WO2019145044A1

WO2019145044A1 - Protection of switching cells for a voltage source converter

Info

Publication number: WO2019145044A1
Application number: PCT/EP2018/051987
Authority: WO
Inventors: Nan Chen; Mikael Davidsson; Muhammad Nawaz; Kalle ILVES; Yuhei OKAZAKI
Original assignee: Abb Schweiz Ag
Priority date: 2018-01-26
Filing date: 2018-01-26
Publication date: 2019-08-01

Abstract

Switching cell (10) for a voltage source converter (1) comprising a switching circuit (13) with a plurality of semiconductor switches (30, 31-34), and a DC source (14). The switching circuit (13) is arranged in parallel to the DC source (14) and configured to selectively connect either side of the DC source (14) to first and second output connections (11, 12), such as AC terminals. The switching cell (10) also comprises a protective circuit (15) comprising a current limiting, or cutting, device (16) and a short-circuit switch (50). The current limiting, or cutting, device (16), such as a fuse, is arranged in a first current path (1) between the DC source (14) and the switching circuit (13). The protective circuit (15) is controllable to selectively provide a second current path (2) for the current through the current limiting, or cutting, device (16), which second current path (2) bypasses the switching circuit (13) and shortens the DC source (14). A method for controlling the switching cell and a voltage source converter comprising the switching cell are provided.

Description

Title: Protection of switching cells for a voltage source converter.

Technical Field

The present invention relates to voltage source converters, especially to switching cells provided with protective circuits in voltage source converters and methods for controlling such a switching cell. The invention concerns high voltage (HV) applications.

Background and Prior Art

A number of documents in the prior art describes different protective measures for semiconductor switches of voltage source converters. For example, SE 1 551 618 (^'618) illustrates a voltage source converter comprising switching cells, each switching cell comprising a plurality of semiconductor switches (S1- S4 in fig. 2 of ^'618) and a capacitor as DC (direct current) source (C in fig. 2, see abstract of ^'618). The semiconductor switches of the switching cells consist e.g. of power transistors, such as Insulated Gate Bipolar Transistors (IGBT) or Bi-mode Insulated Gate Transistors (BiGT). A respective bypass switch is arranged in parallel with each switching cell. Each bypass switch comprises two anti-parallel semi-conducting switching elements, for example thyristors, such as two anti-parallel integrated gate-commutated thyristors (IGCT) (page 10, line 20-22, page 11 , line 7-9).

Similar bypass arrangements can be found in other documents. For example, EP 3 001 552 (^'552) describes a bypass for a voltage source converter. The switching cells of the voltage source converter are provided with a respective bypass, wherein the bypass is provided by means of two thyristors in an anti- parallel arrangement (see abstract of ^'552). DE 10 2011 004328 (^'328) describes a bypass switch protecting a half-bridge switching cell (see abstract of ^'328). CN 106100404 (^'404) describes bypassing half-bridge, or full-bridge switching cells in a voltage source converter by means of a thyristor or pair of thyristors (see figures of ^'404). A drawback with bypass switches is that they need to be dimensioned, or rated, for large currents, and thereby adds to the costs.

EP0227407 (^'407) describes using a fuse to protect a thyristor in a power converter. The fuse is arranged in series with the thyristor and a resistor is arranged in parallel with the fuse, or the serial circuit of the thyristor and fuse (see abstract, claim 1 of ^'407). By using a fuse in series with e.g. a thyristor, the thyristor, which is protected by the fuse, needs only be rated above the rating level of fuse (see page 1 , line 17-22 of ^'407).

On the other hand, a drawback with using a thermal fuse is that the fuse adds resistance to the circuit in order to be able to burn fast, thus, adding losses to the voltage source converter during operation. US2014/0145321 (^'321 ) shows an electro-mechanical bypass switch that includes an electrically controlled actuator in the form of a coil (16 in ^'321 ) operatively connected to a control unit in order to provide selective bypass by driving the contact of the bypass switch by means of a magnetic field. The bypass arrangement also comprises a housing with a compartment (1 , 2 in ^'321 ) provided for transforming a pressure from an exploding semiconductor into motion so that the bypass is closed by passively reacting to the pressure of an exploding semiconductor. The bypass arrangement of ^'321 , thus, constitutes both an electro-mechanical switch and a pyro-mechanical switch. Such a bypass arrangement constitutes a comparably large and complex structure.

