Technical field

The present disclosure relates to the field of circuit breakers for direct current power systems. In particular, the present disclosure relates to bidirectional circuit breakers for direct current power systems.

Technical background

To convert between alternating current (AC) and direct current (DC), voltage-source converters (VSCs) have been developed as a replacement alternative for more traditional line-commutated converters. The VSCs, often constructed using insulated-gate bipolar transistors (IGBTs) and anti-parallel connected diodes may be incapable of handling DC faults (such as e.g. faults between high-impedance and ground, or faults across the converter itself). The anti-parallel diodes may conduct as rectifier bridges to feed the fault, or for example act as short circuits together with e.g. a faulty IGBT. The diodes at the IGBTs may be by-passed and unable to extinguish the fault current on their own. To resolve this issue, insertion of fast and reliable DC breakers with low losses is required.

Modern DC breakers often rely on a combination of a mechanical switch that is complemented with a parallel coupled semiconductor switch. During normal operation, the current is passed through the closed mechanical switch while the semiconductor switch is open. If a fault is detected, the switches are reconfigured such that the current is commutated (redirected) through the semiconductor switch while the mechanical switch is allowed to be opened. Once the mechanical switch has been opened (and the current through it has been driven to zero), the semiconductor switch may once again be opened and any remaining energy may be absorbed by e.g. a varistor.

A further development of such a semiconductor based DC breaker is the commutation booster disclosed in e.g. US 9,148,01 1 , in which an inductive coupling between the branch containing the mechanical switch and the branch containing the semiconductor switch is used to enhance the commutation of the current.

With VSCs appearing in wider ranges of low-voltage (LV), medium- voltage (MV) and high-voltage (HV) systems, more flexible and more efficient DC breakers may however be required.

Summary of the invention

An object of the present disclosure is therefore to at least partially fulfill the above requirements. This and other objects are achieved by means of a circuit breaker for interrupting an electrical current flow between a first node and a second node as defined in the appended independent claim. Other embodiments are defined by the dependent claims.

According to one aspect of the present disclosure, the circuit breaker may include a first switching branch that includes a first winding of a mutual inductor and a mechanical switch connected in series. The circuit breaker may further include a second switching branch that includes a second winding of the mutual inductor and a switching arrangement that includes at least one switching device connected to the second winding. The first switching branch and the second switching branch may be connected in parallel between the first node and the second node, and the second switching branch may be configured to operate bidirectionally (i.e. in either direction between the first node and the second node).

In the present aspect, current may be commutated from the first switching arrangement to the second switching arrangement in any direction, as the second switching arrangement is configured to operate bidirectionally. This may allow for a fault current to be interrupted in any direction, i.e. from both sides of the circuit breaker, and the circuit breaker may be used in systems where fault currents (and also load currents) may appear having different directions. Examples of such systems include energy storage systems which utilize e.g. batteries and voltage-source converters (VSCs), wherein load currents are normally carried in different directions depending on whether the system is currently in a charging or discharging phase, and wherein fault currents may go in different directions through e.g. failing semiconductors in the VSCs.

By allowing for both load and fault currents to have multiple directions, the circuit breaker of the present aspect may be used in transportation systems where accumulator batteries and voltage-source converters for example are located in fixed installations or onboard vehicles or both, e.g. as in electrical vehicles (EVs) such as electric trams, trains, buses, hybrid vehicles or the like. Other systems in which the circuit breaker may be used include power transmission systems and grids, wherein a current may change its direction depending on the location of a fault within the system (such as e.g. a multi-terminal DC power grid).

In one embodiment of the present disclosure, the switching

arrangement may be a bidirectional device (or bipolar device), i.e. a device that may allow current to flow in either directions between the first node and the second node.

In one embodiment of the present disclosure, the at least one switching device may include, or may be, at least one of an insulated-gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor

(MOSFET), a field effect transistor (FET), a gate turn-off (GTO) thyristor, or a capacitor-based arrangement. It is also envisaged that the at least one semiconductor switching device may be any other power transistor or power thyristor suitable to handle high enough current and/or power.

