MODULAR MULTI-LEVEL CONVERTER WITH THYRISTOR VALVES

Abstract

An interface arrangement is configured to couple an alternating current, AC, power system with a direct current, DC, power system, or vice versa. The interface arrangement includes a plurality of series-connected converter modules. Each converter module includes at least one multi-level converter cell configured to provide a voltage contribution to at least a portion of an AC waveform for example based on voltage of the DC power system. Each converter module includes at least one converter valve, electrically connected to the multi-level converter cells and including at least two anti-parallel thyristors. The converter valves are switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell. The converter valves can also serve as fault protection, e.g. to divert overcurrents.

Claims

1-15. (canceled)

16. An interface arrangement configured to couple an alternating current, AC, power system comprising a plurality of phases with a direct current, DC, power system, the interface arrangement comprising: a plurality of converter modules for conversion of DC power to AC power, or vice versa, electrically connected in series, each converter module corresponding to one of the phases and being configured to provide at least a portion of an AC waveform, and each converter module comprising: at least one multi-level converter cell, each multi-level converter cell configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system; and at least one converter valve electrically connected to the at least one multi-level converter cell, wherein the interface arrangement is configured such that the at least one converter valve is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, wherein the at least one converter valve includes at least two anti-parallel thyristors; and a control unit configured to control operation of the at least one converter valve at least with respect to switching, said control with respect to the switching comprising selectively controlling polarity of any voltage contribution provided by the at least one multi-level converter cell, wherein the control unit is configured to, in response to receiving an indication indicating presence of a fault current in the interface arrangement, control switching of the at least one converter valve so as to route the fault current through the at least one converter valve and bypass at least a portion of the at least one multi-level converter cell.

17. The interface arrangement according to claim 16, each converter module further comprising at least one commutation cell electrically connected to the at least one converter valve and switchable so as to selectively cause the at least one converter valve to enter the non-conducting state.

18. The interface arrangement according to claim 17, wherein the at least one commutation cell comprises at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, wherein by switching of the commutation cell it may provide a selected voltage across at least one of the thyristors in order to switch the at least one thyristor into a non-conducting state.

19. The interface arrangement according to claim 17, wherein the at least one commutation cell comprises a full-bridge cell.

20. The interface arrangement according to claim 16, each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.

21. The interface arrangement according to claim 16, wherein the plurality of converter modules are electrically connected in series between a first DC pole and a second DC pole, or between a DC pole and ground.

22. The interface arrangement according to claim 21, wherein the control unit is configured to, in response to receiving an indication indicating presence of a fault current in the interface arrangement caused by a fault at one of the first DC pole and the second DC pole, control switching of the at least one converter valve so as to route the fault current from the one of the first DC pole and the second DC pole at which there is a fault through the at least one converter valve to the other one of the first DC pole and the second DC pole, wherein the at least a portion of the at least one multi-level converter cell is bypassed.

23. The interface arrangement according to claim 16, wherein each converter module comprises: a plurality of multi-level converter cells electrically connected in a multi-level converter cell arm; and a plurality of converter valves electrically connected in a converter valve arm.

24. The interface arrangement according to claim 23, further comprising at least one transformer connected between the AC power system and a midpoint of the converter valve arm and a midpoint of the multi-level converter cell arm.

25. The interface arrangement according to claim 23, wherein the multi-level converter cell arm and the converter valve arm are electrically connected in parallel.

26. The interface arrangement according to claim 17, wherein each converter module comprises a plurality of multi-level converter cells electrically connected in a multi-level converter cell arm, wherein at least one of the multi-level converter cells in the multi-level converter cell arm comprises a full-bridge cell, and wherein the at least one commutation cell comprises or is constituted by the at least one multi-level converter cell which comprises a full-bridge cell.

27. The interface arrangement according to claim 26, wherein each of the plurality of multi-level converter cells in the multi-level converter cell arm comprises a full-bridge cell, and wherein the at least one commutation cell comprises or is constituted by any one of the multi-level converter cells in the multi-level converter cell arm.

