WO2015155112A1

WO2015155112A1 - Modular multilevel converter with redundant converter cells in standby mode

Info

Publication number: WO2015155112A1
Application number: PCT/EP2015/057300
Authority: WO
Inventors: Mario Schweizer; Peter Steimer; Roman Grinberg
Original assignee: Abb Technology Ag
Priority date: 2014-04-07
Filing date: 2015-04-02
Publication date: 2015-10-15

Abstract

A modular multilevel converter (10) comprises a plurality of converter cells (20) connected in series in at least one converter leg or branch (16). Each converter cell (20) comprises a cell capacitor (26), semiconductor switches (24) for connecting the cell capacitor (26) to a converter leg (16) and for bypassing the converter cell (20), and a cell controller (32) for providing gate signals to the semiconductor switches (24). At least one redundant converter cell (20') comprises a bypass switch (30) for bypassing the redundant cell. The modular multilevel converter (10) is configured for supplying the cell controller (32) of the redundant cell (20') with electrical energy, when the bypass switch (30) bypasses the redundant converter cell, such that the redundant converter cell (20') is in a standby mode.

Description

DESCRIPTION

Modular multilevel converter with redundant converter cells in standby mode

FIELD OF THE INVENTION

The invention relates to the field of high power electronics. In particular, the invention relates to a modular multilevel converter and a method for operating such a converter.

BACKGROUND OF THE INVENTION

Modular multilevel converters have a very promising topology for medium voltage (up to 20 kV) and high voltage (up to 50 kV) applications. A modular multilevel converter has a plurality of converter cells with its own cell capacitor, which are connected in series in one or more converter legs.

Supplying a converter cell from its own cell capacitor (further called self-supplied cell concept) may allow reducing the number of auxiliary equipment and may result in a truly modular converter. To reduce the impact of high component count on the converter reliability, redundancy on the cell level is employed.

However, self-supplied converter cells usually have to be kept active, i.e. connected to its converter leg, until they may be used for redundancy. Otherwise it may be possible that the cell capacitor depletes and the cell controller of the self-supplied converter cell is shut down. So, as a rule, adding redundant converter cells in high-reliability application increases converter losses.

DESCRIPTION OF THE INVENTION

If it were possible to keep redundant cells in a standby mode before cell failure occurs in the converter, reduction of total converter losses could be achieved. If such a standby mode (further called warm redundancy) were used, there would be almost no efficiency penalty even at very high redundancy levels. The objective of the invention may be to keep redundant converter cells in a standby mode, while having a low additional component count on the converter cell level and while not interfering with the converter operation.

This objective is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

An aspect of the invention relates to a modular multilevel converter. Such a converter comprises a plurality of converter cells connected in series in at least one converter leg or branch. Each converter cell comprises a cell capacitor, semiconductor switches for connecting the cell capacitor to a converter leg or branch and for bypassing the converter cell, and a cell controller for providing gate signals to the semiconductor switches. For example, the converter cell may comprise an upper and a lower semiconductor switch (such as IGBTs) arranged in a half-bridge that may be controlled by the cell controller.

Furthermore, the converter comprises at least one redundant cell with a bypass switch for bypassing the redundant cell. It has to be understood that also the other converter cells (i.e. non-redundant cells) may comprise a bypass switch. The bypass switches may be used for disconnecting failed cells from a converter leg or branch and for connecting redundant converter cells.

The modular multilevel converter is configured for supplying the cell controller of the redundant cell with electrical energy, when the bypass switch bypasses the redundant converter cell, such that the redundant converter cell is in a standby mode. In such a way, the redundant converter cell may be kept in standby until it needed. Auxiliary energy is provided for the self-supplied redundant converter cell, although this cell may be in a bypassed state. With such an arrangement, it may be possible that one or more redundant converter cells are in warm standby mode during the converter operation. They only have to be connected to the corresponding converter leg or branch, when they are really needed.

