A protection method and device for a three-level converter

Abstract

This application provides a protection method and apparatus for a three-level inverter. When executing the method, firstly, it is determined whether the three-level inverter has malfunctioned; if the three-level inverter has malfunctioned, the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor of the three-level inverter is determined; then, based on the relationship between the upper and lower capacitor voltages, the devices in the three-level inverter are controlled to turn off. In this way, by controlling the turn-off of the devices in the three-level inverter according to the relationship between the upper and lower capacitor voltages, the voltage across the devices in the three-level inverter can be kept within a preset range, thereby preventing damage to the devices. Thus, in the event of a three-level inverter malfunction, the devices can be controlled to turn off simultaneously without damage, solving the problem that existing solutions for dealing with ANPC three-level inverter malfunctions still result in device damage.

Description

Technical Field

[0001] This application relates to the field of three-level converter protection technology, and in particular to a protection method and device for a three-level converter. Background Technology

[0002] An inverter is a converter that transforms direct current (DC) energy into alternating current (AC) energy with fixed frequency and voltage, or with adjustable frequency and voltage. In high-voltage, high-power photovoltaic inverters, active neutral point clamped (ANPC) and neutral point clamped (NPC) three-level inverter topologies are commonly used. Due to its advantages such as flexible control, adaptability to various operating conditions, uniform temperature distribution, and easier heat dissipation, the ANPC three-level inverter topology is increasingly widely used in photovoltaic inverters.

[0003] When an ANPC three-level inverter malfunctions, such as when the current momentarily exceeds the normal value, the devices must be shut down promptly to protect them and keep them operating in a safe zone. Existing solutions for ANPC three-level inverter malfunctions often fail to shut down simultaneously during actual shutdown because the bridge arms do not consist entirely of similar devices with varying parasitic parameters and drive delays. This can lead to device damage. Summary of the Invention

[0004] In view of this, the present application provides a protection method and apparatus for a three-level inverter, which aims to solve the problem that the existing solutions for dealing with the failure of ANPC three-level inverters will damage the devices.

[0005] In a first aspect, embodiments of this application provide a protection method for a three-level converter, the method comprising:

[0006] Determine if a three-level converter has malfunctioned;

[0007] If the three-level converter fails, determine the relationship between the upper capacitor voltage and the lower capacitor voltage of the three-level converter.

[0008] The device in the three-level converter is turned off based on the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor.

[0009] Optionally, controlling the turn-off of devices in the three-level converter based on the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor specifically includes:

[0010] If the voltage of the upper capacitor is equal to the voltage of the lower capacitor, then the state of the fifth and sixth devices before the fault is maintained, while the first, second, third, and fourth devices are turned off.

[0011] Optionally, controlling the turn-off of devices in the three-level converter based on the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor specifically includes:

[0012] If the voltage of the upper capacitor is greater than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously.

[0013] After a preset time period, the fifth device is turned off, and the sixth device is turned on.

[0014] Optionally, controlling the turn-off of devices in the three-level converter based on the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor specifically includes:

[0015] If the voltage of the upper capacitor is less than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously.

[0016] After a preset time period, the sixth device is turned off, and the fifth device is turned on.

[0017] Optionally, the method further includes:

[0018] When the difference between the voltage of the upper capacitor and the voltage of the lower capacitor is less than a preset threshold and the freewheeling current of the inductor is zero, the fifth and sixth devices are turned off.

[0019] Secondly, embodiments of this application provide a protection device for a three-level converter, the device comprising: a first judgment module, a second judgment module, and a control module;

[0020] The first judgment module is used to determine whether the three-level converter has malfunctioned;

[0021] If the three-level converter malfunctions, the second judgment module is used to determine the relationship between the upper capacitor voltage and the lower capacitor voltage of the three-level converter.

[0022] The control module is used to control the shutdown of the devices in the three-level converter according to the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor.

[0023] Optionally, the control module is specifically used for:

[0024] If the voltage of the upper capacitor is equal to the voltage of the lower capacitor, then the state of the fifth and sixth devices before the fault is maintained, while the first, second, third, and fourth devices are turned off.