28. The interface arrangement according to claim 16, wherein the at least one multi-level converter cell comprises a half-bridge cell or a full-bridge cell.

29. A converter station configured to couple an alternating current power system with a direct current power system, the converter station comprising the interface arrangement according to claim 16.

30. A power system including an alternating current, AC, power system and a direct current, DC, power system, the power system comprising the interface arrangement according to claim 16 configured to couple the AC power system with the DC power system.

31. The interface arrangement according to claim 18, wherein the at least one commutation cell comprises a full-bridge cell.

32. The interface arrangement according to claim 17, each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.

33. The interface arrangement according to claim 18, each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.

34. The interface arrangement according to claim 19, each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.

35. The interface arrangement according to claim 17, wherein the plurality of converter modules are electrically connected in series between a first DC pole and a second DC pole, or between a DC pole and ground.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

[0040] FIG. 1 is a schematic circuit diagram of an interface arrangement according to an embodiment of the present invention.

[0041] FIG. 2 is a schematic circuit diagram of a portion of an interface arrangement in accordance with an embodiment of the present invention.

[0042] FIG. 3 is a schematic circuit diagram of a multi-level converter cell in accordance with an embodiment of the present invention.

[0043] FIG. 4 is a schematic circuit diagram of a multi-level converter cell in accordance with an embodiment of the present invention.

[0044] FIG. 5 is a schematic circuit diagram of a converter valve in accordance with an embodiment of the present invention.

[0045] FIG. 6 is a schematic circuit diagram of a commutation cell in accordance with an embodiment of the present invention.

[0046] FIG. 7 is a schematic block diagram of a power system according to an embodiment of the present invention.

[0047] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

[0048] The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

[0049] FIG. 1 is a schematic circuit diagram of an interface arrangement 100 according to an embodiment of the present invention. The interface arrangement 100 is configured to couple an AC power system 102 with a DC power system 103, or vice versa. The interface arrangement 100, the AC power system 102 and the DC power system 103 may be included in or constitute a power system. The interface arrangement 100 comprises three converter modules 104, 105, 106 for conversion of AC power to DC power, or vice versa.

[0050] The converter modules 104, 105, 106 are electrically connected in series. For example, in accordance with the embodiment of the present invention illustrated in FIG. 1, the converter modules 104, 105, 106 may be electrically connected in series between a first DC pole or terminal T1 and a second DC pole or terminal T2. The first DC pole or terminal T1 may be at DC voltage +U.sub.d. The second DC pole or terminal T2 may be at DC voltage −U.sub.d. Alternatively or in addition (not indicated in FIG. 1), the converter modules 104, 105, 106 may be electrically connected in series between DC pole and ground. Thus, the interface arrangement 100 may for example be configured according to a monopole configuration, or a bipole configuration, or an asymmetrical monopole configuration.

[0051] Each of the converter modules 104, 105, 106 may be configured to provide at least a portion of an AC waveform, e.g. an AC voltage waveform. To that end, each of the converter modules 104, 105, 106 may comprise at least one multi-level converter cell (not shown in FIG. 1), wherein each multi-level converter cell is configured to provide a voltage contribution to the AC voltage waveform, e.g. based on (at least) voltage of the DC power system.

[0052] The AC power system 102 may comprise a plurality of phases. According to an example, the AC power system 102 may be a three-phase power system. In accordance with the embodiment of the present invention illustrated in FIG. 1, the AC power system 102 is a three-phase power system, comprising three conductors or phases for coupling the AC power system 102 with the DC power system 103, or vice versa, and each of the converter modules 104, 105, 106 corresponds to one phase, such that there is a one-to-one correspondence between the converter modules 104, 105, 106 and the three phases. However, it is to be understood that the number of phases as well as the number of converter modules as illustrated in FIG. 1 (and also in FIG. 2 described in the following) are according to examples, and that in principle any number of phases and any number of converter modules are possible, e.g. one or two phases, and/or two or four converter modules.

[0053] As illustrated in FIG. 1, the (three) phases, and the (three) converter modules 104, 105, 106 may be electrically connected in series on the DC side so as to share the DC link voltage.

