WO2014132066A1

WO2014132066A1 - Fault current limiter

Info

Publication number: WO2014132066A1
Application number: PCT/GB2014/050586
Authority: WO
Inventors: Jeremy Peter HALL
Original assignee: Faultcurrent Limited
Priority date: 2013-02-28
Filing date: 2014-02-27
Publication date: 2014-09-04
Also published as: GB201303565D0

Abstract

A fault current limiter (FCL) has a core structure including first and second, spaced magnetisable core members; and an AC magnetomotive force source configured to generate a varying magnetic flux in at least a portion of the first and second magnetisable core members. Static magnetomotive force sources are positioned to provide a magnetic circuit within at least part of the magnetisable core members; wherein the static magnetomotive force sources, comprise U shaped magnets arranged to bridge the first and second core members.

Description

Fault Current Limiter.

The present invention relates to a fault current limiter and in particular a fault current limiter having a re-settable static magnetomotive force source.

A Fault Current Limiter (FCL) is a device used to limit, or in its most basic form interrupt, a fault current in a branch of a circuit on occurrence of a fault condition so as to prevent any components in the circuit from being overloaded.

Fuses are an example of a device which interrupts high currents in fault conditions, however these devices must be replaced after a fault condition has occurred and cannot be used in high power systems. It is usually more preferable to employ a re- settable FCL which limits the fault current, rather than interrupts it.

An example of a re-settable FCL which is suitable for low power operations is the Magnetic Current Limiter (MCL) which comprises a permanent magnet sandwiched between a saturable core with an AC wire wound around the core (see figure 1). The permanent magnet 2 causes the core to saturate in the normal operating state. For the device to operate for each half of the AC cycle, two cores are required such that in the first core the magnetic field produced by the AC current flows through the coil since the magnetic field provided by the permanent magnet are additive and in the second core they are subtractive. In the normal operating condition the AC current flowing through the coil is low and both cores are saturated causing the effective impedance in the AC coil 3 to be low. During a fault condition a large AC current value (the fault current) forces each of the cores of the device to come out of saturation in alternative half-cycles. The mostly unsaturated first core in combination with the mostly saturated second core (and vice versa) restricts the flow of the fault current since the inductance of the coil is caused to increase. In this arrangement multiple distinct core elements are used and useful regions of the core, where interaction between the magnetic field associated with the magnet and the AC coil takes place, are limited. Further, the MCL does not perform well in high power alternating systems.

In higher power alternating power systems, series reactors have been implemented so as to protect against excessively large currents under short-circuit, however they have a major disadvantage in that they produce significant I²R losses,

Another system suitable for use in a high power alternating system is the saturated Iron Core FCL which comprises a copper coil with an iron core. The Iron core is maintained in magnetic saturation in normal operation by applying the magnetic field of an additional superconducting wire. The impedance of the device is low in normal operation; however when a fault condition occurs the increased AC current through the normal conducting coil causes the core to depart from saturation so as to cause the impedance of the device to increase. In this arrangement the superconducting wire is exposed only to DC current and therefore always remains in the superconducting state and eliminates the need for a recovery time. The main disadvantages with this system includes the large mass and volume of the device, the high magnetic fields at the superconducting coil and the high cooling costs of the superconducting coil.

A recently developed system for use in high power applications is the Superconducting FCL which relies on a rapid change of resistance with temperature so as to limit the fault current. The superconducting FCL is directly connected in series with the current path to be protected. When a specified current density is reached, which corresponds to a particular temperature, the resistance increases rapidly so as to substantially limit the flow of the fault current. Such arrangements have an array of disadvantages including: a) expensive cooling mechanisms since the superconductor must be cooled to 77 , b) the development of thermal instabilities and c) AC current cooling losses. Further, in order to prevent the excessive heating of the superconductor, so as to avoid long cool down phases, the reduced fault current is only be carried for a few cycles. WO200702924 discloses a fault current limiter device in which an electromagnet DC source is utilised.

Due to the costs associated with the Saturated Iron Core FCL and the Superconducting FCL such systems are not usually desirable for smaller power operations e.g. circuits implementing power electronics devices such as transistors, diodes etc.

It is therefore desirable to provide an improved Fault Current Limiter which addresses at least some of the above described problems and/or which offers improvements generally.

