MULTILEVEL POWER CONVERTER

Abstract

A multilevel power converter has at least one phase module with a plurality of modules (1_1 . . . 1_n; 2_1 . . . 2_n) connected between first and second DC voltage connections. The phase module has a first phase module branch connected to the first DC voltage connection, and a second phase module branch connected to the second DC voltage connection. Each of the modules has at least two electronic switching elements and an electric energy storage unit. A third phase module branch connects the first phase module branch to the second phase module branch. A switching device connects an AC voltage connection of the multilevel power converter to a first connection node between the first phase module branch and the third phase module branch in a first switch position and connects the AC voltage connection to a second connection node between the third phase module branch and the second phase module branch in a second switch position.

Claims

1-20. (canceled)

21. A multilevel power converter, comprising: a first DC voltage terminal, a second DC voltage terminal, an AC voltage terminal, and at least one phase module connected between said first and second DC voltage terminals; said at least one phase module having a plurality of modules, said plurality of modules respectively having at least two electronic switching elements and an electrical energy storage device; said at least one phase module including a first phase module branch connected to said first DC voltage terminal, a second phase module branch connected to said second DC voltage terminal, and a third phase module branch connecting said first phase module branch to said second phase module branch; a switching device configured, in a first switch position, to connect said AC voltage terminal to a first connection point between said first phase module branch and said third phase module branch and, in a second switch position, to connect said AC voltage terminal to a second connection point between said third phase module branch and said second phase module branch.

22. The multilevel power converter according to claim 21, wherein said switching device, in the second switch position, isolates said AC voltage terminal of the multilevel power converter from said first connection point and, in the first switch position, isolates said AC voltage terminal from said second connection point.

23. The multilevel power converter according to claim 21, wherein each of said first phase module branch, said second phase module branch, and said third phase module branch respectively comprises at least two of said modules in a series-connected circuit.

24. The multilevel power converter according to claim 23, wherein each of said first, second, and third phase module branches comprises at least five of said modules connected in series.

25. The multilevel power converter according to claim 21, wherein said switching device comprises thyristors.

26. The multilevel power converter according to claim 21, wherein: said switching device comprises a first power electronics switch which, in the first switch position of said switching device, connects said AC voltage terminal to said first connection point and, in the second switch position of said switching device, isolates said AC voltage terminal from said first connection point; and said switching device comprises a second power electronics switch which, in the second switch position of said switching device, connects said AC voltage terminal to said second connection point and, in the first switch position of said switching device, isolates said AC voltage terminal from said second connection point.

27. The multilevel power converter according to claim 26, wherein said first power electronics switch and said second power electronics switch respectively comprise thyristors.

28. The multilevel power converter according to claim 26, wherein said first power electronics switch and said second power electronics switch respectively comprise antiparallel-connected thyristors.

29. The multilevel power converter according to claim 21, wherein at least one module of said third phase module branch is configured to deliver a voltage output with a positive or a negative polarity.

30. The multilevel power converter according to claim 21, wherein at least one module of said third phase module branch comprises a full-bridge circuit.

31. The multilevel power converter according to claim 30, wherein said full-bridge circuit comprises four electronic switching elements and an electrical energy storage device.

32. The multilevel power converter according to claim 21, wherein said third phase module branch comprises a plurality of electrically series-connected sub-branches, and said switching device has a third switch position, wherein said AC voltage terminal is connected to a third connection point of two said sub-branches.

33. The multilevel power converter according to claim 21, wherein said switching device is configured to assume the switch positions in accordance with an instantaneous value of a voltage that is present at said AC voltage terminal.

34. The multilevel power converter according to claim 33, wherein: said switching device assumes the first switch position when the instantaneous value of the voltage at said AC voltage terminal is greater than zero, and said switching device assumes the second switch position when the instantaneous value of the voltage at said AC voltage terminal is lower than zero; or said switching device assumes the first switch position when the instantaneous value of the voltage at said AC voltage terminal lies within a first preselected voltage range, and said switching device assumes the second switch position when the instantaneous value of the voltage at said AC voltage terminal lies within a second preselected voltage range.