[0054] The interface arrangement 100 may comprise a transformer, which may comprise a primary side for coupling of the transformer to the AC power system 102 and a secondary side for coupling of the transformer to the converter modules 104, 105, 106. In accordance with the embodiment of the present invention illustrated in FIG. 1, the transformer is a three-phase transformer, which can be considered as comprising three (separate) ‘phase transformers’ 107, 108, 109, one for each phase. Each of the phase transformers 107, 108, 109 may comprise a primary side for coupling of phase the transformer 107, 108, 109 to the AC power system 102 and a secondary side for coupling of the phase transformer 107, 108, 109 to the respective converter modules 104, 105, 106. The primary side of the respective phase transformers 107, 108, 109 may comprise a set of primary windings, arranged to be coupled to the AC power system 102. The secondary side of the respective phase transformers 107, 108, 109 may comprise a set of secondary windings, arranged to be coupled to the respective converter modules 104, 105, 106. Each of the phase transformers 107, 108, 109 may be controlled and/or operated independently of the others.

[0055] The interface arrangement 100 may comprise a circuit breaker arranged in a current path between the AC side, or AC bus, of the converter modules 104, 105, 106 and the AC power system 102. Hence, the circuit breaker may be an AC circuit breaker. In accordance with the embodiment of the present invention illustrated in FIG. 1, the circuit breaker can be considered as comprising three (separate) ‘phase circuit breakers’ 110, 111, 112, one for each phase. Further in accordance with the embodiment of the present invention illustrated in FIG. 1, the phase circuit breakers 110, 111, 112 may be arranged in a current path between the respective phase transformers 107, 108, 109 and the AC power system 102. Each of the phase circuit breakers 110, 111, 112 is configured to controllably effect discontinuation of flow of current in the current path upon opening of contacts (not shown in FIG. 1) of the phase circuit breaker 110, 111, 112. The phase circuit breakers 110, 111, 112 may for example be arranged in a current path between the set of primary windings of the respective phase transformers 107, 108, 109 and the AC power system 102. Each of the phase circuit breakers 110, 111, 112 may possibly be controlled and/or operated independently of the others.

[0056] As illustrated in FIG. 1, each of the phases may include a reactor or inductor 113, 114, 115 arranged in a current path between the respective phase transformers 107, 108, 109 and the AC power system 102. For example, according to FIG. 1, the reactors or inductors 113, 114, 115 may be arranged in a current path between the respective phase circuit breakers 110, 111, 112 and the AC power system 102.

[0057] Further as illustrated in FIG. 1, each of the phases may be coupled to the AC power system 102 by way of terminals T3, T4 and T5, respectively.

[0058] It is to be understood that various components which are not illustrated in FIG. 1 may be included in the interface arrangement 100. Such components, which thus are not shown in FIG. 1, may for example include resistors, capacitors, filters, additional transformers and/or other auxiliary elements.

[0059] FIG. 2 is a schematic circuit diagram of a portion of the interface arrangement 100 illustrated in FIG. 1, illustrating an exemplary configuration of the converter modules 104, 105, 106. As mentioned in the foregoing with respect to FIG. 1, each of the converter modules 104, 105, 106 may be configured to provide at least a portion of an AC waveform, e.g. an AC voltage waveform. To that end, each of the converter modules 104, 105, 106 may comprise a plurality of multi-level converter cells, each multi-level converter cell being configured to provide a voltage contribution to the AC voltage waveform, e.g. based on (at least) voltage of the DC power system 103.

[0060] The converter module 104 may include a plurality of multi-level converter cells 141-1, . . . , 141-N and 142-1, . . . , 142-N, electrically connected, e.g. in series as illustrated in FIG. 2, and arranged in a multi-level converter cell arm 161. The multi-level converter cells 141-1, . . . , 141-N of the multi-level converter cell arm 161 constitute an upper multi-level converter cell arm of the converter module 104, and the multi-level converter cells 142-1, . . . , 142-N of the multi-level converter cell arm 161 constitute a lower multi-level converter cell arm of the converter module 104.

