Method for operating a modular multi-level converter and modular multi-level converters

Description

[0001] The invention relates to a method for operating a modular multilevel converter and a modular multilevel converter.

[0002] Converters are power electronic circuits for converting electrical energy. Converters can convert alternating current into direct current, direct current into alternating current, alternating current into alternating current of a different frequency and / or amplitude, or direct current into direct current of a different voltage. Converters can comprise a multitude of similar submodules, which can be electrically connected in series. These submodules each contain at least two electronic switching elements and an electrical energy storage device. Such converters are referred to as modular multilevel converters (MMC or M2C for short).

[0003] By electrically connecting the submodules in series, high output voltages can be achieved. The modular multilevel converters are easily adapted (scalable) to different voltages, and a desired output voltage can be generated relatively precisely. Modular multilevel converters are often used in the high-voltage range, for example, as converters in high-voltage direct current (HVDC) transmission systems or as reactive power compensators in flexible three-phase AC transmission systems (FACTS).

[0004] From the international patent application WO 2017 / 125134 A1, a modular multilevel converter is known in which the submodules each have an optical bridging device which optically bridges the respective submodule in the event of a defect in the submodule.

[0005] Patent application CN 110 635 675 A discloses a method for detecting submodules that can no longer be monitored by a control system. These submodules are referred to as black submodules.

[0006] Patent application CN 110 677 029 A discloses a submodule of a modular multilevel converter that features passive overvoltage protection. The submodule terminals are equipped with a parallel circuit consisting of an electronic switch and a bypass switch.

[0007] From patent application CN 107 728 508 B it is known to detect submodules of a modular multilevel converter that are not able to send status information to a monitoring device.

[0008] One of the challenges in modular multilevel converters is how to deal with defective submodules.

[0009] In the event of a failure of a single submodule of the modular multilevel converter, a shutdown of the entire powered system is generally unacceptable. Therefore, the faulty submodule must be brought into a safe state during normal operation and replaced during the next scheduled shutdown of the system.

[0010] To enable continued operation even with defective submodules, all submodules can be equipped with a bypass switch that can bridge the respective submodule. This bypass switch can be installed between the submodule connections (or connection terminals). This allows the system current to flow through the faulty submodule without causing damage to the modular multilevel converter, particularly through arcing.

[0011] One criterion for closing the bypass switch can be that the submodule automatically closes the bypass switch when the capacitor voltage of a capacitor in the submodule exceeds a threshold (this can also be referred to as the bypass switch threshold). This bypasses the submodule and protects it from further charging. The necessary logic can be installed on the submodule and operate independently of the rest of the submodule's control system.

[0012] This trigger criterion allows the majority of fault cases to be managed, as many fault scenarios can lead to the blocking of the faulty submodule either directly (i.e., through the fault occurrence itself) or indirectly (i.e., through a reaction of the converter control system). If the submodule is blocked, no more control signals are sent to the submodule's switching elements, or the submodule's power electronics are shut down.

[0013] If a submodule is blocked, it can only be charged via freewheeling diodes in the submodule's electronic switching elements. Discharge is no longer possible. If the system continues to operate, the submodule will inevitably be charged by the flowing branch current. Blocking a submodule during operation thus closes the bypass switch.

[0014] This self-protection of the submodules also functions in the event of a communication failure between the submodules and a communication device or control device of the modular multilevel inverter. If the affected submodule no longer receives communication telegrams from the communication device, it blocks itself. If the inverter controller no longer receives communication telegrams from the affected submodule, a command is sent to block the submodule.

[0015] Since the bypass switch cannot be fully tested for functionality during operation, its functionality only becomes apparent in the event of a submodule failure. The scenario of a bypass switch failure must therefore be considered. In this case, an undefined current flow through the submodule, i.e., an arc, can be expected, which could damage the system. In the event of a bypass switch failure, the system must be shut down.

[0016] To ensure that the bypass switch is closed, each submodule can have a bypass switch monitoring feature. For the majority of submodule faults, the communication device or control device thus receives feedback from the submodule as to whether the bypass switch is closed or open. However, in the event of a communication failure, this is no longer possible. Consequently, if communication with a submodule fails and the bypass switch fails at the same time, the communication device or control device cannot detect this.

