ARRANGEMENT WITH A MULTILEVEL CIRCUIT

Description

[0001] The invention relates to an arrangement with a multilevel converter comprising at least one phase module, wherein the phase module comprises a plurality of modules, each having a first electrical module connection and a second electrical module connection. The application further relates to a method for polarity-dependent limiting of a voltage occurring at such a multilevel converter.

[0002] During the operation of a multilevel converter, temporary fluctuations / oscillations of the DC voltage can occur (for example, due to a fault or a singular event on a DC transmission link connected to the multilevel converter). These voltage fluctuations can lead to high negative voltages at the DC terminals of the multilevel converter. Such high negative voltages can occur particularly when the DC transmission link has different conductor types, for example, if the DC transmission link consists partly of a cable and partly of an overhead line. Reflections between the different conductor types can then cause particularly high negative voltages."Negative voltage" here refers to a voltage whose polarity is opposite to the polarity of the voltage ("positive voltage") applied to the first DC terminal and the second DC terminal of the multilevel converter during normal operation.

[0003] To manage such unwanted negative voltages, it is conceivable to increase the number of modules in the modular multilevel converter so that it can absorb the voltages without overloading the individual modules. However, such an increase in the number of modules is complex and expensive. In particular, (more) bipolar modules, such as full-bridge modules, could also be used, which would likewise result in higher costs.

[0004] European patent application EP 3 068 008 A1 discloses a modular multilevel power converter comprising several sub-modules connected in series. A bypass protection circuit, providing overvoltage protection, is connected in parallel to each of the sub-modules connected in series.

[0005] A multilevel converter with three phase modules is known from document CN 106 100 404 A. An electronic bypass switch based on a thyristor is arranged between the first and second DC terminals of the multilevel converter. This electronic bypass switch closes when a fault occurs, connecting the first DC terminal to the second DC terminal. This protects the modules of the multilevel converter.

[0006] Documents CN 106 953 347 A and CN 208 738 855 U each disclose a multilevel converter comprising both half-bridge and full-bridge modules. Some of the full-bridge modules are arranged adjacent to an AC terminal of a phase module, and some of the full-bridge modules are arranged adjacent to a DC terminal of the phase module.

[0007] Document CN 106 953 509 A discloses a module of a multilevel converter which incorporates a capacitor as an energy storage device. A series circuit consisting of a diode and a resistor is connected in parallel with the capacitor. This series circuit protects the capacitor from overcharging.

[0008] The invention is based on the objective of providing an arrangement and a method with which the temporarily occurring negative voltages can be controlled without having to increase the number of modules of the multilevel converter.

[0009] This problem is solved according to the invention by an arrangement and a method according to the independent claims. Advantageous embodiments of the arrangement and the method are specified in the dependent claims.

[0010] Disclosed is an arrangement comprising a multilevel converter having at least one phase module, wherein the phase module comprises a plurality of modules, each having a first electrical module terminal and a second electrical module terminal. The plurality of modules comprise modules of the first type, which are capable of outputting a voltage of only one polarity or zero voltage at their first and second electrical module terminals. The plurality of modules comprise modules of the second type, which are capable of outputting a voltage of one polarity, a voltage of opposite polarity, or zero voltage at their first and second electrical module terminals. The arrangement further comprises a voltage limiting device that limits the voltage (total voltage) appearing across the modules of the second type, depending on the polarity of that voltage.

[0011] The voltage limiting device thus performs polarity-dependent voltage limiting. It is advantageous that the voltage limiting device limits the voltage across the second-type modules depending on the polarity of this voltage. The polarity of the voltage across the second-type modules is the same as the polarity of the voltage between a first DC input and a second DC input of the multilevel converter. Therefore, this voltage limiting device can advantageously limit only the unwanted negative voltage; the positive voltage occurring during normal operation between the first and second DC inputs is not limited. The voltage limiting device can, in particular, bridge the second-type modules. In other words, the voltage limiting device can be connected in parallel with the second-type modules.

[0012] The arrangement is designed in such a way that The phase module has an AC voltage connection, a first DC voltage connection, and a second DC voltage connection. The multilevel converter is then, in particular, an inverter or a rectifier.

[0013] The arrangement is designed in such a way that The modules of the second type are arranged adjacent to the AC voltage terminal of the phase module, and the modules of the first type are arranged adjacent to the first and / or the second DC voltage terminal of the phase module. This results in a series connection of modules of the second type adjacent to the AC voltage terminal.

[0014] The arrangement can also be designed in such a way that The modules each have at least one first electronic switching element, one second electronic switching element and one electrical energy storage device.

[0015] The arrangement can be designed in such a way that The modules of the first type each have the first electronic switching element and the second electronic switching element in a half-bridge circuit.

[0016] The arrangement can also be designed in such a way that The modules of the first type, in parallel with the second electronic switching element, have an additional semiconductor valve, in particular a diode, a thyristor, an IGCT or a GTO.

[0017] This additional semiconductor valve is specifically connected between the first and second electrical module connections. The additional semiconductor valve is connected in the same direction and in parallel with a diode, which is connected antiparallel to the second electronic switching element. The additional semiconductor valve serves to relieve the load on the diode connected antiparallel to the second electronic switching element.

[0018] The arrangement can be designed in such a way that The modules of the second type each comprise the first electronic switching element, the second electronic switching element, a third electronic switching element and a fourth electronic switching element in a full bridge circuit.

