MODULAR MULTILEVEL POWER CONVERTER

Description

[0001] The invention relates to a modular multilevel power converter and a method for monitoring such a power converter.

[0002] Power converters are power electronic circuits for converting electrical energy. Power 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. Power converters can comprise a large number of similar modules connected electrically in series. These modules are also referred to as submodules. These modules each comprise at least two electronic switching elements and an electrical energy storage device. One such power converter is known, for example, from published patent application JP 2015 130743 A. Such power converters are referred to as modular multilevel power converters and belong to the VSC (voltage sourced converter) type of power converter. High output voltages can be achieved by electrically connecting the modules in series.The converters are easily adaptable (scalable) to different voltages, and a desired output voltage can be generated relatively precisely. These converters are often used in the high-voltage range, for example, as converters in high-voltage direct current transmission systems or as reactive power compensators in flexible three-phase transmission systems.

[0003] To ensure continued operation of the power converter even if a defect occurs in one (or multiple) modules, the modules each have a switch that, when closed, electrically bridges (short-circuits) the module. Modules with such switches are known from published patent application WO 2015 / 133365 A1. Instead of the bridged module, a backup module can then be used to operate the power converter; therefore, the power converter often has multiple backup modules.

[0004] Modules with such switches are also known from document CN 103 941 073 A.

[0005] Published patent application US 2016 / 056710 A1 discloses a power converter whose modules can be bridged with a mechanical switch. The mechanical switch has a movable switching element. The modules contain a monitoring device that uses an optical fiber to monitor the position of the movable switching element and thus the switching position of the switch.

[0006] The international patent application WO 2011 / 154047 A1 discloses the possibility of using a measuring sensor to monitor the switching state of a mechanical switch, which detects a switching current flowing through the contacts of the switch.

[0007] The switch for electrically bypassing the module is also called a bypass switch or a bypass switch. The defective module is electrically bypassed (short-circuited) using the switch until it is replaced by a functioning module during converter maintenance. Depending on the time interval between maintenance visits, it may be necessary to electrically bypass a module for an extended period of time using the switch.

[0008] It may happen that the switch for bridging the faulty module is also faulty, i.e.that the switch is not functioning properly. Such a switch fault could be, for example, that the switching contact becomes jammed during the closing process, or that the switch does not close at all, even though a closing command has been sent to the switch. The latter can happen, for example, in the event of an electronic failure. As a result, no current or only a small current is passed through the defective module, meaning that the switch's protective function is not provided or is only partially provided.

[0009] If the module's electrical bypass fails to function properly, current may continue to flow through the defective module, causing a total failure of the power converter (e.g., due to a fire). It's also possible that the defective module interrupts the current flow, causing the power converter to fail.

[0010] The invention is based on the object of specifying a multilevel power converter and a method with which a failure of the multilevel power converter can be avoided.

[0011] This object is achieved according to the invention by a multilevel power converter and by a method according to the independent patent claims. Advantageous embodiments of the multilevel power converter and the method are specified in the respective dependent patent claims.

[0012] A modular multilevel power converter is disclosed, having at least one arrangement comprising a module of the modular multilevel power converter, wherein the arrangement has a switch which, in its closed switching state, electrically bridges the module and an (electrical) measuring sensor for measuring a physical quantity which occurs at the switch and which assumes different values depending on whether the switch is in its open switching state (open switch) or in its closed switching state (closed switch). It is advantageous that the arrangement has the electrical measuring sensor. The physical quantity can be measured using this measuring sensor. The physical quantity allows a conclusion to be drawn as to whether the switch (at the time of measurement) is in its open switching state or in its closed switching state, i.e., whether the switch is open or closed.The arrangement can therefore be used to monitor the proper functioning of the switch.

[0013] The arrangement is designed so that the physical quantity is an electrical voltage. This physical quantity (voltage) allows a conclusion to be drawn as to whether the switch is open or closed.

[0014] The arrangement is designed such that the measuring sensor is a voltage measuring sensor connected in parallel with the switch. The voltage measuring sensor is connected in parallel with the switch. The voltage measuring sensor can be used to determine whether the switch is in its open or closed switching state.

