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Patent Searching and Data


Title:
CURRENT LIMITER
Document Type and Number:
WIPO Patent Application WO/2010/056122
Kind Code:
A1
Abstract:
A current limiter, wherein the limiter is designed to limit three-phase alternating current, having a nominal current intensity of 100 A or more, wherein the limiter is at least provided with: -a first, second and third current conductor (31, 32, 33) to conduct the three-phase alternating current; -a magnetic core (5) of saturable, magnetically permeable material; -a saturation unit (8) to keep the core (5) in a magnetically saturated state during a normal use position; wherein each current conductor (31, 32, 33) is provided with a first flux generator (1, 2, 3) to generate a respective first magnetic flux in the core, and a second flux generator (11, 12, 13) to generate a second magnetic flux opposite to the respective first flux.

Inventors:
VAN RIET MARTINUS JOHANNES MARIA (NL)
FERREIRA JAN ABRAHAM (NL)
DE HAAN SJOERD WALTER HERO (NL)
CVORIC DALIBOR (NL)
Application Number:
PCT/NL2009/050691
Publication Date:
May 20, 2010
Filing Date:
November 17, 2009
Export Citation:
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Assignee:
LIANDON B V (NL)
VAN RIET MARTINUS JOHANNES MAR (NL)
FERREIRA JAN ABRAHAM (NL)
DE HAAN SJOERD WALTER HERO (NL)
CVORIC DALIBOR (NL)
International Classes:
H01F27/32; H01F29/14
Foreign References:
US4122385A1978-10-24
EP1768137A12007-03-28
US5400238A1995-03-21
US3671810A1972-06-20
GB1314270A1973-04-18
Attorney, Agent or Firm:
HATZMANN, M.J. (Johan de Wittlaan 7, JR Den Haag, NL)
Download PDF:
Claims:
CLAIMS

1. A current limiter, wherein the limiter is designed to limit three-phase alternating current, having a nominal current intensity of 100 A or more, wherein the limiter is at least provided with:

-a first, second and third current conductor (31, 32, 33) to conduct the three-phase alternating current;

-a magnetic core (5) of saturable, magnetically permeable material; -a saturation unit (8) to keep the core (5) in a magnetically saturated state during a normal use position; wherein each current conductor (31, 32, 33) is provided with a first flux generator (1, 2, 3) to generate a respective first magnetic flux in the core, and a second flux generator (11, 12, 13) to generate a second magnetic flux opposite to the respective first flux.

2. A current limiter according to claim 1, wherein the core has a mass of more than 5 tons, in particular more than 10 tons. 3. A current limiter according to claim 1 or 2, wherein said flux generators comprise coils, wherein the coils of the first flux generators (1, 2, 3) are all three wound in the same direction with respect to each other, wherein the coils of the second flux generators (11, 12, 13) are wound in the same direction with respect to each other. 4. A current limiter according to any one of the preceding claims, wherein the core substantially consists of laminated steel. 5. A current limiter according to any one of the preceding claims, wherein each saturation unit (8) comprises a coil non-superconducting in use, in particular provided with an aluminum core. 6. A current limiter according to any one of the preceding claims, wherein two orthogonal dimensions of the core (5) are each greater than about 1 meter, in particular greater than about 2 meters.

7. A current limiter according to any one of the preceding claims, wherein the core (5) has a thickness of at least 40 cm, in particular at least 0.5 m.

8. A current limiter according to any one of the preceding claims, wherein said conductors (31, 32, 33) each comprise a current-conducting core having a cross section of more than 150 mm2, in particular at least 180 mm2.

9. A current limiter according to any one of the preceding claims, wherein the flux generators comprise wound power cables, wherein each power cable is provided with an insulation sheath.

10. A current limiter according to any one of the preceding claims, wherein the core (5) is provided with at least a drum, which drum comprises at least two said flux generators, wherein the drum is preferably manufactured from aluminum. 11. A current limiter according to claim 10, provided with two drums, which are each provided with three said flux generators.

12. A current limiter according to any one of the preceding claims, wherein the core (5) is at least provided with a first leg (51) and a second leg (52), wherein at least two of the first flux generators (1, 2, 3) are designed to generate respective fluxes in the first leg, wherein at least two of the second flux generators (11, 12, 13) are designed to generate respective fluxes in the second leg.

