MODULAR MULTILEVEL CONVERTER WITH DIFFERENT SUBMODULATE TYPES

Description

[0001] The invention relates to a modular multilevel power converter which, for each AC voltage phase of a multiphase AC voltage, has a first power converter branch which connects an AC voltage terminal of the AC voltage phase to a first DC voltage terminal, and a second power converter branch which connects the AC voltage terminal to a second DC voltage terminal, wherein each power converter branch has several submodules of two different submodule types connected in series.

[0002] These types of converters are used particularly in converter stations for so-called high-voltage direct current (HVDC) transmission, where electrical energy is transferred between alternating current (AC) grids via a high-voltage direct current transmission line (HVDC link) at high DC voltage. To connect an HVDC link to an AC grid, a converter station is located between the AC grid and one end of the HVDC link, where the conversion between the AC current and voltage of the AC grid and the DC current and voltage of the HVDC link takes place.

[0003] Each submodule of a modular multilevel converter typically contains several semiconductor switches, a capacitor, and a bypass switch. The bypass switch bridges the submodule for its protection if the voltage applied to the submodule exceeds a submodule limit voltage.

[0004] International patent application WO 2012 / 103936 A1 discloses a multiphase modular multilevel converter which has in each converter branch a module group with half-bridge modules and a module group with full-bridge modules.

[0005] Document CN 103 050 955 B discloses a modular multilevel converter design in which each converter branch contains several module groups, each comprising half-bridge modules. A varistor is connected in parallel to each module group.

[0006] International patent application WO 2014 / 166129 A1 discloses a power converter for high-voltage direct current transmission. A spark gap is installed on the high-voltage direct current side as a protective circuit.

[0007] Documents US 2015 / 0229234 Al and WO 2012 / 072168 A2 each disclose a double half-bridge module; document US 2017 / 054294 A1 discloses a double full-bridge module.

[0008] European patent application EP 2 608 344 A1 discloses a module whose terminals can be short-circuited by means of a safety switch in the event of a fault. This prevents overloading of the module's internal intermediate circuit capacitor.

[0009] In a modular multilevel power converter, submodules can be overloaded due to faults, such as insulation failures. To protect the power converter, the submodules are blocked when such faults occur by opening the semiconductors, and the AC grid is switched off.

[0010] A disadvantage of this is that during the period from the blocking of the submodules until the AC grid is switched off, the difference between the DC voltage and the applied AC voltage is present across the converter branches. This leads to a charging of the capacitors in the submodules. Depending on the magnitude and duration of the applied

[0011] A voltage difference can lead to a voltage spike, which may be associated with exceeding the submodule limit voltage in one or more submodules. Exceeding the submodule limit voltage of a submodule due to overload triggers the bypass switch of that submodule, causing it to fail. This means that an overload can lead to the failure of several or even all submodules in a power converter branch.

[0012] One way to protect the converter branches from submodule overload and the associated risk of bypass switch tripping is to increase the number of submodules in the converter branches to such an extent that the submodule limit voltages are highly unlikely to be exceeded. However, this solution is very complex and expensive, increases losses due to the additional submodules, and increases the space required for the submodules.

[0013] The invention is based on the objective of protecting a modular multilevel power converter with submodules of two different submodule types from overloading of the submodules.

[0014] The problem is solved according to the invention by a modular multilevel power converter with the features of claim 1.

[0015] Advantageous embodiments of the invention are the subject of the dependent claims.

[0016] A multiphase modular multilevel power converter according to the invention comprises two DC terminals and, for each AC phase of a multiphase AC voltage, one AC terminal, a first converter branch connecting the AC terminal to a first DC terminal, and a second converter branch connecting the AC terminal to the second DC terminal. Each converter branch has a first module group of several series-connected submodules of a first submodule type, each with several semiconductor switches, and a second module group of several series-connected submodules of a second submodule type different from the first submodule type, each with several semiconductor switches, connected in series with the first module group. The two module groups of each converter branch are independently protected against overvoltages by overvoltage-limiting protection circuits.

