WO2018138723A1

WO2018138723A1 - Dc power supply arrangement

Info

Publication number: WO2018138723A1
Application number: PCT/IL2018/050089
Authority: WO
Inventors: Uri Garbi; Dvir LANDWER; Alex OREN; Vladimir Rozenshtein
Original assignee: Gridon Ltd.
Priority date: 2017-01-24
Filing date: 2018-01-24
Publication date: 2018-08-02
Also published as: GB201701200D0; GB2560297A

Abstract

A DC power supply arrangement is provided that comprises a DC power supply arranged to supply current to an inductive load in a first state of the DC power supply arrangement, and arranged not to supply current to the inductive load in a second state of the DC power supply arrangement; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element; wherein on reception of a signal indicating the second state, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in the second state conditions the circuit element is arranged to enable stored energy to transfer from the inductive load to the capacitance but to prevent energy transfer from the capacitance back to the inductive load.

Description

DC Power Supply Arrangement

The present invention relates to a DC power supply arrangement and a fault current limiter (FCL) including such a DC power supply arrangement.

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 iron 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 saturated core FCL is described in WO2007/ 029224.

It will be appreciated that the DC coils of such FCLs require a DC power supply. More generally, it will be appreciated that there are numerous examples of inductive loads that require DC power supplies. The project leading to this application has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 672530.

The present invention sets out to provide a DC power supply arrangement with improved performance compared to conventional arrangements.

According to a first aspect of the invention there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in a first state of the DC power supply arrangement, and arranged not to supply current to the inductive load in a second state of the DC power supply arrangement; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element; wherein on reception of a signal indicating the second state, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in the second state the circuit element is arranged to enable stored energy to transfer from the inductive load to the capacitance but to prevent energy transfer from the capacitance back to the inductive load. Hence, when there is a transition from the first state to the second state, the current on the inductive load is lowered as it is not connected to the DC power supply and the stored magnetic energy on the inductive load is transferred to the capacitance. The circuit element (e.g. reversed biased diode or suitably controlled switch) prevents transfer of energy from the capacitance back to the inductive load.

The DC power supply arrangement may enable substantially all the current of the inductive load to be eliminated via a single and brief energy transfer to the capacitance within a short time from the first switching mechanism opening the first switch, for example within l to 3 msec.

In some embodiments, the first state represents normal conditions of the inductive load (e.g. an FCL), and the second state represents a fault condition of the inductive load. In some embodiments, the energy transfer branch comprises a resistance in series with the circuit element.

In some embodiments, there is a second switching mechanism connected in parallel with the DC power supply, wherein the second switching mechanism includes a second switch, and wherein the second switch is open in the first state and closed in the second state.

In some embodiments, the second switching mechanism comprises an impedance in series with the second switch.

In some embodiments, the circuit element comprises a reverse biased diode. Hence, the presence of a reverse biased diode in series with the capacitance enables stored energy to transfer from the inductive load to the capacitance but prevents energy transfer from the capacitance back to the inductive load.

In some embodiments, the circuit element comprises an energy transfer switching mechanism, wherein the energy transfer switching mechanism comprises a switch arranged to be closed in a portion of the second state to allow energy transfer from the inductive load to the capacitance, and to be open in another portion of the fault second state to block energy transfer back from the capacitance to the inductive load. Hence, the presence of a such a switch (e.g. in series with the capacitance) enables stored energy to transfer from the inductive load to the capacitance but prevents energy transfer from the capacitance back to the inductive load. It will be appreciated that the switch will need to be controlled appropriately according to the oscillation of the LC circuit formed by the inductive load and the capacitance.

In some embodiments, following a transition from the first state to the second state (e.g. a fault condition), on reception of a signal indicating a recovery condition representing a desired change from the second state back to the first state, the first switching mechanism is arranged to close the first switch to restore the supply of current from the DC power supply to the inductive load.

In some embodiments, on reception of the signal indicating a recovery condition, the second switching mechanism is arranged to open the second switch.

In some embodiments, the energy transfer branch further comprises a dissipating resistance in parallel with the capacitance.

In some embodiments, the DC power supply arrangement further comprises a second energy transfer branch connected in parallel with the inductive load, the second energy transfer branch comprising a second resistance.

In some embodiments, the second energy transfer branch comprises a second circuit element connected in series with the second resistance, wherein the second circuit element is arranged to prevent current from the DC power supply flowing along the second energy transfer branch in the first state, wherein during the second state, current from the inductive load may flow along the second energy transfer branch.

In some embodiments, the second circuit element comprises a second reverse biased diode.

In some embodiments, the second circuit element comprises a second energy transfer switching mechanism, wherein the second energy transfer switching mechanism comprises a switch arranged to be closed in a portion of the second state to allow energy transfer from the inductive load to the second resistance. The DC power supply arrangement may enable substantially all the stored magnetic energy of the inductive load to be transferred to the capacitance once the first switching mechanism opens the first switch, for example within a time of l to 3 msec. Hence, the current of the inductive load may be eliminated.

According to an aspect of the invention there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in normal conditions (first state); a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in normal conditions; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element arranged to prevent charging of the capacitance by the DC power supply in normal conditions; wherein on reception of a signal indicating a fault condition (second state), the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that current from the inductive load charges the capacitance.

According to an aspect of the invention there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in normal conditions (first state); a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in normal conditions; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a reversed biased diode arranged to prevent charging of the capacitance by the DC power supply in normal conditions; wherein on reception of a signal indicating a fault condition (second state), the first switching mechanism is arranged to open the first switch and the reversed biased diode is arranged such that current from the inductive load charges the capacitance.

According to an aspect of the invention, there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in normal conditions; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in normal conditions; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element; wherein on reception of a signal indicating a fault condition, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in fault conditions the circuit element is arranged to enable stored energy to transfer from the inductive load to the capacitance but to the prevent energy transfer from the capacitance back to the inductive load.