In one embodiment of the present disclosure, the switching

arrangement may include a diode bridge.

In one embodiment of the present disclosure, the switching

arrangement may include a first semiconductor switching device and a second semiconductor switching device. The first and the second

semiconductor switching devices may be unidirectional (or unipolar devices) and may be configured to operate in electrically opposite directions between the first node and the second node, i.e. they may allow current to be either blocked from passing through them, or allowed to pass through them in a single direction between the first node and the second node. In one embodiment of the present disclosure, the first semiconductor switching device, the second semiconductor switching device and the second winding are connected from and to the second node as a loop, the first node being connected to the second switching branch through the second winding. In this or other embodiments, the second winding may be at least a bifilar winding.

In one embodiment, the second switching branch may include a first sub branch that includes the second winding connected in series to the first semiconductor switching device, and a second sub branch including the second semiconductor switching device and a third winding of the mutual inductor connected in series (to each other to form the second sub branch). The first sub branch and the second sub branch may be connected in parallel.

In one embodiment of the present invention, the switching arrangement may further include a first diode that is connected in series with the first semiconductor switching device, and a second diode that is connected in series with the second semiconductor switching device. The first diode and the first semiconductor switching device may be connected antiparallel with the second diode and the second semiconductor switching device (i.e. the first diode and the first semiconductor switching device, on the one hand, and the second diode and the second semiconductor switching device, on the other hand, are connected in electrically opposite directions between the first node and the second node).

In one embodiment of the present disclosure, the first semiconductor switching device may be connected antiparallel with the second

semiconductor switching device.

In one embodiment of the present disclosure, the circuit breaker may further include an overvoltage protection circuit.

In one embodiment of the present disclosure, the circuit breaker may further include a third switching branch that includes the overvoltage protection circuit. The third switching branch may be connected in parallel with the first switching branch and the second switching branch between the first node and the second node. It is also envisaged that the third switching branch may be connected in parallel with the switching arrangement, such as e.g. in parallel with a switching device.

In one embodiment of the present disclosure, the self inductance of the first switching branch may be larger than a self inductance of the second switching branch.

In one embodiment of the present disclosure, a conductor cross- section of the first winding may be larger than a conductor cross-section of the second winding.

In one embodiment of the present disclosure, a high voltage direct current power system may be provided which includes at least one circuit breaker according to any embodiment of the present disclosure.

In one embodiment of the present disclosure, an energy storage system may be provided which includes at least one circuit breaker according to any embodiment of the present disclosure.

The present disclosure relates to all possible combinations of features mentioned herein, including the ones listed above as well as other features which will be described in what follows with reference to different

embodiments. Any embodiment described herein may be combinable with other embodiments also described herein, and the present disclosure relates also to all such combinations.

Brief description of the drawings

The above, as well as additional objects, features, advantages and applications of the inventive circuit breaker, will be better understood through the following illustrative and non-limiting detailed description of embodiments. Reference is made to the appended drawings, in which:

Figure 1 is a schematic illustration of a circuit breaker according to one or more embodiments of the present disclosure;

Figure 2 is a schematic illustration of a circuit breaker according to one or more embodiments of the present disclosure;

Figure 3 is a schematic illustration of a circuit breaker according to one or more embodiments of the present disclosure; Figures 4a-4d are schematic illustrations of switching arrangements and devices according to one or more embodiments of the present disclosure;

Figure 5a is a schematic illustration of a high voltage direct current system; Figure 5b is a schematic illustration of a node of a high voltage direct current system, wherein the node includes one or more circuit breakers according to one or more embodiments of the present disclosure;

Figure 6 is a schematic illustration of an energy storage system including one or more circuit breakers according to one or more embodiments of the present disclosure; and

Figure 7 is a schematic illustration of an electrical traction/drive system in a battery powered electric vehicle, including one or more circuit breakers according to one or more embodiments of the present disclosure.

In the drawings, like reference numerals will be used for like elements unless stated otherwise. Unless explicitly stated to the contrary, the drawings show only such elements that are necessary to illustrate the example embodiments, while other elements, in the interest of clarity may be omitted or merely suggested.

