Sub-module, bypass protection method thereof and modular multilevel converter

Abstract

The invention provides a sub-module, a bypass protection method thereof and a modular multilevel converter. Each submodule is provided with a bidirectional turning breakdown thyristor and a mechanical switch, and under the condition that a communication link in the submodule or a part of devices in the submodule breaks down, bypass of the submodule can be achieved through the bidirectional turning breakdown thyristors or the mechanical switches, reliable protection of the submodule is achieved, and the submodule cannot be damaged. Software control protection of the sub-modules can be realized through the mechanical switches, and hardware protection of the sub-modules can be realized through the bidirectional turning breakdown thyristors. That is to say, through dual protection of software and hardware of the sub-module, reliable bypass of the sub-module is realized, so that the sub-module can operate more safely and stably under actual complex working conditions.

Description

Technical Field

[0001] The present application relates to the technical field of DC power transmission, and particularly relates to a sub-module, a bypass protection method thereof, and a modular multilevel converter. Background Art

[0002] The modular multilevel converter (MMC) has become the core converter of the flexible DC power transmission system. The flexible DC power transmission system based on overhead lines can transmit clean energy over long distances. However, due to the large influence of external factors on overhead lines, various DC faults such as short circuits and broken wires are likely to occur.

[0003] To solve DC faults, a hybrid modular multilevel converter with the ability to self-clear DC side faults has emerged. Each arm of the hybrid modular multilevel converter can include a plurality of series-connected half-bridge sub-modules and a plurality of series-connected full-bridge sub-modules. However, in the case of a DC fault occurring in the hybrid modular multilevel converter, the half-bridge sub-modules and the full-bridge sub-modules will lose control, resulting in the continuous increase of the voltage of the support capacitor, even exceeding the limit withstand voltage of the semiconductor devices in the sub-module, causing the semiconductor devices to break down, and further possibly causing safety accidents such as sub-module explosion. Therefore, bypass protection for the sub-module is required.

[0004] Related technologies often set unidirectional thyristors at the output end of the sub-module. The unidirectional thyristor conducts according to the conduction instruction, thereby realizing the bypass of the sub-module, and further achieving the purpose of protecting the sub-module. However, in the case where the unidirectional thyristor cannot reliably receive the conduction instruction or a communication link or internal components inside the sub-module fail, reliable protection for the sub-module cannot be achieved, and the sub-module will still be damaged. That is to say, the sub-module and its bypass protection method provided by the related technology are prone to damage the sub-module and have poor working reliability. Summary of the Invention

[0005] To solve the problems that the sub-module in the prior art is prone to damage and has poor working reliability, the present application provides a sub-module, which may include an equalizing resistor, a support capacitor, a bidirectional snap-back thyristor, a mechanical switch, and a sub-module arm.

[0006] The first ends of the equalizing resistor and the support capacitor are respectively connected to the first end of the sub-module arm, and the second ends of the equalizing resistor and the support capacitor are respectively connected to the second end of the sub-module arm. The first ends of the bidirectional snap-back thyristor and the mechanical switch are respectively connected to the first midpoint of the sub-module arm, and the second ends of the bidirectional snap-back thyristor and the mechanical switch are respectively connected to the second midpoint of the sub-module arm or the second end of the sub-module arm.

[0007] Wherein, the gate and the cathode of the bidirectional snap-back thyristor are connected.

[0008] In some possible implementations, the sub-module arm adopts a full-bridge structure.

[0009] The sub-module arm includes a first fully-controlled semiconductor device, a second fully-controlled semiconductor device, a third fully-controlled semiconductor device, and a fourth fully-controlled semiconductor device.

[0010] The first poles of the first fully-controlled semiconductor device and the third fully-controlled semiconductor device are connected to each other as the first end of the sub-module arm. The second pole of the first fully-controlled semiconductor device is connected to the first pole of the second fully-controlled semiconductor device as the first midpoint of the sub-module arm. The second pole of the third fully-controlled semiconductor device is connected to the first pole of the fourth fully-controlled semiconductor device as the second midpoint of the sub-module arm. The second poles of the second fully-controlled semiconductor device and the fourth fully-controlled semiconductor device are connected to each other as the second end of the sub-module arm.