According to an embodiment of the invention, the cell controller and/or an auxiliary power supply of the redundant cell is connected to a central power supply of the modular multilevel converter as further power source. The cell controller and/or the auxiliary power supply may be arranged in a converter module. The central power supply may be located remote from the converter module, for example in another mechanical distinct component of the converter. The central power source may supply more than one redundant converter cell with electrical power. According to an embodiment of the invention, the redundant converter cell comprises an auxiliary power supply for supplying the cell controller with electrical power. The auxiliary power supply may be connected to the cell capacitor for receiving electrical power and to a further power source for receiving electrical power, when the cell capacitor is depleted. For example, the auxiliary power supply may be connected to a central power supply or to the cell capacitor of another (non-redundant) converter cell.

It has to be understood that the auxiliary power supply may provide a smaller voltage than the voltage that is switched by the semiconductor switches, for example only up to 1 kV.

According to an embodiment of the invention, the auxiliary power supply is connected to a cell capacitor of a second converter cell as a further power source. The redundant cell may be bypassed but kept alive using energy obtained from a neighbouring cell.

According to an embodiment of the invention, the second cell is a neighbouring cell in the converter leg or branch of the redundant converter cell. A neighbour cell may be defined as a converter cell that is in the same branch or leg as a redundant cell, and directly connected to the redundant cell electrically.

According to an embodiment of the invention, the auxiliary power supply is connected via a standby supply circuit with a further power source, for example a cell capacitor of a neighbouring cell and/or a central power supply. The standby supply circuit comprises a switch for connecting the auxiliary power supply to the further power source. For example, this switch may be closed, when the bypass switch of the converter cell is closed (and vice versa).

The auxiliary standby supply circuit may be an integral part of the cell design. Depending on the cell redundancy level in the converter, the circuit may be used in the respective number of redundant cells. For cost reasons it may be that for non-redundant cells the circuit is not populated.

The standby supply circuit may be very simple with a low number of components. The circuit standby supply may be completely independent from other converter cell functionality. So, it may be populated for redundant (or neighbour non-redundant) cells only. Thus, the cost impact of additional hardware may be reduced.

According to an embodiment of the invention, the switch of the standby supply circuit and/or the bypass switch is a mechanical switch, such as a relay. According to an embodiment of the invention, the standby supply circuit comprises a diode connected in series with the switch. The standby supply circuit may be directly connected with the cell capacitor of the redundant cell. In such a way, the cell capacitor may be charged by the standby supply circuit and the cell capacitor may be used as intermediate power storage by the auxiliary power supply of the redundant converter cell.

According to an embodiment of the invention, the cell capacitor, the semiconductor switches, the cell controller and an auxiliary power supply of the redundant cell are arranged on one cell module. A module may be one common housing and/or one common board, the components of the converter cell are mechanically attached to. For example, the converter cell may be exchanged in the converter by simply removing the whole module and inputting another equally designed module.

According to an embodiment of the invention, the cell capacitor, the semiconductor switches and the cell controller of a second cell are arranged on one cell module. The auxiliary power supply of the redundant cell is supplied by the cell capacitor of the second cell with electrical energy. A converter module may comprise two converter cells, a redundant cell and a further cell, supplying the redundant cell with electrical power, while the redundant cell is provided in warm redundancy. Also the standby supply circuit may be arranged in the two-cell converter module.

According to an embodiment of the invention, the converter cell is a half-bridge cell. The converter cell may comprise two semiconductor switches arranged as a half-bridge.

According to an embodiment of the invention, the converter cell is a full-bridge cell. The converter cell may comprise four semiconductor switches arranged as a full-bridge.

In general, every type of modular multilevel converter may be provided with the externally supplied redundant converter cells. For example, the converter may be a DC-to- AC converter comprising two series connected converter legs or branches for each phase, which provide a phase output between them and which are connected to DC outputs. The converter may be a direct AC-to-AC converter with a converter leg or branch interconnecting each input phase with each output phase. The converter may be a cascaded full-bridge converter.

A further aspect of the invention relates to a method for operating a modular multilevel converter. It has to be understood that features of the method as described in the above and in the following may be features of the converter as described in the above and in the following and vice versa. According to an embodiment of the invention, the method comprises: bypassing a redundant converter cell in the converter leg or branch with a bypass switch, supplying a cell controller of the redundant converter cell with electrical energy, when the redundant converter cell is bypassed, detecting a cell fault in a further converter cell of the converter leg or branch, disconnecting the faulty converter cell from the converter leg or branch by closing a bypass switch of the fault converter cell, and connecting the redundant converter cell to the converter leg or branch by opening the bypass switch of the redundant converter cell.