According to an aspect of the invention, there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in a first state of the DC power supply arrangement, and arranged not to supply current to the inductive load in a second state of the DC power supply arrangement; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element in series, wherein the circuit element is arranged to prevent the capacitance from being charged by the DC power supply; wherein on reception of a signal indicating the second state, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in the second state the circuit element is arranged to enable substantially the entire inductive load current to be eliminated (e.g. within 1 to 3 msec) via stored energy transfer from the inductive load to the capacitance but to prevent energy transfer from the capacitance back to the inductive load.

According to an aspect of the invention, there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in a first state of the DC power supply arrangement, and arranged not to supply current to the inductive load in a second state of the DC power supply arrangement; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element, wherein the circuit element is arranged to prevent the capacitance from being charged by the DC power supply in the first state; wherein on reception of a signal indicating the second state, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in the second state the circuit element is arranged to enable stored energy to transfer from the inductive load to the capacitance but to prevent energy transfer from the capacitance back to the inductive load.

According to an aspect of the invention, there is provided a DC power supply arrangement comprising: a DC power supply arranged to supply current to an inductive load in a first state of the DC power supply arrangement, and arranged not to supply current to the inductive load in a second state of the DC power supply arrangement; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element in series, wherein the circuit element is arranged to prevent the capacitance from being charged by the DC power supply; wherein on reception of a signal indicating the second state, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in the second state the circuit element is arranged to enable stored energy to transfer from the inductive load to the capacitance but to prevent energy transfer from the capacitance back to the inductive load. The DC power supply arrangement may enable substantially all the stored magnetic energy of the inductive load to be transferred to the capacitance once the first switching mechanism opens the first switch, for example within a time of l to 3 msec. Hence, the current of the inductive load may be eliminated. According to an aspect of the invention, there is provided a DC power supply arrangement comprising: a DC power supply to supply current to an inductive load in a first state of the DC power supply arrangement, and not to supply current to the inductive load in a second state of the DC power supply arrangement; a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state; an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element in series, wherein the circuit element prevents the capacitance from being charged by the DC power supply; wherein on reception of a signal indicating the second state, the first switching mechanism opens the first switch, wherein in the second state the circuit element enables stored energy to transfer from the inductive load to the capacitance but prevents energy transfer from the capacitance back to the inductive load.

According to an aspect of the invention, there is provided a fault current limiter comprising: a magnetically saturable core; at least one AC coil wound on a first portion of the magnetically saturable core, the at least one AC coil being connected to a first phase AC source; at least one DC coil wound on a second portion of the magnetically saturable core, wherein the at least one DC coil is arranged to magnetically saturate the first portion of the magnetically saturable core in normal conditions; a DC power supply arrangement according to any of the above aspects, wherein the DC power supply arrangement is arranged to supply current to the at least one DC coil; a control mechanism arranged to detect a fault condition; wherein on detection of the fault condition, the control mechanism is arranged to send a signal indicating a fault condition to the DC power supply arrangement. Hence, the at least one DC coil maybe the inductive load of the DC power supply arrangement.

Hence, in fault conditions, the current on the inductive load is lowered as it is not connected to the DC power supply and the stored magnetic energy on the inductive load is transferred to the capacitance.

The signal indicating a fault condition may be equivalent to the signal indicating the second state for the DC power supply arrangement. Hence, in normal conditions of the FCL, the DC power supply arrangement maybe in the first state, and in fault conditions of the FCL, the DC power supply arrangement may be in the second state.

Embodiments of the present invention can lead to a decrease in this coupling between AC and DC coils compared to conventional arrangements as well as removing the DC bias component. In some embodiments, on detection of the fault condition clearing, the control mechanism is arranged to send a signal indicating a recovery condition to the DC power supply arrangement. The signal indicating a recovery condition may be equivalent to a signal indicating the first state DC power supply arrangement.

In some embodiments, the control mechanism comprises a current sensor arranged to detect a change in current on the at least one DC coil.

In some embodiments, the control mechanism comprises a current sensor arranged to detect a change in current on the at least one AC coil.

In some embodiments, the control mechanism comprises a voltage sensor arranged to detect a change in voltage on the at least one AC coil and/ or the at least one DC coil.

In some embodiments, the control mechanism is arranged to detect a fault on the basis of a received signal from an external device. According to an aspect of the invention, there is provided a fault current limiter comprising: a magnetically saturable core; at least one AC coil wound on a first portion of the magnetically saturable core, the at least one AC coil being connected to a first phase AC source; at least one DC coil wound on a second portion of the magnetically saturable core, wherein the at least one DC coil is arranged to magnetically saturate the first portion of the magnetically saturable core in normal conditions; a DC power supply arrangement according to any of the above aspects, wherein the DC power supply arrangement is arranged to supply current to the at least one DC coil; a controller arranged to detect a fault condition; wherein on detection of the fault condition, the controller sends a signal indicating a fault condition to the DC power supply arrangement. The at least one DC coil may be the inductive load of the DC power supply arrangement, and the signal indicating a fault condition may be equivalent to the signal indicating the second state for the DC power supply arrangement.

According to an aspect of the invention, there is provided an electronic device comprising: an inductive load, a DC power supply arrangement according to any of the above aspects arranged to supply the current to the inductive load in the first state, and arranged not to supply current to the inductive load in the second state; a controller to provide a signal to the DC power supply arrangement to indicate the first or second state. 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 shows a schematic illustration of a conventional DC power supply

arrangement;

Figure 4 shows a schematic illustration of a DC power supply arrangement according to a first embodiment of the invention;

Figure 5 shows a schematic illustration of a DC power supply arrangement according to a second embodiment of the invention;

Figure 6 shows a schematic illustration of a DC power supply arrangement according to a third embodiment of the invention;

Figure 7 shows a schematic illustration of a DC power supply arrangement according to a fourth embodiment of the invention;

Figure 8 shows a schematic illustration of an FCL according to an embodiment of the invention;

Figure 9 shows a graph of a simulation of discharge of the current on the inductive load vs. voltage in the capacitor in the energy transfer branch;

Figure 10 shows a graph of a simulation of discharge of the current on the inductive load vs. voltage in the capacitor in the energy transfer branch; and

Figures 11a and 11b show graphs of a simulation of discharge of the current on the inductive load vs. voltage across the inductive load without the energy transfer branch; Figures la and lb show a first illustrative arrangement of an FCL of a type disclosed in WO2013/024462. 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. Hence, the flux direction of the first DC magnetic circuit 12a opposes the flux direction of the second DC magnetic circuit 12b. 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 the 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 2oa will support the DC flux in the second leg 20a, and the AC flux in the third leg 2ob will oppose the DC flux in the third leg 20b.