[0015] The arrangement can be designed so that the switch is a mechanical switch. A mechanical switch is advantageous here because large currents can flow through a mechanical switch even over long periods with low losses.

[0016] The arrangement can be designed such that the module has at least two electronic switching elements and an electrical energy storage device.

[0017] The arrangement is designed such that the two electronic switching elements of the module are arranged in a half-bridge circuit (such a module is also referred to as a half-bridge module or a half-bridge sub-module) or the arrangement is designed such that the module has the two electronic switching elements and two further electronic switching elements, wherein the two electronic switching elements and the two further electronic switching elements are arranged in a full-bridge circuit (such a module is also referred to as a full-bridge module or a full-bridge sub-module).

[0018] The modular multilevel power converter preferably comprises a plurality of the arrangements described above, for example at least 24 such arrangements.

[0019] Also disclosed is a high-voltage direct current transmission system with a modular multilevel converter according to the invention.

[0020] Furthermore, a method is disclosed for monitoring a modular multilevel power converter which has at least one phase module branch with a plurality of modules, each of which is provided with an electrical switch which, in its closed switching state, electrically bridges the respective module, wherein in the method a physical quantity is measured using an (electrical) measuring sensor to form a measured value of the physical quantity, whereby the physical quantity occurs at the switch and assumes different values depending on whether the switch is in its open switching state or in its closed switching state, and the measured value is used to determine whether the switch is in its open switching state or in its closed switching state (at the time the measured value is formed). In this method, the measured value of the physical quantity is used to determine whether the switch is in its open switching state or in its closed switching state (at the time the measured value is measured). The measured value therefore provides feedback as to whether the switch is in its open switching state (open switch) or in its closed switching state (closed switch).

[0021] The procedure is such that The physical quantity measured is the voltage across the switch, forming a switch voltage measurement value. The closed switching state is detected when the switch voltage measurement value falls below a (predetermined) minimum switch voltage value (and the open switching state is detected when the switch voltage measurement value exceeds the minimum switch voltage value). The physical quantity measured and evaluated is the voltage across the switch (i.e., the voltage present across the switch).

[0022] The method can also be implemented in such a way that, once the switch's closed state has been determined, the physical quantity is repeatedly measured at intervals, each time generating a measured value. For each measured value, it is determined whether the switch (at the time the measured value is generated) is in its open state or in its closed state. This method can advantageously be used to determine whether the switch maintains the closed state even over longer periods.

[0023] The method works like this: when the switch is in the closed state, switch voltage measurements are repeatedly taken at time intervals. These switch voltage measurements are then evaluated to determine whether the ohmic contact resistance of the switch is increasing. This method can advantageously be used to determine whether the ohmic contact resistance of the switch is increasing. An increasing ohmic contact resistance of the switch is an indication that the electrical contact of the closed switch is becoming increasingly poor. As a result, the bridging of the module functions increasingly poorly and unwanted electrical losses occur at the ohmic contact resistance. An increasing ohmic contact resistance of the switch is therefore an indication that the switch is increasingly poorly performing its bridging function and should therefore be repaired or replaced.

[0024] The procedure can also be carried out in such a way that It is recognized that the ohmic contact resistance of the switch increases as the switch voltage measurements increase.

[0025] The method is designed such that the module has at least two electronic switching elements and an electrical energy storage device.

[0026] The method is designed such that the two electronic switching elements of the module are arranged in a half-bridge circuit (half-bridge module) or that the module has the two electronic switching elements and two further electronic switching elements, wherein the two electronic switching elements and the two further electronic switching elements are arranged in a full-bridge circuit (full-bridge module).

[0027] The method can be configured such that a switch-open signal is output when the switch of a defective module is in its open state. Such a switch-open signal can, for example, be a warning signal or can be sent as an additional input to a control unit of the power converter. The control unit of the power converter then reacts to the switch being in its open state, even though it should be in its closed state (due to the defective module). The control unit then reduces the output current of the power converter. This prevents overheating damage to the defective module, fires, etc.

