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Title:
FAULT CURRENT LIMITER
Document Type and Number:
WIPO Patent Application WO/2014/128697
Kind Code:
A1
Abstract:
There is provided a fault current limiter that comprises a magnetically saturable core. The FCL comprises a first DC coil wound around a first portion of the core, with the first DC coil being arranged to saturate at least part of the core in normal conditions. The FCL comprises a first AC coil wound around a second portion of the core, wherein the first AC coil is arranged desaturate at least part of the core in appropriate half cycle in fault limiting conditions and it also comprises a second AC coil wound around a third portion of the core, wherein the second AC coil is arranged desaturate at least part of the core in alternative half cycle in fault limiting conditions. The first and second AC coils being connected in series and connected between a first phase AC source and a load which are producing an AC flux in an AC flux direction that alternates with each AC half-cycle in fault limiting conditions. The FCL also comprise electro-magnetic screens placed around appropriate portion of core between AC and DC coils presenting closed electrical circuit and reducing coupling between AC and DC coils.

Inventors:
LANSBERG DMITRY (IL)
ROZENSHTEIN VLADIMIR (IL)
Application Number:
IL2014/050172
Publication Date:
August 28, 2014
Filing Date:
February 17, 2014
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
GRIDON LTD (IL)
International Classes:
H02H9/02
Domestic Patent References:
WO2013024462A12013-02-21
WO2009006666A12009-01-15
WO2011024179A22011-03-03
Foreign References:
CN1728495A2006-02-01
GB1314270A1973-04-18
US5930095A1999-07-27
US6300856B12001-10-09
Attorney, Agent or Firm:
LEVIN, Helena (279 Hayarkon Str.PO Box 6451, Tel -Aviv, IL)
Download PDF:
Claims:
Claims

1. A fault current limiter comprising a magnetically saturable core, wherein the fault current limiter comprises:

a first DC biasing means located proximate a first portion of the core, wherein the first DC biasing means is arranged to saturate at least part of the core in normal conditions;

a first AC coil wound around a second portion of the core;

a second AC coil wound around a third portion of the core, the first and second AC coils being connected in series for connection to a first phase AC source to produce an AC flux in an AC flux direction that alternates with each AC half-cycle; a first electro-magnetic screen, the first electro-magnetic screen comprising a length of conductive material arranged around a portion of the core between the first portion and the second portion of the core;

wherein changes in the AC flux are arranged to induce a current in the first electro-magnetic screen that opposes the AC flux changes, so as to reduce the flux changes in the first DC biasing means.

2. A fault current limiter according to claim l, wherein the fault current limiter further comprises:

a second electro-magnetic screen, the second electro-magnetic screen comprising a length of conductive material arranged around a portion of the core on an opposite side of the first DC biasing means to the first electro-magnetic screen.

3. A fault current limiter according to claim 1 or 2, wherein the first DC biasing means is a first DC coil wound around the first portion of the core, wherein the current induced in the first electro-magnetic screen reduces the current induced in the first DC coil by the AC flux.

4. A fault current limiter according to claim 3, wherein the core comprises: a first leg, with the first DC coil wound on the first leg;

a second leg, with the first AC coil wound on the second leg.

5. A fault current limiter according to claim 4, wherein the first and second legs are orientated in the same direction, and the first and second legs are connected by yokes, wherein the or each electro-magnetic screen is arranged on a said leg or a said yoke.

6. A fault current limiter according to claim 4, wherein the first and second legs are orientated in a different direction, with the first and second legs connected to each other, wherein the or each electro-magnetic screen is arranged on a said leg.

7. A fault current limiter according to any one of claims 4 to 6, wherein the core comprises:

a third leg, with the second AC coil wound on the third leg;

8. A fault current limiter according to claim 7, wherein the first, second legs, and third legs are orientated in the same direction, and the first, second and third legs are connected by yokes.

9. A fault current limiter according to claim 8, wherein the first leg is arranged between the second and third legs.

10. A fault current limiter according to any one of claims 3 to 9, wherein the fault current limiter comprises:

a second DC coil wound around a fourth portion of the core, wherein the second DC coil is arranged to saturate at least part of the core in normal conditions.

11. A fault current limiter according to claim 10, wherein the fault current limiter comprises:

a third electro-magnetic screen, the third electro-magnetic screen comprising a length of conductive material arranged around a portion of the core between the fourth portion and the second portion of the core.

12. A fault current limiter according to claim 11, wherein the fault current limiter further comprises:

a fourth electro-magnetic screen, the fourth electro-magnetic screen comprising a length of conductive material arranged around a portion of the core on an opposite side of the second DC coil to the third electro-magnetic screen.

13. A fault current limiter according to claim 11 or 12, wherein the core comprises:

a fourth leg, with the second DC coil wound on the fourth leg.

14. A fault current limiter according to claim 13, wherein the first, second, third and fourth legs are orientated in the same direction, and the first, second, third and fourth legs are connected by yokes.

15. A fault current limiter according to claim 14, wherein the first, second, third and fourth legs are arranged in order.

16. A fault current limiter according to claim 13, wherein the first and fourth legs are orientated in a first direction, with the second and fourth legs orientated in a second direction orthogonal to the first direction.

17. A fault current limiter according to any one of claims 1 to 3, comprising: a first leg;

a second leg, with the first AC coil wound on the second leg;

a third leg, with the second AC coil wound around the third leg;

a fourth leg; wherein the first, second, third and fourth legs are arranged in order, wherein first ends of the first, second, third and fourth legs are joined by a first yoke and second ends of the first, second, third and fourth legs are joined by a second yoke;

a first magnetic biasing unit arranged to produce a first closed magnetic circuit in the first leg and the second leg that has a first flux direction; and

a second magnetic biasing unit arranged to produce a second closed magnetic circuit in the fourth leg and the third leg that has a second flux direction, wherein the first flux direction opposes the second flux direction;

wherein the first and second AC coils are arranged to produce a first closed AC magnetic circuit in the second and third legs in an AC flux direction that alternates with each AC half-cycle;

wherein the first electro-magnetic screen is arranged around a portion of the core between the first magnetic biasing unit and the first AC coil.

18. A fault current limiter according to claim 17, wherein the first magnetic biasing unit and the second magnetic biasing unit are provided by one or more DC coils.

19. A fault current limiter according to claim 17 or 18, wherein the fault current limiter further comprises: a second electro-magnetic screen, the second electro-magnetic screen comprising a length of conductive material arranged around a portion of the core on an opposite side of the first magnetic biasing unit to the first electro-magnetic screen; and/or

a third electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the second magnetic biasing unit and the second AC coil; and/or

a fourth electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the second magnetic biasing unit and the second AC coil, on the other side of the second magnetic biasing unit to the third electro-magnetic screen.

20. A fault current limiter according to any one of claims l to 19, wherein the or each said electro-magnetic screen comprises a shorted ring.

21. A fault current limiter according to any one of claims 3 to 20, wherein the or each said electro-magnetic screen comprises at least one shorted turn of a said DC coil.

22. A fault current limiter according to any one of claims 1 to 21, wherein the first and second AC coils are arranged to produce a closed AC magnetic circuit.

23. A fault current limiter according to any one of claims 3 to 22, wherein the or each DC coil is arranged to produce a closed DC magnetic circuit.

Description:
Fault Current Limiter

The present invention relates to a fault current limiter (FCL). Faults in electrical power systems cannot be avoided. Fault currents flowing from the sources to a location of the fault lead to high dynamical and thermal stresses being imposed on equipment e.g. overhead lines, cables, transformers and switch gears.

Conventional circuit breaker technology does not provide a full solution to selectively interrupting currents associated with such faults. The growth in electrical energy generation and consumption and the increased interconnection between networks leads to increasing levels of fault current. In particular, the continuous growth of electrical energy generation has the consequence that networks reach or even exceed the limits with respect to their short circuit withstand capability. Therefore, there is a need for devices that are capable of limiting fault currents.