[0017] If the system is not shut down in this case, the system current would continue to charge the affected submodule until an insulation failure occurs at some point. An arc would then form, and the system current would flow undefined across this fault location. Even if such an arc is detected by optical monitoring in a converter hall, the inevitable result is significant damage to the system.

[0018] Even at lower currents, it is to be expected that an arc will escape from the submodule after a few seconds and thus cause damage to the system.

[0019] To safely handle the fault scenario described above, the closure of the bypass switch can be monitored not only by the affected submodule itself, but also by another submodule, such as a neighboring submodule. Two neighboring submodules can form a pair that monitors each other. If a submodule fails with a communication error, the closure of the affected bypass switch is detected by its neighboring submodule and communicated to the communication device or control device.

[0020] In order for such a bypass switch closure monitoring system to reliably detect a bypass switch failure, the temporal charging of the faulty, blocked submodule must be taken into account. Only when the bypass switch threshold is exceeded does it become clear whether the bypass switch in the faulty submodule is functioning.

[0021] If a submodule is blocked and is being charged by the flowing system current, depending on the system's operating point, it may take a very long time for the capacitor voltage to exceed the threshold for closing the bypass switch. For example, if the system is idle or operating at low power, charging may take several tens of seconds. However, waiting too long for charging and the associated closing of the bypass switch is unacceptable, as an arc is likely to escape from the submodule after a few seconds. Therefore, detection of a bypass switch failure must take place within this time.

[0022] To ensure that a bypass switch failure can be detected even at low power or when the system is idle, a minimum system current must be ensured during operation. This ensures a minimal current flow through the submodules, which ensures that the bypass switch threshold is reached after a certain period of time.

[0023] The solution described above for detecting a bypass switch failure requires minimal current flow through the submodules. This current flow causes significant additional losses at low power or idle operation, increasing the system's operating costs.

[0024] The object of the invention is therefore to at least partially reduce the disadvantages of methods known from the prior art for operating modular multilevel converters, in particular to provide an operating method that is particularly cost-effective and enables the detection of a failure of a bypass switch in a particularly reliable manner.

[0025] The above object is achieved by the subject matter of the patent claims, in particular by a method for operating a modular multilevel converter according to claim 1 and by a modular multilevel converter according to claim 8. Further advantages and details of the invention emerge from the subclaims, the description, and the drawings. Features and details disclosed in connection with the method according to the invention naturally also apply in connection with the modular multilevel converter according to the invention, so that reciprocal reference is or can always be made to the individual aspects of the invention with regard to the disclosure.The stated object is achieved according to a first aspect by a method for operating a modular multilevel converter with a plurality of submodules, each of which has at least two electronic switching elements, an electrical energy store, two submodule connections, a bypass switch for bridging its submodule (1, 2, 3, 4, 5, 6) in the modular multilevel converter and a communication element for communicating with a communication device of the modular multilevel converter, the method comprising the steps: . (a) determining that the plurality of submodules includes a defective submodule such that the communication element in the defective submodule does not communicate with the communication device, (b) determining whether a current arm current resulting from an operating point of the modular multilevel converter is below a predetermined threshold, and (c) generating or amplifying an internal circulating current in the modular multilevel converter including the defective submodule if the arm current resulting from the operating point of the modular multilevel converter is below the predetermined threshold of the operating point.

[0026] Consequently, the proposed solution provides for the generation of an internal converter circulating current when needed. This can significantly reduce idle losses. Furthermore, circulating currents can be generated when needed that are significantly larger than the previous minimum system currents, converter currents, or currents in the modular multilevel converter. A defective submodule can thus be charged much more quickly using the proposed method, making bypass switch failure faster to detect and significantly reducing the likelihood of a fault resulting in an arc.