[0028] The arrangement and the method have similar advantages; some of these are mentioned above only in relation to the arrangement and some only in relation to the method.

[0029] In the following description, the term "embodiment" is also used for objects or methods that do not fall within the scope of the claims. The Figures 6, 7 , 8, 9 and 11 The examples shown are not within the scope of the patent claims; they serve to understand the invention.

[0030] In the following, the invention is explained in more detail using exemplary embodiments. The same reference numerals refer to the same or similarly acting elements. Figure 1 shows an embodiment of a power converter having a plurality of modules, Figure 2 shows an embodiment of a module, Figure 3 shows a further embodiment of a module, Figure 4 shows an embodiment of a high-voltage direct current transmission system, Figure 5 shows an embodiment of a power converter as a reactive power compensator, Figure 6 shows an embodiment of an arrangement with a current measuring sensor, Figure 7 shows an embodiment of an arrangement with two current measuring sensors, Figure 8 shows an embodiment of an arrangement with a magnetic field measuring sensor, Figure 9 shows an embodiment of an arrangement with two magnetic field measuring sensors, Figure 10 shows an embodiment of an arrangement with a voltage measuring sensor, Figure 11 shows an exemplary method sequence, and Figure 12 shows a further exemplary method sequence shown.

[0031] Fig. 10reflects the subject matter of the independent claims.

[0032] In Figure 11 shows a power converter 1 in the form of a modular multilevel converter (MMC). This multilevel converter 1 has a first AC voltage connection 5, a second AC voltage connection 7, and a third AC voltage connection 9. The first AC voltage connection 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 1. The end of the first phase module branch 11 facing away from the first AC voltage connection 5 is electrically connected to a first DC voltage connection 16; the end of the second phase module branch 13 facing away from the first AC voltage connection 5 is electrically connected to a second DC voltage connection 17.The first DC voltage terminal 16 is a positive DC voltage terminal; the second DC voltage terminal 17 is a negative DC voltage terminal.

[0033] The second AC voltage connection 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 voltage connection 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.

[0034] The end of the third phase module branch 18 facing away from the second AC voltage connection 7 and the end of the fifth phase module branch 27 facing away from the third AC voltage connection 9 are electrically connected to the first DC voltage connection 16. The end of the fourth phase module branch 21 facing away from the second AC voltage connection 7 and the end of the sixth phase module branch 29 facing away from the third AC voltage connection 9 are electrically connected to the second DC voltage connection 17.

[0035] Each phase module branch comprises a plurality of modules (1_1, 1_2, 1_3, 1_4; 2_1 ... 2_4; etc.) that are electrically connected in series (via their galvanic current connections). Such modules are also referred to as submodules. In the exemplary embodiment of the Figure 1Each phase module branch has four modules (converter modules). The number of modules electrically connected in series (via their galvanic current connections) can vary greatly; at least three modules are connected in series, but 50, 100, or more modules can also be connected in series.

[0036] Each module is assigned a switch (bypass switch) that can bridge (short-circuit) the module (in the event of a module defect). For example, the first module 1_1 is assigned a first switch S1_1. When a defect occurs in the first module 1_1, the first switch S1_1 is controlled by a module-internal switch control device. The first switch then closes; that is, it changes from the open switching state to the closed switching state.

[0037] In Figure 2The basic structure of a module 201 is shown as an example. This can be, for example, module 1_1 of the first phase module branch 11 (or one of the others in Figure 1shown modules). The module is designed as a half-bridge module 201. The module 201 has a first semiconductor valve 202 that can be switched on and off and has a first anti-parallel-connected diode 204. Furthermore, the module 201 has a second semiconductor valve 206 that can be switched on and off and has a second anti-parallel-connected diode 208 and an electrical energy store 210 in the form of a capacitor 210. The first semiconductor valve 202 that can be switched off is a first electronic switching element 202; the second semiconductor valve 206 that can be switched off is a second electronic switching element 206. The first semiconductor valve 202 and the second semiconductor valve 206 are each designed as an IGBT (insulated-gate bipolar transistor). The first switchable semiconductor valve 202 is electrically connected in series with the second switchable semiconductor valve 206.A first galvanic module connection 212 is arranged at the connection point between the two semiconductor valves 202 and 206. A second galvanic module connection 215 is arranged at the connection of the second semiconductor valve 206, which is opposite the connection point. The second module connection 215 is further connected to a first connection of the energy storage device 210; a second connection of the energy storage device 210 is electrically connected to the connection of the first semiconductor valve 202, which is opposite the connection point.