Short circuit currents are rising as transmission and distribution networks expand to address increasing energy demand and connectivity of power generation and intermittent energy sources. These may result in power disruptions, equipment damage and major outages, which have been estimated to cost billions of dollars per year. In order to restrict fault current impact, utility operators have traditionally needed to resort to network segmentation and installation of expensive and lossy protection gear, such as series reactors, capacitors, high rated circuit breakers and high impedance transformers. Such solutions come at the cost of overall reduction of energy efficiency and network stability.

The use of fault current limiters (FCL) allows equipment to remain in service even if the prospective fault current exceeds it rated peak and short-time withstand current. Thus, replacement of equipment (including circuit breakers) can be avoided or postponed to a later time.

A fault current limiter (FCL) can be provided in various forms. One type of fault current limiter involves a fully magnetised (saturated) iron core. Such fault current limiters typically have one or more AC coils wound around an iron core, with the iron core being maintained in a saturated state by a DC bias coil in normal operating conditions. The AC coils are connected to the grid, and in normal conditions the coil is kept saturated, making the FCL virtually transparent to the grid during normal operation.

In a fault condition (e.g. a short-circuit), a current surge will increase the current on the AC coil, causing desaturation of the ion core. As a result of this desaturation of the iron core, the impedance will rise, acting to limit the current on the AC coil. Various arrangements of the saturable core and AC and DC coils are possible. An example of a prior art saturated core FCL is described in WO2007/ 029224. The present invention sets out to provide an FCL with improved performance compared to conventional arrangements.

According to a first aspect of the invention there is provided a a fault current limiter comprising a magnetically saturable core, wherein the fault current limiter comprises: a first DC biasing means located proximate a first portion of the core, wherein the first DC biasing means is arranged to saturate at least part of the core in normal conditions; a first AC coil wound around a second portion of the core; a second AC coil wound around a third portion of the core, the first and second AC coils being connected in series for connection to a first phase AC source to produce an AC flux in an AC flux direction that alternates with each AC half -cycle; a first electro-magnetic screen, the first electro-magnetic screen comprising a length of conductive material arranged around a portion of the core between the first portion and the second portion of the core; wherein changes in the AC flux are arranged to induce a current in the first electro-magnetic screen that opposes the AC flux changes, so as to reduce the flux changes in the first DC biasing means.

In such embodiments, the first electro-magnetic screen reduces the amount of current induced in the first DC biasing means. In such embodiments, in normal conditions, the first DC biasing means (e.g. a DC coil) will saturate the second portion of the core. The AC coil will produce flux that alternates with each half -cycle, and in one half-cycle (e.g. a first half -cycle) will act with the DC flux (pushing the second portion of the core deeper into saturation), and in the other half -cycle (e.g. a second half-cycle) will act against the DC flux. In some embodiments, the first electro-magnetic screen comprises a length of electrically isolated conductive material arranged around a portion of the core between the first portion and the second portion of the core. In fault conditions, the AC current will increase, thus increasing the AC flux. As for normal conditions, in one half-cycle, the AC flux in fault conditions will act with the DC flux, and in the other half -cycle the AC flux will act against the DC flux. Such fault current limiters comprising a magnetically saturable core are arranged such that, in fault conditions, in the half-cycle in which the AC flux acts against the DC flux, the AC flux will cause the second portion of the core to move out of saturation -thus causing an increase in impedance, limiting the fault.

In some embodiments, the core is a single core. In other embodiments the core is a modular core comprising a number of sections. For example, in embodiments in which the core has a plurality of legs, each leg could be a different section. In other embodiments, the legs could be either connected to each other or connected via yokes.

In some embodiments, the fault current limiter further comprises: a second electromagnetic screen, the second electro-magnetic screen comprising a length of conductive material arranged around a portion of the core on an opposite side of the first DC biasing means to the first electro-magnetic screen.

In some embodiments, the first DC biasing means is a first DC coil wound around the first portion of the core, wherein the current induced in the first electro-magnetic screen reduces the current induced in the first DC coil by the AC flux.

In some embodiments, the core comprises: a first leg, with the first DC coil wound on the first leg; a second leg, with the first AC coil wound on the second leg.

In some embodiments, the first and second legs are orientated in the same direction, and the first and second legs are connected by yokes, wherein the or each electromagnetic screen is arranged on a said leg or a said yoke.

In some embodiments, the first and second legs are orientated in a different direction, with the first and second legs connected to each other, wherein the each electro- magnetic screen is arranged on a said leg. In some embodiments, the core comprises: a third leg, with the second AC coil wound on the third leg;

In some embodiments, the first, second legs, and third legs are orientated in the same direction, and the first, second and third legs are connected by yokes.

In some embodiments, the first leg is arranged between the second and third legs.

In some embodiments, the fault current limiter comprises: a second DC coil wound around a fourth portion of the core, wherein the second DC coil is arranged to saturate at least part of the core in normal conditions.

In some embodiments, the fault current limiter comprises: a third electro-magnetic screen, the third electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the fourth portion and the second portion of the core.

In some embodiments, the fault current limiter further comprises: a fourth electromagnetic screen, the fourth electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core on an opposite side of the second DC coil to the third electro-magnetic screen.

In some embodiments, the core comprises: a fourth leg, with the second DC coil wound on the fourth leg.

In some embodiments, the first, second, third and fourth legs are orientated in the same direction, and the first, second, third and fourth legs are connected by yokes.

In some embodiments, the first, second, third and fourth legs are arranged in order. In some embodiments, the first and fourth legs are orientated in a first direction, with the second and third legs orientated in a second direction orthogonal to the first direction.

In some embodiments, the or each said electro-magnetic screen comprises a shorted ring. In some embodiments, the or each said electro-magnetic screen comprises at least one shorted turn of a said DC coil.

In some embodiments, the first and second AC coils are arranged to produce a closed AC magnetic circuit.

In some embodiments, the or each DC coil is arranged to produce a closed DC magnetic circuit. According to another aspect of the invention there is provided a fault current limiter comprising a magnetically saturable core, wherein the core comprises: a first DC biasing means located proximate a first portion of the core, wherein the first DC biasing means is arranged to saturate at least part of the core in normal conditions; a first AC coil wound around a second portion of the core; a second AC coil wound around a third portion of the core, the first and second AC coils being connected in series for connection to a first phase AC source to produce an AC flux in an AC flux direction that alternates with each AC half-cycle; a first electro-magnetic screen, the first electromagnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the first portion and the second portion of the core; wherein changes in the AC flux are arranged to induce a current in the first electro-magnetic screen that opposes the AC flux changes, so as to reduce the current induced in the first DC biasing means by the AC flux.

In some embodiments, the core further comprises: a second electro-magnetic screen, the second electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core on an opposite side of the first DC biasing means to the first electro-magnetic screen.

In some embodiments, the core comprises: a second DC biasing means proximate a fourth portion of the core, wherein the second DC biasing means is arranged to saturate at least part of the core in normal conditions. In some embodiments, the core comprises: a third electro-magnetic screen, the third electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the fourth portion and the second portion of the core. In some embodiments, the core further comprises: a fourth electro-magnetic screen, the fourth electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core on an opposite side of the second DC biasing means to the third electro-magnetic screen.

According to another aspect of the invention there is provided a fault current limiter comprising a magnetically saturable core, wherein the core comprises: a first DC coil wound around a first portion of the core, wherein the first DC coil is arranged to saturate at least part of the core in normal conditions; a first AC coil wound around a second portion of the core, wherein the first AC coil is arranged desaturate at least part of the core in appropriate half cycle in fault limiting conditions; a second AC coil wound around a third portion of the core, wherein the second AC coil is arranged desaturate at least part of the core in alternative half cycle in fault limiting conditions; the first and second AC coils being connected in series and connected between a first phase AC source and a load which are producing an AC flux in an AC flux direction that alternates with each AC half -cycle ; a first electro-magnetic screen, the first electro-magnetic screen comprising an electrically conductive material presenting closed electrical circuit and arranged around a portion of the core between the first DC coil and the first AC coil; wherein the AC and/ or DC flux changes are inducing a current in the first electro-magnetic screen that opposes to these AC or DC flux changes, so as to reduce the voltage induced in the first DC coil by the AC flux changes and vice versa.