Detailed description

Exemplifying embodiments will now be described more fully hereinafter with reference to the accompanying drawings. The drawings show currently preferred embodiments, but the invention may, however, be embodied in many different forms and should not be construed as limited to the

embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

Some embodiments of a circuit breaker according to the present disclosure will now be described with reference mainly to figures 1 , 2 and 3.

Figure 1 illustrates a circuit breaker 100 that may interrupt an electrical current flowing between a first node 102 and a second node 104. The circuit breaker 100 includes a first switching branch 1 10 which includes a first winding 1 20 of a mutual inductor and a mechanical switch 130. The first winding 1 20 and the mechanical switch 130 are connected in series, and during normal operation (i.e. when the circuit breaker 1 00 has not been activated) the current flows from the first node 102 to the second node 104 (or from the second node 104 to the first node 102, depending on the direction of the current flow) through the first switching branch 1 10. The first winding 120 may be on either side of the mechanical switch 1 30.

The circuit breaker 100 further includes a second switching branch 1 12 that includes a second winding 1 22 of the mutual inductor, and a switching arrangement that includes at least one switching device 140. The second winding 1 22 may be on either side of the switching arrangement and the at least one switching device 140. During normal operation, the switching arrangement is opened (i.e. it does not allow current to flow through it, by e.g. keeping the at least one switching device 140 opened or in a current blocking state) and thus no (or very little) current flows through the second switching branch 1 12. The first switching branch 1 10 and the second switching branch 1 12 are connected in parallel between the first node 1 02 and the second node 104.

With the term "mutual" inductor is meant an inductor which is shared between, or which is common to, the first switching branch and the second switching branch. In the present embodiment, the first winding and the second winding of the first switching branch and the second switching branch, respectively, together form the mutual inductor. In these embodiments, the first winding and the second winding are arranged, or spaced, such that they can interact magnetically. In the figures, inductor dot markings (according to the dot convention) have intentionally been left out. In for example Figure 1 , the windings 1 20 and 122 may be arranged such that their dots (if they were included in the figure) would e.g. appear on the same side (e.g. both dots are to the left, or both dots are to the right) of the windings 120 and 122.

The second switching branch 1 12 is configured to operate

bidirectionally. With the term "bidirectionally" is here meant that, when the switching arrangement including the at least one switching device 140 is closed, current is allowed to flow through the second switching branch 1 12 in both directions. In other words, electrical current may flow through the second switching branch 1 12 both from the first node 102 to the second node 104, and/or from the second node 104 to the first node 102. This may be beneficial as the load current (i.e. the current flowing through the circuit breaker 100 during normal operation) and a fault current may e.g. have opposite directions. The circuit breaker 100 may thereby be more flexible and able to interrupt current independently of the electrical direction of the current. This may allow the circuit breaker 1 00 to be used also in DC systems where currents may occasionally change direction (such as in energy storage systems where e.g. a battery may first provide energy to load, i.e. be itself discharging, and then be recharged, with a resulting reversion of the load current). The circuit breaker 100 may e.g. be used in systems where the DC current may have multiple directions, while the polarity of the DC voltage remains unchanged.

If a fault is detected, e.g. by detection of an abnormal increase in load current, the circuit breaker 100 may be activated by allowing current to flow through the switching arrangement (e.g. by closing the at least one

semiconductor switching device 140 and putting it in a non-current blocking state). This may start a commutation (redirection) of the current from the first switching branch 1 10 to the second switching branch 1 12.