[0025] Optionally, the control module is specifically used for:

[0026] If the voltage of the upper capacitor is greater than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously.

[0027] After a preset time period, the fifth device is turned off, and the sixth device is turned on.

[0028] Optionally, the control module is specifically used for:

[0029] If the voltage of the upper capacitor is less than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously.

[0030] After a preset time period, the sixth device is turned off, and the fifth device is turned on.

[0031] Optionally, the control module is further configured to:

[0032] When the difference between the voltage of the upper capacitor and the voltage of the lower capacitor is less than a preset threshold and the freewheeling current of the inductor is zero, the fifth and sixth devices are turned off.

[0033] This application provides a protection method and apparatus for a three-level inverter. When executing the method, firstly, it is determined whether the three-level inverter has malfunctioned; if the three-level inverter has malfunctioned, the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor of the three-level inverter is determined; then, based on the relationship between the upper capacitor voltage and the lower capacitor voltage, the devices in the three-level inverter are controlled to turn off. In this way, by controlling the turn-off of the devices in the three-level inverter according to the relationship between the upper capacitor voltage and the lower capacitor voltage, the voltage across the devices in the three-level inverter can be kept within a preset range, thereby preventing damage to the devices. Thus, it achieves the effect of controlling the simultaneous turn-off of devices when the three-level inverter malfunctions without damaging the devices, solving the problem that existing solutions for dealing with ANPC three-level inverter malfunctions still result in device damage. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in this embodiment or the prior art, the drawings used in the description of the embodiment or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This application provides an active clamping three-level topology diagram for embodiments of the present application.

[0036] Figure 2 This is a fault blocking state diagram provided in an embodiment of this application;

[0037] Figure 3 This is an equivalent state diagram of fault blocking provided in an embodiment of this application;

[0038] Figure 4 A flowchart illustrating a protection method for a three-level converter provided in this application embodiment;

[0039] Figure 5 This is a fault protection first state diagram provided in the embodiments of this application when the upper capacitor voltage and the lower capacitor voltage do not deviate;

[0040] Figure 6 This is a second state diagram for fault protection when the upper capacitor voltage and the lower capacitor voltage do not deviate, as provided in the embodiments of this application.

[0041] Figure 7 This is a third state diagram for fault protection when the upper capacitor voltage and the lower capacitor voltage do not deviate, as provided in the embodiments of this application.

[0042] Figure 8 This application provides a first state diagram for fault protection when the upper capacitor voltage is greater than the lower capacitor voltage.

[0043] Figure 9 This application provides a second state diagram for fault protection when the upper capacitor voltage is less than the lower capacitor voltage.

[0044] Figure 10 This application provides a first state diagram for fault protection when the upper capacitor voltage is less than the lower capacitor voltage.

[0045] Figure 11 This application provides a second state diagram for fault protection when the upper capacitor voltage is less than the lower capacitor voltage.

[0046] Figure 12 A flowchart of an ANPC three-level fault blocking method provided in this application embodiment;

[0047] Figure 13 This is a schematic diagram of the structure of a protection device for a three-level converter provided in an embodiment of this application. Detailed Implementation

[0048] An inverter is a converter that transforms direct current (DC) energy into alternating current (AC) energy with fixed frequency and voltage, or with adjustable frequency and voltage. In high-voltage, high-power photovoltaic inverters, active neutral point clamped (ANPC) and neutral point clamped (NPC) three-level inverter topologies are commonly used. Due to its advantages such as flexible control, adaptability to various operating conditions, uniform temperature distribution, and easier heat dissipation, the ANPC three-level inverter topology is increasingly widely used in photovoltaic inverters.

[0049] When an ANPC three-level inverter malfunctions, such as when the current momentarily exceeds the normal value, the devices must be shut down promptly to protect them and keep them operating in a safe zone. Existing solutions for ANPC three-level inverter malfunctions often fail to shut down simultaneously during actual shutdown because the bridge arms do not consist entirely of similar devices with varying parasitic parameters and drive delays. This can lead to device damage.