[0019] The voltage limiting device is designed such that it limits only a voltage of one polarity appearing across the second type of modules, and in particular leaves a voltage of the opposite polarity appearing across the second type of modules essentially unchanged. The voltage limiting device thus limits only voltages of one polarity appearing across the second type of modules, and in particular leaves voltages of the opposite polarity appearing across the second type of modules essentially unchanged. Therefore, the voltage limiting device also limits only voltages of one polarity appearing between the first DC voltage terminal and the second DC voltage terminal, and in particular leaves voltages of the opposite polarity appearing between the first DC voltage terminal and the second DC voltage terminal essentially unchanged.The "voltage of one polarity" is therefore the unwanted negative voltage; the "voltage of opposite polarity" is the (desired) positive voltage that occurs during normal operation of the power converter.

[0020] The voltage limiting device can be designed so that a current flows through the voltage limiting device only when the voltage of one polarity occurs across the modules of the second type (and thereby limits the voltage of one polarity).

[0021] The voltage limiting device can therefore be designed such that a current flows through it (and thus limits the voltage of one polarity) only when the voltage of one polarity occurs between the first DC terminal and the second DC terminal. The current flowing through the voltage limiting device loads or (fully or partially) short-circuits the voltage of one polarity, thereby limiting it.

[0022] The arrangement is also designed such that the voltage limiting device includes at least one semiconductor valve, in particular a series connection of semiconductor valves. The polarity dependency of the voltage limiting is implemented by the semiconductor valve. The semiconductor valve can be an uncontrolled semiconductor valve (for example, a diode) or a switchable semiconductor valve (for example, a thyristor or a transistor).

[0023] The arrangement can be configured such that the series connection comprises at least 50 semiconductor valves. This comparatively large number of semiconductor valves advantageously allows for a high blocking voltage, ensuring that the (desired) voltage of the opposite polarity remains essentially unchanged.

[0024] The arrangement can be configured such that the semiconductor valve is a diode, a thyristor, or a transistor. In particular, the diode can be a power diode (for example, a disk cell diode); the thyristor can be a power thyristor (for example, a disk cell thyristor). The semiconductor valve can be configured, for example, as an integrated gate-commutated thyristor (IGCT), a gate turn-off thyristor (GTO thyristor), an insulated-gate bipolar transistor (IGBT), or a metal oxide-semiconductor field-effect transistor (MOSFET).

[0025] The arrangement can also be configured such that an electrical resistor is connected in series with the at least one semiconductor valve. Advantageously, the electrical resistor can limit the current flowing through the voltage limiting device. Furthermore, in the event of a fault, the electrical energy stored in the DC transmission line can be converted into heat by the electrical resistor.

[0026] The arrangement can be designed such that the electrical resistance is a varistor (voltage-dependent resistor), in particular a metal oxide varistor. For example, a surge arrester can be used as the varistor. By appropriately selecting the response voltage / threshold voltage, such a surge arrester can ensure that voltage limiting only occurs above a certain level of the (undesired) voltage of one polarity.

[0027] The arrangement can also be configured such that the first DC terminal and the second DC terminal are connected to a DC transmission line, in particular a high-voltage DC transmission line, wherein the DC transmission line has two different conductor types which differ in their electrical capacitance per unit length ("mixed DC transmission line"). The voltage limiting device can be used to particular advantage in such DC transmission lines because, with two different conductor types, particularly high undesirable negative voltages can occur at the DC terminals of the multilevel converter.

[0028] The arrangement can be designed such that the different conductor types are an overhead line, a cable, and / or a gas-insulated line. The cable can, in particular, be a high-voltage cable. For example, a cable has a higher specific electrical capacitance per unit length than an overhead line. As another example, a gas-insulated line also has a higher specific electrical capacitance per unit length than an overhead line.

[0029] The arrangement can also be designed such that the cable is a plastic-insulated cable, an oil-insulated cable, or a mass-impregnated cable. In particular, the direct current transmission line can comprise an overhead line and a cable (in particular a plastic-insulated cable, an oil-insulated cable, or a mass-insulated cable).

[0030] The arrangement can also be designed such that the phase module has modules of the first type and modules of the second type, at least some of which are electrically connected in series.

[0031] The arrangement can be configured such that the multilevel converter has two or three phase modules. These phase modules can be connected in parallel at their first and second DC terminals. This allows the multilevel converter to be used in two-phase or three-phase power grids. In particular, the voltage limiting device (i.e., a single voltage limiting device) can be switchable between the phase modules. Alternatively, at least one of the phase modules can have its own (permanent) voltage limiting device. Thus, one phase module, two phase modules, or all three phase modules can each have their own (individual) voltage limiting device.

[0032] The arrangement can be configured such that the voltage limiting device comprises a first series circuit consisting of at least one semiconductor valve (or valves) and an electrical resistor, and a second series circuit consisting of at least one further semiconductor valve (or valves) and another electrical resistor. The connection point (center connection point) between the first series circuit and the second series circuit is grounded.The terminal of the first series connection opposite this connection point (center connection point) is electrically connected to a first connection point between the modules of the first type and the modules of the second type (in one phase module branch of the phase module); the terminal of the second series connection opposite this connection point (center connection point) is electrically connected to a second connection point between the modules of the first type and the modules of the second type (in the other phase module branch of the phase module). The semiconductor valves and resistors can be configured as described above and in the exemplary embodiments. This arrangement can be used, in particular, in a symmetrical monopoly configuration of a high-voltage direct current transmission system.