Due to its physical properties, the mutual inductor may "resist" (i.e. oppose and/or counter-act) any change in the sum of the current passing through the first switching branch 1 1 0 and the second switching branch 1 12. When an increased amount of current passes through the second switching branch 1 12, the mutual inductor may help to further reduce the amount of current that passes through the first switching branch 1 10, eventually forcing the amplitude of this current to zero at some point (or at least significantly reducing the amplitude of the current). For this purpose, a self inductance of the first switching branch 1 10 may be larger than a self inductance of the second switching branch 1 12. This may be achieved e.g. when a self inductance of the first winding 120 is larger than a self inductance of the second winding 122. At the point when the amplitude of the current that passes through the first switching branch 1 10 goes to zero, the mechanical switch 130 may be opened with less effort and a reduced risk of electrical breakdown (or electrical arc generation) across the gap of the mechanical switch 130. The physical requirements on the mechanical switch 130 may be reduced, which may allow for e.g. lower production cost.

As the second switching branch 1 12 is configured to operate bidirectionally, the circuit breaker is no longer limited to interrupt current of a particular electrical direction and the circuit breaker 100 may be used to interrupt currents also in systems where such situations may occur (e.g. in energy storage systems, or e.g. in multi-terminal HVDC systems or similar). Such flexible, directionally independent, current interruption may not be possible in other circuit breakers in which the commutation branch (e.g. the second switching branch 1 12) is not configured to operate bidirectionally, e.g. in circuit breakers wherein for example the switching arrangement is asymmetric with respect to a direction of the current.

In some embodiments of the circuit breaker 100, the switching arrangement may be bidirectional by using one semiconductor switching device (e.g. an IGBT, or any other type of transistor or for example a thyristor) together with e.g. additional components. One example of such a bidirectional arrangement 420 is illustrated in figure 4c wherein four diodes 424, 425, 426 and 427 are arranged together with a semiconductor switching device 422 in a diode bridge configuration. When the semiconductor switching device 422 is opened (or "turned off", e.g. in a current blocking state), no current may pass from one side to the other (e.g. from left to right). When the semiconductor switching device 422 is closed (or "turned on", e.g. in a non-current blocking state), current may pass from left to right via diode 424, the switching device 422 and diode 425. Likewise, current may pass from right to left via diode 426, the switching device 422 and diode 427, making the switching

arrangement 420 bidirectional.

In some embodiments of a circuit breaker, e.g. the circuit breaker 100, the switching arrangement may include more than one switching device, e.g. a first semiconductor switching device and a second semiconductor switching device, where the first and the second semiconductor switching devices are unidirectional (or unipolar devices). Even though the first and second semiconductor switching devices are unidirectional, the switching

arrangement may be made bidirectional, for example by arranging the first and the second semiconductor switching devices to operate in electrically opposite directions between the first node 102 and the second node 104.

One example of such a switching arrangement 400 is illustrated in figure 4a, in which a first semiconductor switching device 402 is connected in series with a first diode 408, and in which a second semiconductor switching device 404 is connected in series with a second diode 406. The first semiconductor switching device 402 and the first diode 408 are connected antiparallel with the second semiconductor switching device 404 and the second diode 406. When both semiconductor switching devices 402 and 404 are open, no current may flow e.g. from left to right due to the diodes 406 and 408. When the first semiconductor switching device 402 is closed, a current may flow from left to right via the first switching device 402 and the first diode 408. When the second semiconductor switching device 404 is opened, a current may flow from right to left via the second switching device 404 and the second diode 406, making the switching arrangement 400 bidirectional.

A second example of such a bidirectional switching arrangement 410 is illustrated in figure 4b, wherein a first semiconductor switching device 412 and a second semiconductor switching device 414 are connected antiparallel. When both switching devices 41 2 and 414 are open, no current may flow e.g. from left to right. If the first semiconductor switching device 41 2 is closed, current may flow from left to right. If the second semiconductor switching device 414 is closed, current may flow from right to left, and the switching arrangement 410 may be bidirectional.