Warm redundancy of a redundant converter cell may be achieved by supplying the converter cell from a further power source, when the cell capacitor of the converter cell is not charged by the usual (normal) operation of the converter cell.

For example, warm redundancy may be implemented in the following way:

- at converter start-up, all cell capacitors are charged,

- after the charging, all redundant converter cells are bypassed,

- converter operation is started,

- during converter operation, cell capacitors of the redundant cells are kept charged by a further (different) power source,

- once a cell failure happens, a redundant cell in the respective branch or leg deactivates its bypass and starts operating.

Auxiliary power, that is needed for a converter cell to operate, can be generated externally or internally (in a self- supplied converter cell). In the former case, there may be an external, central power supply in the converter. Warm redundancy may be implemented, since losses in the bypassed converter cell may be compensated by external power supply.

The auxiliary power to the converter cell may be supplied either via cables or wirelessly from the centralized power supply.

Another approach to achieve warm redundancy may be energy harvesting. The energy may be taken from the ambient (thermal, mechanical, electromagnetic).

With a dual cell concept, warm redundancy may be achieved in the following way:

- the converter is first started with all converter cells in active mode (i.e. with opened bypass switch),

- all the converter cells (and their cell capacitors) are precharged, - some (redundant) converter cells are bypassed and transferred into warm standby mode and where the required power for internal consumption is obtained from other converter cells in the same converter,

- when a fault occurs in any converter cell on any end of the warm standby supply circuit, the faulty converter cell is disconnected from the healthy one by means of the switch in the warm standby supply circuit, and

- the converter cell that was previously kept bypassed in warm standby mode is transferred into the active state, if required.

In summary, the proposed method and converter allow:

- to increase the converter efficiency, in particular, in high-reliability applications,

- to reduce current stress on redundant converter cells, thus reducing their ageing failures,

- to provide a simple circuitry with low number of components,

- to integrate the circuitry in any converter cell, thus keeping the modular approach, and

- to not assemble the circuitry for all the converter cells, which may save costs.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

Fig. 1 schematically shows an AC-to-DC converter according to an embodiment of the invention.

Fig. 2 schematically shows a converter cell for a converter according to an embodiment of the invention.

Fig. 3 schematically shows an AC-to-DC converter according to a further embodiment of the invention.

Fig. 4 schematically shows a converter cell for a converter according to a further embodiment of the invention.

Fig. 5 schematically shows a converter cell for a converter according to an embodiment of the invention. Fig. 6 schematically shows a double cell module for a converter according to an embodiment of the invention.

Fig. 7 schematically shows auxiliary standby supply circuit for a converter according to an embodiment of the invention.

Fig. 8 schematically shows an AC-to-AC converter according to an embodiment of the invention.

Fig. 9 schematically shows a cascaded full-bridge converter according to an embodiment of the invention.

Fig. 10 schematically shows a converter cell for the converter of Fig. 9.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 shows a modular multilevel converter 10, 10a that is adapted for converting a DC current at its DC outputs 12 into three AC currents at its phase outputs 14. The DC outputs are connected via two converter legs 16 connected in series, which provide a phase output 14 between them. The phase output 14 is provided by a phase impedance 18 between the two converter legs.

Each converter leg 16 comprises a plurality of series-connected converter cells 20.

Fig. 2 shows a converter cell 20, 20a that may be used in the converter shown in Fig. 1. One, two or more of the converter cells 20 may be arranged in a converter module 22, for example a common board or common housing.

The converter cell 20, which is a half-bridge cell 20a, comprises two series-connected semiconductor switches 24 (such as IGBTs and/or with an antiparallel freewheeling diode) that are connected in parallel to a cell capacitor 26. A cell output 28 is provided between the semiconductor switches 24. A second cell output 28 is provided at one end of the two series- connected semiconductor switches 24.

The cell outputs 28 of the converter cell 20 are used for connecting the converter cell 20 with the leg 16 of the converter 10. Furthermore, the cell outputs 28 may be short-circuited by a bypass switch 30, for example a relay. The gates of the semiconductor switches 24 and the bypass switch 30 may be controlled by a cell controller 32 that also may be part of the converter module 22. The cell controller 32 may receive control signals 36 from a central controller 38 of the converter 10.