In a first aspect of the present invention there is provided fault current limiter (FCL) comprising: a core structure having: i) a first magnetisable core member; ii) a second magnetisable core member, spaced from the first core member; an AC magnetomotive force source configured to generate a varying magnetic flux in at least a portion of the first and second magnetisable core members; and; a plurality of static magnetomotive force sources being positioned to provide a magnetic circuit within at least part of the magnetisable core members; wherein the static magnetomotive force sources, comprise U shaped magnets arranged to bridge the first and second core members. The use of a U shaped magnet enables the core members to be spaced by a small gap only whilst enabling a large magnet to be used. The U shaped magnets are spaced and together with the magnetisable core members provide a closed magnetic circuit.

The U shaped magnets (or C magnets/horseshoe magnets) comprise respective Hmbs extending in the same general direction with one another each arranged to contact one of the respective magnetisable core members usually at a common contact plane. The limbs of the U shaped magnet are typically parallel and are preferably orientated substantially perpendicularly to the contact surface of the core members. Alternatively, in the case of the C magnet the respective limbs may extend in the same direction with each limb in the form of an arc or a section of a polygon in order that flux leakage between parallel limbs may be minimised. The U shaped magnets can be electromagnets or permanent magnets depending upon the circumstances of use.

It is preferred that the AC magnetomotive force source and the static magnetomotive forces source are relatively positioned to be orthogonal to each other.

It is preferred that the first and second magnetisable core members are spaced in the same plane. Preferably, the U shaped magnets are spaced in the longitudinal direction of the core members.

It is preferred that the AC magnetomotive force source comprises a coil wound around the first and second core members at a position intermediate the bridging U shaped magnets. This enables the coil to be wound around only the core members. The coil does not have to be wound around the magnets. In a preferred embodiment, a plurality of pairs of U shaped magnets are arranged to bridge the first and second core members. In one embodiment, the arrangement may take the form of a composite structure comprising a second pair of first and second core members, positioned adjacent the first pair of core members; the second pair of core members being provided with a set of respective U shaped magnets arranged to bridge the first and second core members,

In such an embodiment, the pairs of core members may be provided adjacent one another, possibly but not necessarily spaced by a small gap.

In such an embodiment the U shaped magnets for the respective pairs of cores may be provided on opposed sides of the cores, preferably such that the U shaped magnets for the respective pairs of cores are matched positionally opposite one another.

In such an embodiment it may be preferred that the composite structures are stacked or adjacently filed in a repeatable structure in order to maximise the overall capacity of the FCL.

In one embodiment it is preferred that the magnetisable core members comprise an inner core ring and an outer core ring spaced from one another radially.

In such an embodiment it may be preferred that the U shaped magnets bridge the core members in a radial direction.

In one embodiment it is preferred that the magnetisable core members comprise a pair of parallel core rings spaced from one another axially. In such an embodiment it may be preferred that the U shaped magnets bridge the core members in an axial direction.

In one embodiment it may be preferred that the core members include air gaps, preferably at the location of contact of the poles of the U shaped magnets. In this context the core members may be split or segmented at the contact position with the magnets and the flux is bifurcated into each of the magnetisable cores.

The static magnetomotive force sources are positioned to provide magnetic saturation in the core members.

In certain embodiments the static magnetomotive force sources are positioned to provide a bifurcated magnetic field in the core adjacent to the static magnetomotive force source in which the field direction in the same core member branches in opposed directions at the junction with the static magnetomotive force source.

In one embodiment the FCL may comprise a plurality of stacked core structures each comprising a ring structure, the plurality being positioned stacked or in file one adjacent another and each comprising: a core structure comprising: i) a first magnetisable core member; and ii) a second magnetisable core member, spaced from the first core member.

Typically the AC magnetomotive force source is an AC coil having a longitudinal axis and to which is applied an AC current so as to produce an AC magnetic field. Generally, it is envisaged that the U shaped magnet will be formed from a permanent magnetic material having a default magnetisation.

The U shaped magnets cause the core members to become magnetically saturated in normal operation.

In a fault condition the magnitude of the AC magnetic field increases from a normal state value to overcome magnetic saturation in the core members.

The AC coil obtains a higher inductance value when a fault condition occurs.

