Field of the invention

[0001 ] The present invention relates to a fault current limiter (FCL).

[0002] The invention has been developed primarily for a compact high voltage fault current limiter and will be described with reference to that application. However, the invention is not limited to that particular field of use and is also suitable for low voltage, medium voltage, extra-high voltage and ultra-high voltage FCLs.

Background

[0003] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[0004] It is known to use an FCL in an electrical distribution system (EDS) to protect infrastructure - and in particular to protect a transformer in an EDS - during transient and prolonged fault conditions. Such FCLs are by necessity large devices for they carry the load current at the load voltage and are required to meet all the necessary standards for use at those levels of current and voltage.

[0005] Those FCLs that have been developed for commercial use typically include at least one coil of high temperature superconductor (HTS) that is disposed about a metal core of high magnetic permeability. For laboratory testing purposes there has also been use made of small-scale test devices constructed from more traditional conductors. The latter are, however, only intended for researching physical phenomena and are not rated for continuous use and are not constructed for practical use in an EDS.

[0006] A HTS FCL is typically categorised as an inductive FCL - where the load current flows through a metal coil - or a resistive FCL - where the current flows thought the superconductor coil. For an inductive FCL use is made of a HTS DC bias coil for biasing the core into saturation at least in the vicinity of the metal coil. This bias is such that a flow of current through the metal coil above a given threshold will result in the core being progressed out of saturation which, in turn, will change the inductance of the metal coil and have a limiting effect upon the load current.

[0007] The use of a HTS coil in an inductive HTS FCL is problematic due to it being more maintenance intensive, more expensive to purchase, more difficult to manufacture and transport, and it requires more overall footprint due to the need for associated cooling equipment. For resistive HTS FCLs there is the additional problem of controlling the resistance accurately to provide the required limiting, and dealing with the considerable thermal delays and inertia that are inherent in the available designs.

Summary of the invention

[0008] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0009] According to one aspect of the invention there is provided a fault current limiter including:

an input terminal for electrically connecting to a power source that provides an AC load current;

an output terminal for electrically connecting with a load circuit that draws the load current;

a core having: at least two magnetically saturable core members; and at least one solid flux return path for defining, in combination with the core members, one or more magnetic circuits;

an AC coil disposed about the core members for carrying the load current between the input terminal and the output terminal; and

a magnetic field generator having one or more permanent magnets disposed adjacent to the core members for magnetically biasing the members such that the AC coil moves from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

[0010] In a preferred embodiment the AC coil is continuous.

[001 1 ] In a preferred embodiment the core members are spaced apart the AC coil has at least two AC coil segments that are disposed about respective core members.

[0012] In a preferred embodiment the core members are substantially longitudinally coextensive and transversely spaced apart.

[0013] In a preferred embodiment the one or more permanent magnets are disposed in the return path.

[0014] In a preferred embodiment the fault current limiter includes a plurality of return paths for defining a plurality of magnetic circuits.

[0015] In a preferred embodiment the magnetic field generator also includes one or more DC coils. [0016] In a preferred embodiment the one or more DC coils are disposed about the core members.

[0017] In a preferred embodiment the fault current limiter includes a plurality of DC coils disposed about respective core members.

[0018] In a preferred embodiment the core members are defined by at least part of one or more core limbs.

[0019] In a preferred embodiment the core members are defined by at least part of two or more core limbs.

[0020] In a preferred embodiment the core members are defined by at least part of respective core limbs.

[0021 ] In a preferred embodiment the one or more core limbs extend beyond the core members.

[0022] In a preferred embodiment wherein the return path includes at least two return path limbs.

[0023] In a preferred embodiment the return path limbs are parallel.

[0024] In a preferred embodiment the return path limbs are longitudinally coextensive.

[0025] In a preferred embodiment the return path limbs are transversely spaced apart.

[0026] In a preferred embodiment the return path includes at least two yokes for connecting the core limbs and the return limbs.

[0027] In a preferred embodiment the core limbs and the return limbs extend longitudinally and are transversely spaced apart.

[0028] In a preferred embodiment the core limbs are disposed transversely between the return limbs.

[0029] In a preferred embodiment the one or more permanent magnets are disposed closely adjacent to the core members.

[0030] In a preferred embodiment the fault current limiter includes at least four permanent magnets respectively disposed at or adjacent to each end of each core member.

[0031 ] In a preferred embodiment, when the AC coil is in a low and a high impedance state, the core members respectively provides a high and a low magnetic reluctance.

[0032] In a preferred embodiment the return path provides a low magnetic reluctance when the AC coil is in a low and a high impedance state. [0033] In a preferred embodiment the return path provides a low DC magnetic reluctance.

[0034] In a preferred embodiment the return path provides a high AC magnetic reluctance.

[0035] In a preferred embodiment the return path includes a flux modifier for providing the low DC magnetic reluctance and the high AC magnetic reluctance.

[0036] In a preferred embodiment the flux modifier is disposed adjacent to at least one of the one or more permanent magnets.

[0037] According to a second aspect of the invention there is provided a fault current limiter including:

an input terminal for electrically connecting to a power source that provides an AC load current at a load frequency;

an output terminal for electrically connecting with a load circuit that draws the load current;

a core having: at least two magnetically saturable core members; and a flux return path for defining, in combination with the core members, one or more magnetic circuits, wherein the flux return path includes a first magnetic reluctance for a substantially constant first magnetic field and a second reluctance for a second magnetic field varying at or about the load frequency;

an AC coil disposed about the core members for carrying the load current between the input terminal and the output terminal and for generating one or more of the second magnetic fields in at least one of the one or more magnetic circuits; and

a magnetic field generator for magnetically biasing the core members with one or more of the first magnetic fields such that the AC coil moves from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

[0038] In a preferred embodiment the magnetic field generator includes one or more permanent magnets.