[0017] A modular multilevel converter according to the invention thus comprises submodules of two different submodule types. This allows the advantages of different submodule types to be combined. For example, half-bridge modules can be combined with full-bridge modules or other submodule types mentioned below. However, submodules of different submodule types are generally not charged uniformly in the event of fault-induced overvoltages. For example, full-bridge modules can be charged in the blocked state by overvoltages of both voltage directions (or voltage polarity), while half-bridge modules have a freewheeling path for one voltage direction and can therefore only be charged by overvoltages of the other voltage direction.The invention addresses this by connecting the submodules of each submodule type in series to form a module group in each converter branch, and by protecting the module groups against overvoltages through independent protection circuits. This allows the protection circuits to be adapted to the submodule types of the module groups, so that, in particular, no additional submodules are required to protect the converter branches from submodule overload. Compared to increasing the number of submodules, this saves effort, costs, and space in the converter design and reduces its losses.

[0018] One embodiment provides that each module group is protected against overvoltages by a protective circuit connected in parallel to the module group. An alternative embodiment, according to the invention, provides that the first module groups of both converter branches of each AC phase are connected to the AC terminal of the AC phase and are protected against overvoltages by a protective circuit connected in parallel to the first two module groups, and the second module group of each converter branch is protected against overvoltages by a protective circuit connected in parallel to the second module group. The first alternative allows each module group to be protected by a separate protective circuit connected in parallel to it.The second alternative allows for the protection of two identical module groups of a power converter branch by means of a common protection circuit connected in parallel to both module groups, thereby reducing the number of protection circuits compared to the first alternative.

[0019] A further embodiment of the invention provides that at least one protective circuit comprises a varistor or a combination of several varistors. For example, at least one varistor is a metal oxide varistor. Alternatively or additionally, at least one protective circuit comprises a spark gap. These embodiments of the invention thus provide for protecting the module groups against overvoltages by means of varistors, spark gaps, or combinations of varistors and spark gaps. By appropriately designing the varistors with respect to the voltage dependence of their electrical resistances or the spark gaps with respect to their breakdown voltages, the overvoltage protection of a protective circuit can be adapted to the respective module group it protects.

[0020] A further embodiment of the invention provides that the submodules of one of the two module groups of each converter branch are half-bridge modules, each comprising a half-bridge with a semiconductor switch arranged in each of its bridge arms and a capacitor connected in parallel to the half-bridge. Half-bridge modules are suitable as submodules of a modular multilevel converter because they have fewer semiconductor switches than other submodules (for example, full-bridge modules) and are therefore more cost-effective and cause lower losses.

[0021] A further embodiment of the invention provides that the submodules of one of the two module groups of each converter branch are full-bridge modules, each comprising two half-bridges connected in parallel, with a semiconductor switch arranged in each of the bridge arms, and a capacitor connected in parallel to the two half-bridges. Unlike half-bridge modules, full-bridge modules enable the generation of voltages with opposite polarities, thereby increasing the range of applications for the converter compared to the exclusive use of half-bridge modules.

[0022] A further embodiment of the invention provides that the submodules of one of the two module groups of each power converter branch are double half-bridge modules, each comprising two half-bridges, in the bridge arms of which a semiconductor switch is arranged, a capacitor connected in parallel to each half-bridge, a first rectifier circuit that connects a first electrode of a first capacitor to a first electrode of the second capacitor, a second rectifier circuit that connects the second electrode of the first capacitor to the second electrode of the second capacitor, and a coupling semiconductor switch that is connected between the first electrode of the first capacitor and the second electrode of the second capacitor.

[0023] A further embodiment of the invention provides that the submodules of one of the two module groups of each power converter branch are double half-bridge modules, each comprising two half-bridges, in whose bridge arms a semiconductor switch is arranged, a capacitor connected in parallel to each half-bridge and two coupling semiconductor switches that connect the two capacitors crosswise.