The legs and yokes have, in this arrangement, 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 l 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). The operation of the fault current limiter l shown in Figure la in normal (i.e. an example of a first state) and fault conditions (i.e. an example of a second state) will now be explained.

Under normal conditions, the second and third legs are kept in a saturated state (with one leg being more saturated than the other leg) by the DC coils na and lib. Hence, under normal conditions, the AC coils 21a and 21b around saturated legs 20a and 20b have very low impedance, and hence the FCL l is virtually transparent to the grid connected to the FCL l. 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. The magnification of the AC magnetic flux supporting/opposing the DC magnetic flux has the effect of putting one inner leg into very high saturation, whilst putting the other inner leg into an unsaturated state. The effect of one of the inner legs being in the unsaturated state will be that the impedance of the coil on that leg will increase, acting to limit the fault current. In the next half cycle, the situation will reverse and the inner leg that went into very high saturation will become unsaturated, and the inner leg that was unsaturated will go into very high saturation. While the above describes a particular FCL

implementation - the invention described herein below offers improvements to other types of saturated core FCLs as well, including ones with open AC magnetic circuits. Furthermore, it will be appreciated that embodiments of the invention could be applicable to switching of any inductive load.

Furthermore, while the below makes reference to "normal conditions" and "fault conditions", it will be appreciated that these are example states of the DC power supply arrangement. In general terms, the "normal state" referred to below can refer to a first state in which it is desired that the DC power supply arrangement powers the inductive load, and that the "fault state" is in general terms a second state in which it is desired that the DC power supply arrangement does not power the inductive load. While the above described arrangements are substantial improvements over conventional designs, the present invention provides embodiments of FCLs with a number of improvements, such as reduced mass. In particular, the present invention provides FCLs with improved DC bias circuits. Figure 3 shows a schematic illustration of a conventional DC power supply

arrangement in combination with the FCL 1 as shown in Figures la and lb.

The conventional DC power supply arrangement comprises a DC supply 13 that supplies a DC bias current to the DC coils 11 of the FCL 1. In this embodiment, the FCL 1 has two DC coils, with the DC coils connected in series. In other embodiments, the DC coils 11 could be connected in parallel. In some embodiments, the FCL 1 may comprise a different number of DC coils. For example, the FCL 1 may comprise at least one DC coil. As shown in Figure 3, the DC coils 11 can be schematically considered to have a resistive component 11a and an inductive component lib.

In general terms, a known issue for FCLs that have core(s) saturated by DC bias is transformer coupling between the AC coil(s) and the DC coil(s) in fault limiting conditions. In normal operating conditions, with the core in magnetic saturation, this coupling is typically very low, as the core behaves essentially like an air core. In fault limiting conditions, the core is de-saturated by the AC flux caused by the high fault current. Consequently, the coupling between AC and DC coils increases. DC circuit current increases when the AC current operates to desaturate the AC leg according with Lenz's law.

This transformer coupling decreases impedance of the device in the limiting state while impacting limiting capability (performance) of the FCL.

The total current in the DC coil(s) can be considered as a DC bias component (provided by the DC power supply) and coupled current component resulting from the transformer coupling.

Embodiments of the present invention can lead to a decrease in this coupling between AC and DC coils compared to conventional arrangements as well as removing the DC bias component.

Figure 4 shows a schematic illustration of a DC power supply arrangement 100 according to a first embodiment of the invention. The DC power supply arrangement 100 comprises a DC power supply 110, a first switching mechanism 130, and an energy transfer branch 140.

The DC power supply 110 is arranged to supply current to an inductive load 120 in normal conditions.

The first switching mechanism 130 is connected in series between the DC power supply 110 and the inductive load 120. In this embodiment, the first switching mechanism includes a first switch (not shown) that is closed in normal conditions. Hence, in normal conditions, current flows from the DC power supply 110 to the inductive load 120.

The energy transfer branch 140 is connected in parallel with the inductive load 120. In this embodiment, the energy transfer branch 140 comprises a capacitance 141 and a circuit element 142. The circuit element 142 is arranged to ensure a unidirectional flow of energy from the inductive load 120 into the energy transfer branch 140, and not allow for energy to flow back from the energy transfer branch 140 into the inductive load 120.

It will be appreciated that, in general terms, when a capacitor and an inductor are directly connected, and one of the capacitor or inductor starts out in a charged state, the two components will exchange energy between them, back and forth, creating their own AC voltage and current cycles. Hence, without the presence of the circuit element 142, energy from the inductive load 120 will initially flow to the capacitance 141. Once fully charged, the capacitance 141 will discharge and charge the inductor, and this cycle would continue akin to a pendulum.

However, the circuit element 142 enables energy to flow from the inductive load 120 into the energy transfer branch 140 (e.g. to capacitance 141), but not to back from the energy transfer branch 140 into the inductive load 120.

Hence, wherein on reception of a signal indicating fault conditions (second state) the circuit element 142 is arranged such that stored energy from the inductive load charges the capacitance 141, wherein in the second state the circuit element is arranged to enable stored energy to transfer from the inductive load 120 to the capacitance 141 but to prevent energy transfer from the capacitance 141 back to the inductive load 120.

In this embodiment, on reception of a command signal e.g. from a fault detection mechanism (not shown), the first switching mechanism 130 is arranged to open (go from ON to OFF state) and the circuit element 142 is arranged such that current from the inductive load charges the capacitance 141.