[0027] The operating point and the resulting arm current can be determined using the active power and reactive power relative to a DC side or an AC voltage network of the modular multilevel converter. The arm current is the current flowing through a phase module branch of a phase module of the modular multilevel converter. This arm current consists of a DC component and an AC component. The DC component of the arm current depends on the active power exchange with the DC side. The AC component depends on the power exchange (active and reactive power) with the AC side. In the phase module branch, several submodules are connected in series. The phase module branches are each connected to an AC side and a DC side. Two phase module branches form a phase module.Accordingly, the threshold is determined by the magnitude or amplitude of the minimum arm current flowing through the modular multilevel converter. The threshold can be selected such that the arm current is sufficient to charge the electrical energy storage device in a blocked submodule within a short period of time, thus closing the bypass switch of that submodule within that short period of time.

[0028] The electronic switching elements of the submodules can be designed as turn-off semiconductor valves, in particular as transistors, especially as bipolar transistors with an insulated gate electrode. A semiconductor diode, in particular a freewheeling diode, can be connected antiparallel to each of the electronic switching elements.

[0029] The electrical energy storage devices of the submodules can be designed as capacitors.

[0030] The bypass switches of the submodules can be arranged, in particular, between the two submodule connections of the respective submodule. The submodule connections can be designed as terminal connections.

[0031] Submodules of the plurality of submodules can be connected in series using the submodule connectors. Several submodules connected in series can be combined to form a phase module branch. Any two interconnected phase module branches can be combined to form a phase module.

[0032] The internal circulating current flows in the defective submodule. The internal circulating current can be injected, generated, or amplified, in particular in addition to a current flowing at an operating point of the modular multilevel converter, in particular three-phase current. Furthermore, the converter-internal circulating current can be generated or amplified when the modular multilevel converter is in no-load operation. Internal or converter-internal circulating currents are a current flow between phase modules, in particular the three phase modules, of the modular multilevel converter that does not result in any power exchange with an AC or DC side of the modular multilevel converter.

[0033] It is intended that the defective submodule is blocked before the internal circulating current is generated or amplified in the modular multilevel converter with the defective submodule, whereby the electrical energy storage of the defective submodule is charged using the internal circulating current. When the defective submodule is blocked, the electronic switching elements are no longer controlled or a power electronics of the submodule is switched off. Consequently, the current of the modular multilevel converter according to its operating point and / or the converter-internal circulating current flows through the defective submodule, in particular via the freewheeling diodes, to the electrical energy storage and charges it. The blocking can be triggered directly by the occurrence of the defect in the submodule itself or indirectly by a reaction of a control device of the modular multilevel converter.

[0034] It is also provided that the bypass switch of the blocked, defective submodule is closed to bypass the blocked, defective submodule in the modular multilevel converter when the electrical energy storage device of the blocked, defective submodule has reached a predetermined charge level via the internal circulating current. The predetermined charge level of the electrical energy storage device can be a maximum charge level of the electrical energy storage device or correspond to the maximum voltage of the electrical energy storage device. Consequently, this is considered a simple trigger criterion for closing the bypass switch when a defective submodule is blocked.

[0035] It can be provided that, when determining whether the arm current of the modular multilevel converter resulting from the operating point is below the predetermined threshold value of the operating point, an equivalent charging current of the electrical energy storage device in the defective submodule is used. Using the equivalent charging current, a statement can be made about how much of the arm current flowing at the operating point reaches the electrical energy storage device as equivalent charging current when the submodule is blocked. The equivalent charging current can be determined approximately.

[0036] For a modular multilevel converter with half-bridge modules as submodules, the equivalent DC charging current i DC_BPS can be approximately determined using the following formula: i DC _ BPS = i sec _ eff π ⋅ 2 + i d 6 where i sec_eff is the effective value of a secondary AC current of the modular multilevel converter and id is a current DC current (with sign).

[0037] For a modular multilevel converter with full-bridge modules as submodules, the equivalent DC charging current i DC_BPS can be approximately determined using the following formula: i DC _ BPS = i sec _ eff ⋅ 2 π where i sec_eff is the effective value of a secondary AC current of the modular multilevel converter.

[0038] Alternatively or additionally, the arm current resulting from the operating point of the modular multilevel converter can be looked up in a lookup table to determine whether the arm current of the modular multilevel converter resulting from the operating point is below the predetermined arm current threshold and, in particular, to determine the magnitude, frequency, and / or phase position of the internal circulating current. For this purpose, the lookup table can store a magnitude, frequency, and / or phase position of the internal circulating current for each arm current resulting from the operating point of the modular multilevel converter (in particular based on the PQ diagram).