[0039] In a preferred embodiment the one or more permanent magnets are disposed in the return path.

[0040] In a preferred embodiment the magnetic field generator includes one or more DC coils. [0041 ] In a preferred embodiment at least one of the one or more DC coils is disposed about one or more of the core members.

[0042] In a preferred embodiment at least one of the one or more DC coils is disposed about the return path.

[0043] In a preferred embodiment the at least one of the one or more DC coils is adjacent to one or more of the core members.

[0044] In a preferred embodiment the return path includes at least two return limbs and the core members are defined by at least part of one or more core limbs.

[0045] In a preferred embodiment the return path includes at least two yokes for connecting the core limbs and the return limbs.

[0046] In a preferred embodiment the one or more permanent magnets are disposed immediately adjacent to the core limbs.

[0047] In a preferred embodiment the fault current limiter includes at least four permanent magnets respectively disposed at or adjacent to an end of each core limb.

[0048] In a preferred embodiment, when the AC coil is in a low and a high impedance state, the associated core limbs respectively provide a high and a low magnetic reluctance.

[0049] In a preferred embodiment the return path provides a low magnetic reluctance to the substantially constant magnetic field when the AC coil is in a low and a high impedance state.

[0050] In a preferred embodiment:

the load current is supplied at a load frequency; and

the one or more second magnetic fields vary at or about the load frequency.

[0051 ] According to a third aspect of the invention there is provided a method of fault current limiting including the steps of:

electrically connecting an input terminal to a power source that provides an AC load current;

electrically connecting an output terminal with a load circuit that draws the load current;

providing a core having: at least two magnetically saturable core members; and at least one solid flux return path for defining, in combination with the core members, one or more magnetic circuits; disposing an AC coil about the core members for carrying the load current between the input terminal and the output terminal; and

providing a magnetic field generator having one or more permanent magnets disposed adjacent to the core members for magnetically biasing the members such that the AC coil moves from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

[0052] According to a fourth aspect of the invention there is provided a method of limiting fault current including the steps of:

electrically connecting an input terminal to a power source that provides an AC load current at a load frequency;

electrically connecting an output terminal with a load circuit that draws the load current;

providing a core having: at least two magnetically saturable core members; and a flux return path for defining, in combination with the core members, one or more magnetic circuits, wherein the flux return path includes a first magnetic reluctance for a substantially constant first magnetic field and a second reluctance for a second magnetic field varying at or about the load frequency;

disposing an AC coil about the core members for carrying the load current between the input terminal and the output terminal and for generating one or more of the second magnetic fields in at least one of the one or more magnetic circuits; and

magnetically biasing the core members with one or more of the first magnetic fields such that the AC coil moves from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

[0053] According to a fifth aspect of the invention there is provided a method of manufacturing a fault current limiter, the method including the steps of:

constructing a core having: at least two magnetically saturable core members; and at least one solid flux return path for defining, in combination with the core members, one or more magnetic circuits;

disposing an AC coil about the core members for carrying a load current between at least one input terminal and at least one output terminal; and

disposing one or more permanent magnets adjacent to the core members for magnetically biasing the members such that, in use, the AC coil moves from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

[0054] According to a sixth aspect of the invention there is provided a method of manufacturing a fault current limiter, the method including the steps of:

an input terminal for electrically connecting to a power source that provides an AC load current at a load frequency;

an output terminal for electrically connecting with a load circuit that draws the load current;

constructing a core having: at least two magnetically saturable core members; and a flux return path for defining, in combination with the core members, one or more magnetic circuits, wherein the flux return path includes a first magnetic reluctance for a substantially constant first magnetic field and a second reluctance for a second magnetic field varying at or about a predetermined load frequency;

disposing an AC coil about the core members for carrying a load current between at least one input terminal and at least one output terminal and for generating one or more of the second magnetic fields in at least one of the one or more magnetic circuits; and

disposing a magnetic field generator adjacent to the AC coil for magnetically biasing the core members with one or more of the first magnetic fields such that the AC coil moves from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

Brief description of the drawings

[0055] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a three phase fault current limiter (FCL) according to an embodiment of the invention that is disposed in an electrical distribution system (EDS);

Figure 2 is a schematic cross-sectional view taken along a horizontal plane of the FCL of Figure 1 illustrating the three separate modular single phase FCLs included within the three phase FCL;

Figure 3 is a schematic cross-sectional view of one of the single phase FCLs taken along line 3-3 of Figure 2; Figure 4 is a cutaway front perspective view of one of the single phase FCLs included within the three phase FCL of Figure 1 ;

Figure 5 is a cutaway cross-sectional front view of the single phase FCL of Figure 4;

Figure 6 is a front perspective view of the core of the single phase FCL of Figure 4;

Figure 7 is a schematic view of a core limb of the single phase FCL of Figure 4;

Figure 8 is a sectional view of a single phase FCL similar to that FCL of Figure 3, but having a racetrack DC coil, where the section through the single phase FCL is taken along a line corresponding to the line 8-8 as illustrated in Figure 5;

Figure 9 is a front perspective view of an alternative core to that illustrated in Figure 7, which is segmented;

Figure 10 is a cutaway front perspective view of a single phase FCL similar to the single phase FCL of Figure 4, but with differently proportioned core limbs and return limbs;

Figure 1 1 is a cutaway front view of the single phase FCL of Figure 10;

Figure 12 is a sectional view of a single phase FCL similar to that FCL of Figure 10 but having a racetrack DC coil, where the section through the single phase FCL is taken along a line corresponding to the line 12-12 as illustrated in Figure 1 1 ;

Figure 13 is a perspective front view of the core of the single phase FCL of Figure 10;

Figure 14 is a schematic cross-sectional view taken along a horizontal plane of a three phase FCL of another embodiment, which includes two separate modular cores located within a single tank;