In this embodiment, the circuit element 142 comprises a reversed biased diode. In other words, in this embodiment, the circuit element 142 comprises a diode whose polarity blocks current flow from the DC power supply 110 in normal conditions.

Hence, in fault conditions, the magnetic energy stored in the inductive load 120 drives current from the inductive load 120 trying to maintain the same current flow as before (i.e. downwards in Figure 4). This causes the diode of the circuit element 142 to start conducting (i.e. upwards in Figure 4) and the capacitance 141 begins to charge due to this current. In other embodiments, the circuit element 142 may be implemented by other components. For example, the circuit element 142 may comprise an energy transfer switching mechanism comprising a switch arranged to be open (OFF) in normal conditions and closed (ON) in fault conditions, opening (OFF) again once the capacitance 141 is fully charged to avoid the pendulum effect mentioned above. Such a switch could be controlled to close (ON) by the reception of the command signal (e.g. from a fault detection mechanism), and could open (OFF) after a predetermined time. The predetermined time could be determined on the basis of an assumed time to charge the capacitance 141. Hence, it will be appreciated that such a switch will need to be controlled appropriately according to the oscillation of the LC circuit formed by the inductive load and the capacitance.

In other words, in general terms, the circuit element 142 can be implemented by any active or passive component that enables charging of the capacitance 141 by the inductive load 120 in fault conditions, but does not enable the capacitor to return its stored energy back to the inductive load 120.

In some embodiments, there may be a resistance in series with the circuit element 142 in the energy transfer branch 140.

The DC power supply arrangement 100 of this embodiment operates as follows. In normal conditions, the first switch is closed and the DC power supply 110 supplies current to the inductive load 120. As a result of the circuit element 142 (e.g. a reversed biased diode), current does not flow on the energy transfer branch 140 in normal conditions, and the capacitance 141 remains uncharged. As a result, in normal conditions, the first switching mechanism 130 and the energy transfer branch 140 do not have a significant effect on the current or voltage on the inductive load 120.

When a fault is detected, e.g.by a fault detection mechanism (not shown), a command is issued such that the first switching mechanism 130 is controlled to open the first switch. For example, the first switch may be implemented as an electronic switch arrangement (e.g. one or more Insulated Gate Bipolar Transistors, IGBTs) that receives a signal indicating a fault condition as a gate signal, causing the transistor to open (switching OFF). Once the first switching mechanism 130 opens the first switch, the DC power supply 110 can no longer drive current into the inductive load 120. It will, however, be appreciated that the inductive load 120 will have stored magnetic energy. This stored magnetic energy is then transferred in fault conditions to the capacitance 141 of the energy transfer branch 140. Current can flow from the inductive load 120 to the capacitance 141 as a result of the circuit element 142 (i.e. causing) the diode of the circuit element 142 to start conducting.

Hence, in fault conditions, the current on the inductive load 120 is lowered (as it is not connected to the DC power supply 110) and the stored magnetic energy on the inductive load 120 is transferred to the capacitance 141.

This arrangement is beneficial, since the proper selection of the capacitor value enables determining the rate of current reduction in the inductive load, and determining a manageable voltage level in the circuit.

For example, the capacitor value of the capacitance 141 may be chosen to enable substantially all the current of the inductive load 120 to be eliminated via a single and brief energy transfer to the capacitance 141, e.g. within a short time from the first switching mechanism 130 opening the first switch, for example within 1 to 3 msec.

Hence, in this embodiment, the energy transfer branch 140 is arranged to enable substantially the entire inductive load current to be eliminated via stored energy transfer from the inductive load 120 to the capacitance 140, e.g. within 1 to 3 msec of the first switching mechanism 130 opening the first switch.

While the description above mainly relates to FCLs, it would appreciate that this method is also applicable to other applications where rapid discharge of an inductive load is required.

After the fault has passed, e.g. a command is received indicating the fault has passed, or after a predetermined time, the first switching mechanism 130 is controlled to close the first switch (switch back ON). The DC power supply 110 is therefore reconnected to the inductive load 120. This process may be termed recovery. Once the capacitor is discharged, the arrangement is ready to receive another command (e.g. upon recurrence of a fault event). It should be noted that the discharge time can be designed as a parameter according to system needs, and can typically be very fast (e.g. few to tens of milliseconds), but may also be much longer if needed.

In the above arrangement, the capacitance 141 is not charged in normal conditions before a fault. However, in other embodiments, the capacitance 141 may be pre-charged to a negative voltage. This negative voltage can accelerate the action of energy transfer from the inductor (p re-charging circuit not shown).

Figure 5 shows a schematic illustration of a DC power supply arrangement 200 according to a second embodiment of the invention.

The DC power supply arrangement 200 comprises a DC power supply 210, a first switching mechanism 230, an energy transfer branch 240, and a second switching mechanism 250.

The DC power supply 210 is arranged to supply current to an inductive load 220 in normal conditions.

The first switching mechanism 230 is connected in series between the DC power supply 210 and the inductive load 220. In this embodiment, the first switching mechanism 230 includes a first switch (not shown) that is closed (ON) in normal conditions.

The second switching mechanism 250 is connected in parallel with the DC power supply 210. In this embodiment, the second switching mechanism 250 includes a second switch that is open (OFF) in normal conditions.

For example, the first switch and second switch may be respectively implemented as an electronic switch arrangement (e.g. one or more Insulated Gate Bipolar Transistors, IGBTs) that receives a command signal indicating a fault condition as gate signals.

The energy transfer branch 240 is connected in parallel with the inductive load 220. In this embodiment, the energy transfer branch 240 comprises a capacitance 241, a circuit element comprising a reverse biased diode 242, and a dissipating resistance 243. The dissipating resistance 243 is electrically connected in parallel with the capacitance 240. Hence, the reverse biased diode 242 is series connected with the parallel connected capacitance 240 and dissipating resistance 243. Hence, compared to the embodiment of Figure 4, the embodiment of Figure 5 has two main differences. The first is the presence of the second switching mechanism 250 and the second is the presence of the dissipating resistance 243. The dissipating resistance 243 may be a resistor, varistor or any suitable component.