According to another aspect of the invention there is provided a fault current limiter comprising a first magnetically saturable core, the first core including: a first leg; a second leg, with a first AC coil wound on the second leg; a third leg, with a second AC coil wound around the third leg, the first and second AC coils being wound in series and connected to a first phase AC source; a fourth leg; wherein the first, second, third and fourth legs are arranged in order, wherein first ends of the first, second, third and fourth legs are joined by a first yoke and second ends of the first, second, third and fourth legs are joined by a second yoke; a first magnetic biasing unit arranged to produce a first closed magnetic circuit in the first leg and the second leg that has a first flux direction; and a second magnetic biasing unit arranged to produce a second closed magnetic circuit in the fourth leg and the third leg that has a second flux direction, wherein the first flux direction opposes the second flux direction; wherein the first and second AC coils are arranged to produce a first closed AC magnetic circuit in the second and third legs in an AC flux direction that alternates with each AC half-cycle; further comprising a first electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the first magnetic biasing unit and the first AC coil; wherein changes in the AC flux are arranged to induce a current in the first electro-magnetic screen that opposes the AC flux changes, so as to reduce the current induced in the first magnetic biasing unit (e.g. first DC coil) by the AC flux. The first and second magnetic biasing units can be DC coils. Such DC coils could be on the first and fourth legs, e.g. with a first DC coil on the first leg and a second DC coil on the fourth leg. Alternatively, the DC coils could be arranged differently. For example, the DC coils could be located around other parts of the core (e.g. parts of the yoke). Furthermore, the first and second magnetic biasing units can be provided by a single unit, e.g. one DC coil. For example, such a DC coil could be arranged around the second and third legs.

In such an arrangement, in one AC half-cycle the AC flux in the second leg opposes the DC flux in the second leg and the AC flux in the third leg supports the DC flux in the third leg, and in the next half cycle the AC flux in the third leg opposes the DC flux in the third leg and the AC flux in the second leg supports the DC flux in the second leg. Other embodiments can, however, use alternative arrangements for the first and second magnetic biasing units, with the same opposing/supporting effect of the AC flux on the first and second closed magnetic circuits produced by the first magnetic and second units.

In some embodiments, the fault current limiter further comprises a second electromagnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the first magnetic biasing unit and the first AC coil, on the other side of the first magnetic biasing unit to the first electromagnetic screen.

In some embodiments, the fault current limiter further comprises a third electro- magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the second magnetic biasing unit and the second AC coil. In some embodiments, the fault current limiter further comprises a fourth electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the second magnetic biasing unit and the second AC coil, on the other side of the second magnetic biasing unit to the third electro-magnetic screen. In such embodiments, in normal conditions, the second and third legs of the first core are saturated, and hence the impedance of the FCL is low. Hence, in normal conditions, the AC flux in the second and third legs is such that the second and third legs maintain deep saturation. The electro-magnetic screen(s) reduce the amount of current induced in the DC biasing unit.

In such embodiments, the FCL may further comprise a third AC coil wound around the second leg and a fourth AC coil wound around the third leg, the third and fourth AC coils being wound in series and connected to a second phase AC source, wherein the third and fourth AC coils are arranged to produce a second closed AC magnetic circuit in the second and third legs in an AC flux direction that alternates with each AC half- cycle. In addition, the FCL may further comprise a fifth AC coil wound around the second leg and a sixth AC coil wound around the third leg, the fifth and sixth AC coils being wound in series and connected to a third phase AC source, wherein the fifth and sixth AC coils are arranged to produce a third closed AC magnetic circuit in the second and third legs in an AC flux direction that alternates with each AC half-cycle.

Embodiments of the invention will now be described, by way of example and with reference to the accompanying drawings in which:-

Figures la and lb show an FCL according to a first arrangement;

Figure 2 shows a schematic illustration of the magnetic circuits produced in FCL according to the first arrangement;

Figure 3 is a graph of AC current against time for the first arrangement;

Figure 4a shows a model of flux density (B) and Figure 4b shows a model of

AmpTurns/m (H) for the initial state of the first arrangement;

Figure 5a shows a model of flux density (B) and Figure 5b shows a model of

AmpTurns/m (H) for normal operating conditions of the first arrangement;

Figure 6a shows a model of flux density (B) and Figure 6b shows a model of

AmpTurns/ m (H) for fault conditions of the first arrangement;

Figure 7a shows a portion of an FCL according to a second arrangement, and Figure 7b shows a model of flux density (B);

Figure 8 is a graph of AC current on the against time for the second arrangement; Figure 9a shows a portion of an FCL according to a first embodiment of the invention, and Figure 9b shows a model of flux density (B);

Figure 10 is a graph of AC currents on the against time for the first embodiment; Figure 11 shows a portion of an FCL according to a second embodiment of the invention;

Figure 12 shows a portion of an FCL according to a third embodiment of the invention; Figure 13a shows a portion of an FCL according to a third arrangement, and Figure 13b shows a model of flux density (B);

Figure 14 is a graph of AC current on the against time for the third arrangement;

Figure 15a shows a portion of an FCL according to a fourth embodiment of the invention, and Figure 15b shows a model of flux density (B);

Figure 16 is a graph of AC current on the against time for the fourth embodiment; Figure 17a shows a portion of an FCL according to a fifth embodiment of the invention, Figure 17b shows a model of flux density (B), and Figure 17c is a graph of AC current against time for the fifth embodiment;

Figure 18a shows a portion of an FCL according to a sixth embodiment of the invention, and Figure 18b shows a model of flux density (B);

Figure 19 shows an FCL according to a seventh embodiment of the invention;

Figure 20a shows an FCL according to a fourth arrangement, and Figure 20b shows a model of flux density (B);

Figure 21a shows an FCL according to a eigth embodiment of the invention, and Figure 21b shows a model of flux density (B);

Figure 22a shows an FCL according to a ninth embodiment of the invention, and Figure 22b shows a model of flux density (B);

Figure 23a shows an FCL according to a tenth embodiment of the invention, and Figure 23b shows a model of flux density (B);

Figure 24a shows an FCL according to a fifth arrangement, and Figure 24b shows a model of flux density (B) ;

Figure 25a shows an FCL according to an eleventh embodiment of the invention, and Figure 25b shows a model of flux density (B);

Figure 26 shows an FCL according to a twelfth embodiment of the invention; and Figure 27 shows an FCL according to a thirteenth embodiment of the invention.

Figures la and lb show a first illustrative arrangement of an FCL. In this arrangement, the FCL 1 has a single core, and the FCL 1 is arranged to limit fault currents for a single phase AC supply. Figure la shows a front view, whereas Figure lb shows a side view. As shown in Figure la, the FCL 1 has a single core that includes four legs 10a, 20a, 20b and 10b aligned in the same direction. The four legs are joined by a first yoke 30a at one end, and by a second yoke 30b at the other end. In this arrangement, the four legs 10a, 20a, 20b and 10b are aligned vertically, with the two yokes 30a, 30b aligned horizontally. A first DC coil 11a is wound around the first leg 10a, and a second DC coil lib is wound around the fourth leg 10b. Hence, a DC coil is wound around each of the two outer legs 10a and 10b.

A first AC coil 21a is wound around the second leg 20a, and a second AC coil 21b is wound around the third leg 20b. The AC coils 21a and 21b are connected in series, and are connected to the grid. Hence, the two AC coils 21a and 21b are wound around in series around the inner legs.

The DC coils 11a and 11b are wound so that the flux produced by the DC coils in the outer two legs has the same direction. The AC coils are wound such that the flux produced by the AC coils in the inner two legs supports the DC flux in one AC leg and opposes the DC flux in the other AC leg. Hence, the arrangement of Figure 1 has a closed magnetic loop for the DC flux and a closed magnetic loop for the AC flux. This is shown Figure 2, which schematically shows the magnetic circuits produced by the DC and AC coils. The coils themselves are not shown in Figure 2, for ease of illustration.

As shown in Figure 2, the first DC coil 11a produces a first DC magnetic circuit 12a in a closed group around the first leg 10a and the second leg 20a. The second DC coil produces a second DC magnetic circuit 12b in a closed loop around the fourth leg 10b and the third leg 20b. As shown in Figure 2, the first DC magnetic circuit 12a has a clockwise DC flux direction and the second DC magnetic circuit 12b has an

anticlockwise DC flux direction.