Figure 15 is a cross-sectional of one of the cores of the FCL of Figure 13 taken along line 15-15;

Figure 16 is a cutaway front perspective view of one of two like modular units used collectively to define a three phase FCL similar to the FCL of Figure 14 although with the individual units including respective discrete tanks;

Figure 17 is a cutaway front view of the modular unit of Figure 16;

Figure 18 is a sectional view of the modular unit similar to the modular unit of Figure 17 but having a racetrack DC coil, where the section through the modular unit is taken along a line corresponding to the line 18-18 as illustrated in Figure 17; Figure 19 is a front perspective view of the core of the modular unit of Figure 16;

Figure 20 is a view similar to Figure 18 but illustrating the location of two modular units when in use;

Figure 21 is a schematic cross-sectional view taken along a horizontal plane of another three phase FCL similar to the FCL of Figure 1 having three separate modular single phase FCLs;

Figure 22 is a sectional view of one of the single phase FCLs of Figure 20 taken along line 22-22;

Figure 23 is a schematic cross-sectional view taken along a horizontal plane of yet another three phase FCL similar to the FCL of Figure 1 having three separate modular single phase FCLs;

Figure 24 is a sectional view of one of the single phase FCLs of Figure 22 taken along line 24-24;

Figure 25 is a cutaway front perspective view of one of the single phase FCLs included within the three phase FCL of Figure 23;

Figure 26 is a cutaway cross-sectional front view of the single phase FCL of Figure 24;

Figure 27 is a sectional view of a single phase FCL similar to that FCL of Figure 24 but having a racetrack DC coil, where the section through the single phase FCL is taken along a line corresponding to the line 27-27 as illustrated in Figure 24;

Figure 28 is a front perspective view of the core of the single phase FCL of Figure 24;

Figure 29 is a cutaway front perspective view of another single phase FCL suitable for use within a three phase FCL such as the FCL of Figure 23;

Figure 30 is a cutaway cross-sectional front view of the single phase FCL of Figure 29;

Figure 31 is a sectional view of a single phase FCL similar to that FCL of Figure 30 but having a racetrack DC coil, where the section through the single phase FCL is taken along a line corresponding to the line 31 -31 as illustrated in Figure 30;

Figure 32 is a front perspective view of the core of the single phase FCL of Figure 29; Figure 33 is a schematic cross-sectional view taken along a horizontal plane of a three phase FCL of another embodiment, which includes two separate modular cores located within a single tank; and

Figure 34 is a schematic cross sectional view of one of the modular units of Figure 33 taken along line 34-34.

Detailed description

[0056] The following description and Figures make use of reference numerals to assist the addressee understand the structure and function of the illustrated embodiments. Like reference numerals are used in different embodiments to designate features having the same or similar function and/or structure.

[0057] The drawings need to be viewed as a whole and together with the associated text in this specification. In particular, some of the drawings selectively omit features to provide greater clarity about the specific features being described. While this is done to assist the reader, it should not be taken that those features are not disclosed or are not required for the operation of the relevant embodiment. That is, in some embodiments not all features required for an operational FCL are explicitly illustrated for the sake of more clearly exhibiting other features of those embodiments. By way of example only, in Figure 2 there is no illustration of, amongst other things, the AC terminals, the DC terminals and the DC coils. Such features will be understood by the skilled addressee to be required for normal operation of the FCL and their inclusion, given the benefit of the teaching herein, would be a matter of routine design.

[0058] Where use is made of the term "an embodiment" in relation to a feature, that is not to be taken as indicating there is only one embodiment in which that feature is able to be used, or that that feature is not able to be used in combination with other features not illustrated as being in the same embodiment. It will be appreciated by the skilled addressee that while some features are mutually exclusive within a single embodiment, others are able to be combined.

[0059] Referring to Figure 1 , there is illustrated an electrical distribution system (EDS) 1 including a three phase transformer 2 for providing a predetermined maximum AC operating current I _M AX at a predetermined AC operating voltage V _T . Transformer 2 includes three first input terminals 3 (only one shown) for connecting with a three phase electrical power source in the form of a power station 4. The power station provides an AC operating voltage V _s . The transformer also includes three first output terminals 5 (only one shown) that provide a load current LOAD at the predetermined operating voltage V _T . System 1 includes a three phase fault current limiter in the form of FCL 6 that has a housing 7 from which extend three spaced apart second input terminals 10 (only one shown) for electrically connecting to respective terminals 5 of transformer 2. Also extending from housing 7 are three spaced apart second output terminals 1 1 (only one shown) for electrically connecting the FCL with a downstream load circuit 9, which draws load current L _OAD - FCL is an inductive type fault current limiter that moves from a low impedance state to a high impedance state in response to L _OAD exceeding l _MAX .

[0060] In other embodiments, FCL 6 is responsive to one or more other characteristics of L _OAD for moving from the low impedance state to the high impedance state.

[0061 ] The line-to-line voltage V _s in this embodiment is 33 kV RMS AC at 50 Hz. However, in other embodiments different operational voltages or frequencies are used. Examples of commonly used voltages include 132 kV, 66 kV, 33 kV, 1 1 kV and many other voltages that will be known to those skilled in the art. The other common operating frequency is 60 Hz. The voltage provided by power station 4 is greater than 33 kV and it will be appreciated that there is at least one substation intermediate station 4 and transformer 2 for providing the voltage V _s .

[0062] In other embodiments the FCL is a lower voltage FCL installed in a substation or other lower voltage facility. In further embodiments, FCL 6 is configured for operation at higher voltages and currents.

[0063] It will be appreciated that EDS 1 includes a plurality of distributed transformers (not shown) and a plurality of appropriately rated and distributed FCLs for limiting respective fault currents at different parts of the EDS.