35. A high-voltage direct current transmission installation, comprising a multilevel power converter according to claim 21.

36. A method of operating a multilevel power converter, the multilevel power converter having at least one phase module with a plurality of modules connected between a first DC voltage terminal and a second DC voltage terminal, wherein the phase module has a first phase module branch connected to the first DC voltage terminal, a second phase module branch connected to the second DC voltage terminal, and a third phase module branch connecting the first phase module branch to the second phase module branch, the method comprising: measuring an instantaneous value of a voltage that is present on an AC voltage terminal of the multilevel power converter; and depending on the measured instantaneous value, connecting the AC voltage terminal to a first connection point between the first phase module branch and the third phase module branch, or connecting the AC voltage terminal to a second connection point between the third phase module branch and the second phase module branch.

37. The method according to claim 36, which comprises connecting the AC voltage terminal to the first connection point or to the second connection point by way of a switching device which comprises at least one power electronics switch.

38. The method according to claim 37, which comprises: placing the switching device into the first switch position to connect the AC voltage terminal to the first connection point and to isolate the AC voltage terminal from the second connection point; or placing the switching device into the second switch position to connect the AC voltage terminal to the second connection point and to isolate the AC voltage terminal from the first connection point.

39. The method according to claim 36, which comprises: connecting the AC voltage terminal to the first connection point when the instantaneous value of the voltage at the AC voltage terminal is greater than zero, and connecting the AC voltage terminal to the second connection point when the instantaneous value of the voltage at the AC voltage terminal is lower than zero; or connecting the AC voltage terminal to the first connection point when the instantaneous value of the voltage at the AC voltage terminal lies within a first preselected voltage range, and connecting the AC voltage terminal to the second connection point when the instantaneous value of the voltage at the AC voltage terminal lies within a second preselected voltage range.

40. The method according to claim 36, wherein the third phase module branch comprises a plurality of electrically series-connected sub-branches, between which at least one further connection point is provided, and the method further comprises connecting the AC voltage terminal to the first connection point, to the second connection point, or to the further connection point in dependence on the measured instantaneous value.

41. The method according to claim 36, which comprises, in the event of an occurrence of a fault (e.g. short circuit), isolating the AC voltage terminal from the phase module by way of the switching device.

Description

[0042] The invention is described in greater detail hereinafter, with reference to exemplary embodiments. Identical reference numbers refer to identical or identically-functioning elements.

[0043] To this end:

[0044] FIG. 1 represents an exemplary embodiment of a multilevel power converter which is known from the prior art,

[0045] FIG. 2 represents a phase module of the known multilevel power converter,

[0046] FIG. 3 represents an exemplary voltage characteristic of the phase module according to FIG. 2,

[0047] FIG. 4 represents an exemplary embodiment of a phase module of a multilevel power converter according to the invention,

[0048] FIG. 5 represents an exemplary voltage characteristic of the phase module according to FIG. 4,

[0049] FIG. 6 shows an alternative representation of the phase module according to FIG. 4,

[0050] FIG. 7 represents a further exemplary embodiment of a phase module of a multilevel power converter,

[0051] FIG. 8 represents an exemplary embodiment of a multilevel power converter having three phase modules,

[0052] FIG. 9 represents an exemplary embodiment of a module of the multilevel power converter,

[0053] FIG. 10 represents a further exemplary embodiment of a module of the multilevel power converter, and

[0054] FIG. 11 represents an exemplary embodiment of a high-voltage direct current transmission installation.

[0055] FIG. 1 shows a multilevel power converter 1 (modular multilevel converter, or MMC) which is known from the prior art. This multilevel power converter 1 comprises a first AC voltage terminal 5, a second AC voltage terminal 7, and a third AC voltage terminal 9. The first AC voltage terminal 5 is electrically connected to a first phase module branch 11 and to a second phase module branch 13. The first phase module branch 11 and the second phase module branch 13 constitute a first phase module 15 of the power converter 1. The end of the first phase module branch 11 which is averted from the first AC voltage terminal 5 is electrically connected to the first DC voltage terminal 16; the end of the second phase module branch 13 which is averted from the first AC voltage terminal 5 is electrically connected to a second DC voltage terminal 17. The first DC voltage terminal 16 is a positive DC voltage terminal; the second DC voltage terminal 17 is a negative DC voltage terminal.