[0061] Similarly, the converter module 105 may include a plurality of multi-level converter cells 143-1, . . . , 143-N and 144-1, . . . , 144-N, electrically connected, e.g. in series as illustrated in FIG. 2, and arranged in a multi-level converter cell arm 162. The multi-level converter cells 143-1, . . . , 143-N of the multi-level converter cell arm 162 constitute an upper multi-level converter cell arm of the converter module 105, and the multi-level converter cells 144-1, . . . , 144-N of the multi-level converter cell arm 162 constitute a lower multi-level converter cell arm of the converter module 105.

[0062] Similarly, the converter module 106 may include a plurality of multi-level converter cells 145-1, . . . , 145-N and 146-1, . . . , 146-N, electrically connected, e.g. in series as illustrated in FIG. 2, and arranged in a multi-level converter cell arm 163. The multi-level converter cells 145-1, . . . , 145-N of the multi-level converter cell arm 163 constitute an upper multi-level converter cell arm of the converter module 106, and the multi-level converter cells 146-1, . . . , 146-N of the multi-level converter cell arm 163 constitute a lower multi-level converter cell arm of the converter module 106.

[0063] In the embodiment of the present invention illustrated in FIG. 2, the upper multi-level converter cell arm and the lower multi-level converter cell arm of the converter modules 104, 105, 106 each includes N multi-level converter cells, where N is an integer, such as ten, fifteen, or twenty. However, it is to be understood that each of the converter modules 104, 105, 106 may include in principle any number of multi-level converter cells. According to an example, each of the converter modules 104, 105, 106 may include a single multi-level converter cell.

[0064] Referring now to FIG. 3, there is shown an example configuration of the multi-level converter cell 141-1 shown in FIG. 2. It is to be understood that any one of the other multi-level converter cells 141-N, 142-1, . . . , 142-N, 143-1, . . . , 143-N, 144-1, . . . , 144-N, 145-1, . . . , 145-N, 146-1, . . . , 146-N shown in FIG. 2 may be configured in the same manner or in a similar manner as the multi-level converter cell 141-1 illustrated in FIG. 3. According to the example illustrated in FIG. 3, the multi-level converter cell 141-1 comprises two switches or switching elements 191, 192 and a capacitor 193. According to the example illustrated in FIG. 3, each of the switches or switching elements 191, 192 comprises a transistor together with a diode. The transistors may for example comprise insulated gate bipolar transistors (IGBTs). It is to be understood that the switching elements 191, 192 shown in FIG. 3 are according to an example, and that other types of switching elements can be used. Also, the multi-level converter cell 141-1 is not limited to using a capacitor 193 as electrical energy storage element, but other types of electrical energy storage elements may be employed. With reference to FIG. 2, the capacitor 193 can be selectively charged with DC power from the DC power system 103 and selectively discharged. The multi-level converter cell 141-1 can thereby be controlled so as to provide a voltage contribution to the AC voltage waveform based on a voltage of the capacitor 193 (or another electrical energy storage element).

[0065] FIG. 3 illustrates the multi-level converter cell 141-1 configured as a half-bridge circuit, wherein the two switches or switching elements 191, 192 are connected in series across the electrical energy storage element 193, with a midpoint connection between the switches or switching elements 191, 192 and one of the electrical energy storage element 193 terminals as external connections. However, it is to be understood that this configuration is according to a non-limiting example and that variations are possible. For example, the multi-level converter cell 141-1 could be configured as a full-bridge circuit. Configuring the multi-level converter cell 141-1 as a full-bridge circuit may allow for or facilitate insertion of the electrical energy storage element 193 into the circuit in either polarity. FIG. 4 illustrates the multi-level converter cell 141-1 configured as a full-bridge circuit. The multi-level converter cell 141-1 illustrated in FIG. 4 comprises four switches or switching elements 191, 192, 196, 197, each comprising a transistor (e.g., an IGBT) together with a diode. The multi-level converter cell 141-1 illustrated in FIG. 4 further comprises an electrical energy storage element in the form of a capacitor 193.