[0050] The inventors of this application discovered through research that during actual shutdown, the inductor current needs to carry over during the protection instant. This carryover must ensure that the withstand voltage of each device remains within its rated range. Although some solutions propose using clamping transistors for the carryover process, they overlook the significant potential deviation in three-level inverters, which exceeds the withstand voltage of a single transistor. Therefore, the inventors propose that during inverter protection, a reasonable arrangement of the device shutdown sequence ensures that the voltage across each power device does not exceed its normal value. This effectively avoids damage to devices caused by the inability to shut down simultaneously due to factors such as the presence of different types of devices in the bridge arm, varying parasitic parameters, and drive delays. Therefore, this application presents the proposed solution.

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0052] See Figure 1 , Figure 1 The active clamped three-level topology provided in this application embodiment has six power devices in each phase arm. All six power devices can be insulated gate bipolar transistors (IGBTs). T2 and T3 can be silicon carbide (SiC) devices as high-frequency transistors, or T1, T4, T5, and T6 can be SiC devices as high-frequency transistors.

[0053] See Figure 2 , Figure 2 The fault-blocking state diagram provided in this application embodiment illustrates that when an overcurrent or overvoltage fault occurs, if all devices are turned off simultaneously, the inductor current cannot change instantaneously and needs to pass through the diode for freewheeling. In this state, the DC bus voltage is directly applied to transistors T1 and T2. If, due to drive delay or different switching speeds of T1 and T2, transistor T2 turns off first, it may be damaged because it bears the voltage of the entire bus.

[0054] Even if transistors T1 and T2 are turned off simultaneously, if T1 is an IGBT device and T2 is a SiC device, the significant difference in parasitic parameters between the IGBT and SiC devices prevents them from achieving voltage equalization under static conditions.

[0055] See Figure 3 , Figure 3 The fault shielding equivalent state diagram provided in the embodiments of this application will Figure 2 In this configuration, transistors T1 and T2 are equivalent to DS parasitic capacitances. Since the parasitic parameters of IGBT devices and SiC devices differ significantly, according to the capacitance impedance expression 1 / jwc, it can be seen that the static voltage division difference between IGBT devices and SiC devices is substantial. Therefore, under these circumstances, the devices are also at risk of damage.

[0056] The solution provided in this application can effectively address the two situations that may occur when the ANPC three-level inverter is blocked during fault protection.

[0057] See Figure 4 , Figure 4 A flowchart of a protection method for a three-level converter provided in this application embodiment includes:

[0058] S401. Determine if the three-level converter has malfunctioned.

[0059] When a three-level converter malfunctions, overcurrent or overvoltage may occur. The presence of overcurrent or overvoltage can be detected to determine if a fault has occurred. In this case, an ANPC three-level inverter can be selected.

[0060] S402. If the three-level converter malfunctions, determine the relationship between the upper capacitor voltage and the lower capacitor voltage of the three-level converter.

[0061] If a three-level converter malfunctions, the relationship between the upper capacitor voltage Uup and the lower capacitor voltage Udown of the three-level converter is then determined, and the shutdown device method is selected based on the relationship between the upper and lower capacitor voltages of the three-level converter.

[0062] S403. Based on the relationship between the upper capacitor voltage and the lower capacitor voltage, control the turn-off of the devices in the three-level converter.

[0063] When the voltage of the upper capacitor and the voltage of the lower capacitor are not deviated, or the deviation is very small and can be approximated as not deviating; before the three-level converter fails, at least one of devices T5 and T6 is in the conducting state. After the failure occurs, the state of devices T5 and T6 is maintained, while devices T1, T2, T3, and T4 are turned off. Here, T1 is the first device, T2 is the second device, T3 is the third device, T4 is the fourth device, T5 is the fifth device, and T6 is the sixth device.