[0033] Furthermore, a high-voltage direct current transmission system with an arrangement according to one of the variants described above is revealed.

[0034] Furthermore, a method for polarity-dependent voltage limiting in a multilevel converter having at least one phase module is disclosed. wherein the phase module comprises a plurality of modules (1_1 ... 2_n), each having a first electrical module terminal and a second electrical module terminal, the plurality of modules comprising modules of the first type which are capable of outputting a voltage of exclusively one polarity or zero voltage at their first electrical module terminal and their second electrical module terminal, the plurality of modules comprising modules of the second type which are capable of outputting a voltage of one polarity, a voltage of opposite polarity, or zero voltage at their first electrical module terminal and their second electrical module terminal, wherein in the method with a voltage limiting device only a voltage of one polarity appearing across the modules of the second type is limited, and in particular a voltage of the opposite polarity appearing across the modules of the second type is left substantially unchanged.

[0035] In particular, the voltage limiting device ensures that any voltage across the first-type modules remains essentially unchanged. The voltage limiting device can bridge the second-type modules. In other words, the voltage limiting device can be connected in parallel with the second-type modules. The phase module can have one AC terminal, one DC terminal, and one DC terminal. At least some of the majority of the modules can be electrically connected in series.

[0036] The procedure can be designed in such a way that Only when a voltage of one polarity occurs across the modules of the second type does a current flow through the voltage limiting device, thereby limiting the voltage of one polarity.

[0037] This method must therefore be designed in such a way that only when a voltage of one polarity occurs between the first DC voltage terminal and the second DC voltage terminal does a current flow through the voltage limiting device, thereby limiting the voltage of one polarity.

[0038] The process can proceed as follows: The modules of the first type each comprise a first electronic switching element and a second electronic switching element in a half-bridge circuit, wherein an additional semiconductor valve is arranged in parallel to the second electronic switching element, and only when a voltage of one polarity is applied across the modules of the second type will a portion of the current flowing through the modules of the first type flow through or be directed through the additional semiconductor valve. In particular, the majority of the current flowing through the modules of the first type can then be directed through the additional semiconductor valve.

[0039] The described arrangement and method have the same or similar advantages.

[0040] The invention will now be explained in more detail using exemplary embodiments. Identical reference numerals refer to identical or equivalently acting elements. For this purpose, see in Figure 1 shows an embodiment of a multilevel converter with a voltage limiting device, Figures 2 and 3 show further embodiments of a multilevel converter with a voltage limiting device, Figure 4 shows an embodiment of a high-voltage direct current transmission system with such a multilevel converter, Figure 5 shows an embodiment of a first-type module of the multilevel converter in the form of a half-bridge module, Figure 6 shows an embodiment of a second-type module of the multilevel converter in the form of a full-bridge module, and Figure 7 shows a further embodiment of a multilevel converter with a voltage limiting device.

[0041] In Figure 1Figure 1 shows an embodiment of an arrangement 1 with a power converter 3 in the form of a modular multilevel power converter 3. This multilevel power converter 3 has 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 a second phase module branch 13. The first phase module branch 11 and the second phase module branch 13 form a first phase module 15 of the power converter 3. The end of the first phase module branch 11 opposite the first AC voltage terminal 5 is electrically connected to a first DC voltage terminal 16; the end of the second phase module branch 13 opposite the first AC voltage terminal 5 is electrically connected to a second DC voltage terminal 17.The first DC terminal 16 is a positive DC terminal; the second DC terminal 17 is a negative DC terminal.

[0042] The second AC 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 form a second phase module 24. The third AC 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 form a third phase module 31.

[0043] The end of the third phase module branch 18 opposite the second AC terminal 7 and the end of the fifth phase module branch 27 opposite the third AC terminal 9 are electrically connected to the first DC terminal 16. The end of the fourth phase module branch 21 opposite the second AC terminal 7 and the end of the sixth phase module branch 29 opposite the third AC terminal 9 are electrically connected to the second DC terminal 17. The first phase module branch 11, the third phase module branch 18, and the fifth phase module branch 27 form a positive-side converter section 32; the second phase module branch 13, the fourth phase module branch 21, and the sixth phase module branch 29 form a negative-side converter section 33.

[0044] Each phase module branch has a plurality of modules (1_1, 1_2, 1_3, 1_4 ... 1_m; 2_1 ... 2_m; etc.) which are electrically connected in series (by means of their module connections). Such modules are also referred to as submodules. In the exemplary embodiment of the Figure 1 Each phase module branch has m modules. The number of modules electrically connected in series via their module terminals can vary considerably; at least three modules are connected in series, but there can also be, for example, 50, 100, or more modules electrically connected in series. In the exemplary embodiment, m = 36: the first phase module branch 11 therefore has 36 modules 1_1, 1_2, 1_3, ... 1_36. The other phase module branches 13, 18, 21, 27, and 29 are constructed identically.

[0045] Each phase module branch 11, 13, 18, 21, 27, 29 has modules of the first type and modules of the second type. Modules of the first type are modules that can output a voltage of only one polarity or zero voltage between their first electrical module terminal and their second electrical module terminal (depending on the module control). Examples of modules of the first type are so-called half-bridge modules, as used in Figure 5 are shown.

[0046] Second-type modules are modules that can output a voltage of one polarity, a voltage of the opposite polarity, or zero voltage between their first and second electrical module terminals (depending on the module's control method). Examples of second-type modules include so-called full-bridge modules, as used in... Figure 6 are shown.

