GB2491642A - Saturated core fault current limiter - Google Patents

Abstract

A saturated core fault current limiter includes at least one AC coil 31 wrapped around a high magnetic permeability core 33 having an elongated form; and a series of permanent magnets 41,42, 35-38 adjacent the core for saturating the core during non-fault operation of the fault current limiter. There can be at least two cores 33,34 adjacent each other, each wrapped by a corresponding AC coil 31,32 and at least one permanent magnet ring 35-38 surrounding the cores and coils for saturating the cores during normal operation. Permanent magnets 41,42 can extend across the ends of both cores which can be tapered having a reduced cross section at a middle portion and an increased are at the core ends (fig 5, 70-73). There can be six cores and six AC coils surrounding the cores for a three phase fault current limiter (fig 9).

Description

Compact Permanent Magnet Fault Current Limiter (1P0149)

Field Of The Invention

10001] The present invention relates to current limiting and in particular to a fault current limiter (FCL).

çjcground [0002] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common

general knowledge in the field.

[00031 Saturated core fault current limiters are generally known. For example, see United States Patent 4045823 to Parton, and United States Patents 7,551,410 and 7,193,825.

Open core FCLs are known, for example, from PCT Publication WO 2009/121,141 [00041 Saturated core fault current limiters utilize the non-linearity of magnetic saturation of a high magnetic permeability material to induce inductance changes on fault conditions. During normal operation, the high magnetic permeability material is placed into magnetic saturation by a high magnetic field. On the occurrence of a fault, the material desaturates and provides an increased inductance to the fault.

[0005] As disclosed in the aforementioned references, the high magnetic field is normally provided by a superconducting DC coil, or alternatively, any form of DC coil which includes a circulating DC current in accordance with Faraday's law.

[0006] The utilization of superconducting coils often results in a large additional expense in the need for a cryostat, power supply and superconducting coil, Additionally, DC coils often involve the continuous use of a power supply.

Summary of the invention

100071 It is an object of the present invention to provide a useful alternative FCL having a number of advanced features.

10008] In accordance with a first aspect of the present invention, there is provided a saturated core fault current limiter including: at least one AC coil wrapped around a core from a high magnetic permeability material; an high permeability core having an elongated form; a series of permanent magnets adjacent the high penneability core for saturating the high permeability core during non-fault operation of the fault current limiter.

100091 The high magnetic permeability core can be of an elongated form and permanent magnets are preferably arranged at each end thereof Ideally, there is provided two adjacent high magnetic permeability cores, for each phase of an AC signal. In some embodiments, the high magnetic permeability core has a reduced cross sectional area in a middle portion thereof and an expanded cross sectional area at each end.

[00101 In accordance with a farther aspect of the present invention, there is provided a saturated core fault current limiter including: at least a first and second AC coil wrapped around corresponding high magnetic permeability cores; at least a first and second high magnetic permeability core adj acent on another; at least one permanent magnet ring surrounding the first and second AC coils and first and second high magnetic permeability cores for saturating the high magnetic permeability cores during normal operation.

100111 A series of pennanent magnet rings are preferably provided around the first and second high magnetic permeability cores. Further, in some arrangements, at least one permanent magnet located at least one end each high magnetic permeability core. In other arrangements, an end permanent magnet extends across the ends of both first and second high magnetic permeability core. In other arrangements, the AC coils are preferably located centrally around corresponding high magnetic penneability cores and at least one permanent magnet ring can be located around an end portion of the colTespondmg high magnetic permeability core.

[0012) In a three phase system, the number of AC coils can be six and the number of cores can be six with the at least one permanent magnet ring surrounding the six cores and/or a permanent magnet at each end of each core.