13. A current limiter according to claim 12, wherein the first core leg is provided with all said first flux generators, and wherein the second core leg is provided with all said second flux generators.

14. A current limiter according to any one of the preceding claims 12-13, wherein the core (5) comprises two longitudinal legs (53, 54) which connect ends of the first and second leg, wherein the longitudinal legs (53, 54) are preferably designed to provide a short-circuit path when the core (5) has been brought out of a said saturated state.

15. A current limiter according to any one of the preceding claims, designed to limit three-phase alternating current, at a nominal voltage of more than 1 kV, in particular more than 5 kV.

16. A current limiter according to any one of the preceding claims, wherein the saturation unit (8) is designed to consume 5 kW at a maximum, in particular 2 kW at a maximum, and more particularly 1.5 kW at a maximum to keep the core (5) in the magnetically saturated state during the normal use position.

17. A current limiter according to any one of the preceding claims, wherein both the saturation unit (8) and the flux generators (1, 2, 3, 11,12, 13) comprise coils, wherein a ratio between the total number of winding turns Nac of the flux generators and the number of winding turns Ndc of the saturation unit is in the range of 1:2-1:4, for example about 1:3.

18. A current limiter according to any one of the preceding claims, wherein the core is provided with an E-shaped core part and a closing part which connects two outer legs of the E-shaped core part to each other, wherein a gap is present between the closing part and a middle leg of the E-shaped core part.

19. An electricity transport system, provided with at least one electricity generator, for example a wind-drivable generator, which generator is designed to generate three-phase alternating current, and at least one current limiter to limit the alternating current, for example when a short circuit occurs in a downstream part of the transport system, wherein the current limiter is a limiter according to any one of the preceding claims. 20. A method for manufacturing a current limiter according to any one of claims 1-18, wherein the method at least comprises: -providing an E-shaped transformer core part;

-arranging drums on outer legs of the E-shaped transformer core part, which drums have been or are provided with the flux generators (1, 2, 3, 11, 12, 13) to generate respective first magnetic fluxes in the core; and -arranging a core closing part to close the E-shaped transformer core part. 21. A method according to claim 20, wherein end sides of outer legs of the E-shaped transformer core part engage end sides of the closing part to position the closing part with respect to the E-shaped part in a longitudinal direction of that part, during the bringing together of the closing part and the E-shaped part.

Description:
Title: Current limiter

The invention relates to a current limiter.

Current limiters are known in diverse variants. For example, the old US 3,671,810 (from 1962) discloses a limiter based on the principle of a saturated magnetic core, which core can be brought out of the saturated state in the event of a short circuit in a current consumer.

Many variations of this principle have already been developed, such as superconducting variants which are supposed to offer diverse advantages over non-superconducting systems. See, for example, WO2007/029224. The present invention contemplates an improved current limiter.

In particular, the invention contemplates a limiter that can limit relatively high nominal currents, for example a current that is generated by a medium-voltage electricity generator, in a particularly efficient (nearly non-dissipative) manner. The invention moreover contemplates a relatively durable limiter, which requires little maintenance, and which can preferably be manufactured with relatively inexpensive means.

To that end, according to an aspect of the invention, the limiter is characterized by the features of claim 1.

The limiter is designed to limit three-phase alternating current, having a nominal current intensity of 100 A or more, and is at least provided with:

-a first, second and third current conductor to conduct the three-phase alternating current;

-a magnetic core of saturable, magnetically permeable material; -a saturation unit to keep the core in a magnetically saturated state during a normal use position; wherein each current conductor is provided with a first flux generator to generate a respective first magnetic flux in the core, and a second flux generator to generate a second magnetic flux opposite to the respective first flux. The current limiter according to the invention can limit particularly large nominal currents (having a current intensity of at least 100 A, and preferably at least 200 A, in particular at least 400 A). This is to say that the limiter leaves the current substantially alone as long as it remains within the nominal range, and limits the current as soon as the current shoots out significantly beyond the nominal range (for example as a result of a short circuit downstream of the limiter). Moreover, it has been found that the present limiter consumes surprisingly little power, in particular to keep the magnetic core in the saturated state. In particular, it has been found that relatively high core masses benefit the desired efficient operation of the current limiter.