[0047] Each phase module 15, 24, 31 has 500 modules of the first type and 600 modules of the second type; thus, each phase module has a plurality of modules. At least some modules of the first and second types are electrically connected in series. Therefore, at least some modules of the plurality of modules are electrically connected in series. Each phase module branch 11, 13, 18, 21, 27, 29 has n modules of the second type (modules 1_1 ... 1_n; 2_1 ... 2_n, etc.). Furthermore, each phase module branch 11, 13, 18, 21, 27, 29 has (mn) modules of the first type (modules 1_n+1 ... 1_m; 2_n+1 ... 2_m, etc.). The modules of the second type are arranged adjacent to the AC voltage terminal of the phase module. The modules of the first type are arranged adjacent to the first and / or the second DC voltage connection of the phase module.The modules of the second type each form one (central) series circuit in each phase module (at the AC voltage terminal); the modules of the first type each form two series circuits in each phase module (one series circuit at the first DC voltage terminal and one series circuit at the second DC voltage terminal). In particular, inductors (choke coils) can be arranged in the phase modules, for example at the AC voltage terminals or at the DC voltage terminals.

[0048] In the left area of ​​the Figure 1A control unit 35 for modules 1_1 to 6_m is shown schematically. Optical messages or signals are transmitted from this central control unit 35 to the individual modules via an optical communication link 37 (for example, via a fiber optic cable). The message transmission between the control unit and a module is symbolically represented by a line 37; the direction of the message transmission is symbolized by the arrowheads on the lines 37. This is illustrated using the example of modules 1_1, 1_(n+1), and 4_2; messages are sent to and received from the other modules in the same manner. For example, the control unit 35 sends a setpoint for the switching state of the electronic switching elements to each individual module.

[0049] A voltage limiting device 50 is electrically connected to one of the phase modules 15, 24 or 31. In the exemplary embodiment of the Figure 1 The voltage limiting device 50 is electrically connected to the third phase module 31. The voltage limiting device 50 bridges the second-type modules of the third phase module 31. In other words, the voltage limiting device 50 is connected in parallel to the second-type modules.

[0050] The voltage limiting device 50 has a first terminal 53 and a second terminal 56. The first terminal 53 of the voltage limiting device 50 is electrically connected to a connection point 59 (first connection point 59) between the first-type modules and the second-type modules of one phase module branch 27 of the phase module 31. The second terminal 56 of the voltage limiting device 50 is electrically connected to a connection point 62 (second connection point 62) between the first-type modules and the second-type modules of the other phase module branch 29 of the phase module 31. The voltage limiting device 50 thus bridges the second-type modules of the third phase module 31.

[0051] In the exemplary embodiment, the voltage limiting device 50 is connected between the first connection point 59 and the second connection point 62. This voltage limiting device 50 connects the first connection point 59 and the second connection point 62. Thus, the voltage limiting device 50 is connected between the first DC voltage terminal 16 and the second DC voltage terminal 17. The voltage limiting device 50 therefore limits the voltage of one polarity between the first connection point 59 and the second connection point 62. Consequently, the voltage limiting device 50 also limits the voltage of one polarity between the first DC voltage terminal 16 and the second DC voltage terminal 17.

[0052] The voltage limiting device 50 comprises semiconductor valves 65_1, 65_2, 65_3, etc., up to 65_p, which are electrically connected in series. In principle, the voltage limiting device 50 would also function with a single semiconductor valve 65_1. The semiconductor valves 65_1, 65_2, 65_3, ... 65_p form a combined semiconductor valve 65 with a higher blocking voltage than the individual semiconductor valves.

[0053] In this embodiment, p = 50, meaning 50 semiconductor valves are electrically connected in series. However, in other embodiments, larger numbers of semiconductor valves can be connected in series, for example, 100 or 200. If only a low blocking voltage is required, a smaller number of semiconductor valves may suffice.

[0054] The semiconductor valves 65_1, 65_2, 65_3 ... 65_p are configured as diodes 65_1, 65_2, 65_3 ... 65_p in the exemplary embodiment. Specifically, in the exemplary embodiment, the Figure 1 p Diodes are electrically connected in series and form the overall semiconductor valve 65.

[0055] The semiconductor valves 65_1, 65_2, 65_3 ... 65_p form an electrical series circuit with an electrical resistance 68. The resistance 68 can be a linear (ohmic) resistor. However, the resistance 68 can also be a varistor (voltage-dependent resistor), in particular a metal oxide varistor. The resistance 68 can, in particular, act as a surge arrester. The surge arrester 68 ensures that the voltage limiting device 50 only becomes effective above a certain magnitude of the unwanted negative voltage. Furthermore, excess electrical energy can be converted into heat in the resistance 68 or the varistor 68.

[0056] Under normal operating conditions / normal operation, a positive voltage Udc is present between the first DC terminal 16 and the second DC terminal 17 (that is, the electrical potential at the first DC terminal 16 is greater than the electrical potential at the second DC terminal 17).

[0057] Then a positive voltage UVB is also present between the first connection point 59 and the second connection point 62 (that is, the electrical potential at the first connection point 59 is greater than the electrical potential at the second connection point 62). Then, due to the valve action of the semiconductor valves 65_1 ... 65_p, no electric current flows through the voltage limiting device 50. Therefore, during normal operation, the (positive) DC voltage Udc applied to the DC voltage terminals 16, 17 remains unchanged.