Brief Desci-iption of the Drawings [0013] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: [00141 Fig. 1 illustrates a sectional view through a first single phase pennanent magnet FCL arrangement; [0015] Fig. 2 illustrates a sectional view through a second single phase permanent magnet FCL arrangement; [0016] Fig. 3 illustrates a sectional view through a third single phase permanent magnet FCL arrangement; [0017] Fig. 4 illustrates a sectional view through a fourth single phase permanent magnet FCL arrangement; [00181 Fig. S illustrates a sectional view through a fifth single phase permanent magnet FCL arrangement; [0019] Fig. 6 illustrates a sectional view through a sixth single phase permanent magnet FCL arrangement; [0020] Fig. 7 illustrates a sectional view through a seventh single phase permanent magnet FCL arrangement; [0021] Fig. 8 illustrates a sectional view through a eighth single phase permanent magnet FCL arrangement; [0022] Fig. 9 is a side perspective view, partly in section, of a first three phase permanent magnet FCL arrangement; [00231 Fig. 10 illustrates a sectional view through a second three phase permanent magnet FCL arrangement; [0024] Fig. 11 is a side perspective view, partly in section, of the second three phase pemrnnent magnet FCL arrangement; [0025] Fig. 12 illustrates a sectional view through a third three phase pemmnent magnet FCL arrangement; [0026] Fig. 13 is a side perspective view, partly in section, of the third three phase permanent magnet FCL arrangement; [00271 Fig. 14 illustrates voltage traces across a first FCL prototype device in the un-faulted steady state condition; [00281 Fig. 15 illustrates the fault current limiting performance of the first prototype; [00291 Fig. 16 illustrates the voltage trace across a second prototype in the un-faulted steady state condition; and 100301 Fig. 17 illustrates fault cunent traces for the second prototype.

Detailed Description

[00311 In the preferred embodiments there is provided a fault current limiter device which utilizes permanent magnets to assist in saturating the core of the fault current limiter.

[00321 Turning initially to Fig. 1, there is illustrated a sectional view of an initial design of a saturated core FCL 1 having a permanent magnet saturation arrangement. In the design 1, an AC coil 2 is wrapped around a first high magnetic permeability core 3, with a second AC coil wrapped in a reverse sense around core 4. The two cores are 3, 4 are placed into magnetic saturation during normal operation by means of circular pennanent magnets 5,6 which can be of a high intensity rare earth type. The FCL arrangement is surrounded in a bath of transformer oil, in a grounded tank 8. Ideally, the permanent magnets 5, 6 are formed externally to the tank 8 for simplified access.

100331 Fig. 2 illustrates a further alternative arrangement 10 having an increased number of pernianent magnets. In this arrangement, the AC coils 11, 12 are wound around corresponding steel cores 13, 14. Around each steel core are four circular permanent magnets 15-18 for saturating the cores. Again the cores are formed within transformer oil and tank 20.

[0034] It will be evident that the pennanent magnets must be of sufficient strength to provide normal saturation of the core. This necessitates a large volume of high intensity magnetic material. Alternative arrangements can be provided so as to reduce the volume of magnetic material that is required.

[0035] Turning to Fig. 3, there is illustrated a further alternative arrangement 30 wherein a further biasing is provided by top and bottom magnetic material. In the arrangement 30, two AC coils 31, 32 are wound around cores 33, 34. Circular ring permanent magnets 3 5-38 are also provided for saturation of the cores. Additionally, top and bottom permanent magnets 41, 42 are also provided for providing further saturation of the core, Each of the magnets can be of predetermined dimensions WM2, HM2 respectively. The provision of the top and bottom permanent magnets acts to increase the level of saturation of the core or alternatively, they act to reduce the amount of permanent magnetic material required to achieve a certain degree of saturation of the core. The magnets 41, 42 can be large disk shaped.

[0036] Fig. 4 illustrates a ftirther alternative alTangement 50. In this case, the AC coils are wrapped around laminated steel cores 53, 54. The cores are saturated by means of permanent magnets 55-58. The magnets act to saturate the steel cores.

[0037] Tn some arrangements the cores can be tapered. Fig. 5 illustrates a further embodiment having tapered cores. In this arrangement, the two laminated steel cores include tapered steel core ends 70-74 of material with a high magnetic permeability to assist and enhance the functionality of the end blocks of permanent magnets in magnetizing the steel cores. The tapering at each end has been found to increase the degree of saturation of the whole core with a lower volume of permanent magnets. At the end of the cores is formed disc magnets 65-68 for saturating the overall core. In the arrangement 70 no circular ring permanent magnets are provided.

10038] Fig. 6 illustrates an arrangement 80 similar to Fig. 5, however in this case a number of circular ring pernunent magnets 89-92 are provided. In this arrangement, two AC coils 81, 82 are wound around corresponding cores 83, 84. The cores are tapered by means of laminated steel portions 96-99. End disc permanent magnets 85-8 8 are also provided in addition to the circular ring permanent magnets 89-92 so as to saturate the cores under normal operating un-faulted conditions.