[0011] In some other possible implementations, the sub-module arm adopts a half-bridge structure.

[0012] The sub-module arm includes a first fully-controlled semiconductor device and a second fully-controlled semiconductor device.

[0013] The first pole of the first fully-controlled semiconductor device serves as the first end of the sub-module arm. The second pole of the first fully-controlled semiconductor device is connected to the first pole of the second fully-controlled semiconductor device as the first midpoint of the sub-module arm. The second pole of the second fully-controlled semiconductor device serves as the second end of the sub-module arm.

[0014] Exemplarily, the sub-module further includes a control unit. The control unit is connected to the mechanical switch.

[0015] The control unit is configured to: send a bypass instruction to the mechanical switch.

[0016] The mechanical switch is configured to: close according to the bypass instruction.

[0017] Optionally, the sub-module further includes a driving unit.

[0018] The control unit is further connected to the driving unit, and the driving unit is connected to the control poles of the fully-controlled semiconductor devices in the sub-module arm.

[0019] The control unit is further configured to: send a switching instruction to the driving unit.

[0020] The driving unit is configured to: drive the conduction or turn-off of the fully-controlled semiconductor device according to the switching instruction.

[0021] The switching instruction includes a conduction instruction and a turn-off instruction. Among them, the conduction instruction is used to indicate the conduction of the fully-controlled semiconductor device, and the turn-off instruction is used to indicate the turn-off of the fully-controlled semiconductor device.

[0022] On the other hand, the present application also provides a bypass protection method for a sub-module, including:

[0023] Obtain the working state of the sub-module and the voltage of the support capacitor. The working state includes an operating state and a fault state.

[0024] If the working state is a fault state or the voltage of the support capacitor exceeds a preset voltage threshold, some fully controlled semiconductor devices in the sub-module arm are turned on, and the mechanical switch acts according to the received bypass command.

[0025] If the mechanical switch is not closed, the sub-module is bypass protected by a bidirectional thyristor with reverse breakdown.

[0026] Wherein, the gate and cathode of the bidirectional thyristor with reverse breakdown are connected.

[0027] In some possible implementation manners, the sub-module adopts a full-bridge structure. If the mechanical switch is not closed, bypass protecting the sub-module by a bidirectional thyristor with reverse breakdown includes:

[0028] If the mechanical switch is not closed, all fully controlled semiconductor devices in the sub-module arm are turned off.

[0029] If the instantaneous value of the output voltage of the sub-module arm is higher than or equal to the preset forward breakdown voltage of the bidirectional thyristor with reverse breakdown, or the instantaneous value of the output voltage of the sub-module arm is lower than or equal to the preset reverse breakdown voltage of the bidirectional thyristor with reverse breakdown, the bidirectional thyristor with reverse breakdown breaks down forward or reversely, and bypasses the sub-module.

[0030] Optionally, the amplitude of the preset forward breakdown voltage is equal to the amplitude of the reverse breakdown voltage, and both are greater than the voltage threshold.

[0031] In other possible implementation manners, the sub-module adopts a half-bridge structure. If the mechanical switch is not closed, bypass protecting the sub-module by a bidirectional thyristor with reverse breakdown includes:

[0032] If the mechanical switch is not closed, all fully controlled semiconductor devices in the sub-module arm are turned off.

[0033] If the instantaneous value of the output voltage of the sub-module is higher than or equal to the preset forward breakdown voltage of the bidirectional thyristor with reverse breakdown, the bidirectional thyristor with reverse breakdown breaks down forward, and bypasses the sub-module.

[0034] Optionally, the amplitude of the preset forward breakdown voltage is greater than the voltage threshold.

[0035] On yet another aspect, the present application also provides a modular multilevel converter, including a plurality of the above-mentioned sub-modules, and the plurality of sub-modules can be connected in series.

[0036] The modular multilevel converter can be a hybrid modular multilevel converter.

[0037] In another aspect, the present application also provides a DC power transmission system, which may include the above-mentioned modular multilevel converter.