[0024] A further embodiment of the invention provides that the submodules of one of the two module groups of each power converter branch are double full-bridge modules, each comprising two half-bridges, in the bridge arms of which a semiconductor switch is arranged, a capacitor connected in parallel to each half-bridge and three coupling semiconductor switches connected in series, wherein a first coupling semiconductor switch and a second coupling semiconductor switch form a series circuit connected in parallel to a first capacitor and the second coupling semiconductor switch and the third coupling semiconductor switch form a series circuit connected in parallel to the second capacitor.

[0025] The double half-bridge modules and double full-bridge modules mentioned in the aforementioned embodiments of the invention are particularly suitable as an alternative to the use of full-bridge modules. Preferably, a power converter according to the invention comprises a combination of half-bridge modules with full-bridge modules or with the aforementioned double half-bridge modules or double full-bridge modules.

[0026] A further embodiment of the invention provides that each submodule has a bypass switch that bypasses the submodule if a submodule voltage applied to the submodule exceeds a submodule limit voltage, and that each protection circuit limits a module group voltage applied to the series connection of submodules protected by the protection circuit to a module group limit voltage that is less than the sum of the submodule limit voltages of all submodules in this series connection. This embodiment of the invention prevents the bypass switches of the submodules from tripping by means of a corresponding module group-specific design of the protection circuits, which is adapted to the submodule limit voltages at which the bypass switches are triggered.

[0027] In particular, the invention provides for the use of a modular multilevel power converter according to the invention in a power converter station for high-voltage direct current transmission.

[0028] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of exemplary embodiments, which are explained in more detail in conjunction with the drawings. These drawings show: FIG 1 a schematic circuit diagram of a first embodiment of a modular multilevel power converter, FIG 2 schematically a module group of a modular multilevel power converter, FIG 3 a schematic circuit diagram of a second embodiment of a modular multilevel power converter, FIG 4 a circuit diagram of a first embodiment of a submodule of a modular multilevel power converter, FIG 5 a circuit diagram of a second embodiment of a submodule of a modular multilevel power converter, FIG 6a circuit diagram of a third embodiment of a submodule of a modular multilevel power converter, FIG 7 a circuit diagram of a fourth embodiment of a submodule of a modular multilevel power converter, FIG 8 a circuit diagram of a fifth embodiment of a submodule of a modular multilevel power converter.

[0029] Corresponding parts are marked with the same reference symbols in the figures.

[0030] Figure 1Figure 1 shows a schematic circuit diagram of a first embodiment of a three-phase modular multilevel converter 1 in a simplified representation as a single-line diagram, which considers only one AC voltage phase. The converter 1 has one AC voltage terminal 3 and two DC voltage terminals 5, 6 for each AC voltage phase. The AC voltage terminal 3 is connected to an AC voltage phase of a three-phase AC power grid 9 via a transformer unit 7.

[0031] The power converter 1 further comprises, for each AC voltage phase, a first power converter branch 11, which connects the AC voltage terminal 3 to a first DC voltage terminal 5, and a second power converter branch 12, which connects the AC voltage terminal 3 to the second DC voltage terminal 6. Each power converter branch 11, 12 comprises a first module group 13 of several series-connected submodules 15 of a first submodule type, each with several semiconductor switches 17, and a second module group 14, connected in series with the first module group 13, of several series-connected submodules 15 of a second submodule type different from the first submodule type, each with several semiconductor switches 17. Furthermore, each power converter branch 11, 12 comprises an inductor 19 connected in series with the module groups 13, 14.

[0032] Each module group 13, 14 is protected against overvoltages by a protection circuit 21, wherein each protection circuit 21 has a varistor 23 connected in parallel to the module group 13, 14. Each protection circuit 21 is designed for the module group 13, 14 it protects, as described in more detail below; that is, the varistors 23 for different module groups 13, 14 generally differ from one another with respect to the voltage dependence of their electrical resistances. The varistors 23 are, for example, designed as metal oxide varistors.

[0033] Figure 2 Figure 1 schematically shows a module group 13. Each module group 13, 14 can have a large number of submodules 15 connected in series, for example several hundred submodules 15.