[0039] Fig. 7 illustrates a un-them alternative arrangement. In this arrangement 100, the AC coils 101, 102 are wrapped around central cores 103, 104. Each end of the core includes a tapering 112-115. The top end also includes a disc permanent magnet 109 and the bottom includes a further disc magnet 110. The cores are further surrounded by a series of permanent circular ring magnets 105 108.

[0040J Fig. 8 illustrates a further arrangement 120. In this arrangement the AC coils 121, 122 are wrapped around core 125, 126 which include thicker ends. The thicker ends provide improved magnetic coupling with permanent magnets 127, 128. The permanent magnets, being at the ends of the cores can be placed closer to the cores, thereby providing closer magnetic coupling therewith.

[0041] Although the arrangements described so far relate to single phase arrangements, the teachings can be readily extended to multiphase arrangements, such as the phase power systems commonly in use. For example, Fig. 9 illustrates a three phase FCL 130. Three input phases 131-133 are provided. Each input phase includes two arms wrapped around corresponding cores 14 1-146. Around each of the six cores is formed a permanent magnet rings 1 50-153, with the system 130 having four rings. The thickness and number of rings can be adjusted in accordance with their magnetic properties and the desired fault condition suppression characteristics. The output is provided by three outputs 135-137. The system 130 can be fonned within a tanlc 155 in the usual maimer and included transformer oil cooling radiatorsl 56 for providing continual cooling of the immersed transformer oil.

100421 Fig 10 and Fig. 11 illustrate an alternative three phase alTangement. In this arrangement, the coils 161, 162 are formed around cores 163, 164. The cores include thickened top and bottom sections of material with a high magnetic permeability which may be laminated or non-laminated e.g. 165, 166 to aid coupling of the core to corresponding permanent ring magnets 167, 168. The permanent magnets can be formed on the outside of tank 169, which is also filled with transformer oil 170. As shown in Fig. 11, each AC coil 171, 172 and multi-phases e.g 174-176 can be formed within a single tank 169.

100431 Fig. 12 and Fig. 13 illustrate a further alternative three phase arrangement 180. This arrangement is similar to the arrangement 160 of Fig. 10 however, in this arrangement there is included an internal disc magnet e.g. 181, 182 at each end of the core. The disc magnets act to assist the efficient biasing of the cores by providing additional direct magnctisation at the end of the cores.

[0044] It can therefore be seen that the arrangements provided for a compact form of permanent magnet fault current limiter. Various alternative designs are possible, with the incorporation of the end magnets allowing for a more balanced saturation.

[00451 It can be seen that the permanent magnets can bias the steel cores in either a ring s shape or employing consolidated end caps on each end of the steel core structure, or a combination of both. The rings around the cores are efficient at biasing the central portion of the steel cores but not the ends. Similarly, ends caps of permanent magnets are efficient at biasing the ends of the cores but not the central portion. Combining rings of permanent magnets and end caps of permanent magnets is often a better approach and leads to lower overall permanent magnet volumes being required. In addition, the rings of pernianent magnets encompassing the steel cores could alternatively be in the form of a DC coil and be made of copper, High Temperature Supercondutor (HTS) or MgB2 -this hybrid approach can lead to lower power for any required copper coils for example.

Experimental Results [0046] A single phase FCL device in the style of Fig. 6 was constructed and tested with the following parameters: Two steel cores of M4 laminated transformer laminations; Steel cores of dimensions: 80mm x 80mm x 600mm; Steel core cross sectional area = 64cm2; Height of the AC coils = 390mm; Number of AC turns = 60; Test voltage 312 Volts AC ntis line to ground; End cap permanent magnet dimensions = 140mm x 105mm x 45mm; Four sets of magnets employed, one block on each end of each steel core; Total end block magnet volume equal to 2560 cm3 of NdFeB; Ring magnet inner dimensions = 280 x 165 mm; Ring magnet height = 3 60mm; Ring magnet radial build = 40mm; Total ring magnet volume = 15,120cm3; [0047] In addition, provision was made to bias this device by employing a copper DC coil ring around the steel cores only with the following details: Height of the copper DC coil = 220mm; Number of DC turns = 196; Inside diameter of the DC copper coil = 800mm; DC bias applied = 59,000 ampere-turns [0048] Fig. 14 shows the traces of the voltage across the FCL as a fUnction of time showing less than 2.5 Volts ntis across the FCL or 0.8 % of the source voltage for the DC coil biased version 190 and 2.3 volts rams across the FCL or 0.7 % of the source voltage for the permanent magnet biased version 191.