[0038] Compared with the prior art, the beneficial effects of the present application are as follows:

[0039] The sub-module provided by the present application is provided with a bidirectional turn-on thyristor and a mechanical switch. In the case of a failure in the communication link or some internal devices inside the sub-module, the bypass of the sub-module can be realized through the bidirectional turn-on thyristor or the mechanical switch, achieving reliable protection of the sub-module without damaging the sub-module. That is to say, the sub-module provided by the present application has high working reliability.

[0040] The present application can realize software control protection of the sub-module through the mechanical switch and hardware protection of the sub-module through the bidirectional turn-on thyristor. That is to say, through the dual software and hardware protection of the sub-module, the reliable bypass of the sub-module is realized, enabling the sub-module to operate more safely and stably under actual complex working conditions.

[0041] By providing bypass protection for the sub-module, the present application can effectively avoid the breakdown of fully controlled semiconductor devices caused by the malfunction of the mechanical switch or DC faults, thereby avoiding safety accidents such as sub-module explosion, and can also avoid faults such as the tripping of the modular multilevel converter, improving the stability of the modular multilevel converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0043] Figure 1 It is a schematic structural diagram of a sub-module in an embodiment of the present application;

[0044] Figure 2 It is another schematic structural diagram of a sub-module in an embodiment of the present application;

[0045] Figure 3 It is a schematic structural diagram of a modular multilevel converter in an embodiment of the present application;

[0046] Figure 4 It is a schematic flowchart of a method for bypass protection of a sub-module in an embodiment of the present application;

[0047] Figure 5 It is another schematic flowchart of a method for bypass protection of a sub-module in an embodiment of the present application. Detailed implementation manners

[0048] The technical solutions in the present application will be described below with reference to the accompanying drawings.

[0049] In the embodiments of the description, claims and drawings of the present application, terms such as "first", "second", etc. are only used for the purpose of distinguishing descriptions, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying order. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a method, system, product or device comprising a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0050] It should be understood that in the present application, "at least one (item)" means one or more, and "a plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" may represent: only A exists, only B exists, and both A and B exist at the same time. Among them, A and B may be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after. "At least one (one) of the following" or a similar expression means any combination of these items, including any combination of single item (one) or plural items (ones). For example, at least one (one) of a, b or c may represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c may be single or multiple.

[0051] The embodiments of the present application provide a sub-module, as Figure 1 and Figure 2 shown. The sub-module 10 may include an equalizing resistor R, a support capacitor C, a bidirectional thyristor Thy, a mechanical switch K, and a sub-module arm 1.

[0052] Among them, the first ends of the equalizing resistor R and the support capacitor C are respectively connected to the first end of the sub-module arm 1, and the second ends of the equalizing resistor R and the support capacitor C are respectively connected to the second end of the sub-module arm 1. The first ends of the bidirectional thyristor Thy and the mechanical switch K are respectively connected to the first midpoint A of the sub-module arm 1, and the second ends of the bidirectional thyristor Thy and the mechanical switch K are respectively connected to the second midpoint B of the sub-module arm 1 or the second end of the sub-module arm 1.

[0053] Among them, the gate of the bidirectional turn-on thyristor Thy can be connected to the cathode. The bidirectional turn-on thyristor Thy no longer has the external trigger turn-on function of the bidirectional thyristor, and is only used as a semiconductor device that conducts bidirectionally after bidirectional overvoltage breakdown. In addition, according to the design requirements of different projects, bidirectional turn-on thyristors with different forward breakover voltages or reverse breakover voltages can be selected, so as to flexibly adapt to the sub-modules to be protected and meet the voltage control and protection requirements of the support capacitors of the modular multilevel converter.

[0054] In some possible implementation manners, such as Figure 1 shown, the sub-module arm 1 adopts a full-bridge structure.

[0055] The sub-module arm 1 includes a first fully-controlled semiconductor device 11, a second fully-controlled semiconductor device 12, a third fully-controlled semiconductor device 13, and a fourth fully-controlled semiconductor device 14.