[0039] It is provided that each submodule of the plurality of submodules is communicatively connected to a bypass switch of another submodule of the plurality of submodules by means of a respective communication element in order to monitor the state of a bypass switch of each submodule of the plurality of submodules by means of a respective other submodule, wherein the generation or amplification of the circulating current in the modular multilevel converter takes place with the defective submodule if, furthermore, the communication element of a submodule monitoring the defective submodule, which is communicatively connected to the bypass switch of the defective submodule, communicates with the communication device. The state of the bypass switch can be open or closed accordingly. When the bypass switch is open, current flows through the defective submodule. When the bypass switch is closed, the defective submodule is bypassed.The communication connections can be implemented, for example, using optical fibers. In particular, submodules in a pair of submodules can monitor each other. Consequently, one submodule of the pair can be interconnected via a communication element to the bypass switch of the other submodule of the pair, and vice versa. The submodules of the pair of submodules can be adjacent submodules to shorten the communication paths.

[0040] It is further provided that the generation or amplification of the internal circulating current in the modular multilevel converter with the defective submodule occurs if the communication element of the submodule monitoring the defective submodule communicates to the communication device that the bypass switch of the defective submodule is open. In this case, it is not yet clear whether the bypass switch is functional or not, since the electrical energy storage device may not yet be sufficiently charged to close the bypass switch. In order to close the bypass switch as quickly as possible and to ensure that the bypass switch is functional, the converter-internal circulating current must be generated or amplified if the arm current resulting from the operating point of the modular multilevel converter is below the specified threshold.

[0041] Furthermore, it is also preferred that, after a predetermined period of flow of the generated or amplified internal circulating current in the modular multilevel converter with the defective submodule, the submodule monitoring the defective submodule checks whether the bypass switch in the defective submodule is closed. After the predetermined period of flow of the internal circulating current, the bypass switch should be closed. If this is the case, the bypass switch has functioned, so the defective submodule has been bypassed and can be replaced during the next downtime or maintenance.

[0042] In this case, it is again preferred that the specified time period be set depending on the current and / or voltage of the generated or amplified internal circulating current. The time period can be selected depending on the current and / or voltage to optimize power consumption, safety, and reliability.

[0043] It is also preferred that the modular multilevel converter with the defective submodule be shut down if, after the specified time period has elapsed, the submodule monitoring the defective submodule communicates to the communication device via its communication element that the bypass switch of the defective submodule is open. If the bypass switch remains open, this indicates that the bypass switch is defective, so the shutdown of the modular multilevel converter must now be initiated to prevent damage that could otherwise occur.

[0044] Furthermore, it is preferred that the magnitude, frequency, and / or phase position of the internal circulating current in the modular multilevel converter with the defective submodule be adjusted depending on the arm current resulting from the operating point of the modular multilevel converter. The aforementioned lookup table can be used for this purpose. Consequently, the energy consumption of the modular multilevel converter can be optimized by adjusting the degrees of freedom of the magnitude, frequency, and / or phase position of the internal circulating current. For example, if the modular multilevel converter is running in idle mode, a high internal circulating current is generated.However, if the modular multilevel converter runs, for example, with a partial load below the predetermined threshold value for the arm current, a lower internal circulating current is generated, since the total current flowing is then still sufficiently high that damage to the modular multilevel converter can be prevented with similar or equal reliability.

[0045] It is preferred that the threshold value (of the arm current) is or becomes predetermined such that closing the bypass switch in the blocked, defective submodule takes a maximum of 10 seconds, in particular a maximum of 7 seconds, when the modular multilevel converter is operating at the threshold value. This allows a defect in the bypass switch to be detected even before an arc typically forms in the defective submodule, which would otherwise lead to damage to the modular multilevel converter.

[0046] It is also preferred that the submodules be designed as half-bridge modules and / or as full-bridge modules. It is possible for the modular multilevel converter to have only half-bridge modules, only full-bridge modules, or both half-bridge modules and full-bridge modules.

