Method and control system for controlling a converter unit

Abstract

A method and a control system for controlling a converter unit. Disclosed herein is a method for controlling a converter cell (10) of a modular multilevel converter (MMC) (1), the converter cell comprising a fault response circuit. The method comprises monitoring (S1) at least one current derivative di / dt in the converter cell, detecting (S2) a fault in the converter cell, operating the fault response circuit in response to the fault, and classifying (S3) the fault based on the current derivative. A control system (2) performing the method is also disclosed.

Description

Technical Field

[0001] The present disclosure generally relates to a method for controlling a converter unit of a modular multilevel converter, and to a control system. Background Art

[0002] Power converters play a key role in the operation of various electrical systems such as renewable energy systems and high voltage direct current (HVDC) grid systems. In order to ensure the safety of equipment, improve system reliability and protect key components, fault protection is implemented in power converters. In a modular multilevel converter (MMC), each converter unit usually has a protection circuit that is configured to protect the individual converter unit (MMC) and also to protect other components in the grid system.

[0003] An example of a protection system for a converter unit is a combination of a crowbar circuit and a bypass switch. A crowbar circuit is commonly used as a protection mechanism in power converters to quickly and effectively clamp the output voltage to a safe level in the event of an overvoltage fault. It acts as a "short circuit" path across the output terminals of the converter to divert excess energy and protect the connected equipment. A bypass switch is often used to redirect power flow so that a faulty component in the converter is bypassed.

[0004] However, the uptime of the converter unit provided by existing protection systems may be too little, and the reliability of the system may also be too low.Therefore, there remains a need for an improved protection system for a converter unit. Summary of the invention

[0005] It is therefore an object of the present disclosure to provide a method for controlling a converter unit of a modular multilevel converter (MMC) and a control system for such a converter unit, thereby providing higher reliability, higher availability and improved information for handling faults.

[0006] According to a first aspect of the present disclosure, a method for controlling a converter unit of an MMC is provided. The converter unit includes a fault response circuit. The method includes: monitoring at least one current derivative di / dt in the converter unit, detecting a fault in the converter unit, operating the fault response circuit in response to the fault, and classifying the fault based on the current derivative.

[0007] According to a second aspect of the present disclosure, a control system is provided. The control system includes a converter unit of an MMC, and a control unit configured to execute the method according to the first aspect. The converter unit includes: at least one switch element branch, the at least one switch element branch having a first switch element and a second switch element connected in series; and an energy storage device, the energy storage device is connected in parallel with the at least one switch element branch. The converter unit further includes: a crowbar circuit, the crowbar circuit is connected in parallel to the at least one switch element branch; and a first terminal and a second terminal, the first terminal and the second terminal are connected to the at least one switch element branch.

[0008] The MMC topology is widely used in a wide range of applications, such as high voltage direct current (HVDC) systems and STATCOM. Protection of the converter unit is a vital part to avoid faults such as short circuit faults from propagating through the system. When a fault occurs in a converter unit, the protection system usually bypasses the faulty converter unit and thereby disconnects the converter unit from the MMC. Thereafter, the faulty converter unit can be replaced by the operator during the next maintenance period.

[0009] However, there are different types of faults with different severity. It is the inventor's recognition that the severity of a fault can be measured by the current derivative, and that less severe types of faults are generally more common than faults that may cause serious propagation problems. Furthermore, it is realized as part of the present disclosure that faults of lower severity generally have lower current derivatives than faults of higher severity. Thus, faults can be classified based on the magnitude of the current derivative, and thereby based on the expected severity of the fault. Different types of faults may require different types of maintenance, and in some cases, because the converter unit is not damaged, the converter unit does not need to be replaced.

[0010] By measuring the current derivative and thereby classifying the fault into different types of faults based on the measured current derivative, the operator will be provided with improved information about the system status. By knowing the severity of the fault, different types of maintenance can be planned based on this information. Thus, the maintenance period can be shortened and troubleshooting on site can be partially avoided.

[0011] Furthermore, the improved information may also provide the operator with the option of simply checking the status of the converter unit before reintroducing it into operation. However, it should be noted that the classification of faults may not only be based on severity, but also on other classifications, such as for example different fault types leading to different consequences for the system.

[0012] The method may further include operating a fault response circuit based on the classification of the fault.

[0013] The improved information may also provide the operator with the option of performing different fault responses by the fault response circuit based on the classification of the fault. As previously mentioned, typically, most faults have a lower severity. For faults that are classified as more severe, the fault response circuit may be programmed to operate in one way, and for faults that are classified as less severe, the fault response circuit may be programmed to operate in another way. This allows the operator to classify the faults and design the fault response circuit to operate in the best manner for each classification of the fault.