Figure 2 illustrates another embodiment of a circuit breaker 200. As for the circuit breaker 100, the circuit breaker 200 includes a first switching branch 210 that includes a first winding 220 of a mutual inductor and a mechanical switch 230 connected in series. The first winding 220 may be on either side of the mechanical switch 230. The circuit breaker is connected between a first node 202 and a second node 204. A second switching branch 212 is connected in parallel with the first switching branch 210 between nodes 202 and 204, and the second switching branch 212 includes in this

embodiment a first group 214 of electrical components and a second group 216 of electrical components. The first group 214 includes the second winding 222 of the mutual inductor and a first semiconductor switching device 242. The second winding 222 may be on either side of the first semiconductor switching device 242. The second group 216 includes a second semiconductor switching device 244. In the present embodiment, the first semiconductor switching device 242 and the second semiconductor switching device 244 of the switching arrangement may be unidirectional. The two switching devices 242 and 244 and the second winding are connected from and to the second node 204 as a loop (i.e. a closed electric circuit). The first node 202 may be connected to the second switching branch 212 by a tap to the second winding 250 of the first group 214. The first and the second switching devices are in this configuration arranged antiparallel such that current may be passed through the first sub branch 214 in a first direction, and through the second sub branch 216 in a second direction opposite to the first direction. An example of a unidirectional semiconductor switching device 430 is illustrated in figure 4d, where e.g. a transistor 432 allows current to flow from left to right when it is turned on, and otherwise blocks current from flowing either from left to right or from right to left, making the device unidirectional.

In the circuit breaker 200 in figure 2, the first group 214 and the second group 216 may be considered to be connected in parallel, wherein the second switching branch 212 is connected to the node 202 by a tap 250 to the second winding 222 of the mutual inductor. When, after detection of a fault, at least one of the semiconductor switching devices 242 and 244 is turned on (i.e. closed), current will be commutated from the first switching branch 21 0 to the second switching branch 212 and pass through the second winding 222. As described earlier, the ability of the mutual inductor to "resist" any change in the sum of the current through the first and second switching branches will alleviate the subsequent closing of the mechanical switch 230 by forcing the current through the first switching branch 210 to zero at some point in time.

By using two sub groups 214 and 216, through which current may pass in different directions, unidirectional semiconductor switching devices 242 and 244 may be used without the need of additional components in the switching arrangement such as parallelly or serially connected diodes. This may reduce e.g. power losses otherwise related to the use of such diodes and/or other additional components, and/or reduce e.g. the number and cost of the required components.

Another embodiment of a circuit breaker 300 is illustrated in figure 3. As for the circuit breakers 1 00 and 200, the circuit breaker 300 includes a first switching branch 310 including a first winding 320 of a mutual inductance and a mechanical switch 330 connected in series. The circuit breaker 300 is connected between a first node 302 and a second node 304. The circuit breaker 300 also includes a second switching branch 312 that in turn includes a first sub branch 314 and a second sub branch 316. The first sub branch 314 includes a second winding 322 of the mutual inductor and a first

semiconductor switching device 342 connected together (in series), and the second sub branch 316 includes a second semiconductor switching device 344.

The second sub branch 316 includes a third winding 324 of the mutual inductor connected (in series) to the second semiconductor switching device 344. The first and second sub branches are connected in parallel to form the second switching branch 312, and the second switching branch 31 2 is connected in parallel with the first switching branch 310 between the nodes 302 and 304. As a third winding 324 is used, no tap to any winding is needed to connect the second switching branch 312 to the first switching branch 310 in this embodiment. It may however be envisaged that the embodiment of the circuit breaker 300 is functionally equal to the embodiment of the circuit breaker 200, if e.g. the combined lengths (number of turns, e.g. the inductances) of the second winding 322 and the third winding 324 matches that of the second winding 222 in the circuit breaker 200, and the tap 250 is in the middle of the second winding 222.

A circuit breaker 100, 200, 300 according to the present disclosure may also include an overvoltage protection circuit, such as a snubber (e.g. a varistor, an RC snubber, an RCD snubber or a capacitor). The overvoltage protection circuit may be included in a third switching branch that is connected in parallel with the first and second switching branches (between the first node and the second node). The third switching branch may also be connected in parallel not with the whole of the first and/or second switching branches, but only with e.g. a switching arrangement (such as the switching arrangement 140 in figure 1 ) or a switching device (such any of the switching devices 242, 244 in figure 2 and 342, 344 in figure 3). It is also envisaged that the third switching branch may include an additional winding of the mutual inductor, and that this additional winding is connected in series with e.g. the overvoltage protection circuit. The current may be commutated to the third switching branch e.g. by opening also the switching arrangement, and dissipated by the overvoltage protection circuit.