[0047] The object stated at the outset is achieved according to a second aspect by a modular multilevel converter according to the features of the first aspect of the invention, wherein the modular multilevel converter is configured to carry out the method according to the first aspect of the invention.

[0048] Accordingly, the modular multilevel converter has a plurality of submodules, each of which has at least two electronic switching elements, an electrical energy storage device, two submodule connections, a bypass switch for bridging it in the modular multilevel converter and a communication element for communicating with a communication device of the modular multilevel converter.

[0049] In particular, the modular multilevel converter can further comprise a control device. The control device can comprise the communication device or be connected to it via communication technology. The control device can be configured to execute the operations of the modular multilevel converter, for example, setting the operating point of the modular multilevel converter, shutting down the modular multilevel converter, but also blocking a defective submodule and generating or amplifying the internal circulating current in the modular multilevel converter. Consequently, the control device can be configured, in particular, to carry out the method according to the first aspect of the invention.

[0050] Further measures improving the invention will become apparent from the following description of various exemplary embodiments of the invention, which are schematically illustrated in the figures. All features and / or advantages apparent from the claims, the description, or the figures, including structural details and spatial arrangements, may be essential to the invention both individually and in various combinations.

[0051] The invention is explained in more detail below with reference to the accompanying drawings. In the drawings: Figure 1 is a schematic view of a circuit diagram of a modular multilevel converter according to an embodiment of the invention, Figure 2 is a schematic view of a half-bridge module in the modular multilevel converter from Figure 1 , and Figure 3 is a schematic view of a full-bridge module in the modular multilevel converter of Figure 1 .

[0052] Elements with the same function and mode of action are listed in the Figures 1 to 3 Each is provided with the same reference numerals. Multiple identical elements in a figure are provided with consecutive numbering, separated from their reference symbol by a dot or underscore.

[0053] Figure 1 shows a schematic view of a circuit diagram of a modular multilevel converter 10 according to an embodiment of the invention.

[0054] The modular multilevel converter 1 has a first AC voltage terminal 7, a second AC voltage terminal 8, and a third AC voltage terminal 9. The first AC voltage terminal 7 is electrically connected to a first phase module branch 41 and a second phase module branch 42. The first phase module branch 41 and the second phase module branch 42 form a first phase module 51 of the modular multilevel converter 10.

[0055] The end of the first phase module branch 41 facing away from the first AC voltage terminal 7 is electrically connected to a first DC voltage terminal 47. The end of the second phase module branch 42 facing away from the first AC voltage terminal 7 is electrically connected to a second DC voltage terminal 48. The first DC voltage terminal 47 is a positive DC voltage terminal, and the second DC voltage terminal 48 is a negative DC voltage terminal.

[0056] The second AC voltage terminal 8 is electrically connected to one end of a third phase module branch 43 and to one end of a fourth phase module branch 44. The third phase module branch 43 and the fourth phase module branch 44 form a second phase module 52.

[0057] The third AC voltage terminal 9 is electrically connected to one end of a fifth phase module branch 45 and to one end of a sixth phase module branch 46. The fifth phase module branch 45 and the sixth phase module branch 46 form a third phase module 53.

[0058] The end of the third phase module branch 43 facing away from the second AC voltage connection 8 and the end of the fifth phase module branch 45 facing away from the third AC voltage connection 9 are electrically connected to the first DC voltage connection 47. The end of the fourth phase module branch 44 facing away from the second AC voltage connection 8 and the end of the sixth phase module branch 46 facing away from the third AC voltage connection 9 are electrically connected to the second DC voltage connection 48.

[0059] Each phase module branch 41, 42, 43, 44, 45, 46 has a plurality of submodules (1 1, 1_2, 1_3, ... 1_n; 2 1 ... 2_n, etc.), which, in particular by means of their submodule connections 11.1, 11.2 (see Fig. 2 and 3 ), are electrically connected in series. In each of the phase module branches 41, 42, 43, 44, 45, 46, an arm current resulting from the operating point of the modular multilevel converter 1 flows. The operating point and the resulting arm current are determined from the active power and reactive power across the DC voltage terminals 47, 48 and the AC voltage terminals 7, 8, 9 of the modular multilevel converter 1.