[0014] The classification of the fault may be determined by whether the current derivative exceeds a predetermined threshold. That is, the fault response circuit may be programmed to operate in one manner when the fault exceeds the predetermined threshold and in another manner when the current derivative is below the predetermined threshold. The predetermined threshold may be set by an operator to a level known to be less severe. The predetermined threshold for the current derivative may be between 2 kA / μs and 12 kA / μs, and more preferably between 4 kA / μs and 6 kA / μs.

[0015] However, it should be noted that the classification of the fault can also be determined based on multiple intervals of the current derivative, and also classified based on the interval in which the current derivative rises. Thus, the fault can be classified into more than two categories. In the discussion, an example of two classifications will be used, but the discussion is equally applicable to systems with a larger number of predetermined thresholds and therefore a larger number of classifications.

[0016] Operating the fault response circuit of the converter unit may include shutting down the converter unit and activating a crowbar circuit of the converter unit. The activation of the crowbar circuit may be adjusted based on the classification of the fault.

[0017] Turning off the converter unit and activating the crowbar circuit is an example of how the fault response circuit can be operated. In order to protect the system, when a fault occurs, the converter unit is usually turned off directly. As described above, the crowbar circuit is a common method for releasing the energy of the energy storage device of the converter unit. However, by adjusting the activation of the crowbar circuit based on the classification of the fault, the converter unit and the operator will be provided with the flexibility of how to operate the system and for what type of faults to use the crowbar circuit. Some faults may not require the release of the energy of the energy storage device, and therefore it may be preferred not to power off the energy storage device, while for other faults, it may be necessary to directly release the energy of the energy storage device. As an example, if the current derivative exceeds a threshold, the crowbar circuit can release energy.

[0018] The fault response circuit of operating the converter unit may further include activating a bypass circuit of the converter unit. The activation of the bypass circuit may also be adjusted based on the classification of the fault. In such an embodiment, the control system of the second embodiment may include a bypass switch connected between the first terminal and the second terminal.

[0019] For some fault classifications, it may be necessary to bypass the entire converter unit to protect it from the current due to the fault. However, this may not be necessary for all types of faults, and with information from the current derivative-based classification, the operator may decide not to bypass the faulty converter unit. When bypassing the converter unit, the risk of fault propagation may be reduced.

[0020] Operating the fault response circuit of the converter unit may further include restarting operation of the converter unit and modifying the classification of the fault if the current derivative of the fault is below a predetermined threshold if the fault is detected within a preset duration or if the current derivative approaches zero during shutdown.

[0021] For some types of faults, restarting the operation of the converter unit may be sufficient to mitigate the fault and return the converter unit to normal operation. By attempting to restart the operation of the converter unit, and if successful in mitigating the fault and thereby returning the converter unit to normal operation, the availability of the converter unit may be increased.

[0022] Compared to existing systems that bypass converter units when a fault occurs, restarting the converter units can mitigate the fault regardless of the severity of the fault and allow the converter units to remain operational when the fault severity is lower. Because fewer converter units may be bypassed due to faults and awaiting maintenance periods, operators can design systems with fewer redundant units, thereby reducing the cost of the system while still meeting the reliability and availability requirements of the power grid.

[0023] When the converter unit is restarted, and if a fault is detected within a preset duration from the first fault, the classification of the fault can be modified by, for example, raising the classification to a category with a higher severity. The fault detected within the preset duration may be an earlier fault that has not yet been alleviated or a new fault that has occurred due to an earlier fault. Another method for detecting whether the fault has been alleviated is through the current derivative. When the converter unit is successfully shut down and the fault is alleviated, the fault current will be reduced and a negative current derivative can therefore be measured. However, if the amplitude of the current derivative remains small and close to zero, it indicates that the fault current is still flowing. In any of these cases where a fault occurs within a preset duration from restart or when the amplitude of the current derivative is close to zero during shutdown, the fault response circuit may need to take further action. Therefore, the classification of the fault can be raised to a level, allowing the fault response circuit to take further action to reduce the impact of the fault.

[0024] To improve the understanding of how the fault response circuit can operate with different features, an example will be provided. It will be appreciated that this is merely an example of how the fault response circuit can be arranged for a particular system. Other combinations of operations may also be used. When a fault occurs in a converter unit, the current derivative will increase due to the fault current from the fault. The magnitude of the current derivative is measured, and the fault is classified based on whether the current derivative exceeds a predetermined threshold.