The core members respond respectively to the positive and negative halves of the AC current cycle received by the AC coil.

In certain embodiments the magnetisable cores contain at least one air gap or comprise core members separated from adjacent core members by an air gap.

Beneficially the at least one air gap in the magnetisable core is positioned adjacent the surface of a pole face of the U shaped magnet.

In circumstances in which the static magnetomotive force source is a permanent magnet, the permanent magnet is preferably selected to have an ability to recover its default magnetised state on cessation of a fault current event.

The at least magnetisable core members may be formed of strip steel or other ferromagnetic metal alloy, a soft ferrite material or an amorphous or nanocrystalline soft magnetic alloy. The broadest aspect of the invention can be characterised by one or more preferred features of the aspects described above or by technical features described in relation to the specific embodiments which follow.

The present invention will now be described by way of example only with reference to the following illustrative figures in which:

Figure 1 shows a side view of a Magnetic Current Limiter in accordance with the prior art.

Figure 2A is a perspective view of a FCL falling outside the scope of the invention, but useful for explanatory purposes;

Figure 2B is a detailed view of figure 2A showing bifurcation of the field.

Figure 3 is a perspective view of a FCL according to the invention;

Figure 4 is a perspective view of an alternative embodiment of FCL in accordance with the invention;

Figure 5 is a perspective view of an alternative embodiment of FCL in accordance with the invention, similar to figure 2 but with additional magnets;

Figure 6 is a perspective view of an alternative embodiment of FCL in accordance with the invention, similar to figure 4 but with additional magnets;

Figure 7 is a perspective view of an alternative embodiment of FCL in accordance with the invention;

Figure 8 is a perspective view of an alternative embodiment of FCL in accordance with the invention; Referring to the drawings, Figure 2 shows a magnetisable core 1 formed of two core sections la, lb. The first core section la has a picture frame shape and the second core section lb is a mirror image of the first core section. The first and second sections la, lb are arranged in a face-to-face parallel arrangement, the first core section comprises a first closed magnetic circuit and the second core section comprises a second closed magnetic circuit distinct from the first closed magnetic circuit. A first DC magnetomotive force source 2a, for example a magnet, such as a magnet made from permanently magnetic material (hereafter referred to as a permanent magnet) is arranged to bridge the gap between between the two opposing faces of the first and second core sections la, lb, so as to link a first arm lc of the first core section with a first arm Id of the second core section. The permanent magnet 2a is also known as a static magnetomotive force source and has a magnetic dipole moment associated with it. It is an advantage of the arrangement of the invention that for the FCL, re-coil of the permanent magnet is not a primary concern since the flux linkage through the magnet during fault helps to maintain the moments of magnets. It is of concern in the case of the prior art shown in figure 1 where the moment of the magnets face full force of the AC mmf head on. This is required so that the Fault Current Limiter can automatically reset following a fault current event thereby ensuring that the permanent magnet is not permanently demagnetised. In this embodiment the permanent magnet is one which possesses good re-coil capability such as a rare earth metallic alloy or a hard ferrite. A second permanent magnet 2b possessing a good re-coil capability and having a magnetic dipole moment opposing the direction of the magnetic dipole moment of the first magnet 2a is arranged between the two opposing faces so as to link a second arm of the first core section le and a second arm of the second core section If.

The first and second permanent magnets 2a, 2b are in a parallel arrangement and the first and second arms lc, Id, le, If are parallel arms in the frame arrangement. The flux set up by the first and second permanent magnets 2a, 2b forms a complete magnetic circuit through the soft magnetic material of the core 1 i.e. a magnetic field flows from the north pole of the first magnet 2a through the first core section la to the south pole of the second magnet 2b and the north pole of the second magnet 2b flows through the second core section lb to the south pole of the first magnet 2a. Therefore the magnetic field flows in opposite directions in the first and second core sections la, lb. The relative geometries of the core sections la, lb and the permanent magnets 2a, 2b are so as to maximise the ratio of magnetic flux interacting to non-interacting volume of the core. Therefore, ideally under normal operating conditions the entire volume of the soft magnetic material of the core 1 remains in the magnetic saturated state.