[0060] In this exemplary embodiment, each phase module branch 41, 42, 43, 44, 45, 46 has n submodules 1_1 to 6_n. The number of submodules 1_1 to 6_n electrically connected in series can vary; at least two submodules 1_1 to 6_n are connected in series, but at least 50 or at least 100 submodules 1_1 to 6_n can also be electrically connected in series, for example. In this exemplary embodiment, n = 36. The first phase module branch 41 therefore has 36 submodules 1_1, 1_2, 1_3, ... 1_36.

[0061] In the left area of ​​the Fig. 1 A communication device 30 of the modular multilevel converter 10 for the submodules 1_1 to 6_n is shown schematically. The communication device 30 can also be designed as a control device, or a control device can comprise the communication device 30. Optical messages are transmitted from the communication device 30 to the individual submodules 1_1 to 6_n.

[0062] The message transmission between the control device 30 and a submodule 1_1 to 6_n is represented by a dashed line 33, with the direction of the message transmission symbolized by the arrowhead on the dashed lines 33. In this case, communication takes place via first optical fibers 33.1, 33.12, 33.3, 33.4, 33.5, 33.6, which are shown by the dashed lines. The communication device 30 sends optical messages to the submodules 1_1 to 6_n via second communication outputs 31.1, 31.2, 31.3, in this case optical outputs, and receives optical messages from the individual submodules via second communication inputs 32.1, 32.2, 32.3, in this case optical inputs. This is illustrated using the example of the submodules 1_1, 1_n, and 4_3; Optical messages are sent to the other submodules 1_1 to 6_n in the same way or received by these submodules 1_1 to 6_n.

[0063] Figure 2 shows a schematic view of a submodule 1_1 designed as a half-bridge module in the modular multilevel converter from Fig. 1 .

[0064] This can, for example, be the submodule 1_1 of the first phase module branch 41 (or one of the other Fig. 1 The submodule 1_1 is configured as a half-bridge module 1_1. The submodule 1_1 has a first electronic switching element 12.1 in the form of a turn-off semiconductor valve with a first antiparallel-connected diode 13.1. Furthermore, the submodule 1_1 has a second electronic switching element 12.2 in the form of a turn-off semiconductor valve with a second antiparallel-connected diode 13.2, as well as an electrical energy storage device 14 in the form of a capacitor. The electronic switching elements 12.1, 12.2 are each configured as bipolar transistors with an insulated gate electrode.

[0065] The first electronic switching element 12.1 is electrically connected in series with the second electronic switching element 12.2. A first submodule connection 11.1 is arranged at a connection point between the two electronic switching elements 12.1, 12.2. A second submodule connection 11.2 is arranged at the connection of the second electronic switching element 12.2, which is opposite this connection point. The second submodule connection 11.2 is further connected to a first connection of the electrical energy storage device 14. A second connection of the energy storage device 14 is electrically connected to the connection of the first submodule connection 12.1, which is opposite the connection point.

[0066] The energy storage device 14 is thus electrically connected in parallel to the series connection of the electronic switching elements 12.1, 12.2. By appropriately controlling the electronic switching elements 12.1, 12.2 by a control circuit (not shown) with a sub-module-internal communication element 20, it can be achieved that between the first sub-module connection 11.1 and the second sub-module connection 11.2 either the voltage of the energy storage device 14 is output or no voltage is output (i.e., a zero voltage is output). Through the interaction of the submodules of the individual phase module branches 41, 42, 43, 44, 45, 46, a desired output voltage of the modular multilevel converter 10 can be generated.

[0067] A bypass switch 15 is located between the submodule terminals 11.1 and 11.2. When the bypass switch 15 is closed, the current flows through the closed bypass switch 15 instead of to the electronic switching elements 12.1 and 12.2 and the electrical energy storage device 14.

[0068] For communication outside the submodule, the submodule 1_1 has a first communication input 21, in this case an optical input, and a first communication output 22, in this case an optical output. The communication element 20 is connected to the first communication input 21 and the first communication output 22. For communication outside the submodule, first optical fibers 33.1, 33.2 are connected to the first communication input 21 and the second communication output 22, each of which is connected to one of the second communication outputs 31 and the second communication input 32. This allows the control circuit or the communication element 20 to detect states of the submodule 1_1 and report them to the communication device 30.