[0034] Figure 3Figure 1 shows a schematic circuit diagram of a second embodiment of a three-phase modular multilevel converter 1. This embodiment differs from the one shown in Figure 2. Figure 1 The embodiment shown differs only with regard to the design of the protective circuits 21. Firstly, unlike the one in Figure 1 In the illustrated embodiment, the first module groups 13 of both converter branches 11, 12, each connected to an AC voltage connection 3, are protected by a common protection circuit 21, while every second module group 14 is protected as in the embodiment shown. Figure 1 In the illustrated embodiment, each protective circuit is protected separately by a protective circuit 21. Furthermore, each protective circuit 21, unlike the one shown in Figure 1 In the illustrated embodiment, a spark gap 25 is used instead of a varistor 23. Analogous to the one in Figure 1In the illustrated embodiment, each protection circuit 21 is designed for the module group 14 it protects or for the module groups 13 it protects, i.e., a spark gap 25 for a second module group 14 generally differs from the spark gap 25 for the first module groups 13 with respect to its flashover voltage.

[0035] The Figures 4 to 8 show circuit diagrams of submodules 15 of different submodule types that are suitable as the first or second submodule type of a modular multilevel power converter 1 according to the invention.

[0036] Figure 4Figure 1 shows a submodule 15 designed as a half-bridge module. This submodule 15 has a half-bridge 27, in each of whose bridge arms a semiconductor switch 17 is arranged, and a capacitor 29 connected in parallel to the half-bridge 27. The submodule 15 further has a first submodule terminal 33 connected to the bridge branch of the half-bridge 27 and a second submodule terminal 34 connected to a first electrode 29.1 of the capacitor 29.

[0037] Figure 5Figure 1 shows a submodule 15 designed as a full bridge module. This submodule 15 has two half-bridges 27, 28 connected in parallel, each with a semiconductor switch 17 arranged in its bridge arm, and a capacitor 29 connected in parallel to the two half-bridges 27. The submodule 15 also has a first submodule terminal 33 connected to the bridge arm of a first half-bridge 27 and a second submodule terminal 34 connected to the bridge arm of the second half-bridge 28.

[0038] Figure 6Figure 1 shows a submodule 15 designed as a double half-bridge module. This submodule 15 has two half-bridges 27, 28, in whose bridge arms a semiconductor switch 17 is arranged, two capacitors 29, 30, two rectifier circuits 37, 38 and a coupling semiconductor switch 39. A first capacitor 29 is connected in parallel to a first half-bridge 27. The second capacitor 30 is connected in parallel to the second half-bridge 28. A first rectifier circuit 37 connects a first electrode 29.1 of the first capacitor 29 with a first electrode 30.1 of the second capacitor 30. The second rectifier circuit 38 connects the second electrode 29.2 of the first capacitor 29 with the second electrode 30.2 of the second capacitor 30. The coupling semiconductor switch 39 is connected between the first electrode 29.1 of the first capacitor 29 and the second electrode 30.2 of the second capacitor 30.Each rectifier circuit 37, 38 is configured as a series connection of a rectifier diode 43 and a resistor 45, wherein the cathode of the rectifier diode 43 of the first rectifier circuit 37 is connected to the first electrode 29.1 of the first capacitor 29, and the cathode of the rectifier diode 43 of the second rectifier circuit 38 is connected to the second electrode 29.2 of the first capacitor 29. The submodule 15 further comprises a first submodule terminal 33 connected to the bridge branch of the first half-bridge 27 and a second submodule terminal 34 connected to the bridge branch of the second half-bridge 28.