An AC magnetomotive force source 3, or AC conductive element is wound around each perpendicular arm of the first and second core la, lb in a parallel arrangement so as to provide an AC coil. The AC magnetomotive force source 3 and the static magnetomotive force source 2a,2b are relatively positioned to be orthogonal to eachother. Therefore the longitudinal axis of the AC coil is orthogonal (or perpendicular) to the dipole moment of the magnet i.e. the north to south direction of the magnet. Alternatively this can be thought of as the coil being arranged to provide an orthogonal AC field to the DC field generated in the region of the core close to the pole face of the DC magnetomotive force source (e.g. permanent magnet). The important interaction between the AC and DC fields is where there is a parallel interaction between the AC and DC fields within the soft magnetic cores.

Under normal operating conditions the AC current which passes along the AC coil experiences minimal impedance. As current passes along the AC coil a magnetic field in a direction perpendicular to the magnetic moment of the first and second permanent magnet is produced. As mentioned above this may also be defined with respect to the pole face whereby the AC field in the region near to the pole face of the permanent magnet is perpendicular to the DC magnetic field generated by the magnet at the pole face and in the region close to the pole face.

The inductance L of the coil 3 can be approximated with the following equation: L=Ufl.^.N².A/l where uo- permeability of free space (constant), μ_{- relative permeability of the magnetic core 1, N- number of turns of the coil 3, A- cross section area of the coil 3, I- magnetic path length of the coil 3. The N, A and 1 are linked with the physical design of the coil 3 (inductor) thereby making it relatively difficult to change them gradually over a wide range. The permeability of free space μο is a constant. The relative permeability , is a measure of how easy the material of the core 1 can be magnetised and it is usually measured for closed magnetic circuits. The μ_Γ varies with many factors, the most important being the level of magnetisation and for a ferromagnet material this can vary from tens of thousands (at the peak) to one (at extremely high magnetisation).

Therefore, in normal operation the inductance of the AC coil 3 is low since the core (comprising of the first and second core sections la, lb) is saturated by the presence of the magnetic circuit within the core caused by the magnet arrangement. When a fault condition occurs a high current (a fault current) flows through the AC coil 3 and the magnetic field generated by the AC coil 3 increases in magnitude and becomes strong enough to overcome the magnetic saturation in regions of the core where the AC field and permanent magnetic field interact in opposite directions, i.e. where the fields are subtractive.

For example firstly considering the effect in the first section of the core la. When considering the positive half cycle of the AC signal, the magnetic field produced by the AC coil 3 is subtracted from the magnetic field produced by the permanent magnet 2a in the regions where the two fields oppose causing at least part of the region in the first core section to become unsaturated. However, when considering the region of the first core section la where the fields are in the same direction the core in this region is driven deeper into saturation, there is no change in the relative permeability μ_Γ in this region. Therefore the combined net effect provides a net increase in relative permeability μ,- which, in accordance with equation 1, provides an increase in the AC coil inductance value. This increase in inductance limits the passage of the fault current through the coil 3.

The second core section lb functions in the same way, however the saturated and unsaturated portions are reversed compared to that of the first core section la. This results from the permanent magnetic fields in the second core section lb being in opposing directions to those in the first core section 1 a.

When considering the effect of the negative half of the AC cycle on the first core section la, the first core section now behaves in the same way as the second core section lb does for the positive half cycle i.e. the regions where the AC magnetic field and the permanent magnetic field coincided previously are now experiencing opposing fields, therefore the two fields subtract to give an unsaturated (or less saturated) region, and the regions which were opposing now coincide (adding to give a more saturated region). The inversion is also applied to the second core element lb. It is noted that the cyclic nature of the AC current passed through the coil 3 causes the direction of the AC magnetic field to vary (or alternate) whereas the magnetic fields caused by the permanent magnets are fixed in direction, therefore they are said to be DC fields or static fields.

Therefore for both halves of the AC cycle (i.e. the positive and negative parts), the overall effect of the magnetic fault current limiter is an increase in inductance of the AC coil 3 due to an increase in μ_Γ of the core (in accordance with equation 1) as a fault current passes through the AC coil 3. Therefore the passage of the fault current can be limited for each half of the AC current cycle.