[0069] The first communication input 21 of submodule 1_1 forwards the incoming messages to the communication element 20. The communication element 20 outputs (changed or unchanged) messages to the first communication output 22 of submodule 1_1.

[0070] Furthermore, the electronic switching elements 12.1, 12.2 are connected to the communication element 20 by means of second optical fibers 23.1, 23.1, for example to transmit a status of the electronic switching elements 12.1, 12.2 (blocked or not blocked) to the communication device 30 or to receive a message from the communication device 30 that the electronic switching elements 12.1, 12.2 should be blocked.

[0071] Submodule 1_1 is communicatively connected to the electrical energy storage device 14 via the third optical fiber 24. This allows the control circuit or communication element 20 to detect the charge state of the electrical energy storage device 14 and report it to the communication device 30.

[0072] Furthermore, the bypass switch 15 is connected to the communication element 20 via a fourth optical fiber 25. This allows the communication element 20 to change and / or query the state of the bypass switch 15, i.e., whether it is closed or open. In particular, the bypass switch 15 can be closed when the submodule 1_1 is blocked and the electrical energy storage device 14 is fully charged.

[0073] In addition, a fifth optical fiber 26 leads from the bypass switch 15 to the outside of submodule 1_1. This fifth optical fiber 26 leads to a communication element 20 of a submodule 1_2 adjacent to submodule 1_1. Using the adjacent submodule 1_2, it can be checked whether the bypass switch 15 is closed or not, even if communication between the communication device 30 and the communication element 20 of submodule 1_1 fails, as long as the communication between the communication device 30 and the communication element 20 of the adjacent submodule 1_2 is functioning.

[0074] Figure 3 shows a schematic view of a full-bridge module 1_1 in the modular multilevel converter from Fig. 1 .

[0075] The submodule 1_1, designed as a full-bridge module, has, compared to the half-bridge module, Fig. 2four electronic switching elements 12.1, 12.2, 12.3, 12.4, and four diodes 13.1, 13.2, 13.3, 13.4. It is characterized by the fact that, with appropriate control of the four electronic switching elements 12.1, 12.2, 12.3, 12.4, between the first submodule connection 11.1 and the second submodule connection 11.2, either the positive voltage of the energy storage device 14, the negative voltage of the energy storage device 14, or a voltage of zero (zero voltage) can be output. Thus, the polarity of the output voltage can be reversed using the full-bridge module. The modular multilevel converter 10 made of Fig. 1 can have either only half-bridge modules, only full-bridge modules or also half-bridge modules and full-bridge modules. Reference symbol

[0076] 1 first submodule 2 second submodule 3 third submodule 4 fourth submodule 5 fifth submodule 6 sixth submodule 7 first AC voltage connection 8 second AC voltage connection 9 third AC voltage connection 10 modular multilevel converter 11 submodule connection 12 electronic switching element 13 diode 14 electrical energy storage device 15 bypass switch 20 communication element 21 first communication input 22 first communication output 23 second optical fiber 24 third optical fiber 25 fourth optical fiber 26 fifth optical fiber 30 communication device 31 second communication output 32 second communication input 33 first optical fiber 41 first phase module branch 42 second phase module branch 43 third phase module branch 44 fourth phase module branch 45 fifth phase module branch 46 sixth phase module branch 47First DC voltage connection 48Second DC voltage connection 51First phase module 52Second phase module 53Third phase module