Thus, a further elaboration features a core having a mass of more than 5 tons, in particular more than 10 tons. According to an advantageous embodiment, the flux generators comprise coils, preferably with aluminum current conductors. In particular, the relatively heavy core can lead to a relatively low current density in the flux generators and the saturation unit, which can reduce losses considerably.

In an extra advantageous embodiment, the coils of the first flux generators are wound in the same direction with respect to each other, while the coils of the second flux generators are also wound in the same direction with respect to each other.

The present limiter is durable and relatively simple, in particular by not utilizing superconductors. Use of low -temperature cooling means, with all its disadvantages, can therefore be omitted.

Preferably, the saturation unit is designed to consume 5 kW at a maximum, in particular 2 kW at a maximum and more in particular 1.5 kW at a maximum to keep the core in the magnetically saturated state during the normal (i.e. non-limiting) use position. In this manner, the current limiter consumes particularly little energy to control the three-phase current. Further, it has been found that surprisingly good results can be obtained when the core is sizeable. In a particularly efficient current limiter for high nominal currents, two orthogonal dimensions of the core (for example a length and a width) are each greater than about 1 meter, in particular even greater than about 2 meters. Further, it has been found to be advantageous if the core has a thickness of at least 40 cm, in particular at least 0.5 m. In a cross section, the core can locally measure, for example, at least 0.4 m x 0.4 m, in particular at least 0.5 m x 0.5 m.

The invention further provides an electricity transport system, provided with at least one electricity generator, for example a wind-drivable generator, which generator is designed to generate three-phase alternating current, and at least one current limiter to limit the alternating current, for example when a short circuit occurs in a downstream part of the transport system, wherein the current limiter is a limiter according to the invention. The present current limiter can be used with advantage, for example, to control electricity delivered by windmill generators, but can also be used in combination with other kinds of generators. The limiter can be disposed, for example, near the generator, and can serve to protect the generator against problems (e.g. short circuit) elsewhere in the network. In view of its reliability and durability, the limiter in such a case is highly advantageous, for example, in combination with a (typically remotely disposed) windmill generator.

According to a preferred embodiment, the current limiter is used to limit three-phase alternating current at a relatively high nominal voltage of more than 1 kV, in particular in the range of 5-36 kV. The limiter can protect such a current in a particularly reliable manner. The current limiter can be manufactured in different ways. The invention offers a particularly advantageous method for manufacturing an extra advantageous embodiment of the current limiter. The method comprises at least: -providing an E-shaped transformer core part;

-arranging drums on the outer legs of the E-shaped transformer core part, which drums have been or are provided with the flux generators to generate respective first magnetic fluxes in the core; and

-arranging a core closing part to close the E-shaped transformer core part.

In this manner, a particularly solid limiter can be obtained, which is considerably less complex and less costly than known superconducting systems. Moreover, the limiter can thus be manufactured from relatively few components. The drums mentioned are preferably manufactured from aluminum, and may, for example, be accurately provided with the flux generators before (or after) the drums are (have been) slipped on respective legs of the core. The drums are preferably designed not to allow any circular currents in respective drum material; to this end, the drums may, for example, be provided with an interruption or an electrically insulating seam, for example if the drums are manufactured from aluminum. Further, it is preferred that parts of the above-mentioned saturation unit, in particular coil parts, be also provided on the drums.

A saturation unit as mentioned (to keep the core in a magnetically saturated state) can be arranged at various moments, on a suitable core part or drum part, for example prior to, or following, placement of the closing part. Further advantageous elaborations of the invention are described in the subclaims. Presently, the invention will be clarified with reference to an exemplary embodiment and the drawing. In the drawing:

Fig. 1 shows schematically a front view of an exemplary embodiment of the invention; Fig. 2 shows a side elevational view of the exemplary embodiment;

Fig. 3 shows a top plan view of a part of the exemplary embodiment;

Fig. 4 shows schematically an electricity transport system provided with an exemplary embodiment of the invention; and

Fig. 5 shows a part of a method for manufacturing the exemplary embodiment.

The same or corresponding features in this application are designated with the same or corresponding reference signs. Figs. 1-3 show a non-limiting exemplary embodiment of a current limiter K, which is designed to limit three-phase alternating current, having a nominal current intensity of 100 A or more, and preferably at least 400 A. Fig. 5 shows a part of the limiter K, in a disassembled position, in a method of assembling the limiter. As will be apparent from the following, the current limiter K has a relatively simple construction, while the limiter can provide a particularly reliable operation, in an energy-saving manner, without use of superconduction.