[0058] However, if, for example, a singular event or fault occurs in a DC transmission line connected to DC terminals 16 and 17, a voltage U of reverse polarity (negative voltage) may appear between the first DC terminal 16 and the second DC terminal 17. Such negative voltages can occur, for example, due to unwanted oscillations in the DC transmission line. In this case, the electrical potential at the first DC terminal 16 is, at least temporarily, lower than the electrical potential at the second DC terminal 17. Consequently, the electrical potential at the first connection point 59 is also, at least temporarily, lower than the electrical potential at the second connection point 62.

[0059] Then a current I flows from the second DC terminal 17 via the second connection point 62 through the resistor 68 and the semiconductor valves 65_1 ... 65_p to the first connection point 59 and then to the first DC terminal 16. This loads and limits / attenuates the unwanted negative voltage U between the two DC terminals 16 and 17. In extreme cases, the unwanted negative voltage U is short-circuited. If the resistor 68 is a surge arrester, then the current I only flows through the voltage limiting device 50 when the negative voltage is so large that the response voltage of the surge arrester is exceeded.

[0060] Depending on the maximum rate of change of the current I flowing through the voltage limiting device 50, it may be advantageous to use fast diodes (e.g., IGCT diodes). The semiconductor valves can be selected, in particular, based on their i 2 < t value.

[0061] In Figure 2 Figure 1 shows a further embodiment of an arrangement 200. This arrangement 200 differs from arrangement 1 of the Figure 1This is achieved simply by the fact that the semiconductor valves are designed as thyristors 265_1 ... 265_p. When an unwanted negative voltage occurs, these thyristors 265_1 ... 265_p are essentially simultaneously controlled by a (not shown) control circuit, causing them to enter a conducting state and allowing a current flow I from the second connection point 62 to the first connection point 59 (and thus from the second DC voltage terminal 17 to the first DC voltage terminal 16). The thyristors can have a parallel-connected protection circuit with an RC network. Also, the in Figure 1 The diodes shown can have such a protective circuit. Instead of thyristors, other switchable semiconductor devices, such as transistors, can also be used.

[0062] In Figure 3Figure 1 shows a further embodiment of an arrangement 300. This arrangement 300 differs from the embodiment of Figure 2. Figure 1 simply by omitting resistor 68. The voltage limiting device 50 therefore only has the semiconductor valves 65_1 ... 65_p. In this case, the unwanted negative voltage is limited to the sum of the forward voltages of the semiconductor valves 65_1 ... 65_p. The current I flowing through the voltage limiting device 50 is not limited by resistor 68, so the semiconductor valves 65_1 ... 65_p must be sufficiently robust. This also applies to the embodiment of Figure 2 The resistor or surge arrester 68 can be omitted.

[0063] In Figure 4Figure 1 shows an embodiment of a high-voltage direct current transmission system 400. This high-voltage direct current transmission system 400 has an arrangement 1 with a multilevel converter 3 and a voltage limiting device 50 according to Figure 1. Figure 1 as well as a direct current transmission line 405. In the exemplary embodiment, the direct current transmission line 405 has the following features: Figure 4Two different conductor types are provided: a first part 410 of the DC transmission line 405 is designed as an overhead line 410 (first conductor type 410); a second part 412 of the DC transmission line 405 is designed as a cable 412 (second conductor type 412), in particular as a plastic-insulated cable 412. The cable 412 can also be a different type of cable, in particular an oil-insulated cable or a mass-insulated cable. At the end of the DC transmission line 405 facing away from the arrangement 1, a further arrangement 1' with a further multilevel converter 3 and an optional further voltage limiting device 50 can be arranged.

[0064] The two arrangements 1 and 1' (and thus the two converters 3) are electrically connected to each other on the DC side via the high-voltage direct current transmission line 405. Electrical energy can be transmitted over long distances by means of such a high-voltage direct current transmission system 400; the high-voltage direct current transmission line 405 then has a correspondingly long length. Such a high-voltage direct current transmission system can also be a multi-terminal high-voltage direct current transmission system.

[0065] In the modular multilevel converter 3 or the further modular multilevel converter 3, for example, the first DC terminal 16 or the second DC terminal 17 can be grounded (especially in a bipole configuration). This grounding can be direct ("hard grounding") or indirect, for example via a surge arrester or an impedance. Alternatively, the first DC terminal and the second DC terminal can be ungrounded (especially in a symmetrical monopole configuration); grounding then takes place on the AC side of the multilevel converter. A neutral point of the AC network can be grounded; in particular, a neutral point of an AC-side transformer or an additionally created high-impedance neutral point ("artificial neutral point").

[0066] In Figure 5Figure 500 is an embodiment of module 500 of the modular multilevel converter 3. Module 500 is a "first-type module". Module 500 could, for example, be one of the modules described in Figure 3. Figure 1 The modules shown are 1_(n+1) ... 1_m to 5_(n+1) ... 5_m and / or 6_(n+1) ... 6_m.

[0067] Module 500 is designed as a half-bridge module 500. Module 500 comprises a first (switchable) electronic switching element 502 (first switchable semiconductor valve 502) with a first antiparallel diode 504. Furthermore, module 500 comprises a second (switchable) electronic switching element 506 (second switchable semiconductor valve 506) with a second antiparallel diode 508, as well as an electrical energy storage device 510 in the form of a capacitor 510. The first electronic switching element 502 and the second electronic switching element 506 are each designed as an IGBT (insulated-gate bipolar transistor). The first electronic switching element 502 is electrically connected in series with the second electronic switching element 506. A first module connection 512 is arranged at the connection point between the two electronic switching elements 502 and 506.A second module connection 515 is arranged at the terminal of the second electronic switching element 506, which is opposite the connection point. The second module connection 515 is further electrically connected to a first terminal of the energy storage device 510; a second terminal of the energy storage device 510 is electrically connected to the terminal of the first electronic switching element 502, which is opposite the connection point.