[0056] The second AC voltage terminal 7 is electrically connected to one end of a third phase module branch 18 and to one end of a fourth phase module branch 21. The third phase module branch 18 and the fourth phase module branch 21 constitute a second phase module 24. The third AC voltage terminal 9 is electrically connected to one end of a fifth phase module branch 27 and to one end of a sixth phase module branch 29. The fifth phase module branch 27 and the sixth phase module branch 29 constitute a third phase module 31.

[0057] The end of the third phase module branch 18 which is averted from the second AC voltage terminal 7 and the end of the fifth phase module branch 27 which is averted from the third AC voltage terminal 9 are electrically connected to the first DC voltage terminal 16. The end of the fourth phase module branch 21 which is averted from the second AC voltage terminal 7 and the end of the sixth phase module branch 29 which is averted from the third AC voltage terminal 9 are electrically connected to the second DC voltage terminal 17. The first phase module branch 11, the third phase module branch 18 and the fifth phase module branch 27 constitute a positive-side power converter section 32; the second phase module branch 13, the fourth phase module branch 21 and the sixth phase module branch 29 constitute a negative-side power converter section 33.

[0058] The first AC voltage terminal 5, the second AC voltage terminal 7 and the third AC voltage terminal 9 can be electrically connected to an AC electricity transmission system (not represented).

[0059] Each phase module branch comprises a plurality of modules (1_1 . . . 1_n; 2_1 . . . 2_n; etc.) which (by means of their galvanic power terminals) are electrically connected in series. Modules of this type are also described as submodules. In the exemplary embodiment according to FIG. 1, each phase module branch comprises n modules. The number of modules which are electrically connected in series by means of their galvanic power terminals can be highly variablealthough at least two modules are connected in series, it is also possible for 5, 50, 100 or more modules to be electrically connected in series. In the exemplary embodiment, n=36: the first phase module branch 11 thus comprises 36 modules 1_1, 1_2, 1_3, . . . 1_36. The other phase module branches 13, 18, 21, 27 and 29 are of identical design.

[0060] From an unrepresented control device of the power converter 1, optical messages or optical signals are transmitted via an optical communication link (for example via an optical wave guide) to the individual modules 1_1 to 6_n. For example, the control device transmits a target value to each of the individual modules for the magnitude of the output voltage which is to be delivered by the respective module.

[0061] FIG. 2 represents the first phase module 15 of the multilevel power converter 1. On this first phase module 15, various voltages are constituted. A first voltage U.sub.p is applied across the first phase module branch 11, and a second voltage U.sub.n is applied across the second phase module branch 13. Between the first AC voltage terminal 5 and a ground terminal 202, an alternating voltage U.sub.ac is applied. Between the first DC voltage terminal 16 and the ground terminal 202 the voltage U.sub.dc is applied, wherein U.sub.dc is the total DC voltage which is present between the first DC voltage terminal 16 and the second DC voltage terminal 17. Between the ground terminal 202 and the second DC voltage terminal 17, the voltage U.sub.dc is also applied.

[0062] In FIG. 3, these voltages are shown in an exemplary graphic representation, plotted against time t. One cycle of the alternating voltage U.sub.ac is represented, which occurs on the first AC voltage terminal 5. At time point t=T/4 (which corresponds to one quarter of the cycle period of the AC voltage), the maximum positive value (the positive maximum instantaneous value) of the AC voltage U.sub.ac occurs, whereas at time point t= T the maximum negative value of the AC voltage occurs. At time point t=T/4, virtually the entire DC voltage U.sub.dc is present in the form of the voltage U.sub.n on the second phase module branch 13. At time point t= T, virtually the entire DC voltage U.sub.dc is present in the form of the voltage U.sub.p on the first phase module branch 11. This means that both the first phase module branch 11 and the second phase module branch 13 must be configured such that they are respectively capable of being energized with the full DC voltage U.sub.dc. In other words, a correspondingly large number of modules 1_1 . . . 1_n, 2_1 . . . 2_n is required in the phase module branches 11, 13, as each module is only rated for a predefined maximum voltage loading.