In particular, the limiter K is designed to limit the three-phase alternating current at a medium voltage, at least, at a nominal voltage of more than 1 kV, in particular more than 5 kV. The active area of the present limiter K is, for example, in the range of 5-36 kV nominal voltage.

Alternatively, the current limiter K can be used to protect three- phase current at higher voltages. Accordingly, the current limiter K may be used, for example, in an upstream part of an electricity transport system, for example to protect a medium -voltage generator against short-circuiting (for example short- circuiting of one of the phases R, S, T to ground).

Fig. 4 shows an example of a system that includes the current limiter K. As Fig. 4 shows, the limiter K can be part of, for example, an electricity transport system, provided with at least one electricity generator W, for example a wind-drivable generator. The generator W is designed to generate three-phase alternating current. The current limiter K can limit the alternating current, for example when a short circuit occurs in a downstream part D of the transport system. The limiter K may be disposed with advantage, for example, near the generator W, or be integrated therewith, but this is not requisite. As Fig. 4 shows, for example electricity transport means 130 are typically at least provided with power cables, and further, for example, with distribution stations and the like (not shown), arranged to link up the different components W, K, D with each other.

The limiter K is provided with a first, second and third current conductor 31, 32, 33 to conduct the three-phase alternating current. The limiter K further comprises a magnetic core 5 of (magnetically) saturable, magnetically permeable material. A saturation unit 8 is provided to keep the magnetic core 5 in a magnetically saturated state during a normal use position (in particular, if the nominal current is below a particular threshold value). In addition, each current conductor 31, 32, 33 is provided with a first flux generator 1, 2, 3 to generate a respective first magnetic flux in the core 5 and a second flux generator 11, 12, 13 to generate a second magnetic flux opposite to the respective first flux in the core 5.

In particular, a first phase of the alternating current (usually designated with R) is conducted via the first conductor 31, a second phase (usually designated with S) via the second conductor 32, and a third phase (usually designated with T) via the third conductor 33. Each of the current conductors 31, 32, 33 may be designed in different ways. Preferably, each conductor 31, 32, 33 is provided with a power cable, or a series of power cables or conductor parts connected to each other. The current conductors 31, 32, 33, during use, are connected to the electricity transport means 130, or are part thereof, to conduct the current to be limited. Connection between the conductors 31, 32, 33 and electricity transport means 130 can be achieved in various manners, for example utilizing plug means and the like.

As Fig. 1 shows, each phase conductor 31, 32, 33 of the limiter K may in itself, for example, be subdivided into different consecutive sub-conductor parts. Conductor parts 31, 32, 33 are associated with the first flux generators 1, 2, 3. Conductor parts 31", 32", 33" are associated with the second flux generators 11, 12, 13. Further, intermediate conductor parts 31', 32', 33' are provided, to couple the first flux generators 1, 2, 3 and second flux generators 11, 12, 13 to each other in a suitable manner. The intermediate conductor parts 31', 32', 33' can be designed in different manners, and may be kept at a suitable distance from other parts of the limiter K (and from each other), for example, by suitable spacers (not shown). In the example, each of the intermediate conductor parts 31', 32', 33' is provided with ground sheath branch sockets 45. Preferably, the flux generators are provided with insulation material, to make the current limiter K touch-safe. Preferably, each of the phase conductor parts mentioned consists of a suitable power cable, for example a coaxial cable, provided with a current-conducting core and an insulating sheath surrounding the core. In an extra advantageous embodiment, each cable further comprises at least a grounding sheath (of electrically conductive material), and a second insulating protective layer surrounding the grounding sheath.

Good results are obtained if the current-conducting cable core is manufactured of metal, and has a cross section of more than 150 mm 2 , a cross section of the cable including sheath being, for example, greater than about 500 mm 2 .