[0068] The energy storage device 510 is therefore electrically connected in parallel to the series connection of the first electronic switching element 502 and the second electronic switching element 506. By appropriately controlling the first electronic switching element 502 and the second electronic switching element 506 via a control unit of the power converter, it is possible to output either the voltage of the energy storage device 510 or no voltage (i.e., zero voltage) between the first module terminal 512 and the second module terminal 515. The first-type module 500 is thus capable of outputting either a voltage of only one polarity or zero voltage between the first module terminal 512 and the second module terminal 515. Through the interaction of the modules of the individual phase module branches, the desired output voltage of the power converter can be generated.

[0069] The energy storage device can be a capacitor (or a circuit of several capacitors); the voltage of the energy storage device is then the capacitor voltage (or the total voltage of the circuit).

[0070] Optionally, an additional semiconductor valve 520 can be connected between the first module terminal 512 and the second module terminal 515. This additional semiconductor valve 520 can be a bypass switching element 520. Upon activation, the bypass switching element 520 bypasses the module 500. In the exemplary embodiment, the bypass switching element 520 is designed as a thyristor 520, i.e., as a switchable bypass switching element 520. In the exemplary embodiment, the bypass switching element 520 is connected such that (in the switched-on state) it can conduct a current that flows in the same direction and in parallel with the current flowing through the diode 508 (second antiparallel-connected diode 508). This relieves the diode 508 in the event of a negative voltage that needs to be limited.In this case, the bypass switching element 520 must be switched on / off; in the exemplary embodiment, the thyristor 520 is switched on / triggered when a negative voltage to be limited occurs. In the event of a DC fault, the active switching on / off of the bypass switching element 520 can therefore be advantageous. As an alternative to the bypass switching element 520, the second antiparallel diode 508 can also be designed to be correspondingly robust and / or the second antiparallel diode 508 can be reinforced by means of one or more further diodes connected in parallel (for example, press-pack diodes). In this case, the additional semiconductor valve 520 is therefore at least one diode. However, the additional semiconductor valve 520 can also be designed differently, for example as an IGCT or a GTO. The additional semiconductor valve 520 is connected in parallel with the second antiparallel diode 508.connected in antiparallel to the second electronic switching element 506.

[0071] In Figure 6 Figure 600 shows another embodiment of a module 600 of the modular multilevel converter 3. Module 600 is a "second-type module". Module 600 can, for example, be one of the modules described in Figure 3. Figure 1 The modules shown are 1_1 ... 1_n, 2_1 ... 2_n to 6_1 ... 6_n.

[0072] In addition to the already from Figure 5 The known first electronic switching element 502, second electronic switching element 506, first freewheeling diode 504, second freewheeling diode 508 and energy storage device 510 exhibits the in Figure 6The illustrated module 600 includes a third electronic switching element 602 with an antiparallel-connected third freewheeling diode 604 and a fourth electronic switching element 606 with a fourth antiparallel-connected freewheeling diode 608. The third electronic switching element 602 and the fourth electronic switching element 606 are each designed as an IGBT. In contrast to the circuit of the Figure 5 The second module connection 615 is not electrically connected to the second electronic switching element 506, but to a midpoint (connection point) of an electrical series circuit consisting of the third electronic switching element 602 and the fourth electronic switching element 606.

[0073] The module of Figure 6This is a so-called full-bridge module 600. This full-bridge module 600 is characterized by the fact that, with appropriate control of the four electronic switching elements between the first module terminal 512 and the second module terminal 615, either the positive voltage of the energy storage device 510, the negative voltage of the energy storage device 510, or a voltage of zero (zero voltage) can be output. Thus, the polarity of the output voltage can be reversed using the second type 600 module. The second type 600 module is therefore capable of outputting a voltage of one polarity, a voltage of the opposite polarity, or a voltage of zero between its first electrical module terminal 512 and its second electrical module terminal 615.

[0074] In Figure 7 Figure 1 shows a further embodiment of an arrangement 700. This arrangement 700 differs from arrangement 1 of the Figure 1The voltage limiting device 50 comprises a first series circuit 704 consisting of the semiconductor valves 65_1, 65_2, ... 65_p and the electrical resistor 68, and a second series circuit 708 consisting of semiconductor valves 764_1, 764_2, ... 764_p and an electrical resistor 768. A first terminal of the first series circuit 704 is connected to the second connection point 62. A second terminal of the first series circuit 704 is connected to a first terminal of the second series circuit 708. A second terminal of the second series circuit 708 is connected to the first connection point 59. The connection point 712 (center connection point 712) between the first series circuit 704 and the second series circuit 708 is grounded. This arrangement 700 is particularly advantageous in a symmetrical monopoly configuration of a high-voltage direct current transmission system.

[0075] In the figures, the voltage limiting device 50 is shown by way of example arranged on the third phase module 31. In other embodiments, however, the voltage limiting device 50 can also be arranged on a different phase module. In yet another embodiment, the voltage limiting device 50 can also be selectively (switchably) connected to different phase modules. Alternatively, one, several, or all phase modules can each have their own (permanently) assigned voltage limiting device.