The AC coils 21a and 21b are wound such that the there is a closed AC magnetic circuit 22. The direction of the flux in closed AC magnetic circuit 22 is such that the AC flux in one of the inner legs will oppose the DC flux in that leg, whereas the AC flux in the other leg will support the DC flux in that leg. The situation will reverse in the next half- cycle of the AC current. Hence, Figure 2 shows a snapshot in time the AC flux in the second leg 20a opposes the DC flux in the second leg 20a, whereas the AC flux in the third leg 20b supports the DC flux in the third leg 20b. In the next half -cycle, the direction of the AC flux will reverse (i.e. it will switch from being clockwise to anticlockwise), and the AC flux in the second leg 20a will support the DC flux in the second leg 20a, and the AC flux in the third leg 20b will oppose the DC flux in the third leg 20b.

The legs and yokes have, in this embodiment, interleaved, mitred, step-lapped joints. However, other embodiments can employ simpler arrangements, using non-mitred, butt-lapped joints. The core is built from grain-oriented sheet steel laminations, though other embodiments could use alternative core structures.

The coils (AC and DC) are made of electrolytic grade copper in this arrangement.

However, other arrangement could use alternative materials for the coils.

The FCL 1 of this arrangement can further comprise a tank (not shown) arranged to house the core. The tank can be partially or completely filled with a dielectric fluid. Any suitable dielectric fluid could be used, for example mineral oil or vegetable oil (which have been found to be suitable as a dielectric for voltages up to 30okV and beyond).

In the arrangement of Figure la, which represents a FCL rated lokVA, the outer DC legs (first leg 10a and fourth leg 10b), top yoke 30a and bottom yoke 30b are each 60mm wide and 32mm deep. The inner AC legs (second leg 20a and third leg 20b) are each 40mm wide and 32mm deep. The leg centres are 103mm between the AC legs and 149mm between the AC and DC legs. As will be explained in more detail, the longer distance between AC and DC legs reduces the AC voltage induced in DC coils during short circuit.

In the illustrated arrangement, each DC coil 11a, lib has 60 turns and 50 A rms normal current, and each AC coil 21a, 21b has 48 turns and 14.5 A rms normal current. It will be, of course, appreciated that the example values and dimensions of the FCL mentioned above are purely for illustrative purposes. FCLs according to such arrangements may, for example, have much higher current ratings and may be much larger in size.

The operation of the fault current limiter 1 shown in Figure la in normal and fault conditions will now be explained. The single phase configuration was simulated as a 3D Transient Electromagnetic model, and the results are shown in Figures 3 - 6, which will be used to explain the operation of the fault current li miter l shown in Figure la in normal and fault conditions

Figure 3 shows a graph of current versus time for the series connected AC coils 21a and 21b shown in Figure la. The darker line shows the current with the FCL 1 in operation, and the lighter line shows the prospective short circuit current of the system if the FCL 1 were not in the circuit. In this example, the prospective short circuit current is simulated as i43.5Arms. Up to a time of 30.5 milliseconds, the FCL 1 is under normal conditions. Hence the AC current up to a time of 30.5 milliseconds is sinusoidal. The lighter line shows the AC current that would flow if the FCL 1 were not in the circuit in the event of a short circuit. The darker line shows the limited short circuit current resulting from the fault current limitation of the FCL 1.

Figure 4a shows a model of flux density (B) and Figure 4b shows a model of

AmpTurns/m (H) for the initial state (t = o milliseconds) of the first embodiment of the invention. The initial (transient) state (t = o milliseconds) is used to set up the transient analysis. At this point, the AC current is oA, and the DC current in each DC coil is 50A. As shown in the flux plot of Figure 4a, the flux produced by each DC coil 11a, lib returns through the nearest AC leg. Hence, the first DC coil 11a produces flux in the first DC magnetic circuit 12a that flows around the first leg 10a and the second leg 20a. The second DC coil 11b produces flux in the second DC magnetic circuit 12b that flows around the fourth leg 10b and the third leg 20b. The flux in the first DC magnetic circuit 12a flows in a different direction to the second flux in the DC magnetic circuit 12b. There is no current on the AC coils, and hence there is no flux produced by the AC coils.

Hence, in the illustrated arrangement, the flux in the first DC magnetic circuit 12 a flows in a clockwise direction and the flux in the second DC magnetic circuit 12b flows in an anti-clockwise direction. As a result of this arrangement of the flux, as shown in the saturation plot 4b, each of the first 11a, second 20a, third 20b and fourth 10b legs are in saturation (light colour in Figure 4b. As shown in Figure 4b, the AC legs 20a and 20b are deeper in saturation (lighter in colour) than the DC legs as they have a smaller cross-section in this embodiment. As shown in Figure 4b, the four legged arrangement of the FCL 1 is such to keep the first 10a, second 20a, third 20b and fourth 10b legs in saturation, while having areas of the yokes 30a and 30b out of saturation. As shown in Figure 4b, the corner regions of the yokes 31a, 31b, 31c and 3id are out of saturation, as are the regions 32a and 32b between the two inner legs (second leg 20a and third leg 20b).

Hence, as shown in Figure 4b, each DC coil couples to its nearest AC coil, with the flux concentrated on the four legs, with maximum saturation on the two inner legs.

Figures 5a and 5b show flux density (B) and AmpTurns/ m (H) plots at a snapshot in time in the normal current state (t = 5 milliseconds). As shown in Figure 3, a time of 5 milliseconds shows peak AC current in the normal current state.

Figure 5a shows the net magnetic flux in the FCL 1 (i.e. adding the flux contributions of the AC and DC magnetic circuits). At the AC current peak occurring at time of 5 milliseconds, the flux produced by the AC coils 21a and 21b closed AC magnetic circuit 22 is anticlockwise in this half cycle. Hence, the flux produced by the AC coil 21a in the second leg 20a supports the DC flux produced by the first DC coil 11a in the second leg 20a, whereas the AC flux produced by the second AC coil 21b in the third leg 20b opposes the DC flux produced by the second DC magnetic circuit 12b in the third leg 20b.

As a result, as shown in Figure 5b, the second leg 20a is put deeper in saturation (lighter in colour) when compared to the equivalent plot of the second leg 20a in Figure 4b under transient conditions, whereas the third leg 20b is made less saturated (darker in colour) when compared to the third leg 20b in Figure 4b.

This arrangement of the AC magnetic circuit supporting/opposing the DC magnetic circuits will reverse in the next half cycle, with the third leg 20b becoming more saturated and the second leg 20a becoming less saturated than under the transient conditions. Under normal conditions, the second and third legs are kept in a saturated state (with one leg being more saturated than the other leg). Hence, under normal conditions, the coils around saturated legs 20a and 20b have very low impedance, and hence the FCL 1 is virtually transparent to the grid connected to the FCL 1.

In the above described arrangement, the AC legs have a smaller cross-section than the DC legs. This has the advantage that the AC legs are easier to saturate than the DC legs, which helps ensure low impedance in normal conditions. As shown in Figure 3, at a time of 30.5 milliseconds, a short circuit state is simulated. Figures 6a and 6b show flux and saturation plots at a snapshot in time in this short circuit state (t = 63.5 milliseconds). A time of 63.5 milliseconds represents a region near the fourth AC current peak after the short circuit. The arrangement of the DC and AC magnetic circuits in Figures 6a and 6b are the same as those described in the normal current state in Figures 5a and 5b, except the magnitude of the AC flux is increased due to the higher AC current in the short circuit state. Hence, in the short circuit state, the effect of the AC magnetic circuit supporting the DC flux in one leg and opposing the DC flux in the other inner leg is magnified.

As shown in Figure 6b, the magnification of the AC magnetic circuit

supporting/opposing the DC magnetic circuits has the effect of (in this half cycle) putting the second leg 20a into very high saturation, whilst putting the third leg 20b into an unsaturated state. The effect of the third leg 20b being in the unsaturated state in Figure 6b will be that the impedance of the coil on the right-hand leg will increase, acting to limit the fault current.