[0063] FIG. 4 represents an exemplary embodiment of a phase module 400 according to the invention. This phase module 400, in addition to the first (positive-side) phase module branch 11 and the second (negative-side) phase module branch 13, comprises a third (central) phase module branch 404. On the third phase module branch 404, a third voltage U.sub.s occurs. The phase module 400 is arranged between the first DC voltage terminal 16 and the second DC voltage terminal 17, wherein the first phase module branch 11 is connected to the first DC voltage terminal 16. The second phase module branch 13 is connected to the second DC voltage terminal 17. Each phase module branch comprises a plurality of modules. In the exemplary embodiment, the first phase module branch 11 comprises n modules 1_1 . . . 1_n, each of which can be configured as a half-bridge circuit (half-bridge module) or as a full-bridge circuit (full-bridge module). For example, module 1_1 is constituted as a full-bridge module, and module 1_n as a half-bridge module. The second phase module branch 13 likewise comprises n modules 2_1 . . . 2_n, each of which can be configured as a half-bridge circuit (half-bridge module) or as a full-bridge circuit (full-bridge module). Accordingly, the first phase module branch 11 and the second phase module branch 13, for example, can respectively comprise half-bridge modules only (or, respectively, full-bridge modules only).

[0064] The third phase module branch 404 comprises at least one module 408_1 having a full-bridge circuit (a full-bridge module 408_1), together with further modules 408_2 to 408_n, each of which can be configured as a half-bridge circuit (half-bridge module) or as a full-bridge circuit (full-bridge module).

[0065] The third phase module branch 404 connects the first phase module branch 11 to the second phase module branch 13 by the constitution of a first connection point 412 between the first phase module branch 11 and the third phase module branch 404, and by the constitution of a second connection point 416 between the third phase module branch 404 and the second phase module branch 13. To this end, a switching device 420 is designed, in a first switch position 421, to electrically connect the first AC voltage terminal 5 to the first connection point 412 and, in a second switch position 422, to electrically connect the first AC voltage terminal 5 to the second connection point 416. The switching device 420 is further designed, in the second switch position 422, to electrically isolate the AC voltage terminal 5 from the first connection point 412 and, in the first switch position 421, to electrically isolate the first AC voltage terminal 5 from the second connection point 416.

[0066] The switching device 420 comprises, by way of switching elements, a first thyristor T1, a second thyristor T2, a third thyristor T3 and a fourth thyristor T4. More specifically, the switching device comprises a first power electronics switch 424 and a second power electronics switch 428. The first power electronics switch 424 comprises the first thyristor T1 and the second thyristor T2; the second power electronics switch 428 comprises the third thyristor T3 and the fourth thyristor T4. The first power electronics switch 424 thus comprises antiparallel-connected thyristors T1 and T2; the second power electronics switch 428 comprises antiparallel-connected thyristors T3 and T4.

[0067] The representation of individual thyristors T1, T2, T3 and T4 is to be understood as symbolic only. In practice, for example, a larger number of thyristors can be connected in series and/or in parallel, in order to permit the achievement of the requisite voltage and current values.

[0068] The first power electronics switch 424, in the first switch position of the switching device 420, electrically connects the first AC voltage terminal 5 to the first connection point 412; the first power electronics switch 424, in the second switch position of the switching device 420, electrically isolates the first AC voltage terminal 5 from the first connection point 412. The second power electronics switch 428, in the second switch position of the switching device 420, electrically connects the first AC voltage terminal 5 to the second connection point 416; the second power electronics switch 428, in the first switch position of the switching device 420, electrically isolates the first AC voltage terminal 5 from the second connection point 416.