[0025] If the measured current derivative is below a predetermined threshold, the fault response circuit attempts to restart the faulty converter unit. If this operation is successful and the fault is mitigated by the restart, the converter unit will enter normal operation. Therefore, the system can maintain normal operation.

[0026] However, if the magnitude of the current derivative approaches zero during shutdown, this may indicate that the fault still exists, or if restarting is unsuccessful and the fault is still occurring or reoccurring during a preset duration, this may also indicate that the fault still exists. Therefore, further measures are required by the fault response circuit to handle the fault. The fault may be classified as a more severe fault, and the converter unit may be shut down and the crowbar circuit may be activated to release the energy of the energy storage device. The converter unit may then be bypassed by the bypass switch.

[0027] Detecting a fault may be performed by calculating a voltage signal based on a current derivative. The current derivative may be converted to a voltage signal by an integrator circuit. The fault may be detected based on the magnitude of the determined voltage signal.

[0028] The current derivative can be measured for each switching element of the converter unit. This makes it possible to determine faults at the switching element level.

[0029] The control system may further comprise a gate drive element for each switching element.Each gate drive element may be configured to control a respective switching element and provide a measured value of the current derivative to the control unit.

[0030] The measured current derivative can be processed by the gate drive element before being transmitted to the control unit. If the current derivative measurement value is digitized on the corresponding gate drive element, the stress and processing time at the control unit can be reduced. This may make the reaction time from the fault to the start of operation of the fault response circuit faster. It should be noted that all corresponding measured current derivatives can be processed in one gate drive board to which each gate drive element is connected. However, additional circuits can also be added to the control unit and the processing can be performed in the control unit instead of in the gate drive element. The additional circuit in the control unit allows the control of the switching of the switching element and the control of the current derivative to be separated, which can facilitate the operation of the control unit and reduce delays.

[0031] Each gate drive element may include a local controller configured to classify the fault and communicate the classification to the control unit.

[0032] This allows the gate drive element to classify the fault in the local controller and thereby send a signal to the control unit only when an actual fault is present. Thus, the signal can be binary and there is no need to digitize the signal at the gate drive. This reduces the delay at the control unit in deciding what fault response is required. As mentioned previously, in this embodiment, additional circuitry can be added to the control unit to separate the control of the switching of the switching element and the control of the current derivative.

[0033] Each gate drive element may include a local controller configured to classify the fault and directly operate the crowbar circuit.

[0034] By allowing the local controller to directly operate the crowbar circuit, communication delays between the control unit and the crowbar circuit are eliminated. As a result, the crowbar circuit can be activated faster than the information passes through the control unit.

[0035] The at least one switch element branch may include a first switch element branch and a second switch element branch, each switch element branch having a first switch element and a second switch element connected in series. The first terminal may be connected between the first switch element and the second switch element of the first switch element branch, and the second terminal may be connected between the first switch element and the second switch element of the second switch element branch.

[0036] Further scope of applicability of the present disclosure will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present disclosure, are given by way of illustration only, because various changes and modifications within the scope of the present disclosure will become apparent to those skilled in the art from the detailed description.

[0037] Therefore, it should be understood that the present disclosure is not limited to the specific components of the described apparatus or steps of the described methods, as such apparatus and methods may vary. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present disclosure will be described in more detail, by way of example, with reference to the accompanying drawings, which show example embodiments of the present disclosure.

[0039] Figure 1 A circuit diagram of a modular multilevel converter is shown.

[0040] Figure 2A control system with a half-bridge converter unit is illustrated.

[0041] Figure 3 A control system with a full-bridge converter unit is illustrated.

[0042] Figure 4 An overview of the control scheme is shown.

[0043] Figure 5 A method for controlling a converter unit is illustrated.

[0044] Figure 6 A flow chart for controlling a converter unit is illustrated. DETAILED DESCRIPTION

[0045] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which presently preferred embodiments of the present disclosure are shown. However, the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for comprehensiveness and completeness, and to fully convey the scope of the present disclosure to the skilled person.

[0046] Figure 1 A modular multilevel converter MMC 1 is illustrated that connects three phases 101a to 101c of an alternating current (AC) system 101 to a high voltage direct current (HVDC) system 100. Each of the phases 101a to 101c is connected to an upper branch 4a to 4c and a lower branch 5a to 5c of the MMC 1. Each upper branch 4a to 4c and lower branch 5a to 5c includes a plurality of converter units 10 that perform switching operations to convert voltage between AC and DC. All or some of the converter units 10 may be converter units according to any embodiment of the converter unit 10 to be described below.