[0038] The energy storage device 210 is thus electrically connected in parallel to the series circuit comprising the first semiconductor valve 202 and the second semiconductor valve 206. By appropriately controlling the first semiconductor valve 202 and the second semiconductor valve 206 by an internal module electronic control circuit 220, it can be achieved that between the first galvanic module connection 212 and the second galvanic module connection 215, either the voltage of the energy storage device 210 is output or no voltage is output (i.e., a zero voltage is output). Through the interaction of the modules of the individual phase module branches, the respective desired output voltage of the power converter can be generated.

[0039] In Figure 3A further embodiment of a module 301 is shown. This module 301 can, for example, be module 1_2 (or one of the other modules shown in Figure 1). In addition to the module already shown in Figure 2 known first semiconductor valve 202, second semiconductor valve 206, first diode 204, second diode 208 and energy storage 210, the Figure 3 The module 301 shown has a third turn-off semiconductor valve 302 with an antiparallel connected third diode 304 and a fourth turn-off semiconductor valve 306 with a fourth antiparallel connected diode 308. The third turn-off semiconductor valve 302 is a third electronic switching element 302; the fourth turn-off semiconductor valve 306 is a fourth electronic switching element 306. The third turn-off semiconductor valve 302 and the fourth turn-off semiconductor valve 306 are each designed as an IGBT. In contrast to the circuit of the Figure 2 the second galvanic module connection 315 is not electrically connected to the second semiconductor valve 206, but to a center point of an electrical series circuit comprising the third semiconductor valve 302 and the fourth semiconductor valve 306.

[0040] Module 301 of the Figure 3is a so-called full-bridge module 301. This full-bridge module 301 is characterized by the fact that, with appropriate control of the four semiconductor valves between the first galvanic module connection 212 and the second galvanic module connection 315, either the positive voltage of the energy storage device 210, the negative voltage of the energy storage device 210, 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 301. The power converter 1 can have either only half-bridge modules 201, only full-bridge modules 301, or also half-bridge modules 201 and full-bridge modules 301. Instead of the described half-bridge module or the described full-bridge module, the module can also be another multi-level module, for example a three-level voltage inverter module or a five-level voltage inverter module.

[0041] In Figure 4 An exemplary embodiment of a high-voltage direct current transmission system 401 is shown schematically. This high-voltage direct current transmission system 401 has two power converters 1, as shown in Figure 1 are shown. These two power converters 1 are electrically connected to one another on the DC side via a high-voltage direct current connection 405. The two positive DC voltage terminals 16 of the power converters 1 are electrically connected to one another by means of a first high-voltage direct current line 405a; the two negative DC voltage terminals 17 of the two power converters 1 are electrically connected to one another by means of a second high-voltage direct current line 405b. By means of such a high-voltage direct current transmission system 401, electrical energy can be transmitted over long distances; the high-voltage direct current connection 405 then has a corresponding length.

[0042] In Figure 5 An embodiment of a power converter 501 is shown, which serves as a reactive power compensator 501. This power converter 501 has only the three phase module branches 11, 18, and 27, which form three phase modules 505, 507, and 509 of the power converter. The number of phase modules 505, 507, and 509 corresponds to the number of phases of an AC voltage network 511 to which the power converter 501 is connected.

[0043] The three phase module branches 11, 18, and 27 are connected to each other in a star configuration. The end of each of the three phase module branches opposite the star point is electrically connected to a phase line 515, 517, and 519 of the three-phase AC voltage network 511. (In another embodiment, the three phase modules 505, 507, and 509 can be connected in a delta configuration instead of a star configuration.) The power converter 501 can supply the AC voltage network 511 with reactive power or draw reactive power from the AC voltage network 511.