[0076] The described arrangement and method offer several advantages. Depending on the polarity, the voltage appearing across the second type of phase module (i.e., the voltage between the first connection point 59 and the second connection point 62) can be limited or attenuated. This also limits or attenuates the voltage appearing between the first DC terminal 16 and the second DC terminal 17, depending on the polarity.

[0077] This avoids the need to equip the multilevel converter with additional modules to accommodate the negative voltage. This allows for cost savings in the multilevel converter. This is particularly advantageous for multilevel converters that have modules capable of providing only one polarity (e.g., half-bridge modules) and also modules capable of providing both polarities (e.g., full-bridge modules). With such converters, the number of full-bridge modules can be kept small, resulting in further cost savings. Such full-bridge modules would otherwise be necessary to provide the negative voltages that occur in the event of a fault. In other words, the voltage limiting device advantageously decouples the converter from the DC transmission line.

[0078] In the described arrangement, the polarity-dependent voltage limiting device is connected in parallel to the modules of the second type (and thus indirectly between the DC terminals of the multilevel converter), thereby limiting the voltage. This limits the maximum value of the (undesired) negative voltage, thus protecting the multilevel converter. Oversizing the multilevel converter (by adding modules) is advantageously avoided. Such a polarity-dependent voltage limiting device can be implemented, in particular, as a series connection of semiconductor valves and an electrical resistor (especially a surge arrester). Optionally, however, the surge arrester can be omitted, so that only a series connection of semiconductor valves is used.The voltage limiting device 50 can also be described as a voltage-asymmetric voltage limiting device or a voltage-asymmetric surge arrester, particularly suitable for high-voltage direct current (HVDC) transmission systems with mixed DC transmission lines. In such mixed DC transmission lines, undesirable high negative voltages can arise, for example, due to reflections between the different conductor types (especially reflections between an overhead line and a cable). The polarity-dependent voltage limiting device can be particularly advantageous in the case of a cable, as cables are sensitive to sudden voltage reversals.

[0079] The voltage limiting device 50 could also be connected directly between the first DC voltage terminal 16 and the second DC voltage terminal 17. The voltage limiting device 50 could therefore be connected directly between the DC-side converter terminals. Such an arrangement is described in the international patent application with the application number PCT / EP2019 / 051251. However, in this case, the DC voltage present at the converter (i.e., the nominal voltage of the DC path) determines the voltage load on the voltage limiting device 50 and thus the number of semiconductor valves of the voltage limiting device 50 to be connected in series. Several hundred semiconductor valves may well be necessary to achieve the required voltage withstand capability.

[0080] In contrast, the solution described here has the advantage that fewer semiconductor valves need to be connected in series in the voltage limiting device 50. This is because the second antiparallel diode 508 (or the bridging circuit element 520, or the diodes connected in parallel to the second antiparallel diode 508 (load-relief diodes)) present in the modules of the first type can each absorb a portion of the voltage. Thus, some of the components of the modules of the first type can be used multiple times: firstly, for their usual function in the module of the first type, and secondly, for the voltage relief of the voltage limiting device 50. These components of the modules of the first type can therefore implement part of the functionality of the voltage limiting device 50 (in particular, providing a blocking voltage).

[0081] The number of semiconductor valves to be connected in series in the voltage limiting device 50 can therefore be reduced in practice by approximately 15%, and even by a greater proportion under specific boundary conditions. Depending on the modulation level of the power converter and the ratio of the number of modules of the first type to the modules of the second type, the DC voltage load on the semiconductor valves of the voltage limiting device 50 can be reduced to values ​​of, for example, between 32% and 55%, which allows for an even greater reduction in the number of semiconductor valves to be connected in series in the voltage limiting device 50. (The modulation level of the power converter describes the ratio between the magnitude of the AC voltage (in particular the amplitude ûac of the AC voltage) and the magnitude of the DC voltage Udc. For example, the modulation level M can be given by M = 2ûac / Udc.)

[0082] This results in significant cost advantages. The reduced number of semiconductor valves required opens up possibilities for further optimization. In particular, the power converter can be optimally designed with regard to overall costs; the efficiency of the power converter can be significantly improved.

[0083] An arrangement and a method were described with which polarity-dependent voltage limiting can be carried out in a multilevel power converter.

Claims

1. An assembly having a multilevel power converter (3) which has at least one phase module (31), - wherein the phase module (31) has an alternating current voltage terminal (9), a first direct current voltage terminal (16) and a second direct current voltage terminal (17), - the phase module (31) has a plurality of modules (5_1 ... 5_m, 6_1 ... 6 m) which each have a first electrical module terminal (512) and a second electrical module terminal (515, 615), - the plurality of modules has modules of a first type (5_(n+1) ... 5_m, 6_(n+1) ... 6_m, 500), which are able to output a voltage of only one polarity or zero voltage at their first electrical module terminal (512) and their second electrical module terminal (515), - the plurality of modules has modules of a second type (5_1 ... 5_n, 6_1 ... 6_n, 600), which are able to output a voltage of one polarity, a voltage of opposite polarity or zero voltage at their first electrical module terminal (512) and their second electrical module terminal (615), and - having a voltage limiting device (50), characterized in that - the modules of the second type (5_1 ... 5_n, 6_1 ... 6_n, 600) are arranged adjacent to the alternating current voltage terminal (9) of the phase module (31), and the modules of the first type (5_(n+1) ... 5_m, 6_(n+1) ... 6_m, 500) are arranged adjacent to the first direct current voltage terminal (16) and the second direct current voltage terminal (17) of the phase moule (31), - the voltage limiting device (50) is designed to limit only a negative voltage (UVB) occurring across the modules of the second type and to leave a voltage (UVB) of opposite polarity occurring across the modules of the second type substantially unchanged, and - the voltage limiting device (50) comprises at least one semiconductor valve (65_1), in particular a connection in series of semiconductor valves (65_1 ... 65_p).