The situation in the next AC half cycle will reverse, with the second leg 20a being put out of saturation (and hence its impedance will rise), with the third leg 20b being more saturated. Hence, during fault conditions, in every half-cycle, one of the second or third legs (i.e. the inner legs) will be out of saturation, ensuring a high impedance state. This alternation of raising impedance in the AC coils on one of the inner legs continues until the fault is cleared. Table 1 shows computed AC impedance for the normal state and for five half cycles after the simulated short circuit. Table 1

As can be seen from Table l, the ratio of the short circuit to normal impedance varies from 5.9 to 12.6 for the five successive half cycles after short circuit.

While the above described arrangements are substantial improvements over conventional designs, the present invention provides embodiments that reduce or prevent transformer coupling between AC coils and DC coils in fault limiting conditions.

In general, saturable core FCLs can take a variety of different forms, with different arrangements of DC and AC coils on one or more cores, with each core having one or more legs. All such FCLs work on a principle of the DC coil(s) saturating the core (or at least a portion of the core) in normal conditions, with the flux from the AC coil either acting for or against the DC flux but not overcoming it in normal conditions. In such arrangements, in a fault condition, the AC flux will bring the core out of saturation, acting to limit the fault.

Furthermore, saturable core FCLs can have magnetically open or magnetically closed circuits for the AC and/or DC flux.

In general terms, one of main problems for FCLs that have core saturated by DC bias is transformer coupling between the AC coil(s) and the DC coil(s) in fault limiting conditions.

In particular, this transformer coupling may cause high voltage induced on DC circuit which can lead to the failure of the DC power supply. On the other hand, low output impedance can reduce limiting performance of the FCL. Additional problem from this phenomenon is DC current increasing at the time which AC current tries to desaturate the AC leg according with Lenz's law.

In such fault conditions, a large voltage will be applied across the AC coils and coupling with DC coils may cause a number of unwanted affects. For example, it may lead to damage to the DC power supply (which typically is low voltage device) as a result of a high induced voltage on its terminals vs. ground.

Furthermore, this transformer coupling can lead to a decreased impedance of the device in limiting state thus decreasing limiting capability (performance) of the FCL. In order to provide deep saturation, the number of turns of the DC coils is typically few times larger than the number of turns of the AC coils. Hence, the impedance which the AC coil will see will be less than the output impedance of the DC circuit. For a transformer with turn ratio N/n (where N is larger number of turns of the first transformer coil and n is the small number of turns of the second transformer coil) this last coil with small number of turns will see impedance Z connected to the first coil by Z_2nd = [(n/N) A 2]*Z_first.

In addition, this transformer coupling can lead to large high harmonic contents in voltage induced on DC coil and thus a risk of damage to DC power supply.

This transformer coupling can lead to each DC coil in limiting state being twice saturated/desaturated during the AC current cycle. Thus, a second harmonic exists in flux change and induced voltage. Additional high harmonic contents provided by fast flux changes during the saturation/desaturation of the AC legs which may cause sharp peaks getting inducedon the DC coils.

Embodiments of the present invention can lead to a decrease in this coupling between AC and DC coils compared to conventional arrangements. In the following description embodiments of the invention will be explained with the help of further illustrative arrangements.

Figure 7a shows a schematic of a portion of an FCL according to a second arrangement. In this portion of the FCL 100, there is a section of ferromagnetic core 101, around which there is a DC coil 111 and an AC coil 121. Hence, the DC coil 111 and AC coil 121 are wound on a single leg. It will be appreciated that the portion shown in Figure 7a may form part of the FCL 100. For example, the FCL 100 could comprise two cores 101, each with a DC coil and AC coil as shown in Figure 7a, with the two AC coils connected in series. In such a configuration, these two cores with 2 AC and 2 DC coils will provide an FCL with open- AC - open-DC magnetic path configuration (open-open).

Figure 7b shows a model of flux density (B) for the arrangement of Figure 7a at a maximum AC current point, and Figure 8 shows a graph of current against time for this arrangement.

In the portion of the FCL 100 in Figure 7a, it is assumed that the DC coil 111 saturates the core 101. The current in the AC coil 121 produces a flux whose direction varies with each half cycle. It will be appreciated that the flux from the AC coil 121 will induce a current in the DC coil 111, and this induced current is shown in Figure 8, where the induced current in DC winding is of 97 A rms.

The effect of this induced current is shown in Figure 7b, where the flux from the AC coil induces an opposing flux on the DC coil 111. As can be seen, the AC arrows coming from the AC coil 121 penetrate deeply into the area of the DC coil 11.

As shown in Figure 7b, there is transformer coupling between the DC coil 111 and AC coil 121, which for the various reasons discussed above is disadvantageous.

Figure 9a shows a portion of an FCL 100a according to a first embodiment of the invention. In the portion of the FCL 100a of Figure 9a, there is a section of

ferromagnetic core 101 on which there is a DC coil 111 and an AC coil 121. Furthermore, there is a shorted ring 131 around the core 101 that acts as an electro-magnetic screen between the DC coil 111 and an AC coil 121. The electro-magnetic screen acts as a flux isolator, as will be described in detail below.

In this embodiment the shorted ring 131 is made out of copper. However, in other embodiments, the shorted ring 131 could be made out of any suitable conductor.

It will be appreciated that the portion shown in Figure 9 would form part of the FCL iooa. For example, the FCL 100a could comprise two legs, each with a DC coil and AC coil as shown in Figure 9, with the two AC coils connected in series. Figure 9b shows a model of flux density (B) for the FCL 100a of Figure 9a at a maximum AC current point, and Figure 10 shows a graph of current against time for this arrangement. As for the FCL 100 of Figure 7a, the flux from the AC coil 121 in the FCL 100a of Figure 9a will (in the appropriate half cycle) flow towards the DC coil 111. However, in this arrangement the shorted ring 131 reduces the induced voltage in the DC coil 111.

As can be seen from Figure 9b, the AC arrows coming from the AC coil 121 do not penetrate deeply into the area of the DC coil 111. This is because the flux from the AC coil 121 induces a voltage and current in the shorted ring 131 that creates a field that opposes the AC flux. Hence, the shorted ring acts to prevent the AC flux from penetrating the area of the DC coil 111. As shown in Figure 10, the induced current in the DC coil 111 lowered down to 46 A rms, which is a 52% suppression effect when compared to Figure 8. The current induced in shorted ring reaches 5139 A rms.

Figure 11 shows a portion of an FCL 100b according to a second embodiment of the invention.

In the portion of the FCL 100b there is a section of ferromagnetic core 101, around which there is a DC coil 111, a first AC coil 121a and a second AC coil 121b, with the first and second AC coils 121a, 121b connected in series. In the FCL 100b, there is a shorted ring 131 that is arranged on a portion of the core 101 that is between the first AC could 121a and the DC coil 111. In this embodiment, the shorted ring 131 will prevent flux from the first AC could 121a from penetrating the area of the DC coil 111.

Figure 12 shows a schematic of a portion of an FCL looc according to a third embodiment of the invention.

In this embodiment, there is a section of ferromagnetic core 101, around which there is a DC coil 111, a first AC coil 121a and a second AC coil 121b, with the first and second AC coils 121a, 121b connected in series. In this embodiment, there is a first shorted ring 131a that is arranged on a portion of the core 101 that is between the first AC could 121a and the DC coil 111, and a second shorted ring 131b that is arranged on a portion of the core 101 that is between the second AC could 121b and the DC coil 111.

In this embodiment, the first shorted ring 131a will prevent flux from the first AC coil 121a from penetrating the area of the DC coil 111, and the second shorted ring 131b will prevent flux from the second AC coil 121a from penetrating the area of the DC coil 111. Hence, the first and second shorted rings 131a, 131b reduce the induced voltage in the DC coil 111. Figure 13a shows a portion of an FCL 200 according to a third illustrative arrangement. The FCL 200 has a ferromagnetic core 201 that has a first leg 202a and a second leg 202b, with the ends of the first and second legs connected by yokes 203 a and 203b. A DC coil 211 is wound on the first leg 202a and an AC coil 221 is wound on the second leg 202b.