[0044] In Figure 6 An embodiment of an arrangement 601 with the module 1_1 and the switch S1_1 of the power converter 1 is shown. The module 1_1 and the switch S1_1 are part of the power converter 1 of the Figure 1. The first module terminal 212 is electrically connected to a first node 605; the second module terminal 215 is electrically connected to a second node 607. A bridging branch 610 comprising the switch S1_1 electrically connects the first node 605 to the second node 607. In other words, the bridging branch 610 bridges the module 1_1. The switch S1_1 is a mechanical switch, i.e., a switch having mechanical switching contacts.

[0045] In the bridging branch 610, an electrical measuring sensor 615 is electrically connected in series with the switch S1_1. In the exemplary embodiment of Figure 6, the electrical measuring sensor 615 is a first current measuring sensor 615. By means of this first current measuring sensor 615, the electrical current 620 flowing in the bridging branch 610 is measured, forming a current measured value M620. This current measured value is referred to below as the switch current measured value M620. The electrical current 620 in the bridging branch 610 is also referred to below as the switch current 620.

[0046] A current 622 flows through the arrangement 601, which current corresponds to the total current flowing through the phase module branch 11, cf. Figure 1This current 622 is also referred to as phase module branch current 622. When the phase module branch current 622 is measured (with a measuring sensor not shown), the phase module branch current 622 has the phase module branch current measured value M622. The phase module branch current 622 splits at the second node 607 into the switch current 620 and a module current 625. The module current 625 is the current flowing through module 1_1.

[0047] If the switch S1_1 (as in Figure 6shown) is in its open switching state, then no current flows through switch S1_1 and the first current measuring sensor 615 measures the value zero as the switch current measured value: M620 = 0 A. When the switch S1_1 is in its closed switching state, then almost the entire current 622 flowing in the phase module branch 11 flows through the switch S1_1. In its closed switching state, the switch S1_1 has a lower ohmic resistance than the module 1_1. Therefore, when the switch S1_1 is in the closed switching state, the first current measuring sensor 615 measures a switch current measured value that essentially corresponds to the phase module branch current 622: M620 = M622.

[0048] In Figure 7 a further arrangement 701 is shown. This further arrangement 701 differs from the arrangement 601 of the Figure 6in that this arrangement 701 has a further current measuring sensor 718 (second current measuring sensor 718). This second current measuring sensor 718 is arranged between the first module connection 212 and the first node 605. The second current measuring sensor 718 measures the module current 625 flowing through the module 1_1, forming a module current measured value M625. As current measuring sensors 615, 718 in the arrangements of Figures 6 and 7 For example, so-called Rogowski coils can be used, with which the current is measured using an induction method.

[0049] In Figure 8 An embodiment of an arrangement 801 is shown, which has a first magnetic field measuring sensor 815. This arrangement differs from that in Figure 6The arrangement 601 shown differs only in that the first magnetic field measuring sensor 815 is arranged instead of the first current measuring sensor 615. The first magnetic field measuring sensor 815 measures the magnetic field 820 generated by the switch current 620 flowing through the switch S1_1, forming a magnetic field measurement value. From this magnetic field measurement value, the switch current 620 flowing through the bridging branch 610 can be determined. The measurement using the first magnetic field measuring sensor 815 is therefore an alternative method for measuring the switch current 620.

[0050] The first magnetic field measuring sensor therefore measures the physical quantity "magnetic field." When switch S1_1 is closed, switch current 620 flows through bypass branch 610. Consequently, first magnetic field measuring sensor 815 measures the magnetic field resulting from this switch current 620. However, when switch S1_1 is open, no current flows through bypass branch 610, and first magnetic field measuring sensor 815 measures a magnetic field value of zero. The constantly existing Earth's magnetic field or the very weak magnetic field of distant electrical currents can be neglected here.