[0064] The internal structure of FCL 6 will now be described in more detail with reference to Figures 2 to 7, where corresponding features are denoted by corresponding reference numerals. Referring first to Figure 2, there is illustrated a schematic cross-sectional view taken along a horizontal plane of FCL 6. More particularly, FCL 6 includes three like, separate, elongate, transversely co-extending, generally parallel, and longitudinally spaced apart modular single phase FCLs 21 , 22 and 23. The FCLs 21 , 22 and 23 are associated with respective phases of the three phase power supply and operate in a like way with those respective phases. Given this, the following description will be limited to FCL 21 on the basis that the skilled addresses will understand that FCL 22 and 23 operate similarly but for respective other phases.

[0065] FCL 21 includes, as shown in Figure 1 , input terminal 10 for electrically connecting to terminal 5 of transformer 2. This transformer, in this embodiment, is a power source that provides an AC load current. Output terminal 1 1 electrically connects with a load circuit downstream of FCL 21 that draws the load current. As best shown in Figures 3 to 6, FCL 21 includes a core 29 having two spaced apart, magnetically saturable, coextensive and generally cylindrical and parallel core members 31 and 32. Core 29 also includes two solid flux return paths 33 and 34 for defining, in combination with members 31 and 32, two magnetic circuits. An AC coil includes two series connected helically wound coil segments 35 and 36 that are respectively disposed about members 31 and 32 for carrying the load current between terminal 1 0 and terminal 1 1 . A magnetic field generator including four spaced apart permanent magnets 37, 38, 39 and 40 are disposed adjacent to members 31 and 32 for in part magnetically biasing those members such that respective segments 35 and 36 move from a low impedance state to a high impedance state in response to one or more characteristics of the load current.

[0066] The magnetic field generator provides a magnetic field that is directed upwardly through both members 31 and 32. In other embodiments the magnetic field is directed downwardly through both members. Segments 35 and 36 are wound in an opposite sense such that the half cycles of the load current will be individually impacted should I _M AX be exceeding in that respective half cycle. In other embodiments use is made of two magnetic fields for providing the magnetic bias to respective segments.

[0067] Specific reference is now made to Figure 7 where there is illustrated schematically member 31 which is not to scale. More particularly, member 31 is defined by a portion (which is indicated in the Figure with shading) of a core limb 41 of core 29. That shaded portion of limb 41 underlies and is adjacent to segment 35. It will be appreciated that member 32 is similar to member 31 .

[0068] Limb 41 is elongate and constructed from laminated steel. This limb extends between a first end 43 and a second end 44 that both protrude beyond segment 35 and member 31 . In this embodiment member 31 and segment 35 are centred on limb 41 . However, in other embodiments member 31 and segment 35 asymmetrically extend along limb 41 .

[0069] It will be appreciated that core 29 includes a further like core limb 45, where segment 36 is disposed about core limb 45, and member 32 is defined therefore by the portion of core limb 45 that directly underlies segment 36.

[0070] Core 29, the AC coil (defined by segments 35 and 36) and the associated mountings and electrical interconnections (not shown) are contained within a sealed metal tank 47. This tank is of a generally prismatic form and provides physical protection to the components housed within and added structural rigidity to FCL 21 , particularly as required during fault conditions. The tank is also a reservoir for a predetermined volume of transformer oil 48 that acts both as a heat exchange fluid to aid cooling of the components within the tank and to provide a higher dielectric constant than air to reduce the electrostatic clearances required in the design of FCL 21 . That is, housing 7, in this embodiment, is defined collectively by the three separate tanks 47. In further embodiments housing 7 includes further elements such as a mesh enclosure within which tanks 47 are located.

[0071 ] In other embodiments, tanks 47 of FCLs 21 , 22 and 23 are fluidically interconnected and share a common volume of transformer oil. In further embodiments, use is made of a common tank rather than separate tanks for the individual single phase FCLs.

[0072] Tank 47 includes a base 51 , two broad and opposed front and rear walls 52 that extend upwardly from base 51 , two elongate and opposed walls 53 that extend upwardly from base 51 and between walls 52, and a top 54 that is engaged continuously with the upper peripheral edge of all the walls. Tank 47 includes two heat exchanger elements 57 and 58 that extend transversely outwardly from respective walls 53 and which have upper and lower pipes 59 for fluidically connecting elements 57 and 58 with the oil within tank 47 and for allowing that oil to circulate through the elements and tank due to the operation of convection currents.

[0073] A sealed closure (not shown) is provided in top 54 for allowing a partial vacuum to be applied to tank 47 to assist with moisture removal. The transformer oil is then passed through the closure into the tank and the closure moved to a closed position to seal the tank.

[0074] As best shown in Figure 6, members 31 and 32 are part of core limbs 41 and 45 and are constructed from laminated high magnetic permeability steel. As mentioned above, members 31 and 32 are that part of core limbs 41 and 45 in the zone directly within coil segments 35 and 36. That is, limbs 41 and 45 both extend beyond that zone and both terminate in opposite ends 43 and 44. Those parts of the magnetic circuits that are not directly under segments 35 and 36 - that is, those parts of the magnetic circuits that are not in the zone refereed to - are part of the return path.

[0075] Core limbs 41 and 45 are generally cylindrical and extend along respective parallel and spaced part axes. In other embodiments, limbs 41 and 45 are coaxial. An example of such other embodiments includes the magnetic circuits being arranged in a stack, with one being disposed on top of the other. [0076] In further embodiments (not shown) limbs 41 and 45 have a differently shaped cross section, such as a generally rectangular cross section, or a generally square cross section, or a generally oval cross section, or a complex cross section. In other embodiments, limbs 41 and 45 are profiled or tapered, in that the cross section of the limbs varies along the length of the limbs. Preferentially, limbs 41 and 45 are of like shape so as to substantially uniformly bear the forces that occur during a fault.