Summary of Invention

An aim of the present invention is therefore to provide protection to the semiconductor switches of a switching cell of a voltage source converter, and provide said protection without the drawbacks of the prior art. Especially, the invention is provided for, but not limited to, protection on the DC side of the switching cell. According to a first aspect, the present invention provides a switching cell for a voltage source power converter. The switching cell comprises a first and a second output connection, such as a first and a second AC connection, for connection to a phase of the voltage source converter, a switching circuit comprising a plurality of semiconductor switches, and a DC source. The switching circuit is arranged in parallel to the DC source and configured to selectively connect the DC source to the first and the second connection during use of the switching cell. Especially, the switching cell further comprises a protective circuit comprising a current limiting device or a current cutting device, such as a thermal fuse, and a short-circuit switch. The current limiting, or current cutting, device (such as a fuse) is triggered by the current running through the device. The protective circuit is arranged in parallel to the DC source and in parallel to the switching circuit. The current limiting, or current cutting, device is arranged in a first current path between the DC source and the switching circuit. The protective circuit is controllable to selectively provide a second current path for the current that, during use of the converter, passes through the current limiting, or current cutting, device. The second current path bypasses the switching circuit and shortens the DC source.

Thus, the protective circuit is controllable to short circuit the DC source via the current limiting, or current cutting, device. The DC source may comprise one or more capacitors, which will be de-energized through the second current path until the current limiting, or current cutting, device is triggered.

The current limiting, or current cutting, device may comprise, or consist of, a fuse, a resettable PTC fuse (positive temperature coefficient fuse), a

semiconductor component which fails in open circuit at high current, such as a wire bonded normally-on MOSFET, or a non-linear resistor. When the current cutting device, such as a fuse, is triggered, or blows, the current paths becomes interrupted and the DC source becomes disconnected from the switching circuit. A non-linear resistor may be configured to give high resistance and limit the current, but will not totally cut the current.

An advantage of this combination of short-circuit switch and current limiting, or current cutting, device is that the current limiting, or current cutting, device may more easily be dimensioned and does not depend on a small tolerance between normal operation and guaranteed triggering. The current limiting, or current cutting, device may therefore have comparably low resistance and cause comparably low loss during use of the switching cell. The first and second output connection may be connected to a phase of the voltage source converter e.g. via further switching cells.

In an embodiment of the first aspect, the switching cell is configured with a first inductive component between the plurality of semi-conductor switches of the switching circuit and the DC source. The first inductive component (17) is arranged, in the electrical path, between the plurality of semi-conductor switches of the switching circuit and the protective circuit, and, thus, the first inductive component is provided in the first current path. The second current path provides less inductance and will carry a larger portion of the current from the DC source.

Preferably, the impedance of first inductive component is larger than the impedance of the second inductive component, so that the second current path provided by the protective circuit will carry a large fraction of the current through the fuse, and thereby the large fraction of the current will be bypassed by the protective circuit from the switching circuit. In an embodiment, the DC source comprises at least one capacitor, or a DC supply having a first and a second DC bus bar. In an embodiment, the DC source comprises a plurality of capacitors connected in parallel. Preferably, the protective circuit provides a respective fuse for each capacitor. Preferably also, the fuses are interconnected at a connection to the short-circuit switch, and the protective circuit may only include one short-circuit switch for adding a second current path for the current through the fuses.

In an embodiment, the DC source comprises a plurality of capacitors connected in series. Preferably, the protective circuit provides a single fuse for the series of capacitors. In an embodiment, the DC source comprises a plurality of capacitors in a plurality of series, the series being arranged in parallel. Thus, the DC source of this embodiment comprises a plurality of capacitors arranged in parallel and in series. In an embodiment, the short-circuit switch comprises a semi-conductor switch, preferably a thyristor. Alternatively, the short-circuit switch may be an electro- mechanical switch.