[0066] With further reference to FIG. 2, any one of the multi-level converter cells 141-1, . . . , 141-N, 142-1, . . . , 142-N, 143-1, . . . , 143-N, 144-1, . . . , 144-N, 145-1, . . . , 145-N and 146-1, . . . , 146-N may for example comprise a half-bridge, or two-level, cell or a full-bridge, or three-level, cell.

[0067] With further reference to FIG. 2, each of the converter modules 104, 105, 106 may comprise a plurality of converter valves. The plurality of converter valves are electrically connected to the multi-level converter cells and are controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, so as to selectively control polarity of any voltage contribution provided by the respective multi-level converter cells.

[0068] The converter module 104 may include a plurality of converter valves 151-1, . . . , 151-N and 152-1, . . . , 152-N electrically connected, e.g. in series as illustrated in FIG. 2, and arranged in a converter valve arm 171. The converter valves 151-1, . . . , 151-N of the converter valve arm 171 constitute an upper converter valve arm of the converter module 104, and the converter valves 152-1, . . . , 152-N of the converter valve arm constitute a lower converter valve arm of the converter module 104.

[0069] Similarly, the converter module 105 may include a plurality of converter valves 153-1, . . . , 153-N and 154-1, . . . , 154-N electrically connected, e.g. in series as illustrated in FIG. 2, and arranged in a converter valve arm 172. The converter valves 153-1, . . . , 153-N of the converter valve arm 172 constitute an upper converter valve arm of the converter module 105, and the converter valves 154-1, . . . , 154-N of the converter valve arm constitute a lower converter valve arm of the converter module 105.

[0070] Similarly, the converter module 106 may include a plurality of converter valves 155-1, . . . , 155-N and 156-1, . . . , 156-N electrically connected, e.g. in series as illustrated in FIG. 2, and arranged in a converter valve arm 173. The converter valves 155-1, . . . , 155-N of the converter valve arm 173 constitute an upper converter valve arm of the converter module 106, and the converter valves 156-1, . . . , 156-N of the converter valve arm constitute a lower converter valve arm of the converter module 106.

[0071] In the embodiment of the present invention illustrated in FIG. 2, the upper converter valve arm and the lower converter valve arm of the converter modules 104, 105, 106 each includes N converter valves, where N is an integer, such as ten, fifteen, or twenty. However, it is to be understood that each of the converter modules 104, 105, 106 may include in principle any number of converter valves. According to an example, each of the converter modules 104, 105, 106 may include a single converter valve.

[0072] Referring now to FIG. 5, there is shown an example configuration of the converter valve 151-1 shown in FIG. 2. It is to be understood that any one of the other converter valves 151-N, 152-1, . . . , 152-N, 153-1, . . . , 153-N, 154-1, . . . , 154-N, 155-1, . . . , 155-N, 156-1, . . . , 156-N shown in FIG. 2 may be configured in the same manner or in a similar manner as the converter valve 151-1 illustrated in FIG. 5. According to the example illustrated in FIG. 5, the converter valve 151-1 includes at least two anti-parallel thyristors 194, 195. As illustrated in FIG. 5, the thyristors 194, 195 are electrically connected in parallel and have their polarities reversed with respect to each other. Thereby, the converter valve 151-1 may exhibit a capability or capacity of a controllable, bidirectional switch.

[0073] With further reference to FIGS. 1 and 2, the phase transformers 107, 108, 109 may be connected between the AC power system 102 and a midpoint of the corresponding converter valve arm 171, 172, 173 and a midpoint of the corresponding multi-level converter cell arm 161, 162, 163.

[0074] The midpoint of the multi-level converter cell arms 161, 162, 163 may be defined as being a point connecting the upper multi-level converter cell arm of the respective multi-level converter cell arm 161, 162, 163 on one side, and the lower multi-level converter cell arm of the respective multi-level converter cell arm 161, 162, 163 on the other side. For example when the multi-level converter cells in the multi-level converter cell arm 161, 162, 163 are electrically connected in series, the midpoint may be defined as a point where half or approximately half of the multi-level converter cells are provided on one side of the midpoint and the remaining ones of the multi-level converter cells are provided on the other side of the midpoint.