The use of permanent magnets for the magnetomotive source elements 2a 2b can be extremely beneficial in that there is no requirement to use superconducting systems or electromagnets that require significant power supply. The concern with the use of permanent magnets is their potential de-magnetisation during a fault current event and the result that the FCL would thereafter not re-set to a usable state. This concern is ameliorated by the way in which the permanent magnets are incorporated into the magnetic circuit of the device. Although the permanent magnets 2a, 2b may experience brief periods of demagnetisation, they are not easily permanently demagnetised and 'spring back' or 're-coil' into the original (or default) magnetised state following a fault current event.

The static magnetomotive force sources (permanent magnets 2a, 2b) are positioned to provide a bifurcated magnetic field in the core la, lb in which the field direction in the same core member branches in opposed directions at the junction with the static magnetomotive force source (permanent magnets 2a, 2b). This is shown most clearly in figures 2A and 2B. This bifurcation of the static magnetic field at the junction with the core members, particularly where permanent magnets are used to bridge the gap between core members, provides protection against de-magnetisation of the static magnetomotive force source, during a fault current event. The arrangement provides common mode rejection of the AC field across the poles of the static magnetomotive force source. Such an arrangement is not disclosed in, for example WO2007029224.

The use of one or more pairs of permanent magnetomotive force sources to bridge the gap between core members provided benefits also in that a low reluctance DC flux path is provided to aid saturation and protection of the magnets, whilst at the same time allowing high levels of flux linkage through the magnetic circuit to give good current limiting inductance during a fault. The ability to include magnets distributed in this way provides distributed DC mmf around the magnetic circuit, aiding core saturation.

In accordance with the present invention significant benefits can be achieved by utilising this technology, particularly employing U shaped magnets to bridge thereon core members.

Embodiment of Figure 3. Single pair of U shaped magnets 102a 102b set along parallel iron core members 103a 103b.

In this first embodiment, two parallel cores 103a 103b formed from soft magnetic material (such as iron or electrical steel) are set in a plane. A pair of U-shaped permanent magnets 102a 102b bridges the gap between the parallel cores 103 a, 103b. The flux from a permanent magnet passes from the North pole N of the magnet into the facing iron core. As the flux passes into the iron core it passes in a direction towards the South pole S of the adjacent magnet. A similar pattern is set up in the adjacent iron core with flux passing in the opposite direction with flux linkage completed through the magnets. See the arrows in the figure The magnets bias the iron cores such that each iron core is substantially saturated. The direction of saturation in each iron core in the regions between adjacent magnets will be opposite. The distance of separation between the parallel iron cores and, likewise, between the poles of the U-shaped magnets is sufficient to minimise flux leakage across the gap. A benefit of using this arrangement with U-shaped magnets allows the iron cores to be biased using relatively large ferrite or Alnico magnets without requiring large area AC coil windings to encompass the whole core assembly. The AC coil windings 101 can be simply around the core members 103a 103b, which can be relatively closely spaced.

The AC electrical coils 101 are wound to encompass both of the adjacent iron cores 103a 103b. Coils 101 are located in the positions between the U-shaped sides of adjacent magnets 102a 102b. As the coils encompass the iron rings only and not the magnets the area of coil may be minimised to reduce the inductance of the coil according to 1 =Ν²μο μκ Area/Length. A fault current creates an alternating magnetic field which overcomes saturation in the iron cores in the region where the DC flux is in the opposing direction. When the iron core is no longer saturated μρ_. will become significantly large. When the iron is not saturated the flux linkage through the iron determined by L = Flux linkage/Current becomes significant. An additional benefit of coils which encompass the iron cores only is a reduced tendency to demagnetise the permanent magnets under high AC fields.

Embodiment of Figure 4.

In a further embodiment, shown in figure 2 a second similar set of iron cores 103c 103d and magnets 102c 102d can be arranged as the mirror image of the first. Here the poles of the opposing U-shaped magnets are set in opposition. In this embodiment the effect of the directly opposing pole faces is to reduce the leakage of magnetic flux into the surrounding air and enhance the degree of saturation of the iron cores. The AC electrical coils 101 are wound to encompass all of the adjacent iron cores in the position between the U-shaped sides of adjacent magnets. There may or may not be a small gap between the mirror-facing iron cores. A small gap may aid the manufacturing process. A minimum practicably realisable gap would aid the minimisation of unwanted flux leakage from the magnetic circuit.