[0051] In Figure 9 An embodiment of an arrangement 901 is shown. This arrangement 901 differs from the one in Figure 7The arrangement 701 shown differs in that, instead of the first current measuring sensor 615 and the second current measuring sensor 718, the first magnetic field measuring sensor 815 and a further magnetic field measuring sensor 918 are present (second magnetic field measuring sensor 918). The second magnetic field measuring sensor 918 measures the magnetic field generated by the module current 625 flowing through the module 1_1, forming a magnetic field measured value. From this magnetic field measured value, the module current 625 flowing through the module 1_1 can be determined. Such magnetic field measuring sensors 815, 918 are also referred to as Hall sensors. Using Hall sensors, the current is measured indirectly using the magnetic field (Hall effect).

[0052] In Figure 10 An embodiment of an arrangement 1001 with a voltage measuring sensor 1020 is shown. In accordance with the arrangements of Figures 6 to 9This arrangement comprises the module 1_1 and the bridging branch 610 with the switch S1_1. In contrast to the arrangements of the Figures 6 to 9 However, no electrical measuring sensor is arranged in the bridging branch 610 (measuring sensor-free bridging branch 610); instead, an electrical measuring sensor 1020 in the form of the voltage measuring sensor 1020 is connected in parallel with the switch S1_1 (and thus also in parallel with the module 1_1). Using this voltage measuring sensor 1020, the voltage Us (switch voltage Us) occurring across the switch S1_1 is measured, forming a voltage measured value. This voltage measured value is referred to below as the switch voltage measured value.

[0053] The voltage measuring sensor 1020 measures the physical quantity voltage across switch S1_1. When switch S1_1 is closed, switch S1_1 has a very low ohmic resistance, so only a very small voltage appears across switch S1_1, which is measured by voltage measuring sensor 1020 (ideally zero voltage). However, when switch S1_1 is open, the switch has a very high ohmic resistance, and voltage measuring sensor 1020 essentially measures the voltage appearing at module 1_1.

[0054] In the following, a method for monitoring the modular multilevel power converter is described using the example of the arrangement 601 of the Figure 6explained. Using the first current measuring sensor 615, the current 620 flowing through the switch (switch current 620) is measured, forming a switch current measured value M620. Furthermore, a current measured value M622 of the total current 622 flowing through the phase module branch 11 of the power converter (phase module branch current 622) is known. In the example, this current phase module branch current measured value M622 is available at the control device (not shown) of the power converter 1. The current phase module branch current measured value M622 serves as a comparison value.

[0055] The control device now calculates the ratio between the switch current measurement value M620 and the phase module branch current measurement value M622. If this ratio exceeds a predetermined threshold (for example, threshold 10), the closed switching state of switch S1_1 is detected. This is because the majority of the current then flows through switch S1_1; this indicates that switch S1_1 is in the closed switching state. However, if this ratio falls below the predetermined threshold, the open switching state of the switch is detected (because a significant portion of the current then flows through module 1_1 rather than switch S1_1). Thus, by evaluating the switch current measurement value, it can be determined whether the switch is in its open switching state or its closed switching state.

[0056] If the phase module branch current measurement value M622 is not known, the module current measurement value M625 can be used as a comparison value. This procedure is based on the arrangement of the Figure 7 explained. As a comparison value, the current 625 flowing through module 1_1 (module current 625) is measured here using the second current measuring sensor 718, forming a module current measured value M625. The ratio of the switch current measured value M620 to the module current measured value M625 is then formed. If this ratio is greater than the predetermined threshold value, the closed switching state of switch S1_1 is detected; if the ratio is less than the predetermined threshold value, the open switching state of switch S1_1 is detected.

[0057] The procedure for orders under the Figures 8 and 9with the only difference that instead of the current measurement values, the corresponding magnetic field measurement values of the first magnetic field measurement sensor 815 and / or the second magnetic field measurement sensor 918 are used.

[0058] The order 1001 according to Figure 10 The method does not require a comparison value. Only the voltage Us across switch S1_1 is measured, forming a switch voltage measurement value MUs. If this switch voltage measurement value MUs falls below a predetermined minimum switch voltage value, then the closed switching state is detected; if the switch voltage measurement value MUs exceeds the minimum switch voltage value, then the open switching state of switch S1_1 is detected. The minimum switch voltage value can be 100 V, for example.