[0064] When the ANPC three-level inverter is operating normally, transistors T5 and T6 act as clamping transistors, and at least one of them must be in the conducting state. In case of a fault, T5 and T6 are maintained in their current states, while transistors T1, T2, T3, and T4 are quickly turned off. After the inductor freewheeling current ends, transistors T5 and T6 are turned off. The possible states after this protection method are as follows: Figure 5 , Figure 6 , Figure 7 As shown, Figure 5 This is a fault protection first state diagram provided in an embodiment of this application when the upper capacitor voltage and the lower capacitor voltage are not deviated. In this case, device T6 is turned on before the fault, the current flows in the output direction, the freewheeling current flows through the diode connected in parallel with device T3, the voltage across T3 is zero, and T1 and T2 share half of the DC bus voltage; the voltage across T4 is half of the bus voltage. In the figure, n is the midpoint potential.

[0065] Figure 6 The second state diagram of fault protection provided in the embodiment of this application is when the upper capacitor voltage and the lower capacitor voltage are not deviated. In this case, device T5 is turned on before the fault, the current flows in the output direction, the freewheeling current flows through the diode connected in parallel with devices T3 and T4, the voltage across T3 and T4 is zero, and the voltage across T1 and T2 is half of the DC bus voltage.

[0066] Figure 7 The third state diagram of fault protection provided in this application embodiment is when the upper capacitor voltage and the lower capacitor voltage are not deviated. In this case, before the fault, devices T5 and T6 are both turned on, the current flows in the output direction, the freewheeling current flows through the diode connected in parallel with device T3, the voltage across T3 is zero, T1 and T2 share half of the DC bus voltage; the voltage across T4 is half of the bus voltage.

[0067] The above three scenarios ensure that the voltage across each device does not exceed half of the DC bus voltage, thus achieving the rated voltage withstand capability of each device and preventing device damage.

[0068] When the voltage of the upper capacitor is greater than the voltage of the lower capacitor, transistors T1, T2, T3, and T4 are simultaneously turned off. After a preset time period, T5 is turned off, and T6 is turned on. The preset time can be set to 3µs. The midpoint potential of a three-level inverter is difficult to control. When a fault occurs, the voltage of the upper capacitor is very high. If transistor T5 is used for conduction clamping, transistor T1 will bear the entire voltage of the upper bus, which can easily damage transistor T1. Therefore, transistor T6 is used for conduction clamping. When the voltage difference between the upper and lower capacitors is within 30V and the freewheeling current drops to 0, the clamping transistor T6 is turned off. The state after protection is as follows: Figure 8 , Figure 9 As shown. Figure 8 The first state diagram of fault protection provided in this application embodiment is when the upper capacitor voltage is greater than the lower capacitor voltage. The current flows in the output direction. The freewheeling current flows through the diode connected in parallel with device T3. The voltage across T3 is zero. T1 and T2 share more than half of the DC bus voltage. The voltage across T4 is less than half of the bus voltage. Figure 9 The second fault protection state diagram provided in this application embodiment shows that when the upper capacitor voltage is less than the lower capacitor voltage. The current flows in the input direction, and the freewheeling current flows through the diodes connected in parallel with devices T1 and T2. T3 and T4 share the DC bus voltage. Both protection states ensure that the voltage across each device does not exceed half of the DC bus voltage, thus keeping the withstand voltage of each device within the rated range and preventing device damage.

[0069] When the voltage of the upper capacitor is less than the voltage of the lower capacitor, transistors T1, T2, T3, and T4 are simultaneously turned off. After a preset time period, T6 is turned off, and T5 is turned on. The preset time can be set to 3µs. If the voltage of the lower capacitor is too high during a fault, using transistor T6 for clamping would cause transistor T4 to bear the entire voltage of the upper bus, which could easily damage transistor T4. Therefore, transistor T5 is used for clamping. When the voltage difference between the upper and lower capacitors is within 30V and the freewheeling current drops to 0, the clamping transistor T5 is then turned off. The state after protection is as follows: Figure 10 , Figure 11 As shown. Figure 10 The first state diagram of fault protection provided in this application embodiment is when the upper capacitor voltage is less than the lower capacitor voltage. The current flows in the output direction. The freewheeling current flows through the diodes connected in parallel with devices T3 and T4. T1 and T2 share the DC bus voltage. Figure 11 The second state diagram for fault protection provided in this application embodiment is when the upper capacitor voltage is less than the lower capacitor voltage. The current flows in the input direction, the freewheeling current flows through the diode connected in parallel with device T2, T3 and T4 share more than half of the DC bus voltage, and T1 shares less than half of the bus voltage.