Embodiment of Figure 5.

Multiple pair of magnets set along parallel iron cores.

In the embodiment of figure 5 two parallel cores 103a 103b formed from soft magnetic material (such as iron or electrical steel) are set in a plane. An even number of pairs of U-shaped permanent magnets 102a 102b 102c 102d etc bridge the gap between the parallel cores. The flux from a permanent magnet passes from the North pole N of the magnet into the facing iron core. As the flux passes into the iron core it bifurcates such that half of the flux passes in one direction towards the South pole of the adjacent magnet and half passes in a other direction through the iron core towards the South pole of the corresponding adjacent magnet. A similar pattern is set up in the adjacent iron core with flux linkage completed through the magnets 102. See the arrows in the figure. The magnets bias the iron core such that each adjacent parallel section of the iron cores is substantially saturated in opposing directions. The distance of separation between the parallel iron cores and, likewise, between the poles of the U-shaped magnets is sufficient to minimise flux leakage across the gap. Magnets and iron cores may be split at the one, several or all of the places located at centreline of static flux bifurcation in order to modify the inductance profile (by inclusion of a small air gap between sections) and to simplify the process of manufacture by allowing the FCL core to be manufactured as an assembly of sub-sections.

AC electrical coils 101 are wound to encompass both of the iron cores. Coils are located in the positions between each of the U-shaped sides of adjacent magnets. As the coils encompass the iron rings only and not the magnets the area of coil may be minimised to reduce the inductance of the coil according to

μ¾ Area/Length. A fault current creates an alternating magnetic field which overcomes saturation in the iron cores in the regions where the DC flux is in opposition. When the iron core is no longer saturated μ_¾ will become significantly large. When the iron is not saturated the flux linkage through the iron determined by L = Flux linkage/Current becomes significant. An additional benefit of coils which encompass the iron cores only is a reduced tendency to demagnetise the permanent magnets under high AC fields. The winding sense of the coils may be the same for all coils or in alternation for each adjacent coil.

Embodiment of Figure 6,

In the embodiment of figure 6 a second similar set of iron cores 103c 103d and magnets 102e 102f 102g etc can be arranged as the mirror image of the first, Here the poles of the opposing U-shaped magnets are set in opposition. In this embodiment the effect of the directly opposing pole faces is to reduce the leakage of magnetic flux into the surrounding air and enhance the degree of saturation of the iron cores. The AC electrical coils are wound to encompass all of the adjacent iron cores in each of the positions between the U-shaped sides of adjacent magnets. There may or may not be a small gap between the mirror-facing iron cores. A small gap may aid the manufacturing process. A minimum practicably realisable gap would aid the minimisation of unwanted flux leakage from the magnetic circuit.

Embodiment of Figure 7.

Single pair or multiple pairs of U shaped magnets set around concentric iron ring core members.

In this embodiment, two concentric ring core members 103 a 103 b formed from soft magnetic material (such as iron or electrical steel) are set in a plane. An even number of U-shaped permanent magnets 102a bridge the gap between the concentric rings. The flux from a permanent magnet passes from the, say, North pole N of the magnet into the facing iron ring core member 103a. As the flux passes into the iron ring it bifurcates such that half of the flux passes in a direction clockwise towards the South pole of the adjacent magnet and half passes in a counter-clockwise direction through the iron ring towards the South pole of the corresponding adjacent magnet. A similar pattern is set up in the adjacent concentric iron ring flux passing in this case from the, say, South pole and bifurcates into clockwise and counterclockwise directions in the iron ring towards the corresponding North poles of the adjacent magnets with flux linkage completed through the magnets - see the arrows in the figure. The magnets bias the iron rings such that each iron ring is substantially saturated. The direction of saturation in each iron ring in the regions between adjacent magnets will be opposite. The distance of separation between the concentric iron rings and, likewise, between the poles of the U-shaped magnets is sufficient to minimise flux leakage across the gap. At least two pairs of magnets allows the iron rings to be made from sections which are straight (or substantially straight in the case of toriodal iron coreswith a large overall diameter), thus allowing the shortest path through the substantially saturated iron between the magnet pole faces. In the case of fully straight sections between poles, a closed core is formed through use of concentric polygonal cores and angled magnets positioned at each corner of the polygon. Therefore whist the embodiment shown is in a circular ring form, polygonal ring forma are envisaged. Magnets and iron core members may be split at the one, several or all of the corners at centreline of static flux bifurcation in order to modify the inductance profile (by inclusion of a small air gap between sections) and to simplify the process of manufacture by allowing the FCL core to be manufactured as an assembly of sub-sections.