In an embodiment, the switching cell further comprises a bypass switch arranged at the first and the second output connection. The bypass switch is configured for bypassing the switching cell.

In a preferred embodiment, the current limiting, or cutting, device is a fuse, such as a resettable fuse.

In another preferred embodiment, current limiting, or cutting, device is a non- linear resistor providing a comparably larger resistance above a certain threshold current, or a semiconductor switch dimensioned to fail above certain threshold current, such as a normally-on MOSFET.

According to a second aspect, the present invention provides a method for controlling the switching cell of the first aspect. The method comprises:

- monitoring a current or voltage level of the switching cell,

- determining a violation of the level,

- providing the second current path for current through the current limiting, or current cutting, device, which second current path bypasses the switching circuit and shortens the DC source of the switching cell.

In an embodiment of the second aspect, the voltage or current is monitored at the switching circuit or the DC source of the switching cell, preferably at one or more of the semi-conductor switches.

In an embodiment of the second aspect, the monitoring comprises monitoring the current through the switching circuit.

In an embodiment of the second aspect, the second current path is provided by closing the short-circuit switch of the switching cell.

In an embodiment of the second aspect, the control method further includes performing an attempt to block the switching circuit. The attempt is preferably performed prior to providing the second current path. The delay between performing a blocking attempt and providing the second current path should be small, such as in the order of some ms (milli-seconds), or smaller.

In an embodiment of the second aspect, the switching cell is a switching cell of the first aspect provided with a bypass switch at its output connections, wherein the method further comprises bypassing the switching cell by means of the bypass switch. Preferably, the switching cell is bypassed after the short-circuit switch has been closed. In this way the current from the DC source will have been reduced before the bypass is activated.

According to a third aspect, the present invention provides a voltage source converter having at least one phase for a power system. The voltage source converter comprises a controller and a plurality of interconnected switching cells, where each switching cell is a switching cell of the first aspect. The controller is configured to control the switching cells in accordance with the method of the second aspect.

Alternatively, the short-circuiting switch may be an electro-mechanical switch.

Brief Description of Drawings

The invention will be described with examples of different embodiments presented in the drawings. The invention is not limited to these embodiments, but may be provided in different forms within the scope of the appended claims. In the drawings:

Figure 1 is a circuit diagram of an embodiment of a switching cell according to the invention;

Figures 2A and 2B illustrate embodiments of switching circuits;

Figure 3 illustrates a protective circuit;

Figure 4A and 4B illustrate DC sources;

Figure 5 illustrates an embodiment of a switching cell in a circuit diagram;

Figure 6 illustrates a further embodiment of a switching cell;

Figure 7 shows an embodiment of a control method according to the invention; Figure 8 illustrates a further embodiment of the method;

Figures 9A-B illustrate a wye-connected and a delta-connected voltage source converter, respectively, provided with switching cells according to the invention. Description of Embodiments

Figure 1 describes a switching cell 10 comprising a DC source 14, a switching circuit 13 and a first connection 11 and a second AC connection 12 suitable for connecting the switching cell 10 to an AC phase leg of a voltage source converter (100 in figs. 9A-B)). The switching circuit 13 is configured for selectively connecting either side of the DC source 14 to the first and second AC connection 11 , 12 and apply a voltage to the first and second connections 11 , 12. A protective circuit 15 is arranged electrically between the DC source 14 and the switching circuit 13 to provide a connection between the DC source and the switching circuit 13. The protective circuit 15 is arranged in parallel to the DC source 14 and provided to selectively short-circuit the DC source 14. The protective circuit 15 comprises a short-circuit switch 50 for shortening the DC source and a current limiting, or cutting, device 16, such as a fuse. The current limiting, or cutting, device 16 is arranged in series with the short-circuit switch 50. The current limiting, or cutting, device 16 is arranged between, and connects, the DC source 14 to the switching circuit. A controller 61 is provided for the switching cell 10. The controller 61 may be a part of an overall control system 60 of a voltage source converter 100. The controller 61 is configured for selectively closing the short-circuit switch 50 so that the DC source 14 is shortened and the shortening current from the DC source 14 triggers the current limiting, or cutting, device 16 so that the connection between the DC source 14 and the switching circuit 13 is interrupted, or limited, i.e. the current between the the DC source 14 and the switching circuit 13 is limited by e.g. a non-linear resistor.