[0070] Both protection states ensure that the voltage across each device does not exceed half of the DC bus voltage, thus keeping the withstand voltage of each device within the rated range and preventing device damage.

[0071] This application provides a protection method for a three-level inverter. When executing the method, firstly, it is determined whether the three-level inverter has malfunctioned; if the three-level inverter has malfunctioned, the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor of the three-level inverter is determined; then, based on the relationship between the upper capacitor voltage and the lower capacitor voltage, the devices in the three-level inverter are controlled to turn off. In this way, by controlling the turn-off of the devices in the three-level inverter according to the relationship between the upper capacitor voltage and the lower capacitor voltage, the voltage across the devices in the three-level inverter can be kept within a preset range, thereby preventing damage to the devices. Thus, it achieves the effect of controlling the simultaneous turn-off of devices when the three-level inverter malfunctions without damaging the devices, solving the problem that existing solutions for dealing with ANPC three-level inverter malfunctions still result in device damage.

[0072] In an optional embodiment of this application, after fault protection is performed, regardless of the state of devices T5 and T6 before the fault occurs, when the difference between the upper and lower capacitors is less than a preset threshold (the preset threshold can be 30V) and the freewheeling current drops to 0, devices T5 and T6 are turned off.

[0073] See Figure 12 , Figure 12 The flowchart of an ANPC three-level fault blocking method provided in this application embodiment first determines whether a fault has occurred. If a fault has occurred, it determines whether the voltage deviation between the upper and lower capacitors exceeds a threshold. If the deviation is small, T1, T2, T3, and T4 are simultaneously turned off, and T5 and T6 are maintained in their pre-fault state until the freewheeling current drops to 0. If the deviation is large, it determines whether the voltage of the upper capacitor is greater than that of the lower capacitor. If it is, T1, T2, T3, and T4 are simultaneously turned off, and after 3µs, T6 is turned on and T5 is turned off. The protection ends when the voltage difference between the upper and lower capacitors is within 30V and the freewheeling current drops to 0. If the voltage of the upper capacitor is not greater than that of the lower capacitor, T1, T2, T3, and T4 are simultaneously turned off, and after 3µs, T5 is turned on and T6 is turned off. The protection ends when the voltage difference between the upper and lower capacitors is within 30V and the freewheeling current drops to 0.

[0074] The above are some specific implementations of a protection method for a three-level converter provided in the embodiments of this application. Based on this, this application also provides a corresponding protection device for a three-level converter. The device provided in the embodiments of this application will be described below from the perspective of functional modularity.

[0075] See Figure 13 , Figure 13 This is a schematic diagram of a protection device for a three-level converter provided in an embodiment of the present application. The device includes: a first judgment module 1301, a second judgment module 1302, and a control module 1303.

[0076] The first judgment module 1301 is used to determine whether the three-level converter has malfunctioned;

[0077] If the three-level converter fails, the second judgment module 1302 is used to determine the relationship between the upper capacitor voltage and the lower capacitor voltage of the three-level converter.

[0078] The control module 1303 is used to control the shutdown of the devices in the three-level converter according to the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor.

[0079] This application provides a protection device for a three-level inverter, used to execute a corresponding protection method for a three-level inverter. When executing the method, it is first determined whether a fault has occurred in the three-level inverter; if the three-level inverter has faulted, the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor of the three-level inverter is determined; then, based on the relationship between the upper capacitor voltage and the lower capacitor voltage, the devices in the three-level inverter are controlled to turn off. In this way, by controlling the turn-off of the devices in the three-level inverter according to the relationship between the upper capacitor voltage and the lower capacitor voltage, the voltage across the devices in the three-level inverter can be kept within a preset range, thereby preventing damage to the devices. Thus, it achieves the effect of controlling the simultaneous turn-off of devices when the three-level inverter fails, without damaging the devices, solving the problem that existing solutions for dealing with faults in ANPC three-level inverters still result in device damage.