Optionally, the converter cell 20 may comprise an auxiliary power supply 34 for supplying the cell controller with electrical power. The auxiliary power supply 34 is connected to the cell capacitor 26, such that it may draw power from the cell capacitor 26, when it is charged, for example during the normal operation of the converter cell 20.

Furthermore, the auxiliary power supply 34 may be supplied with electrical power from a power source external to the converter cell 20, for example, when the cell capacitor 26 is depleted.

Alternatively or additionally, the cell converter 20 may be supplied directly from an external power source.

Fig. 3 shows a DC-to-AC converter 10, 10a with a central isolated auxiliary power supply 40 as external power source. The cell controllers 32 of the converter cells 20 may be directly supplied from the auxiliary power supply 40. Alternatively or additionally, the local auxiliary power supplies 34 of the converter cells 20 may be supplied by the central auxiliary power supply 40 of the converter.

The converter 10, 10a of Fig. 3 may comprise converter half-bridge cells 20a as shown in Fig. 2.

Fig. 4 shows a further type of converter cell 20, 20b that may be used in the converter 10, 10a of Fig. 1 or Fig. 3. Fig. 4 shows a full-bridge cell 20b that comprises two pairs of series connected semiconductor switches 24 that are connected in parallel to the cell capacitor 26. A cell output 28 is provided between the semiconductor switches 24 of each pair.

Some of the converter cells 20 of the converter 10 may be redundant cells 20', i.e. they may be only used, when one of the other converter cells 20 fails. For example, each leg 16 of the converter may comprise one, two or more redundant converter cells 20'.

During start-up of the converter 10, the controller 38 may bypass all redundant converter cells 20' with their bypass switches 30. The cell controller 32 of the redundant converter cells 20' is then supplied with electrical energy from the power source external to the corresponding cell 20', for example from the central auxiliary power supply 40, when the redundant converter cell 20' is bypassed. In such a way, the redundant converter cells 20' may be kept in standby mode or in warm redundancy. When the central controller 38 and/or the cell controller 32 detects a cell fault in a further converter cell 20 of the converter leg 16, the faulty converter cell is disconnected from the converter leg 16 by closing its bypass switch 30 and the redundant converter cell 20' is connected to the converter leg 16 by opening its bypass switch 30.

Since the redundant cells 20' are not operated, when they are in standby mode, converter cell losses may be reduced, in particular, in applications requiring high redundancy levels and high efficiency (e.g. Subsea or HVDC). This may also result in reduced cooling effort, higher reliability and higher converter efficiency.

Furthermore, if a converter cell 20 is shorted by accident, the converter cell 20 may be restarted with the aid of the external power supply, also in the case, when the cell capacitor 26 is depleted.

Fig. 5 shows more details of a converter cell 20 and in particular of the cell controller 36. The cell controller 36 may comprise IGBT gate units 42, a bypass gate unit 44, a microcontroller 46 and a sensing and protection unit 48.

Furthermore, Fig. 5 shows that a balancing resistor 50 may be connected in parallel to the cell capacitor 26 and the auxiliary power supply 34, which also is connected in parallel to the cell capacitor 26.

Fig. 6 shows a further possibility of how a redundant cell 20' may be supplied with electrical power. A first cell 20 and a second (redundant) cell 20' are neighbouring cells in a converter leg 16. For example, both cells 20, 20' may be assembled in one converter module 22.

Both converter cells 20 ' are equally designed and have an auxiliary power supply 34. A cell controller supplied by the auxiliary power supply 34 has been omitted in Fig. 6.

The second converter cell 20' uses the cell capacitor 26 of the first converter cell 20' as further power source. An auxiliary standby supply circuit 52, which also may be part of the converter module 22, connects the two cell capacitors 26 of the two cells 20, 20'.

For example, when the bypass switch 30 of the second cell 20' is closed, the auxiliary power supply 34 of the second cell 20' may be supplied via the cell capacitor 26 of the first cell 20. The circuit 52 is connected between the two cells 20, 20' that when the redundant cell 20' is bypassed (in which case the cell outputs 28 of the cell 20' are directly connected), the two cell capacitors are connected in parallel. So, no further connection besides the circuit 52 is required. Is has to be noted that the auxiliary standby supply circuit 52 only has to be provided for a limited number of converter cells according to the redundancy level. This may save further cost.