[0077] In other embodiments, limbs 41 and 45 are other than parallel.

[0078] Core 29 includes two return path limbs 61 and 62 that are generally parallel and coextensive, and which are spaced apart to flank limbs 41 and 45. Limbs 61 and 62 have substantially uniform and square cross sections and are constructed from laminated mild steel. In other embodiments use is made of high magnetic permeability steel.

[0079] An upper yoke 65 and an opposed and spaced apart generally parallel lower yoke 66 coextend transversely and engage with the upper and lower ends of limbs 61 and 62. Yokes 65 and 66 have a substantially identical and generally square cross section. The yokes have substantially uniform and square cross sections and are constructed from laminated mild steel. In other embodiments use is made of high magnetic permeability steel.

[0080] Core 29 also includes four non-laminated mild steel blocks 69 that extend between the permanent magnets and yokes 65 and 66. These blocks act as "flux blockers" to the time varying magnetic field in the two magnetic circuits that is generated by the flow of I LOAD through segments 35 and 36. That is, I LOAD is supplied at a load frequency, which in this embodiment is 50 Hz. The AC coil (that is, segments 35 and 36) induce in members 35 and 36 and the respective magnetic circuits a consequential time varying magnetic field having a frequency of 50 Hz. The return paths provided by the magnetic circuits, due to blocks 69, provide a first magnetic reluctance for the consequential magnetic field and a second magnetic reluctance for the substantially constant magnetic field provided by the magnetic field generator. The first reluctance is greater than the second reluctance due to the skin effect that takes place within blocks 69. For it will be appreciated that the magnetic flux at 50 Hz will be concentrated in an annular zone of blocks 69 that extends from the periphery of the blocks to a depth of about 1 mm. Whereas the magnetic flux at zero frequency will be spread across all the effective cross sectional area of blocks 69. In this embodiment, the diameter of blocks 69 is about 100 mm - making the effective cross sectional area about 7,850 mm ² - whereas the effective cross sectional area seen by the consequential time varying magnetic field is about 156 mm ² , or less than about 2% of the effective cross sectional area. In other embodiments blocks 69 have other diameters or shapes. In those embodiments the effective cross sectional area seen by the consequential time varying magnetic field is typically kept to less than about 5% of the effective cross sectional area.

[0081 ] In other words, the return paths have: a DC magnetic reluctance; and an AC magnetic reluctance that is higher than the DC magnetic reluctance. In some embodiments this is achieved by employing a bulk material - that is, a non-laminated material - such that the magnetic field at the AC load frequency is forced to return through the skin depth of the material. In other embodiments different structures are used.

[0082] The solid flux return path 33 for the magnetic circuit including member 31 is defined sequentially by end 44 of limb 41 , magnet 38, adjacent block 69, the adjacent portion of upper yoke 65, limb 61 , the adjacent portion of lower yoke 66, the adjacent block 69, magnet 37 and end 43 of limb 41 . Whereas the solid flux return path 34 for the magnetic circuit including member 32 is defined sequentially by end 44 of limb 45, magnet 40, adjacent block 69, a portion of upper yoke 65, limb 62, a portion of lower yoke 66, the adjacent block 69, magnet 39 and end 43 of limb 45.

[0083] Although the two magnetic circuits are physically joined by common yokes 65 and 66, the magnetic operation of those circuits is, for practical purposes, separate. However, the integral construction is used, in this embodiment, to gain specific advantages in manufacturing of the FCL. That is, the core structure as shown, having common yokes in the two magnetic circuits, is very similar to manufacture to cores used in transformers. This allows FCL 21 to be manufactured in an existing facility by personnel who are skilled in the transformer art whereas conventional commercially available FCLs require specialist manufacturing facilities and specialist manufacturing skills. It will be appreciated by those skilled in the art, with the benefit of the present teaching, that a number of changes to the standard transformer manufacturing process are required to accommodate the manufacture of the FCLs of the embodiment. For example, for the relevant embodiments, there is a need to accommodate the use of permanent magnets and other features of FCL 21 . However, the heavy construction related to the manufacture of core 29 and coil segments 35 and 36 is familiar to personnel accustomed to the manufacture of transformers, even though those personal will likely be unfamiliar with both fault current limiters per se and the usual methods of manufacture for such fault current limiters.

[0084] In another embodiment use is made of the segmented core 70 illustrated in Figure 9. That is, the two magnetic circuits are separated both functionally and physically.

[0085] Referring again to Figures 4 and 5, the magnetic field generator of FCL 21 includes, in addition to magnets 37, 38, 39 and 40, two series connected helically wound DC coils 75 and 76. These DC coils are disposed about segments 35 and 36 for constructively cooperating with the magnets to collectively provide a substantially constant steady state magnetic field that, during normal operating conditions, biases members 31 and 32 into magnetic saturation. Coils 75 and 76, when formed, are wound about respective formers in the same sense - although in use are disposed along respective parallel and spaced apart axes - such that the magnetic fields are both similarly directed.

[0086] Coils 75 and 76 are disposed coaxially with but radially outwardly from the respective coil segments 35 and 36 and extend along almost the full length of those segments. In other embodiments, coils 75 and 76 are of lesser relative length. In further embodiments, coils 65 and 66 are segmented. An example of such a further embodiment including a Helmholtz coil configuration having two or more axially spaced apart DC coil segments.

[0087] FCL 21 includes two DC terminals 77 and 78 that extend outwardly from front wall 52 for connecting with a DC power supply (not shown) which energises coils 75 and 76 to provide the required steady state magnetic bias to complement that provided by magnets 37, 38, 39 and 40.

[0088] In other embodiments use is made of the permanent magnets only to provide the required magnetic bias to members 31 and 32.

[0089] In other embodiments, use is made of HTS coils to complement the magnetic bias provided by the permanent magnets.