[0069] The switching device 420 assumes its switch positions (i.e. the first switch position and the second switch position) in accordance with the instantaneous value of the AC voltage which is present on the AC voltage terminal 5. More specifically, the switching device 420 is actuated by an (unrepresented) actuating device, such that it assumes its switch positions (i.e. the first switch position and the second switch position) in accordance with the instantaneous value of the AC voltage which is present on the AC voltage terminal 5. The switching device 420 thus assumes the first switch position, if the instantaneous value of the AC voltage is greater than (or equal to) zero. The first AC voltage terminal 5 is then electrically connected to the first connection point 412. The switching device 420 assumes the second switch position, if the instantaneous value of the AC voltage which is present on the AC voltage terminal 5 is lower than zero. The first AC voltage terminal 5 is then electrically connected to the second connection point 416.

[0070] The multilevel power converter can also be designed such that the switching device assumes the first switch position, if the instantaneous value of the voltage which is present on the AC voltage terminal lies within a first preselected voltage range, and the switching device assumes the second switch position, if the instantaneous value of the voltage which is present on the AC voltage terminal lies within a second preselected voltage range. The first preselected voltage range can be, for example, the voltage range between 0 and + U.sub.dc; the second preselected voltage range can be, for example, the voltage range between U.sub.dc and 0.

[0071] s FIG. 5 represents an exemplary embodiment of the voltages occurring in the phase module 400. During the time interval between t=0 and t= T (i.e. during the time period in which the instantaneous values of the AC voltage U.sub.ac are greater than zero) only the relatively low voltage U.sub.p is present on the first phase module branch 11. The larger voltage (U.sub.dcU.sub.p) is distributed on the second phase module branch 13 (in the form of the voltage U.sub.n) and on the third phase module branch 404 (in the form of the voltage U.sub.s). As a result, a maximum voltage U.sub.dc is present on the first phase module branch 11, on the second phase module branch 13 and on the third phase module branch 404 respectively. Consequently, it is only necessary for each phase module branch 11, 13 and 404 to be rated for half the maximum DC voltage U.sub.dc in each case. In comparison with the previously known phase module 15 according to FIG. 2, fewer modules are therefore required. A 25% economy in modules can be achieved, as the modules on the third phase module branch 404 are optionally assigned, so to speak, either to the first phase module branch 11 or the third phase module branch 404 (provided that equal numbers of modules are present in each phase module branch). During the second cycle period (i.e. in the region between t= T and t=T), only the relatively low voltage U.sub.n is present on the second phase module branch 13, whereas the larger voltage (U.sub.dcU.sub.n) is distributed on the first phase module branch 11 and the second phase module branch 404. Analogously to the processes in the first half of the cycle period of the AC voltage, a maximum voltage of U.sub.dc again occurs on each phase module branch.

[0072] The phase module 400 according to FIG. 4 is again shown in FIG. 6, in a simplified representation. To this end, the first phase module branch 11, the second phase module branch 13 and the third phase module branch 404 are each symbolically represented as a square, marked with a diagonal line. The first power electronics switch 424 and the second power electronics switch 428 are each represented as a smaller square, marked with two diagonal lines. In FIG. 6, by means of voltage arrows, it is further represented how the AC voltage U.sub.ac present on the first AC voltage terminal 5 vis--vis the ground terminal 202 proceeds during the first half-cycle of the AC voltage (U.sub.ac>0) between the first connection point 412 and the ground terminal 202. During the second half-cycle (U.sub.ac<0), the AC voltage U.sub.ac is present between the second connection point 416 and the ground terminal 202 (this representation assumes that the first power electronics switch 424 and the second power electronics switch 428 behave as ideal switches, i.e. there is no voltage drop across these switches 424 and 428, in the switched-on state).