Figure 1 further illustrates how a first current path 1 is provided from the DC source 14 to the switching circuit 13. A second current path 2 for the current from the DC source is provided by means of closing the short-circuit switch 50. This second current path shortens the DC source 14, and bypasses the switching circuit 13.

The switching cell 10 is suitably also provided with a bypass switch 19 arranged at the first and second AC connections 11 , 12. The bypass switch 19 is configured for selective interconnection of the first and second connection 11 , 12, wherein the switching circuit 13, and thereby the switching cell 15, becomes bypassed at the AC side. The bypass may comprise two power semi-conductor switch in an anti-parallel arrangement, such as thyristors, e.g. IGCTs. The bypass may alternatively be an electrically or electronically controlled

mechanical switch provided a fast switching action can be ensured, for example such a mechanical switch can be an electro-mechanical switch driven by a Thompson coil.

The controller 61 is operatively connected to the bypass switch 19 as well as to the short-circuit switch 50. The controller 61 is further configured to monitor a voltage level or current level of the switching circuit 13 and selectively short circuit the DC source by means of the short-circuit switch 50. The controller 61 may, preferably, also monitor other operational properties of the switching circuit 13, or the DC source 14, such as the temperature of the switching circuit 13, and be configured to, for example, short circuit the DC source 14 and/or activate the bypass switch 19 in response to the temperature of the switching circuit 13 exceeding a safe operating level of the switching cell 10. Preferably, the controller 61 is also provided to utilize the protective circuit 15 as a response to system failure of the control system 60 or the voltage source converter 100, or in other cases to maintain a safe operation of the voltage source converter.

Figures 2-4 describe embodiments of the switching circuit 13, the protective circuit 15 and the DC source 14. In practice, these units may be integrated as one single unit, or e.g. the switching circuit 13 and the protective circuit 15 may be provided in one unit with a first and a second AC connection 11 , 12 and a first and a second DC connection for the DC source 14. The protective circuit 15 may be provided as a separate device and be connected between DC sources and switching circuits of existing switching cells. Switching circuits 13 for switching cells 10 that can be used in connection with the present invention can be of different types. Figures 2A and 2B illustrates two embodiments of switching circuits 13. Figure 2A and 2B illustrates a switching circuit 13 connected to the first and second AC connections 11 , 12 and comprising a first and a second DC connection at its DC side connecting the switching circuit 13 to the DC source 14, via the protective circuit 15. The switching circuit 13 comprises a first inductive component 17 at the connection to the protective circuit, which provides the first inductive component in the first current path (1 in figure 1 ).

The switching circuit 13 of figure 2A comprises four semiconductor switches 31 - 34 arranged in a full bridge configuration for the DC source 14, whereas the switching circuit 13 of figure 2B comprises two semiconductor switches 31 , 32 arranged in a half bridge for the DC source 14.

The examples of switching circuits 13 in figures 2A and 2B are of common types. The invention may however be provided for many different types of and the invention is not limited to these examples. For example, US 2015/0365011 and WO 2014/005634 shows other layouts where the invention can be employed.

The semiconductor switches 31 -34 of the embodiment of figures 2A, 2B may be of any known type of power semiconductor suitable for a voltage source converter (100 in figs. 9A-B), such as IGBTs, BiGTs, or IGCTs (Integrated Gate- Commutated Thyristors), and be based on silicon or silicon carbide. The semiconductor switches 31 -34 should be of a turn-off type. The semiconductor switches are of high voltage type, above 1 kV, for example about 6 kV, but may use silicon carbide based semiconductors of even higher voltages.