[0059] Another variant provides for the physical quantity (voltage, current, or magnetic field) to be repeatedly measured at intervals using one of the methods described above when the switch is in the closed state (i.e., when a defective module is bridged by the switch), thereby generating a measured value each time. Based on the respective measured value, it can then be repeatedly determined whether the switch is (still) in its closed state or whether the switch is in its open state. The presence of an open state would indicate a fault, because a switch, once closed, should not be opened again until the defective module is replaced with a functioning module.

[0060] For all of the described methods, the following option is available: If the switch of a defective module is in its open state, a switch-open signal is output. Such a switch-open signal can, for example, serve as a warning signal or be sent as an additional input to a control unit of the power converter, which then reduces the output current of the power converter. This prevents overheating damage due to the (unwanted) current flow through the defective module, subsequent fires, and the like.

[0061] Another variant of the method provides for repeated measurements of the physical quantity (switch current measured values or switch voltage measured values) when the switch is in the closed state. These switch current measured values or switch voltage measured values, measured at time intervals, represent a temporal measured value curve. This temporal measured value curve is then evaluated to determine whether the ohmic contact resistance of the switch is increasing. Such an increase in the ohmic contact resistance of the switch can occur, for example, due to aging processes (material fatigue, oxidation of the contacts, etc.). For example, it is recognized that the ohmic contact resistance of the switch increases when the ratio between the switch current measured values M620 and the associated phase module branch current measured values M622 increases or when the switch voltage measured values MUs increase.In this way, the long-term quality (or aging behavior) of the electrical contact of the closed switch can be monitored. If it turns out that the contact quality is deteriorating (due to increasing ohmic resistance), the switch can be replaced, or the defective module can be replaced with a new, functioning one.

[0062] In Figure 11 An example process sequence with a current measurement value is summarized in a flow chart.

[0063] Method step 1102: Measuring, by means of the electrical measuring sensor, the current flowing through the switch to form a switch current measurement value.

[0064] Method step 1104: Comparing the switch current measured value M620 with a comparison value, wherein the comparison value is the current flowing through the phase module branch (phase module branch current measured value M622) or the current flowing through the module (module current measured value M625).

[0065] Method step 1106: Detecting the closed switching state when the ratio exceeds a predetermined threshold.

[0066] Method step 1108: Detecting the open switching state when the ratio falls below the predetermined threshold value.

[0067] Process step 1110 (optional): Output of an alarm signal if the open switching state has been detected, although the switch should have the closed switching state.

[0068] In Figure 12 An example process sequence with a voltage measurement value is summarized in a flow chart.

[0069] Method step 1202: Measuring, by means of the electrical measuring sensor, the voltage occurring at the switch to form a switch voltage measured value MUs.

[0070] Method step 1204: Detecting the closed switching state when the switch voltage measured value MUs falls below a predetermined threshold value.

[0071] Method step 1206: Detecting the open switching state when the switch voltage measured value MUs exceeds the predetermined threshold value.

[0072] Process step 1208 (optional): Output of an alarm signal if the open switching state has been detected, although the switch should have the closed switching state.

[0073] A system and method have been described that allow the function of the switch and its contact properties to be monitored by measuring a physical quantity. This allows the functional reliability of the bridging of defective modules to be monitored and malfunctioning switches to be detected. This significantly improves the functionality of the multilevel power converter (and thus the functionality of the power transfer via the multilevel power converter).