AC electrical coils 101 are wound to encompass both of the iron cores. Coils are located in the positions between each of the U-shaped sides of adjacent magnets. As the coils encompass the iron rings only and not the magnets the area of coil may be minimised to reduce the inductance of the coil according to Ι_=Ν²μο μκ Area Length. A fault current creates an alternating magnetic field which overcomes saturation in the iron cores in the regions where the DC flux is in opposition. When the iron core is no longer saturated ι* will become significantly large. When the iron is not saturated the flux linkage through the iron determined by L = Flux linkage/Current becomes significant. An additional benefit of coils which encompass the iron cores only is a reduced tendency to demagnetise the permanent magnets under high A.C. fields. The winding sense of the coils may be the same for all coils or in alternation for each adjacent coil.

Embodiment of Figure 8.

A similar arrangement of iron core members 103c 103d and U shaped magnets 102b can be set up as the mirror image of the first. Here the poles of the opposing U- shaped magnets are in opposition in order that flux is preferably directed through the iron cores along the long direction. In this embodiment the effect of the directly opposing pole faces is to reduce the leakage of magnetic flux into the surrounding air and enhance the degree of saturation of the iron cores. The AC electrical coils are wound to encompass all of the adjacent iron cores in each of the positions between the U shaped sides of adjacent magnets. There may or may not be a small gap between the mirror-facing iron cores. A small gap may aid the manufacturing process. A minimum practicably realisable gap would aid the minimisation of unwanted flux leakage from the magnetic circuit.

Embodiment of Figure 9.

Stacked arrangement.

In figure 9 the embodiment the FCL shown comprises a plurality of stacked core structures (which may be a ring structure) a ring structure. The AC coil may be wound between the magnets 102a 102b around groups of adjacent core members 103a 103b 103c 103d or around the entire stack. Stacking enables a maximum density of saturated core to be packed in a given space

In an alternative embodiment the permanent magnets could be replaced by DC conducting or superconducting coils containing a soft magnetic or an air core. This would provide adjustability of the DC static magnetic flux. The interaction of the AC and DC fluxes are the same as described when selecting permanent magnet configuration. It is also envisaged that a combination of a DC coil and a permanent magnet can be implemented too.

As an alternative to the embodiment of figure 7 in which the concentric ring core members are spaced radially, it is possible that ring cores could be arranged coaxially and spaced in the axial direction. In such an embodiment the U shaped magnets bridge the core in the axial direction rather than the radial direction.

Advantages include that the FCL is always ready as a) it responds equally to each half of the AC cycle and b) the permanent magnets are not easily permanently demagnetised, springing back to their original magnetic condition after a fault current event has occurred. The FCL greatly reduces the use of costly materials compared to the prior art and is also operable over a broad power range in single and three phase alternating power systems such that the FCL can be used in low power (i.e. more numerous) applications and high power applications. Further, the relative orthogonal arrangement between the DC/static magnetomotive force source and the varying magnetomotive force source protects the permanent magnets and aids saturation of the core material since the DC/static magnetomotive force can be more distributed without increasing the AC reluctance of the core material (since the relative permeability of the permanent magnet material is low and is seen by the AC magnetomotive force as a high reluctance element in the magnetic circuit).

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements except when specifically stated as such and vice-versa. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage.

Claims

1. A fault current limiter (FCL) comprising: a core structure comprising: i) a first magnetisable core member; ii) a second magnetisable core member, spaced from the first core

member; an AC magnetomotive force source configured to generate a varying magnetic flux in at least a portion of the first and second magnetisable core members; and; a plurality of static magnetomotive force sources being positioned to provide a magnetic circuit within at least part of the magnetisable core members; wherein the static magnetomotive force sources, comprise U shaped magnets arranged to bridge the first and second core members.