A cable core as mentioned may be manufactured, for example, of copper, but for reasons of cost and for the purpose of mass reduction, use of an aluminum cable core is preferred. In that case, the cross section of the core can be more than 500 mm 2 , for example about 630 mm 2 . A particularly advantageous embodiment, to this end, comprises use of 3-fold extruded cables with an aluminum core. Each cable 31, 32, 33 is preferably provided with a conducting (aluminum) core, provided with a first insulation sheath, around which a grounding screen is provided. An insulating top layer is provided around the grounding screen. The grounding screen may, for example, be separated from the first insulation sheath by an extra intermediate layer, for example an (electrically conductive) swelling tape. The grounding screen of each cable 31, 32, 33 can in itself comprise, for example, two aluminum windings wound in opposite direction, or may be designed in another manner. An outer layer (or layers) of the cable 31, 32, 33 provided on the grounding screen can comprise, for example, one or more layers of mica, or may be configured in another way.

The grounding sheaths of the conductors of the flux generators 1, 2, 3, 11, 12, 13 are preferably coupled to earth E. In the example, the cables are provided with branch sockets 45, in which branches of the grounding sheaths are arranged, to couple those sheaths (via suitable conductors 45) to ground. In the example, these sockets 45 are arranged in-between the cable parts 31, 32, 33 of the first flux generators 1, 2, 3 and the cable parts 31", 32", 33" of the second flux generators 11, 12, 13, viz. on the intermediate cable parts 31', 32', 33'. Preferably, the ground branch sockets are arranged at the middle of the respective intermediate cable parts 31', 32', 33'.

As is further apparent from the drawing, the branches of the grounding sheaths of the cables 31, 32, 33 may be coupled, for example, to the limiter magnetic core 5, with the core 5 itself being grounded (by coupling to earth E). Alternatively, cable grounding sheaths may be coupled to earth bypassing the core 5.

The limiter magnetic core

The present limiter magnetic core 5 is a closed core, designed to provide a loop, closed upon itself, of magnetically saturable core material. The core 5 is of relatively sizeable and heavy design. Preferably, the core 5 alone already has a mass of more than 5 tons, in particular more than 10 tons. According to an advantageous elaboration, the core 5 consists substantially of a suitable magnetic core material, preferably laminated steel (preferably also provided with silicon), in particular transformer tin. In the example, two orthogonal dimensions X, Y of the core 5 (see Fig. 5) are each greater than about 1 meter, in particular greater than about 2 meters. The core 5 preferably has a thickness Z (see Fig. 3) of at least 0.3 m, preferably at least 0.4 m, in particular at least 0.5 m. Preferably, the core 5 is coupled to earth E (to ground the core).

The core can be configured in different manners. In the example, the core 5 is of mirror-symmetrical design, and provided with a first leg 51 and a second leg 52. In addition, the core 5 comprises two longitudinal legs 53, 54, which connect ends of the first and second leg. The four legs 51-54 enclose an opening H. In the example, the first and second leg 51, 52 are parallel, while the longitudinal legs (also mutually parallel) extend perpendicularly between them. Each of the legs 51-54 in this example has a substantially square cross section. The legs 51-54 may also be differently shaped, for example with a polygonal, rectangular, octagonal, oval, circular or differently shaped cross section. Each of the legs 51-54 is manufactured from the core material mentioned, for example from a laminate of transformer tin (in particular comprising superposed layers of magnetic core material having a layer thickness of less than 1 mm, for example approximately 0.3 mm. A transversal width B of each of the legs 51-54 (see Fig. 5) is, for example, greater than or equal to approximately 0.3, preferably 0.4 m. Width B may be, for example, approximately equal (for example with a margin of about +/- 10%) to the core thickness Z.

The longitudinal legs 53, 54 are preferably designed to provide a short-circuit path when the core 5 has been brought out of a saturated state as mentioned. To this end, the second longitudinal leg 54 may be provided, for example, with a central middle leg 55 extending at right angles to that leg 54 and reaching towards the opposite first longitudinal leg 53.

Between an end side of the middle leg 55 and the first longitudinal leg 53 there is an air gap Q (which is part of the earlier-mentioned opening H surrounded by the core). The middle leg 55 is, for example, parallel to the first and second legs 51, 52, and has, for example, approximately the same thickness as the longitudinal legs 53, 54 together. The end side of the middle leg 55 may extend, for example, parallel to an opposite inner side of a core leg 53.