[0056] The first poles (which can be collectors) of the first fully-controlled semiconductor device 11 and the third fully-controlled semiconductor device 13 are connected to serve as the first end of the sub-module arm 1. The second pole (which can be the emitter) of the first fully-controlled semiconductor device 11 is connected to the first pole (which can be the collector) of the second fully-controlled semiconductor device 12 to serve as the first midpoint A of the sub-module arm 1. The second pole (which can be the emitter) of the third fully-controlled semiconductor device 13 is connected to the first pole (which can be the collector) of the fourth fully-controlled semiconductor device 14 to serve as the second midpoint B of the sub-module arm 1. The second poles (which can be the emitters) of the second fully-controlled semiconductor device 12 and the fourth fully-controlled semiconductor device 14 are connected to serve as the second end of the sub-module arm 1. Figure 1 In, u out represents the instantaneous value of the output voltage of the sub-module.

[0057] Optionally, the above semiconductor devices (including the first fully-controlled semiconductor device 11, the second fully-controlled semiconductor device 12, the third fully-controlled semiconductor device 13, and the fourth fully-controlled semiconductor device 14) can be insulated gate bipolar transistors (IGBTs), integrated gate-commutated thyristors (IGCTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), etc. In the full-bridge structure, the first fully-controlled semiconductor device 11, the second fully-controlled semiconductor device 12, the third fully-controlled semiconductor device 13, and the fourth fully-controlled semiconductor device 14 all adopt IGBTs, and diodes D are anti-parallel connected to the IGBTs.

[0058] In some other possible implementation manners, such as Figure 2 shown, the sub-module arm 1 may adopt a half-bridge structure.

[0059] The sub-module arm 1 includes a first fully-controlled semiconductor device 11 and a second fully-controlled semiconductor device 12.

[0060] The first pole (which may be the collector) of the first fully-controlled semiconductor device 11 serves as the first end of the sub-module arm 1. The second pole (which may be the emitter) of the first fully-controlled semiconductor device 11 is connected to the first pole (which may be the collector) of the second fully-controlled semiconductor device 12, serving as the first midpoint A of the sub-module arm 1. The second pole (which may be the emitter) of the second fully-controlled semiconductor device 12 serves as the second end of the sub-module arm 1. Figure 2 In, u out represents the instantaneous value of the output voltage of the sub-module. In the half-bridge structure, both the above-mentioned first fully-controlled semiconductor device 11 and second fully-controlled semiconductor device 12 adopt IGBTs, and diodes D are anti-parallel connected to the IGBTs.

[0061] Exemplarily, the sub-module 10 further includes a control unit ( Figure 1 and Figure 2 not shown in). The control unit is connected to the mechanical switch K.

[0062] The control unit is configured to: send a bypass instruction to the mechanical switch K.

[0063] The mechanical switch K is configured to: close according to the bypass instruction.

[0064] Optionally, the sub-module 1 further includes a driving unit ( Figure 1 and Figure 2 not shown in). The control unit is further connected to the driving unit, and the driving unit is connected to the control poles of the fully-controlled semiconductor devices (i.e., the first fully-controlled semiconductor device 11, etc.) in the sub-module arm 1.

[0065] The control unit is further configured to: send a switching instruction to the driving unit.

[0066] The driving unit is configured to: drive the conduction or turn-off of the fully-controlled semiconductor device according to the switching instruction.

[0067] Among them, the switching instruction may include a conduction instruction and a turn-off instruction. Among them, the conduction instruction is used to indicate the conduction of the fully-controlled semiconductor device, and the turn-off instruction is used to indicate the turn-off of the fully-controlled semiconductor device.

[0068] Of course, the sub-module may further include an energy extraction unit, etc. The energy extraction unit can supply power to the control unit and the driving unit, which will not be introduced in detail in the embodiments of the present application.

[0069] The embodiment of the present application also provides a modular multilevel converter (MMC), which includes a plurality of the above-mentioned sub-modules, and the plurality of sub-modules can be connected in series.