It will be appreciated that the portion shown in Figure 13a may form part of the FCL 200. For example, the FCL 200 could comprise two cores, each configured like Figure 13a, with the two AC coils connected in series. Figure 13b shows a model of flux density (B) for the FCL 200 of Figure 13 a at a maximum AC current point, and Figure 14 shows a graph of current against time for this arrangement.

The current in the AC coil 221 produces a flux whose direction varies with each half cycle. It will be appreciated that the flux from the AC coil 221 will induce a current in the DC coil 211, and this induced current is shown in Figure 14, where the maximum induced current in DC winding is of ~+i90 A. The effect of this induced current is shown in Figure 13b, where the flux from the AC coil induces an opposing flux on the DC coil 211. As can be seen, the AC arrows coming from the AC coil 221 penetrate deeply into the area of the DC coil 211.

Figure 15a shows a portion of an FCL 200a according to a fourth embodiment of the invention. In the arrangement 200a of Figure 15a, there are first and second legs 202a, 202b connected by yokes 203a and 203b. A DC coil 211 is wound on the first leg 202a and an AC coil 221 is wound on the second leg 202b. In the embodiment 200a there is a shorted ring 231 acting as an electro- magnetic screen between the DC coil 111 and an AC coil 121, at a position that is proximate the DC coil 111. It will be appreciated that the portion shown in Figure 15a may form part of the FCL 200a. For example, the FCL 200a could comprise two cores 201, each configured like Figure 15a, with the two AC coils connected in series.

Figure 15b shows a model of flux density (B) for the embodiment of Figure 15a at a maximum AC current point, and Figure 16 shows a graph of current against time for this embodiment

As for the arrangement 200 in Figure 13a, the the flux from the AC coil 221 will (in the appropriate half cycle) flow towards the DC coil 211. However, in this embodiment the shorted ring 231 reduces the induced current in the DC coil 211.

As can be seen from Figure 15b, the AC arrows coming from the AC coil 221 do not penetrate deeply into the area of the DC coil 211. This is because the flux from the AC coil 221 induces a voltage and current in the shorted ring 231 that creates a field that opposes the AC flux. Hence, the shorted ring acts to prevent the AC flux from penetrating the area of the DC coil 211. Hence, the shorted ring 231 acts as an electromagnetic screen between the DC coil 211 and an AC coil 221.

As shown in Figure 16, the maximum induced current in the DC coil 211 is lowered down to 125 A, which is a 34% suppression effect when compared to Figure 8. The current induced in shorted ring reaches 1552 A rms.

Figure 17a shows a portion of an FCL 200b according to a fifth embodiment of the invention that is a modification of the embodiment of Figure 15a. In this embodiment, the portion of the FCL 200b has two shorted rings 231a, 231b, with one arranged at either side of the DC coil 211.

Figure 17b shows a model of flux density (B) for the embodiment of Figure 17a at a maximum AC current point. As can be seen, there is a significant reduction in the amount of AC flux penetrating the area of the DC coil 211 when compared to the arrangement of Figure 13 a. Furthermore, the presence of two shorted rings 231a, 231b will ensure that this reduction of flux is efficiently performed in both AC half cycles. Figure 17c is a graph of AC current on the against time for the fifth embodiment, showing the reduction of the current induced in the DC coil as compared to Figure 16.

In Figures 13a, 15a, and 17a both the DC coil and AC coil are trying to drive magnetic flux upwards (DC coil clockwise and AC coil counterclockwise). When number of shorted rings increases, less AC flux penetrates DC leg and DC coil succeeds in driving more flux upwards.

Figure 18a shows a further modification of the arrangement of Figure 15a. In this embodiment 200c, instead of a shorted ring, one of the turns 211a of the DC coil 211 is shorted. Hence, the shorted turn 211a of the DC coil 211 acts as an electro-magnetic screen in a similar way to the shorted ring 231 of Figure 15a.

Figure 18b shows a model of flux density (B) for the arrangement of Figure 18a at a maximum AC current point. As can be seen, there is a significant reduction in the amount of AC flux penetrating the area of the DC coil 211 when compared to the arrangement of Figure 13 a.

The embodiment of Figure 18a has the advantage that an electro-magnetic screen be provided by simply shorting one turn of the DC coil. Hence, no separate material for a shorted ring (or shorted coil) is required. In other embodiments, a different number of turns of the DC coil are shorted to provide the electro- magnetic screen.

In the embodiment of Figure 18a, there is a single shorted turn 211a of the DC coil 211 at the edge of the DC coil 211 closest to the AC coil 221 that forms a first

electromagnetic screen. In other embodiments, the first electromagnetic screen could comprise more than one shorted turn of the DC coil 211 at the edge of the DC coil 211 closest to the AC coil 221. Furthermore, in other embodiments, the FCL could comprise a second electromagnetic screen comprising another set of one or more shorted turns of the DC coil 211 at the opposite end of the DC coil to the first electromagnetic screen.

Figure 19 shows a schematic of an orthogonal FCL 200d according to a seventh embodiment of the invention. In this embodiment, the FCL 200d has a ferromagnetic core 201 that has a first leg 202a and a second leg 202b, with one end of the first and second legs connected by a third leg 202c and the other end of the first and second legs connected by a fourth leg 202d. A first DC coil 211a is wound on the first leg 202a and a second DC coil 211b is wound on the second leg 202b. A first AC coil 221a is wound on the third leg 202c and a second AC coil 211b is wound on the fourth leg 202d, with the first and second AC coils 221a, 221b wound in series. In this arrangement, the first and second AC coils 221a, 221b are arranged to produce a closed magnetic circuit, whose flux direction changes with each half cycle.

In this embodiment, there are first and second shorted rings 231a and 231b arranged either side of the first DC coil 211a. There are also third and fourth shorted rings 231c and 23id arranged either side of the second DC coil 211b. As a result, it will be appreciated that the shorted rings will prevent flux from the first and second AC coils 221a, 221b from penetrating into the respective areas of the DC coils 211a, 211b.

Figure 20a shows a schematic of an FCL 300 according to a fourth illustrative arrangement. In this arrangement, the FCL 300 has a ferromagnetic core 301 that has a first leg 302a, a second leg 302b, and a third leg 302c, with one end of the first to third legs connected by a first yoke 303a and the other end of the first to third legs connected by a second yoke 303b. A first AC coil 321a is wound on the first leg 302a, and a second AC coil 321b is wound on the third leg 302c, with the first and second AC coils 321a, 321b connected in series. A DC coil 311 is wound around the second leg 302b. In this arrangement, the first and second AC coils 321a and 321b are arranged to produce AC flux that flows in the same directions (e.g. flux from AC coil 321a in clockwise direction and flux from AC coil 321b also in clockwise direction) in one half cycle in a magnetically closed arrangement and in oppose direction in another half -cycle.

Hence, the AC flux from the first AC coil 321a will tend to flow from the fist leg 302 a to the second leg 302b via the first yoke 303a, and then back to the fist leg 302a via the second yoke 303b. Similarly, the AC flux from the second AC coil 321b will tend to flow from the third leg 302c to the second leg 302b via the first yoke 303a, and then back to the third leg 302c via the second yoke 303b.

Figure 20b shows a model of flux density (B) for the arrangement of Figure 20a at a maximum AC current point. As can be seen, there is a significant amount of flux from the first and second AC coils 321a and 321b in the area of the DC coil 311. Figure 21a shows an FCL 300a according to an eighth embodiment of the invention. The FCL 300a has a ferromagnetic core 301 that has a first leg 302a, a second leg 302b, and a third leg 302c, with one end of the first to third legs connected by a first yoke 303a and the other end of the first to third legs connected by a second yoke 303b. In the arrangement 300a of Figure 21a, the first and second AC coils 321a and 321b are wound and connected in the same way as for Figure 20a.

The FCL 300a has a shorted ring 331 acting as an electro- magnetic screen between the DC coil 311 and first and second AC coils 321a and 321b, at a position that is proximate the DC coil 311 between the DC coil 311 and the first yoke 303a.