[0047] Figure 2 A control system 2 for controlling a converter unit 10 of an MMC 1 is illustrated. The converter unit 10 includes a switch element branch 20 having a first switch element 22 and a second switch element 24 connected in series. The first switch element 22 and the second switch element 24 have different possible switch states that can be used to actively manipulate the voltage and current characteristics of an electrical signal. By adjusting the timing and duration of the switching events, the converter unit 10 can convert voltage between AC and DC and provide a desired output.

[0048] The converter unit further comprises an energy storage device 30 connected in parallel with the one switching element branch 20. Figure 2, the energy storage device 30 is a capacitor. The energy storage device 30 is used to store energy during operation of the MMC 1. The converter unit 10 further includes a crowbar circuit 40 connected in parallel with the at least one switching element branch 20. In order to protect the system in which the converter unit 10 operates, the converter unit 10 is usually directly shut down when a fault occurs. The crowbar circuit 40 is a common method for releasing the energy of the energy storage device 30 of the converter unit 10 when the converter unit 10 has been shut down. The crowbar circuit 40 allows the energy storage device 30 to release at least most of the energy of the converter unit 10 into a crowbar element such as a thyristor so as to limit the energy released into the switching elements 22, 24.

[0049] exist Figure 2 , a first terminal 26 and a second terminal 28 configured to connect the converter unit 10 to an external circuit are provided. The first terminal 26 is connected between the first switching element 22 and the second switching element 24, and the second terminal 28 is connected between the first switching element 22 and the energy storage device 30. By connecting the first terminal 26 and the second terminal 28 in this way, the external circuit is connected in parallel with the first switching element 22. This is a common terminal connection when a half-bridge topology is used for the converter unit 10. A bypass switch 50 is provided between the first terminal 26 and the second terminal 28. In other words, the bypass switch 50 is connected in parallel with the first switching element 22. When the bypass switch 50 is closed and activated, the bypass switch 50 is configured to bypass the entire converter unit 10.

[0050] The control system 2 further comprises a control unit 60. The control unit 60 is configured to execute the method 100 for controlling the converter unit 10 and may also be configured to control the switching operation of the switching elements 22, 24. The method 100 will be combined with Figures 4 to 6 However, a first another embodiment of the control system 2 will be described.

[0051] exist Figure 3 In the Figure 2 The control system in is similar to the control system 2. Figure 3 Control system 2 in Figure 2The difference of the control system 2 in the embodiment is mainly that the converter unit 10 includes two switch element branches 20a, 20b. The first switch element branch 20a is connected in parallel with the second switch element branch 20b, and each of the first branch and the second branch includes a first switch element 22a, 22b and a second switch element 24a, 24b connected in series. Each of the first switch element branch 20a and the second switch element branch 20b is connected in parallel to the energy storage device 30. The first terminal 26 is connected between the first switch element 22a and the second switch element 24a of the first switch element branch 20a, and the second terminal 28 is connected between the first switch element 22b and the second switch element 24b of the second switch element branch 20b. Thus, a full-bridge converter unit 10 is provided.

[0052] The control unit 60 is configured to be provided with measured values ​​of the current derivatives of the switching elements 22, 24 from the gate drivers. There may be a single gate driver providing the measured values ​​to the control unit, or a plurality of gate drivers may be provided. Figure 3 and Figure 4 As shown, the control system 2 may further include a gate drive element 62a to 62n for each switching element 22, 24. These gate drive elements 62a to 62n are also configured to perform switching of the switching elements 22, 24. Figure 4 In the embodiment of the present invention, the control unit 60 has a cell controller 66 for controlling the switching performed by the gate drive elements 62a to 62n, and an additional circuit 64 for processing the measured current derivatives provided by the gate drive elements 62a to 62n. However, it should be noted that there are different methods for processing the measured current derivatives. The methods can be performed directly in the gate driver or by a local controller in the respective gate driver allowing the operation of the crowbar circuit 40 and the bypass switch 50.