[0073] FIG. 7 represents a further exemplary embodiment of a phase module 700. In this phase module 700, by way of distinction from the phase module according to FIG. 6, the third phase module branch is divided into three sub-branches: a first sub-branch 704, a second sub-branch 708 and a third sub-branch 712. The first phase module branch 11 is electrically connected to the first sub-branch 704, thereby constituting the first connection point 412. The first sub-branch 704 is electrically connected to the second sub-branch 708, thereby constituting a third connection point 716. The second sub-branch 708 is electrically connected to the third sub-branch 712, thereby constituting a fourth connection point 720, and the third sub-branch 712 is electrically connected to the second phase module branch 13, thereby constituting the second connection point 416.

[0074] A third power electronics switch 724 electrically connects the third connection point 716 to (or isolates the latter from) the AC voltage terminal 5. A fourth power electronics switch 728 electrically connects the fourth connection point 720 to (or isolates the latter from) the first AC voltage terminal 5. The switching device 732, in addition to the first switch position and the second switch position known from FIG. 6, further comprises a third switch position and a fourth switch position. In the third switch position, the first AC voltage terminal 5, by means of the third power electronics switch 724, is electrically connected to the third connection point 716 (and, by means of the remaining power electronics switches 424, 728 and 428, is electrically isolated from the connection points 412, 720 and 416). In the fourth switch position of the switching device 732, the first AC voltage terminal 5 is electrically connected to the fourth connection point 720 by means of the fourth power electronics switch 728 (and is electrically isolated from the connection points 412, 716 and 416 by means of the power electronics switches 424, 724 and 428). A voltage range of the AC voltage is thus assigned to each of the power electronics switches 424, 724, 728 and 428. These voltage ranges are non-overlapping voltage ranges. If the instantaneous value of the voltage which is present on the AC voltage terminal 5 lies within one of the voltage ranges, the associated power electronics switch closes, and connects the first AC voltage terminal 5 to the respective connection point. The AC voltage terminal 5 is then electrically isolated from the remaining connection points.

[0075] The first preselected voltage range can be, for example, the voltage range between + U.sub.dc and + U.sub.dc; the second preselected voltage range can be, for example, the voltage range between 0 and + U.sub.dc; the third preselected voltage range can be, for example, the voltage range between U.sub.dc and 0; the fourth preselected voltage range can be, for example, the voltage range between U.sub.dc and U.sub.dc.

[0076] The phase module 700 permits an even more flexible assignment of the modules of the individual sub-branches to the first phase module branch 11 or to the second phase module branch 13. Accordingly, these modules, by way of further differentiation, can be connected to the first phase module branch 11 or to the second phase module branch 13.

[0077] FIG. 8 represents an exemplary embodiment of a multilevel power converter 800. This three-phase multilevel power converter 800 comprises three phase modules, in accordance with FIG. 6.

[0078] FIG. 9 represents an exemplary embodiment of a module 900 of the multilevel power converter. This can be, for example, one of the modules of the phase modules represented in FIG. 4 or 7.

[0079] The module 900 is configured as a half-bridge module 900. The module 900 comprises a first (interruptible) electronic switching element 902 (a first interruptible semiconductor valve 902) having a first antiparallel-connected diode 904. The module 900 further comprises a second (interruptible) electronic switching element 906 (a second interruptible semiconductor valve 906) having a second antiparallel-connected diode 908 and an electrical energy store 910 in the form of a capacitor 910. The first electronic switching element 902 and the second electronic switching element 906 are each configured as an IGBT (insulated-gate bipolar transistor). The first electronic switching element 902 is electrically connected in series with the second electronic switching element 906. At the connection point between the two electronic switching elements 902 and 906, a first galvanic module terminal 912 is arranged. On the terminal of the second electronic switching element 906 which is situated opposite the connection point, a second galvanic module terminal 915 is arranged. The second module terminal 915 is additionally electrically connected to a first terminal of the energy store 910; a second terminal of the energy store 910 is electrically connected to the terminal of the first electronic switching element 902 which is situated opposite the connection point.