[0075] Similarly, the midpoint of the converter valve arms 171, 172, 173 may be defined as being a point connecting the upper converter valve arm of the respective converter valve arm 171, 172, 173 on one side, and the lower converter valve arm of the respective converter valve arm 171, 172, 173 on the other side. For example when the converter valves in the converter valve arm 171, 172, 173 are electrically connected in series, the midpoint may be defined as a point where half or approximately half of the converter valves are provided on one side of the midpoint and the remaining ones of the converter valves are provided on the other side of the midpoint.

[0076] As illustrated in FIG. 2, for any one of the converter modules 104, 105, 106, the multi-level converter cell arm 171, 172, 173 and the converter valve arm 161, 162, 163 in the respective converter module 104, 105, 106 may for example be electrically connected in parallel.

[0077] With further reference to FIG. 2, any one of the converter modules 104, 105, 106 may comprise a commutation cell 181, 182, 183 electrically connected to the respective converter valves 151-1, . . . , 151-N, 152-1, . . . , 152-N, 153-1, . . . , 153-N, 154-1, . . . , 154-N, 155-1, . . . , 155-N, 156-1, . . . , 156-N, and switchable so as to cause the respective converter valves 151-1, . . . , 151-N, 152-1, . . . , 152-N, 153-1, . . . , 153-N, 154-1, . . . , 154-N, 155-1, . . . , 155-N, 156-1, . . . , 156-N to enter the non-conducting state.

[0078] Referring now to FIG. 6, there is shown an example configuration of the commutation cell 181 shown in FIG. 2. It is to be understood that any one of the other commutation cells 182, 183 shown in FIG. 2 may be configured in the same manner or in a similar manner as the commutation cell 181 illustrated in FIG. 6. In general, each of the commutation cells 181, 182, 183 may include at least one electrical energy storage element, such as a capacitor, which can be selectively charged with DC power from the DC power system 102 and selectively discharged, wherein by switching of the commutation cell 181, 182, 183, it may provide a selected voltage across at least one of the thyristors 194, 195 in a converter valve 151-1, . . . , 151-N, 152-1, . . . , 152-N, 153-1, . . . , 153-N, 154-1, . . . , 154-N, 155-1, . . . , 155-N, 156-1, . . . , 156-N, and switchable so as to cause the respective converter valves 151-1, . . . , 151-N, 152-1, . . . , 152-N, 153-1, . . . , 153-N, 154-1, . . . , 154-N, 155-1, . . . , 155-N, 156-1, . . . , 156-N in order to switch the at least one thyristor 194, 195 into a non-conducting state. According to the example illustrated in FIG. 6, the commutation cell 181 comprises two electrical energy storage elements 186-1, 186-2 in the form of capacitors. Each of the capacitors 186-1, 186-2 is arranged in a corresponding full-bridge cell 184, 185 having four switching elements 187-1 to 187-4 and 187-5 to 187-8, respectively. According to the example illustrated in FIG. 6, the switching elements 187-1 to 187-8 comprise a transistor together with a diode. The transistors may for example comprise IGBTs. It is to be understood that the switching elements 187-1 to 187-8 shown in FIG. 6 are according to an example, and that other types of switching elements can be used. Thus, any one of the commutation cells 181, 182, 183 may preferably comprise a full-bridge cell, which is in accordance with the embodiment of the present invention illustrated in FIG. 6. However, this is not necessary. Any one of the commutation cells 181, 182, 183 may for example comprise a half-bridge cell. The commutation cells 181, 182, 183 are not limited to using a capacitors 186-1, 186-2 as electrical energy storage elements, but other types of electrical energy storage elements may be employed. As illustrated in FIG. 2, the commutation cells 181, 182, 183 may for example be arranged at the midpoint of the multi-level converter cell arms 161, 162, 163, respectively.