2. The assembly according to claim 1, wherein - the modules each have at least a first electronic switching element (502), a second electronic switching element (506) and an electrical energy store (510).

3. The assembly according to claim 2, wherein - the modules of the first type (500) each have the first electronic switching element (502) and the second electronic switching element (506) in a half-bridge circuit.

4. The assembly according to claim 2 or 3, wherein - the modules of the first type (500) have an additional semiconductor valve (520) connected in parallel with the second electronic switching element (506), in particular a diode, a thyristor, an IGCT or a GTO.

5. The assembly according to one of claims 2 to 4, wherein - the modules of the second type (600) each have the first electronic switching element (502), the second electronic switching element (506), a third electronic switching element (602) and a fourth electronic switching element (606) in a full-bridge circuit.

6. The assembly according to any one of the preceding claims, wherein - the voltage limiting device (50) is designed such that a current (I) only flows through the voltage limiting device (50) when the negative voltage (UVB) occurs across the modules of the second type.

7. The assembly according to any one of the preceding claims, wherein - the connection in series has at least 50 semiconductor valves (65_1 ... 65_p).

8. The assembly according to any one of the preceding claims, wherein - the at least one semiconductor valve (65_1 ... 65_p) is a diode (65_1), a thyristor (265_1), an IGCT, a GTO or a transistor.

9. The assembly according to any one of the preceding claims, wherein - an electrical resistor (68) is connected in series with the at least one semiconductor valve (65_1 ... 65_p).

10. The assembly according to claim 9, wherein - the electrical resistor is a varistor (68).

11. The assembly according to any one of the preceding claims, wherein - the assembly has a direct current transmission link (405), in particular a high-voltage direct current transmission link (405), and - the first direct current voltage terminal (16) and the second direct current voltage terminal (17) are connected to the direct current transmission link (405), wherein the direct current transmission link (405) has two different kinds of conductors (410, 412) differing with respect to their length-related capacitance.

12. The assembly according to claim 11, wherein - the different kinds of conductors are an overhead line (410), a cable (412) and / or a gas-insulated line.

13. The assembly according to claim 12, wherein - the cable is a plastic-insulated cable (412), an oil-insulated cable or a ground-insulated cable.

14. The assembly according to any one of the preceding claims, wherein - the multilevel power converter (3) has two or three phase modules (15, 24, 31), wherein the voltage limiting device is switchable between the phase modules or a dedicated voltage limiting device is associated with each of at least one of the phase modules.

15. A high-voltage direct current transmission installation (400) having an assembly (1) according to any one of claims 1 to 14.

16. A method for polarity-dependent limiting of a voltage in a multilevel power converter (3) which has at least one phase module (31), - wherein the phase module (31) has an alternating current voltage terminal (9), a first direct current voltage terminal (16) and a second direct current voltage terminal (17), - the phase module (31) has a plurality of modules (5_1 ... 5_m, 6_1 ... 6 m) which each have a first electrical module terminal (512) and a second electrical module terminal (515, 615), - the plurality of modules has modules of a first type (5_(n+1) ... 5_m, 6_(n+1) ... 6_m, 500), which are able to output a voltage of only one polarity or zero voltage at their first electrical module terminal (512) and their second electrical module terminal (515), - the plurality of modules has modules of a second type (5_1 ... 5_n, 6_1 ... 6_n, 600), which are able to output a voltage of one polarity, a voltage of opposite polarity or zero voltage at their first electrical module terminal (512) and their second electrical module terminal (615), - the modules of the second type (5_1 ... 5_n, 6_1 ... 6_n, 600) are arranged adjacent to the alternating current voltage terminal (9) of the phase module (31), and the modules of the first type (5_(n+1) ... 5_m, 6_(n+1) ... 6_m, 500) are arranged adjacent to the first direct current voltage terminal (16) and the second direct current voltage terminal (17) of the phase moule (31), wherein, in the method, - with a voltage limiting device (50), only a negative voltage (UVB) occurring across the modules of the second type (5_1 ... 5_n, 6_1 ... 6_n, 600) is limited and a voltage (UVB) of opposite polarity occurring across the modules of the second type is left substantially unchanged, wherein the voltage limiting device (50) comprises at least one semiconductor valve (65_1), in particular a connection in series of semiconductor valves (65_1 ... 65_p).

17. The method according to claim 16, wherein - a current (I) only flows through the voltage limiting device (50) when the negative voltage (UVB) occurs across the modules of the second type (5_1 ... 5_n, 6_1 ... 6_n, 600), thereby limiting the negative voltage.

18. The method according to claim 16 or 17, wherein - the modules of the first type (500) each have a first electronic switching element (502) and a second electronic switching element (506) in a half-bridge circuit, wherein an additional semiconductor valve (520) is arranged connected in parallel with the second electronic switching element (506), and - only when the negative voltage (UVB) occurs across the modules of the second type (5_1 ... 5_n, 6_1 ... 6_n, 600), a part of the current flowing through the modules of the first type (500) will flow through the additional semiconductor valve (520).