Claims

1. Method for operating a modular multilevel converter (10) comprising a plurality of submodules (1, 2, 3, 4, 5, 6), each of which has at least two electronic switching elements (12), an electrical energy store (14), two submodule connections (11), a bypass switch (15) for bridging its submodule (1, 2, 3, 4, 5, 6) in the modular multilevel converter (10), and a communication element (20) for communication with a communication device (30) of the modular multilevel converter (10), the method comprising the following steps: (a) ascertaining that the plurality of submodules (1, 2, 3, 4, 5, 6) have a submodule (1, 2, 3, 4, 5, 6) which is defective such that the communication element (20) in the defective submodule (1, 2, 3, 4, 5, 6) does not communicate with the communication device (30), characterized by (b) determining whether a present arm current resulting from an operating point of the modular multilevel converter (10) is below a predetermined threshold value, (c) generating or amplifying an internal circulating current in the modular multilevel converter (10) with the defective submodule (1, 2, 3, 4, 5, 6) if the arm current resulting from the operating point of the modular multilevel converter (10) is below the predetermined threshold value, the internal circulating current flowing via the electrical energy store (14) of the defective submodule (1, 2, 3, 4, 5, 6), the defective submodule (1, 2, 3, 4, 5, 6) being blocked before generating or amplifying the internal circulating current in the modular multilevel converter (10), as a result of which the electrical energy store (14) in the defective submodule (1, 2, 3, 4, 5, 6) is charged by means of the internal circulating current, and the bypass switch (15) of the blocked defective submodule (1, 2, 3, 4, 5, 6) being closed in order to bridge the blocked defective submodule (1, 2, 3, 4, 5, 6) in the modular multilevel converter (10) if the electrical energy store (14) of the blocked defective submodule (1, 2, 3, 4, 5, 6) has reached a predetermined charging by means of the internal circulating current, and (d) monitoring the closing of the bypass switch not only by the relevant submodule itself, but also by another submodule, in particular by a neighboring submodule, each submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) being connected to a bypass switch (15) of another submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) in terms of communication technology by means of a respective communication element (20) in order in each case to monitor the state of a bypass switch (15) of each submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) by means of another submodule (1, 2, 3, 4, 5, 6) in each case, generating or amplifying the circulating current in the modular multilevel converter (10) with the defective submodule (1, 2, 3, 4, 5, 6) being effected if furthermore the communication element (20) of a submodule (1, 2, 3, 4, 5, 6) which monitors the defective submodule (1, 2, 3, 4, 5, 6) and which is connected to the bypass switch (15) of the defective submodule (1, 2, 3, 4, 5, 6) in terms of communication technology communicates to the communication device (30) the fact that the bypass switch (15) of the defective submodule (1, 2, 3, 4, 5, 6) is open.

2. Method according to Claim 1, characterized in that after a predefined time duration of the flowing of the generated or amplified internal circulating current in the modular multilevel converter (10) with the defective submodule (1, 2, 3, 4, 5, 6), by means of the submodule (1, 2, 3, 4, 5, 6) which monitors the defective submodule (1, 2, 3, 4, 5, 6), a check is made to establish whether the bypass switch (15) in the defective submodule is closed.

3. Method according to Claim 2, characterized in that the predefined time duration is set depending on the current intensity of the generated or amplified converter-internal circulating current.

4. Method according to Claim 2 or 3, characterized in that the modular multilevel converter (10) with the defective submodule (1, 2, 3, 4, 5, 6) is switched off if the submodule (1, 2, 3, 4, 5, 6) which monitors the defective submodule (1, 2, 3, 4, 5, 6), after the predefined time duration has elapsed, communicates by means of its communication element (20) to the communication device (30) the fact that the bypass switch (15) of the defective submodule (1, 2, 3, 4, 5, 6) is open.

5. Method according to any of the preceding claims, characterized in that a magnitude, frequency and / or phase angle of the internal circulating current in the modular multilevel converter (10) with the defective submodule (1, 2, 3, 4, 5, 6) are / is set depending on the arm current resulting from the operating point of the modular multilevel converter (10).

6. Method according to any of the preceding claims, characterized in that the threshold value of the arm current resulting from the operating point has been or is predetermined in such a way that closing of the bypass switch (15) in the blocked defective submodule (1, 2, 3, 4, 5, 6) has a duration of at most 10 seconds when the modular multilevel converter (10) is operated at the threshold value.

7. Method according to any of the preceding claims, characterized in that the submodules (1, 2, 3, 4, 5, 6) are embodied as half-bridge modules and / or as full-bridge modules.

8. Modular multilevel converter (10) according to the features of any of the preceding claims, wherein the modular multilevel converter (10) is configured such that it carries out a method according to any of the preceding claims.