Figure 21b shows a model of flux density (B) for the embodiment of Figure 21a at a maximum AC current point. As can be seen, the shorted ring 231 reduces the induced flux in the DC coil 311. The AC arrows coming from the first and second AC coils 321a and 32 lb do not penetrate deeply into the area of the DC coil 311. This is because the flux from the first and second AC coils 321a and 321b induces a voltage and current in the shorted ring 331 that creates a field that opposes the AC flux direction. Hence, the shorted ring 331 acts to prevent the AC flux from penetrating the area of the DC coil 311ยท

Figure 22a an FCL 300b according to a ninth embodiment of the invention. The FCL 300b has two shorted rings 331a, 331b, with one arranged at either side of the DC coil 311. Figure 22b shows a model of flux density (B) for the embodiment of Figure 22a at a maximum AC current point. As can be seen, there is a significant reduction in the amount of AC flux penetrating the area of the DC coil 211 when compared to the arrangement of Figure 20a. Furthermore, the presence of two shorted rings 331a, 331b will ensure that this reduction of flux is efficiently performed in both AC half cycles. The configuration (described in 22a) operates in a different way compared to some of the other embodiments. In the nominal state, DC flux may saturate AC legs and cause low impedance at the same manner as in other embodiments. However, in fault current limiting state large magneto-motive force (MMF) of the AC coils cannot (because of the shorted rings) effectively change DC flux in the DC leg.

In such a way DC flux will "close" in the DC leg and the DC leg will be left in deep saturation state. Thus AC flux will flow via close magnetic circuit : core 302a-yoke 303a-core 302c-yoke 303b in clockwise direction in one half cycle and in the opposite direction in the other half cycle.

In Figures 20a, 21a, 22a and 23a both the DC coil is trying to drive flux upwards. The AC coils are trying to drive flux clockwise direction, that is left hand side AC coil tries to drive flux upwards and right hand side AC coil tries to drive flux downwards.

As the number of shorted rings increases, less AC flux penetrates DC leg and DC coil succeeds in driving more flux upwards.

Figure 23a shows an FCL 300c according to a tenth embodiment of the invention. The FCL 300c has four shorted rings 331a, 331b, 331c, 33id. The first and second shorted rings 331a and 331b are located on the first yoke 303a, at positions either side of the second leg 302b. Hence, the first shorted ring 331a is between the DC coil 311 and the first AC coil 321a and the second shorted ring 331b is between the DC coil 311 and the second AC coil 321b. The third and fourth shorted rings 331c and 33id are located on the second yoke 303b, at positions either side of the second leg 302b. Hence, the third shorted ring 331c is between the DC coil 311 and the first AC coil 321a and the second fourth ring 33id is between the DC coil 311 and the second AC coil 321b.

Figure 23b shows a model of flux density (B) for the embodiment of Figure 23a at a maximum AC current point. As can be seen, there is a significant reduction in the amount of AC flux penetrating the area of the DC coil 211 when compared to the arrangement of Figure 20a. Furthermore, the presence of four shorted rings 331a, 331b, 331c, and 33id will ensure that this reduction of flux is efficiently performed in both AC half cycles.

Figure 24a shows a schematic of an FCL 400 according to a fifth illustrative arrangement. The FCL 400 is a four legged arrangement 400 with a ferromagnetic core 401 that has a first leg 402a, a second leg 402b, a third leg 402c, and a fourth leg 402d. One end of the first to fourth legs is connected by a first yoke 403a and the other end of the first to fourth legs is connected by a second yoke 403a.

A first DC coil 411a is wound on the first leg 402a, and a second DC coil 411b is wound on the fourth leg 402d. A first AC coil 421a is wound on the second leg 402b and a second AC coil 421b is wound on the third leg 402 c, with the first and second AC coils 421a, 421b wound in series. A DC coil 311 is wound around the second leg 302b. In this arrangement, it will be appreciated that the fault current limited is structurally similar to the FCL shown in Figure 1, and the AC coils are wound so as to each produce an AC magnetic circuit within the two inner limbs (second leg 402b and third leg 402c) that opposes the DC flux in one leg and supports the DC flux in the other leg, with the situation reversing in the next half cycle.

Figure 24b shows a model of flux density (B) for the arrangement of Figure 24a at a maximum AC current point. As can be seen, there is a significant amount of flux from the first and second AC coils 421a and 421b in the area of the first and second DC coils 411a and 411b.

Figure 25a shows an FCL 400a according to an eleventh embodiment of the invention. The FCL 400a has a ferromagnetic core 401 that has a first leg 402a, a second leg 402b, a third leg 402c, and a fourth leg 402d. One end of the first to fourth legs is connected by a first yoke 403a and the other end of the first to fourth legs is connected by a second yoke 403b.

In this embodiment there are two shorted rings either side of the first and second DC coils 411a and 411b. There is a first shorted ring 431a between the first DC coil 411a and the first yoke 403a, and a second shorted ring 431b between the first DC coil 411a and the second yoke 403b. There is also a third shorted ring 431c between the second DC coil 411b and the first yoke 403a, and a fourth shorted ring 43id between the second DC coil 411a and the second yoke 403b.

Figure 25b shows a model of flux density (B) for the embodiment of Figure 25a at a maximum AC current point. As can be seen, there is a significant reduction in the amount of AC flux penetrating the area of the first and second DC coils 411a and 411b when compared to the arrangement of Figure 24a. Furthermore, the presence of four shorted rings 431a, 431b, 431c, and 43 id will ensure that this reduction of flux is efficiently performed in both AC half cycles.

Figure 26 shows an FCL 400b according to a twelfth embodiment of the invention. The FCL 400b has a ferromagnetic core 401 that has a first leg 402a, a second leg 402b, a third leg 402c, and a fourth leg 402d. One end of the first to fourth legs is connected by a first yoke 403a and the other end of the first to fourth legs is connected by a second yoke 403b. In this embodiment, there are four shorted rings acting as electro-magnetic screens between the AC coils and the DC coils. There is a first shorted ring 431a on the first yoke between the first leg 402a and the second leg 402b, a second shorted ring 431b on the first yoke 403a between the third leg 402c and the fourth leg 402d, a third shorted ring 431c on the second yoke between the first leg 402a and the second leg 402b, and a fourth shorted ring 43id on the second yoke 403c between the third leg 402c and the fourth leg 402 d.

In the embodiment of Figure 26, the four shorted rings between the AC coils and the DC coils prevent flux from the AC coils from penetrating into the areas of the DC coils.

In the nominal state, the DC flux saturates both AC legs as illustrated in Fig.24. Thus, the shorted turns on yokes do not strongly influence the nominal performance.

However, in fault limiting conditions due to the shorted turns on the yokes working as electro-magnetic screen, we will have an AC closed magnetic circuit for both AC coils which is isolated from the DC magnetic circuit. Thus it may be possible to get more limiting impedance and limiting capability in some configurations.

Figure 27 shows an FCL 400c according to a thirteenth embodiment of the invention.. In this embodiment, the FCL 400c has a single core, and the FCL is arranged to limit fault currents for each phase of a three-phase AC supply. Figure 27 shows a side view of the FCL.

In the embodiment shown in Figure 27, there is a single core with four legs 402a, 402b, 402c and 402d aligned in the same direction, with a first yoke 403a joining one end of the four legs, and a second yoke 403b joining the other ends of the four legs. In this embodiment, the four legs 402a, 402b, 402c and 402d are aligned vertically, with the two yokes 403 a, 403b aligned horizontally.

A first DC coil 411a is wound around the first leg 402a, and a second DC coil 411b is wound around the fourth leg 402d (i.e. around the two outer legs). There are two AC coils connected in series for each of the three phases of the AC supply. As shown in Figure 27, a first AC coil 42iRa and a second AC coil 42iRb are connected in series to the first (R) phase of the three-phase supply. The first AC coil 42iRa is wound around the second leg 402b, and the second AC coil 42iRb is wound around the third leg 402c. A third AC coil 42iSa is connected in series to a fourth AC coil42iSb, and the third and fourth AC coils 42iSa and 42iSb are connected to the second (S) phase of the three- phase supply. The third AC coil 42iSa is wound around the second leg 402a, and the fourth AC coil 42iSb is wound around the third leg 402c.