[0070] In the embodiment of the present application, the modular multilevel converter can be a hybrid modular multilevel converter, as Figure 3 shown. Each arm can include a plurality of series-connected half-bridge sub-modules (i.e., HBSM1 to HBSMn) and a plurality of series-connected full-bridge sub-modules (i.e., FBSM1 to FBSMn). Figure 3 In, L is the arm reactance.

[0071] Among them, the full-bridge sub-module can adopt the Figure 1 shown sub-module, and the half-bridge sub-module can adopt the Figure 2 shown sub-module.

[0072] The embodiment of the present application also provides a DC power transmission system, which can include the above-mentioned modular multilevel converter.

[0073] The embodiment of the present application provides a bypass protection method for a sub-module, as Figure 4 shown. The bypass protection method includes the following steps:

[0074] Step S21: Obtain the working state of the sub-module 10 and the voltage of the support capacitor C. The working state includes the operating state and the fault state.

[0075] Step S22: If the working state is the fault state or the voltage of the support capacitor C exceeds the preset voltage threshold, some fully controlled semiconductor devices in the sub-module arm 1 are turned on, and the mechanical switch K acts according to the received bypass command.

[0076] It can be understood that for the half-bridge sub-module, the second fully controlled semiconductor device 12 in the sub-module arm 1 is turned on. For the full-bridge sub-module, the first fully controlled semiconductor device 11 and the third fully controlled semiconductor device 13 in the sub-module arm 1 are turned on, or the second fully controlled semiconductor device 12 and the fourth fully controlled semiconductor device 14 in the sub-module arm 1 are turned on.

[0077] Step S23: If the mechanical switch K is not closed (i.e., the mechanical switch K refuses to act, which means that the voltage of the support capacitor C continues to rise), the sub-module 10 is bypass protected through the bidirectional thyristor Thy.

[0078] Among them, the gate and cathode of the bidirectional thyristor Thy are connected.

[0079] In some embodiments, the sub-module 10 adopts as Figure 1The full-bridge structure shown. If the mechanical switch K is not closed, the sub-module 10 is bypassed and protected through the bidirectional turn-breakdown thyristor Thy, including:

[0080] If the mechanical switch K is not closed, all the fully controlled semiconductor devices in the sub-module arm are turned off.

[0081] If the instantaneous value of the output voltage of sub-module arm 1 (which can be a positive voltage) is higher than or equal to the preset forward turn-breakdown voltage of the bidirectional turn-breakdown thyristor Thy, or, if the instantaneous value of the output voltage of sub-module arm 1 (which can be a negative voltage) is lower than or equal to the preset reverse turn-breakdown voltage of the bidirectional turn-breakdown thyristor Thy, the bidirectional turn-breakdown thyristor Thy breaks down forward or reversely, and the bidirectional turn-breakdown thyristor Thy is in a short-circuit state, and the sub-module 10 can be bypassed to realize the protection of the sub-module 10. It can be understood that if the instantaneous value of the output voltage of sub-module arm 1 is lower than the preset forward turn-breakdown voltage of the bidirectional turn-breakdown thyristor Thy, the bidirectional turn-breakdown thyristor Thy will not break down.

[0082] Optionally, the amplitudes of the preset forward turn-breakdown voltage and the reverse turn-breakdown voltage are equal and both are greater than the voltage threshold.

[0083] In some embodiments, the sub-module 10 adopts the half-bridge structure as shown in Figure 2 In step S1, if the mechanical switch K is not closed, the sub-module 10 is bypassed and protected through the bidirectional turn-breakdown thyristor Thy, including

[0084] If the mechanical switch K is not closed, all the fully controlled semiconductor devices in the sub-module arm are turned off.

[0085] If the instantaneous value of the output voltage of the sub-module 10 (which is a positive voltage) is higher than or equal to the preset forward turn-breakdown voltage of the bidirectional turn-breakdown thyristor Thy, the bidirectional turn-breakdown thyristor Thy breaks down forward, and the bidirectional turn-breakdown thyristor Thy is in a short-circuit state, and the sub-module 10 can be bypassed to realize the protection of the sub-module 10. It can be understood that if the instantaneous value of the output voltage of the sub-module 10 (which is a positive voltage) is lower than the preset forward turn-breakdown voltage of the bidirectional turn-breakdown thyristor Thy, the bidirectional turn-breakdown thyristor Thy will not break down. If the voltage of the support capacitor drops to the normal range, the bypass protection of the sub-module will start again.