In some embodiments, a switch may be provided in parallel with the capacitance 240 to allow discharge of the capacitance 240, e.g. after a fault. The DC power supply arrangement 200 of this embodiment operates as follows. In normal conditions, the first switch is closed and the second switch is open and hence the DC power supply 210 supplies current to the inductive load 220. As a result of the reversed biased diode 242, current does not flow on the energy transfer branch 240 in normal conditions, and the capacitance 241 remains uncharged.

As a result, in normal conditions, the first switching mechanism 230, the second switching mechanism 250 and the energy transfer branch 40 do not have a significant effect on the current or voltage on the inductive load 220. When a fault is detected, e.g.by a fault detection mechanism (not shown), a command is issued such that the first switching mechanism 230 is controlled to open the first switch and the second switching mechanism 250 is controlled to close the second switch. In this embodiment, when a fault is detected, a fault detection mechanism (not shown) sends a first fault signal to the first switching mechanism 230 (to open the first switch) and a second fault signal to the second switching mechanism 250 (to close the second switch). For example, if the first and second switching mechanisms both comprise transistors of the same polarity, the first and second fault signals may be of different polarities. Alternatively, it will be appreciated that the functionality of the first and second switching mechanisms could be implemented with switches that operate in the appropriate way using a single fault signal. A small delay (e.g. few microseconds) may be implemented between the switching commands to 230 and 250. This may provide a make before break function for the DC supply current.

Hence, in fault conditions, not only is the DC power supply 210 not driving current into the inductive load 220, there is also an alternate path for the DC power supply current via the parallel connected second switching mechanism 250. Also, it will be appreciated that practical implementations of this embodiment may utilise resistors and/or inductors in series with the second switching mechanism 250 on the bypass path to appropriately limit the DC power supply 210 transient current during this switching of the second switching mechanism. The alternate path created by 250 enables the DC power supply to keep driving regulated current such that upon recovery - the current would be restored faster into the inductive load.

In normal conditions, the dissipating resistance 243 does not affect the circuit.

However, in fault conditions, the dissipating resistance 243 provides a discharge path for the capacitance 241. This ensures that the capacitor is discharged in time for a subsequent fault operation. It also allows safe operation of the circuit, such that there is a guaranteed decay of the voltage on the capacitor to zero. Furthermore, the choice of the value of the dissipating resistance 243 can be used to tune both the charge and the discharge time of the capacitance 241. This enables the reduction of the inductive load current to near zero in a controlled and known time. The resistor serves as a damping element to the circuit, and dissipates part of the stored magnetic energy in the inductive load, also allowing to limit the voltage level on the capacitance.

Hence, in fault conditions, the current on the inductive load 220 is lowered (as no current is driven into it by the DC power supply 210) and the stored magnetic energy on the inductive load 220 is transferred to the capacitance 241, which can discharge via the dissipating resistance 243.

After the fault has passed, e.g. detected by a fault detection mechanism (not shown), the first switching mechanism 230 is controlled to close the first switch and the second switching mechanism 250 is controlled to open the second switch. The DC power supply 210 is therefore capable of driving current to the inductive load 220.

Figure 6 shows a schematic illustration of a DC power supply arrangement 300 according to a third embodiment of the invention.

The DC power supply arrangement 300 comprises a DC power supply 310, a first switching mechanism 330, an energy transfer branch 340, a second switching mechanism 350. These components function in the same way as the equivalent features discussed in relation to Figure 5, and thus a description of these components will not be repeated. The DC power supply arrangement 300 further comprises a second energy transfer branch 360, that comprises a second resistance 361 and a second reversed biased diode 362.

As a result of the second reverse-biased diode 362, current does not flow in the second energy transfer branch 360 in normal conditions, and energy is not lost via the second resistance 361. In some cases, when the resistance 361 is high, the reverse biased diode may be omitted.

However, in fault conditions, current will flow from the inductive load 320 via both the first energy transfer branch 340 and the second energy transfer branch 360. Hence, in fault conditions, energy from the inductive load 320 will be dissipated by the second resistance 361 in a portion of the duration of the fault condition, while a portion of the current will keep being transferred to the capacitance 341. As a result, the magnetic energy stored in the inductive load 320 drives current from inductive load 320 coil trying to maintain the same current flow as before (i.e. downwards in Figure 6). This causes the first and second diodes 342 and 362 to start conducting (i.e. upwards in Figure 6) and current flows on the first energy transfer branch 340 and on the second energy transfer branch 360. It will be appreciated that branch 360 serves as a secondary energy transfer branch, and will typically conduct during only a portion of the fault condition.

Figure 7 shows a schematic illustration of a DC power supply arrangement 400 according to a fourth embodiment of the invention.

The DC power supply arrangement 400 comprises a DC power supply 410, a first switching mechanism 430, an energy transfer branch 440, a second switching mechanism 450, and a second energy transfer branch 460.

The DC power supply 410 is arranged to supply current to an inductive load 520 in normal conditions. In this embodiment, the inductive load 520 represents the DC coils of an FCL and can be schematically considered to have a resistive component and an inductive component. The first switching mechanism 430 is connected in series between the DC power supply 410 and the inductive load 520. In this embodiment, the first switching mechanism 430 comprises one or more series connected switches, in this example a first IGBT 431. The IGBT 43 is controlled by means of a control signal Ci from a fault detection system (not shown).

It will be appreciated that other embodiments may achieve the appropriate function of the first switching mechanism 430 using a different combination of switches (e.g.

IGBTs or other switching devices) in series and/or in parallel.