[0039] Figure 7Figure 1 shows a further submodule 15 configured as a double half-bridge module. This submodule 15 has two half-bridges 27, 28, in each of whose bridge arms a semiconductor switch 17 is arranged, two capacitors 29, 30, and two coupling semiconductor switches 39, 40. A first capacitor 29 is connected in parallel to a first half-bridge 27. The second capacitor 30 is connected in parallel to the second half-bridge 28. A first coupling semiconductor switch 39 is connected between a first electrode 29.1 of the first capacitor 29 and a second electrode 30.2 of the second capacitor 30. The second coupling semiconductor switch 40 is connected between the second electrode 29.2 of the first capacitor 29 and the first electrode 30.1 of the second capacitor 30.

[0040] The coupling semiconductor switches 39, 40 thus connect the capacitors 29, 30 crosswise to each other. The submodule 15 further has a first submodule connection 33 connected to the bridge branch of the first half-bridge 27 and a second submodule connection 34 connected to the bridge branch of the second half-bridge 28.

[0041] Figure 8 Figure 1 shows a submodule 15 designed as a double full-bridge module. This submodule 15 has two half-bridges 27, 28, in whose bridge arms a semiconductor switch 17 is arranged, two capacitors 29, 30 and three coupling semiconductor switches 39, 40, 41. A first capacitor 29 is connected in parallel to a first half-bridge 27. The second capacitor 30 is connected in parallel to the second half-bridge 28. A first coupling semiconductor switch 39 and a second coupling semiconductor switch 40 form a connection to the first

[0042] Capacitor 29 is connected in parallel in a series circuit. The second coupling semiconductor switch 40 and the third coupling semiconductor switch 41 form a series circuit connected in parallel with the second capacitor 30. The third coupling semiconductor switch 41 is connected between a first electrode 29.1 of the first capacitor 29 and a first electrode 30.1 of the second capacitor 30. The second coupling semiconductor switch 40 is connected between the first electrode 29.1 of the first capacitor 29 and the second electrode 30.2 of the second capacitor 30. The first coupling semiconductor switch 39 is connected between the second electrode 29.2 of the first capacitor 29 and the second electrode 30.2 of the second capacitor 30. The semiconductor switches 17 of the first half-bridge 27, the first coupling semiconductor switch 39 and the second coupling semiconductor switch 40 thus form a full bridge, the bridge branch of which has the first capacitor 29.The semiconductor switches 17 of the second half-bridge 28, the second coupling semiconductor switch 40, and the third coupling semiconductor switch 41 form a full bridge, the bridge branch of which includes the second capacitor 30. Therefore, the following is shown. Figure 8 The submodule 15 shown here is referred to as a double full-bridge module. The submodule 15 further comprises a first submodule connection 33 connected to the bridge branch of the first half-bridge 27 and a second submodule connection 34 connected to the bridge branch of the second half-bridge 28.

[0043] Each of the in the Figures 4 to 8In the submodule 15 shown, a bypass switch 35 is connected between the two submodule terminals 33, 34. The bypass switch 35 bypasses the submodule 15 for its protection by closing the bypass switch 35 if a submodule voltage applied to the submodule 15 between the submodule terminals 33, 34 exceeds a submodule limit voltage of the respective submodule 15.

[0044] The semiconductor switches 17 and the coupling semiconductor switches 39, 40, 41 are present in all of the Figures 4 to 8 The submodules 15 shown are each configured as an insulated-gate bipolar transistor (IGBT). A freewheeling diode 31 is connected antiparallel to each semiconductor switch 17 and coupling semiconductor switch 39, 40, 41.

[0045] Each half-bridge 27, 28 is further connected to the capacitor 29, 30 connected in parallel to it in such a way that the emitter of a semiconductor switch 17 of the half-bridge 27, 28 is connected to the first electrode 29.1, 30.1 of the capacitor 29, 30 and the collector of the other IGBT semiconductor switch 17 of the half-bridge 27, 28 is connected to the second electrode 29.2, 30.2 of the capacitor 29, 30.

[0046] At the in Figure 6 In the submodule 15 shown, the emitter of the coupling semiconductor switch 39 is connected to the first electrode 29.1 of the first capacitor 29 and the collector is connected to the second electrode 30.2 of the second capacitor 30.