Claims

1. Modular multilevel converter (1) having at least one arrangement (601) comprising a module (1_1) of the modular multilevel converter (1), wherein the module comprises at least two electronic switching elements (202, 206) and an electrical energy storage device (210), and wherein the arrangement (601) comprises a switch (S1_1) which in its closed switching state electrically bypasses the module (1_1), wherein - the two electronic switching elements (202, 206) of the module are arranged in a half-bridge circuit, or - the module (301) comprises the two electronic switching elements (202, 206) and two additional electronic switching elements (302, 306), wherein the two electronic switching elements (202, 206) and the two additional electronic switching elements (302, 306) are arranged in a full-bridge circuit, wherein the module comprises a switch actuation device inside the module, wherein said switch actuation device is configured to close the switch (S1_1) in the event of a fault in the module (1_1), wherein the arrangement comprises a measuring sensor (615) for measuring a physical variable (620) which arises at the switch (S1_1) and which assumes different values depending on whether the switch (S1_1) is in its open switching state or in its closed switching state, and wherein the modular multilevel converter (1) comprises a control unit, characterized in that - the physical variable is an electrical voltage (Us), and the measuring sensor is a voltage measuring sensor (1020) connected in parallel with the switch (S1_1), and the arrangement is designed so that - the physical variable measured is the voltage (Us) arising across the switch (S1_1) forming a switch voltage measurement value (MUs), and the closed switching state is identified when the switch voltage measurement value (MUs) falls below a minimum switch voltage value, and the open switching state is identified when the switch voltage measurement value exceeds the minimum switch voltage value, - the control unit of the multilevel converter reduces an output current of the multilevel converter if, in the case of a faulty module, the respective switch is in its open switching state, and - if the switch (S1_1) is in its closed switching state, switch voltage measurement values (MUs) are repeatedly determined at intervals, and these switch voltage measurement values (MUs) are evaluated to determine whether the ohmic contact resistance of the switch (S1_1) increases.

2. Modular multilevel converter (1) according to Claim 1, characterized in that - the switch is a mechanical switch (S1_1).

3. High-voltage DC transmission system (401) having a modular multilevel converter (1) according to Claim 1 or 2.

4. Method for monitoring a modular multilevel converter (1) having at least one phase module branch (11) having a plurality of modules (1_1, 1_2, 1_3, 1_4 each having at least two electronic switching elements (202, 206) and an electrical energy storage device (210), wherein - the two electronic switching elements (202, 206) of the module (201) are arranged in a half-bridge circuit, or the module (301) comprises the two electronic switching elements (202, 206) and two additional electronic switching elements (302, 306), wherein the two electronic switching elements (202, 206) and the two additional electronic switching elements (302, 306) are arranged in a full-bridge circuit, and - the modules are each provided with an electrical switch (S1_1, S1_2, S1_3, S1_4) which in its closed switching state electrically bypasses the respective module, wherein, in the method, - a measuring sensor (615) is used to measured a physical variable (620) by forming a measurement value (M620) of the physical variable, wherein the physical variable (620) arises at the switch (S1_1) and assumes different values depending on whether the switch (S1_1) is in its open switching state or in its closed switching state, and - the measurement value (M620) is used to determine whether the switch (S1_1) is in its open switching state or in its closed switching state, wherein the modules comprise a switch actuation device inside the module, said switch actuation device being configured to close the switch (S1_1) in the event of a fault in the respective module (1._.1), characterized in that - the physical variable measured is the voltage (Us) arising over the switch (S1_1) forming a switch voltage measurement value (MUs), the closed switching state is identified when the switch voltage measurement value (MUs) falls below a minimum switch voltage value, and the open switching state is identified when the switch voltage measurement value exceeds the minimum switch voltage value, - a control unit of the multilevel converter reduces an output current of the multilevel converter if, in the case of a faulty module, the respective switch is in its open switching state, and - if the switch (S1_1) is determined to be in its closed switching state, switch voltage measurement values (MUs) are repeatedly determined at intervals, and these switch voltage measurement values (MUs) are evaluated to determine whether the ohmic contact resistance of the switch (S1_1) increases.

5. Method according to Claim 4, characterized in that - if the switch (S1_1) is determined to be in its closed switching state, the physical variable (620) is measured repeatedly at intervals, each time forming a measurement value (M620), and - it is determined for each of the measurement values (M620) whether the switch (S1_1) is in its open switching state or in its closed switching state.

6. Method according to Claim 4 or 5, characterized in that - it is identified that the ohmic contact resistance of the switch (S1_1) increases as the switch voltage measurement values (MUs) increase.