In this embodiment, the first switching mechanism 430 also comprises a snubber arrangement 433 in parallel with the IGBT 431. The snubber arrangement in this embodiment comprises a capacitance and a resistance (not shown). The snubbers act to reduce the transient voltage caused by parasitic inductances when the switches interrupt current. It will be appreciated that the snubber arrangement 433 could be implemented in a number of ways which are well known to those experienced in the art.

The first IGBT 431 is closed in normal conditions, and open on reception of the control signal Ci indicating a fault.

The second switching mechanism 450 is connected in parallel with the DC power supply 410. In this embodiment, the second switching mechanism 450 comprises a second IGBT 451. The IGBT 451 is controlled by means of a control signal C2 from a fault detection system (not shown).

It will be appreciated that other embodiments may achieve the appropriate function of the second switching mechanism 450 using a different combination of switches (e.g. IGBTs or other switching devices) in series and/or in parallel. In this embodiment, the second switching mechanism 450 also comprises a snubber arrangement 453 in parallel with IGBT 451. The snubber arrangement 453 comprises a capacitance and a resistance in this embodiment (not shown). It will be appreciated that the snubber arrangement could be implemented in a number of ways, e.g. using RC snubbers or varistor snubbers. A person skilled in the art would appreciate that other snubber topologies can be adopted. The IGBT 451 is open in normal conditions, and closes on reception of the control signal C2 indicating a fault.

Also shown in Figure 7 is an optional limiting resistance 470 that acts to limit current flow across the second switching mechanism 450 during a fault in order to protect the power supply from an overcurrent surge.

The energy transfer branch 440 is connected in parallel with the inductive load 520. In this embodiment, the energy transfer branch 440 comprises a capacitance 441, a circuit element 442, and a dissipating resistance 443. The dissipating resistance 443 is electrically connected in parallel with the capacitance 440. Hence, the first circuit element 442 is series connected with the parallel connected capacitance 440 and dissipating resistance 443. Furthermore, as shown in Figure 7, the circuit element 442 comprises a reverse biased diode. In some embodiments it maybe beneficial to add another resistance in series with circuit element 442. This resistance can further help reducing the voltage on the capacitance 441. In other embodiments a non-linear resistance (e.g. metal oxide varistor) maybe added in parallel with the capacitance 441 to limit high transient voltages on the capacitance. The second energy transfer branch 460 comprises a second resistance 461 and a second circuit element 462. The second circuit element 462 in this embodiment comprises a reversed biased diode.

As a result of the circuit element 442, current does not flow on the energy transfer branch 440 in normal conditions (due to the states of the first and second switching mechanisms), and the capacitance 441 is not charged. As a result of the second circuit element 462, current does not flow on the second energy transfer branch 460 in normal conditions, and energy is not lost via the second resistance 461. However, in fault conditions, current will flow from the inductive load 520 via both the first energy transfer branch 440 and the second energy transfer branch 460. Hence, in fault conditions, energy from the inductive load 520 will be dissipated by the second resistance 461 as well as being transferred to the capacitance 441.

In other embodiments, the first energy transfer branch 440 may comprise (instead of or in addition to a reversed biased diode) a circuit element that comprises an energy transfer switching mechanism comprising a switch arranged to be open in normal conditions and closed in fault conditions. Such a switch could be controlled by the reception of the signal indicating the fault condition (e.g. from a fault detection mechanism). For example, such a switch could be controlled by signal Ci or C2 (or by whatever control signal or signals are used for the other switches).

Embodiments may provide an electronic device (not shown) comprising: an inductive load, a DC power supply arrangement according to any of the above embodiments arranged to supply the current to the inductive load in the first state, and arranged not to supply current to the inductive load in the second state; and a controller (not shown) to provide a signal to the DC power supply arrangement to indicate the first or second state. The DC power supply arrangement of any of the above embodiments may enable substantially all the current of the inductive load to be eliminated via a single and brief energy transfer to the capacitance in the energy transfer branch, e.g. within a short time from the first switching mechanism opening the first switch, for example within 1 to 3 msec.

The operation of the embodiment of Figure 7 will now be explained by reference to Figure 8.

Figure 8 shows the DC power supply arrangement 400 used with a FCL 500.

The FCL 500 comprises a magnetically saturable core 510, an AC coil 530 wound on a first portion of the magnetically saturable core 510. The AC coil 5301s connected to a first phase AC source 600 and to a load 700.

The FCL 500 also comprises a DC coil 520 wound on a second portion of the magnetically saturable core. The DC coil 520 forms the inductive load discussed in relation to Figure 7. The DC power supply arrangement 400 supplies current to the at least one DC coil, and hence DC coil 520 is arranged to magnetically saturate the first portion of the magnetically saturable core in normal conditions.

The FCL 500 further comprises a control mechanism 550 arranged to detect a fault condition. The control mechanism 550 is arranged to output on detection of the fault condition, the control mechanism is arranged to send one or more signal indicating a fault condition to the DC power supply arrangement. For example, in this embodiment, the control mechanism 550 may send signals Ci and C2 (see above) to indicate the presence and/or absence of a fault. However, depending on how the various switches area arranged, the control mechanism 550 may send a different number of signals. In addition, the FCL 500 further comprises a current sensor 540 arranged to sense the current on the AC coil 530 of the FCL 500.

In normal conditions, the first and second switching mechanisms 430, 450 do not have a significant effect on the current or voltage on the DC coil 520 or the operation of the FCL 500.

In this embodiment, the control mechanism 550 is arranged to detect a fault based on a rise of the current on the AC coil 530, and the current sensor 540 is a current transformer. The fault detection system could be based on the AC circuit current of the FCL 500 and/ or the AC circuit current rate of change. For example the current transformer 540 could be used to detect an increase in the AC current on the AC coils of the FCL 500. For example, if the AC circuit current rises over 10-20% above the maximum normal level, and/ or if the rate of change rises over 10-20% above the maximum normal level, a fault is detected.