A transversal width U of the middle leg 55 in the example is greater than width B of the other core legs 51-54. In an advantageous embodiment, the middle leg 55 has a width U of at least 0.5 m, in particular at least 0.8 m, and more in particular at least 1 m. The intermediate space between a middle leg 55 and a first and second leg 51, 52, respectively, is at least intended for placement of the flux generators 1, 2, 3, 11, 12, 13, and, for example, also for placement of saturation coils 8a. A width L of an intermediate space can be, for example, greater than 0.2 m. According to a further elaboration, this width is in the range of about 20-80 cm.

The middle leg 55 and the gap Q constitute a flux short -circuiter, which flux short-circuiter has a lower magnetic permeability than a permeability of the core legs 51-54.

As Fig. 5 shows, the present core is provided with, for example, an E-shaped core part 51, 52, 54, 55 and a closing part 53 which connects two outer legs of the E-shaped core part to each other. To this end, the middle leg 55, the respective longitudinal leg 54 and both the first and the second leg 51, 52 may be manufactured in one piece. The other longitudinal leg 53 then serves as closure of the core. In an alternative embodiment, each longitudinal leg may be provided with a central middle leg, the central middle legs then reaching towards each other to define between them a gap as mentioned.

The flux generators

In the example, the flux generators comprise coils 1, 2, 3, 11, 12, 13, provided with wound power cables, each power cable provided with an insulation sheath. Advantageous cable configurations, provided with a grounding screen, and different insulation layers/sheaths, have been described hereinabove.

In particular, first cable parts 31, 32, 33 of the current conductors mentioned are part of the first flux generators 1, 2, 3, by being wound to form respective coils. Second cable parts 31", 32", 33" of the current conductors in a similar manner are part of the second flux generators 11, 12, 13. Good results are obtained when conductors 31, 32, 33 each comprise a current-conducting aluminum core, with a cross section of more than 500 mm 2 , and preferably at least 600 mm 2 .

Preferably, each flux generator 1, 2, 3, 11, 12, 13 is provided with a relatively small number (N ac ) of coil turns. Good results are obtained if each flux generator comprises not more than 100 winding turns, for example not more than 50 winding turns, and, for example, more than 10 winding turns.

A further elaboration is particularly advantageous, in which the coils of the first flux generators 1, 2, 3 are all three wound in the same direction with respect to each other (e.g. clockwise or counterclockwise, with respect to a coil axis). This yields the particularly great advantage in the use of the three-phase current that in case of fully balanced currents (of the three phases) the magnetic fluxes generated by the three first flux generators 1, 2, 3 can cancel each other out completely. Moreover, a relatively small imbalance between the three current phases (e.g., maximum 10% of the nominal current) can be taken up well by the magnetic limiter (without blocking current passage).

The coils of the second flux generators 11, 12, 13, also, are all three wound in the same direction with respect to each other (e.g., clockwise or counterclockwise), in order to provide the same advantage.

The first flux generators 1, 2, 3 are designed to generate respective first magnetic fluxes in the first core leg 51; the second flux generators 11, 12, 13 are designed to generate respective second magnetic fluxes in the second core leg 52. To this end, the first core leg 51 is provided with all first flux generators mentioned, while the second core leg 52 is provided with all second flux generators mentioned.

The configuration of the flux generators is then such that a flux during normal use generated in core 5 by the first flux generator 1, associated with the first current phase (conductor 31), is opposite to a second flux which, during use, is generated in the core 5 by the second flux generator 11 of the first current phase (conductor 31"). The first and second fluxes generated under the influence of the first current phase (via generators 1, 11) can therefore substantially cancel each other out during normal use. The configuration of the flux generators is moreover such that a flux during normal use generated in core 5 by the first flux generator 2, associated with the second current phase (conductor 32), is opposite to a second flux which, during use, is generated in the core 5 by the second flux generator 12 of the second current phase (conductor 32"). Furthermore, the configuration of the flux generators is such that a flux during normal use generated in core 5 by the first flux generator 3, associated with the third current phase (conductor 33), is opposite to a second flux which, during use, is generated in the core 5 by the second flux generator 13 of the third current phase (conductor 33"). The saturation unit

The saturation unit 8 comprises a coil system 8a, non-superconducting during use, in particular comprising one or more coils of electrically conductive windings, preferably with an aluminum core, for example wound coaxial cables.