[0053] Go to Figure 5 , illustrates a method 100 for controlling a converter unit 10 during a fault. When a fault, such as a short circuit fault, occurs in the converter unit 10, there is a risk that the fault current propagates in the power system. Therefore, methods for mitigating the risk of propagating the fault current are crucial. However, there are different types of faults with different severities. The inventors have recognized that the severity of the fault can be measured by the current derivative, and that less severe types of faults are generally more common than faults that may cause serious propagation problems. Some less severe faults can even be mitigated by correct operation of the control unit 60. The method 100 for controlling the converter unit 10 includes monitoring S1 at least one current derivative di / dt in the converter unit 10. Monitoring S1 can be performed by the gate drive elements 62a to 62n, as described above in combination with Figure 4 as described.

[0054] The method further includes detecting S2 a fault in the converter unit 10. The control unit 60 may detect the fault by the measured current derivative by calculating a voltage signal based on the current derivative. The current derivative may, for example, be converted into a voltage signal by passing through an integrator circuit. Thereafter, the fault may be detected based on the amplitude of the determined voltage signal. When a fault is detected, the method further includes classifying the fault based on the current derivative S4. The classification of the fault may be determined by whether the current derivative exceeds a predetermined threshold. Faults of lower severity generally have lower current derivatives than faults of higher severity. Thus, the faults may be classified based on the magnitude of the current derivative and therefore based on the expected severity of the fault. This provides the operator with the option of executing different fault responses based on the classification of the fault by the fault response circuit. As previously mentioned, generally, most faults have a lower severity. For faults classified as more severe, the fault response circuit may be programmed to operate in a certain way, while for faults classified as less severe, the fault response circuit may be programmed to operate in another way. This allows the operator to classify the fault and design the fault response circuit to operate in the best way for each classification of the fault.

[0055] Thereafter, the control unit operates S3 a fault response circuit in response to the fault. The fault response circuit may be constructed from different components for mitigating or stopping the propagation of the fault current. Figure 2 and Figure 3 In FIG. 4 , the fault response circuit includes a crowbar circuit 40 and a bypass switch 50. However, if another response from the fault response circuit is required, the fault response circuit may also include other fault protection components.

[0056] A common response that the control unit 60 may perform is to shut down the converter unit 10 and activate the crowbar circuit 40 of the converter unit 10. This allows the energy stored in the energy storage device 30 to be released and thereby the current in the converter unit 10 to be reduced. However, by operating the crowbar circuit 40 based on the classification of the fault, flexibility is introduced to the control system 2. Some faults may not require the release of energy of the energy storage device 30, and therefore it may be preferred not to de-energize the energy storage device 30, while for other faults it may be necessary to release the energy of the energy storage device 30 directly. As an example, if the current derivative exceeds a threshold value, the crowbar circuit 40 may release energy.

[0057] Another response that the control unit 60 can perform is to activate the bypass circuit and close the bypass switch 50. For some classifications of faults, it may be necessary to bypass the entire converter unit 10 to protect it from the current caused by the fault. However, this may not be necessary for all types of faults, and with information from the classification based on the current derivative, the operator may decide on another action besides bypassing the converter unit 10.

[0058] One response that may be suitable for less severe faults is to perform a restart operation of the converter unit 10. For some types of faults, restarting the operation of the converter unit 10 may be sufficient to mitigate the fault and return the converter unit 10 to normal operation. In prior art systems, the converter unit is bypassed when a fault occurs regardless of the severity of the fault. Restarting the converter unit 10 can mitigate the fault and allow the converter unit to remain in operation when the fault severity is low. However, if the fault is detected within a preset duration, or if the current derivative is close to zero during shutdown, the control unit can modify the classification of the fault to perform other operations to protect the converter unit 10. Whether the fault is mitigated can be detected by studying the current derivative during shutdown. When the converter unit 10 is successfully shut down and the fault is mitigated, it will cause the fault current to decrease and therefore a negative current derivative can be measured. However, if the magnitude of the current derivative remains small and close to zero, it indicates that the fault current is still flowing. In other cases, the fault can be mitigated by restarting, but the fault may reappear due to, for example, some faulty components. In this case, if the fault is detected within a preset duration, the classification of the fault can be modified.

[0059] Figure 6 The flowchart in the figure shows how Figure 2 An example of how the control unit 60 shown in FIG. 1 and the fault response circuit may function (here using the example from Figure 2 ).

[0060] The current derivative is monitored / measured and will increase due to the current from the fault when a fault occurs in the converter unit 10. If a fault is detected, the control unit 60 will shut down the converter unit 10 as a first protection measure for the converter unit 10. The magnitude of the current derivative is measured and the fault is classified based on whether the current derivative exceeds a predetermined threshold.

