of the invention, the current limiter 10 may be designed to limit the magnitude of the fault current for a short period of time before external circuit breakers are actuated to open the circuit and thereby extinguish the fault current .

The current limiter 10 may be reused by demagnetizing the core element after the fault current is extinguished, so as to return the core element 12 to its unmagnetized state.

Preferably the hard magnetic material is selected so that the magnetizing force required to achieve saturation in the core element 12 is higher than the magnetizing force arising from the fault current. This is because when a magnetic material reaches saturation, its magnetic relative permeability reduces to one, which results in a low coil inductance and thereby may result in inadequate performance from the current limiter 10.