[0086] Optionally, the amplitude of the preset forward turn-breakdown voltage is greater than the voltage threshold.

[0087] It can be understood that the bidirectional turn-breakdown thyristor Thy can break down only once, and once it breaks down, a permanent sub-module bypass is formed and will not be affected by external electromagnetic interference or other factors for the bypass state. The sub-module can be put back into use only after being replaced during the next power outage for maintenance.

[0088] As Figure 5 shown, taking the full-bridge sub-module shown in Figure 1 as an example, the bypass protection method provided by the embodiments of the present application may include the following steps:

[0089] Step 31: Obtain the working state of the sub-module 10 and the voltage of the support capacitor C.

[0090] Step 32: Determine whether the working state is a fault state or whether the voltage of the support capacitor C exceeds a preset voltage threshold. If so, execute Step 22; otherwise, return to Step 31.

[0091] Step 33: The first fully-controlled semiconductor device 11 and the third fully-controlled semiconductor device 13 are turned on, or the second fully-controlled semiconductor device 12 and the fourth fully-controlled semiconductor device 14 are turned on, and the mechanical switch K acts according to the received bypass command.

[0092] Step 34: Determine whether the mechanical switch K is closed. If so, execute Step 35; otherwise, continue to execute Step 34.

[0093] Step 35: All fully-controlled semiconductor devices are turned off, and the mechanical switch K acts according to the received bypass command.

[0094] Step 36: Determine whether the instantaneous value of the output voltage of the sub-module 10 is higher than or equal to the preset forward breakover voltage of the bidirectional breakover thyristor. If so, execute Step 37; otherwise, return to Step 35.

[0095] Step 37: The bidirectional breakover thyristor Thy breaks down forward or backward, and the sub-module 10 is bypassed.

[0096] Those skilled in the art should understand that the embodiments of the application can be provided as a method, a system, or a computer program product. Therefore, the application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0097] The application is described with reference to the flowcharts and / or block diagrams of methods, apparatus (systems), and computer program products according to the embodiments of the application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in one flow Figure 1 one flow or multiple flows and / or blocks Figure 1 or multiple blocks.

[0098] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured article including instruction means, and the instruction means implement the functions specified in one flow Figure 1 one flow or multiple flows and / or blocks Figure 1 or multiple blocks.

[0099] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one flow Figure 1 one flow or multiple flows and / or blocks Figure 1 or multiple blocks.

[0100] The above are only embodiments of the application and are not used to limit the application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the application are included in the scope of the claims of the pending application for the invention.

Claims

1. A submodule, characterized in that: It includes voltage grading resistor, supporting capacitor, bidirectional breakdown thyristor, mechanical switch and submodule bridge arm; The first ends of the voltage-equalizing resistor and the supporting capacitor are respectively connected to the first end of the submodule bridge arm, and the second ends of the voltage-equalizing resistor and the supporting capacitor are respectively connected to the second end of the submodule bridge arm; the first ends of the bidirectional breakover thyristor and the mechanical switch are respectively connected to the first midpoint of the submodule bridge arm, and the second ends of the bidirectional breakover thyristor and the mechanical switch are respectively connected to the second midpoint of the submodule bridge arm or the second end of the submodule bridge arm; Wherein, the gate and cathode of the bidirectional breakdown thyristor are connected.

2. The submodule according to claim 1, characterized in that: The submodule bridge arm adopts a full-bridge structure; The submodule bridge arm includes a first fully-controlled semiconductor device, a second fully-controlled semiconductor device, a third fully-controlled semiconductor device and a fourth fully-controlled semiconductor device; The first poles of the first fully-controlled semiconductor device and the third fully-controlled semiconductor device are respectively connected to serve as the first end of the sub-module bridge arm; the second pole of the first fully-controlled semiconductor device is connected to the first pole of the second fully-controlled conductor device to serve as the first midpoint of the sub-module bridge arm; the second pole of the third fully-controlled semiconductor device is connected to the first pole of the fourth fully-controlled semiconductor device to serve as the second midpoint of the sub-module bridge arm; the second poles of the second fully-controlled conductor device and the fourth fully-controlled semiconductor device are respectively connected to serve as the second end of the sub-module bridge arm.