[0080] The energy store 910 is thus electrically connected in parallel with the series-connected arrangement comprised of the first electronic switching element 902 and the second electronic switching element 906. By the corresponding actuation of the first electronic switching element 902 and the second electronic switching element 906 by means of an (unrepresented) electronic control apparatus of the power converter, it can be achieved that, between the first module terminal 912 and the second module terminal 915, either the voltage output from the energy store 910 is delivered, or no output voltage is delivered (i.e. a voltage output of zero). By the interaction of the modules on the individual phase module branches, the desired output voltage of the power converter can thus be generated in each case.

[0081] FIG. 10 represents a further exemplary embodiment of a module 1000 of the multilevel power converter. This module 301 can be, for example, module 408_1 of the phase module 400 represented in FIG. 4 (or one of the other modules of the multilevel power converter). In addition to the first electronic switching element 902, the second electronic switching element 906, the first freewheeling diode 904, the second freewheeling diode 908 and the energy store 910 which are already known from FIG. 9, the module 1000 represented in FIG. 10 comprises a third electronic switching element 1002 having an antiparallel-connected third freewheeling diode 1004, and a fourth electronic switching element 1006 having an antiparallel-connected fourth freewheeling diode 1008. The third electronic switching element 1002 and the fourth electronic switching element 1006 are respectively configured as an IGBT. By way of distinction from the circuit represented in FIG. 9, the second module terminal 1015 is electrically connected, not to the second electronic switching element 906, but to a mid-point of an electrical series-connected arrangement comprised of the third electronic switching element 1002 and the fourth electronic switching element 1006.

[0082] The module 1000 according to FIG. 10 is a full-bridge module 1000. This full-bridge module 1000 is characterized in that, by the corresponding actuation of the four electronic switching elements, between the first (galvanic) module terminal 912 and the second (galvanic) module terminal 1015, optionally, either the positive voltage of the energy store 910, the negative voltage of the energy store 910, or a voltage with a value of zero (zero voltage) can be delivered as an output. Accordingly, by means of the full-bridge module 1000, the polarity of the output voltage can be reversed. The multilevel power converter can comprise only half-bridge modules 900, only full-bridge modules 1000, or both half-bridge modules 900 and full-bridge modules 1000.

[0083] FIG. 11 shows a schematic representation of a high-voltage direct current transmission installation 1100. This high-voltage direct current transmission installation 1100 comprises two power converters 800, of the type represented in FIG. 8. These two power converters 800 are electrically interconnected on the DC voltage-side by means of a high-voltage direct current link 1105. Both positive DC voltage terminals 16 of the power converters 800 are thus electrically interconnected by means of a first high-voltage direct current line 1105a; both negative DC voltage terminals 17 of the two power converters 800 are electrically interconnected by means of a second high-voltage direct current line 1105b. By means of a high-voltage direct current transmission installation 1100 of this type, electrical energy can be transmitted over long distances; the high-voltage direct current link 1105 is then of a corresponding length. In FIG. 11, by way of an example, a high-voltage direct current transmission installation 1100 in a symmetrical monopole configuration is represented. Naturally, however, the power converter can also be employed in other high-voltage direct current transmission installations, for example in bipole installations.

[0084] For the operation of a multilevel power converter according to FIG. 8, the instantaneous value of the voltage which is present on the AC voltage terminal 5 of the multilevel power converter is thus measured. Depending upon the measured instantaneous value, the AC voltage terminal 5 is then electrically connected to the first connection point 412 or the second connection point 416 (or optionally to one of the further connection points, such as e.g. to the third connection point 716 or the fourth connection point 720). In each case, the AC voltage terminal 5 is thus electrically connected to only one of the connection points; the AC voltage terminal 5 is electrically isolated from the other connection points by means of the other power electronics conductors.

[0085] In the event of a multilevel power converter having phase modules according to FIG. 6, the AC voltage terminal 5 is then electrically connected to the first connection point 412 if the instantaneous value of the voltage which is present on the AC voltage terminal 5 is greater than (or equal to) zero. The AC voltage terminal 5 is then electrically connected to the second connection point 416, if the instantaneous value of the voltage which is present on the AC voltage terminal is lower than zero.