In other embodiments, the control mechanism 550 can be arranged to detect a fault based only a rise of the current on the AC coils or on the DC coils. In such a case, only one of the AC or DC sensors would be required. Furthermore, in other embodiments, the control mechanism 550 could be arranged to detect a fault based only a rise of the voltage on the AC coils and/ or the DC coils. In some embodiments, the control mechanism 550 can be arranged to detect a fault based on a received signal from an external system (e.g. a remote fault detection system) and/ or based on the reception of a user command. As discussed, as a result of the transformer coupling between the AC coil of the FCL 530 and the DC coil 520 in fault conditions, the total current on the DC coil 520 can be considered as a DC bias component provided by the DC power supply 4ioand the coupled current component resulting from the transformer coupling. It will be appreciated that during a fault, the total current through the DC coils 520 will decrease (decreasing both DC bias component and coupled current component). This reduction in the current through the DC coil 520 significantly suppresses the transformer coupling.

Reducing the current through the DC coil 520 can also help fault limiting performance. This is because the smaller the current through the DC coil 520, the smaller the DC bias effect on the FCL 500. A smaller DC bias will increase the impendence of the FCL 500, acting to improve limitation of the fault current

In some embodiments, a fault can be detected within up to 1-3 msec. The DC power supply arrangement 400 rapidly eliminates the DC bias from the DC coil of the saturated core unit within about 2-3 msec. With the DC interrupted, the performance of the saturated core FCL 500 is improved.

At some point after the detection of the fault, the control mechanism 550 will control the first switching mechanism 430 to close the first IGBTs 431a, 431b (e.g. supply an appropriate value of signal Ci) and control the second switching mechanism 450 (e.g. supply an appropriate value of signal C2) to open the second IGBT 451. This process is referred to as recovery. In some embodiments, the control mechanism 550 can detect when the conditions on the FCL 550 have returned to normal. For example, by monitoring the current and/or voltage of the DC and/or AC coils of the FCL and then close the first switching mechanism 430 and open the second switching mechanism 450. Determining that the conditions have returned to normal may include the return of the AC current to nominal or lower level, and/ or that the AC voltage across the FCL has dropped to below its nominal level.

In other embodiments, the control mechanism can close the first switching mechanism 430 and open the second switching mechanism 450 after a predetermined time from detection of the fault condition. For example, this predetermined time may be about 10 ms. A time of 10ms has been found to be sufficient for fault current reduction improvement in the first few peaks and cycles after the start of the fault.

By using such a DC power supply arrangement, the FCL may be 2-5 times more light and compact compared to an FCL with similar fault limiting properties using a conventional FCL. Figures 9 and 10 both show graphs of simulations of discharge of the current on the inductive load vs. voltage in the capacitor in the energy transfer branch. Figure 9 uses a DC power supply arrangement with a dissipation resistor in the energy transfer branch (i.e. equivalent to resistor 242 in Figure 5), and Figure 10 uses a DC power supply arrangement without a dissipation resistor in the energy transfer branch (i.e. equivalent to Figure 4). In these simulations, the inductive load was a coil, and a reverse biased diode was used as the circuit element in the energy transfer branch. Considering Figure 9, initially constant current (125A DC in this simulation) flows through the coil from the DC power supply. The capacitor voltage in this state is zero and no current flows through it since the diode is reverse biased.

At t=io msec, the switching command is received, and the first switching mechanism goes from ON to OFF state within a few microseconds. The magnetic energy stored in the coil drives current from the coil trying to maintain the same current flow as before (i.e. downwards in Figure 5). This causes the diode to start conducting (i.e. upwards in Figure 5) and the capacitor begins to charge due to this current. The charge accumulating in the capacitor causes voltage on the capacitor to increase in magnitude. The polarity of the capacitor voltage is positive at the bottom, negative at the top (i.e. opposite to the original power-supply voltage). At the same time, part of the current from the coil runs into the resistor in parallel with the capacitor. When the inductor current reaches zero the capacitor is fully charged and the diode turns from conducting condition to voltage blocking condition so the charge doesn't convert back to current and into magnetic energy in the inductor. This results in dissipated energy (heat) and damping of the LC circuit. Due to the resistor's presence, the capacitor is discharged, and its voltage decreases rapidly towards zero. The overall effect is that the energy which was stored in the coil gets converted to electrical energy in the capacitor (and then dissipated) and the rest is directly dissipated in the resistor. For example, it is possible to achieve this in 1-3 msec. The end result is that the coil current drops to zero quickly.

Considering Figure 10, if there is no resistance in parallel with the capacitor, the entire coil energy gets converted into electrical charge in the capacitor. Once the coil current drops to near zero - the diode stops conducting, and the voltage remains on the capacitor. Since there is no resistance in this scenario, the voltage on the capacitor will not decay. The end of the plot shows of Figures 9 and 10 show recovery operation of the circuit towards normal coil current.

Figures 11a shows a graph of simulations discharge of the current on the inductive load vs. voltage on the first switch (i.e. the series switching device) without an energy transfer branch. Figure lib shows a zoomed in portion of Figure 11a.

It is apparent that the transient voltage on the switching mechanism when it is switched OFF is much higher than in the case with the capacitor branch. Furthermore, instead of rapidly going to zero and staying there - the current goes to a large value in the opposite direction and takes much longer time to decay to zero when compared to embodiments of the invention.

As discussed, embodiments of the invention provide a DC power supply arrangement that may be used for FCLs, which the FCL having a large variety of different magnetically saturable core configurations.

For example, a DC bias circuit according to some embodiments of the invention may be used with an FCL of the type shown in Figure la and lb, i.e. a four leg arrangement with two AC coils (one on each inner legs) and two DC coils (one on each outer leg). In such a case, the FCL has a first AC coil wound on the first portion of the magnetically saturable core, a first DC coil wound on the second portion of the magnetically saturable core, a second AC coil wound on a third portion of the magnetically saturable core, the first and second AC coils being connected in series and connected to the first phase AC source, a second DC coil wound on a fourth portion of the magnetically saturable core. The DC power supply is arranged to supply DC current to the first and second DC coils.