Figure 3 shows an embodiment of the protective circuit 15. The protective circuit includes the current limiting, or cutting, device 16, exemplified as a fuse 16a, and the short-circuit switch (50 in figure 1 ) exemplified as a controllable thyristor 51. The current limiting, or cutting, device is illustrated as a fuse (16a-n) in figures 3, 5 and 6 and a fuse, such as a resettable fuse, is a preferred

embodiment thereof. Flowever, the embodiments illustrated in figs. 3, 5 and 6 may alternatively comprise, or consist of, another of the preferred embodiments of the current limiting or cutting device 16, such as a non-linear resistor. The protective circuit also includes an inductive component 18 (second inductive component 18 of the switching cell 10) arranged in series with the fuse 16a or other current limiting, or cutting, device (16), which serial connection of fuse 16a (or other current limiting, or cutting, device (16)) and inductive component 18 provides a connection for current between the DC source 14 and the switching circuit 13 (current paths 1 and 2 in figure 1 ). Although the second inductive component 18 is not illustrated in figure 1 , the switching cell 10 of the present invention preferably provides such a second inductive component 18 in all embodiments of the switching cell 10 and the protective circuit 15. The second inductive component 18 should preferably be smaller than the first inductive component 17 so that the second current path (2 in figure 1 ) will carry a large fraction of the current from the DC source 14.

Figures 4A and 4B illustrate embodiments of the DC source 14. Figure 4A illustrates a DC source in the form of a capacitor 41 , whereas figure 4B illustrate a DC source in the form of a DC supply 42 and a pair of DC bus bars 43, 44. The DC supply 42 may comprise a battery storage, and/or comprise a voltage source converter providing DC voltage for supplying the switching cell 10. In an embodiment illustrated in figure 6, the DC source 14 comprises a plurality of capacitors 41 a-n.

The DC source 14 may alternatively comprise capacitors in a serial

arrangement. Thus, alternatively, the capacitor 41 of figure 4, or each of the capacitors 41 a-n of figures 6, may consist of a series of individual capacitors (not illustrated), so that each capacitor 41 , 41 a-n illustrated in figures 4-6 are made up of a number of capacitors (not illustrated). Figure 5 illustrate how an embodiment of the switching cell 10 of figure 1 may be provided in larger detail. The half bridge switching circuit 13 of figure 2B is used for exemplifying the switching circuit 13 in the example given in figure 5. The DC source 14 is exemplified as comprising a (single) capacitor 41. Figure 5 further illustrates how the current in the first current path 1 passes the fuse 16a (or other current limiting, or cutting, device (16)) and is affected by both the first and the second 17, 18 inductive components. Upon closing the short-circuit switch 50, exemplified and illustrated as a thyristor, the second current path 2 is provided which passes the fuse 16a (or current limiting, or cutting, device) and is affected by the second inductive component 18. The first and the second current paths 1 , 2 has a common portion 3 through the fuse 16 and the second inductive component 18.

Figure 6 illustrates an embodiment where the DC source 14 comprises a plurality of capacitors 41 a-n connected in parallel. Each capacitor 41a, 41 b,..., 41 n is connected to the switching circuit 13 via a respective fuse 16a, 16b, ..., 16n or alternatively via a respective other type of current limiting, or cutting, device (16), so that each capacitor 41a, 41 b...,, 41 n is connected to the switching circuit 13 and provides a respective third current path 3a, 3b,..., 3n through the fuse 16a, 16b, ..., 16n which current paths are joined into the first current path 1 leading to the switching circuit 13. A short-circuit switch 50, exemplified as a thyristor 51 in figure 6, is arranged in parallel to the capacitors 41 a, 41 b,..., 41 n and is connected between the switching circuit 13 and each fuse 16a, 16b, ..., 16n in order to provide a second current path 2 for the current when the switch/thyristor 51 is closed. Thus, each capacitor 41 a,

41 b,..., 41 n is connected in series with its respective fuse 16a, 16b, ..., 16n, and these serial connections of capacitor and fuse are arranged in parallel and interconnected to each other, wherein the first and second current paths 1 , 2 are provided to the switching circuit 13 and thyristor 51 , respectively, after the interconnection.