[0090] It will be appreciated that having the magnetic field generator that is open to being constructed from one or more different forms of magnetic field generation components provides the designer of an FCL with considerable flexibility to accommodate different footprints and other dimensions, as well as being able to take account of the total lifetime cost of operating the FCL. Preferentially in the embodiments, the design is limited to one or both of permanent magnets and DC coils due to the ease of inclusion of these into traditional transformer manufacturing processes.

[0091 ] In Figure 2 coils 75 and 76 have not been illustrated purely for the sake of clarity.

[0092] Reference is now made to Figure 8 where there is illustrated a further embodiment of the invention in the form of an FCL 79. For this FCL the magnetic field generator includes, in addition to magnets 37, 38, 39 and 40, a single racetrack DC coil 80 that extends about both coil segments 35 and 36. Coil 80, during manufacture, is wound about an external surface of a tubular former 81 having the desired "racetrack" cross section. Former 81 , together with wound coil 80, is symmetrically disposed about segments 25 and 36. The former is made of fibreglass and includes an internal, integrally formed, continuous metal foil layer for acting as an electromagnetic shield between coil 80, on the one hand, and segments 35 and 36, on the other.

[0093] In some embodiments FCL 79 includes a separate electromagnetic shield (not shown) in the form of two cylindrical tubular shields that are respectively disposed about segments 35 and 36. In further embodiments, a single shield is used that is disposed about both segments 35 and 36 and which is formed separately from former 81 .

[0094] The shield or shields help reduce the risk of triggering immediately upstream protection circuitry in the associated EDS by reducing the time derivative of the load current during fault conditions.

[0095] Magnets 37, 38, 39 and 40 are disposed immediately adjacent to the respective ends of limbs 41 and 45, and closely adjacent to the ends of members 31 and 32. This is done: to gain optimum benefit from the constant magnetic field these magnets contribute to the magnetic bias of members 31 and 32; and to contain the height of core 29 and hence FCL 79. To that same end, the length of the exposed ends 43 and 44 of limbs 41 and 45 are kept preferentially small. However, it will be appreciated that to facilitate the manufacture of FCL 79, it is desirable for ends 43 and 44 to extend sufficiently beyond members 31 and 32 to offer a convenient site to secure limbs 41 and 45 to the adjacent magnets and/or to yokes 65 and 66. For, during fault current conditions, there are considerable physically stresses experienced within core 29 and, as such, all the separate elements of the core are, as a matter of design, fixedly secure at least to adjacent elements, and more often held in relative position under a compressive load.

[0096] As a matter of practice, segments 35 and 36 have an axial length of L _ax and ends 43 and 44 extend beyond those coils by at least 1 .01 x L _ax . It has been found that if the extent is greater than this, that the permanent magnets will not be exposed to the saturation of core 29 that occurs during a fault current. The saturation during a fault current will be for half of the cycle in the opposite direction to that provided by the magnets. Accordingly, by keeping the magnets out of the zone experiencing that saturation, the magnets are protected from demagnetisation.

[0097] Moreover, as the segments are of laminated high magnetic permeability material, this too assists in favourably directing the magnetic flux provided by the magnets.

[0098] Segments 35 and 36, during manufacture, are wound about respective tubular and cylindrical formers 85 and 86. These formers are also constructed from fibreglass. Once wound, the formers, together with the respective segments, are disposed about limbs 41 and 45 to thereby respectively define members 31 and 32.

[0099] It will also be appreciated that any coils used in the above-described FCLs are, once disposed about the relevant limb, maintained in position under a compressive load. This is to ensure that the coils (be they AC coils or coil segments or DC coils or otherwise) remain practically fixed even during the considerable physical stresses that are experienced by these coils during a fault condition.

[00100] Reference is now made to Figures 10, 1 1 and 13 which illustrate a single phase FCL 91 and where corresponding features are denoted by corresponding reference numerals. In summary, core 92 of FCL 91 makes use of a greater mass of steel in the return path to provide a lower reluctance for that return path than for FCL 21 . Whilst this increases the overall dimensions of FCL 19 relative to FCL 21 , it also reduces the ampere turns that have to be provided by coils 75 and 76 to affect the required magnetic bias of members 31 and 32. Accordingly, the DC current flowing through coils 75 and 76 is able to reduced relative to that which would otherwise be required, or less turns need to be included in those DC coils helping to reduce the material cost of the FCL. The design of FCL 91 is predisposed to installation where there is a relatively high energy cost (for example, in remote locations) and the overall cost of the FCL for its operational lifetime is better managed by accepting a slightly higher upfront material cost - in the form of the additional mass of steel that is required.

[00101 ] Reference is now made to Figure 12 where there is illustrated a further single phase FCL 95 having a single racetrack coil 96. It is also illustrated that limbs 61 and 62 have generally rectangular cross section rather than the generally square cross section of the corresponding limbs of FCL 79. The area of the cross sections in both embodiments is the same and hence the return paths offer substantially the same magnetic reluctance. However, in FCL 95, the design has been optimised for a given footprint. That is, for the installation into which FCL 95 is to be retrofitted the design requirements are for a reduced longitudinal dimension, which has been achieved whilst still allowing many others of the components used within FCL 95 to remain the same as those used within FCL 79.

[00102] Reference is now made to Figures 14 and 15 where there is illustrated a three phase FCL 101 having two like, parallel, spaced apart, longitudinally coextensive modular core units 102 that are contained within a single unitary tank 47. The core 29 of each unit 102 includes, in addition to limbs 41 and 45, a third like limb 103 that is disposed centrally intermediate limbs 41 and 45. Limbs 103 are received within a respective segment 35 and 36 of an AC coil through which flows one of the three phases of L _OAD - Another of the two phases flows through segments 35 and 36 that are disposed about limbs 41 of the respective modular units. The third of the three phases flows through segments 35 and 36 that are disposed about limbs 45 of the respective modular units.