Fig. 7 shows details of an auxiliary standby supply circuit 52 that may be used in Fig. 6 or that, in general, may be used for connecting a redundant converter cell 20' with a further power source additionally to the cell capacitor 26, such as the central auxiliary power supply 40.

The circuit 52 may be a very simple additional circuit with low component count. The circuit may comprise a resistor 54, a switch 56 (for example a relay) and a diode 58 connected in series. Furthermore, the circuit 52 may comprise a driver circuit 60 for the switch 56.

When the switch 56 is closed, the cell capacitor 26 of a redundant cell 20' is kept charged and its voltage is equal to the voltage of the cell capacitor of the respective non-redundant cell 20'. Before the redundant cell 20' is triggered from a bypassed to an active state, the switch 56 may be opened and the circuit 52 may be disconnected.

A triggering of the switch 56 may be performed from the cell controller 32 of the non- redundant cell 20. The diode 58 may prevent the flow of current from a redundant cell 20' to a non-redundant cell 20 and/or may prevent multiple charging/discharging due to cell capacitor voltage ripples. The resistor 54 may be needed to limit an inrush current at the beginning of the charging.

The control of the switch 56 and the bypass switches 30 may be distributed among the central controller 38 and the local cell controllers 32.

In case of a cell failure of a non-redundant cell 20, that is not a neighbouring cell, the switch 56 may be opened by the central control. The bypass switch for the redundant cell 20 also may be opened by the central controller 38.

In case of a cell failure of a non-redundant neighbouring cell 20, the switch 56 may be opened by the central controller 38 or the local controller 32 either of the redundant cell or the neighbouring cell 20. It may be beneficial to use a latching relay type as switch 56 to protect against the case of a cell power supply failure. The bypass switch for the redundant cell 20 may be opened by the central controller 38.

In the case of a cell failure of a redundant cell 20', the switch 56 may be opened by the central controller 38. The bypass switch 30 may be kept closed without extern control, since it is based on latching relay and/or it may be kept close by the central controller 38 or the cell controller 32.

Fig 8 shows an AC-to-AC converter 10, 10b that comprises three primary side phase outputs 14 and three secondary side phase outputs 14. Each phase output 15 of the primary side is connected with each phase output of the secondary side via a converter leg 16 comprising a plurality of series-connected converter cells 20.

The converter cells 20 may be designed like in Fig. 4. The converter 10b (as well as the converter 10a) may comprise double cells like shown in Fig. 6, which are connected by a circuit 52 like shown in Fig. 7.

It also may be possible that the converter cells 20 are supplied by a central auxiliary power supply 40 as shown in Fig. 3. In this case, it may be possible that the cells do not have a local auxiliary power supply.

Fig. 9 shows a cascaded full-bridge converter 10, 10c that comprises an transformer 62 that on the primary side provides three phase outputs 14 and that on the secondary side provides a winding for each converter cell 20, 20c.

The converter has three legs 16 of series-connected converter cells 20, 20c, which on one side are star-connected and on the other side provide three secondary side phase outputs 14.

Optionally, the converter cells 20, 20c may be supplied by a central auxiliary power supply 40.

Fig. 10 shows a converter cell 20, 20c that may be used in the converter 10c of Fig. 9. The converter cell 20 comprises three AC phase inputs 64, which are connected to the secondary windings of the transformer 62. The phase inputs 64 are provided by a passive rectifier 66 which is connected to a cell capacitor 26 in parallel. The rest of the cell 20c is equally designed to the full-bridge cell 20b shown in Fig. 4.