[0079] As indicated in the foregoing, the multi-level converter cells 141-1, . . . , 141-N, 142-1, . . . , 142-N, 143-1, . . . , 143-N, 144-1, . . . , 144-N, 145-1, . . . , 145-N and 146-1, . . . , 146-N of the converter modules 104, 105, 106 are configured to provide a voltage contribution to the AC voltage waveform, e.g. based on (at least) voltage of the DC power system 103. The multi-level converter cells 141-1, . . . , 141-N, 142-1, . . . , 142-N, 143-1, . . . , 143-N, 144-1, . . . , 144-N, 145-1, . . . , 145-N and 146-1, . . . , 146-N can hence be used in order to synthesize a desired AC voltage waveform in order to satisfy the requirements of either the AC power system 102 or the DC power system 103. The interface arrangement 110 can hence be operated as a Voltage Source Converter, wherein DC side voltage establishes the AC side voltage. By way of the converter cells 141-1, . . . , 141-N, 142-1, . . . , 142-N, 143-1, . . . , 143-N, 144-1, . . . , 144-N, 145-1, . . . , 145-N and 146-1, . . . , 146-N being multi-level converter cells, each multi-level converter cell 141-1, . . . , 141-N, 142-1, . . . , 142-N, 143-1, . . . , 143-N, 144-1, . . . , 144-N, 145-1, . . . , 145-N and 146-1, . . . , 146-N is configured so as to be capable of providing a multiple of voltage levels, such as two or more voltage levels, which may be used in forming the AC voltage waveform.

[0080] The forming of the AC voltage waveform by operation and control of the (components of the) converter modules 104, 105, 106 may be carried out using general principles which as such are known in the art. For each phase, depending on which of the switches or switching elements 191, 192 in each multi-level converter cell is switched on (i.e. is in a conducting state), the electrical energy storage element 193 can either be bypassed or connected into the circuit. Each multi-level converter cell can thereby act as a possibly independent, separate, controllable voltage source. In accordance with the embodiment of the present invention, the multi-level converter cell 141-1 is a two-level converter, which can generate either zero voltage or the voltage across the electrical energy storage element (e.g., a capacitor) 193. With a number of multi-level converter cells electrically connected, e.g. in series such as illustrated in FIG. 2, with the series-connected multi-level converter cells forming multi-level converter cell arms 161, 162, 163, a number of voltage levels can be provided which can be used to synthesize a stepped voltage waveform. Another way to describe this is that the voltage at the AC output of each phase may be controllably switched between a number of discrete voltage levels which are based on or correspond to the electrical potentials at the first DC pole or terminal T1 and at the second DC pole or terminal T2, +U.sub.d and −U.sub.d, respectively. The polarity of voltage contributions provided by the multi-level converter cells can be controlled by the converter valves (so as to produce positive or negative voltage contributions), thereby allowing for a stepped voltage waveform to be synthesized which for example can approximate a sine wave or sinusoid.

[0081] Referring now to FIG. 7, there is shown a schematic block diagram of a power system 200 according to an embodiment of the present invention. The power system 200 includes an AC power system 102 and a DC power system 103 coupled or connected by way of terminals T1 and T3, respectively, to an interface arrangement 100 configured to couple the AC power system 102 with the DC power system 103, or vice versa. The interface arrangement 100 may for example be configured in accordance with any one of the embodiments of the present invention described above with reference to FIGS. 1-5. As illustrated in FIG. 7, the interface arrangement 100 may comprise a control unit 101 configured to control operation of one or more other components of the interface arrangement 100. For example with reference to FIG. 2, the control unit 101 may be configured to control the converter valves 151-1, . . . , 151-N, 152-1, . . . , 152-N, 153-1, . . . , 153-N, 154-1, . . . , 154-N, 155-1, . . . , 155-N, 156-1, . . . , 156-N, for example with respect to switching thereof.

[0082] In conclusion, an interface arrangement configured to couple an AC power system with a DC power system, or vice versa, is disclosed. The interface arrangement comprises a plurality of series-connected converter modules. Each converter module comprises at least one multi-level converter cell configured to provide a voltage contribution to at least a portion of an AC waveform for example based on voltage of the DC power system. Each converter module comprises at least one converter valve, electrically connected to the at least one multi-level converter cell and including at least two anti-parallel thyristors. The at least one converter valve is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell.

[0083] While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present 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 in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. 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.