2. A FCL according to claim 1 , wherein the AC magnetomotive force source and the static magnetomotive forces source are relatively positioned to be orthogonal to each other.

3. A FCL according to claim 1 or claim 2, wherein the first and second magnetisable core members are spaced in the same plane.

4. A FCL according to any preceding claim, wherein the U shaped magnets are spaced in the longitudinal direction of the core members.

5. A FCL according to any preceding claim, wherein the AC magnetomotive force source comprises a coil wound around the first and second core members at a position intermediate the bridging U shaped magnets.

6. A FCL according to any preceding claim, wherein a plurality of pairs of U shaped magnets arranged to bridge the first and second core members.

7. A FCL according to any preceding claim, including a composite structure comprising a second pair of first and second core members, positioned adjacent the first pair of core members; the second pair of core members being provided with a set of respective U shaped magnets arranged to bridge the first and second core members.

8. A FCL according to claim 7, wherein the pairs of core members are provided adjacent one another, preferably spaced by a small gap.

9. A FCL according to claim 7 or claim 8 wherein the U shaped magnets for the respective pairs of cores are provided on opposed sides of the cores.

10. A FCL according to any of claims 7 to 9 wherein the U shaped magnets for the respective pairs of cores are matched positionally opposite one another.

11. A FCL according to any of claims 7 to 9 comprising a series of stacked or adjacently filed composite structures.

12. A FCL according to any preceding claim, wherein the magnetisable core members comprise an inner core ring and an outer core ring spaced from one another radially.

13. A FCL according to claim 12, wherein the U shaped magnets bridge the core members in a radial direction.

14. A FCL according to any preceding claim, wherein the magnetisable core members comprise a pair of parallel core rings spaced from one another axially.

15. A FCL according to claim 14, wherein the U shaped magnets bridge the core members in an axial direction.

16. A FCL according to any preceding claim, wherein the core members include air gaps, preferably at the location of contact of the poles of the U shaped magnets.

17. A FCL according to any preceding claim, wherein the static magnetomotive force sources are positioned to provide magnetic saturation in the core members.

18. A FCL according to any preceding claim, wherein static magnetomotive force sources are positioned to provide a bifurcated magnetic field in the core adjacent to the static magnetomotive force source in which the field direction in the same core member branches in opposed directions at the junction with the static magnetomotive force source.

19. A FCL according to any preceding claim, wherein the FCL comprises a plurality of stacked core structures each comprising a ring structure, the plurality being positioned stacked or in file one adjacent another and each comprising: a core structure comprising: i) a first magnetisable core member; and ii) a second magnetisable core member, spaced from the first core member.

20. A fault current limiter according to any preceding claim, wherein the AC magnetomotive force source is an AC coil having a longitudinal axis and to which is applied an AC current so as to produce an AC magnetic field.

21. A fault current limiter according to any preceding claim, wherein the U shaped magnet is formed from a permanent magnetic material having a default magnetisation.

22. A fault current limiter according to any preceding claim, wherein the U shaped magnets cause the core members to become magnetically saturated in normal operation.

23. A fault current limiter according to any preceding claim, wherein in a fault condition the magnitude of the AC magnetic field increases from a normal state value to overcome magnetic saturation in the core members.

24. A fault current limiter according to any preceding claim, wherein the AC coil obtains a higher inductance value when a fault condition occurs.

25. A fault current limiter according to any preceding claim wherein the core members respond respectively to the positive and negative halves of the AC current cycle received by the AC coil.

26. A fault current limiter according to any preceding claim, wherein the magnetisable cores contain at least one air gap.

27. A fault current limiter according to claim 24, wherein the at least one air gap in the magnetisable core or between 2 core components comprising a core member is positioned adjacent the surface of a pole face of the U shaped magnet.

28. A fault current limiter according to any preceding claim, wherein the static magnetomotive force source is a permanent magnet and the permanent magnet recovers its default magnetised state on cessation of a fault current event.

29. A fault current limiter according to any preceding claim, wherein the at least one magnetisable core member is foimed of strip steel or other ferromagnetic metal alloy, a soft ferrite material or an amorphous or nanocrystalline soft magnetic alloy.

30. A circuit including the fault current limiter of any preceding claim.

31. An alternating current power system including the fault current limiter of any preceding claim.