[0080] In an optional embodiment of this application, the control module 1303 is specifically used for:

[0081] If the voltage of the upper capacitor is equal to the voltage of the lower capacitor, then the state of the fifth and sixth devices before the fault is maintained, while the first, second, third, and fourth devices are turned off.

[0082] In an optional embodiment of this application, the control module 1303 is specifically used for:

[0083] If the voltage of the upper capacitor is greater than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously.

[0084] After a preset time period, the fifth device is turned off, and the sixth device is turned on.

[0085] In an optional embodiment of this application, the control module 1303 is specifically used for:

[0086] If the voltage of the upper capacitor is less than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously.

[0087] After a preset time period, the sixth device is turned off, and the fifth device is turned on.

[0088] In an optional embodiment of this application, the control module 1303 is further configured to:

[0089] When the difference between the voltage of the upper capacitor and the voltage of the lower capacitor is less than a preset threshold and the freewheeling current of the inductor is zero, the fifth and sixth devices are turned off.

[0090] In the embodiments of this application, the terms "first" and "second" in the names such as "first device" and "second device" are only used as name identifiers and do not represent the order of first and second.

[0091] As can be seen from the above description of the embodiments, those skilled in the art can clearly understand that all or part of the steps in the methods of the above embodiments can be implemented by means of software plus a general-purpose hardware platform. Based on this understanding, the technical solution of this application can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as a read-only memory (ROM) / RAM, magnetic disk, optical disk, etc., including several instructions to cause a computer device (which may be a personal computer, a server, or a network communication device such as a router) to execute the methods described in various embodiments or some parts of the embodiments of this application.

[0092] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on its differences from other embodiments. In particular, the apparatus embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0093] The above description is merely an exemplary implementation of this application and is not intended to limit the scope of protection of this application.

Claims

1. A protection method for a three-level converter, characterized in that, The method includes: Determine if a three-level converter has malfunctioned; If the three-level converter fails, determine the relationship between the upper capacitor voltage and the lower capacitor voltage of the three-level converter. The device in the three-level converter is turned off based on the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor. The step of controlling the turn-off of devices in the three-level converter based on the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor specifically includes: If the voltage of the upper capacitor is equal to the voltage of the lower capacitor, then the state of the fifth and sixth devices before the fault is maintained, while the first, second, third and fourth devices are turned off. If the voltage of the upper capacitor is greater than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously. After a preset time period, the fifth device is turned off and the sixth device is turned on; If the voltage of the upper capacitor is less than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously. After a preset time period, the sixth device is turned off, and the fifth device is turned on.

2. The method according to claim 1, characterized in that, The method further includes: When the difference between the voltage of the upper capacitor and the voltage of the lower capacitor is less than a preset threshold and the freewheeling current of the inductor is zero, the fifth and sixth devices are turned off.

3. A protection device for a three-level converter, characterized in that, The device includes: a first judgment module, a second judgment module, and a control module; The first judgment module is used to determine whether the three-level converter has malfunctioned; If the three-level converter malfunctions, the second judgment module is used to determine the relationship between the upper capacitor voltage and the lower capacitor voltage of the three-level converter. The control module is used to control the shutdown of the devices in the three-level converter according to the relationship between the voltage of the upper capacitor and the voltage of the lower capacitor. The control module is specifically used for: If the voltage of the upper capacitor is equal to the voltage of the lower capacitor, then the state of the fifth and sixth devices before the fault is maintained, while the first, second, third and fourth devices are turned off. If the voltage of the upper capacitor is greater than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously. After a preset time period, the fifth device is turned off and the sixth device is turned on; If the voltage of the upper capacitor is less than the voltage of the lower capacitor, then the first device, the second device, the third device and the fourth device are turned off simultaneously. After a preset time period, the sixth device is turned off, and the fifth device is turned on.

4. The apparatus according to claim 3, characterized in that, The control module is also used for: When the difference between the voltage of the upper capacitor and the voltage of the lower capacitor is less than a preset threshold and the freewheeling current of the inductor is zero, the fifth and sixth devices are turned off.

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