The circuit of figure 6 may be operated as follows. The voltage level at, or the current through, each of the third current paths 3a, 3b, ..., 3n, i.e. from each capacitor 41 a, 41 b,..., 41 n through each fuse 16a, 16b, ..., 16n, is monitored and the thyristor 51 is closed when any one of the voltages or currents exceeds the triggering level so that the second current path 2 is provided for handling the exceeding voltage or current and de-energize the capacitor 41 a, 41 b, ...41 n, and wherein the corresponding fuse 16a, 16b, ..., 16n is triggered. Alternatively, the circuit of figure 6 may be operated by monitoring the current through the first current path 1 , and/or the voltage level at the switching circuit 13, e.g. at each of the semiconductor switches 31 -34 that provides a connection to the first current path 1. Figure 7 illustrates an embodiment of a control method. The method includes 101 monitoring the current or voltage, preferably monitoring the current at the semiconductor switches (30, 31 - 34 in figures 2A, 2B, 5, 6) of the switching circuit (13 in figure 1 ), and detect 103 if the monitored voltage or current exceeds an allowed level for voltage or current in the switching circuit 13. If the level is violated, the method continues with closing the short-circuit switch 50, and thereby shortening 109 the DC source while conducting the shortening current through the fuse. The control method may also include bypassing 110 the switching cell by means of the bypass switch (19 in figure 1 ). Bypassing 110 is preferably performed with a short delay compared to the closure of the short- circuit switch in order to decrease the current at the bypass switch 19. The method may also include closing 105 the short-circuit, upon detecting 103A other critical failures, such as overheating of semiconductor switches 30, 31-34, over-voltage at the DC source 14, or upon receiving a system failure signal. Figure 8 illustrates an embodiment of the control method that includes performing 105 an attempt to block the semiconductor switches (30, 31 -34) if the voltage or current level of the switching circuit is too high, or if there is any other violation of safety measures, such as semiconductor temperature too high, too high voltage of DC source, e.g. capacitor, or a system failure signal. The attempt is preferably performed 105 prior to the short-circuiting 109 of the DC source. After detecting 103 a violation of a safety level of the voltage or current at the switching circuit 13, or detecting another violation, the method includes determining whether a blocking 104 of the semiconductor switches 30- 34 may be appropriate to avoid further failure. If blocking 104 is determined as suitable, the method continues with performing the blocking attempt 105. If blocking 104 is determined not to be possible or suitable to counteract the malfunction, the method continues with shortening 109 the DC source. The method further includes determining 107 if a performed blocking attempt was successful in which case shortening 109 of the DC source may not need being performed. If blocking attempt was not successful, the method continues with shortening 109 the DC source. The delay between the blocking attempt 105 and the shortening 109 of the DC source should be very small, in the order of milli seconds. The method includes bypassing 110 the switching cell, preferably at a short delay after shortening 109 the DC source, so that the voltage level of the DC source 14 may be lowered before bypassing 110. Figures 9A-B illustrate a wye-connected and a delta-connected voltage source converter 100, respectively. The voltage source converter comprises switching cells 10 and a control system 60, which control system 60 is configured to control each switching cell 10 of the voltage source converter 100 in

accordance with the functionality of the switching cells 10 as previously described and in accordance with the methods of figures 7 and 8. The switching cells 10 of each leg of the voltage source converter 100 are arranged in series to provide connections for a respective phase A, B, C of an AC power system. Figures 9A-B only show to common arrangements, the delta and the wye, and other arrangements known in the art are possible, wherein the voltage source converter 100 comprises a plurality of switching cells 10 where each switching cell 10 is provided in accordance with the present invention,

The embodiments have described a switching cell 10 for a voltage source converter 100 comprising a switching circuit 13 with a plurality of semiconductor switches 30, 31-34, and a DC source 14. The switching circuit 13 is arranged in parallel to the DC source 14 and configured for selectively connecting either side of the DC source 14 to first and second output connections 11 , 12, such as AC terminals. The switching cell 10 also comprises a protective circuit 15 comprising a current limiting or cutting device 16, such as a fuse, and a short- circuit switch 50. The current limiting, or cutting, device 16 is arranged in a first current path 1 between the DC source 14 and the switching circuit 13. The protective circuit 15 is controllable to selectively provide a second current path 2 for the current through the current limiting, or cutting, device 16, which second current path 2 bypasses the switching circuit 13 and shortens the DC source 14. Embodiments describing a method for controlling the switching cell 10 and a voltage source converter 100 comprising the switching cell have also been provided. The present invention is not limited to these embodiments but further embodiments within the scope of the claims will be apparent to a person skilled in the art.