[0047] At the in Figure 7In the submodule 15 shown, the emitter of the first coupling semiconductor switch 39 is connected to the first electrode 29.1 of the first capacitor 29, the collector of the first coupling semiconductor switch 39 is connected to the second electrode 30.2 of the second capacitor 30, the emitter of the second coupling semiconductor switch 40 is connected to the first electrode 30.1 of the second capacitor 30, and the collector of the second coupling semiconductor switch 40 is connected to the second electrode 29.2 of the first capacitor 29.

[0048] At the in Figure 8In the submodule 15 shown, the emitter of the first coupling semiconductor switch 39 is connected to the second electrode 30.2 of the second capacitor 30, the collector of the first coupling semiconductor switch 39 is connected to the second electrode 29.2 of the first capacitor 29, the emitter of the second coupling semiconductor switch 40 is connected to the first electrode 29.1 of the first capacitor 29, the collector of the second coupling semiconductor switch 40 is connected to the second electrode 30.2 of the second capacitor 30, the emitter of the third coupling semiconductor switch 41 is connected to the first electrode 30.1 of the second capacitor 30, and the collector of the third coupling semiconductor switch 41 is connected to the first electrode 29.1 of the first capacitor 29.

[0049] In the Figure 1 and 3In the illustrated embodiments of a modular multilevel converter 1, for example, one of the two module groups 13, 14 of each converter branch 11, 12 is a series connection of in Figure 4 The submodules 15 shown are formed. The other module group 13, 14 of each converter branch 11, 12 is either as a series connection of in Figure 5 shown submodules 15 or as a series connection of in Figure 6 shown submodules 15 or as a series connection of in Figure 7 shown submodules 15 or as a series connection of in Figure 8 The submodules shown are 15 in formation.

[0050] Each protection circuit 21 limits a module group voltage that is applied to the series connection of submodules 15 protected by the protection circuit 21 to a module group limit voltage that is less than the sum of the submodule limit voltages of all submodules 15 of this series connection.

[0051] The embodiments described with reference to the figures can be modified in various ways to form further embodiments. For example, at least one protection circuit 21 can have a series and / or parallel connection of varistors 23 and / or of at least one varistor 23 and a spark gap 25. Reference symbol list

[0052] 1 Modular multilevel converter 3 AC connection 5, 6 DC connection 7 Transformer unit 9 AC network 11, 12 Converter branch 13, 14 Module group 15 Submodule 17 Semiconductor switch 19 Inductor 21 Protection circuit 23 Varistor 25 Spark gap 27, 28 Half-bridge 29, 30 Capacitor 29.1, 29.2, 30.1, 30.2 Electrode 31 Freewheeling diode 33, 34 Submodule connection 35 Bypass switch 37, 38 Rectifier circuit 39, 40, 41 Coupling semiconductor switch 43 Rectifier diode 45 Resistor

Claims

1. A multiphase modular multilevel power converter (1) comprising - two DC voltage terminals (5, 6), and - for each AC voltage phase of a multiphase AC voltage, an AC voltage terminal (3), a first power converter branch (11) connecting the AC voltage terminal (3) to a first DC voltage terminal (5), and a second power converter branch (12) connecting the AC voltage terminal (3) to the second DC voltage terminal (6), wherein - each power converter branch (11, 12) has a first module group (13) of a plurality of submodules (15) connected in series of a first sub-module type, each with a plurality of semiconductor switches (17), and a second module group (14) connected in series to the first module group (13) of a plurality of sub-modules (15) connected in series of a second sub-module type different from the first sub-module type, each with a plurality of semiconductor switches (17), characterized in that - the two module groups (13, 14) of each power converter branch (11, 12) are protected against surge voltages independently of one another by surge voltage-limiting protective circuits (21), wherein the first module groups (13) of both power converter branches (11, 12) of each AC voltage phase are connected to the AC voltage terminal (3) of the AC voltage phase and are protected against surge voltages by a protective circuit (21) connected in parallel to the two first module groups (13), and the second module group (14) of each power converter branch (11, 12) is protected against surge voltages by a protective circuit (21) connected in parallel to the second module group (14).