[0061] If the measured current derivative is below a predetermined threshold, the fault response circuit attempts to turn on the faulty converter unit 10 again. If this operation is successful and the fault is mitigated by restarting, the converter unit 10 will enter normal operation. Thereafter, the system can maintain normal operation.

[0062] However, if the magnitude of the current derivative approaches zero during shutdown, this may indicate that the fault still exists, or if restarting is unsuccessful and the fault is still occurring or reoccurring during the preset duration, the fault response circuit may need to take additional measures to handle the fault. Therefore, the fault may be classified as a more severe fault, i.e., a fault exceeding a predetermined threshold, and the converter unit 10 may be shut down and the crowbar circuit 40 may be activated to release the energy of the energy storage device 30. The converter unit may then be bypassed via the bypass switch 50.

[0063] This operation provides the possibility for the converter unit 10 to return to normal operation instead of being directly bypassed and replaced during the next maintenance window. This results in increased availability among the converter units and requires fewer redundant converter units.

[0064] Those skilled in the art will recognize that the present disclosure is by no means limited to the preferred embodiments described above. On the contrary, various modifications and variations within the scope of the appended claims are possible. In addition, by studying the drawings, the present disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments when implementing the claimed invention.

Claims

1. A method (100) for controlling a converter unit (10) of a modular multilevel converter MMC (1), the converter unit comprising a fault response circuit; The method comprises: monitoring (S1) at least one current derivative di / dt in the converter unit, detecting (S2) a fault in the converter unit, In response to the fault, operating (S3) the fault response circuit, and The fault is classified based on the current derivative (S4).

2. The method according to claim 1, further comprising: The fault response circuit is operated (S4) based on the classification of the fault.

3. The method according to claim 2, wherein: A fault response circuit for operating the converter unit comprises: shutting down the converter unit; activating a crowbar circuit of the converter unit; Therein, activation of the crowbar circuit is adjusted based on the classification of the fault.

4. The method according to claim 3, wherein: The fault response circuit for operating the converter unit further comprises: activating a bypass circuit of the converter unit; Wherein, activation of the bypass circuit is adjusted based on the classification of the fault.

5. A method according to any one of the preceding claims, wherein: The classification of the fault is determined by whether the current derivative exceeds a predetermined threshold.

6. A method according to any one of the preceding claims, wherein: The predetermined threshold value of the current derivative is between 2 kA / μs and 12 kA / μs, and more preferably between 4 kA / μs and 6 kA / μs.

7. The method according to claim 5 or 6, wherein: The fault response circuit for operating the converter unit further comprises: The operation of the converter unit is restarted and if the fault is detected within a preset duration or if the current derivative approaches zero during shutdown, the classification of the fault is modified if the current derivative for the fault is below the predetermined threshold.

8. A method according to any one of the preceding claims, wherein: Detecting the fault is performed by calculating a voltage signal based on the current derivative.

9. The method according to claim 8, wherein: The fault is detected based on the determined magnitude of the voltage signal.

10. A method according to any one of the preceding claims, wherein: A current derivative is measured for each switching element of the converter unit.

11. A control system (2), comprising: a converter unit (10) of a modular multilevel converter MMC, and a control unit (60) configured to execute the method according to claims 1 to 10, Wherein, the converter unit comprises: at least one switching element branch (20), the at least one switching element branch having a first switching element (22) and a second switching element (24) connected in series, an energy storage device (30) connected in parallel with the at least one switching element branch, a crowbar circuit (40) connected in parallel to the at least one switching element branch, and A first terminal and a second terminal (26, 28) connected to the at least one switching element branch.

12. The control system according to claim 11, wherein: The converter unit further comprises a bypass switch (50) connected between the first terminal and the second terminal.

13. A control system according to claim 11 or 12, further comprising a gate drive element (62a-62d) for each switching element, wherein Each gate drive element is configured to control a respective switching element and to provide a measurement of the current derivative to the control unit.

14. The control system according to claim 13, wherein: Each gate drive element comprises a local controller configured to classify the fault and communicate the classification to the control unit.

15. The control system according to claim 13, wherein: Each gate drive element includes a local controller configured to classify the fault and directly operate the crowbar circuit.

16. A control system according to any one of claims 11 to 15, wherein: The at least one switch element branch comprises a first switch element branch and a second switch element branch (20a, 20b), each switch element branch having a first switch element (22a, 22b) and a second switch element (24a, 24b) connected in series, wherein the first terminal is connected between the first switch element and the second switch element of the first switch element branch, and the second terminal is connected between the first switch element and the second switch element of the second switch branch.

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