With "switching device", it is envisaged any device which may be used to selectively block a current to pass through it. Examples of such devices, preferably suitable to handle high currents and/or high power, may include various semiconductor switching devices such as transistors (e.g. IBGTs, FETs, MOSFETs) and various thyristors (such as for example a gate turn-off, GTO, thyristor). A switching device may also include a capacitor.

In figures 1 -3, a mutual inductance between windings/coils is illustrated with two parallel lines. The inductors may be made with air-coils only, with flux-carrying magnetic materials (with a relative magnetic permeability larger than one), or with combinations thereof.

The windings 122, 222, 322 or 324 are normally idling (i.e. they do not carry current), and are only active during a period when the switching device handles a fault. During this period, it may be acceptable for the normally idling windings to give rise to temporary losses and to be subjected to an adiabatic temperature increase. In addition to the adiabatic temperature increase, the cross sectional area of the conductors of a normally idling winding may therefore be reduced. A cross sectional area of a conductor in a winding that is normally active (such as windings 1 20, 220 and 320) may be increased (e.g. by reallocating material, such as copper, from a normally idling winding to a normally active winding). A normally active winding may therefore carry current with reduced normal conduction losses, which may lead to an improved efficiency of the circuit breaker. As an example, a cross sectional area of a winding that is normally active may be 10-30 times (or more) larger than a cross sectional area of a normally idling winding. It is envisaged that a circuit breaker according to the present disclosure may be included in various types of systems, and especially systems in which VSCs are present. Examples of such systems may be energy storage systems, data centers, transportation systems (such as electrical vehicles, EVs) and HVDC power transmission systems. The circuit breaker may be used in DC systems having many terminals, either internal terminals or terminals for connecting to AC systems. Examples of such systems may e.g. be windmill farms where the generated AC power from a windmill is converted to DC power before being transported away on a DC grid. Other systems include electrical buses, trams, and trains where AC motors are driven from a DC power supply (such as e.g. from batteries). In particular, a circuit breaker according to any embodiment of the present disclosure may be useful in DC systems where loading currents and fault currents may flow in different directions, locations within the system, i.e. in situations where traditional or recently disclosed DC breakers may not be flexible enough.

In figure 5a, a multi-terminal HVDC system 500 is illustrated. The system 500 has three nodes 510, 512 and 514 which are connected to each other in a triangular fashion via DC power lines 520, 522 and 524. At each node, a voltage-source converter (VSC) connects the respective node to an external AC system. An external AC system may for example be a windmill, a water turbine, or any other electrical device or system consuming or generating AC power. If a fault occurs on the DC side of the system 500, it is preferable if the fault can be detected and isolated quickly, such that the system 500 may be restored to its normal operating state as soon as possible.

In figure 5b, the first node 510 of the multi-terminal HVDC system 500 is illustrated in more detail. To convert between AC and DC, the node 510 includes a VSC circuit 540 constructed at least partially from IGBTs antiparallel coupled with respective freewheeling diodes. If a fault occurs on the DC side of the system 500, the IGBTs of the VSC may not be able to stop the fault current. Instead, the freewheeling diodes may act as a bridge rectifier and continue to feed the fault. To at least partially resolve such a problem, the node 510 is equipped with circuit breakers 560 and 562, according to one or more of the above described embodiments, positioned at the terminals of the VSC circuit 540. Additional circuit breakers 564, 565, 566 and 567 are also positioned at the ends of the DC power lines 520 (including the two wires 520a and 520b) and 524 (including the two wires 524a and 524b). In some embodiments, only the circuit breakers 560 and 562 may be required, while in some embodiments only the additional circuit breakers 564, 565, 566 and 567 may be required. The node 510 is also equipped with DC switches 570, 571 , 572 and 573 positioned on respective wires of the power lines 520 and 524.