Claims

1. Switching cell (10) for a voltage source converter (100) and comprising a first and a second output connection (11 , 12) for connection to a phase of the voltage source converter (1 ), a switching circuit (13) comprising a plurality of semiconductor switches (30, 31 -34), and a DC source (14), wherein the switching circuit (13) is arranged in parallel with the DC source (14) and configured to selectively connect the DC source (14) to the first and the second connection (11 , 12) during use of the switching cell (10),

c h a r a c t e r i z e d in further comprising a protective circuit (15) comprising a current limiting, or cutting, device (16) and a short-circuiting switch (50), said current limiting, or cutting, device (16) being triggered by the current running through the device (16) itself,

- said protective circuit (15) being arranged in-between the DC source (14) and the switching circuit (13), wherein said current limiting, or cutting, device (16) is arranged in a first current path (1 ) between the DC source (14) and the switching circuit (13), and wherein the short-circuiting switch (50) is arranged in parallel with the switching circuit (13), wherein the protective circuit (15) is controllable to selectively provide a second current path (2) for the current that passes through the current limiting, or cutting, device (16) which second current path (2) bypasses the switching circuit (13) and shortens the DC source (14).

2. Switching cell (10) according to claim 1 , configured with a first inductive component (17) between the plurality of semi-conductor switches (31 -34) of the switching circuit (13) and the protective circuit (15), wherein said first inductive component (17) is provided in the first current path (1 ).

3. Switching cell (10) according to claim 2, configured with a second inductive component (18) between the DC source (14) and the protective circuit (15), said second inductive component being provided in the first current path (1 ) and in the second current path (2).

4. Switching cell (10) according to claim 3, wherein the first inductive

component (17) is larger than the second inductive component (18).

5. Switching cell (10) according to any of claims 1 to 4, wherein the DC source (14) comprises at least one capacitor (41 ), or a battery, or a DC supply (42) having a first and a second DC bus bar (43, 44).

6. Switching cell (10) according to claims 5, wherein the DC source (14) comprises a plurality of capacitors (41A-N) connected in parallel.

7. Switching cell (10) according to any of claims 1 to 6, wherein the short-circuit switch (50) comprises a semi-conductor switch, such as a thyristor (51 ).

8. Switching cell (10) according to any of claims 1 to 7, further comprising a bypass switch (19) arranged at the first and the second AC connection (11 , 12) and configured for bypassing the switching cell (10).

9. Method for controlling a switching cell (10) according to any of claims 1 -8, said method comprising:

- monitoring (101 ) a current or voltage level of the switching cell (10),

- detecting (103) a violation of a safe operation condition criterial,

- providing (109) a second current path (2) for current through the current limiting, or cutting, device (16), which second current path (2) bypasses the switching circuit (13) and shortens the DC source (14) of the switching cell (10).

10. Method according to claim 9, said voltage or current being monitored at the switching circuit (13) or the DC source (14) of the switching cell (10).

11. Method according to claim 10, wherein said monitoring (101 ) comprises monitoring the current through the switching circuit (13).

12. Method according to any of claims 9 to 11 , said second current path (2) being provided by closing the short-circuit switch (50) of the switching cell (10).

13. Method according to any of claims 9 to 12, further including performing (105) an attempt to block the switching circuit (13), said attempt being performed prior to providing the second current path (2).

14. Method according to any of claims 9 to 13, and wherein the switching cell (13) is provided with a bypass switch (19) at its first and second output connections (11 , 12), said method further comprising bypassing (110) the switching cell by means of the bypass switch (19).

15. Voltage source converter (100) for at least one phase of a power system comprising a control system (60) and a plurality of interconnected switching cells (10), each switching cell according to any of claims 1 to 8, said control system (60) being configured to control the switching cells (10) in accordance with the method of any of claims 9-14.