[00103] As best shown in Figure 19, core 29 also includes two additional permanent magnets 105 and 106 and blocks 69.

[00104] In other embodiments, such as that illustrated in Figure 16 and 17, an FCL 107 includes two units 102 that are disposed in respective tanks 47. As also illustrated in those Figures, FCL 107 includes a third DC coil 108 for contributing the magnetic bias of the underlying limb and member and a further DC terminal 109 for allowing a DC voltage to be applied to coil 108.

[00105] Referring to Figure 18 there is illustrated a modular unit 1 10, similar to unit 102, but having a single racetrack DC coil 1 1 1 . Two units 1 10 are used, as illustrated in Figure 20, to provide the required current limiting in a three phase power system.

[00106] In the embodiments illustrated in Figures 14 to 20, each modular unit includes one AC coil segment for each phase of the three phase system. This design allows for a different footprint than would be achieved with a three phase FCL having three modular cores as occurs in some other embodiments. Moreover, it provides flexibility in allowing for the transport of the FCL, once constructed. For example, for high voltage and ultra high voltage applications the inclusion of the FCL within a single tank results in a very large and heavy device that is difficult to transport, handle and install. Through use of the modular approach, it is possible to have the FCL transported more easily, whilst not requiring any significant change in installation procedures. That is, notwithstanding the FCL is able to be transported in a number of separate pieces, the assembly and installation of those pieces at the relevant site is not complex.

[00107] Given the benefit of the teaching herein it will be appreciated that the above embodiments, in combination, allow a designer of a three phase FCL to select from a single tank, two tanks, or three tanks, so as to optimise the design for the given site, the associated logistics and other factors associated with installing and operating an FCL at that site.

[00108] Reference is now made to Figures 21 and 22 where there is illustrated a further FCL 121 . The return paths provided by core 29 omit magnets 37 and 39. That is the magnetic field generator is comprised of magnets 38 and 40 and coils 75 and 76 that are coaxial with respective coils 35 and 36 but axially offset. In the earlier described embodiments coils 75 and 76 are disposed radially outwardly of and axially overlying coils 35 and 36. In this embodiment, however, an axial asymmetry is provided and coils 75 and 76 overlie respective magnets 38 and 40.

[00109] Coils 75 and 76 are disposed intermediate yoke 65 and respective coil segments 35 and 36. In other embodiments, coils 75 and 76 are disposed intermediate yoke 66 and respective coils 35 and 36. In still further embodiments, magnets 38 and 40 are omitted and magnets 37 and 39 included. That is, the coils 75 and 76 need not overlie the permanent magnets, although this design feature is often included to better contain the height of the resulting FCL.

[001 10] Reference is now made to Figures 23 and 24 where there is illustrated a further three phase FCL 122 having three like single phase FCLs 123, 124 and 125. FCL 123 is illustrated in more detail in Figures 25, 26, 28 and 29. Whilst only FCL 123 will be described in more detail, it will be appreciated that FCL 124 and 125 include like features, are constructed similarly, and operate similarly.

[001 1 1 ] FCL 123 includes eight flux modifier blocks 130 that are arranged in pairs to sandwich magnets 37, 38, 39 and 40. Blocks 130 are a non-laminated solid (or bulk) material of preferentially relatively high magnetic permeability. Suitable materials include transformer steel, mild steel, ferromagnetic powder, other ferromagnetic material and any other material with a high magnetic permeability. In other embodiments use is made of a combination of such materials.

[001 12] These blocks 130 function similarly to blocks 69 in that they ensure that the magnetic reluctance of the associated return path is lower for the substantially constant magnetic field provided by the magnetic field generator than for a time varying magnetic field such as that induced in the core by the current flowing through the coil segments. This, in turn, reduces the impact upon the permanent magnets of the time varying magnetic field which, during fault conditions, can be strongly opposed to the magnetic field provided by the permanent magnets. It will be appreciated that blocks 130 provide for a greater magnetic reluctance to the time varying magnetic field than is the case of only blocks 69 are used. In some embodiments, blocks 69 are laminated and blocks 130 provide all if not substantially all of the flux modification in the relevant return path.

[001 13] In other embodiments less than eight blocks 130 are used. For example, in one such embodiment only four of blocks 130 are used, one from each of the pairs.

[001 14] In further embodiments, some but not all of the magnets are sandwiched by blocks 130. For example, in one such embodiment only magnets 38 and 40 are sandwiched between adjacent blocks 130. [001 15] In further embodiments one or more of the blocks is disposed within the return path, but not adjacent to segments 35 and 36. For example, in some embodiments one or more of the blocks is disposed in one of the yokes, or in one of the outer limbs. More usually, however, the blocks are disposed between two other elements. For example, between a yoke and an adjacent end of a limb.

[001 16] The blocks mentioned above are of a solid substantially uniform material. In other embodiments one or more of the blocks include a metal support structure that is filled with a ferromagnetic powder. In other embodiments, the blocks include a stack of laminations where each lamination extends substantially normal to the flux path. In some of these other embodiments, at least one of the laminations are of one material, while at least one other of the laminations is of another material.

[001 17] Referring to Figure 27 there is illustrated a further single phase FCL 133 that is similar to FCL 123, but which includes a racetrack DC coil 134.

[001 18] Referring to Figures 30 to 32 there is illustrated a further single phase FCL 140 that is similar to FCL 133, but which includes limbs 61 and 62 which have a different cross- sectional shape to the corresponding limbs in FCL 133. In this embodiment the cross section of limbs 61 and 62 has been optimised for a different installation having a different footprint and different operational lifetime cost requirements.