The auxiliary power supply 36 is optional. The auxiliary power supply 36 and/or the cell controller 32 may be supplied by a further power source such as the central auxiliary power supply 40 and/or a second cell 20c, analogously to Fig. 6 and 7.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising 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 or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

10 converter

10a AC-to-DC converter

12 DC output

14 phase output

16 converter leg

18 phase impedance

20 converter cell

20a half-bridge cell

20b full-bridge cell

20' redundant converter cell

22 converter module

24 semiconductor switch

26 cell capacitor

28 cell output

30 bypass switch

32 cell controller

34 auxiliary power supply

36 control signal

38 central controller

40 central auxiliary power supply

42 gate unit

44 bypass switch unit

46 microcontroller

48 sensing and protection unit

50 balancing resistor

52 auxiliary standby supply circuit

54 resistor

56 switch

58 diode

60 switch driver

62 transformer

64 AC input

66 rectifier

Claims

1. A modular multilevel converter (10) comprising a plurality of converter cells (20) connected in series in at least one converter leg or branch (16),

wherein each converter cell (20) comprises a cell capacitor (26), semiconductor switches (24) for connecting the cell capacitor (26) to a converter leg or branch (16) and for bypassing the converter cell (20), and a cell controller (32) for providing gate signals to the semiconductor switches (24);

wherein at least one redundant converter cell (20') comprises a bypass switch (30) for bypassing the redundant converter cell;

wherein the modular multilevel converter (10) is configured for supplying the cell controller (32) of the redundant converter cell (20') with electrical energy, when the bypass switch (30) bypasses the redundant converter cell, such that the redundant converter cell (20') is in a standby mode.

2. The converter (10) of claim 1,

wherein the cell controller (32) and/or an auxiliary power supply (34) of the redundant cell (20') is connected to a central power supply (40) of the modular multilevel converter (10) as further power source.

3. The converter (10) of claim 1 or 2,

wherein the redundant converter cell (20') comprises an auxiliary power supply (34) for supplying the cell controller (32) with electrical power;

wherein the auxiliary power supply (34) is connected to the cell capacitor (26) for receiving electrical power and to a further power source for receiving electrical power, when the cell capacitor (26) is depleted.

4. The converter (10) of claim 3,

wherein the auxiliary power supply (34) is connected to a cell capacitor (26) of a second converter cell (20) as a further power source.

5. The converter ( 10) of claim 4,

wherein the second cell (20) is a neighbouring cell in the converter leg (16) of the redundant converter cell (20').

6. The converter (10) of one of claims 3 to 5,

wherein the auxiliary power supply (34) is connected via a standby supply circuit (52) with a further power source (26, 40),

wherein the standby supply circuit comprises a switch for connecting the auxiliary power supply to the further power source.

7. The converter (10) of claim 6, wherein the switch is a mechanical, electromechanical or semiconductor switch.

8. The converter (10) of claim 6 or 7,

wherein the standby supply circuit comprises a diode connected in series with the switch.

9. The converter (10) of one of the preceding claims,

wherein the cell capacitor, the semiconductor switches, the cell controller and an auxiliary power supply of the redundant cell are arranged on one cell module.

10. The converter (10) of claim 9,

wherein the cell capacitor, the semiconductor switches and the cell controller of a second cell are arranged on one cell module; and/or

wherein the auxiliary power supply of the redundant cell is supplied by the cell capacitor of the second cell with electrical energy.

11. The converter (10) of one of the preceding claims,

wherein the converter cell is a half-bridge cell (20a); and/or

wherein the converter cell is a full-bridge cell (20b).

12. The converter (10a) of one of the preceding claims,

wherein the converter is a DC-to-AC converter comprising two series connected converter legs or branches (16) for each phase, which provide a phase output (14) between them and which are connected to DC outputs (12).

13. The converter (10b) of one of the preceding claims,

wherein the converter is a direct AC -to-AC converter with a converter leg or branch (16) interconnecting each input phase with each output phase.

The converter (10c) of one of the preceding claims,

wherein the converter is a converter with cascaded cells.

15. A method for operating a modular multilevel converter (10) comprising a plurality of converter cells (20) connected in series in a converter leg or branch (16), the method comprising:

bypassing a redundant converter cell (20') in the converter leg or branch (16) with a bypass switch (30);

supplying a cell controller (32) of the redundant converter cell (20') with electrical energy, when the redundant converter cell (20') is bypassed;

detecting a cell fault in a further converter cell (20) of the converter leg or branch

(16);

disconnecting the faulty converter cell (20) from the converter leg or branch (16) by closing a bypass switch (30) of the fault converter cell (20);

connecting the redundant converter cell (20') to the converter leg or branch (16) by opening the bypass switch (30) of the redundant converter cell (20').