A fifth AC coil 42iTa is connected in series to a sixth AC coil 421Tb, and the fifth and sixth AC coils 42iTa and 421Tb are connected to the third (T) phase of the three-phase supply. The fifth AC coil 42iTa is wound around the second leg 402b, and the sixth AC coil 421Tb is wound around the third leg 402c.

The AC coils on second leg 402b are placed top to bottom as first 42iRa, third 42iSa and fifth 42iTa respectively. In other words, the first 42iRa, third 42iSa and fifth 32iTa AC coils are arranged in order on the second leg 402b. The AC coils on third leg 402c are placed top to bottom as sixth 421Tb, fourth 42iSb and second 42iRb respectively. In other words, the AC coils on the third 402c leg are arranged in an opposite order of the R, S, T phases when compared to the second leg 120a. Other sequential arrangements of R, S and T phases may be used on the AC legs in other embodiments.

The AC coils for each of the three phases are wound in a similar way to the AC coils 21a and 21b in Figure la. In other words, they are wound so as to each produce an AC magnetic circuit within the two inner limbs (second leg 402b and third leg 402c) that opposes the DC flux in one leg and supports the DC flux in the other leg, with the situation reversing in the next half cycle.

In this embodiment, there are four shorted rings between the AC coils and the DC coils. There is a first shorted ring 431a on the first yoke between the first leg 402a and the second leg 402b, a second shorted ring 431b on the first yoke 403a between the third leg 402c and the fourth leg 402d, a third shorted ring 431c on the second yoke between the first leg 402a and the second leg 402b, and a fourth shorted ring 43 id on the second yoke 403c between the third leg 402c and the fourth leg 402d.

As discussed above, embodiments of the invention provide one or more electro- magnetic screens (e.g. a shorted ring or coil) on a portion of the core between a DC coil and an AC coil. These electro- magnetic screens reduce the induced voltage in the DC coil caused by flux from the AC coil. Reducing this induced voltage is associated with many benefits, for example those discussed above.

Furthermore, it will be appreciated that the above configurations that use a shorted ring as the electro-magnetic screens could use one or more shorted turns at the edge of one of the DC coils closest to an AC coil.

While the electro-magnetic screens of embodiments of the present invention are suitable for any FCL configuration, they are particularly suited for saturated core FCLs with "closed" AC magnetic circuit configuration, for example the three and four legged examples discussed above. In these cases, the FCLs are often designed such that in a large portion of the cycle in the limiting state AC leg approaches the "reverse saturation" state. This is because this design provides better weight/dimension characteristics. As a result of this, a high harmonic content may be observed in voltage across the FCL and even in limited fault current. This phenomenon may cause problems for grid equipment such as transformers, protection relays, circuit breakers etc that may be present, and also for the FCL as well. High harmonics in fault currents can lead to undesired forces and vibration in transformer coils and cores. Protection relays may operate incorrectly in cases where harmonic content is high. Circuit breakers may be impacted by transient overvoltage associated with this phenomenon.

Furthermore, increasing of losses in iron core can lead to thermal hot spots and other problems. Core laminations joint points, that are typically associated with higher flux density (induction) magnitude and flux changes, may develop higher heat, causing degradation of their insulation.

Furthermore, eddy currents developing in this area may create a constant over heating called a hot spot. High harmonic content may also cause increased vibration and acoustic noise of the core which may exceed standards and potentially lead to FCL damage.

Another type of problem may be caused by very fast changing of flux in AC and DC legs. This may lead to developing of high voltage spikes in the FCL and an increased risk of damage to the FCL. All these problems benefit from the merits of some embodiments of this invention : high harmonic contents decrease dramatically (especially for large prospective currents) and flux changes become more gradual. The shorted turns present electro-magnetic screen and thus each fast change of flux (caused of DC coils MMF or AC coils MMF) lead to current being induced in the shorted turn which reduces this change (Lenz's law).

The above embodiments have been discussed with reference to the electromagnetic screen being a shorted ring, or alternatively one or more shorted turns of a DC coil (for example one or more turns at the edge of the DC coil closest to the AC coil). More generally, the electro-magnetic screen could be any shape around the core. For example, the electro-magnetic screen may be a partial ring shape or a coil with any number of turns (or even not a full turn). Such electromagnetic screen acts to provide counter flux with any flux change experienced by the electro- magnetic screen. Such a flux change provides electro-motive force (EMF) which causes current flow in the electro-magnetic screen which provides the counter flux.

In the above mentioned embodiments, the shorted ring(s) is/are made of copper. However, in other embodiments, the shorted ring(s) could be made out of any suitable conductor.

As discussed above, embodiments of the invention provide fault current limiter that comprises a magnetically saturable core. The core comprises a first DC coil wound around a first portion of the core, with the first DC coil being arranged to saturate at least part of the core in normal conditions. First and second AC coils are provided with the first AC coil being wound around a second portion of the core, and the second AC coil being wound around a third portion of the core, with the first and second AC coils being wound in series and connected to a first phase AC source to produce an AC flux in an AC flux direction that alternates with each AC half-cycle. A first more electromagnetic screen is provided that comprises a length of conductive material arranged around a portion of the core between the first portion and the second portion of the core. In such arrangements, the AC flux from the AC coils is arranged to induce a current in the first more electro-magnetic screen that opposes the any flux change AC or DC, so as to reduce the voltage and current induced in the first DC coil by the AC flux. Embodiments of the invention can also provide a fault current limiter comprising a magnetically saturable core, the fault current limiter including: a first leg; a second leg, with a first AC coil wound on the second leg; a third leg, with a second AC coil wound around the third leg, the first and second AC coils being wound in series and connected to a first phase AC source; a fourth leg; wherein the first, second, third and fourth legs are arranged in order, wherein first ends of the first, second, third and fourth legs are joined by a first yoke and second ends of the first, second, third and fourth legs are joined by a second yoke; a first magnetic biasing unit arranged to produce a first closed magnetic circuit in the first leg and the second leg that has a first flux direction; and a second magnetic biasing unit arranged to produce a second closed magnetic circuit in the fourth leg and the third leg that has a second flux direction, wherein the first flux direction opposes the second flux direction; wherein the first and second AC coils are arranged to produce a first closed AC magnetic circuit in the second and third legs in an AC flux direction that alternates with each AC half -cycle; further comprising a first electro-magnetic screen comprising a length of electrically isolated conductive material arranged around a portion of the core between the first magnetic biasing unit and the first AC coil; wherein changes in the AC flux are arranged to induce a current in the first electro-magnetic screen that opposes the AC flux changes, so as to reduce the current induced in the first magnetic biasing unit (e.g. first DC coil) by the AC flux. The first and second magnetic biasing units can be DC coils. Such DC coils could be on the first and fourth legs, e.g. with a first DC coil on the first leg and a second DC coil on the fourth leg. Alternatively, the DC coils could be arranged differently. For example, the DC coils could be located around other parts of the core (e.g. parts of the yoke).

Furthermore, the first and second magnetic biasing units can be provided by a single unit, e.g. one DC coil. For example, such a DC coil could be arranged around the second and third legs.

In the above embodiments the core is built from grain -oriented sheet steel laminations, though other embodiments could use alternative core structures. The various legs and yokes have, in some embodiments, interleaved, mitred, step-lapped joints. However, other embodiments can employ simpler arrangements, using non-mitred, butt-lapped joints.

The coils (AC and DC) are made of electrolytic grade copper in this arrangement.

However, other arrangement could use alternative materials for the coils. In embodiments of the invention, the AC coils may be formed from any suitable material, such as aluminium or copper. Furthermore, the DC coils can be any suitable material, for example aluminium, copper or a high temperature superconductor. In other embodiments, the DC coils could be replaced by a suitable DC biasing means.

Some embodiments employ fluid around the windings, such as mineral oil, vegetable oil or cryogenic fluid.

Some embodiments, for example for small FCLs, may employ dry type solid insulation and air around the windings with a tank/enclosure.

The AC and DC windings can have various shapes, such as circular, rectangular, oval or race-track shapes. Furthermore, the core legs and yokes can have circular (cruciform), oval or rectangular cross-section. The AC and DC coils can be wound on circular, oval or rectangular formers.

Many further variations and modifications will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only, and which are not intended to limit the scope of the invention, that being determined by the appended claims.