In some embodiments, the DC power supply arrangement may be retrofitted onto existing DC power supply arrangements or existing FCLs, for example, that already include a DC power supply. In general, the FCLs used in embodiments of the invention have at least one AC coil wound on a first portion of the magnetically saturable core, the first AC coil being connected to a first phase AC source, and at least one DC coil wound on a second portion of the magnetically saturable core. The first portion and second portions of the magnetically saturable core can be different portions of the core or the same portion (e.g. if the AC and DC coils were wound concentrically).

In some embodiments, the FCL comprises two AC coils and two DC coils. For example, a first AC coil and a first DC coil may be wound on a first leg of the magnetically saturable core, and a second AC coil and a second DC coil may be wound on a second leg of the magnetically saturable core.

Furthermore, it will be appreciated any of these variations could be combined in any suitable way.

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) can be 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, low temperature superconductor or a high temperature superconductor. In other embodiments, the DC coils could be replaced by a suitable DC biasing means. In other embodiments, the DC coils could be

supplemented by permanent magnets.

Some embodiments employ fluid around the all or part of 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.

Claims

1. A DC power supply arrangement comprising:

a DC power supply arranged to supply current to an inductive load in a first state of the DC power supply arrangement, and arranged not to supply current to the inductive load in a second state of the DC power supply arrangement;

a first switching mechanism connected in series between the DC power supply and the inductive load, wherein the first switching mechanism includes a first switch, and wherein the first switch is closed in the first state;

an energy transfer branch connected in parallel with the inductive load, the energy transfer branch comprising a capacitance and a circuit element in series, wherein the circuit element is arranged to prevent the capacitance from being charged by the DC power supply;

wherein on reception of a signal indicating the second state, the first switching mechanism is arranged to open the first switch and the circuit element is arranged such that stored energy from the inductive load charges the capacitance, wherein in the second state the circuit element is arranged to enable stored energy to transfer from the inductive load to the capacitance but to prevent energy transfer from the capacitance back to the inductive load.

2. A DC power supply arrangement according to claim l, wherein the first state represents normal conditions of the inductive load, and the second state represents a fault condition of the inductive load.

3. A DC power supply arrangement according to claim 1 or 2, wherein the energy transfer branch comprises a resistance in series with the circuit element.

4. A DC power supply arrangement according to any preceding claim, further comprising a second switching mechanism connected in parallel with the DC power supply, wherein the second switching mechanism includes a second switch, and wherein the second switch is open in the first state and closed in the second state.

5. A DC power supply arrangement according to claim 4, wherein the second switching mechanism comprises an impedance in series with the second switch.

6. A DC power supply arrangement according to any preceding claim, wherein the circuit element comprises a reverse biased diode.

7. A DC power supply arrangement according to any preceding claim, wherein the circuit element comprises an energy transfer switching mechanism, wherein the energy transfer switching mechanism comprises a switch arranged to be closed in a portion of the second state to allow energy transfer from the inductive load to the capacitance, and to be open in another portion of the second state to block energy transfer back from the capacitance to the inductive load.

8. A DC power supply arrangement according to any preceding claim, wherein, following a transition from the first state to the second state, on reception of a signal indicating a recovery condition representing a desired change from the second state back to the first state, the first switching mechanism is arranged to close the first switch to restore the supply of current from the DC power supply to the inductive load.

9. A DC power supply arrangement according to claim 8 when dependent on claim 4, wherein on reception of the signal indicating a recovery condition, the second switching mechanism is arranged to open the second switch.

10. A DC power supply arrangement according to any preceding claim, wherein the energy transfer branch further comprises a dissipating resistance in parallel with the capacitance.

11. A DC power supply arrangement according to any preceding claim, wherein the DC power supply arrangement further comprises a second energy transfer branch connected in parallel with the inductive load, the second energy transfer branch comprising a second resistance.

12. A DC power supply arrangement according to claim 11, wherein the second energy transfer branch comprises a second circuit element connected in series with the second resistance, wherein the second circuit element is arranged to prevent current from the DC power supply flowing along the second energy transfer branch in the first state, wherein during the second state, current from the inductive load may flow along the second energy transfer branch.

13. A DC power supply arrangement according to claim 11 or 12, wherein the second circuit element comprises a second reverse biased diode.

14. A DC power supply arrangement according to any one of claims 11 to 13, wherein the second circuit element comprises a second energy transfer switching mechanism, wherein the second energy transfer switching mechanism comprises a switch arranged to be closed in a portion of the second state to allow energy transfer from the inductive load to the second resistance.

15. A fault current limiter comprising:

a magnetically saturable core;

at least one AC coil wound on a first portion of the magnetically saturable core, the at least one AC coil being connected to a first phase AC source;

at least one DC coil wound on a second portion of the magnetically saturable core, wherein the at least one DC coil is arranged to magnetically saturate the first portion of the magnetically saturable core in normal conditions;

a DC power supply arrangement according to any one of claims 1 to 14, wherein the DC power supply arrangement is arranged to supply current to the at least one DC coil;

a control mechanism arranged to detect a fault condition;

wherein on detection of the fault condition, the control mechanism is arranged to send a signal indicating a fault condition to the DC power supply arrangement.

16. A fault current limiter according to claim 15, wherein on detection of the fault condition clearing, the control mechanism is arranged to send a signal indicating a recovery condition to the DC power supply arrangement.

17. A fault current limiter according to claim 15 or 16, wherein the control mechanism comprises a current sensor arranged to detect a change in current on the at least one DC coil.

18. A fault current limiter according to any one of claims 15 to 17, wherein the control mechanism comprises a current sensor arranged to detect a change in current on the at least one AC coil.

19. A fault current limiter according to any one of claims 15 to 18, wherein the control mechanism comprises a voltage sensor arranged to detect a change in voltage on the at least one AC coil and/ or the at least one DC coil.

20. A fault current limiter according to any one of claims 15 to 19, wherein the control mechanism is arranged to detect a fault on the basis of a received signal from external device.