2. The power converter (1) according to claim 1, wherein at least one protective circuit (21) has a varistor (23) or a plurality of interconnected varistors (23).

3. The power converter (1) according to claim 2, wherein at least one varistor (23) is a metal oxide varistor.

4. The power converter (1) according to any one of the preceding claims, wherein at least one protective circuit (21) has a spark gap (25).

5. The power converter (1) according to any one of the preceding claims, wherein the sub-modules (15) of one of the two module groups (13, 14) of each power converter branch (11, 12) are half-bridge modules, each having a half-bridge (27), in the bridge arms of which a respective semiconductor switch (17) is arranged, and a capacitor (29) connected in parallel to the half-bridge (27).

6. The power converter (1) according to claim 5, wherein the sub-modules (15) of the other of the two module groups (13, 14) of each power converter branch (11, 12) are full-bridge modules, each having two half-bridges (27, 28) connected in parallel to one another, in the bridge arms of which a respective semiconductor switch (17) is arranged, and a capacitor (29) connected in parallel to the two half-bridges (27, 28).

7. The power converter (1) according to claim 5, wherein the sub-modules (15) of the other of the two module groups (13, 14) of each power converter branch (11, 12) are dual half-bridge modules, each having two half-bridges (27, 28), in the bridge arms of which a respective semiconductor switch (17) is arranged, a capacitor (29, 30) for each half-bridge (27, 28) connected in parallel thereto, a first rectifier circuit (37) connecting a first electrode (29.1) of a first capacitor (29) to a first electrode (30.1) of the second capacitor (30), a second rectifier circuit (38) connecting the second electrode (29.2) of the first capacitor (29) to the second electrode (30.2) of the second capacitor (30), and a coupling semiconductor switch (39) connected between the first electrode (29.1) of the first capacitor (29) and the second electrode (30.2) of the second capacitor (30).

8. The power converter (1) according to claim 5, wherein the sub-modules (15) of the other of the two module groups (13, 14) of each power converter branch (11, 12) are dual half-bridge modules, each having two half-bridges (27, 28), in the bridge arms of which a respective semiconductor switch (17) is arranged, a capacitor (29, 30) for each half-bridge (27, 28) connected in parallel thereto, and two coupling semiconductor switches (39, 40) connecting the two capacitors (29, 30) crosswise to one another, wherein the first coupling semiconductor switch (39) is connected between a first electrode (29.1) of the first capacitor (29) and a second electrode (30.2) of the second capacitor (30), and the second coupling semiconductor switch (40) is connected between a second electrode (29.2) of the first capacitor (29) and a first electrode (30.1) of the second capacitor (30).

9. The power converter (1) according to claim 5, wherein the sub-modules (15) of the other of the two module groups (13, 14) of each power converter branch (11, 12) are dual full-bridge modules, each having two half-bridges (27, 28), in the bridge arms of which a respective semiconductor switch (17) is arranged, a capacitor (29, 30) for each half bridge (27, 28) connected in parallel thereto, and three coupling semiconductor switches (39, 40, 41) connected in series, wherein a first coupling semiconductor switch (39) and a second coupling semiconductor switch (40) form a series circuit connected in parallel to a first capacitor (29), and the second coupling semiconductor switch (40) and the third coupling semiconductor switch (41) form a series circuit connected in parallel to the second capacitor (30).

10. The power converter (1) according to any one of the preceding claims, wherein each sub-module (15) has a bypass switch (35) bypassing the sub-module (15) when a sub-module voltage applied to the sub-module (15) exceeds a sub-module limit voltage, and each protective circuit (21) limits a module group voltage applied to the series circuit of sub-modules (15) protected by the protective circuit (21) to a module group limit voltage which is smaller than the sum of the sub-module limit voltages of all sub-modules (15) of this series circuit.

11. A use of a modular multilevel power converter (1) designed according to any one of the preceding claims in a power converter station for high-voltage direct current transmission.