If a DC fault is detected, such as when a current is confirmed to exceed a predefined threshold value, the respective circuit breaker may start to interrupt the current as previously described herein. Once the current is interrupted, the respective DC switch may be opened and the faulted line may be isolated. As the one or more circuit breakers according to the present disclosure does not require current to pass through any switching device (such as a semiconductor switching device) during normal operation, protection against e.g. overcurrents in a HVDC system may be achieved with reduced losses and increased cost-effectiveness and robustness.

Conventional alternatives, such as systems using IGBT circuit breakers instead of circuit breakers according to the present disclosure, would require semiconductor switches to carry current also during normal operation and higher losses would therefore be expected. In addition, such conventional circuit breakers would also not be able to block the current in any direction. An IGBT circuit breaker is a unidirectional device, and a fault current flowing in the direction of the freewheeling diode would not be interrupted. Such a fault current may result from e.g. a fault occurring on the converter side of a node, and the interruption of such a current may have to rely on the IGBTs in the converter itself.

In figure 6, a medium voltage (MV) energy storage system 600 is illustrated wherein multiple batteries 620 and 622 are connected to an AC line 610 via voltage-source converters (VSCs). Only a single phase of the AC line 610 is shown, and it is envisaged that similar arrangements may exist for other phases. In the energy storage system 600, each battery is connected to the line 610 via cascaded H-bridge inverters 630 and DC-DC chopper circuits 640.

As previously described, the VSCs themselves are vulnerable to DC faults and normal current control may be discontinued, especially since currents in the system 600 may flow in different directions depending on whether a battery is discharging or charging. Furthermore, fault currents may go internally through failing semiconductors in the VSC. To at least partially resolve this problem, to offer protection against DC current faults in the system 600, circuit breakers 660, 662 and 624 according to one or more embodiments of the present disclosure are inserted. It is envisaged that more or fewer circuit breakers may be required. In figure 6, circuit breakers 660 and 662 are inserted between the respective batteries 620 and 622 and the DC- DC chopper circuits (such as circuit 640), while circuit breaker 624 is inserted between the stack of cascaded battery and inverter circuits and the AC line 610. Due to their ability to isolate fault currents flowing in multiple directions, the circuit breakers may offer protection also in systems like the energy storage system 600, with reduced losses as no semiconductor devices in the circuit breaker carry current during normal operation.

In figure 7, a low voltage (LV) electrical traction/drive system 700 in a battery powered electrical vehicle (EV) is illustrated. The system 700 includes a battery 720, coupled to an AC motor 730 via an inverter circuit 710 that includes multiple IGBTs and antiparallel coupled diodes. The battery 720 may be charged using suitable charging techniques, illustrated schematically by the charging connections 750 and 752. Charging may be provided as DC (using e.g. charging connection 750, preferably provided with protection according to the present disclosure), or as AC (using e.g. charging connection 752). Depending on whether the battery 720 is discharging (e.g. when the vehicle is running) or charging, current (such as normal current, or fault current) may flow in multiple directions. Circuit breakers 760, 762, 764 and 766 according to one or more of the previously described embodiments of the present disclosure are inserted to provide protection from DC faults in the system 700, independent of the direction of current and with reduced losses. In figure 7, the circuit breaker 760 is inserted between the inverter circuit 710 and the battery 720, while the circuit breakers 762, 764 and 766 are inserted on a respective phase between the inverter circuit 710 and the AC motor 730. It may be envisaged that fewer or more circuit breakers are used if needed.

Other traction/drive systems in which circuit breakers according to one or more embodiments of the present disclosure may be used includes various transportation systems, such as for example trains, trams and electrical buses. In such systems, a battery is not necessarily required and power may be received from e.g. an external conductor (such as an overhead line, or a third rail). Other systems may be e.g. hybrid vehicles wherein batteries are charged using a combustion engine, and where current flow may depend on whether the batteries are charging or recharging.

Although features and elements are described above in particular combinations, each feature or element may be used alone without the other features and elements or in various combinations with or without other features and elements.

Additionally, variations to the disclosed embodiments may be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to

advantage.