3. The submodule according to claim 1, characterized in that: The submodule bridge arm adopts a half-bridge structure; The submodule bridge arm includes a first fully-controlled semiconductor device and a second fully-controlled semiconductor device; The first electrode of the first fully-controlled semiconductor device serves as the first end of the bridge arm of the submodule; The second pole of the first fully-controlled semiconductor device is connected to the first pole of the second fully-controlled semiconductor device as the first midpoint of the submodule bridge arm; the second pole of the second fully-controlled semiconductor device serves as the second end of the submodule bridge arm.

4. The submodule according to claim 1, characterized in that: The submodule further includes a control unit; the control unit is connected to the mechanical switch; The control unit is used to: send a bypass instruction to the mechanical switch; The mechanical switch is used to close according to the bypass instruction.

5. The submodule according to claim 4, characterized in that: The submodule also includes a drive unit; The control unit is also connected to the driving unit, and the driving unit is connected to the control electrode of the fully controlled semiconductor device in the bridge arm of the submodule; The control unit is also used to: send a switch instruction to the drive unit; The driving unit is used to: drive the fully-controlled semiconductor device to be turned on or off according to the switching instruction; The switch instruction includes a turn-on instruction and a turn-off instruction; wherein the turn-on instruction is used to instruct the fully-controlled semiconductor device to turn on, and the turn-off instruction is used to instruct the fully-controlled semiconductor device to turn off.

6. A bypass protection method for a submodule, characterized in that: include: Obtaining the working status of the submodule and the voltage of the supporting capacitor; The working state includes an operating state and a fault state; If the working state is a fault state or the voltage of the support capacitor exceeds a preset voltage threshold, part of the fully controlled semiconductor devices in the bridge arm of the submodule are turned on, and the mechanical switch is actuated according to the received bypass instruction; If the mechanical switch is not closed, the submodule is bypass-protected by a bidirectional break-down thyristor; Wherein, the gate and cathode of the bidirectional breakdown thyristor are connected.

7. The bypass protection method according to claim 6, characterized in that: The submodule adopts a full-bridge structure; if the mechanical switch is not closed, the submodule is bypass-protected by a bidirectional break-down thyristor, including: If the mechanical switch is not closed, all fully controlled semiconductor devices in the submodule bridge arm are turned off; If the instantaneous value of the output voltage of the submodule bridge arm is higher than or equal to the preset forward breakover voltage of the bidirectional breakover thyristor, or the instantaneous value of the output voltage of the submodule bridge arm is lower than or equal to the preset reverse breakover voltage of the bidirectional breakover thyristor, the bidirectional breakover thyristor breaks down forward or reversely, and the submodule is bypassed.

8. The bypass protection method according to claim 7, characterized in that: The amplitude of the preset forward breakover voltage is greater than the preset voltage threshold.

9. The bypass protection method according to claim 6, characterized in that: The submodule adopts a half-bridge structure; if the mechanical switch is not closed, the submodule is bypass-protected by a bidirectional break-down thyristor, including: If the mechanical switch is not closed, all fully controlled semiconductor devices in the submodule bridge arm are turned off; If the instantaneous value of the output voltage of the submodule is higher than or equal to the preset forward breakover voltage of the bidirectional breakover thyristor, the bidirectional breakover thyristor breaks down forward, and the submodule is bypassed.

10. The bypass protection method according to claim 9, characterized in that: The amplitude of the preset forward breakover voltage is equal to the amplitude of the reverse breakover voltage, and both are greater than the preset voltage threshold.

11. A modular multilevel converter, characterized in that: The method comprises a plurality of submodules according to any one of claims 1 to 5; the plurality of submodules are connected in series.

12. A direct current transmission system, characterized in that: Comprising the modular multilevel converter as claimed in claim 11.

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