Preferably, the cable of each saturation unit 8 is configured similarly to the cables 31, 32, 33 of the flux generators, for example at least provided with an aluminum core, a first insulation sheath, a grounding screen, and an outer protective sheath. In the example, the saturation unit 8 is provided with two coils 8a, which are arranged, for example, on the core 5, for example on a longitudinal leg 54 (i.e., a part 54 of the core 5 is surrounded by each coil 8a). Further, the unit is provided with a current source 8b to supply the coils 8a with a direct current, for example a direct current of more than 100 A, in particular more than 400 A, for example about 450 A.

Alternatively, the coils 8a may be provided, for example, next to the coils of the flux generators, for example on the drums 81, 82.

During normal use, the saturation unit 8 generates so high a magnetic flux in the core 5 (under the influence of the direct current mentioned) that the core 5 is in a magnetically saturated state, in order that the flux generators 1, 2, 3, 11, 12, 13 experience no impedance, or hardly any, from the magnetic core. The core saturation brought about by the saturation unit 8 is such that the saturation can be broken by a flux generator if the current phase conducted via that conductor, for example, exceeds a threshold value (for example in the event of short circuit in the respective current phase). In particular, the core saturation generated by saturation unit 8 is such that a particular imbalance that can occur in the three-phase electricity network during normal use has been taken into account. In other words, during use an imbalance does not yet lead to the saturation being broken. Preferably, the current limiter is designed to limit the current as soon as the current phase conducted by a flux generator, for example, exceeds a threshold value that is 10% of the nominal current of the network. In this manner, the current limiter can also act to prevent an unduly great imbalance (of more than 10% nominal current), in order that disadvantages of such an imbalance (e.g., stationary losses) can be prevented.

It is advantageous if the coils 8a of the saturation unit comprise relatively few winding turns. Good results are obtained, for example, when the total number of winding turns Ndc of the coils 8a of the saturation unit 8 is lower than 50, preferably lower than 20. A combined electrical resistance of the coils 8a is preferably lower than 20 mΩ (milli-Ohm), preferably lower than 10 mΩ, for example about 7 mΩ or less. Moreover, a cross section of the coils 8a is relatively large (at least equal to a width B and/or thickness Z of the core leg 54), so that a relatively small number of direct current winding turns Ndc already prove sufficient to achieve a suitable core saturation.

Particularly good results can furthermore be obtained when both the saturation unit (8) and the flux generators (1, 2, 3, 11, 12, 13) comprise coils where a ratio between the total number of winding turns N ac of the flux generators and the number of winding turns Ndc of the saturation unit is in the range of 1:2-1:4, for example approximately 1:3.

Coil drums

It is particularly advantageous when the core 5 is provided with separate drums 81, 82, which drums comprise at least the flux generators. Each drum 81, 82 may be manufactured of, for example, aluminum, or another non-magnetizable material. The example comprises two drums 81, 82, which are each provided with three flux generators (i.e. respective conductor cable windings). Preferably, each drum 81, 82 is provided with optional partitions (see Fig. 5), in order to separate the windings of neighboring flux generators. The drums 81, 82 may, for example, each be provided with a central passage to receive a core leg 51, 52 with a close fit.

The drums 81, 82 may, for example, each be interrupted, for example by being provided in the longitudinal direction thereof with a gap or an electrically insulating seam. This prevents electrical currents starting to circulate in the aluminum drums 81, 82 themselves.

The flux generators may, for example, be accurately arranged on the drums 81, 82 before the drums 81, 82 are arranged on respective parts 51, 52 of the core 5. It is extra advantageous here to provide extra cable length on the drums 81, 82, for example, cable length for the purpose of forming the intermediate cable parts 31', 32', 33', and possibly for the purpose of furnishing external connections to provide, for example, network couplings. After providing the drums 81, 82 on the core 5, the extra cable length can, for example, be unwound from the drums 81, 82 to form the intermediate cable parts 31', 32', 33' and the like.

Fig. 5 schematically shows by way of example a method where an E-shaped transformer core part as mentioned is provided. On the outer legs 51, 52, the drums 81, 82 can be arranged (arrow Pl). Thereupon, the core closing part 53 can be arranged (arrow P2), to close the core 5. The coils 8a of the saturation unit can be arranged at different times, for example, before or after the arrangement of the coil drums 81, 82. According to an advantageous alternative embodiment, each drum 81, 82 has been or is provided with a saturation unit coil 8a, 8b.