[001 19] Reference is now made to Figures 33 and 34 where these is illustrated a three phase FCL 150. This FCL is similar to FCL 101 of Figures 14 and 15, but includes sixteen blocks 130 that are arranged in pairs to sandwich the permanent magnets. In other embodiments less than all of the permanent magnets are sandwiched by blocks 130.

[00120] In the above FCLs, during normal operation, have the core members - for example, members 31 and 32 - magnetically saturated due to the application by the magnetic field generator of a magnetic bias to those members. As a result, the core that is underlying the AC coils - that is, which underlies the AC coil segments - will approximate an air core and those segments will provide a relatively low reluctance to the flow of the load current through those coils. It will also be appreciated that the flow of the load current through the coil segments generates respective time varying magnetic fields in the core that during respective half cycles constructively and destructive interfere with the substantially constant magnetic field provided in the core by the magnetic field generator.

[00121 ] The magnetic field generator is tuned such that when the peak load current exceeds the predetermined current maximum the time varying magnetic field in a half cycle of a coil segment is sufficiently large to progress the underlying member, during one half cycle, to move toward magnetic desaturation. This instantaneously increases the impedance of the coil segment and hence limits the load current to the predetermined maximum current. As the load current falls below the predetermined maximum current, the FCL will automatically and instantaneously resume normal non-fault operation as the magnetic bias provided by the magnetic field generator will continue to ensure that the core members are again magnetically saturated. In this way, the AC coils move between a low and a high impedance state in response to the load current equalling or falling below the predetermined maximum current.

[00122] Furthermore, when the AC coil is in a low and a high impedance state, the core members respectively provides a high and a low magnetic reluctance. However, at all times, the return paths are not saturated and provide a substantially constant and low magnetic reluctance. As mentioned above, the reluctance provided by the return paths to a substantially constant magnetic field is lower than that provided for a time varying magnetic field.

[00123] It will be appreciated that two coil segments are used in the AC coils so that current limiting occurs in both half cycles of the AC supply. In some embodiments the AC coil or coils are continuous and the magnetic field generator is differently arranged to allow current limiting in both half phases.

[00124] The permanent magnets are each constructed from a plurality of individual magnet modules, where the number of modules used depends upon the magnetic field strength required from the resultant permanent magnets and the available space in which those modules can be placed.

[00125] The permanent magnets are disposed closely adjacent to the core members both to better contribute to the saturation of those members and to assist in containing the height of the FCL.

[00126] The conventional design wisdom is that any permanent magnet should be mounted as far away as possible from the AC coil to protect it from demagnetisation during a fault. This in turn requires a large and powerful permanent magnet to provide the required magnetic field strength to result in the core members being saturated during normal operation. This convention wisdom has resulted in very little use being made of permanent magnets in both laboratory FCLs and commercial FCLs. It has been found by the present inventors it is possible to dispose the permanent magnets closely adjacent to the core members. It has been found by developing and using sophisticated time domain transient finite element analysis techniques that the saturation of the core elements by the time varying fault current is limited mostly to the core members. That is, the saturation is substantially limited to the zone directly underlying the AC coil segments. Whilst the saturation does extend beyond that zone, it has found to be substantially zero at a distance of 1 % of the length of the coil segment.

[00127] The invention is also applicable to other embodiments and configurations of FCLs. For example, the invention is able to be applied in the embodiments disclosed in Australian Patent Application No. 201 1905130 filed on 9 December 201 1 . The disclosure within that earlier patent specification is incorporated herein by way of cross reference.

[00128] The major advantages of the embodiments of the invention include:

• Permanent magnets are able to be used.

• A combination of permanent magnets and DC coils are available to provide a bias magnetic field.

• The low magnetic reluctance return path in the magnetic circuit that remains out of saturation during all conditions.

• The ability to make use of standard transformer manufacturing techniques.

• Flexibility in design. That is, the design principles are applicable to a broad range of installations and are able to be optimised for one or a combination of factors whilst still making use of more traditional transformer manufacturing techniques and methodologies.

• The use of a low magnetic reluctance return path for the substantially steady state magnetic field reduces the number of ampere turns required from the DC coil or the volume (and cost) of permanent magnets.

[00129] The limbs and yokes used in the embodiments are able to be configured from components familiar to transformer manufacturers and assembled with techniques familiar to those manufacturers. For example, limbs 61 and 62 and yokes 65 and 66 have a square or rectangular cross section, whilst limbs 41 and 45 have a circular cross section. Typically the cross section of limbs 61 and 62 and yokes 65 and 66 will be the same in value and shape. In addition, it has been found by the inventor that the cross section of limbs 41 and 45 is preferentially at least about 0.8 x the cross section of limbs 61 and 62, and no more than about to 1 .5 x the cross section of limbs 61 and 62.

[00130] The term "footprint" as used herein, unless otherwise specified, should be understood as the underlying surface area required to accommodate a structure or device. The footprint available to accommodate an FCL is often a critical design parameter as it is common to retrofit an FCL in an existing electrical sub-station or other facility where the available surface area is limited due to the need to maintain safe physical separation of disparate pieces of equipment that are operating at high voltages. The footprint is often expressed in terms of available area on a surface. The specification can be in terms of an absolute maximum area or footprint on the surface, or an area or footprint having one or more of a maximum length and a maximum breadth on the surface. It will be appreciated that the term "footprint" can also be interpreted as meaning the area taken up by some object, or the space or area of a 2-dimensional surface enclosed within a boundary. That is, the shape of the footprint need not be regular and is, in some embodiments, defined by a complex or irregular shape.

[00131 ] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[00132] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[00133] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[00134] It is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected", along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other. [00135] Those skilled in the art will recognise that these are examples applied to specific designs that were manufactured and that detailed results for other designs with different construction details will differ. The main conclusions and pattern of results are to be considered.

[00136] Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that it may be embodied in many other forms.