[0086] In the event of a multilevel power converter having phase modules according to FIG. 7 (i.e. phase modules, wherein the third phase module branch comprises a plurality of sub-branches), the AC voltage terminal 5 is electrically connected in each case, by means of the switching device 732, to the connection point within the associated preselected voltage range of which the instantaneous value of the alternating voltage U.sub.ac (which is present on the AC voltage terminal) lies. The AC voltage terminal 5 is electrically isolated from the remaining connection points.

[0087] The switching device 420, 732, in the event of the occurrence of faults, can also be employed for fault detection in a particularly advantageous manner. If, for example, in a multilevel power converter having phase modules 400 according to FIG. 4, a short-circuit occurs on the DC voltage side, the switching device 420 can then assume a specific position (which will be described here as the fault position), in which the switching device electrically isolates the first AC voltage terminal 5 from all the connection points 412, 416 of the phase module. As a result, the energy flux from the AC voltage grid system which is connected to the AC voltage terminal of the power converter is interrupted at the location of the DC voltage-side fault. This electrical interruption/isolation can be executed very rapidly. Specifically, if the switching device is equipped with thyristors, the transition to the fault position can be executed simply, wherein the actuation signals for the thyristors are deactivated, such that the thyristors, at the next zero-crossing of the AC voltage, assume a blocking state. The alternating current can be very rapidly disconnected as a result.

[0088] A multilevel power converter and a method for operating said multilevel power converter have been described wherein, in comparison with a conventional multilevel power converter, a lower number of modules is sufficient. As a result, both costs and structural space can be economized, and power losses occurring in the switching elements of the modules can be reduced.

[0089] An option for the combination of a multilevel power converter with thyristor valves/thyristor switches has been described. This novel power converter topology permits a reduction in the number of series-connected modules in the phase modules. Antiparallel thyristor valves are employed, in order to reduce the maximum voltage occurring on each of the individual phase module branches. For example, by the employment of a third phase module branch, it is possible to reduce the voltage from 200% U.sub.dc to 150% U.sub.do.

[0090] In the novel phase module, in comparison with the known embodiment of phase modules (which comprise only a first positive-side phase module branch and a second negative-side phase module branch), the phase module is subdivided into three phase module branches: a first phase module branch constitutes a positive-side phase module branch, a second phase module branch constitutes a negative-side phase module branch, and a third phase module branch constitutes a central phase module branch. The antiparallel thyristor valves permit the voltage of 100% U.sub.dc delivered by one phase module branch to be limited to 50% U.sub.dc. The requisite power converter capacity is thus reduced from two times 100% to three times 50%. This corresponds to an economization of 25% of the requisite capacity. This can result in an economization of 25% in the requisite modules.

[0091] A further advantage is provided in that, by means of the thyristor valves/thyristor switches, faults can be detected more rapidly, in comparison with fault detection by means of a power switch which is arranged on the AC voltage-side of the multilevel power converter. As a result, the loading of the multilevel power converter in the event of a fault can be reduced.

[0092] Once a protective command has been tripped by way of fault detection, the trigger pulse of the thyristors is deactivated. Thereafter, the thyristors assume a blocking state, such that the AC current flowing rapidly moves towards zero. This occurs within a few milliseconds. Conversely, a conventional AC-side power switch requires switching times of approximately 2.5 to 3 cycle periods, or between approximately 50 ms and 60 ms at an AC frequency of 50 Hz. By means of the switching apparatus described, faults can thus be detected significantly more rapidly than by means of a conventional AC-side power switch.

[0093] Moreover, in comparison with modules having IGBT switching elements, the thyristors feature substantially lower forward power losses (a lower loss per component), higher blocking voltages (thus requiring a lower number of series-connected components for the achievement of the same voltage) and consequently lower costs. The increased expenditure associated with additional thyristor valves is thus more than offset by the economization of modules.

[0094] The above-mentioned multilevel power converter and the above-mentioned method can be employed in installations of a wide variety of types, for example in high-voltage direct current transmission systems using underground cables or overhead lines.