[0049J The two experimental arrangements were then tested for fault current limiting performance and the results are detailed in Fig. 15. The fault current without the FCL in the test circuit 192 was established to be 1330 Amps rrns; The fault current with the DC copper coil biasing arrangement 193 was found to be 640 Amps rms; The fault current with the permanent magnet biasing alTangement 194 was found to be 477 Amps rms; [00501 The results of Fig 14 show that the permanent magnets employed effectively bias the FCL arrangement and result in a device with low insertion impedance and voltage drop. Fig. 15 shows conclusively that employing permanent magnets as an alternative magnetic saturating biasing mechanism can produce a device capable of limiting fault culTents.

[0051] A second single phase FCL device which could withstand higher fault currents was Is then analysed. The prospective fault current allowed for in this case was 15 kA which is representative of the fault current levels present in practical electric utility applications. In this device, both copper DC coils and end blocks of permanent magnets were employed in order to confirm that end blocks of permanent magnets could decrease the total DC bias required from the copper DC coils.

[0052] The device built to withstand a prospective fault current of 15 kA was as follows: Two steel cores of M4 laminated transformer laminations; Steel core cross sectional area = 200cm2; Height of steel cores = 2 100mm; Height of the AC coils = 1500mm; Number of AC turns = 31; Test voltage = 1500 Volts AC rms line to ground; End cap permanent magnet dimensions 140mm x 105mm x 45mm; Four sets of magnets employed, one block on each end; Total end block magnet volume equal to 0.11 m3 of NdFeB; DC coil inside diameter = 800mm; Number of DC turns = 196; DC coil height = 220mm [00531 It was found that the FCL could be biased with 200,000 Ampere-turns from the DC copper coil alone. Employing the end blocks of permanent magnets reduced this DC bias requirement to 110,000 Ampere-turns. Fig. 16 shows a trace of the voltage across the FCL as a function of time for the DC copper coil only biased device 195 with 200 iT total DC bias applied and the hybrid device employing both DC copper coils and end blocks of permanent magnets 196 with 110 1CAT of DC bias applied from the copper DC coils. The DC bias contribution from the copper DC coils was tuned so that both FCL device alTangements would have essentially the same steady state un-faulted voltage drop across them and hence the same FCL device impedance. In this way, it can be assured that any observed fault current limiting perfonnance from either device is due to the steel core dc-saturating and not any parasitic inherent impedance.

[0054] Fig. 17 shows the fault current traces as a function time for a prospective fault current of 15 kA ntis 197. The FCL prototype device which is biased only with the DC copper coil limited the fault current to 8.9 kA rms 198 and this is matched by the hybrid device employing both end blocks of permanent magnets and DC copper coils 199. However, it should be noted that the later was achieved at a much reduced DC bias contribution from the copper DC coils (110 kAT for the hybrid device compared to 200 kAT for the device biased with the DC copper coils only). Both the devices were found to provide the same amount of fault current limiting of 41%.

[0055] It can therefore be seen that the utilization of permanent magnet devices provides for efficient and practical saturation of the FCL core material while still allowing the device to behave as an effective and practical fault current limiter Interpretation [0056] The following description and figures make use of reference numerals to assist the addressee understand the structure and function of the 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 including all features in all instances 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.

-10 - [0058] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0059] Similarly it should be appreciated that in the above description of exemplary to enthodiments of thc invention, various features of the invention are sometimes grouped together in a single embodiment, Fig., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

[0060] 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.

100611 Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an clement described herein of an apparatus embodiment is an example of a means for -11 -carrying out the function perfonned by the element for the purpose of carrying out the invention.

[0062] 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.

[0063] Similarly, 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 tenns 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.

[0064] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks.

Steps may be added or deleted to methods described within the scope of the present invention.