As Fig. 5 also shows, end sides of the outer legs 51, 52 can, for example, engage end sides of the closing leg 53, in order to position the closing leg 53 in a longitudinal direction of that part as the closing part and the E-shaped part are being brought together. In the example, to that end, the end sides referred to are of oblique design; it will be clear that the mutually facing sides of the legs 51, 52, 53 to be fitted onto each other may also be shaped differently to bring about a desired positioning and/or engagement, for example with a finger weld configuration, conical engaging surfaces, integral hole-pin couplings and the like.

Use of the exemplary embodiment comprises keeping the core 5 in a magnetically saturated state, by passing a high direct current through the coils 8a of the saturation unit. The three-phase alternating current is passed via the respective conductors 31, 32, 33; the associated flux generators 1, 2, 3, 11, 12, 13 then experience no impedance, or hardly any, from the magnetic core. As the coils of the first flux generators 1, 2, 3 are wound in the same direction, the fluxes generated can substantially cancel each other out (as long as the three phases are in balance); the same holds for the second flux generators 11, 12, 13.

The limiter K can limit the current through the conductors 31-33 very fast, within a microsecond, for example when one of the phase currents, under the influence of a network fault, such as short circuit (for example, elsewhere in a respective transport network), exceeds a threshold value.

A threshold value can comprise, for example, three times a nominal current, preferably 2.5 times the nominal current, and more preferably twice the nominal current. The threshold value can be, for example, more than 1,000 A. As soon as the threshold value of, for example, a phase current passed through a first conductor 31, is exceeded, the core 5 drops out of the magnetically saturated state (as a result of an increased, saturation- counteracting flux generated by one of the respective flux generators 1, 11 that conducts the short-circuit current), and magnetic flux is diverted via the flux short-circuiter (the middle leg 55 and the gap Q).

The dropping out of the saturated state leads directly to the limitation of the currents. In this manner, the three-phase alternating current, having a high nominal current intensity, is protected particularly reliably. Moreover, the present current limiter K consumes surprisingly little energy. A further elaboration of the limiter K has been found to require 5 kW at a maximum, in particular 2 kW at a maximum, and more particularly 1.5 kW at a maximum to keep the core 5 in a suitable magnetically saturated state during the normal use position (to protect a nominal three-phase alternating current of 400 A).

In particular, it has been found that the three-phase currents of a medium- or high-voltage transport system can be relatively well balanced, for example with respect to low-voltage networks (such as a 230V network). An explanation for this may be that the impedance at high voltages is relatively low. An underlying idea behind the invention is that in such a case a relatively low saturation level of the core 5 already suffices to have the limiter K provide the desired protection. Here, use can be made of a relatively small direct current through the saturation coils 8a, for example a current in the range of about 400-500 A (for example about 450 A), with a limited number of winding turns in the coils 8a, which leads to relatively low losses.

Example A Saber computer simulation was carried out to calculate the efficiency of a design of the current limiter. Use was made of the following parameters:

-core material=transformer tin (silicon steel);

-first orthogonal dimension X of the core=206 cm; -second orthogonal dimension Y of the core=252 cm;

-width B = thickness Z of core legs=53 cm;

-width of intermediate leg U=106 cm;

-gap width V=90 cm;

-interspace width L=20 cm; -number of turns of flux generator coils N ac = 50; -number of turns of saturation coils

-total mass of core=16 tons;

-cross section copper core current conductors=180 mm 2 ;

-total resistance flux generator coils=7 mΩ; -nominal alternating current=400 A;

-nominal line voltage=5.8 kV; and

-current intensity direct current flux generator coils=450 A

In this advantageous configuration, the limiter is found to entail only a loss of about 0.18% of the nominal power, per phase. Further, it is found that the saturation unit in this case requires only 1.4 kW to keep the core in the suitable saturated state, which is only 0.06% of the nominal power. The limiter in itself therefore consumes surprisingly little energy, without use of usually relatively costly superconducting magnets (which moreover require substantial maintenance). The invention is thus based in particular on the idea of providing such a massive, heavy and sizeable current limiter to protect three-phase alternating currents (of high nominal current intensity), which has been found to result in a surprisingly economical limiter system.

To those skilled in the art it will be clear that the invention is not limited to the exemplary embodiments described. Various modifications are possible within the framework of the invention as set forth in the following claims.